EFFECT OF DEEP COLD TREATMENT ON TWO CASE HARDENING STEELS

Transcription

1 Acta Metall. Sin.(Engl. Lett.) Vol.21 No.1 pp1-7 Feb EFFECT OF DEEP COLD TREATMENT ON TWO CASE HARDENING STEELS C.H. Surberg V-Research Center of Competence for Tribology and Technical Logistics GmbH, Dornbirn, Austria P. Stratton The Linde Group, Unterschleiβheim, Germany K. Lingenhöle Lingenhöle Technology GmbH, Feldkirch, Austria Manuscript received 9 July 2007; in revised form 28 September 2007 Although cold treatments have been used to reduce the retained austenite in the cases of carburised steel for many years, there is little data on deep cold temperatures below 70 C or treatment times longer than an hour or two. This study set out to determine the effects of such deep cold treatments at temperatures 150 C for 24 h. The study investigated the effects of deep cold on the microstructure, hardness profile, residual stress and internal oxidation on two typical carburising steels, 16MnCr5 and 21NiCrMo2. The study found that for both 16MnCr5 and 21NiCrMo2 carburised to a case depth of approximately 0.8 mm, the longer and colder the deep cold treatment, the more the austenite retained in the case was converted to martensite and the harder it became. After low temperature tempering, the hardness difference was smaller, but still significant. In both steels, the case appeared more refined and homogeneous after deep cold treatment. Deep cold treatment had a negligible effect on the core properties of either steel. KEY WORDS Carburised steel; Deep cold; Retained austenite 1. Introduction Cold treatment has been used for many years to reduce the austenite retained near the surface in case hardened components [1]. Although most common as a remedial treatment, when the component has been carburised to too high a carbon level, cold treatment can also be used to obtain the very hardest possible carburised surface in some applications, when wear is critical [2]. Fig.1 shows that for a low alloy steel, the hardness would continue to rise as carbon increased to its saturation level if all the austenite were converted to martensite [3]. Although the process has been used for a long time, there are few actual data available to users to help them determine the best treatment for a particular steel and what properties will result, and almost none on the effect of temperatures below 70 C. Lower Corresponding author. Tel: address: paul.stratton@boc.com

2 2 temperatures for longer times can have beneficial effects on tool steel [4]. The study reported in this article set out to determine the effect of various deep cold treatments on the properties of two common carburising steels (16MnCr6 and 21NiCrMo2), both with and without tempering after the cryogenic treatment. 2. Experimental Method Two typical carburizing steels were selected for the study. The samples were commercial components with the compositional specifications shown in Table 1. Fig.1 The variation of hardness with carbon content for martensite and steel. The carburizing cycles were typical of industrial practice and were carried out in an industrial sealed quench furnace. The cycle for each steel is detailed below: 16MnCr5 Heat to 920 C Carburize for 120 min at a carbon potential set point of 1% Cool to 850 C Diffuse for 90 min at acarbon potential set point of 0.8% Quench in oil at 70 C 21NiCrMo2 Heat to 930 C Solution treat for 190 min at a carbon potential set point of 0.4% Carburize for 345 min at a carbon potential set point of 0.9% Cool to 850 C Diffuse for 150 min at acarbon potential set point of 0.75% Quench in oil at 70 C Table 1 The compositions of the steels used (wt pct) Steel C Si Mn P S Cr Mo Ni 16MnCr NiCrMo The residual stress analysis was carried out by the X-ray diffraction method using a Stresstech xstress 3000 device. The samples were prepared by electro-chemical metal removal in steps of 100 µm using a Struers Tegrapol device. The retained austenite was measured by X-ray diffraction analysis according to ASTM E on polished surfaces. The calculation of the retained austenite content was made by the Rietveld analysis method. The hardness analysis was carried out using a Zwick microhardness tester in accordance with DIN EN ISO 2639.

3 3 3. Experimental Results MnCr5 Examination showed that the effective case depth after hardening was 0.93 mm, reducing to 0.84 mm after tempering. Photomicrographs of the case (martensite with 8.7% retained austenite) and the core (martensite with some ferrite islands) are shown in Fig.2. Samples of carburized 16MnCr6 were subjected to various cold treatments and then examined under a light microscope. Fig.3 shows that the cold treatments had little or no visible effect on the structure of the core. On the other hand, there is a significant effect Fig.2 The microstructures of the case (a) and core (b) of 16MnCr6 after carburizing. Fig.3 The microstructures of the core of 16MnCr6 after various cold treatments (a) oil quenched (OQ) from 850 C; (b) oil quenched from 850 C, 90 C for 30 min; (c) oil quenched from 850 C, 90 C for 24 h; (d) oil quenched from 850 C, 150 C for 24 h.

4 4 Fig.4 The microstructures of the case of 16MnCr6 after various cold treatments (the retained austenite is the white phase between martensite laths) (a) oil quenched from 850 C; (b) oil quenched from 850 C, 90 C for 30 min; (c) oil quenched from 850 C, 90 C for 24 h; (d) oil quenched from 850 C, 150 C for 24 h. on the case structure, as Fig.4 shows. As might have been expected, the two 90 C treatments greatly reduce retained austenite and the case structure appears more refined and homogeneous for the longer-term treatment. The 150 C result is however somewhat anomalous. A suggested cause is austenite stabilization, because of a time delay between quenching and the cold treatment. The effect is shown even more clearly in Fig.5. However, it should be noted that the technique used to measure retained austenite has a detection limit of 2%. The comparison of the hardness profiles before and after cold treatment and/or tempering in Fig.6 clearly shows the improvement in hardness produced by cold treatment. In general terms, the longer and colder the treatment, a higher hardness can be achieved. Tempering alone reduces hardness as, due to the low carbon content of the case, there is insufficient retained austenite to transform to martensite by tempering. Fig.5 Retained austenite in the case of 16MnCr5 after various treatments. E1 850 C, OQ E2 850 C, OQ +180 C, 2 h E3 850 C, OQ; 90 C, 30 min E4 850 C, OQ; 90 C, 30 min+180 C, 2 h E5 850 C, OQ; 90 C, 24 h E6 850 C, OQ; 90 C, 24 h+180 C, 2 h E7 850 C, OQ; 120 C, 24 h E8 850 C, OQ; 120 C, 24 h+180 C, 2 h E9 850 C, OQ; 150 C, 24 h E C, OQ; 150 C, 24 h+180 C, 2 h

5 5 Tempering after cold treatment does not further improve either hardness or retained austenite NiCrMo2 The results with regard to microstructure were very similar except that a higher level of austenite, 40%, was retained in this steel. The effective case depth was almost the same after treatment. These samples were subjected to the same deep cold treatments as 16MnCr5. Fig.7 illustrates the reduction in retained austenite produced by longer and colder treatment and Fig.8 quantifies the reduction. microstructure to a certain extent. Fig.6 The case hardness profile of 16MnCr5 before and after cold treatment. As well as reducing retained austenite, the treatment also refines the case Fig.9 shows how the reduction in the retained austenite affects the case hardness profile. The surface hardness increases as the treatment temperature decreases. The difference between the 120 and 150 C treatments in terms of both surface hardness and case depth to 550 HV is only small but still significant. Fig.7 A comparison of the case microstructures of 21NiCrMo2 after various cold treatments (the retained austenite is the white phase between martensite laths). (a) quenched (b) quenched then 120 C/1 h (c) quenched then 150 C/24 h Fig.8 A comparison of the retained austenite of 21NiCrMo2 after various cold treatments. Fig.9 The case hardness profile of 21NiCrMo2 after various cold treatments.

6 6 After tempering at 175 C, the improvements in surface hardness are even less marked for both cold treatments, but none the less significant, as shown in Fig.10. It will be noted that for this steel, because of the higher surface carbon and hence retained austenite, tempering also transforms sufficient retained austenite to increase the surface hardness. 4. Discussion It is known that in highly alloyed steels, the austenite retained after quenching tends to stabilize with time. It is usually recommended to carry out deep cold treatment Fig.10 The case hardness profiles of 21NiCrMo2 after tempering following cold treatment. within 1 h of quenching to avoid this effect. However, it has been reported that stabilization does not normally occur in low alloy steels [4]. The results found here for 16MnCr5 may indicate that this simplistic view needs refinement. Further experimental study will therefore be undertaken. Although longer treatment at lower temperatures improved all the other properties, preliminary residual stress measurements (Fig.11) suggested that the optimum profile was reached after 1 h with the 120 C treatment. This is probably only relevant for components Fig.11 Residual stress for 21NiCrMo2.

7 7 where fatigue is the normal failure mode. For wear conditions treatment at 150 Cfor 24 h is likely to give better results. The effect of cold treatment on the depth of penetration and morphology of the internal oxidation produced during carburising in a conventional, endothermically generated type of atmosphere was also examined. It was found that neither the treatment temperature nor the time had any effect on the internal oxidation in either steel. While deep cold treatment is obviously beneficial for both steels tested when the case contains retained austenite, it is not possible to say that deep cold treatment is beneficial in itself. Since deep cold treatment of martensite has little impact on its hardness [5],the test methods used in this study would not reveal this effect and a wear test study is needed. 5. Conclusions For both carburized 16MnCr5 and carburised 21NiCrMo2, the longer and colder the deep cold treatment, the more of the retained austenite in the case was converted to martensite and the harder it became. After low temperature tempering the hardness difference was less, but still significant. In both steels, the case appeared more refined and homogeneous after deep cold treatment. Deep cold treatment had a negligible effect on thecorepropertiesofeithersteel. Acknowledgements This research project was funded by the Austrian Kind-program and the results were obtained at the Kplus-center of materials, Leoben MCL and at the Kind-centre of competence, V-Research. The authors are also grateful to Lingenhöle Technologie GMbH in Feldkirch, Austria and Linde Gas Corp. in Munich, Germany for setting up and supporting the research project. REFERENCES [1] C. Moore, Heat Treatment 73 (The Metals Society, London, 1975, Book No 163) p.157. [2] P.F. Stratton and L. Sproge, Heat Treatment of Metals 31(3) (2004) 65. [3] E.C. Bain and H.W. Paxton, Alloying Elements in Steel (ASM, Metals Park, OH) p [4] P.F. Stratton, BOC Technology Magazine (BOC Ltd., Guildford, Surrey, UK, November 1997). [5] D.N. Collins, Heat Treatment of Metals 23(2) (1996) 40.