The presence of bainite and retained austenite in advanced high-strength steels (AHSS) has grown a great interest in the automotive industry as simultaneously provides strength and the transformation-induced plasticity (TRIP) effect. This type of microstructure is present in modern trailing arms. A trailing arm is part of the air suspension system in heavy vehicles. In spring steels, such as the ones used to produce trailing arms, this combination of bainite and retained austenite is obtained by a heat treatment named austempering.

In this work, the microstructural distribution of a silicon spring steel (Fe-0.6C-1.63Si- 0.97Mn-0.48Cr) was investigated after austempering for 1 hour at 300o C. Microstruc- tural heterogeneity was investigated at different scales using optical microscopy, scan- ning electron microscopy (SEM), electron probe micro analysis (EPMA), X-ray diffraction (XRD), and hardness measurements. Observed heterogeneities were related to the pres- ence of thermal gradient, chemical segregation, and carbon gradient.

The results gave insights into the effect of the processing route on the microstruc- ture. After austempering, bainitic ferrite, martensite-austenite islands, and carbide par- ticles were observed. The thermal gradient was confirmed by the identification of auto- tempered M-A islands due to the slower cooling rate in the bulk of the material. More- over, chemical segregation of Si, Mn, and Cr parallel to the rolling direction was observed and confirmed from optical microscopy, SEM, and EPMA results. In regions with high concentrations of substitutional elements, the Bs temperature was locally reduced and the bainitic ferrite formation kinetics was slow. Carbon gradient in M-A islands was ob- served as carbide precipitation occurred at the center of the island indicating that the carbon content was lower compared to at the edges of the constituent. Furthermore, in areas of the trailing arm with lower fractions of retained austenite (≺ 8%), bainite forma- tion was ceased based on the To curve (austenite reached a specific carbon content over which growth of bainite based on the diffusionless theory is not possible). These results indicate that bainitic ferrite advances non-uniformly through the trailing arm.

Based on these indications, the carbon gradient had a more significant effect on the bainitic ferrite formation in the spring steel during austempering compared to the ther-mal gradient and chemical segregation.