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.
Highlights
As a mature structural material, low-carbon, micro-alloyed, high-strength steel is widely used in energy, transportation, construction, and other industries
The results showed that due to the increased precipitation of fine carbides during the cryogenic process, the specimen that was subjected to subsequent tempering underwent a reduction in compressive residual stress
The impact on the changes in length caused by caused by the temperature dependence of the thermal expansion coefficient can be deducted the temperature dependence of the thermal expansion coefficient can be deducted using the using the second non-isothermal tempering step as a baseline
Summary
As a mature structural material, low-carbon, micro-alloyed, high-strength steel is widely used in energy, transportation, construction, and other industries. In order to reduce the material cost and improve the welding performance, grain-boundary strengthening, precipitation strengthening, and phase transformation strengthening has become the main way to develop high-strength steel materials [1]. Research has focused on reducing the internal stress of quenching and obtaining high-strength steel products with stable structures and ideal properties [4,5]. From the perspective of mechanical properties, Ritter et al [7] investigated the non-isothermal stress relaxation of carbon-manganese steel. They found that most the relaxation occurred in the heating stage of the thermal cycle, Metals 2019, 9, 709; doi:10.3390/met9060709 www.mdpi.com/journal/metals
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