Effect of Heat Treatment and Bake Hardening on the Mechanical Properties and Microstructure of Dual Phase Steel

Abstract

This research is concerned with study of the effect heat treatment and the bake hardening process on dual phase steels which can be obtain by intercritical annealing process. Hardness, microstructure and tensile specimens were prepared according to standards. Used specimens was with different carbon content (0.12, 0.096 and 0.064). The normalizing heat treatment was conducted on low carbon steel, to remove the effect of the previous manufacturing processes. Intercritical annealing was used as a heat treatment to obtain the dual phase steel, the carbon steel was quenched from760 ºC this temperature lies between the critical points. It has been found that after intercritical annealing process has transformation accrue from pearlite structure resulting from the normalization process to a new structure contain ferrite and martensite. The hardness of dual-phase steel obtained by intercritical annealing is higher than of the steel that was normalized as well as the carbon content of the steel affects the hardness. Tensile test results showed that the ultimate tensile strength and yield strength increased after intercritical annealing process compared to other specimens were normalized ,content of carbon also effect on the ultimate tensile strength and yield point. The dual phase steel is characterized with no visible yield point due to the dislocations movement are restricted. Dual phase steel yield point re-appear after pre-strained at 3, 5, 8, 10%., and baked to 170 ° C. The ultimate tensile strength and the yield point for the pre-strained and baked specimens were higher than the other tensile specimens which conducted with normalizing or intercritical annealing only due to the diffused carbon atoms that pin dislocations and prevent it is movement. Baking process is affected by several factors such as carbon ratio and the amount of prestrain. It is found that when carbon
 content increase, the number of diffused carbon atoms will increase. Amount of pre-strain effect on the mechanical properties after baking process, where it was found that when the mount of pre-strain increase the ultimate tensile strength and yield point stress will increase