Effect of Carbide Precipitation on the Evolution of Residual Stress during Tempering

Abstract

The evolution of microstructure and residual stress during the tempering of 700 L low-carbon micro-alloyed steel was studied using a crack compliance method for measuring residual stress. Additionally, a non-isothermal tempering dilatation test, Vickers micro-hardness test, and transmission electron microscopy were used. The evolution of residual stress during tempering consists of two stages. The first stage coincided with cementite precipitation. Under the initial residual stress, the transformation plasticity due to cementite precipitation leads to partial relaxation of the micro-stress evoked by the austenite-to-ferrite transformation during quenching. It also caused the material surface and the core to exhibit different residual stress evolution trends. After tempering at 300 ∘ C for 30 min, the residual stress was reduced from 487 MPa to 200 MPa; however, the elastic strain energy remained unchanged. The second stage coincided with alloy carbide precipitation and Mn partitioning, but the precipitation of the alloy carbide only reduced the elastic strain energy by 8.7%. Thus, the change in activation energy was the main reason for the relaxation of residual stress at this stage. After tempering at 600 ∘ C for 30 min, the residual stress was reduced to 174 MPa, the elastic strain energy was reduced by 72.72%, and the residual stress was controlled.

Effect of Prestrain and Tempering on the Residual Stress of Low‐Carbon Microalloyed Steel

“…The first derivatives of the difference in the specimen lengths obtained by performing nonisothermal tempering with a heating rate of 2 °C min −1 after treating with 5% and 10% prestrain were plotted with respect to the nonisothermal tempering temperature, as shown in Figure . Reference presents specific data‐processing methods …”

“…The change in the elastic strain energy during the tempering process after normalization processing (Figure c) shows that the 10% PST specimen started to decline substantially at 450 °C, whereas the temperature at which the elastic strain energy of the DT specimen declined was 600 °C. At 450 °C, the elastic strain energy of the PST specimen was 24.8% of the initial value, but that of the DT specimen was 97% of the initial elastic strain energy.…”

“…Previous studies have shown that the evolution of residual stress in low‐carbon microalloyed steel during DT includes two stages: surface residual stress reduction and elastic strain energy reduction. The aforementioned test data also indicated that during the PST process, the residual stress evolution had obvious two‐stage characteristics, but the residual stress evolution temperature decreased significantly, showing that the mechanisms controlling the residual stress differed.…”

“…During the tempering process at 300 °C, some cementite precipitates appeared in the DT specimen matrix. The volumetric changes of the cementite precipitates under the initial residual stress of the 700 L specimen were superimposed in the stress direction to produce precipitation plasticity, causing decreased residual stress at the material surface and a change in the stress direction. Therefore, in the process of DT, the control mechanism of residual stress at a low temperature is precipitation plasticity.…”

“…Therefore, many manufacturers have added a tempering process after hot rolling to reduce the residual stress of the high‐strength steel. In an earlier study, the effect of direct tempering (DT) on the residual stress of a low‐carbon microalloyed high‐strength steel was investigated, and the results showed that after 30 min of tempering at 600 °C, the residual stress in the high‐strength steel decreased by 64.3%. Younger and Eckelmeyer studied the evolution of residual stress during the aging treatment of an aluminum alloy satellite box and found that the aging treatment reduces the residual stress by only 40%, and can also lead to reduced material performance.…”