Project and Design of the Porcelain Enamelled Steel … · Enamelling Steel Cold Rolled Steel: Chemical Composition & Mechanical Characteristics Steel Quality Re(MPa)/ Ys Rm(MPa)

Turkish Ceramic Society (TDS) Porcelain(Vitreous) Enamels course

Project and Design of the

Porcelain Enamelled SteelComposites

by Silvano PagliucaI E I S t G lI.E.I. Secretary General

ContentContent

M h i l Th l d S i ti f• Mechanical, Thermal and Sagging properties ofEnamelling grade steel.

• Mechanical and Phisical Properties of Porcelain• Mechanical and Phisical Properties of PorcelainVitreous Enamel.

• Mechanical and Phisical properties of the• Mechanical and Phisical properties of the Composite Steel‐Porcelain VitreousEnamel (Bilamina).Enamel (Bilamina).

• Stress/Strain Analysis of the Composite Steel‐Porcelain Vitreous Enamel.

• Designing Links and Guidelines of enamelledware.

Enamelling SteelEnamelling Steel

1. Chemical Physical Properties

2 M h i l P ti2. Mechanical Properties

3 Thermical Properties3. Thermical Properties

4. Fishscaling properties4. Fishscaling properties

5. Surface reactivity

Enamelling steelg

Iron - Carbon Equilibrium Diagram

Liquid GraphiteLiquid(Fe - δ)

C Austenite

Graphite solubility in liquid iron (Fe - δ)

Liquid + Fe -δ

Liquid

Peritectic level 1495 °C

Austenite +

ferrite

Tem

pera

ture

°C

Cementite (Fe3C)

empe

ratu

re °C

Austenite γ

Liquid + austenite

Ferrite + Cementite

Te

Eutectic level

Weight % of carbon

Weight % of carbon

Steel LatticeSteel is a crystalline aggregate with more than 99% Iron

atoms , containing less than 1% Carbon atoms and some

additional elements.

Its structure can have several geometrical configurations :Its structure can have several geometrical configurations :

e.g. BCC (body centred cubic), representing structure

α‐ferrite and FCC (face‐centered cubic) structure, called

austenite or γ‐iron.

phase of steel.

BCC

phase of steel.

These structures and their defects, depending on

h i l d h l f ll hchemical and thermal treatment of metallurgy, have

direct consequences on the macroscopic behaviour of

steel.steel.

FCC


• Low‐carbon enamelling steels

D b i d t l• Decarburized steels• Interstitial free steels• Interstitial free steels• Titanium‐stabilized steelsTitanium stabilized steels

• Common cold‐rolled steels

• Common hot‐rolled steels

Enamelling Steel GradeEnamelling Steel Grade

Enamelling Steel GradeEnamelling Steel Grade


Cold Rolled Steel: Chemical Composition &

Mechanical CharacteristicsMechanical Characteristics

Steel Quality Re(MPa)/Ys

Rm(MPa)/UYs

A % per s < 3mm

C % Ti %/U s < 3mm

DC01EK 270 max 270-390 30 min 0,08 max --

DC04EK 220 max 270-350 36 min 0,08 max --

DC06EK 190 max 270-350 38 min 0 02 max 0 30 maxDC06EK 190 max 270-350 38 min 0,02 max 0,30 max

K Key

Cold rolled steel for drawing and enamelling (EN 10209)

Enamelling Steel : QUALITY FOR 2C / 1F ENAMELLING

• Cold rolled steel for 2C/1F enamelling : Grade DC04ES, DC06EK, DC07EK and (SOLFER®)(SOLFER )

– DC04ES grade is cold rolled steel principally intended for 2 coat / 1 fire enamelling process after only degreasing.

– This grade is suitable for light and deep drawing and, after degreasing, suited to 2 coat / 1 fire enamelling by wet / powder or wet/wet process. It also offer an excellent resistance to fish scales defectIt also offer an excellent resistance to fish scales defect.

_ DC04ES is particularly suitable for domestic appliances (hobs, cover, side panels,…) and cookware industry. This steel can also be used for

hi l larchitectural panels.

– DC06EK, DC07EK and SOLFER® (Aluminium killed grade) are also

compatible with this process by using adapted ground enamelscompatible with this process, by using adapted ground enamels.

Enamelling Steel : QUALITY FOR 2C / 1F ENAMELLINGg Q /

• Requirements concerning grades DC04ES, DCO6EK and DC07EK

– Fine grains microstructure

– Resistance to fish scaling

– Deep drawing application

– Easy clean surface for enamel

– No possible use for DWE

• Different types of metallurgy

– Al‐killed steel with high Sulphur content and controlled carbon content

– IF‐Ti steels with high Sulphur and Nitrogen content

Enamelling Steel Grades: I-F Enamelling Steel

This is a vacuum decarburized aluminum killed titanium

Enamelling Steel Grades: I F Enamelling Steel

This is a vacuum decarburized, aluminum killed, titaniumstabilized, continuous cast enamelling steel. I-F EnamelingSteel is suitable for ground coat enamels but will not developSteel is suitable for ground coat enamels but will not developadequate adherence in a direct-on cover coat application. Itwill be free from carbon boiling because the small amount ofwill be free from carbon boiling because the small amount ofcarbon left is tied up with the titanium.Nowdays this product is guaranteed against enamelNowdays this product is guaranteed against enamelfishscale, as well. It has superior formability to othersproducts, because of its inherently high r-value.products, because of its inherently high r value.Yield strength after firing porcelain is reasonable good.Sag resistance for this product is superior very goodSag resistance for this product is superior-very good.

Enamelling Steel GradeEnamelling Steel GradeHot Rolled Sheet Steel Chemical Composition (%) Mechanical Characteristics

Steel Quality C Max Mn Max P Max S Max Si Max Al Max Ti Max Re L (MPa) Rm (MPa)

DD11 0.100 0.500 0.040 0.030 0.030 0.010 / 170-330 280-430

DD12 0.080 0.450 0.025 0.025 0.030 0.015 / 200-230 320-410

DD13 0.080 0.400 0.025 0.025 0.030 0.020 / 200-310 310-400

DD14 0.080 0.350 0.025 0.025 0.030 0.020 0.010 200-290 290-370

Standards References

CEE France Germany Italy G.B. Spain Japan U.S.A.

EN10111 NF A36-301

DIN1614 Chap.1

UNI 5867 BS1449 Part 1

UNE 36-086/11

JIS- G 3131

ASTM SAE Designa-tionN°

DD11 StW22 FeP11 HR3 AP11 SPHD A 659 CQ 1010 1.0332

DD12 RStW23 SPHE A 621 DQ 1008 1.0398

DD13 StW24 FeP13 HR1 AP13 SPHE AK A 622DQ-AK 1006 AK 1.0335

DD14 3CT A 622DQ-SK

1.0389

Enamelling Steel (Volume)Properties

Volume properties : tensile testSt d d t i th• Stressed under tension, the behaviour of a calibrated sample is as shown by the load/elongation (stress/strain) curve :

Load

F

S

OE : Elastic area

ESR : Plastic deformationL S

R

ESR : Plastic deformation (SR : localised deformation called necking)

Fmax

E Localised necking

g)

UE% : uniform elongation

TE% : total elongation

Elongation eO UE% TE%

UTs = Fmax/S : uniaxialtensile strength (UYs)

Elongation eUE% TE%

Enamelling Steel Properties (Volume)

Tensile test : the stress/strain curve

S• Stress is defined as σ = F/S• Strain is defined as ε = ln(1+∆l)Strain is defined as ε ln(1 ∆l)

σ

(∆l= Elongation)

S

R E : Young’s modulus (slope of the elastic part)

Ys : Yield stress

Eσ = K εn

Ys : Yield stress

n : strain hardening coefficient (Hollomon law)Ys

εONote : n = εh = ln (1+UE%)

ε


Transverse anisotropy

idth i tiwidth variation

• Elongation causes a thinning and a width variation of the sample

thinning

a width variation of the sample• The magnitude of these variations

depends on steel quality

Ability to thinning and width decreasing due to a tensile stress applied

to a steel specimen is defined by a anisotropy coefficient or Lankford

coefficient, r :

reductionwidthrthinning

r =

Initial sectionAfter 20% elongation: g

Small r valueHigh r value


Transverse anisotropy

idth i tiwidth variation

• Elongation causes a thinning and a width variation of the sample

thinning

a width variation of the sample• The magnitude of these variations

depends on steel quality

Ability to thinning and width decreasing due to a tensile solicitation of a

steel is defined by a anisotropy coefficient or Lankford coefficient, r :

reductionwidthrthinning

r =

Initial sectionAfter 20% elongation: g

Small r valueHigh r value

Enamelling (Volume)-Steel Properties

Planar anisotropy

90° 45°0°

r depends on the direction of the sample cutting

B f d diff t thi hBecause of process, r0°, r45°, and r90°, are different : this phenomenon

is called planar anisotropy, and quantified by an average value :

42 90450 °°° ++= rrrr

THE CONCEPT OF FORMING LIMIT DIAGRAM

During forming steel behaviour cannot be described only by uni-During forming, steel behaviour cannot be described only by uni

axial tensile test, because it occurs in the 3 dimensions of space.

Looking only at the two principal strains in the plan of the sheet

(ε1 and ε2, with ε2 < ε1 by convention) the other ways of deformation (ε1 a d ε2, t ε2 ε1 by co e t o ) t e ot e ays o de o at o

are mainly :biaxial expansion (with the particularity of equi-biaxial expansionbiaxial expansion (with the particularity of equi biaxial expansion

where ε1 = ε2)

large tensile strain (with the particularity of plane strain where ε2 = 0)g ( p y p 2 )

Uni-axial tensile strain (ε2 = ε1 r / (r+1) )

compression


Forming Limit Diagram (FLD)This concept is introduced to describe all the possible different

combinations of deformation when forming a steel sheet

FLD d d th t l litFLD depends on the steel qualityε1

Experimental Forming Limit Curve

Stretching

Plan

e st

rain

Biaxial expansionCompression

thinning

CompressionCompressionthickening

ε2


FLD is a good tool to predict formabilitySimulationSimulation

ε1

Crack

neckingsafety margin(e.g. 10%)

Risk

Safe

ε2

Experimentalmeasurements

LINK BETWEEN MECHANICAL PROPERTIES AND FORMABILITY

A high formability of steel is related to :

low Ys and high UTS (meaning low YS/UTS)

high value of n (implies good ability to distribute deformations)

high value of r (implies good ability to thinning)

high value of TE%

Sag Characteristicsg

• Sag is the permanent deformation or creep of steel due to its own weight during enamel firing.

• The sag resistance of steel is related to its strength at elevated temperatures and to the temperature at which the steel starts to transform from ferrite to austenite on heating.

• In general, the higher the strength and the higher the transformation temperature, the better the sag p , gresistance of the steel.

Sag Characteristics

Comparison of sag resistance ofresistance of selected enamelling steel:enamelling steel:

• A, low‐carbon

enamellingenamelling

steels;

• B decarburizedB, decarburized

steel

• C interstitial‐freeC, interstitial free

steels.

Sag characteristic of enamelling Steel sheets

Enamelling Steel: Thermical Properties

To be able to know internal stresses as well as the Strain development with T°Cas the Strain development with T C changes it is necessary to focus on:

–forming process

–enamel firing

–Usage Conditions (pyrolyticl h ti )cycle, pan heating up…)

Enamelling Steel: Thermical Properties

4 kinds of data are necessary:

–Young modulus evolution with T°CYoung modulus evolution with T C

– Expansion coefficients

–Glass Transition Temperature

– Steel Mechanical Properties EvolutionSteel Mechanical Properties Evolution

with T°C

Young’s Modulus• General Features• General Features:

– Proportionality between stress and strain (E).

– Decrease with T°C

At t t– At room temperature

for steel α : ~210GPa : for enamel : ~ 80GPa (strongly dependant on Enamel Oxide Composition)

All steel grade for enamelling present similar behaviour :

- Curves present almost linear behaviour until

Difficult measurement for enamelHigh dependence of the Young Modulus on chemical composition.

E E ol tion ith températ re

500-600°C - faster decrease over T> 600°C

p2 categories can be defined

Cover and ground pyrolytic enamelconventional ground enamel 2C/2F

E Evolution with température

175 0200,0225,0250,0

Pa)

DC04EKDC06EKSOLFER

Young's modulous of some enamel

100

100,0125,0150,0175,0

E (G

20406080

E (G

Pa)

blanc titanemasse 2C/2Cmasse pyro

0 200 400 600 800Temperature (°C)

00 100 200 300 400 500 600

Temperature (°C)

Coefficient of Expansion

• Curves present linear behaviour until 650°C

Steel expansion coefficient

19C)p

• Small collapse at T ~ 700°C

• Very little differences between all grades :

– 11 ÷ 12∙10‐6 m/m/°C at room temperature 579

1113151719

alph

a (1

0-6/

°C

DC04EKDC06EKSOLFERSteel & enamel expansion coefficient

/ / p

– 13 ÷ 14 ∙10‐6 m/m/°C at T ~ 600/700°C5

0 200 400 600 800 1000

Temperature (°C)

a

Very different from one enamel to another 79

1113151719

pha

(10-

6/°C

)

DC04EKDC06EKSOLFER

masse 2C/2C

blanc Ti

Very different from one enamel to anotherEnamel composition : generally

- SiO2, F2, ZnO, MgO, P2O5, … : decrease expansionEnamel expansion coefficient15

C)

57

0 200 400 600 800 1000

Temperature (°C)

alp masse pyro

- alcalins, B2O3, CaO, Sb2O3, …: increase expansion

results - ground enamel 2C/2C : Te=490°C, Tg=475°C,

6 0

5

10

coef

. (10

-6/°C

masse 2C/2C

blanc Ti

masse pyro

α = 8 · 10-6 m/m/°C- ground enamel pyro & white Ti : Te=525°C,

Tg=505°C, a = 7· 10-6 m/m/°C

00 100 200 300 400 500 600

Temperature (°C)c

Basic thermo‐mechanical principles

Starting point :cooling phase during enamel firingEnamel

TemperatureLength associated to the thermal expansion

EnamelSteel

p

T

+-

traction

Te

TgTn

enameltraction

compressionIf f

Firing period

steelIf free

If freef f

Klotz testing sampleKlotz testing sample• Experimental measuramentp

– Coling of a one‐side enamelled sample

men

t

Stre

sses

Dis

plac

em

T tTemperature

COMPRESSION

TENSION

Tn TeTg= 350 °C

• Numerical simulationU d f th lid ti f FEM d l– Used for the validation of FEM model

Enamelling Steel: Thermical Propertiesa e g Stee e ca ope t es

Strength Retention. Some steels will exhibit gcritical grain growth and resulting loss of strength during the firing of the enamel coating 790 to 840 °C g g gtemperature range.

The fired enamel coating is normally fracturedThe fired enamel coating is normally fractured when the base metal is strained beyond the elastic limit of 0 002 mm/mm of the steel and the ability oflimit of 0.002 mm/mm of the steel and the ability of a steel substrate to retain its strength after firing becomes an important factor when selecting steelsbecomes an important factor when selecting steels for many service applications.

Strenght RetentionStrenght Retention

C i fComparison of yield strength (Ys) of four enamelling t l ft fi isteels after firing at 870 °C :

A, low‐carbon lli t lenamelling steels

(drawing quality);

B, low‐carbon ll lenamelling steels;

C, interstitial‐free steels;

D, decarburized steel

Strenght RetentionStrenght Retention

Porcelain or Vitreous Enamel Elasticity

Objective : Evaluation of maximum Porcelain (Vitreous) EnamelElongation before Fracturing/ Cracking

Dynamometer

Method:Dynamometer

+

Spark+

Lowvoltage spak test

+Optical

Microscope FractureMicroscope Fracture

Porcelain (Vitreous) Enamel Elasticity

Fracture in Enamel coating appearing only over the Steel plastic

Testing of two different enamelling steel Grades• Fracture in Enamel coating appearing only over the Steel plastic

deformation point (over Ys).• Fracture in Enamel coating not in correlation with Enamel Thickness

P l i (Vit ) E l ti l ti it f ll i l ti• Porcelain (Vitreous) Enamel coating elasticity following elasticdeformation of the steel without fracturing.

E il 300 THR 1000 THR 1000Email 300

0,20%

THR 1000 THR 1000

0,34%

0,20%

Enamelling Steel GradeHYDROGEN AND FISH SCALINGHYDROGEN AND FISH SCALING

• During the enamel firing, hydrogen solubility in steel increases

– Air humidity (H O) which is in the

• During cooling, hydrogen solubility in steel decreases

– Enamel solidification– Air humidity (H2O) , which is in the furnace comes in enamel and migrates to the interface enamel / steel

– H2O decomposition

– Enamel solidification

– One part of hydrogen is in excess (decreasing of its solubility) and has to go out of steel2 p

– Oxygen participate to the enamel adhesion on steel

– Hydrogen comes in steel

– This hydrogen migrates to the interface enamel / steel and then stays here. It has no possibility to go out because enamel became solid

– Hydrogen pressure increases at the interface and then enamel cracks FISH SCALE DEFECTSCALE DEFECT

HYDROGEN AND FISH SCALING• Aggravating parameters

– Wet firing atmosphere

• Hydrogen trapping( 2 ways out)– Physical trapping

• To allow the formation of Fe C– Insufficient drying of wet enamels

– Non suitable steel for enamelling

• To allow the formation of Fe3C• Presence of impurities in steel

– Chemical trappingSt l t i i tit i• Steel containing titanium or boron (HRS + CRS)

• Remedy

– To use a suitable steel, whose metallurgy is such that hydrogen

• Consequences– Special metallurgy for CRS : allow metallurgy is such that hydrogen

absorption capacity is high

– To control enamel conditions

p gythe formation of Fe3C

• HYDROGEN TRAPS ARE OBTAINED at expenses pOF MECHANICALCHARACTERISTICS

– Generally, no enamelling possible onGenerally, no enamelling possible on both sides for HRS for enamelling

Enamelling Steel – Surface reactivity

Porcelain (Vitreous) Enamel Definition

• Porcelain (Vitreous) Enamel is a borosilicate glass, whosechemical nature can be expressed by means of an empiricaloxide composition.

Th diff h i l b d d b f• The different anphoteric elements are bonded by means ofcovalent bond, building the backbone of the glass matrix, while metallic elements are bonded mainly by electrostaticwhile metallic elements are bonded mainly by electrostaticbonds.

• The physical structure of Porcelain Vitreous Enamel is that ofThe physical structure of Porcelain Vitreous Enamel is that ofan overcooled liquid.

• The physical properties of P.V.E. can be considered isotropic. p y p p p

• International P.V.E. Identification codes are:‐EINECS N. 266‐047‐ 6‐CAS N. 65997‐18‐ 4

Example of Oxides Vitreous Enamel Composition ListExample of Oxides Vitreous Enamel Composition List

Substance Min Max Substance Min Max Substance Min MaxSubstance Min Max Substance Min Max Substance Min Max

SiO2 40 80 MgO 0 2 Fe2O3 0 3

B2O3 5 15 CeO2 0 15 MoO3 0 3B2O3 5 15 CeO2 0 15 MoO3 0 3

Na2O 5 20 ZnO 0 10 P2O5 0 5

K2O 1 5 Al2O3 0 5 SnO2 0 5K2O 1 5 Al2O3 0 5 SnO2 0 5

Li2O 0,5 5 CoO 0 3 TiO2 0 10

CaO 1 10 NiO 0 3 ZrO2 0 20CaO 1 10 NiO 0 3 ZrO2 0 20

BaO 0 5 CuO 0 2 F 0 5

SrO 0 5 MnO2 0 5SrO 0 5 MnO2 0 5

Mendeleev’s Periodic Table

Vitreous Enamel Raw Materials Functionality

Vitreous Enamel Raw Materials Functionality

FIG 1: Relationship between properties of FIG 2: Relationship between specific volume crystalline and glassy substances and temperature. and heat content of glasses with temperature. (1) crystalline, (2) glassy. (1) heat content , (2) specific volume.

Porcelain (Vitreous) Enamel Properties


Typical Frit Heating Microscope Curve recorded by HSM.


T i l E l H ti Mi Th l P i t d d b HSMTypical Enamel Heating Microscope Thermal Points recorded by HSM.

Porcelain Enamel PropertiesPorcelain Enamel Properties

• Si ,

O Oxygen

Structure ofStructure of lattice of crystallinecrystalline silica

Porcelain (Vitreous) Enamel Properties( ) p

• Si ,

O Oxygen

structure of a networknetwork for quartz glassglass.

Porcelain (Vitreous) Enamel Properties( ) p

• Si, ,

O Oxygen,

@ Na@ Na

Structure of disodium‐

silicate lglass.

Porcelain Enamel Properties p• Silica

kbBackbone

of Glass.

B• Borum

Backbone

of Glass.

Porcelain (Vitreous)Enamel‐AdherenceE l Fi i CEnamel Firing Curve

Impermeable to O2 and reactions between enamel oxides and iron oxides

Temperature °C

l lidifi tienamel fusion enamel solidification

Stop of chemical ti

Porous enamel and iron oxidation

reactions

Time (minutes)

Porcelain (Vitreous) Enamel‐Adherence

Stage 1, Support Oxidation

Applicazione: TambienteCottura: 150°CCottura: 300°CCottura: 450°CHeating up: 600°C O2 (aria)H2O

O O ↑Ground Fe + H2O → FeO + H2↑

2 Fe + O2 → 2 FeO

FeO, Fe3O4, Fe2O3

4 Fe + 3 O2 → 2 Fe2O3

SteelFeO, Fe3O4, Fe2O3

Porcelain (Vitreous) Enamel‐Adherence

Stage 2, Iron Oxide Stabilization

Heating p: 600 → O ( i )Heating up: 600 →800°C

O2 (air)

Ground

Fes2+, Fes

3+

Steel

Porcelain (Vitreous) Enamel‐Adherence• Stages 3 and 4 Supports Corrosion and Dendrite FormationStages 3 and 4, Supports Corrosion and Dendrite Formation

Firing: 800°C O2 (air)Firing: 800 C O2 (air)

Reduction Reactions:

Ground Ni2+ + 2 e- → Ni0Co2+ + 2 e- → Co0Co + 2 e → Co

Oxidation Reactions:

Steel Fe0 → Fes2+ + 2 e-

Fes2+ → Fes

3+ + e-

CO → CO2

Porcelain (Vitreous) Enamel‐Bubble StructureE l C S i Sh i T i l G d C AdhEnamel Cross‐Section Showing Typical Ground Coat Adherence at Steel‐Enamel Interface After Firing

ENAMEL

GLASS-METAL INTERFACE

STEEL

Porcelain Enamel‐Adherence SEM EDS maps of the ground coat/steel interfaceSEM EDS maps of the ground coat/steel interface

GroundCoat

MetallicMetallic particles

St lSteel


A glass contains a number of oxides, A, B, C,t h th t i i ht f hetc., whose the percentage in weight of each is respectively, PA, PB, PC, etc., and the magnitude of the property for each, is respectively, XA, XB, XC, etc., the following p y, A, B, C, , gadditive formula may be applied to obtain the specific property of the glass:specific property of the glass:

K = PA ∙ XA + PB ∙ XB + PC ∙ XC + etc.,

where K represents the specific property. (In some glasses the accuracy of such a calculation is very ( g y ygreat, but in others an approximation only is obtained).

DensityTh d i f b i d d h iThe density of a substance is a term used to denote the ratio of the mass of the substance to its volume :

d (m/v)d = (m/v)

where d is the density in kg/dm3, m is the mass in kg, and vis the volume in dm3is the volume in dm .

The reciprocal of the density, or specific volume, is an additive property which can be used in calculating density,additive property which can be used in calculating density, and the results are fairly reliable. Winkelmann and Schott's 12

formula for density is:

100 = P1 + P2 + P 3d d1 d2 d31 2 3

where d = density of the glass,

Pi = percentage of the oxide,i p g

di = density of the oxide used.

ElasticityElasticity is the capacity of a body, to changes its length under tension or compression andits length under tension or compression and to assume the previous dimensions and shape after removal of the loadafter removal of the load.

The elongation Δl of a rod, of length l and cross section S, subjected to load P, is equal to:to:

Δl = P ∙ l / E ∙ S

where E is the Young’s modulus or the elasticity or the elastic modulus.y

ElasticityWhen l = 1, S = 1, Δl = 1, E = P, that is the modulus of elasticity is numerically equal to y y qthat load which gives rise to an elongation in the rod equal to the original length with anthe rod equal to the original length, with an initial cross sectional area of the rod equal to

d ith i i l d l th l lone, and with an original rod length also equal to one. The modulus of elasticity is expressed in newtons/ square metre (N/m2).

The modulus of elasticity characterizes theThe modulus of elasticity characterizes the elastic properties of the material.

The lower the modulus, the higher the elasticity

HardnessBy hardness, we normally mean the resistance of a material to be scratched by another stronger material.

Although hardness is not as easily determinedAlthough hardness is not as easily determined as some of the other properties, the following

h h d d b A b hrepresents the method used by Auerbach:

H = p1h1 + p2h2 + p3h3p1 1 p2 2 p3 3

where: H = hardness of the glass,

pi = percentage of the oxide,

hi = factor for the oxide.hi factor for the oxide.

HardnessHardness : 5 – 7 Mohs’ scaleEN 101 Mi l S lEN 101 Minerals Scale

Minerale Durezza Mohs

Talc 1 Gypsum 2Calcite 3Fluorite 4A tit 5Apatite 5Feldspate 6Quarz 7Quarz 7Topaz 8Corundum 9

63

Corundum 9Diamont 10

Typical Hardness range on different scales of Vitreous Enamels

Hardness

Typical Hardness range of gPorcelain (Vitreous)(Vitreous) Enamels onon different scales.

Tensile StrengthTensile strength (Ys) is defined as the load P whichTensile strength (Ys) is defined as the load P which breaks down a rod of cross section S equal to 1mm2. It is determined by stretching a round specimens ofIt is determined by stretching a round specimens of enamel on a breaking machines. Knowing the area S and the applied load it is possible to calculate theand the applied load, it is possible to calculate the tensile strength from the formula:

T (Y ) (P/S) N/ 2Ts (YS ) = (P/S) N/m2.

Winkelmann calculated the tensile strength of glasses successfully by means of the following formula:

TS = p1t1 + p2t2 + p3t3S p1 1 p2 2 p3 3

where TS = tensile strength of the glass,

p = percentage of the oxidepi = percentage of the oxide,

ti = factor for the oxide used.

Compressive/ Crushing StrengthCompressive strength is the load P producing the crushing of an enamel cube with edges 1mm long. g g gThe compressive strength can be calculated by means of the formula: CS = (P/S) N/m2.S ( )where P is the compressive force at the moment of failure of the specimen in Newtons and S is the cross‐section area of the specimen in m2.

The compressive strength of enamels varies in the range 800 / 2‐ 500 MN/m2. The consequence is that enamels operate

under compression 15‐20 times better than under tension. In practice the best working conditions of the enamel is to bepractice the best working conditions of the enamel is to be always under compression, and never in tension. The compressive strength of the coatings largely depends on the p g g g y pstrains which exist in the enamel layer, as well.

Bending strength and torque strength

The bending strength of materials is a term d d b h l d d d b kused to describe that load needed to break

the specimen under bending action. The bending strength for a rod of round cross section is determined from the formula:section is determined from the formula:

BS = (8P∙l/π∙n∙d3) n/m3

• where P is the load at the moment of break, lis the distance between the supports, and d is pp ,the diameter of the rod.

YOUNG’s MODULUS / ELASTIC MODULUSIn solid mechanics, Young's modulus (E) is a measureof the stiffness of an isotropic elastic material. It isalso known as the Young modulus, modulus ofelasticity, elastic modulus (though Young's modulus isactually one of several elastic moduli such as the bulk modulus and the shear modulus) or tensilemodulus. It is defined as the ratio of the uniaxialstress over the uniaxial strain in the range of stress in which Hooke's Law holds. The modulus of elasticity is numerically equal to that load(P) which gives rise to an elongation in the rod equal to the original length(Δl = 1), assuming that l = 1, S = 1.

E = P ∙ l / Δl ∙ S

YOUNG’s MODULUS / ELASTIC MODULUSThis can be experimentally determined from the slopeThis can be experimentally determined from the slope of a stress‐strain curve created during tensile tests conducted on a sample of the materialconducted on a sample of the material.

Young's modulus, E, can be calculated by dividing the t il t b th t il t itensile stress by the tensile strain:

Where:

E is the Young's modulus (modulus of elasticity)

F is the force applied to the object;

A is the original cross sectional area through which the force is applied;A0 is the original cross‐sectional area through which the force is applied;

ΔL is the amount by which the length of the object changes;

L0 is the original length of the object. 0 g g j

YOUNG’s MODULUS / ELASTIC MODULUS

T i l S V S i di f D ilStres Strain Curve Typical Stress Vs. Strain diagram of a Ductile Material with the various stages of deformation .

YOUNG’s MODULUS / ELASTIC MODULUSYOUNG s MODULUS / ELASTIC MODULUS• A stress–strain curve typical of structural steel

1 Ultimate Strength1. Ultimate Strength

2. Yield Strength

3 Rupture3. Rupture

4. Strain hardening regionregion

5. Necking region.

A: Apparent stress (F/A0)A: Apparent stress (F/A0)

B: Actual stress (F/A)


• Stress Strain Curve for Brittle materials Brittle materials such as concrete and Carbon Fibersconcrete and Carbon Fibers do not have a yield point, and do not strain‐harden which means that thewhich means that the ultimate strength and breaking strength are the

A t l tsame. A most unusual stress‐strain curve is shown in the figure. Typical brittle materials like GLASS do not show any plastic deformation but fail while the deformation is Elastic.


Approximate Young's modulus for various materialsMaterial GPa

RUBBER (small strain) 0.01-0.1

PTFE (Teflon) 0.5

POLYPROPILENE 1 5 2POLYPROPILENE 1.5-2

High-strength concrete (under compression) 30

ALUMINIUM 69

Glass (see chart) 50-90

Brass and bronze 100-125

Steel 200Steel 200

Sapphire (Al2O3) along C-axis 435

Silicon carbide (SiC) 450

Tungsten (W) 400-410

Tungsten carbide (WC) 450-650

Diamond (C) 1220


Specific Heat CapacitySpecific heat(Specific Heat Capacity) is that amount of heat (in Joules) needed to heat 1 kg of substance by 1°C at

lconstant pressure or at constant volume.

The average specific heat from t1 to t2 is expressed by the tiratio :

S(cp) = Q J/kg ∙ deg

( t t )m( t2 – t1)

where Q is the amount of heat in J, m is the mass in kg, t2 – t1is the rise in temperature in degreesis the rise in temperature in degrees.

Specific heat may be calculated from the additive formula:

S p s + p s + p sS = p1s1 + p2s2 + p3s3where S = specific heat of the glass

t f th idpi = percentage of the oxide

si = factor for the oxide

Specific Heat Capacityp p ySubstance Phase

S(cp)J/(g·K)

Substance solid 0 92Substance solid 0.92

Rick solid 0.84

Concrete solid 0.88

Glass, silica solid 0.84

Glass, crown solid 0.67

Glass, flint solid 0.50

Glass, pyrex solid 0.75

Granite solid 0.79

Gypsum solid 1.09

Marble, mica solid 0.88Marble, mica solid 0.88

Sand solid 0.84

Soil solid 0.80

Wood solid 0.42

Thermal ConductivityTh l d i i f b i h i d f hThermal conductivity of a substance is the magnitude of the

coefficient of thermal conductivity, which is determined from the formula:the formula:

λ = ( Q ∙ a ) / ( S ∙ Δt∙ z )h Q i th t f h t i th h th S fwhere Q is the amount of heat passing through the area S, of

thickness a in time z with a temperature difference in the walls of the layer Δt . Dimension of λ is in watts/(m∙deg).walls of the layer Δt . Dimension of λ is in watts/(m deg).

Paalhorn determined factors for the calculation of the thermal conductivity:y

λ = p1 λ 1 + p2 λ 2 + p3 λ 3

where: e e

λ = thermal conductivity,

pi = percentage of the oxide,pi percentage of the oxide,

λi = factor for the oxide.

Thermal ConductivitySome rappresentative Thermal Conduttive Factors

Material Thermal conductivity [W/(m·K)]Material Thermal conductivity [W/(m K)]

Air 0.025Alcohols and oils 0.1 - 0.21

Aluminium 237 (pure)120—180 (alloys)120 180 (alloys)

Cement, Portland 0.29Concrete, stone 1.7Copper 401Diamond 900 - 2320Epoxy (silica-filled) 0.30Epoxy (unfilled) 0.59Glass 1.1Gold 318Hollow Fill Fibre Insulation Polartherm 0.042Hollow Fill Fibre Insulation Polartherm 0.042Ice 2Lead 35.3LPG 0.23 - 0.26Mineral oil 0.138Polypropene 0.12 Porcelain (Vitreous) Enamel 0.84 – 1.26Porcelain (Vitreous) Enamel (Foamed) < 0.46Rubber 0.16Sandstone 2.4S ds o e .Silica Aerogel 0.004 - 0.04Silver 429Soil 1.5Stainless steel 12.11 ~ 45.0Thermal epoxy 1 - 7Thermal grease 0.7 - 3Water (liquid) 0.6Wood 0.04 - 0.4

Thermal ConductivityDepending on the composition of the enamel the coefficientDepending on the composition of the enamel, the coefficient of thermal conductivity varies between 0,84 and 1.26 watt/m∙deg. With rise in temperature it increases in value. / g pThe bubble structure of the glass affect on thermal conductivity of enamels (Table 1). The higher the bubbles structure the lower the coefficient of thermal conductivity. This make sense if we consider that the conductivity of oxygen is 28 5 and of nitro gen 27 7 milliwatts/m degoxygen is 28.5, and of nitro‐gen 27.7 milliwatts/m∙deg respectively at 50°C. The low conductivity of an enamel coating adversely affects thermal‐shock resistance. g y ff

If the surface of the products is cooled rapidly and the heat from the internal layers gets away too slowly, then the enamel will develop dangerous tensile strains. Thermal conductivity can be increased by introducing substances into the

iti hi h hi h th l d ti it fcomposition which possess high thermal conductivity, for example, metal powders.

Thermal ConductivityRelationship between thermal conductivity and bulk density of

Porcelain Vitreous EnamelsPorcelain Vitreous Enamels

Type of enamelBulk density of

enamelkg/dm3

Coefficient of thermal conductivity (at )

watt/m degkg/dm3 watt/m·deg

O di 2 48 1 17Ordinary 2.48 1.17

With some bubbles 2.45 1.09

Containing many small bubbles

2.36 1.05

Slight1y foamed 2 28 0 8Slight1y foamed 2.28 0.8

Foamed 2.16 0.46

Viscosity

If a layer of glassy material inside a melt is forced to move in a certain direction at a certain velocity, this layer will compel adjacent layers to move with it.

The velocity of the adjacent layers gradually y j y g ydiminishes with the distance from the layer which was originally forced to move. The force moving the g y gadjacent layers is called the internal friction of the melt. This force f acting between two adjacent f g jparallel layers will be proportional to the area of contact of the layers S, and the gradient of velocity y g ydv/dx:

f = η ∙S ∙ (dv/dx)f = η ∙S ∙ (dv/dx)

ViscosityGlassy Material Melt Viscosity Model y y

ViscosityWhere η is the proportionality factor characterizing the nature and state of the melt and is the so‐called internal‐f f lfriction factor, or simply, viscosity.

When S = 1 and (dv/dx) = 1

then η = f.Hence, the viscosity represents the force of friction between two parallel layers of Porcelain/Vitreous Enamel Melt in contact with each other over an area of S = 1 and with a velocity gradient equal to unityvelocity gradient equal to unity.

Viscosity is expressed in newtons per second per square metre (n sec/m2) The dimension of viscosity [η] = kg (m sec )metre (n.sec/m ). The dimension of viscosity [η] = kg (m.sec.).

The magnitude which is the inverse of viscosity [1/ η] is called fluidity. Porcelain Enamel melts have quite high viscosityfluidity. Porcelain Enamel melts have quite high viscosity values .

ViscosityViscosityViscosity of some liquids, silicate melts, and porcelain vitreous enamels

Substance Temperature Viscosity °C / oF n.sec./m2

Water 20 / 68 1 x 10-3Water 20 / 68 1 x 10 3

Glycerine 20 / 68 1 x 10

Molten Na2O·SiO2 1100 / 2012 1.4 x 10

Boric anhydride 900 / 1652 1.18 x 10

Enamels: Ground boric for steelNon-boric ground for steelLead for iron enamelling by the wet method

965 / 1769 6 x 10

975 / 1787 8.5 x 10g y

840 / 1544 7.5 x 10

M lt ili 2000 / 3632 1 10 3Molten silica 2000 / 3632 1 x 10-3

Viscosity

• Relationship between change in

Firing temperature Viscosity

log(viscosity)

of glass and

102 -103 Nsec/m2

of glass and temperature

ViscosityT i th t t t b l hi h th lTg is that temperature below which the glass becomes brittle. This temperature corresponds to the viscosity of 1012 Nsec/m2

Ts corresponds to the melting temperature ofTs corresponds to the melting temperature of the glass. At this temperature the melt has a viscosity of about 101 Nsec/m2viscosity of about 101 Nsec/m2

The temperatures Tg and Ts are determined by the chemical composition of the glass. For industrial glasses temperature Tg is aroundindustrial glasses temperature Tg is around 500°C and temperature Ts = 1400‐1500°C. For porcelain vitreous enamels these temperaturesporcelain vitreous enamels these temperatures are somewhat lower.

ViscosityThe viscosity is very important factor when enamelling. The firing temperature of the porcelain vitreous enamel depends

h h h h h h fon its viscosity. The greater the viscosity, the higher the firing temperature will be.

Vi it i l l i i t t l d i th fi iViscosity is also playing an important role during the firing process of the enamel. A high viscosity of the enamel melt slows down evolution of gas bubbles from it, increase theslows down evolution of gas bubbles from it, increase the flatness of the surface of the articles, etc.

On the other side a low viscosity melt during firing may give y g g y gexcessive flow in the enamel on the articles and lead to irregular coating thicknesses.

The viscosity of enamels during firing should be kept at a level of about 102 ‐103 Nsec/m2.

Viscosity

Surface TensionSurface tension, σ is the needed work to form a surface unit, or the work spent on increasing the f p gsurface of a material by one unit. This work equals the force acting tangentially to the surface over unit length and tending to reduce the surface. Surface tension is expressed in J/m2 or N/m. Molten porcelain (vitreous) enamels, like molten silicates, have a relatively high surface tension, the level of which is much higher than for liquids and molten salts .Surface tension depends on the nature of the medium in contact with the surface of the melt. It is well known that the surface tension of enamels in relation to air and to metals is different.

Surface TensionSurface tension of some liquids and melts

Substance Temperature °C / oF Surface tension in mJ/m2

Glycerine 20 / 68 63

Water 20 / 68 73

NaCl 801 / 1474 114

Na2SO4 884 / 1623 196

PbO·SiO2 1000 / 1832 200

Na2O·SiO2 1130 / 2066 302

Steel enamels: Boric ground coat 1000 / 1832 247

Non-Boric ground coat 1000 / 1832 2771000 / 1832 277

Titania cover coat 1000 / 1832 229

Acid Resistant 1000 / 1832 315Acid Resistant 1000 / 1832 315

Surface Tension

Surface tension of porcelain vitreous enamels plays a very important role during enamellingplays a very important role during enamelling. It is at the basis of the wetting mechanism of the metal by the enamel and the resultingthe metal by the enamel and the resulting bonding forces. The low surface tension of the porcelain enamel melt allows better wetting of metal surface and facilitates the elimination of craters and indentations formed in the layer of enamel, upon the escape of gases from theof enamel, upon the escape of gases from the melt.

Surface TensionThe capacity of the porcelain enamel melt to wet a solid body depends on the ratio of surface tension on three boundaries:

l ( )‐melt‐air ( σl/g )

‐solid body‐air ( σs/g )

‐solid‐liquid ( σ /l )solid liquid ( σs/l ).

The droplet of the porcelain enamel melt will be in a state of equilibrium if the following condition is fulfilled:

σs/g = σl/g ∙ cosθ + σs/lwhere:

cosθ = ( σ σ ) / σcosθ = ( σs/g ‐ σs/l ) / σl/gThe measurement of wetting property of the porcelain enamel melt can be expressed either by the cosθ, or by the contact angle θ. The lower the value of θ (the greater cosθ), the better the wetting action. Complete wetting occurs when θ = 0 (cosθ = 1). Vice versa, non‐wetting corresponds to values of θ = 180°, cosθ = ‐ 1.,

Surface TensionSurface Tension

Wetting of solid‐body by a frit‐melt.σs/g = solid/gas

σs/l = solid/liquid

σl/g = liquid/gasl/g

Surface TensionFrom the above equation it follows that a reductionFrom the above equation it follows that a reduction in the surface tension of the porcelain enamel melt on the boundary with air (σ ) in general favourson the boundary with air (σl/g) in general favours wetting. However, cases are known when melts with the same surface tension but having differentthe same surface tension but having different compositions, are quite different in terms of their wetting capacity This suggests that σ / does not playwetting capacity. This suggests that σl/g does not play the decisive role in the wetting process. An important part in these processes is played by theAn important part in these processes is played by the surface tension of the solid body on the boundary with the melt (σ / ) However the magnitude of σ / iswith the melt (σs/l ). However, the magnitude of σs/l is difficult to determine experimentally, since it is connected with the chemical processes occurring onconnected with the chemical processes occurring on the boundary of solid and liquid phases.

Surface TensionThe wetting of solid surfaces by porcelain enamelThe wetting of solid surfaces by porcelain enamel

melts depends on the chemical nature of the surface. It is known that to ensure the wetting of metal, itsIt is known that to ensure the wetting of metal, its surface needs to be oxidized. The process of wetting is seriously affected by theThe process of wetting is seriously affected by the gaseous atmosphere and temperature. Porcelain enamel melts have a poor wetting action on metalsenamel melts have a poor wetting action on metals in vacuum and atmospheres in which oxygen is absent. Wetting is improved in air and in oxygen.absent. Wetting is improved in air and in oxygen. Also, an increase in the temperature favours the wetting action.wetting action. The capacity of the porcelain enamel melt to wet a solid body can be altered over wide limits by so d body ca be a te ed o e de ts byintroducing small amounts of surface‐active agents.

Surface TensionSurface Tension

• Illustration of how lower contact angle leads to reduction of puddle depthto reduction of puddle depth

Surface TensionSurface tension of various liquids in dyn/cm against air

Mixture %'s are by weightdyne/cm is also called mN/m (milli-Newton per meter) in S.I. units

Liquid Temperature °C Surface tension, γ

Acetic acid 20 27 6Acetic acid 20 27.6

Acetic acid (40.1%) + Water 30 40.68

Acetic acid (10.0%) + Water 30 54.56

Acetone 20 23.7

Di h l h 20 17 0Diethyl ether 20 17.0

Ethanol 20 22.27

Ethanol (40%) + Water 25 29.63

Ethanol (11.1%) + Water 25 46.03

Glycerol 20 63Porcelain (vitreous) enamel: Boric ground coat for steel

1000 247

Hydrochloric acid 17.7M aqueous solution 20 65.95y q

Isopropanol 20 21.7

Mercury 15 487

Methanol 20 22.6

n‐Octane 20 21.8

Sodium chloride 6.0M aqueous solution 20 82.55

Sucrose (55%) + water 20 76.45

Water 0 75.64

Water 25 71.97

Water 50 67.91

Water 100 58.85

Thermal Expansion

The Thermal expansion is the increase in the length or in volume of a body during heating. The linear y g gthermal expansion coefficient “ α “, is the relative increase in the length of a material specimen when g ptemperature rises by one degree. The linear thermal expansion coefficient of material is determined pfrom the ratio:

α 1 ΔI (m/m/°C)α = 1 X ΔI (m/m/ C)

l Δtwhere l is the length of the specimen, and

Δl is the elongation obtained by means of heating g y gthrough Δt°C.

Thermal Expansion

The change in volume of the body during heating through 1°C is called volume thermal expansionthrough 1 C is called volume thermal expansion coefficient “ β “. For isotropic bodies (amorphous substances glasses) the volume coefficient ofsubstances, glasses) the volume coefficient of expansion can be assumed with sufficient accuracy to be: β = 3∙αto be: β = 3∙α

The thermal expansion is one of the most important properties of enamelsimportant properties of enamels.

Strong bond between enamel and metal can be obtained during enamelling, only if the expansion coefficients of metal and enamel are very close each other.

Thermal ExpansionIf the coefficient of the enamel is greater than that of the metal,

during cooling the enamel would shrink more rapidly than the l h l l d h l lmetal. Since the enamel layer and the metal are strongly

bonded, then the metal will resist the shrinkage of the enamel Under these conditions the enamel layer will remainenamel. Under these conditions the enamel layer will remain in tension developing cracks, when the tensile strength of enamel is exceeded. If the coefficient of expansion of the pmetal is too high, then the enamel layer will potentially chip or delaminate, when enamel compressive strength is

h i h i l 1 20 iovercome. The compressive strength is almost 15‐20 times greater than the tensile strength. Therefore, compressive stresses are less dangerous than tensile onesstresses are less dangerous than tensile ones.

In the normal enamelling practice the coefficient of expansion of the enamel is always lower than that of theexpansion of the enamel is always lower than that of the metal.

Thermal ExpansionVolume Thermal Expansion Coefficients of some metals and enamels

MaterialsCubical Thermal Expansion

Coefficient 3·α x 10-7(m/m/°C)TemperatureRange, °C

Iron 426 0 - 500Steel:

Low carbon 465 27 - 760Titanium 410 27 - 760

Cast Iron 378 - 390 20 - 500Copper 540 0 - 500Gold 450 0 - 1000 100Silver 600 0 - 100Platinum 270 0 - 100Aluminium 600 - 720 0 - 100Aluminium 600 720 0 100Enamels : 0 - 100

Ground Coat for steel 240 - 300 0 - 100Top Coat for steel 280 - 360 0 - 100Top Coat for steel 280 360 0 100Ground Coat for iron 290 - 320 0 - 100Enamels Coats for non ferrous and precious

300 - 350 0 - 100ferrous and precious metals

Enamel Coat for Aluminium 400 - 450 0 - 100

STRESS & STRAIN IN PORCELAIN ENAMEL COMPOSITESCOMPOSITES

• Porcelain‐enamelled metals are, in general, non‐equilibrium composites, since the enamel (glass) never exactly fits the p , (g ) ymetal (iron) over a range of temperatures.

• Since glass is a brittle material, and it almost always fails in tension, enamels are designed to be in a state of compression with respect to the metal on which they are applied.

• This is accomplished by so compounding the enamels that they have a lower over‐all thermal contraction and expansion than the metal so that in cooling the enamel layer is putthan the metal, so that in cooling the enamel layer is put under compression and the metal under tension.

• All enamel‐metal composites therefore contain residualAll enamel metal composites, therefore, contain residual stresses which influence the over‐all physical and mechanical properties of the system.

TYPICAL PHYSICAL PORCELAIN (VITREOUS) ENAMEL PROPERTIESPROPERTIES

Density 2.4 – 2.7 grams/cm3 (Variable due to composition)

Specific Heat 0.20 – 0.30 gram‐Cal. per Degree Celsius

Conductivity 1.698 – 2.796 g‐Cal. cm2 per sec. @ 0 Deg. C.

Conductivity, Glass (Wikipedia) 1.1 W/mK

Emission (Emissivity) 0.85 for Ground Coats( y)

Absorbance 0.85 for Ground Coats (Dependent on Colour)

Heat of Fusion Net endothermic reaction for melting soda‐lime glass, 81.3 Btu/lb (86 103 joules)

Fusion Temperature 1400°F to 1500°F (760° to 815°C)

Young’s Modulus (Mod. of Elasticity) (Typical) 8 × 106 psi / 55 GpaYoung s Modulus (Mod. of lasticity) (Typical) 8 0 psi / 55 Gpa

Poisson’s Ratio 0.20 to 0.24 (0.22 Typical)

Residual Stress 15 to 30 × 103 psi / 103 – 206 Mpa

Linear Thermal Expansion 80 to 100 × 10‐7 m/m/ͦ CFor Acid Resisting Enamels closer to 80 to 85 × 10‐7 m/m/°CFor Acid Resisting Enamels, closer to 80 to 85 × 10 m/m/ C

Moh’s Hardness 5 – 7

Tensile Strength 4,900 – 15,000 psi ( 34 – 103 Mpa)

Compressive Strength 2 – 4 × 105 psi (1380 – 2760 Mpa)

Dielectric Constant 5 ‐ 6Dielectric Constant 5 ‐ 6


It can be observed that l h h h l halthough the metal has a straight line thermal expansion relationshipexpansion relationship with temperature, the curves for enamels are not straight and undergo a radical change in direction

b h " i lat about the "equivalent rate temperature".

The equivalent rateThe equivalent rate temperature is also known as the “glass transitionas the glass transition temperature, Tg.”

STRESS & STRAIN IN PORCELAIN ENAMEL COMPOSITES• P.(V.)E. has, therefore, with increase in temperature from

l i ffi i h iroom temperatures, a lower expansion coefficient than iron, but as it approaches the break in the enamel curve its expansion increases to where its coefficient of expansion isexpansion increases to where its coefficient of expansion is greater than that of the iron.

• In fact, the lines become parallel to each other as the averageIn fact, the lines become parallel to each other as the average rate of expansion of the enamel catches up on the average rate of expansion of the iron.

• As the composite is heated above its equivalent rate temperature(Tg), the enamel expands at an increasing rate, b h f ll ff l d h l d bbut the full effect is not realized, as the stress is relieved by the increasing mobility of the enamel.

Th l h d t F d F' b t thi i f il• The enamel curves show a drop at F and F' but this is a failure in the structural strength as the enamel actually continues to expand as indicated by the dotted line. It has also beenexpand as indicated by the dotted line. It has also been described as the Dilatometric Softening Point or DSP based on dilatometer expansion measurement techniques.


Measured thermal expansion of the three ground coat frits

Ts Ts T0.008

0.009

T

s Ts

0.006

0.007

SoftTg

Tg Tg0.0040.005 Medium

Hard

0.002

0.003 Steel

α

0

0.001

25 125 225 325 425 525 625 F°

STRESS & STRAIN IN PORCELAIN (VITREOUS) ENAMEL COMPOSITES

Cubic Thermal Expansion (CTE )value for Various enamel systemCubic Thermal Expansion (CTE )value for Various enamel system

Enamel coating Typical CTE (β = 3·α)(X 10-7 m/m/°C)

Pyrolytic Range 255-285

Range Tops 290-320

Burner Caps 345-375Range Grates 355-385Sanitaryware 280 310Sanitaryware 280-310Range Tops 290-320

Hot Water Tank 315-345Barbecue grills 270-300Cast Iron white 320-335


• The strength of enamel coatings depends not only on th lit f th b di b t th d dthe quality of the bonding between the ground and metal and the coating enamel, but above all on the t i d l d i th l t l tstrains developed in the enamel-metal system.

• The development of permanent stress is due to many factors, the main one being the difference in the coefficients of thermal expansion of enamel and

ll d l h d h li i ienamelled metal. If the stress exceed the limiting compressive strength of the enamel or the tensile

h h h i i f h l l istrength, then the continuity of the enamel layer is broken to form cracks or crazing, since the

h i l h f h l i l h hmechanical strength of the enamel is lower than that of the metal.

STRESS & STRAIN IN PORCELAIN ENAMEL COMPOSITESThe tangential deflection firing and cooling curves for a g g gground coat enamel applied on only one side of an iron sheet is shown in the next Figures. The flat gpiece on being placed in the furnace at the firing temperature, does not develop measurable stresses p pif it is heated uniformly, and the ground coat as a bisque does not develop perceptible stresses and no q p p pstrain appears. As the enamel melts, however, stresses may develop as the enamel is expanding faster than the iron. Only in cooling these stresses are noted. The enamel chills as the ware is removed from the furnace and since the enamel contracts faster than the iron it is under tension, as shown in the diagram, and the specimen curves toward the enamel.

Thermal deflection behaviour of an enamelled iron strip from bisque to fired state.

C iComparison of the tangentialtangential deflection and the thermal expansion, cooling with id ti lidentical cooling rates and priorand prior heat treatments.

STRESS & STRAIN IN PORCELAIN ENAMEL COMPOSITES

• The development of permanent stress is due to many factors, the main one being the difference in the coefficients of thermal expansion of enamel and enamelled metal.expansion of enamel and enamelled metal.

• If the stress exceed the limiting compressive h f h l h il hstrength of the enamel or the tensile strength,

then the continuity of the enamel layer is broken to form cracks or crazing, since the mechanical strength of the enamel is lower g fthan that of the metal.


σe ∙ (1 – μe) + σm ∙ (1 ‐ μm ) = Δα ∙ ΔT

E EEe Emwhere σe and σm are the stress in the enamel e mand metal, μe and μm are the Poisson ratios for enamel and metal E and E are thefor enamel and metal, Ee and Em are the modules of elasticity for enamel and metal, Δα is the difference bet een the coefficientsΔα is the difference between the coefficients of expansion for metal and enamel, and ΔT is the difference between the solidifying temperature of the enamel and the final pcooling temperature.

STRESS & STRAIN IN PORCELAIN ENAMEL COMPOSITES• it follows that stress in the enamel layer, developed as a result y , p

of differences in coefficient of thermal expansion of enamel and metal, depend on the magnitude of this difference, on the softening temperature of the enamel, and on its elasticity, and of the metal. If the coefficients of expansion of enamel and metal are the same no stress develop But in practice thismetal are the same, no stress develop. But in practice this does not take place. Therefore, it is customary to use enamels whose coefficients of thermal expansion are somewhat lower pthan in the metal, as a result of which the enamel is under compression.

• If the tearing forces exceed the strength of the enamel‐metal bond or exceed the tensile strength of the enamel, then the

l l ll f th t lenamel layer pulls away from the metal

• With higher thickness of metal, the second component of above equation practically disappears and then the stressabove equation practically disappears, and then the stress will develop only in the enamel layer.


• The magnitude of the temporary stress in the enamel• The magnitude of the temporary stress in the enamel layer is largely affected by the low thermal conductivity of the enamel as a result of which theconductivity of the enamel, as a result of which, the internal and external layers of enamel and metal are irregularly heated or cooled and consequently theyirregularly heated or cooled, and consequently they expand or contract to different degrees. Under these conditions an important role is played by theconditions, an important role is played by the thickness of the enamel and metal. In enamelled products the distribution of stress is much moreproducts, the distribution of stress is much more complicated as a result of the heterogeneity of the structure of the enamel layer lamination of certainstructure of the enamel layer, lamination of certain layers of enamel of different composition, irregular cooling in different sections of the articles aftercooling in different sections of the articles after firing, etc.

Thermal deflection of an enamelled metal strip

Th it d f thThe magnitude of the normal stress, in addition to the shape of the surface and radius of curvature, depends on:

• the difference in thethe difference in the coefficient of thermal expansion of enamel and metalmetal

• the softening temperature of the enamel

• ‐the thickness of the enamel and steel enamel and steel

• ‐the elasticity of the enamel and metal and

• ‐other factors.

Designing Links and Guidelines for enamelled ware.

Edges folded overThe internal radius of the fold must be greater thanThe internal radius of the fold must be greater than 3.05 mm for 1C+1F and 4.82 mm for 1C+1F.

Recommended Not recommended

Designing Links and Guidelines forenamelled ware


• Folding and ribbingThe internal radius of the fold must be ≥ 4.82 mm for 2C+2F.


The minimum radius recommended for the two different enamel systems should also beobserved for embossing of a panel. The enamel tends to pull away from sharp pointsbecause of surface tension. This results in thin, dark‐appearing lines over sharp edges., pp g p gThe excessive thickness of porcelain enamel required to cover these lines increases

the chances of spalling, chipping and crazing

Designing Links and Guidelines of enamelled ware

Sharp edges and CurlingSharp edges created by cutting cause problems when enamelling.Sharp edges created by cutting cause problems when enamelling.

They should be rounded off, rolled over or finished with open curls, also for aesthetics reasons.


Roll Forming• Long edges may sometimes be roll formed instead of flangedLong edges may sometimes be roll formed instead of flanged

to provide additional rigidity.

• Beads, or rolled edges, may be employed if parts are , g , y p y psymmetrical and a good, uniform edge is needed.

• The bead must be large enough to avoid burn‐off during firing and a minimum outside radius of 0.19 in.(4.82 mm) is used. If this minimum radius is not maintained, brushing should be

l d t t lliemployed to prevent spalling.

Designing Links and Guidelines for enamelled wareInterlocking joints and WeldingInterlocking joints and Welding

Interlocking joints may also generate puddles where liquids are d h l h h h k f hretained.They also create areas where the thickness of the

coating is not homogeneous, a defect that is undesirable during firing. Whenever possible, end‐to‐end welds should be firing.Whenever possible, end to end welds should bepreferred to interlocking joints.



HolesThe holes needed for process purposes (balancing, dipping, brushing and hooking the part during the pp g, g g p gvarious stages of the process)must be drilled in not visible positions of the finished product, mainly for p p , yaesthetic reasons.They should be free of burrs and cracksThey should be free of burrs and cracks.

It is advisable to make all holes either circular or lli ti l i helliptical in shape.

Holes Construction detailsHoles Construction details

R d d N t d dRecommended Not recommendedC.I.S.P.

Hole DimensionsClearance holes should be large enough to prevent filling with porcelainClearance holes should be large enough to prevent filling with porcelain

enamel. Holes with small diameters will require an extra reaming operationor the use of special reaming screws. For normal use, bolt holes should be at l t 0 062i /1 57 l th th i l b lt di t t ll f thleast 0.062in./1.57 mm larger than the nominal bolt diameter to allow for the

enamel buildup around the edge of the hole. Where good coverage and appearance are necessary, bolt holes should be dimpled to avoid the condition experienced when the porcelain enamel pulls away from and burns off sharp

edges

Holes Construction detailsHoles Construction details


Embossed Edge Forming or BeadingWaviness along flat edges may often be prevented by incorporating edge formations(see Figure 12a) With this type of design the metal is worked from the center to the(see Figure 12a). With this type of design, the metal is worked from the center to theedge (Figure 12b), eliminating the “oil can” tendency encountered with other panels.

Finite Element Analysis(FEA)Objectives:Objectives:

– To evaluate the possible risks related to the design ofenamelled partsenamelled parts

– To identify , include and understand the origins ofpotential problemsp p

– To set up preventive and corrective means

Means:Means:

– Consider all the possible variables of the problem

– Development of Design Of Experiments (DOE)Development of Design Of Experiments (DOE)

1. example: influence of deformation on enamel adherence

– To develop a basic predictive test FMEA(Failure Mode andTo develop a basic predictive test FMEA(Failure Mode and

Effects Analysis) to:

1. predict risk before tools are madep ed ct s be o e too s a e ade

2. test influence of modifications

Stress generationStress generationStamping

State 1Stamped part

Surface treatment

Enamel application

State 2σ2 ε2 σ2 y

INPUT-MC, E-Friction Coef.

M i l l

SpringbackState 3σ2,ε2

Curing at

INPUTCM (T), E(T)

(T)

State 0Initial blank

σ1,ε1 σ2,ε2, σ2,y- Material laws Curing atTannealing

α(T)MicrostructureTAnnealing

State 6σ6 ε6 σ6 y

Firing at

State 4σ4=0,ε4,σ4,y

State 5σ5,ε5, σ5,y

σ6,ε6, σ6,y Cooling down at

Tannealing

Tmax

State 7σ7,ε7

Cooling at

Troom

In use

State 8σ8,ε8

INPUTCM enamel(T)E enamel (T)αenamel (T)

In-use behaviour

Cooling at Tenamel solid

INPUTT enamel solidification

Cooling at Tenamel solid

Reality and numerical simulation

Oven cavity manufacturing process

• Steel process

• FormingStamping trimming bending springback

• Perfect blank – No thickness variation– No stresses

Reality ... … and numerical simulation

– Stamping, trimming, bending, springback• Stamping, bending&springback• Trimming & parts separation

– Mantel

• Forming– Exact process seldomly

knowni h i• Stamping

• Bending• Welding

– Welding of all parts

• High impact on stress level

– Trimming difficultForming process often left id

Welding of all partsThickness variationStresses field

• Enamelling

asideNo thickness variationNo stresses

• Enamelling– application– firing

• expansion + initial stresses• ~550°C : melt of enamel

• Enamelling– Modeling of multi‐layer

material enamel‐steel‐enamel

– Phase transformation not550 C : melt of enamel• ~700°C : annealing, recrystallisation

– cooling• Enamel solidification

t ti

Phase transformation not feasible

– Forming & Thermomechanicalsimulation incompatibility• contraction

p y• FMEA • Field transfer not yet

feasible

Production & In Use Thermal cycleProduction & In‐Use Thermal cycle

800 - 860 °C

450 - 500 °C

Enamel Solidification

Tr 20 °C

Enamelling In use behavior

Heating‐up phaseHeating up phase

• Evaluate steel part deformation during heating phase

– Thermal expansion induced stresses

– Eventually, forming process stresses to be added

Recrystallization

Annealing

800 - 860 °C

Enamel Solidification

Ta 20 °C

Solidification

• Difficulties

– Realistic simulation of complex phenomena

• Annealing (of internal stresses in the steel)

• Crystallization• Crystallization

– which also depend on pre‐deformation, heating process speed

Cooling phase & In‐use behaviourg pPrinciple

- if heating phase = OK 450 - 500 °CEnamel

- Initial stateT° = T(enamel solidification) or Tneutrallenamel = no stress

Enamel solidification

steel = expansion stresses only- Final state

T° ambient

Ta 20 °C

T° in use (measured or calculated)

Results- Stresses in enamel at different stages- Deformation during cooling- To test Virtual best solution

Main manufacturing stages of an enamelled part

• Forming– Stamping R t

• Numerical Simulation– StampingStamping

– Bending– Trimming

• Assembling

– Rupture– Wrinkle– Shape defect

Stamping– Springback

- Minimization of in-Assembling– Welding– Riveting,

clinching– In use behaviour

Minimization of inuse constrain

• Thermomechanical• Enamelling

– Surface treatment– Enamel application – Distortion – Distortion during

• Thermomechanical simulation

Enamel application– Firing– Cooling

• In use properties

– CracksDistortion during firing

– Warping during cooling• In-use properties

– Thermal cycles– Mechanical loading

– In-use distortionA i /

– Distortion during operation

– Ageing / cracking – Thermal fatigue

Solutions linked to the products ProcessSolutions linked to the products‐Process

• Product properties– Adaptation of expansion coefficient between enamel and steel

– Low sag steel property

– Good thermal characteristics of steel

– More flexible enamel ( Optimization of Coefficient of Expansion & Chemical Resistance)

• Process : lower enamelling temperatureg p– Lower level of stresses due to cooling (to be verified case by case)

– More flexible enamelsMore flexible enamels


HOTWATERTANK

“ STRESS ‐ STRAIN ANALYSIS” STRESS ‐ STRAIN ANALYSISBy

Finite Elements Analysis

CUSTOMER’S DATA, CAD AND MESHING OF THE MODEL

• Incoming data : plan of a commercial boiler with an inner diameter of 400 mm and a total length of 790 mm

• two geometrical options for:

– Basket( normal and handle shape)

– Collar ( round and elliptical shape)– Collar ( round and elliptical shape)

use of the CAD software CATIA

• Meshing of the model with PATRAN

CUSTOMER’S DATA, CAD AND MESHING OF THE MODEL

• For symmetry reason the model is developed on half boilerhalf boiler

CUSTOMER’S DATA, CAD AND MESHING OF THE MODELMODEL

• Zoom on the collar

NUMERICAL SIMULATION OF THE PRESSURE TEST

• Numerical simulation of the pressure test

–by ABAQUS STANDARD V6 2 software–by ABAQUS STANDARD V6.2 software–all calculations were made in the elastic domain

R l l i• Results analysis

–on the basis of the Von Mises stresseson the basis of the Von Mises stresses

–on the inner skin

COMPARISON OF THE DIFFERENT END CAP SHAPESSHAPES

Localisation of the Von Mises maximum stress

Elliptical hshape



Basket-h dlhandle shape



WithWith collar


Around the collar

COMPARISON OF THE DIFFERENT END CAP SHAPES

Comparison of the stress levels among theComparison of the stress levels among the different configurations:

Boiler shape Materielthickness

Variation of the VonMises stress

Elli ti l 18 REFERENCEElliptical 1.8 mm REFERENCEElliptical 1.5 mm + 20 %Basket-handle 18mm +26%Basket handle 1.8 mm 26 %Basket-handle 1.5 mm + 52 %Elliptical with

ll1.8 mm + 30 %

collarElliptical withcollar

1.5 mm + 61 %

MECHANICAL CHARACTERISTICS OF CERTAIN STEEL GRADES USED FOR BOILERS

• Boilers are submitted to pressure testing after forming, assembling and enamelling (high temperature annealing)

• We simulated these different stages in our elongation tests on standard samples :– Material as delivered

– After simulation of enamel firing (at 850°C for 10 min.)g ( )

– After a pre‐elongation up to 15 % and simulation of enamel firing

MECHANICAL CHARACTERISTICS OF CERTAIN STEEL GRADES USED FOR BOILERSSTEEL GRADES USED FOR BOILERSResults on the Hot Rolled Steel grades

CONCLUSION

• F E A can show the great influence of• F.E.A. can show the great influence of certain geometrical parameters on the stress level due to enamelled boiler pressure testing.

• Need to consider the steel grade as a• Need to consider the steel grade as a design parameter and to make a choice in d i i h b il i iorder to optimise the boiler in‐service

performances.

Documents

Project and Design of the Porcelain Enamelled Steel … · Enamelling Steel Cold Rolled Steel: Chemical Composition & Mechanical Characteristics Steel Quality Re(MPa)/ Ys Rm(MPa)