Numerical and Experimental Study on Hot Forming by Partition Cooling of 38MnB5Nb

Abstract

Tailored properties hot forming of 38MnB5Nb by partition cooling can be configured on-demand. A gradient distribution microstructural characteristic can be generated by undertaking an appropriate cooling-tempering process for the regions with different properties requirements before forming. A unified viscoplastic constitutive model coupled with the primary cooling temperature and related material constants is established based on genetic algorithm. Meanwhile, the use of the mixture of jet air and dry ice particles to cool the partition region is essential to achieve different primary cooling temperatures. In this paper, the inverse heat conduction problem is solved to obtain the relationship between the interfacial heat transfer coefficient and different cooling conditions in the partition cooling process. The U-shaped part is taken as an example to simulate the change of temperature, stress-strain, thickness, and spring-back in the process of partition cooling and tailored hot forming properties. The results show that the gradient microstructural characteristic formed by partition cooling has a great influence on the stress field distribution. The maximum stress of 345 MPa can be reached after complete pressure holding in the partition slow cooling tailored properties of the hot forming process. The maximum stress can reach 743 MPa in the partition fast cooling tailored properties of the hot forming process due to the relatively high deformation resistance of the tempered martensite at the bottom center. The maximum residual stress in the two processes after spring-back drops to 305 MPa and 545 MPa, respectively. The spring-back is small under the two processes, with a maximum spring back angle of no more than 1°.