Effect of Cooling Mode on Transformation Plasticity and Residual Stress of NM400
Background Information
NM400 is a low alloy wear-resistant steel with high strength and toughness. It has very good weldability and formability and excellent wear resistance. In the design of sheet forming process, builders must consider the forming effect due to material properties such as strength, ductility and formability. The internal structure and properties of the steel, especially transformation plasticity and residual stress behavior, are very sensitive to cooling mode. Therefore, the selection and optimization of cooling mode become the key factor for successful design.
Effect of Cooling Mode on Transformation Plasticity
Transformation plasticity is the ability to undergo plastic deformation during the transformation from austenite to martensite. It is one of the important indicators for the performance of steel. Changing the cooling mode change the structure and properties of steel, leading to different transformation plasticity.
For NM400, it is found that when cooled in water, more austenite will transform into ferrite-cementite than when cooled in air. This great transformation further promotes the transformation of austenite to martensite, improving the transformation plasticity of the steel. In addition, the cooling rate can also affect the transformation plasticity of NM400. With increasing cooling rate, the transformation of austenite to martensite is quicker and the transformation plasticity of NM400 is improved. Besides, cooling intensity also plays a role in transformation plasticity. Under the same cooling rate, the higher cooling intensity can result in greater transformation of ferrite-cementite to martensite, enhancing the transformation plasticity of steel.
Effect of Cooling Mode on Residual Stress
In sheet forming process, the generated residual stress significantly affects the part's performance in terms of formability, springback, dimensional accuracy and so on. A proper cooling system is beneficial to reduce the impact of residual stress and obtain desirable forming effect.
For NM400, cooling in water can effectively reduce residual stress comparing to cooling in air. Besides, with increasing cooling rate, the cooling time is shortened and the residual stress is reduced. Meanwhile, with increasing cooling intensity, the residual stress is decreased, which means the sheet is easier to be deformed and the part has less springback.
Conclusion
The cooling mode influences the transformation plasticity and residual stress of NM400 significantly. Water cooling and high cooling rate are beneficial to plastic transformation and reduce residual stress. The cooling intensity also has an obvious effect on transformation plasticity and residual stress, which is beneficial to obtain desirable forming effect with less springback.
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