Short cycle heat treatment of cold-work tool steels

4ème Séminaire traitements thermiques & Revêtements Innovants des Matériaux Métalliques, Lyon, France

Short cycle heat treatment of cold-work tool steels - metallurgical aspects, main advantages and limitations WELS

Short cycle heat treatment of cold-work tool steels - metallurgical aspects, main advantages and limitations

Reinhold S. E. Schneider

Upper Austrian Univ. of Applied Sciences – Campus Wels



Content

Introduction

Low-alloyed Steels (example: bending tools)

Medium to high alloyed steels (example: cutting tools)

Practical application and possible failures

Conclusions



Content

Introduction




Conclusions



Possibilities of surface hardening of tool steelsPlastics processing HSSHot working cold working

Not for molds

Some applications for extruders

Not for molds and dies

Possible for cutting and

forming tools(large tools!)

Not for large tools

Thoughhardening for e.g. small drills



Tempering (equilibrium)Quenching (disequilibrium)

Carbide - precipitation

Tem

pera

tur i

n °C

Time in hours

1200

1000

800

600

400

200

200 ~10 ~20

Martensite transformationRetained austenite

Carbide - precipitationTransformation of retained austenite

Austenitising, Grain growthCarbide dissolution, Relaxation

Ac1

Ferrite+ Carbides

Austenite+ Carbides

Martensite

Mechanisms during the heat treatment of tool steels

Source: Böhler-Edelstahl, DI Mayerhofer



Surface hardening of cold work tool steelslow distortion as well as lower energy consumption



Investigation methodsQuenching-Dilatometer Bähr 805 A/D

Microstructure: optical microscopy, SEM, XRD

Hardness: micro hardness [HV 0,5]Impact test: max. 15 [J] hammer

Sample geometry:



Content

Introduction




Conclusions



Low alloy gradesExample: laser hardened bendig tools

Typical steels are:C45, 42CrMo4,

51CrV4, 100Cr6 …

No through-hardenin

g behaviour necessary



51CrV4 100Cr6

850°C 30min

1100/1025/950°C 16 sec.

850°C 30min

1100/1025/950/925°C 16 sec.

source for the TTA-diagrams: J. Orlich, A. Rose und P. Wiest, Atlas zur Wärmebehandlung der Stähle Band 3, Verlag Stahleisen M.B.H., Düsseldorf, 1973

Austenitising (TTA-Diagrams) for two low alloy steels

Optimum Temp. Optimum

Temp.

Overheating



74NiCr2 100Cr6

950 bzw. 900°C/16sec

impa

ct to

ughn

ess

[J/c

m²]

hardness [HV1]

blue brittleness

Hardness Toughness-Relationship - Tempering51CrV4

ConventionalShort cycle +

standard tempering


standard tempering


standard tempering

Typ. range of application



950°C 16sec 950°C 16sec + 200°C 60min

51CrV4:

950°C 16sec + 500°C 60min

950°C 16sec + 250°C 60min

martensitic fracture

micro-dimples

blue brittlenessintercrystalline

fracture

micro-dimples

Tempering effects on the fracture surface



hardness [HV1]

74NiCr2 100Cr6

1025°C 16 Sek. + tempering






1100°C/16sec+300°C/60min

impa

ct to

ughn

ess

[J/c

m²]

Effect of overheating during austenitising51CrV4



850°C 30min + 300°C 60min 900°C 16sec + 300°C 60min 1100°C 16sec + 300°C 60min

100Cr6:

Grain growthbrittle fracture brittle fracture intercrystalline brittle

fracture

Effect of overheating on the fracture behaviour

conventional short-cycle short-cycle, overheated



Content

Introduction




Conclusions



Medium to high alloy gradesExample laser hardened cutting tools



Medium to high alloyed steels

EN AISI C [%] Si [%] Mn [%] Cr [%] W [%] Mo [%] V [%]

60WCrV7 ~ S7 0,60 0,60 0,3 1,1 2,0 - 0,2X153CrMoV12 ~ D2 1,55 0,4 0,4 11,5 - 0,8 0,8

Cold-work tool steels

M7C3

Austenite

MC

Ferrite LM3C

M6C MC

TC TC

Austenitising: 870 – 900°C, > 30 min 1020 - 1080 °C, > 30 min.

Quenching: Water / Oil Oil / (Salt bath) / Gas



Austenitizing: 1000 - 1200°C 2 + (0) 2– 8 sec.

Hardness after different austenitising durations

Effect of the carbide dissolutionFull austenitising Continuing carbide dissolution

TA = 1000, 1150°C TA = 1000, 1100, 1200°C



Microstructure X153CrMoV12 (after different austenitising times for 1100°C)

2 s SEM 4 s SEM 8 s SEM

2 s optical 4 s optical 8 s optical

Network formation due to carbide dissolution and Cr (V,Mo) - diffusion



Hardness after different tempering temperatures

Continuous drop & hardness plateau (400 -600°C

Continuous drop in hardness

tT = 2 + 2 sec. tT = 2 + 8 sec.

Tempering:200 – 500 (600)°C

2, 8 s

Austenitising:1000/1150°C, 2+2 s60WCrV7

Austenitising:1100°C, 2+4 sX153CrMoV12



Distortion and stress potentialEffect of the austenitising temperature

Rising volume with rising austenising temperature(dissolution of carbides – expanded martensite)

High C – high alloy grades: reduced volume (due to retained austenite)



Distortion and stress potentialEffect of the tempering temperature (short-cycle)

Reduced volume due to tempering effects (carbide precipitaion)

Risk of tensile stresses after tempering above 300°C



Hardness & Toughness for different tempering

Short-cycle HT(4 s + quench

+ 8 s)

No secondary hardening peakRise in toughness (200 - 400°C)

Holding at 700°C for 30 sec.

Tempering

200-600°C2, 8 s

Austenitising:1100°C, 2+4 sX153CrMoV12

Conventional Heat treatment

(30 min + 60 min)

Toughness curve with minima in the range of the secondary hardening



Hardness – Impact Toughness relationship60WCrV7 & X153CrMoV12

at different heat treatment conditions

Similar trend - Deviations at the secondary hardening

Similar trend - Reduced scatter with increased tempering duration



Content

Introduction




Conclusions



Cutting of Advanced high strength steels (AHSS)

Tensile strength Rm [MPA]

Elon

gatio

n A

80 [%

]

Multi-Phase steels(AHSS)

Conv. High strength steels

(HSS)

TRIP SteelsDual Phase steelsComplex Phase Steels

Material C Si + Mn Cr + Mo Al V+Nb+Ti Rp0,2 Rm A80% % % % % MPa MPa %

CP1000 ~0.15 ~2.2 ~ 0.45 ~0.05 <0.05 810 1050 9

Investigated steel: CP1000 (HCT980 + ZE75/75) - AHSS

Ferrite, Bainite & Martensite



Experimental proceduresCutting press

Semi-industrial 630 kN cutting-/forming-press 200 strokes/min. Feed of the 40 mm wide 1 (1.8) mm thick steel strip: 5 and 15 mm / stroke



Results on wear measurement

Cutting edge profilesWear after 100.000 strokes

straight cutting

Caldie Laser hardened

(1mm/15%)

X153CrMoV12Conv. HT

(1mm/15%)

X153CrMoV12 nitrided

(1mm/10%)

Tool steels: Standard: 153CrMoV12 (D2) Laser hardened: „Caldie“ (~“70CrMoV5-2)



Results on wear measurementCutting edge profiles Wear after 100.000 strokes

X153CrMoV12 Conv. HT (1.8mm/10%)

Caldie, laser hardened

(1.8mm/10%)

Reason for failure:Double cutting

Reason for failure:???



Failure Analysis - Effects of insufficient surface hardening

tool flank

Plastic deformationof the cutting edge

Insufficient hardening dept after laser hardening



Content

Introduction




Conclusions



ConclusionsCold-work tool steels are the most promising area to apply short cycle heat

treatment in the field of tool steelsIf applied correctly: similar results and performance and less distortion

Caldie Laser hardened

(1mm/15%)

X153CrMoV12Conv. HT

(1mm/15%)

Medium-C – medium-alloyed are better suitable than high-C – high-alloy grades

Low to medium tempering temperatures can improve the toughness properties without initiating tensile stresses

Single uniform hardening without overheating of the cutting edges is essential

tool flank



Thank you for your attention!

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Short cycle heat treatment of cold-work tool steels