Thermomechanical processing of 42CrMoS4 steel

Thermomechanical processing of 42CrMoS4 steel M. Nabil1, O. Taha1, T. Elbitar2 and I. El Mahallawi*1 Billets of 42CrMoS4 steel were subjected to a programme including forging and rolling to different reduction ratios, followed by quenching and tempering to simulate online thermomechanical treatment (TMT) during rolling. The mechanical properties obtained were compared with those obtained by conventional heat treatment (CHT) (quenching from 860uC and tempering). It was found that increasing the hot reduction ratio from 18 to 60%, accompanied by a decrease in the finish rolling temperature from 900 to 750uC, enhances strength only at the expense of elongation, while rapid quenching instead of air cooling from the same finish rolling temperature yields improvements in both strength and ductility. It was also found that CHT provides higher hardness, whereas TMT provides higher impact toughness. TMT will confer major economic savings since the heat treatment is achieved online. Keywords: Thermomechanical processing, Conventional heat treatment, Hot reduction ratio, Finish rolling temperature

Introduction Heat treatable low alloy steels, or medium carbon, quenched and tempered low alloy steels, are strengthened by quenching to form martensite and tempered to the desired strength level. They are available for direct forging applications; 42CrMoS4 steel is one of the most widely used heat treatable medium carbon, low alloy steels used for vehicles and machine construction. It is used in the heat treated condition, the heat treatment consisting of hardening and tempering. In addition, 42CrMoS4 steel can be surface hardened for demanding applications.1 Hot rolling is a thermomechanical treatment which plays an important part in the processing of many steels. Hot rolling, followed by hardening and tempering, is a typical industrial process route for many heat treatable steels. Direct quenching from above Ac3 would lead to a fully martensitic structure; the martensite is then tempered to make it suitable for engineering applications.2,3 Thermomechanical treatment (TMT) improves the mechanical properties of steel. It involves the simultaneous application of heat and a deformation process to an alloy, in order to change its shape and refine the microstructure. It has been studied for many years, the results of the most recent investigations that analyse the influence of TMT on the mechanical properties of the steel being reported in Ref. 4. In this study, 42CrMoS4 steel is hot rolled by a multipass technique, so that the final thickness required is achieved in the austenite phase, but very near to Ac3. After the final 1

Faculty of Engineering, Cairo University, Cairo, Egypt, Professer El Mahallawi is currently with British University in Egypt (BUE) 2 Central Metallurgical Research and Development Institute, CMRDI, Cairo, Egypt *Corresponding author, email [email protected]

ß 2010 IHTSE Partnership Published by Maney on behalf of the Partnership DOI 10.1179/174951410X12646901266726

rolling pass, the steel is directly quenched into water or oil. The effect of reduction and finish rolling temperature on the tensile properties and hardness following conventional heat treatment (CHT) has been studied. In addition, the effect of cooling media and tempering temperature on the mechanical properties for TMT were investigated. Comparative data were also obtained on the hardness and impact toughness of 42CrMoS4 steel at different tempering temperatures. The effect of tempering temperature on the fracture mode was also studied.

Experimental programme Material preparation A square billet of 42CrMoS4 steel of composition given in Table 1 (average of nine analysis samples from the heat) was cut into three pieces of dimensions 52665670 mm, and each piece was subjected to specific processing consisting of reduction and/or heat treatment cycle aimed at simulating the production conditions of TMT and CHT. First, the samples were forged to a size suitable for rolling (150672622 mm) at 1050uC using a blowing hammer. Then the forged pieces were rolled to final size (to obtain the final dimensions and simulate the actual rolling process) with different thicknesses using a two/ four high reversing mill (3206320 mm diameter for working and supporting rolls with 450 mm barrel length; 75 hp motor with maximum rolling capacity 150 t rolling load and 3600 kg m rolling torque). Subsequently, three heat treatment cycles were carried out as follows: (i) a group of five samples was subjected to different amounts of reduction then allowed to cool in air immediately after rolling. The specimens were then hardened from a temperature of 860uC, followed by quenching in oil

International Heat Treatment and Surface Engineering

2010

VOL

4

NO

2

87

Nabil et al.

Thermomechanical processing of 42CrMoS4 steel

consisted of lath martensite, which became finer as the hot deformation increased from 27 to 60%. Water quenching and tempering

The microstructure of a hot rolled and water quenched (immediately after rolling with finish rolling temperature 900uC) and tempered sample at 500uC for 45 min is given in Fig. 4. It is seen that the microstructure consists of tempered martensite.

Mechanical properties Conventional heat treatment

Effect of the amount of reduction on mechanical properties The effect of reduction on the tensile properties of the hot formed samples is shown in Fig. 5. Generally, increasing the reduction from 18 to 60% leads to a slight increase in yield strength and ultimate tensile strength. The slight increase in yield strength up to 36% hot deformation may be attributed to refinement of the prior austenite grain size, depending on the alloy chemical composition.5,7,11 Small amounts of reduction cause large grains to be produced. However, with increasing amounts of hot deformation, recovered and recrystallised structures will dominate the microstructure. In general, recrystallised austenite grain size decreases markedly with increasing total reduction.12 For ferritic– pearlitic steels, controlled deformation is reported to produce grain size in the range 3?6–4?2 mm.13 Figures 6 and 7 illustrate the effect of hot reduction on percentage elongation and hardness. It can be seen from Fig. 7 that the hardness increases as the amount of hot reduction increases. This may be explained by the fact that the hot reduction redistributes and increases the nucleation sites of the fine precipitates.12 These fine precipitates create higher hardness with good ductility.12–14 The dispersion of the carbides caused by the hot deformation causes an increase in the hardness, which may also be a reflection of the effects of chemical composition of the alloy. Additional increases in hardness are due to grain size refinement and carbide precipitation.12 The strengthening effect obtained by the deformation causes a decrease in the elongation.

1 Microstructure of as cast billet

(ii) the second group, consisting of five samples, was subjected to quenching in water immediately after rolling (water TMT) (iii) the third group, consisting of five samples, was subjected to quenching in oil immediately after rolling (oil TMT). Finally, all specimens were tempered at various temperatures in the range 100–600uC.

Mechanical tests and microstructural examination Tensile tests were performed by means of universal testing machine according to ASTM 370. Rockwell hardness testing (HRC) was conducted on the specimens according to the specification ASTM E18-98. Also, impact test were carried out on standard specimens of 55610610 mm according to ASTM E23-98. Samples for microstructural examination were taken from the specimens, ground, polished and finally etched with 2% nitric acid in ethyl alcohol. Observation of the structures was carried out by light microscopy.

Results and discussion Microstructural study As cast and hot rolled material

The as cast microstructure reveals a ferritic/pearlitic microstructure with an inhomogeneous distribution of the microstructural constituents and grain sizes, as shown in Fig. 1. Similar structures are observed after finish rolling at 900 and 750uC, with grain refinement and some non-uniformity of the microstructural constituents. A noticeable refinement in grain size and enhancement in structural uniformity is obtained when the finish rolling temperature was decreased from 900 to 750uC,4 which agrees with previous observations.5,6

Effect of finish rolling temperature The effect of finish rolling temperature on tensile properties and hardness of samples hot deformed with nearly 60% reduction is represented in Figs. 8–10. It is obvious that an increase in finish rolling temperature results in a significant decrease in all mechanical properties, and hardness. Decreasing the finish rolling temperature from 900 to 750uC yields a finer grain size, higher dislocation density and more carbide precipitation. The deformation temperature has a dramatic effect on the rate of recrystallisation. The driving force for grain growth will be decreased with decreasing finish rolling temperature; this gives a tendency for more nucleation sites with less growth rate also less atomic diffusion after rolling.12,15–19

Water and oil quenched TMT samples

The microstructure obtained after TMT treatment using direct oil or water quenching immediately after rolling with a finishing temperature of 900uC is represented in Fig. 2. It is clear from comparison with microstructures published in the literature7–10 that the microstructure consists of lath martensite. Samples subjected to water quenching immediately from the finish rolling temperature (Fig. 3)

TMT materials

Effect of cooling medium The effect of cooling rate on hardness after rolling, for TMT samples processed at different deformations, is given in Fig. 11. Water and oil were used as quenchants. Hardness values are compared with those obtained from air cooling immediately after rolling. It is clear that

Table 1 Chemical composition of steel billet used in the study Element

C

Si

Mn

P

S

Cr

Mo

Content, wt-% 0.39 0.23 0.76 0.008 0.027 0.944 0.158

88


2010

VOL

4

NO

2

Nabil et al.


a oil quenched; b water quenched 2 Microstructure of hot rolled samples, with finish rolling temperature 900uC: quenching was carried out immediately after rolling; samples etched using 2% nital

a total reduction 27%; b total reduction 60% 3 Micrographs (SEM) of TMT processed samples using direct water quenching from rolling finish temperature of 900uC with two reduction levels

4 Microstructure of hot rolled sample water quenched immediately after rolling from 900uC and tempered at 500uC for 45 min

5 Effect of reduction on yield strength and ultimate tensile strength

increasing the rate of cooling from air to oil to water increases the hardness. At 45% reduction, hardness values are 29 HRC for air cooling, 49 HRC for oil cooling and 54 HRC for water cooling. This may be explained by the fact that oil and water quenching immediately after rolling gives rise to a martensitic structure (as shown in the section on ‘Water and oil quenched TMT samples’) which possesses high hardness due not only to the martensitic phase itself, but also because of the increasing number of dislocations and the dispersion of carbides. The microstructure of air cooled samples, on the other hand, consists of ferrite plus pearlite with some bainite.10

6 Effect of reduction on elongation


2010

VOL

4

NO

2

89

Nabil et al.


10 Effect of finish rolling temperature on hardness

7 Effect of reduction on hardness

11 Variation of hardness with cooling rate at different amounts of deformation

toughness for hot rolled samples water quenched from 900uC respectively. Three different tempering temperatures, 550, 600 and 650uC, were chosen. Increasing the tempering temperature leads to a decrease in hardness (Fig. 12). This is due to loss of martensite tetragonality and the formation of new phases with lower intrinsic hardnesses, together with the coarsening of iron carbides above 250uC and the coarsening of alloy carbides formed subsequently.2,9,10 From Fig. 13, it can be seen that, in general, tempering yields higher toughness for samples having higher reduction with the same tempering temperature. At tempering temperatures of 600 and 650uC, the impact toughness shows a decrease for 27% reduction (versus 18%) and then increases as the amount of deformation increases from 27 to 54%. The observed decrease may be due to temper embrittlement and is not observed for the steel tempered at 550uC.

8 Effect of finish rolling temperature on yield strength and ultimate tensile strength

Comparison of CHT and TMT samples after tempering

Tempering curves were drawn for samples with a fixed deformation of 60% against both hardness and impact toughness readings, as shown in Figs. 14 and 15 respectively. Figure 14 shows that increasing the tempering temperature from 0 to 600uC leads to a decrease in hardness from 56 to 25 HRC for the CHT samples and from 54 to 22 HRC for the TMT samples. The change in hardness over the tempering range from 0 to 200uC is relatively insignificant and it becomes more significant between 200 and 500uC. Greater reduction in hardness is obtained at temperatures over 500uC. The

9 Effect of finish rolling temperature on elongation

Effect of tempering temperature Tempering is performed on the quenched martensitic structure to enhance its mechanical properties and make it suitable for dynamic loading in accordance with the design requirements. Figures 12 and 13 illustrate the effect of tempering temperature on hardness for hot rolled samples oil quenched from 900uC and on impact

90


2010

VOL

4

NO

2

Nabil et al.


12 Variation in hardness with percentage of hot reduction

14 Hardness tempering curves for water quenched TMT and CHT samples

13 Variation in impact toughness with percentage of hot reduction

15 Impact tempering curves for water quenched TMT and CHT samples

a as quenched; b tempered at 400uC; c tempered at 600uC 16 Fractographs of impact samples, oil quenched from 850uC and tempered for 90 min: all samples tested at room temperature

generally lower hardness of TMT samples may be explained by the fact that TMT yields partially recrystallised austenite with fine grain size, which confers less hardenability than CHT.8,20 Increasing the tempering temperature causes rearrangement of dislocations and low angle boundaries and causes a greater drop in hardness for TMT samples tempered at over 200uC, compared with CHT processed samples.19,20

The impact toughness increased from 10 to 45 J cm22 and from 10 to 60 J cm22 for CHT and TMT samples respectively (Fig. 15). Over the tempering range from 0 to 200uC, the increase in impact toughness, indicated by impact strength, increased insignificantly for both CHT and TMT samples. Above 500uC, impact strength increased rapidly for CHT and TMT processed samples. In general, the impact strength for TMT samples over the


2010

VOL

4

NO

2

91

Nabil et al.


3. G. F. Vander Voort: ‘Atlas of time temperature diagrams for irons and steels’; 1991, Materials Park, OH, ASM International. 4. O. M. Taha: ‘Effect of thermo-mechanical treatment on microstructure and mechanical properties of 42CrMoS4 steel’, MSc thesis, Cairo University, Cairo, Egypt, 2008. 5. I. Tamura, H. Sekine, T. Tanaka and C. Ouchi: ‘Thermomechanical processing of high strength low alloy steels’; 1988, London, Butterworth-Heinemann. 6. ASM International: ‘ASM handbook’, Vol. 9, ‘Metallography and microstructure’; 1991, Materials Park, OH, ASM International. 7. O. Yasuya: ‘Microstructural evolutions with precipitation of carbides in steels’, ISIJ Int., 2001, 41, (6), 554–565. 8. A. Ardehali Barani, F. Li, P. Romano, D. Ponge and D. Raabe: ‘Design of high-strength steels by microalloying and thermomechanical treatment’, Mater. Sci. Eng. A, 2007, A463, (1–2), 138– 146. 9. H. K. D. H. Bhadeshia and R. W. K. Honeycombe: ‘Steels microstructure and properties’, 3rd edn; 2006, Amsterdam, Elsevier. 10. G. Krauss: ‘Martensite in steel: strength and structure’, Mater. Sci. Eng. A, 1999, A273–A275, 40–57. 11. S.-J. Lee, Y.-M. Park and Y.-K. Lee: ‘Relationship between austenite grain size, martensite start temperature, and transformation kinetics of AISI 4340 steel’, Yonsei University, Unpublished Work. 12. M. Abbas, A. Ismail, S. El-Ghazaly and T. El-Bitar: ‘Inter-critical deformation of low alloy steel’, Can. Metall. Q. J., 2004, 43, (1), 109–115. 13. N. Tsuchida, Y. Tomota and K. Nagi: ‘High speed deformation for ultra fine grained ferrite–pearlite Steel’, ISIJ Int., 2002, 42, (12), 1594–1596. 14. L. Storjeva, D. Ponge, R. Kaspar and D. Raabe: ‘Development of microstructure and texture of medium carbon steel during heavy warm deformation’, Acta Mater., 2004, 52, 2209–2220. 15. J. V. Bee and D. V. Edmonds: ‘A metallographic study of the high-temperature decomposition of austenite in alloy steels containing Cr and Mo’, Mater. Charact., 1997, 39, (2–5), 361– 379. 16. T. El-Bitar, A. Ismail, A. El-Morsy and A. Amer: ‘Deformation of special steels’, Steel Grips, 2004, (5), 364–371. 17. T. El-Bitar and A. Amer: ‘Inadequate hot working parameters and its effect on microstructure banding of wrought carbon steel’, ISS Mech. Work. Steel Process. Conf. Proc., 2001, 39, 97–105. 18. E. Evangelista, M. Masini, M. El Mehetedi and S. Spigarelli: ‘Hot working and multipass deformation of 41Cr4 steel’, J. Alloys Compd, 2004, 378, 151–154. 19. L. L. Teoh: ‘Thermo-mechanical processing and microstructure of microalloyed steel bar and wire rod products’, J. Mater. Process. Technol., 1995, 48, 475–481. 20. H. de Boer and H. Beenken: ‘Thermo-mechanical treatment of structural steels – fundamentals and industrial implementation’; 1998, Dusseldorf, Verlag Stahleisen mbH. 21. W. J. Nam, D. S. Kim and S. T. Ahn: ‘Effects of alloying elements on microstructural evolution and mechanical properties of induction quenched-and-tempered steels’, J. Mater. Sci., 2003, 38, 3611– 3617. 22. M. Gojic, L. Kosec and Matkovic: ‘The effect of tempering temperature on mechanical. properties and microstructure of low alloy Cr and CrMo steel’, J. Mater. Sci., 1998, 33, 395–402. 23. K. B. Lee, S. H. Yoon, S. I. Hong and H. Kwon: ‘On intergranular tempered martensite embrittlement’, Scr. Metall. Mater., 1995, 32, (8), 1197–1201. 24. A. J. Papworth and D. B. Williams: ‘Segregation to prior austenite grain boundaries in low alloy steel’, Scr. Mater., 2000, 42, 1107– 1112. 25. H. K. D. H. Bhadeshia: ‘The theory and significance of retained austenite in steels’, PhD thesis, University of Cambridge, Cambridge, UK, 1979.

whole tempering range is higher than that for CHT samples. The increase in impact toughness for TMT may be due to grain refinement of the prior austenite and more carbide dispersion. Tempering of TMT samples produces less of a depression (indicative of temper embrittlement) of impact strength in the range 350–400uC than of CHT samples.8,20–22 The impact toughness at 350uC is 16 J cm22 for TMT samples, compared with 14 J cm22 for CHT samples. The effect of TMT in reducing the effect of segregation of minor elements to the prior austenite grain boundaries has been reported by many researchers.23–25

Fracture study Fractographs of 42CrMoS4 impact specimens, quenched and tempered at different temperatures, are represented in Fig. 16. It can be seen that, for the as quenched structure, the fracture mode is intergranular. At lower tempering temperatures (400uC), the fracture mode is a mixture of ductile rupture, quasi-cleavage and intergranular fracture. The fracture mode changes at the higher temperatures of 500 and 600uC where ductile rupture becomes the predominant fracture mechanism.20

Conclusions 1. Water quenching immediately after TMT from a finish rolling temperature of around 900uC yields higher toughness and lower hardness over the tempering range 0–600uC, compared with using CHT for 42CrMoS4 steel. 2. Increasing the tempering temperature from 0 to 650uC causes a continuous decrease in hardness accompanied by a continuous increase in impact toughness. The drop in hardness accompanying the increase in tempering temperature starts earlier (at around 200uC) for the TMT samples than for conventionally treated samples (around 400uC). 3. Temper embrittlement occurs for both TMT and conventionally treated samples at tempering temperatures between 350 and 400uC. 4. The fracture mode changes from brittle fracture for as quenched samples to mainly ductile fracture at higher tempering temperatures.

Acknowledgement The authors wish to thank the Arab Company for Special Steels, Arco Steel for offering the materials and facilities available at their site, and without which this work could have not been carried out.

References 1. G. Tack, K. Forch and A. Sartorius: ‘Heat-treatable and surfacehardening steels for vehicle and machine construction’, in ‘Steel’, Vol. 2, ‘Applications’, 118–176; 1993, Berlin, Springer-Verlag. 2. ASM International: ‘ASM handbook’ Vol. 4, ‘Heat treating’; 1991, Materials Park, OH, ASM International.

92


2010

VOL

4

NO

2