Key Engineering Materials Vols. 86-87 (1993) pp. 47-54 online at http://www.scientific.net © (1993) Trans Tech Publications, Switzerland

CRYOGENIC TREATMENT OF THREE HEAT-TREATED TOOL STEELS

K. Moore and D.N. CollinsDepartment of Mechanics! Engineering, University College, Dublin, Ireland

Keywords: tool steel, cryogenic, heat treatment

ABSTRACT

This paper reports on experiments carried out to determine the effects o f various parameters on the hardness of three cryogenically treated steels as compared to those only quenched to room temperature. It was found that increases in hardness were very dependent on cryogenic temperature and that stabilisation can affect obtainable hardness. It was confirmed that for two of the steels the hardness was independent o f treatment time. The optimum austenitising temperatures and cryogenic temperatures within the liquid nitrogen range were identified.

1. INTRODUCTION

Hardening of steel involves three processes - austenitising, quenching and tempering. During austenitising carbon and other alloys, having greater solubility in austenite than ferrite, begin to dissolve and homogenise in the austenitic by a process of diffusion. Once the austenite is cooled below its critical temperature it becomes unstable and will transform, the products being dependent on the cooling conditions. For slow cooling rates the transformation is diffusion controlled, and relatively soft constituents will be formed. Under rapid cooling conditions the structure formed by the resulting shear process is a body-centred tetragonal or highly distorted body-centred cubic crystal structure called martensite, the hardness of which results from lattice distortion caused by supersaturation. The temperature at which this transformation begins is designated the martensite start, or Ms temperature . This transformation process is normally athermal, i.e. dependent on temperature only. The temperature must be continuously reduced for the transformation to continue. The temperature at which 100% transformation from austenite to martensite occurs is designated the martensite finish, or Mf, temperature.

Increasing the carbon and alloy content of the austenite, such as by increasing the austenitising temperature or holding time, will produce a harder martensite, but will simultaneously lower the Ms and M f temperatures. The M f temperature o f most commercially used steels is below room temperature, so that after conventional hardening processes, there will be some retained austenite in the steel. A tool steel can typically have 20-30% retained austenite after recommended treatment. One other factor pertaining to retained austenite is stabilisation, a phenomenon that reduces the ability of austenite to transform to martensite, which occurs during slow or interrupted cooling before complete transformation.

Multiple tempering has been the traditional approach taken to deal with the problem of retained austenite. Room temperature is a convenient stopping point for heat treaters but has no significance in the austenite-martensite transformation. The transformation will continue if the steel is

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continuously cooled below room temperature. This is the principle behind the cryogenic treatment of heat-treated steels.

2. TEST PROCEDURE

Three tool steels were investigated in this series of tests, as detailed in Table 1. Steels 1 and 2 will be referred to by their AISI designations, H13 and D2 respectively. Steel 3 will be referred to by its trade name Vanadis 4. The investigation was designed to evaluate the effect of the following factors on hardness of cryogenically treated steels, with the aim of establishing the optimum heat/cold treatment, with the emphasis on maximising hardness.

Table 1: Steels Used

Trade Name Ovar Supreme Sverker 21 Vanadis 4AISI Designation H13 D2 None - falls between A7 and D7General description C hromium-Moly-Vanadium

hot work tool steelHigh-carbon, high-chromium, moly, vanadium cold work tool steel

C hromium-Moly-Vanadium powder metallurgical cold work tool steel

Typical Analysis 0.38% C, 1.0% Si, 0.4% Mn 5.3% Cr, 1.3% Mo, 0.9% V

1.55% C, 0.3% Si, 0.3% Mn 12.0% Cr, 0.8% Mo

1.5% C, 1.0% Si, 0.4% Mn, 8.0% Cr, 1.5% Mo, 4.0% V

2.1 Cryogenic Temperature (Test 1)Steels austenitised at 1170°C. Samples quenched directly to one of three cryogenic temperatures and held for between 38 minutes and 20 hours.

2.2 Holding Time at Cryogenic Temperature (Test 2)Details as above

2.3 Holding at Room Temperature Before Cryogenic Treatment (Test 3)Steels austenitised at 1150°C and held at room temperature for up to 20 hours before a two hour cryogenic treatment.

2.4 Austenitising Temperature (Test 4)Steels austenitised at temperatures between 950°C and 1150°C, before treating half at -196°C for one hour.

2.5 Tempering Treated and Untreated Steels (Test 5)Steels austenitised at 1075°C before treating half at -196°C for 15 minutes. Treated samples were subjected to a one, two hour temper. Untreated samples were tempered twice for two hours.

2.6 Optimum Cryogenic Temperature, Cooling Rate and Effect of Long Term Holding at Room Temperature Before Cryogenic Treatment (Test 6)Steels austenitised at 1075°C and held at room temperature for 22 days, or at 1150°C and held at room temperature for 18 days before a 5 minute cryogenic treatment at progressively lower temperatures.

Notes1. All specimens were austenitised in a vacuum furnace. A forced nitrogen quench was used taking

30-40 minutes to cool to 50°C.

Key Engineering Materials Vols. 86-87 49

2. The samples in Test 6 were cooled in stages with a 5 minute hold at each test temperature. Total cycle time to -196°C approximately 40 minutes. All other specimens cooled by placing directly in nitrogen atmosphere/liquid nitrogen at temperature in question.

3. All specimens were removed from cryogenic unit and allowed to return to room temperature in air.

4. All hardness readings were taken at room temperature. Results are the average of at least 5 readings (10 in the case of the first two series of tests).

3. RESULTS AND DISCUSSION

3.1 Cryogenic Temperature (figs. 1, 2, and 3)The results were as expected. Hardnesses increased as if retained austenite was continuing to transform to martensite. Although -100°C would appear to be the optimum temperature to achieve maximum hardness it was not possible to say whether complete transformation to martensite had occurred. The increase in hardness of each steel is a reflection of the original quantity of retained austenite in that steel. The greatest increase of over 16 HRc corresponds to a retained austenite content in the D2 steel of well over 50% (from trade literature).

3.2 Holding Time at Temperature (figs. 4, 5, and 6)The graphs for D2 and Vanadis 4 indicate that there is no change in hardness with time. Further investigations confirmed that any variations in hardness at a given temperature were due to random effects. Although athermal martensite transformations are the dominant type of transformation in heat treated steels, isothermal transformations have been observed in Fe-N-Mn and Fe-Ni-Cr alloys (Krauss). It appears that the transformation of martensite in H13 may also be partially isothermal, and occurs more rapidly as the temperature decreases.

3.3 Holding at Room Temperature (fig. 7)The results show that holding specimens at room temperature for periods up to 20 hours has little or no effect on the hardness of the samples treated at -196°C. This indicates that stabilisation of retained austenite is not significant under these conditions. If anything, D2 and Vanadis 4 show slight increases in the hardness as holding time increases.

3.4 Austenitising Temperature (figs. 8, 9, and 10)It was assumed from Test 1 that -196°C would be sufficient to achieve maximum hardness and for convenience this temperature was used. The hardness of the H13 continues to increase well beyond 1050 C (contrary to published literature). The small increases in hardness after cryogenic treatment (about 0.5 HRc) even at 1150°C indicate that only small amounts of retained austenite are present (less than 10% - source, trade literature). This is to be expected considering the low carbon content (0.3%) and hence high Mf temperature.

The higher the austentizing temperature of a steel, the more carbon and alloying elements go into solution, and thus the more distorted and harder will be the martensite. Thus as the austentizing temperature increases, the hardness of the martensite and corresponding steel should increase or at least plateau. In this case, although the actual rise in hardness between treated and untreated D2 and Vanadis 4 increases with increasing austenitising temperature, due to the larger amounts of retained austenite being transformed, their final hardnesses do not continue to rise. Their hardness peaks, just like the specimens cooled to room temperature, albeit at a higher hardness and austentizing temperature. (It was assumed that after cryogenic treatment at -196°C a practically complete transformation to martensite would have occurred)

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3.5 Tempering (figs. 11 and 12)As expected, the graphs of the untreated specimens exhibit secondary hardening characteristic of high alloy tool steels. The treated steels are obviously harder than their untreated counterparts because of the differences in martensite content, but the differences start to decrease after 300°C as the transformation of retained austenite in the untreated steels starts to accelerate, until at about 500°C the untreated steels become harder than the treated ones.

These results have two important implications for the heat/cryo treatment of steels and their application. It is obvious that, if maximum hardness is a priority, then a low temperature temper o f a cryogenically treated steel is in order. However, it should also be possible to improve the toughness of some high-alloy tool steels whilst maintaining optimum hardness. These types of steels sometimes exhibit temper embrittlement when tempered in the secondary hardening temperature range, so that hardness is increased at the expense of toughness. It should be possible to achieve this level of hardness by tempering a cryogenically treated steel at a lower temperature, so avoiding temper embrittlement.

3.6 Cooling Regimes, and Long-term Holding at Room Temperature (figs. 13-17)The most important point to note, regarding the non-stabilised specimens, is the lower hardness of about 0.5 HRc obtained at -140°C for D2 and Vanadis 4 austenitised at 1075°C, as opposed to that achieved on cooling to -100°C or -196°C. The consistency of results and the presence o f this effect to some degree in three out of the four non-stabilised conditions for these steels suggests that this is not due to random error. Moreover hardness results from preliminary tests carried out by Moore on a T1 steel also "Showed a low optimum between -80°C and -120°C” as observed here.

The assumption from Test 1 (which failed to show up this effect), that -196°C would be sufficient to obtain maximum hardness within the liquid nitrogen range may have been erroneous, and a higher hardness may be obtainable at an intermediate temperature. The trend suggests that this is most likely to occur at lower austenitising temperatures.

These observations seem to confirm the existence of another cryogenic transformation mechanism, as proposed by others (although not necessarily the same one). It is impossible to draw any conclusions about its characteristics from these tests, except that it is not as prominent at higher austenitising temperatures. Changes in hardness due to austenite transformation could be masked by changes due to this other mechanism.

It is evident that stabilisation can have a significant effect on the hardness o f cryogenically treated steels. The existence o f a secondary Ms or Mss temperature is particularly evident, as is its dependence on the amount of existing martensite. The D2 steel, austenitised at 1150°C, demonstrates this. The Mss for the other steels fall between -20°C and -60°C. The hardnesses thereafter behave as one would have expected i.e. increasing to a plateau, although the D2 steels exhibit a slight drop at - 140°C reminiscent o f the non-stabilised case. The quantity o f austenite that would have transformed at the Mss temperature in the non-stabilised case, and which Cohen (1949) says is permanently stabilised, does not appear to be evident.

The implications of stabilisation are obvious. Holding at room temperature for long periods o f time should be avoided, especially if one considers the case of the D2 steel austenitised at (the optimum) 1075°C. The untreated hardness of 63.5 HRc increases to 65.5 HRc in the non-stabilised case when treated to -60°C, as opposed to no increase in the stabilised steel To optimise on hardness while economising on the cryogenic treatment, a temperature in the -100°C range is recommended. However if stabilisation is unavoidable then the optimum temperature is lowered to -140°C (Note that these figures only relate to the steels and conditions encountered in these tests).

Key Engineering Materials Vols. 86-87 51

It is important to consider these results in the context of Test 3, which showed no stabilisation effects. For parameters used in this test there will certainly be a reduction in hardness at ~196°C after stabilisation. This is contrary to other information from Cohen and others that the "rate" of stabilisation decreases as time progresses and that stabilisation occurs more rapidly in high alloy steels.

Another fascinating outcome of these tests is the results obtained for H13. For test 4 the maximum increase in hardness for a cryogenic temperature of -196°C was from 54.5 to 55.3 HRc, a 0.8 increase. In Test 6 the improvement was from 55.3 to 56.6 HRc at -140°C an improvement of 1.3, with a further improvement expected at -196°C. The difference between these tests is the cooling rate. Test 4 specimens were immersed in liquid nitrogen, whereas cooling in Test 6 was a gradual process. This indicates that the hardness of HI 3 is cooling rate dependent.

To put these results in context - the highest hardnesses achieved for D2 and Vanadis 4 were 65.9 and 66.0 HRc, an improvement of over 2 HRc on the highest hardnesses achieved throughout these tests on untreated specimens.

4. CONCLUSIONS

4.1 The hardness of D2 and Vanadis 4 are dependent only on cryogenic temperature. The hardness of H13 appears to be additionally dependent on holding time at cryogenic temperature and also on cooling rate.

4.2 The optimum austentizing temperature for cryogenic treatment for non-stabilised D2 and Vanadis 4 at -196°C is between 1075°C and 1100°C, higher than that for conventional treatment. Stabilisation and actual cryogenic temperature are two separate variables which are likely to affect the position and magnitude o f this peak. (H13 did not exhibit a peak for the austenitising temperatures investigated).

4.3 Tempering cryogenically treated D2 or Vanadis 4 steels at their secondary hardeningtemperature should be avoided. Actual hardness will not be better than conventionally treated steel. By tempering the treated steels at lower temperatures the user should benefit from the higher hardness due to the cryogenic treatment.

4.4 Holding at room temperature for long periods before treatment should be avoided. Not onlyis transformation delayed but attainable hardness is lowered as a result o f stabilisation.

4.5 The drop in hardness observed in some tests at -140°C seems to confirm the existence o f a second transformation mechanism, as yet unidentified, at cryogenic temperatures.

REFERENCES

Cohen, M. (1949) "Retained Austenite”, ASM Transactions. Vol. 4L, 35-94

Krauss G., (1980), Principles of Heat Treatment, ASM. Metals Park, Ohio 1980.

Moore, c. Development o f BOC Ellenite Process (Cold Treatment o f Metals in Liquid Nitrogen)

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H a rd n e s s (H R C ) H a rd n e s s (H R C ) H a r d n e s s (HRC)

C r y o g e n ic T e m p e ra tu re (°C)

Fl-1C r y o g e n ic Temperature (°C)

K g . 2 F ig . 3 _

H a rd n e s s (H R C ) H a rd n e s s (H R C ) H a r d n e s s (H R C )

C ryogenic H o ld in g T im a (M in s .) C r y o g e n ic H o ld in g T im e (M in s .) C r y o g e n ic H o ld in g T im e (M in s . )

F ig . 4 P ig- 5 F ig . 6

Hardness (HRC)

-+~V4 -198-C

V4 ~40*C

0 2

: '

T........ ................

..t---1---1---«---1---L i- -L.. .i0 200 400 600 600 1,000 1,200

Stabilisation Tims at Room Temp. (Mins.)H g . 7

Key Engineering Materials Vols. 86-87 53

Hardness (HRC) Hardness (HRC) Hardness (HRC)

Austenitizing Tem perature (*C) F ig . 8

Austenitizing Temperature (°C)

F ig . 9

Austenitiz ing Temperature (®C)

F ig . 10

Hardness (HRC)

Tem pering Tem perature (*C) Fig. 11

Hardness (HRC)

Tem pering Tem perature (°C)

F ig . 12

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Hardness (HRC)

Cryogenic Temperature (’C) K g. 13

Hardness (HRC)

Cryogenic Temperature (*C)Fig. 14

Hardness (HRC)

Cryogenic Temperature fC )

Fig. 15

Hardness (HRC)

Cryogenic Temperature (*C)

Fig. 16

Hardness (HRC)

Cryogenic Temperature (*C)

Fig . 17