STRUCTURE AND PROPESTIES OF SUB-ZERO PROCESSED VANADIS 6 PM LEDEBURITIC TOOL STEEL.pdf

18. - 20. 5. 2011, Brno, Czech Republic, EU

STRUCTURE AND PROPESTIES OF SUB-ZERO PROCESSED VANADIS 6

P/M LEDEBURITIC TOOL STEEL

Jana SOBOTOV a, Peter JURI a, Ji CEJP a, Petra SALABOV b, Otakar PRIKNER b

a Czech Technical University in Prague, Faculty of Mechanical Engineering, Karlovo nm. 13, 121 35 Prague 2, Czech Republic, [email protected] b Prikner - tepeln zpracovn kov, Martnkovice 279, 550 01, Czech Republic

Abstract

The P/M Vanadis 6 cold work steel was austenitized, quenched and tempered at various combinations of processing parameters. For selected sets of specimens, also sub-zero period, made at different temperatures and processing dwell times was inserted between quenching and tempering. The microstructure and mechanical properties have been investigated as a function of austenitizing temperature, parameters of sub-zero processing and tempering. As-quenched structure is composed of martensite, retained austenite and carbides. Sub-zero processing led to the decrease of the retained austenite amount and increase of tetragonality of the martensitic lattice. Therefore, the as-quenched hardness of sub-zero processed samples was than that of no sub-zero processed steel. On the other side, the use of sub-zero period results in a slight hardness decrease after tempering. Furthermore it was found that the three point bending strength, decreased markedly with increased austenitizing temperature. The lowering of bending strength with increasing austenitizing temperature can be considered as natural because of the austenitic grain growth. The situation for sub-zero treated steel seems to be more complex. Bending strength for the material processed at -90C was lower while the sub -zero processing at -196C led generally to increas e of the bending strength.

Key words: P/M cold work steel, heat-treatment, sub-zero treatment, bend strength, microstructure

1. INTRODUCTION

Vanadis 6 is a powder metallurgical cold work tool steel offering a combination of very high wear resistance and good toughness. High compressive strength and very good dimensional stability during heat treatment are also typical properties of this material. Based on these characteristics, Vanadis 6 is suitable for long tooling of cold work materials (blanking, forming operations, knives). Demanded tool life is given by means of heat treatment. Suitable heat treatment depends on application. For optimum wear resistance [1], recommended regime is a hardening from a temperature of 1150C and 3x tempering at 560C, resulting in hardness from the range 63-65 HRC. For optimum ductility is directed heat treatment with lower temperatures (1000C, 2x 250C, resulting hardness is stated 60-62 HRC). During the austenitizing, eutectoid and a part of secondary carbides are dissolved in the austenite and results in high hardness of the material after heat treatment. Other part of carbides, which does not undergo the dissolution, hinders the austenite grains coarsening and makes the steels wear resistant. Blek et al. [2] reported that there are two types of carbides in Vanadis after austenitizing. The M7C3 carbides underwent intensive dissolution in the austenite and they were not detected above the temperature of 1100C. On the other side, MC carbides remained almost completely unaffected and symptoms of dissolution were found only at the temperature of 1200C. The saturation o f the austenite with carbon, chromium and partly also vanadium results to an increased hardness of the as-quenched material, with the maximum at the austenitizing temperature of 1025C. Preceding publ ication [3] states that higher austenitizing temperature


results in hardness increase of Vanadis 6 and lowering of three point bending strength, since increased austenitizing temperature results in the grain coarsening. After the quenching, Cr-V ledeburitic steels contain martensite, retained austenite and undissolved carbides. For a more complete martensitic transformation, sub-zero treatment can be inserted between quenching and tempering. There are not doubtless opinions on the effect of sub-zero processing on properties of ledeburitic steels. In [1] it is reported a hardness increase of 1HRC after sub-zero treatment (-70 and -80C/1-3 ho urs). Juri in [3] reported increasing hardness for Vanadis 6 after sub-zero treatment (-90C/4 hours) compared to not sub-zero, but it is valid only before tempering. As-tempered hardness of sub-zero material is by 2,5 - 3,5 HRC lower than that of non-sub-zero processed. This opinion is in good agreement with [4]. Berns [5], on the other hand, reported a significant hardness increase for the sub-zero processed X290Cr12 ledeburitic steel. Authors [3,4] reported a decrease of three point bending with the application of sub-zero processing of Vanadis 6. The nature of this effect is not clear yet and should by subjected to further investigations. It is well known that the microstructure is one of the most important parameters, which determines the wear resistance of steel. Particularly for tool steels, the content and the distance between primary and secondary carbides are the key microstructural variables, controlling the tribological response [6]. Stratton [7] reported for tool steels dramatically improved wear resistance by deep cold treatment (-196C for minimum of 24 h).

2. EXPERIMENTAL

The experimental material was the PM ledeburitic steel Vanadis 6 with nominally 2,1 %C, 1,0 %Si, 0,4 %Mn, 6,8 %Cr, 1,5 %Mo, 5,4 %V and Fe as balance. The hardness of as-received material was 284 HV 10. There were made two types of specimens. Three samples of structural investigation and hardness measurement ( 17x10 mm) and 5 samples for the three point bend testing (10x10x100 mm) which were fine ground to surface roughness of 0,2 0,3 m were heat treatment in each set. Heat treatment was the following: vacuum austenitizing at the temperatures from the range 1000 1075C for 30 min, nitrogen gas quenchi ng at 5 bars pressure, sub-zero treatment at a temperature -196C/4 or 10 hours and 2x tempering at 530C per two hours. Three point bending tests have been 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.

3. RESULTS

It was stated recently [3] that three point bending strength stated decreased with the increasing austenitizing temperature. This reality is connected with grain coarsening with increasing of austenitizing temperature. The effect of sub-zero processing on three point strength depends on conditions. Figure 1 presents that the three point strength after sub-zero treatment (-90C/4 hours) and tempering 2x530C is lower and, afte r sub-zero treatment (-196C/4 hours) is generally higher . Although there is a difference in the bending strength for the given sub-zero treatment regimes, the resulting hardness for the material processed by these two regimes is practically the same, Fig. 2. In addition, the hardness of sub-zero processed material is of about 2,5 HRC lower than that of no sub-zero processed steel. Figure 3 brings a comparison of the obtained three point bending strength results of the material without sub-zero processing and sub-zero processed steel at -196C for 4 and 10 hours, respectively. The hardn ess plot for the material as a function of the austenitizing temperature and sub-zero processing done at -196C is in Fig.4. Although there is a slight difference in bending strength for monitored times of sub-zero processing at -196C (lowering with longer soaking ti me), the hardness practically does not depend on the time of the sub-zero processing. These results are rather surprising, since one can expect higher hardness after sub-zero processing compared to that after no sub-zero treatment.


Fig.1. The effect of sub-zero treatment on bending strength (tempering 2x 530C)

Fig.2. The effect of sub-zero treatment on hardness (tempering 2x 530C)

Also, the three point bending strength can be expected to be lower due to sub-zero treatment. But, the situation can be considered to be more complex since various effects should be taken into account in the assessment of the heat treatment response of the examined material. There are at least three phenomena happened during the tempering. Firstly, the tempering of the martensite proceeds, which is always connected with the hardness decrease. The transformation of the retained austenite during the cooling down from the tempering temperature, corresponding to the temperature of the secondary hardness peak, induces


the increase of hardness. The last process is the precipitation of carbides during the hold at the tempering temperature, which also results in an increase of hardness.

Fig.3. The effect of the time of sub-zero processing on bending strength (tempering 2x 530C)

Fig.4. The effect of the time of sub-zero processing on hardness (tempering 2x 530C)

The amount of retained austenite was found to be three times lower for the sub-zero processed steel than that of on sub-zero processed [8]. In the same literature, there was an increased tetragonality of the martensite in sub-zero processed material established. These facts confirm that the microstructure of sub-zero processed Vanadis 6 steel differ from that of no sub-zero processed in many factors. In addition, Stratton et al. assumed that a precipitation of nano-sized particles can happen during the hold at a temperature of deep-cooling [7]. However, this effect can only hardly be verified since the diffusion at such a


low temperature is practically impossible. Thus, the size of these carbides can be expected well below the resolution of common TEMs. But, based on the results presented here [8] and the Strattons consideration, it should be noted that the contribution of the retained austenite transformation to the final as-tempered hardness can be expected to be much lower when the material was sub-zero processed. Regarding to the martensite state after quenching and/or sub-zero processing, there is no doubtless interpretation of the effect of higher martensite supersaturation (due to sub-zero period) on the as-tempered hardness. However, it is known that the as-tempered hardness of plain carbon steels with a near eutectoid carbon content (but slightly different) does not differ significantly. Therefore, one can also assume that the as-tempered hardness of the Vanadis 6 steels martensite developed by the sub-zero process (with higher tetragonality) would also not differ significantly from that generated by simple quenching. This consideration seems to be a logical interpretation of lower as-tempered hardness of sub-zero processed Vanadis 6 steel at the current state of knowledge.

-196C/4 hours -196C/10 hours

Fig.5. The effect of the time of sub-zero processing -196C on microstructure Vanadis steel (austenitizing 1000C, tempering 2x 530C).

-196C/4 hours -196C/10 hours

Fig.6. The effect of the time of sub-zero processing -196C on microstructure Vanadis steel (austenitizing 1075C, tempering 2x 530C).


The microstructures of the material quenched from 1000C and 1075C, sub-zero processing -196C/4 or 10 hours and tempering 2x 530C are show n in Fig.5.and 6. The material consists of the matrix and carbides. The matrix contains mainly the tempered martensite, the second phase is the retained austenite. Some portion of the lower bainite could be also expected [3]. Carbides are very fine and uniformly distributed in the structure. The material quenched from 1000C contains more carbides than that quenc hed from 1075C, since the carbides, mainly the M 7C3, are dissolved in the austenite in a larger extent. This fact is in accordance with [2]. Nevertheless, possibility of magnification of light metallography, are not enough for investigation changes of structure after sub-zero processed Vanadis 6.

4. CONCLUSIONS

a) Three point bending strength of the Vanadis 6 steel is generally higher for sub-zero processed samples at -196C for 4 h than these of the materia l processed at -90C for 4 h or no sub-zero processed. This becomes an importance with increasing austenitizing temperature.

b) Prolonging of the sub-zero period to 10 h (at -196C) does not bring any benefits with respect to t he three point bending strength.

c) The hardness of sub-zero processed (-196C/4 and 10 hours) is of about 2,5 HRC lower than that of no sub-zero processed Vanadis 6, and practically the same as that after deep cooling to -90 oC/4h, and does not depend on the sub-zero processing time.

d) The microstructure of sub-zero processed Vanadis 6 steel probably differ from that of no sub-zero processed in many factors. Nevertheless, they can not be evaluated with the light microscopy and more detailed checking with TEM seems to be necessary.

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.] http://www.bucorp.com/vanadis_6_c.htm [2.] BLEK. P., SOBOTOV, J., JURI, P. Evaulation of Structural changes in Cr-V Ledeburitic Tool Steels

Depending on Temperature Austenitization, Materials and Technology, paper accepted for publication.

[3.] JURI, P. aj. Efect of Sub-zero Treatment on Mechanical Properties of Vanadis 6 PM Ledeburitic Tool Steel . In Metal 2010: 19.mezinrodn konference metalurgie a materil: 18. - 20. 5. 2010. Ronov pod Radhotm , Hotel Relax, esk republika [CD-ROM]. Ostrava: TANGER: Kvten, 2010, s.518-523. ISBN 978-80-87294-15-4.

[4.] JURI, P., USTARI, B., LESKOVEK, V. Fracture Charakteristic of the Cr-V Ledeburitic Steel Vanadis 6, Materials and Technology , 2010, 44, 1, pp.79 86.

[5.] BERNS, H. Restaustenit in ledeburitischen Chromsthlen und seine Umwandlung durch Kaltumformen, Tiefkhlen und Anlassen, HTM, 1974, 29, 4, pp.236 247.

[6.] FONTALVO, G.A. et al. Microstructural Aspect Determining the Adhesive Wear of Tool Steels, WEAR, 2006, 260, pp.1028 1034.

[7.] STRATTON, P.F. Process Optimatization for Deep Cold Treatment of Tool Steels. In proceedings 1stInternational Conference on Heat Treatment and Surface Engineering of Tools and Dies, Zagreb, IFHTSE, 2005, pp.11 19.

[8.] JURI, P. Cr-V Ledeburitic Cold Work Tool Steels, Materials and Technology, paper accepted for publication.

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STRUCTURE AND PROPESTIES OF SUB-ZERO PROCESSED VANADIS 6 PM LEDEBURITIC TOOL STEEL.pdf