Molinari2001.pdf

Journal of Materials Processing Technology 118 (2001) 350±355

Effect of deep cryogenic treatment on the mechanical properties of tool steels A. Molinaria,*, M. Pellizzaria, S. Gialanellaa, G. Straffelinia, K.H. Stiasnyb a

b

University of Trento (I), Trento, Italy GKN Bir®eld AG, Brunico (BZ), Brunico, Italy

Abstract The effect of deep cryogenic treatment ( 1968C) on the properties of some tool steels was studied by means of both ®eld tests on real tools and laboratory tests. The execution of the deep cryogenic treatment on quenched and tempered high speed steel tools increases hardness, reduces tool consumption and down time for the equipment set up, thus leading to cost reductions of about 50%. A laboratory investigation on an AISI M2 and an AISI H13 steel con®rms the possibility of increasing the wear resistance and toughness by carrying out the treatment after the usual heat treatment. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Deep cryogenic; Tool steels

1. Introduction Cold treating is widely used for high precision parts and components, since it enhances the transformation of austenite to martensite. The common practice identi®es 60 to 808C as the optimum temperature, according to material and the quenching parameters. Many companies use this kind of treatment to improve surface hardness and thermal stability. Deep cryogenic treatment in the range 125 to 1968C improves certain properties beyond the improvement obtained by normal cold treatment [1±4]. The main reason for this is the complete transformation from austenite into martensite plus the formation of very small carbides dispersed in the tempered martensitic structure [5]. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering. However, a signi®cant improvement can be obtained even by treating the tools at the end of the usual heat treatment cycle, i.e. the ®nished tools. This last solution is more ¯exible than the other one and can extend the use of the treatment to many practical applications. In order to check the potential of the deep cryogenic treatment on the performances of the ®nished products, ®eld tests were carried out on some high speed steel tools, and the results were recorded for a long time, in order to get reliable * Corresponding author. Tel.: ‡10-39-461-881919; fax: ‡10-39-461-881999.

data from the industrial production. At the same time, a speci®c laboratory investigation was carried out on two steels: the AISI M2 high speed steel and the AISI H13 hot work tool steel. These experiments were aimed at the study of the effect of the treatment on some mechanical properties and on the wear resistance of the two steels. In the laboratory tests, the deep cryogenic treatment was carried out at different stages of the usual heat treatment route, i.e. both after quenching and after tempering. The results of the tests are presented and discussed here. 2. Field tests on HSS Fig. 1 is a schematic representation of the cryogenic equipment. It comprises an insulated box (cryo box), one motor with a circulating fan, one thermocouple to measure the cryogenic temperature inside the box connected to a temperature controller and programmer, a liquid nitrogen tank and a solenoid valve for the gas inlet. The actual temperature of the mass loaded in the box is recorded by a thermocouple inserted in a 30 kg steel block. Fig. 2 shows the cryogenic cycle: the thin and the bold lines represent the set up temperature and the test probe temperature, respectively. One of the most critical parameters is the cooling rate which must not exceed 20±308C/h in order to prevent the rupture of the components because of the cooling stresses. The soaking time at the minimum temperature is about 35 h; a more prolonged period does

0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 7 3 - 6

A. Molinari et al. / Journal of Materials Processing Technology 118 (2001) 350±355

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Fig. 1. Block diagram of the cryogenic equipment.

not have any signi®cant effect. The total duration of the treatment is about 100 h. The deep cryogenic treatment has a noticeable effect on the hardness of the tools. However, the effect depends on the

material. For instance, the microhardness of an ASP 60 spline forming tool was increased from 953  20 HV0:05 to 1234  23 HV0:05 (‡29.5%). In another case, an AISI M2 drill, the microhardness did not change after the deep

Fig. 2. Cryogenic cycle.

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A. Molinari et al. / Journal of Materials Processing Technology 118 (2001) 350±355

Table 1 Comparison of normal and cryogenic treated centring drills Number of parts produced per drill Normal condition drills Cryogenic treated drills

175 385

190 490

326 161

Average of parts produced with one drill 64 962

297 821

335

116

970

210 530

cryogenic treatment (890 HV0.1), but its distribution on the cross-section improved, the standard deviation being decreased from 64 to 22. Field tests were carried out on different tools. As an example, Table 1 compares the number of parts produced by centring a 21NiCrMo2 annealed steel, in the usual production conditions. The data reported shows a noticeable increase in the number of parts produced per drill due to the deep cryogenic treatment. Moreover, the data dispersion indicates the need to carry out a signi®cant number of tests in order to get reliable information on the effect of the cryo treatment. The treatment was even experimented on the tools for spline cold forming a C50 steel, heat treated to 200±250 HB. In this case, the tools were made of AISI M2 HSS. The tests were carried out by using two tools: one treated in the usual conditions, the other in cryogenic treated conditions, working together, in order to make the comparison as reliable as possible. In fact, in this way the two tools are working the same material, with the same machine and set up and with the same operator. Already after 20,000 parts machined, evidently more wear appeared on the non-cryogenic treated tool, which failed after 48.700 parts machined due to excessive wear. Fig. 3 compares the surface morphology of the two tools after 48.700 parts produced. 3. Laboratory tests on HSS Tests were carried out on an AISI M2 steel. The base treatment was the usual one, comprising vacuum quenching from 12208C and two tempering cycles at 5508C for 2 h each (A). The deep cryogenic treatment was carried out both after the two temperings (B) and after quenching; in this case one (C) and two (D) tempering cycles were carried out. Table 2 summarises the specimens investigated (Q is the quenching, T the tempering, and C the deep cryogenic). Wear tests were carried out on an Amsler trybotester (disk on disk geometry). Dry sliding tests were carried out by using a 100Cr6 steel hardened to 64 HRc as a counterface material. After some preliminary tests, the following test conditions were set up: load, 150 N and sliding speed, 0.8 m/ s for a total sliding distance of 5000 m. For example, Fig. 4 shows the wear curve of specimen B. Since the experimental points can be interpolated by a straight line in all the experiments, the slope of the line was calculated to measure the wear rate. Table 3 shows the wear rate and the hardness measured on the different specimens.

Fig. 3. Surface morphology of normal treated (a) and cryogenic treated (b) spline rolling tools.

Table 3 shows that the deep cryogenic treatment increases both the dry wear resistance and the hardness of AISI M2 and that the two effects are correlated, i.e. the higher the hardness, the lower the wear rate. The analysis of the wear

A. Molinari et al. / Journal of Materials Processing Technology 118 (2001) 350±355 Table 2 Investigated specimens Code

Treatment

A B C D

Q‡T‡T Q‡T‡T‡C Q‡C‡T Q‡C‡T‡T

Fig. 4. Wear curve obtained for specimen B.

Table 3 Results of wear tests Specimen

Wear rate …g=m  10 6 †

HV30

A B C D

3.7 1.8 2.2 2.4

826 894 888 874

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debris allows the wear mechanism to be identi®ed: oxidative wear with the contribution of abrasive wear caused by the hard carbides dispersed in the microstructure of the two counterfacing materials. Since both mechanisms are in¯uenced by hardness, the correlation between hardness and wear rate can be explained: an increase in hardness increases the abrasion resistance and the load bearing capacity of the material, i.e. the ability to support the surface layers where frictional heating causes oxidation. Therefore, it may be concluded that the deep cryogenic treatment increases the hardness of AISI M2 steel and, in turn, dry sliding wear resistance. Moreover, the results in Table 3 shows that the effect of the deep cryogenic treatment is highest when the treatment is carried out on the quenched and tempered steel. The execution of the treatment before tempering leads to a lower improvement of the properties of the steel. Impact tests on V-notched Charpy specimens were carried out on materials A and B only, because they represent the extreme behaviour. Impact energy was the same for the two materials (2 J). Contrarily, three points bending tests revealed a signi®cant increase in both strength (as computed by the Navier equation, since the bending behaviour was purely brittle, as shown in Fig. 5 relating to the specimen A) and absorbed energy after the deep cryogenic treatment: 3230 MPa and 0.56 J for material B, 2680 MPa and 0.38 J for material A. Given the purely brittle behaviour, the increase in the bending properties can be attributed to the increased hardness of material B. Impact tests are not sensitive to the increase in hardness likely because of the higher strain rate than bending tests. In order to interpret the above results, materials A and B were characterised at optical microscope and scanning electron microscope, without revealing any differences caused by the deep cryogenic treatment. In addition, the residual austenite was lower than the detection limit of the X-ray diffractometry (2%) in both cases. Therefore, the

Fig. 5. Bending test curve of material A.

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ongoing in order to con®rm and study the phenomenon in more detail. The reason why the deep cryogenic treatment has a lower effect when carried out after quenching and followed by the usual tempering is not yet clear. The results here presented contrast with those of Yun et al. [5]. However, in that case the steel has been quenched from a higher temperature (12808C), which leads to a greater oversaturation of the virgin martensite and to a higher retained austenite content. This could in¯uence the transformation during tempering and modify the effect of the deep cryogenic. Further experiments are in course. Fig. 6. Microstructure of tempered martensite in material A.

effect of the deep cryogenic treatment must be sought in the submicroscopic microstructure of the steel. Some authors attribute the effect of the deep cryogenic treatment, when carried out after quenching and prior to tempering, to the activation of the tempering transformations of the virgin martensite, because of its high oversaturation attained at 1968C [5]. Because of this, the carbide precipitation occurs with a higher activation energy, thus leading to a higher nucleation rate and, in turn, to ®ner dimensions and a more homogeneous distribution. However, in our experiments, the deep cryogenic treatment was carried out after two tempering cycles, and therefore on a less oversaturated martensite, so that the effect on the carbides is lower. Preliminary TEM analyses did not reveal signi®cant differences in the carbides dispersed in the tempered martensite. Instead, Figs. 6 and 7 show the submicrostructure of the tempered martensite in materials A and B, respectively. While the tempered martensite shows a twinned submicrostructure, that cryogenically treated does not show any twins. This result was con®rmed by several observations. No evidence was found in the literature of this phenomenon (tempered martensite detwinning). Further experiments are

4. Laboratory tests on hot work tool steels Tests were carried out on an AISI H13 hot work tool steel. The base treatment comprises vacuum quenching from 10208C and two tempering cycles at 5708C for 3 h each (A). Also in this case, the deep cryogenic treatment was carried out both after the two temperings (B) and after quenching; again one (C) and two (D) tempering cycles were carried out on the quenched and deep cryogenic treated steel. Therefore, Table 3 summarises the AISI H13 specimens investigated, as well. First of all, hardness, impact energy (E) and toughness (KIC) were measured on the different specimens, obtaining the results reported in Table 4. Both impact and fracture mechanics tests were carried out on V-notched Charpy specimens, with a 0.25 mm notch radius. The results show that the cryogenic treatment, when carried out after the usual heat treatment, increases toughness and does not in¯uence hardness and impact energy; in other words, the increase in toughness is attained without reducing hardness. A toughness increase can be obtained, even if lower, by carrying out cryogenic treatment just after quenching. In this case, as for the usual heat treatment procedure, the number of tempering cycles cannot be reduced to one, because of the excessive brittleness of the single-tempered material. The same wear tests as above were carried out on specimens A and B. Given the lower hardness of AISI H13 than AISI M2, the predominant mechanisms are oxidative wear, abrasion and delamination, as detected by the debris analysis (Fig. 8). Even in this case the experimental points of the wear curves can be interpolated by a straight line over the whole sliding distance, and the wear rate can be determined by the Table 4 Results of mechanical tests on the AISI H13

Fig. 7. Microstructure of tempered martensite in material B.

Specimen

HRc

E (J)

KIC (N/mm3/2)

A B C D

46.5 46.7 49.7 47.9

17.9 18.1 13.6 16.9

42.8 49.1 45.4 45.0

A. Molinari et al. / Journal of Materials Processing Technology 118 (2001) 350±355

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Fig. 8. Diffraction pattern of the collected wear debris.

slope of the wear diagram. Wear rates are one order greater than in AISI M2, and a signi®cant difference between the two specimens was determined: 2:1  10 5 for specimen A, 1:5  10 5 for specimen B. The deep cryogenic treatment is therefore able to strongly reduce the wear rate of the hot work tool steel. This result can be interpreted on the basis of increased toughness, because in the presence of delamination the ability of the material to oppose crack propagation can really increase the mechanical stability of the wear surface and the load bearing capacity. Therefore, even if the deep cryogenic treatment does not in¯uence the hardness of the AISI H13 steel, it increases both toughness and wear resistance. This effect can have an important effect on the performances of the tools, in particular those used for Al extrusion and for the hot forming of steels, where wear resistance and toughness are frequently the key properties. Also, in this case the metallurgical interpretation of the results must be found in the submicroscopic microstructure of the steel. A speci®c investigation based on TEM analyses is ongoing. 5. Conclusions The deep cryogenic treatment ( 1968C) of quenched and tempered high speed steel tools improves their properties; in particular, it increases the hardness and improves the hardness homogeneity, reduces the tool consumption and the

down time for the equipments set up, thus leading to about 50% cost reduction. In addition to the ®eld tests, laboratory tests were carried out on two different steels in order to study the effect of the deep cryogenic treatment on some mechanical properties and on the wear resistance. While in the AISI M2 steel the increase in wear resistance can be attributed to the increased hardness, in the case of the AISI H13 steel the increased wear resistance can be correlated to the increased toughness. When the cryogenic treatment is carried out after quenching and followed by the usual tempering cycle, its in¯uence on the properties of steel is negligible. The results presented here are not exhaustive; TEM analyses are being carried out in order to investigate the submicrostructure of the treated materials in the different treatment conditions. References [1] K. Moore, D.N. Collins, Cryogenic treatment of three heat treated tool steels, Key Eng. Mater. 86±87 (1993) 47. [2] D.N. Collins, Deep cryogenic treatment of tool steels: a review, Heat Treatment Met. 2 (1996) 40. [3] D.N. Collins, J. Dormer, Deep cryogenic treatment of a D2 cold-work tool steel, Heat Treatment Met. 3 (1997) 71. [4] P.L. Yen, Formation of fine eta carbides in special cryogenic and tempering process key to improved properties of alloy steels, Ind. Heating 1 (1997) 40. [5] D. Yun, L. Xiaoping, X. Hongshen, Deep cryogenic treatment of highspeed steels and its mechanism, Heat Treatment Met. 3 (1998) 55.