Thermal stability of retained austenite in TRIP steels studied by synchrotron X-ray diffraction during cooling
Introduction
Recently, low-alloyed transformation induced plasticity (TRIP) steels have attracted growing interest because of their high strength and good formability [1], [2], [3]. TRIP steels possess a multiphase microstructure containing ferrite (α-Fe), bainite, and (metastable) austenite (γ-Fe). The so-called TRIP effect in these steels arises from a martensitic transformation of the metastable retained austenite phase induced by external stress. It was found experimentally [4] and theoretically [5] that the austenite volume fraction, the carbon concentration in the austenite grains, and the grain size of the retained austenite play a crucial role in the TRIP properties as they significantly affect the mechanical stability of the retained austenite. Despite recent efforts, the understanding of the physical mechanism that controls the stability of the retained austenite in TRIP steels is still limited.
In previous experiments we have studied the volume fraction of retained austenite as a function of the chemical composition and the bainitic holding time by dilatometry [6], X-ray diffraction [7], and SQUID magnetisation [8]. The average austenite grain size was characterised by neutron depolarisation measurements [9]. By in situ X-ray diffraction measurements at a synchrotron source [10] we characterised the phase stability of the retained austenite under applied stress. These measurements allowed us to correlate the stability of the austenite grain to the relative orientation with respect to the applied stress.
An alternative way to study the phase stability of the retained austenite in TRIP steels with higher accuracy is to cool the material to low temperatures, and thereby increase the temperature-dependent chemical driving force for the martensitic transformation of the metastable retained austenite. In previous magnetisation measurements [11], [12], we have demonstrated that part of the retained austenite in TRIP steels can indeed become unstable during cooling. The grains with the lowest carbon concentration have the highest martensite start temperature (Ms) and are therefore expected to be relatively unstable during cooling [13]. The difference in thermal expansion of the ferrite matrix [14] and the austenite grains [15] can induce weak strains in the material during cooling, which may assist the martensitic transformation of the retained austenite.
In the current study, we performed in situ X-ray diffraction measurements at a synchrotron source to monitor the stability of retained austenite in TRIP steels during cooling. Part of the retained austenite phase is unstable when the material is cooled from room temperature to 100 K and transforms into martensite. In these measurements the austenite fraction and the average interstitial carbon concentration was monitored as a function of temperature for TRIP steels with different compositions and bainitic holding times. The analysis of the thermal stability of individual austenite grains will be the subject of a future paper.
Section snippets
Experimental
Three TRIP steels with different Al and P concentrations were studied. The chemical compositions of the Al0.4, Al0.4P0.1, and Al1.8 steels are listed in Table 1. Samples from the hot-rolled sheet material were machined into cylinders with a diameter of 0.50 mm and a length of 2.0 mm. The cylindrical axis of the sample was chosen along the rolling direction of the sheet material. The samples were heated in a salt bath to the intercritical holding temperature Ti listed in Table 2, to form a
Diffracted intensity
In Fig. 1, the scattered intensity of the γ-{2 0 0}, γ-{2 2 0}, and γ-{3 1 1} austenite powder reflections is shown as a function of the scattering angle 2θ for the Al0.4 steel with a bainitic holding time of tbh = 60 s. The scattered intensity was obtained by adding all diffraction patterns for sample rotations in the angular range from −30° up to 30°. A radial average over constant scattering angles was performed to obtain 1D powder data from the individual reflections on the 2D diffraction patterns
Discussion
In Fig. 7, the average carbon concentration in the retained austenite phase 〈xC〉 is plotted as a function of the volume fraction retained austenite fγ for the Al0.4 steel with different bainitic holding times during cooling. The results are consistent with our previous observations that the average carbon concentration within the austenite phase increases when the austenite fraction decreases. It is remarkable that the temperature dependent data for the samples with the longer bainitic holding
Conclusions
We have performed in situ X-ray diffraction measurements in order to study the thermal stability of the retained austenite phase in TRIP steels during cooling from room temperature to 100 K. A powder analysis of the data shows the general characteristics of the austenite to martensite transformation during cooling. The prime conclusions are:
- 1.
Part of the initial retained austenite formed in the TRIP steels was found to transform into martensite during cooling, while another part remained stable
Acknowledgements
We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and we would like to thank L. Margulies for assistance in using beamline ID11. This work was financially supported in part by the Foundation for Fundamental Research on Matter (FOM) of the Netherlands Organisation for Scientific Research (NWO) and the Netherlands Institute for Metals Research (NIMR).
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