STRUCTURE AND PROPERTIES OF SUB-ZERO PROCESSED...

15. - 17. 5. 2013, Brno, Czech Republic, EU

STRUCTURE AND PROPERTIES OF SUB-ZERO PROCESSED LEDEBURITIC TOOL STEELS

Jana SOBOTOVÁ1, Jiří CEJP1 , Peter JURČI2, Petra SALABOVÁ3, Otakar PRIKNER3

1Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Materials

Engeneering, Karlovo nám. 13, 121 35 Praha 2, Czech Republic, EU, [email protected]

2 Slovak University of Technology Bratislava, Faculty of Material Science and Technology Trnava,

Department of Materials, Paulinska 16, 91724 Trnava, Slovakia, EU

3 Prikner - tepelné zpracování kovů, Martínkovice 279, 550 01, Czech Republic, EU

Abstract

Three types of ledeburitic cold work steels were austenitized at 1025 °C, nitrogen gas quenched and double

tempered. Sub-zero period, made at different temperatures and dwell times, was also inserted between

quenching and tempering. For the research was used steel Sverker 21 made by conventional way and two

P/M steels (Vanadis 4 Extra and Vanadis 6). The microstructure, mechanical properties and surface

resistance to cyclic impact load have been investigated as a function of parameters of heat treatment. There

were used metallographic and rtg analysis, hardness measurement, three point bending test and impact test

for the evaluation effect of sub-zero processing to observed materials. Analysis of impact craters was made

by laser confocal microscopy and SEM. The results proved small differences in behaviour of observed

material, which can, under specific conditions, be determinative for lifetime period of tools.

Keywords: cold work steel, heat-treatment, sub-zero treatment, three point bending test, impact test

1. INTRODUCTION

It is well known, that cryogenic treatment (CT) has been reported to improve mechanical properties of

materials. The research about CT has been validated by the first results on machinery tools, which have

shown remarkably enhancement in hardness and durability [1]. Sub-zero treatments are classified as

Shallow Cryogenic Treatment (SCT) when the samples are placed in a freezer at -80 °C and then they are

exposed to room temperature and Deep Cryogenic Treatment (DCT) when the samples are cooled to

-196 °C, hold down and gradually warmed to room temperature [1, 2]. Table 1 gives some general indication

about CT effects on mechanical properties reported by the literature on tool steels [1].

Table 1 Effects of SCT and DCT on mechanical properties of tool steels

It is necessary to take some facts into consideration.

There exist a lot of kinds of tool steels with different

chemical composition, mechanical properties and

request on the usable properties. There is not still

perfectly study influence of parameters CT to final

properties of tool steels. For example [3] reported,

that of the parameters relating to DCT, only the

holding time and the heating rate have a significant

effect on properties of the steel Sverker 21. On the

other side in [4] is stated, that the hardness of sub-

zero processed Vanadis 6 does not depend on the

sub-zero processing time. Paper [5] observed, as it is well known, that CT increases hardness by decreasing

SCT DCT

Hardness not available improvement

Wear resistance improvement improvement

Bending strength not available improvement

Yeild strength not available not available

Fatigue life not available not available

Toughness not available improvement


retained austenite. Authors in the paper found, that the hardness of only quenched Vanadis 4 is about

865 HV whereas the hardness of quenched (Q), DCT and double tempered (DT) Vanadis 4 is about 785 HV.

In accordance with it, Baldissera and Delprete [1] state, that the choice of optimal treatment parameters of

CT requires specific investigations on each material.

The current paper deals with study of influence CT to properties of three kinds of ledeburitic cold work tool

steels.

2. EXPERIMENTAL

The chemical composition of used steels is seen in Table 2.

Table 2 The chemical composition of used steels (wt%)

Steel C Si Mn Cr Mo V

Sverker 21 (AISI D2) 1.55 0.30 0.40 11.80 0.80 0.80 C/M steel

Vanadis 4 extra 1.40 0.40 0.40 4.70 3,50 3.70 P/M steel

Vanadis 6 2.10 1.00 0.40 6.80 1.50 5.40 P/M steel

C/M steel made by conventional way

All observed steels are characterized by very high abrasive-adhesive wear resistance, high compressive

strength, very good dimensional stability at heat treatment and in service, there have good through-

hardening properties too.

5 samples for the three point bending test (10x10x100 mm with surface roughness of 0,2 – 0,3 µm) were

heat treated in each set. Heat treatment was the following: vacuum austenitizing at 1025 °C for 30 min,

nitrogen gas quenching at 5 bars pressure, sub-zero treatment at temperature -90 °C and -196 °C per 4 or

10 hours and 2x tempering at 530 °C per two hours for Vanadis 4 extra and Vanadis 6, 2x tempering at

480 °C per two hours for Sverker 21.Three point bending tests were carried out at following parameters: the

distance between supports was 80 mm, loading in the central region and loading rate of 1 mm/min, up to the

moment of the fracture. Microstructure of the material was analysed by the light microscopy, retained

austenite by rtg analysis (RTG diffractometer Philips PW1710 – norm ASTM E975-03). Impact test was carry

out on electromagnetic impact tester, number of hits 10 000 and frequency 10 Hz. It was done 3 craters on

each sample. There were measured diameters of craters. Value of the hardness is average from

5 measurements.

3. RESULTS

Results of three bending test for investigate materials and states are shown in Fig. 1. Not surprisingly there

are the lowest values of bending strength for Sverker 21, bigger for Vanadis 6 and the biggest for Vanadis 4

extra. Whereas CT of Vanadis 6 does not have influence on bending strength, from results is evident

increase this characterization for Vanadis 4 extra compared with conventional treatment e.g. without

cryogenic treatment (CONT). It was proved for Sverker 21 small increase of bending strength for -90 °C/4 h,

bending strengths after the other states of CT are comparable to CONT.

From Fig.2 is evident, that CT does not have influence to hardness for Sverker 21. For P/M steels CT cause

decrease hardness about 1-2 HRC compared with CONT P/M steels.


0

500

1000

1500

2000

2500

3000

3500

4000

4500

50000

-90°C

/4h

-90°C

/10

h

-196

°C/4

h

-196

°C/1

0h 0

-90°C

/4h

-90°C

/10

h

-196

°C/4

h

-196

°C/1

0h 0

-90°C

/4h

-196

°C/4

h

-196

°C/1

0h

Sverker 21 Vanadis 4 extra Vanadis 6

Be

nd

ing

str

en

gth

[M

Pa

]

Fig.1 Bending strength of used steels for observed conditions of the heat treatment

57

57,5

58

58,5

59

59,5

60

60,5

61

61,5

62

0

-90°C

/4h

-90°C

/10

h

-196

°C/4

h

-196

°C/1

0h 0

-90°C

/4h

-90°C

/10

h

-196

°C/4

h

-196

°C/1

0h 0

-90°C

/4h

-196

°C/4

h

-196

°C/1

0h


HR

C

Fig.2 Bending strength of used steels for observed conditions of the heat treatment

There are some different in results of impact test for used steels and observed conditions of the heat

treatment Fig.3. The smallest diameter of crater was measured form CONT Vanadis 4 and CT Sverker 21

(-90°C/4 h, -196°C/10 h Fig.4a).


The worst results were accomplished for DCT Vanadis 6 (-196°C/4 h, -196°C/10 h Fig.4b). Nevertheless

there were not observed any cracks in the area of crater on these samples either. On the basis of this it is

possible to presume for the steels Sverker 21, Vanadis 4 extra and Vanadis 6 that there are not observed

any obvious unwanted characteristics during this state of loading depending on CT of the steels.

0

50

100

150

200

250

300

350

400

450

500

0

-90°C

/4h

-90°C

/10

h

-196

°C/4

h

-196

°C/1

0h 0

-90°C

/4h

-90°C

/10

h

-196

°C/4

h

-196

°C/1

0h 0

-90°C

/4h

-196

°C/4

h

-196

°C/1

0h


siz

e o

f c

rate

r [ m

m]

Fig.3 Size of crater measured after impact test of used steels for observed conditions of the heat treatment

a) Sverker 21 -196 °C/10 h b) Vanadis 6 -196 °C/10 h

Fig.4 Craters after impact test for samples Q + DTC + DT

It is well known, that high content of Cr causes decrease of temperature of finish martensite transformation

and thus increase of content of retained austenite. For the steels Vanadis 4 extra and Vanadis 6, which


Table 3 Content of retained austente for Sverker 21 for observed conditions of the heat treatment

compared with Sverker 21 have approximately

half content of Cr, was not surprisingly

measured content of retained austenite less

than 1% after all investigated states of

treatment. It was found out decrease content of

retained austenite for CT Sverker 21 compared

with CONT Table 3. The lowest content of retained austenite was measured for longer time of cryogenic

treatment. Present results of measurement of retained austenite is in good agreement with previous results

and literature as showed Tab.4

Table 4 Results of an X-ray diffraction of Vanadis 4 quenching of 1040°C and quenching, cryogenic

treatment -196 °C/30 min and double tempered 525 °C/30 min and Vanadis 6 after quenching of 1025°C and

quenching, cryogenic treatment -196 °C/4 h and double tempered 530°C/2 h

Steel Heat treatment Content of retained austenite [%] Reference

Vanadis 4 Q 17,7 [5]

Vanadis 4 Q + DCT + DT 1,9 [5]

Vanadis 6 Q 17,76 [6]

Vanadis 6 Q + DCT + DT < 0,5 current paper

The microstructures of the Sverker 21 quenched from 1025 °C, cryogenic treatment -196 °C/4 hours and

tempering 2x 480 °C are shown in Fig.5. The material consists of the matrix and carbides. The matrix

contains mainly the tempered martensite, the second phase is the retained austenite. Distribution and size

of carbides is accordance with C/M steel.

The microstructures of the Vanadis 4 extra quenched from 1025 °C, cryogenic treatment -196 °C/4 hours

and tempering 2x 530 °C are shown in Fig.6. The material consists of the matrix and carbides. The matrix

contains mainly the tempered martensite, the minimal second phase is the retained austenite. Distribution

and size of carbides is accordance with P/M steel.

Nevertheless, resolution of light metallography, is not appropriate for investigation of structural changes after

cryogenic treatment Sverker 21 and Vanadis 4 extra.

a) Q + DT b) Q + DCT + DT

Fig.5 Microstructure of Sverker 21(1025 °C, -196 °C/4 h, 2x480 °C)

Sverker 21 (1025 °C, 2x480 °C)

Cryogenic treatment Content of retained austenite [%]

0 11,7

-90°C/4h 3,4

-90°C/10h 2,3

-196°C/4h 2,9

-196°C/10h 1,6


a) Q + DT b) Q + DCT + DT

Fig.6 Microstructure of Vanadis 4 extra (1025 °C, -196 °C/4 h, 2x530 °C)

4. CONCLUSIONS

Cryogenic treatment does not have influence on bending strength for Vanadis 6, but does for Vanadis 4 extra.

It is evident, that cryogenic treatment does not have influence to hardness for Sverker 21. For Vanadis 6 and Vanadis 4 extra cryogenic treatment cause decrease hardness about 1-2 HRC compared with P/M steels without cryogenic treatment.

For investigated samples does not prove predisposition to damage by impact workload.

It was found out decrease content of retained austenite of cryogenic treatment Sverker 21 compared with conventional treatment.

ACKNOWLEDGEMENTS

Authors wish to thank the Ministry of Industry and Trade of the Czech Republic for the financial

support for the solution of the Project TIP FR-TI1/003.

REFERENCES

[1] BALDISSERA, P., DELPRETE, C. Deep Cryogenic Treatment: A Bibliographic Review.. The Open Mechanical

Engineering Journal, 2008, 2, p. 1-11.

[2] DEBDULAL, D. et. al. Influence of sub-zero treatments on fracture toughness of AISI D2 steel. Materials Science

and Engineering A 528, 2010, p. 589 - 603

[3] OPPENKOWSKI, A., WEBER, S., THEISEN, W. Evaluation of factors influencing deep cryogenic treatment that

affect the properties of tool steels. Journal of Materials Processing Technology 210, 2010, p.1948 -1955

[4] SOBOTOVA. J. et al. Structure and properties of sub-zero processed Vanadis P/M ledeburitic tool steel, In Metal

2011: 20th Anniversary International Conference on Metallurgy and Materials: 18. - 20. 5. 2011. Brno, Hotel

Voronež I, Czech Republic [CD-ROM]. Ostrava: TANGER: May, 2011, ISBN 978-80-87294-22-2.

[5] ARSLAN, F.K. Characterization of cryogenic heat treatment Vanadis 4 PM cold work tool steel. Vacuum, Volume

86, Issue 4,11 November 2011, p.370 -373

[6] SOBOTOVA, J. et. al. Diagnostics of the Microstructural Changes in Sub-zero Processed Vanadis 6 P/M

Ledeburitic Tool Steel, Materials and technology, Volume 47, no.1, 2013, p.93 – 98, ISSN: 1580-2949

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