Unusual Tempering Behavior of Fe–Cr–C Martensite

Abstract

The tempering behavior of an as-quenched high Cr-high C steel consisting of lath martensite, retained austenite and carbide was studied. The change in the total austenite volume fraction was monitored during tempering by in situ neutron diffraction. The results were compared with those obtained by X-ray diffraction, scanning electron microscopy/electron back scatter diffraction and transmission electron microscopy observations and ex situ neutron diffraction. The volume fraction of the blocky austenite grains increased by tempering at 573 K, whereas the thin film austenite decomposed to ferrite and cementite. The tempering behavior of the blocky austenite grains is an unusual phenomenon.

1. Introduction

High Cr-high C steel has widely been used for press forming or forging dies because of excellent wear resistance, hardness and fatigue strength.^1,2,3,4) These properties are greatly influenced by microstructure and hence heat treatment condition is very important. In earlier works, the influence of carbides, M₇C₃ and M₂₃C₆, was focused to study. Recently, Yaso et al. have pointed out the importance of the matrix microstructure formed by quenching and tempering.^5,6) Then, quite recently, Tanaka et al.^7,8) have reported that the volume fraction of the retained austenite of a quenched specimen increased with low temperature tempering, using scanning electron microscopy/electron back scatter diffraction (SEM/EBSD), transmission electron microscopy (TEM) and X-ray diffraction. It should be confirmed that such an unusual tempering phenomenon occurs inside of a bulk specimen, because these techniques provide only the information of surface layer of a bulk specimen or thin film. Therefore, in this study, ex situ and in situ neutron diffraction measurements were employed to examine the tempering behavior of a commercially available Cr–C steel (JIS SKD11) and compared the obtained results with those by X-ray diffraction, SEM/EBSD and TEM observations. Neutron diffraction has been demonstrated to give the globally averaged volume fraction of metastable austenite.^9,10)

2. Experimental Procedures

The chemical compositions of the steel used in this study were 12.21Cr-1.43C-0.19Si-0.41Mn-0.017P-0.0012S-0.82Mo-0.42Ni-0.252V in mass%. The steel was hot caliber-rolled to a rod with a diameter of 38 mm through an industrial process. As shown in Fig. 1(a), specimens for X-ray diffraction, SEM/EBSD, TEM and neutron diffraction were austenitized at 1323 K for 3.6 ks in vacuum followed by oil quenching or air cooling (cooling rate was approximately 0.2 K/s around M_s temperature (523 K)).

Fig. 1.

Heat treatment processes: (a) quenching and tempering for X-ray diffraction, SEM/EBSD and TEM observations and ex situ neutron diffraction and (b) heat schedules for in situ neutron diffraction measurements. (Online version in color.)

Some specimens were tempered at 573 K or 793 K for 5.4 ks. The EBSD specimens were prepared by mechanical grinding and finished by polishing with colloidal silica slurry. EBSD measurements were performed at an accelerating voltage of 20 KV with a tilt angle of 70° and a step size of 0.1 μm using a Hitachi 4300 scanning electron microscope equipped with a TSL EBSD measuring system. X-ray diffraction measurements were carried out using RIGAKU ULTIMA IV with a wavelength of 0.154 nm (Cu-anode) at 40 kV/30 mA. TEM foils were prepared by a twin jet electrolytic polishing. TEM observations were performed using a 200 KV FE-TEM (JEM-2100F) equipped with electron dispersion analysis (EDS). The EDS measurements were conducted with a probe size of 1 nm. Nano-beam diffractions with a 10 nm spot size were also examined to identify constituent phases. Neutron diffractions were measured using an engineering neutron diffractometer, TAKUMI at MLF/J-PARC in Japan. Ex situ neutron diffraction measurements at room temperature (RT) were carried out for 6 × 6 × 10 mm specimens that were cooled in air and tempered either at 573 K or 793 K for 5.4 ks. The air-cooled and oil-quenched specimens with 6 mm diameter and 20 mm length were used for in situ neutron diffraction. In this experiment, temperature was increased step by step to capture diffraction profiles at various temperatures, which is shown in Fig. 1(b). The specimens were set in such a way that the longitudinal direction, i.e., rolling direction, to be 45 degrees with respect to the incident beam, so that the diffraction profiles for the axial and transverse directions were simultaneously obtained by the two 90-degree detector banks.¹¹⁾ Neutron diffraction profiles were analyzed using the Z-Rietveld software,^12,13) by which the volume fraction, lattice parameter and full width at half maximum (FWHM) could be determined.

3. Results and Discussion

3.1. Microstructural Change with Tempering Observed with SEM/EBSD

Figure 2(a) illustrates the preparation of specimens for X-ray diffraction, OM, SEM/EBSD and TEM observations, where the observation surface is parallel to the longitudinal axis of the rod. Fig. 2(b) shows the microstructure of a specimen air-cooled from 1323 K observed with OM. Large carbide particles (M₇C₃) were found to be distributed in the martensite matrix aligned along the rolling direction.

Fig. 2.

Schematic illustration on specimen preparation for X-ray diffraction, OM, SEM/EBSD and TEM observations (a) and optical microstructure of an air-cooled specimen (b).

To investigate the changes in the retained austenite by tempering, the same area of a specimen was tried repeatedly to observe with SEM/EBSD. The inverse pole figure (IPF) map for austenite obtained by EBSD in an oil-quenched specimen is shown in Fig. 3(a), where the same colored (same crystalline orientation) regions were estimated to be prior austenite grains at 1323 K. A central black region in the figure is carbide particle (M₇C₃). A green colored region indicated by an arrow was extracted and its phase map is presented in (b), in which red and green refer to ferrite and austenite, respectively. Then the specimen was tempered at 573 K, and austenite IPF map and phase map were measured with EBSD after slight polishing of the surface with colloidal silica slurry. Although the observed plane was a little changed, it could be tracked almost the same prior austenite grain in (c) and (d). It is evidently found that the volume fraction of blocky austenite grains increased. This result is consistent with Tanaka et al’s observations although they did not observe the same grain.⁸⁾ Continuously, the same specimen was tempered at 793 K, and IPF and phase maps obtained are shown in Figs. 3(e) and 3(f), respectively, indicating little amount of austenite. Exactly similar results were obtained for the air-cooled specimen. Hence, in conclusion, the volume fraction of blocky austenite grains was confirmed to increase with tempering at 573 K although it was unusual.

Fig. 3.

Influence of tempering on the retained austenite: (a) is an austenite IPF map of an oil-quenched specimen and (b) is a phase map of the area framed by dashed lines in (a) where green and red refer to austenite and martensite, respectively. The specimen was tempered at 573 K and (c) and (d) were observed, and finally subsequently tempered at 793 K and (e) and (f) were obtained. The same region was intended to track the change in austenite.

3.2. TEM Microstructures

Typical TEM micrographs obtained from the specimens, air-cooled, tempered at 573 K and at 793 K are presented in Fig. 4. In the air-cooled specimen, the microstructure was found to consist of lath martensite (α′), retained austenite (A) and carbide. As long as examined by nano-beam diffractions and EDS, the carbide was mostly determined as M₇C₃. Thin film austenite was frequently observed between martensite lathes as shown in (a), where the austenite grains have the identical crystal orientation, as an electron diffraction pattern taken from (a) is shown in (b). Figs. 4(c) and 4(d) is a little complicated microstructure observed in the 573 K tempered specimen. The carbide M₇C₃ particle is found in the central region of the figure. Near this carbide particle, several dark areas were observed and all of them showed the same orientation of austenite. Hence it is postulated the area circled by dashed line must be a blocky austenite grain, which was clearly observed by EBSD in Fig. 3. Bright colored grains inside this area were speculated to be fresh martensite (not tempered). In fact, the blocky austenite grains were hardly found with TEM observations and the reason is believed that austenite was transformed to martensite during the polishing to prepare TEM foils. Most of thin film austenite like in (a) was vanished in the 573 K tempered specimen and cementite diffraction spots were detected. Cementite particles were more frequently observed in the TEM microstructure of the 793 K tempered specimen presented in (e) and (f), and austenite grains were hardly found showing good consistence with Fig. 3(f).

Fig. 4.

TEM microstuctures and electron diffraction patterns: (a) and (b) air-cooled, (c) and (d) tempered at 573 K, and (e) and (f) tempered at 793 K in which cementite diffraction patterns were observed at the circled areas. (Online version in color.)

3.3. Characteristic Features in Neutron Diffraction Profiles

Examples of in situ neutron diffraction profiles are presented in Fig. 5. The upper diffraction profile was obtained by the south bank, i.e., the profile for the axial direction of the specimen, whereas the lower by the north bank, i.e., the radial direction, at 573 K for the oil-quenched specimen. The whole profile was refined to be composed of austenite, bct martensite and M₇C₃ carbide using the Z-Rietveld software. Comparing these two profiles for the orthogonal directions, it is found that textures of austenite and bct martensite are weak but that the M₇C₃ carbide shows a strong preferred orientation; {210} M₇C₃ is observed only in the axial direction whereas {040} M₇C₃ in the transverse, being expected from Fig. 2. It should be noted here that the intensity of austenite is high even at 573 K. Focusing on {110}{101} martensite, {111} austenite and {210} M₇C₃, the changes in diffraction profiles during tempering are depicted in Fig. 6. As seen, austenite is stable even at 873 K and completely disappears at 973 K. The peak boarding in martensite decreases with increasing of temperature. The M₇C₃ peak shows little difference throughout the tempering.

Fig. 5.

In situ neutron diffraction results at 573 K for the oil-quenched specimen, in which peaks austenite (A), martensite (α′) and carbide (M₇C₃) are visible. (Online version in color.)

Fig. 6.

Changes in austenite (A), martensite (α′) and carbide (M₇C₃) peaks of the oil-quenched specimen obtained by in situ neutron diffraction during tempering.

3.4. Volume Fractions and Carbon Concentrations Determined from Neutron Diffraction Profiles

The relative volume fraction of austenite (V_A) was estimated by the following equation,   

$$V_A=\frac{\frac{1}{n}\sum {\hspace{0.17em}}^n\frac{I^A{\hspace{0.17em}}_{\text{hkl}}}{{\text{R}}^A{\hspace{0.17em}}_{\text{hkl}}}}{\left(\frac{1}{\text{m}}\sum {\hspace{0.17em}}^{\text{m}}\frac{I^{{\alpha }^{\prime }}{\hspace{0.17em}}_{\text{hkl}}}{{\text{R}}^{{\alpha }^{\prime }}{\hspace{0.17em}}_{\text{hkl}}}+\frac{1}{n}\sum {\hspace{0.17em}}^n\frac{I^A{\hspace{0.17em}}_{\text{hkl}}}{{\text{R}}^A{\hspace{0.17em}}_{\text{hkl}}}\right)}$$(1)

Here, n and m refer to the number of diffraction peaks of austenite (A) and martensite (α′), respectively, ${\mathrm{R}}_{\mathrm{h}\mathrm{k}\mathrm{l}}^{{\alpha }^{\prime }}$ and ${\mathrm{R}}_{\mathrm{h}\mathrm{k}\mathrm{l}}^A$ are the theoretical integrated intensities of martensite and austenite, respectively, given by the Z-Rietveld software¹³⁾ assuming free-texture, and I_hkl stands for measured {hkl} diffraction integrated intensity. The volume fractions of carbides were neglected. The obtained volume fractions were plotted in Fig. 7(a). The solid lines indicate the results of the in situ neutron diffractions, where nearly identical results were obtained for the oil-quenched and air-cooled specimens. The dashed lines show the results of the ex situ neutron diffractions and X-ray diffraction at RT. The difference between these in situ and ex situ neutron diffraction measurements at higher tempering temperatures around 773 K suggests that the austenite transformed to martensite upon cooling from tempering temperature to RT. X-ray diffraction measurements were resulted in showing smaller amounts of austenite than neutron diffraction. Because the penetration depth of Cu-KαX-ray into iron is approximately 15 μm,¹⁴⁾ it indicates that the austenite near the surface transforms easily to martensite. Hence, comprehensive comparison of the above different experimental methods can show that metastable austenite is believed to transform to martensite more easily in a thin foil, then the surface layer of a bulk specimen and finally the inner part due to the constrictive effect for the transformation shape and size change. The results obtained by TEM, SEM/EBSD, X-ray diffraction and neutron diffraction provide such characteristic features.

Fig. 7.

Changes in the relative volume fraction of austenite (a) and carbon concentrations of austenite and martensite (b) as a function of temperature.

The carbon concentrations were estimated from lattice parameters determined from diffraction profiles. The temperature dependence of lattice parameter has been described by Eq. (2),^15,16)   

$$a_T^X=[1+k(T-298)]*a_0^X$$(2)

Here, $a_T^X$ and $a_0^X$ refer to the lattice parameter at temperature T (K) and room temperature, respectively. X refers to carbon concentration in mass% and k is the thermal expansion coefficient of austenite: k = 2.03 × 10^–5 K^–1 referring to Ref. 16). Using Eq. (2), the lattice parameter at a certain temperature T was converted to that at RT and then the carbon concentration of austenite X can be evaluated by Eq. (3),¹⁶⁾   

$$a_0^X=a_0+0.033*X$$(3)

where the value of a₀ was taken as 0.3573 nm. Here, the effects of other alloying elements were neglected. On the other hand, the tetragonality (the axial ratio of c/a) of martensite has been reported to decrease with tempering. The relationship between c/a and carbon concentration X (mass%) for martensite has been reported as Eq. (4) by Roberts.¹⁷⁾   

$$\frac{c}{a}=1.000+0.045*X$$(4)

The obtained carbon concentrations in austenite as well as martensite are plotted in Fig. 7(b) as a function of tempering temperature. The air-cooled specimens and oil-quenched ones are found to show almost same results. In austenite, it looks to decrease slightly at 473 K, then increase around 573–673 K and finally decrease above 773 K showing good coincidence of the austenite stability found in Fig. 7(a). On the contrary, the carbon concentration in martensite decreases monotonously accompanying a plateau around 573–673 K with increasing of tempering temperature. It is unclear why cementite was precipitated at the thin film austenite but not in the blocky austenite. The reasons might include higher carbon concentration and higher dislocation density in the thin film austenite than in the blocky austenite. Podder et al. have claimed that the coarser austenite is more thermally stable than the film form because of its lower carbon concentration in bainitic steel.¹⁸⁾ In the “quenching and partitioning (Q&P)” process for a low C-high Cr martensitic stainless steel,^19,20,21,22) Si addition was effective to suppress the precipitation of cementite and a large amount of Cr was considered to suppress bainite transformation. Hence, the role of Cr for the present results should be investigated in future.

4. Conclusions

The as-quenched and tempered microstructures of a high Cr-high C steel were studied using SEM/EBSD, TEM, X-ray diffraction and ex situ neutron diffraction, and the change in the total austenite volume fraction was monitored by in situ neutron diffraction. The results obtained in this study would be concluded as follows.

(1) The volume fraction of the blocky austenite grains increases with absorbing carbon atoms from martensite by tempering at 573 K, although the austenite films decompose to ferrite and cementite.

(2) The total austenite volume fraction hardly changes or slightly increases by 573–773 K tempering but its stability is not so high. The critical tempering temperature of martensite transformation in the blocky austenite upon cooling to RT is about 773 K.

(3) Metastable austenite transforms to martensite more easily in a thin foil, then the surface layer of a bulk specimen and finally the inner part due to the constrictive effect on the transformation shape and size change. The results obtained by TEM, SEM/EBSD, X-ray diffraction and neutron diffraction reflect such characteristic features.

Acknowledgements

We are grateful to Profs. S. Morito and T. Hayashi of Shimane University for helpful discussion, Drs. K. Aizawa and T. Kawasaki of JAEA for their help on neutron diffraction measurements and Prof. T. Suzuki of Ibaraki University for acceptance of S. He through the exchanging program between Ibaraki University and Wuhan University of Science and Technology (Oct. 2013–Aug. 2014). This study was supported by a Grant-in-Aid for Scientific Research on Innovative Area “Bulk Nano-structured Metals” through MEXT, Japan (No. 22102006). The neutron diffraction measurements were performed in the JAEA project beam time of 2014P0100.

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