3. The influence of residual compressive stress on the workpiece
The reason why carburized surface strengthening is widely used as a method to improve the fatigue strength of steel parts. On the one hand, because it can effectively increase the strength and hardness of the workpiece surface, improve the wear resistance of the workpiece, on the other hand, carburizing can effectively improve the stress distribution of the workpiece, obtain a large residual compressive stress on the surface layer of the workpiece, and improve the fatigue strength of the workpiece. If the isothermal quenching is carried out after carburizing, the residual compressive stress of the surface layer will be increased, and the fatigue strength will be further improved.
Isothermal quenching has higher surface residual compressive stress than the usual quenching low temperature tempering process. The surface residual compressive stress is higher than that of low temperature tempering after isothermal quenching even after low temperature tempering. Therefore, it can be concluded that the surface residual compressive stress obtained by isothermal quenching after carburizing is higher than the usual low temperature tempering of carburizing quenching. From the point of view of the beneficial influence of the residual compressive stress on the fatigue resistance of the surface layer, the isothermal quenching process of carburizing is an effective method to improve the fatigue strength of carburizing parts.
Why can carburizing and quenching process obtain surface residual compressive stress?
There are two main reasons for this:
One reason is that the specific volume of the surface high-carbon martensite is larger than that of the low carbon martensite in the core, and the volume expansion of the surface surface is large after quenching, while the volume expansion of the low carbon martensite in the heart is small, which restricts the free expansion of the surface surface and causes the stress state of the surface compression core.
Another more important reason is that the beginning transition temperature (Ms) of the transition from high-carbon supercooled austenite to martensite is lower than the beginning temperature (Ms) of the transition from supercooled austenite to martensite with low carbon content in the core. That is to say, during the quenching process, it is often the heart that first produces martensitic transformation causing the volume expansion of the heart and is strengthened, and the surface is not cooled to its corresponding martensitic beginning transition point (Ms), so it is still in the supercooled austenite state with good plasticity, and will not seriously suppress the volume expansion of the martensitic transformation of the heart.
With the continuous decrease of quenching cooling temperature, the surface temperature drops below the (Ms) point, and martensitic transformation occurs in the surface layer, causing the expansion of the surface volume. However, at this time, the heart has already been transformed into martensite and strengthened, so the heart will play a great role in suppressing the volume expansion of the surface surface, so that the surface surface can obtain residual compressive stress. When the isothermal quenching is carried out after carburizing, when the isothermal temperature is above the martensitic beginning transition temperature (Ms) of the carburizing layer, the appropriate temperature isothermal quenching below the martensitic beginning transition temperature (Ms) point of the core is more than continuous cooling quenching to ensure the sequential characteristics of this transformation (that is, to ensure that the surface martensitic transition only occurs in the cooling process after isothermal.
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