The microstructure evolution of hot work tungsten tool ...

C o l l o q u i u m 19-20 April 2011

Institute for Microstructural and Mechanical Process Engineering: The University of Sheffield

INTRODUCTION

The authors gratefully acknowledge the Ministry of National Education of Indonesian Government and Institut Teknologi Nasional Bandung for their financial support

The microstructure evolution of hot work tungsten tool steel during conventional heat treatment

M. Nurbanasari, P. Tsakiropoulos, and E.J. Palmiere

Hot work tungsten tool steels (Group H) are developed to meet the industrial need for materials with good mechanical properties for forming operations at high working temperature. These steels are highly alloyed specially with tungsten and chromium which affects the volume fraction and nature of the carbides. The critical issue of these steels is the low toughness. They are prone to brittleness at normal working temperatures and are inclined to distort during hardening process. The carbides of these steels are very brittle and do not dissolve in austenite by heating. A choice of heat treatment process and their parameter must be considered.

The potential of these steels is:

Resistance to deformation at the elevated working temperature.

High hot hardness,

High compressive strength

Wear resistance at elevated temperatures

Resistance to heat checking

Good machinability in the annealed condition

AIM To provide data that would allow the design of tool steels with a balance of satisfactory hardness and toughness.

STRATEGY

Select heat treatment parameters that give combination of satisfactory hardness and toughness Investigate phase transformations during conventional heat treatment process

HOT WORK TUNGSTEN TOOL STEELS

1. The main alloying elements are strong carbide formers [3]

2. Choice of proper heat treatment processes [3]

3. Create secondary hardening (SH) during heat treatment process

Table 2: The chemical composition of investigated hot work tungsten tool steels (%wt):

cavity

10

10

10

Fig 10 :Heat treated sample

Austenizing temperature of both tool steels : 1100, 1250 oC

First and second tempering temperatures respectively of both tool steels: 650, 750, 800oC

Conventional heat treatment process

Fig 11: Schematic of heat treatment process

AIM AND STRATEGY OF PROJECT

THE STRENGTHEN OF TOOL STEEL

REFERENCES

HARDNESS AND XRD RESULT

CONCLUSIONS

MICROSTRUCTURE EXPERIMENTAL PROCEDURE

Fig 12: Tempering curve of H21

472

226 214

422

247 236

0

100

200

300

400

500

600 650 700 750 800 850

Har

dn

ess

(V

HN

)

Double tempering (DT) temperature (oC)

austenising temp 1250 oC


413,1

300,5 285,4

257,7 254,5 235,0

0,0

100,0

200,0

300,0

400,0

500,0

600 650 700 750 800 850

Har

dn

ess

(V

HN

)

Double tempering (DT) temperature (oC)



Fig 13 : Tempering curve for H23

Table 3: Hardness of as cast and as quenched condition

The as quenched hardness of H23 is lower than as cast because of absence of martensite The hardness of both tool steel decreased with higher tempering temperatures because the carbide size

became bigger The hardness increased during tempering process due to secondary carbides produced at this condition Higher austenizing temperatures increased the hardness of tempered samples, due to the higher

dissolution of M6C in the austenite matrix

as castas quenched

(Tg 1100 oC)

as quenched

(Tg 1250 oC)H21 483 574 536

H23 356 265 284

Hardness (VHN)Type of

tool steel

As cast condition

Fig 15: SE image of cast H21

Type position V Cr Fe W phase

H21 matrix 0.6±0.4 4.8±1 92±1.5 2.7±0.5 ferrite

carbide 1.5 4.6 64.7 29.3 M6C

H23 matrix 1.1±0.6 13.5±1 79.9±1 5.5±2 ferrite

Carbide 1 3.9 21.2 49.5 25.3 M6C

Carbide 2 2.5 16.2 52.2 29.1 M6C

Table 4: EDS analysis of as cast steels (%wt)

Fig 16 : SE image of as cast H23

Heat treated condition

The primary carbide for both tool steels was M6C and was dominantly located along the grain boundaries

the differences between the composition of the carbides and matrixs refer to the levels of W, Cr, V and mainly Fe

Fig 18 : The microstructural changes of heat treated H21 (etchant 98 % picric acid + 2% HCl)

1. G.A Roberts. G.Krauss, Kennedy. 1998, Tool Steels, 5th edition, ASM, Metals Park, Ohio.

2. G.F. Vander Voort., E.P. Manilova., J.R Michael. 2004, A Study of Selective Etching of Carbides in Steels, Micros. Microanal. 10(suppl 2), 76-77

3. LS.Kremnev. 2008, Alloying Theory and Its Use for Creation of Heat Resistant Tool Steels and Alloys”, Material Science and Heat Treatment, Vol.50, 18-27

4. M. Kroneis. 1979, Tungsten in Steel, in: Proc.1st International Tungsten Symposium, Stockholm, Mining Journals Books, Ltd, London, 96-107

code position V Cr Fe W phase

A1 matrix 0.7 4.0 83.3 12.0 martemper

Carbide 1 1.5 3.1 28.4 67 M6C

Carbide 2 1.1 4.1 46.0 48.8 M6C

D6 matrix 0.7 13.4 78.8 7.1 ferrite

Carbide 1 2.6 10.8 47.8 38.8 M6C

Carbide 2 3.0 38.1 42.3 16.6 M7C3

Carbide 3 3.1 40.0 38.4 18.5 M23C6 Fig 19: BSE images of D6 Fig 20: SE images of A1

Table 5: EDS analysis of some heat treated samples (%wt)

• The highest hardness the H21 tool steel was 472 VHN and of the H23 tool steel was 412 VHN with austenizing temperature 1250 oC and double tempered 650oC

• After double tempered process, some of the carbides were still networked and mainly located along the grain boundaries

• The primary carbide for both tool steel was the M6C and after heat treatment the M7C3 and M23C6 were found in the H23 tool steel as secondary carbides

DH1 :Tg 1250 oC , quenched

D1: Tg 1250 oC, quenched, DT 800 oC


D3 : Tg 1250 oC, quenched, DT 650 oC

DH2 :Tg 1100 oC , quenched




• The morphology of carbides in H23: M6C (FCC structure): round, rod,

square, irregular M7C3 (hexagonal structure): square

and irregular M23C6 (FCC structure): square and

irregular

• The distance between the carbides increased in their higher austenizing temperature

• The morphology of M6C (FCC structure) in H21 was mostly round, irregular

• After double tempering, the carbides still made a network but were less interconnected than in the cast structure

• Higher austenizing temperature increased the volume fraction of the retained austenite in the as quenched condition

AH1: Tg 1250 oC , quenched

A1: Tg 1250 oC, quenched, DT 800 oC



AH2: Tg 1100 oC , quenched




Fig 9 : As cast ingot (a,b,c) and dimension of ingot in mm and sample position taken for chemical analysis and as cast microstructure (d)

1: sample taken for chemical analysis (centered 30 mm from the bottom and 120 mm from the top (size: 10x10x10 mm)

2:sample taken for as cast microstructure (size: 10x10x10mm)

Fig 17:The microstructural changes of heat treated H23 (etchant:Groesbeck’s: MC and M23C6 carbides are not etched and M7C3 – M6C in blue or yellow [2])

Fig 14 : Diffraction pattern of as cast and heat treated H21 (a) and H 23 (b) tool steel

(b) (a)

code phase JCPDS code phase JCPDS code phase JCPDS

M6C : Fe3W3C 41-1351 M7C3 : (Cr,Fe)7C3 5-720 gretained 52-512

M23C6 : Cr23C6 85-1281 Ferrite (a) 54-331 6-696

martensite 44-1292

Fig 3: Hot Forging die (H23) [1]

Fig 4: Die casting die (H21) [1]

Fig 5: Vacuum Remelted Hot Work Tool Steel (BOHLER W403 VMR)

Fig 1: Classification of tool steel (AISI)

Fig 6 : Schematic heat treatment process for tool steel

Fig 7: Secondary hardening mechanism during tempering process

Fig 2: Schematic effect of tungsten on hardness [4]

• The main eutectic carbide in the microstructure of both tool steel was Fe3W3C

• The as quenched H21 tool steel contained Fe3W3C and martensite and the quenched H23 contained ferrite, gretained, FeW3C3 and (Cr,Fe)7C3

• After second tempering, the X-ray analysis of H23 confirmed the presence of undissolved Fe3W3C, (Cr,Fe)7C3, Cr23C6 and ferrite meanwhile for H21 only Fe3W3C with tempered martensite as matrix

Why double tempered ?

Fig 8: Microstructural changes during tempering process

Chemical composition analysis using XRF method

Casting of material into 4 ingots (Vacuum induction furnace and slow cooling)

1 1 2

SH

(a) (b) (c) (d)

75

10

70

120 mm

Top of ingot

cavity

10 30

120

285 cavity

2

top bottom

bottom top

steel denotation

Position C Si Mn P S Cr Mo Ni W V Co

H21 Top 0.24 0.33 0.25 0.023 0.022 3.18 <0.02 0.3 7.7 0.43 <0.02 Bottom 0.27 0.32 0.25 0.028 0.022 3.13 <0.02 0.3 7.43 0.42 <0.02

H23 Top 0.33 0.49 0.36 0.014 0.029 12.35 <0.02 0.4 12.3 1.16 <0.02 Bottom 0.36 0.49 0.36 0.014 0.032 12.33 <0.02 0.4 12.47 1.16 <0.02

matrix

matrix

matrix

matrix

matrix

matrix

The microstructure evolution of hot work tungsten tool ...

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