Ch.14 Surface Hardening and Modification of Metalstriangle.kaist.ac.kr/lectures//MS371/2019 spring/Chap 14. Surface... · /MS371/ Structure and Properties of Engineering Alloys Carburizing

/MS371/ Structure and Properties of Engineering Alloys

Ch.14 Surface Hardening and

Modification of Metals

/MS371/ Structure and Properties of Engineering Alloys/MS371/ Structure and Properties of Engineering Alloys

Introduction

• Surface Treatment

– Thermochemical treatments to the surface part: C, N

– Also called hardening

– May or may not require quenching

– Interior to remain

• Reason for Surface Treatment

– Increase resistance

– Increase surface strength for carrying (crush resistance)

– Induce suitable residual and compressive

– Improve fatigue life

– Impact resistance


Carburizing of Steels

• C, wt%, introduced to

→ C content of surface to

increase to 0.8~1.0 wt%

• C to have very low in

bcc-ferrite

→ no

• Temp must be above for the

steel to be in fcc-austenite

• Carburizing is usually done,oC

• Widely used for

: gear, bearing, and shaft

γ-austenite

723oC

850~950oC


Carburizing Steels

• Many variables :carbon and alloy content, grain characteristics, machinability and cost

Chemical compositions of selected steels

for carburizing

• Common carburizing steels

Where high-strength core

properties are not required

– Ni, Cr, Mo

→ low-C (lath-type) martensitic core

→ improved strength and toughness

– S (0.1~0.3%) → to improve machinability

– should be Al-killed (deoxidized) to

prevent austenitic grain coarsening

during long high-temp carburizing

treatment

Alloying elements

Plain-carbon steels

Low-alloy steels


Gas-Carburizing

Gas carburizing furnace

• By maintaining a steady flow of the carrier gas and varying the flow of

hydrocarbon enriched gas

• Close process control being an of the gas-carburizing

process over liquid or solid carburizing processes


Gas-Carburizing Process

• Carburizing gases

– : methane (CH4), ethane, and propane

• Carrier gases

– N2 : inert and to act only as a

– the carrier gas entering the furnace composed of and N2 (major) as well as

CO2-CH4-H2-H2O (minor)

• Carburizing reactions

CH4 + CO2 → 2CO + 2H2

CH4 + H2O → CO + 3H2

2CO ↔ C(γ-Fe) + CO2

CO + H2 ↔ C(γ-Fe) + H2O

primary source for C for

carburizing

Carbon to diffuse into

the steel surface by

overall rxn


Carbon gradients for various times

Carbon gradient in test of 1022 steel.

Test bar was carburized at 920oC.


Carbon Concentration Gradients in Carburizing Steels

• Cs = concentration of element in gas diffusing into the surface

C0 = uniform concentration of element in solid

Cx = concentration of element at distance x from surface at time t

x = distance from surface

D = diffusivity of diffusing solute element

t = time

erf = error function (values for the error function can be obtained from table)


Carbon Concentration Gradients in Carburizing Steels

• Example Problem 14-1, 14-2.


Quenching and Tempering of Carburized Parts

Alloy 4620 steel, gas-carburized 4h at

955oC, austenitized 30 min at 820oC,

and oil-quenched.

Martensitic structure

* Application for the parts: not critical

with respect to cracking and chipping

tempering

• low-temp tempering

– 150~190oC

– little loss of hardening

– increased

Effect of tempering on hardness for

carburized cases of 8620 steel.


Carbonitriding of Steels

• Carbonitriding

– modified carburizing

( + carburizing gas)

• To produce a hard, wear-resistant

in steels

• Nitrogen effects

– to increase the of steel

– stabilizer → retained austenite

• Carried out at a lower temp and for a

shorter time than gas carburizing

→ thinner case (0.075~0.75 mm)

• Lower T → lower cost

• Maximum hardness and less

• Limitation of depth : 0.75 mm

Alloy 8617 steel bar, carbonitrided 4h

at 845oC in 8% ammonia, 8%

propane, and oil-quenched; held 2h at

-75oC; and tempered 1.5h at 150oC

* Scattered carbides in matrix

of tempered martensite


Nitriding of Steels

• Nitridingnitrogen in the atomic (N) form is

introduced into the surface of steel

– Temp : 495~595oC

– Nitride formation →

effect

• Nitriding effects

– high surface hardness

– high wear resistance and

antigalling properties

– long fatigue life

– heat-resistant surface

4140 steel, oil-quenched from 845oC,

tempered 2h at 620oC, surface-activated

in manganese phosphate, and gas-nitride

24h at 525oC

* White layer of Fe2N, Fe3N and

Fe4N, and tempered martensite

NH3 ↔ N + 3H


Surface Hardening of Steels

• Induction hardening

– to rapidly heat the surface of a steel into the condition

– to quickly quench : transformed into a hard case

Click above video !


Surface Hardening of Steels

• Flame hardening

– rapid and quick

– for so large parts : large gears, dies, and rolls (not practical in a )

– for small sections : end of valve stems and push rods

• Laser hardening

– intense heating

– workpiece itself to act as a cooling sink

– to harden a relatively small area in complex shapes

– but, high

Flame hardening Laser hardening


Plasma Surface Treatment

• Plasma

– fully or partially gas consisting of a collection of and

– Pashen’s law:

threshold for initiating plasma to be determined by P·d

( P : pressure, d : distance between electrodes)

Pashen curve of Ne and Ar



Ar(gas)

collision with gas

Generation of plasma

• Generation of a plasma

– particle to with neutral particles

– stable atoms to be excited or ionized by metastable atoms

Penning ionization

X* + Y → X + Y+ + e

Penning excitation

X* + Y → X + Y*



• Plasma Nitriding

– surface chemical reaction process

– nitrided layer on the steel : mm

– than by gas nitriding

• Plasma Carburizing

– thermochemical glow-discharge-type surface treatment

– in vacuum-type furnace with carburizing gas (propane and methane)

– very good uniformity of carburized layer

Schematic of discharge process Plasma nitriding


Plasma-Sprayed Coating

• The plasma torch uses the energy in a thermally ionized gas produced by an

electric arc to propel partially melted powder particles into prepared surfaces.

• Thin layer coating : ㎛

• Important application

– for corrosion and oxidation protection of gas turbine parts

– to protect successfully superalloy gas turbine blades and vanes used for

aerospace, industrial, and marine application

Design of plasma torch Practical plasma torch


Ion Implantation

• Any ion can be implanted into

any surface layer

• High-energy ions (10~500 keV)

• Depth (10~1000 nm)

• The ion implantation process is

carried out in high vacuum

• Thus, clean target is needed

• Significant lattice damage in the

form of vacancy-interstitial pairs

(Frenkel defect)

→ compressive stresses

→ very high strength and

hardness

• process

→ no metallurgical change

→ no adhesion problemThe implanted

concentration profile

A cascade region of

high defect density


Physical Vapor Deposition (PVD)

• Reactive sputtering

– formation of the film using the reactive gas

• Advantage

– easy control of the film’s

– deposition rate

• Disadvantage

– damage to the vacuum gauge

– layer formation on the target surface

Schematic of reactive sputtering Practical sputtering

Gas

particle

Reactive

particle

substrate

e-

ArM

target


Summary

• Surface-hardening technique

– gas carburizing, carbonitriding, and induction surface heating

– for hard-wearing surface layer and tough inner cores

• Localized surface-hardening technique

– flame and laser hardening

• Plasma carburizing & nitriding surface treatments

• Plasma spray coating

– for oxidation protection on Ni-base superalloys for gas turbines

• Ion implantation and physical vapor deposition technique

– for improved hardness and wear

Documents

Ch.14 Surface Hardening and Modification of Metalstriangle.kaist.ac.kr/lectures//MS371/2019 spring/Chap 14. Surface... · /MS371/ Structure and Properties of Engineering Alloys Carburizing