Research On The Rolling Technology Of High Strength 42CrMo Steel For Bolts Manufacturing

Fasteners is one of important fundamental mechanical parts, its level directly decides the performance, quality and reliability of major equipment and host product. High strength fastener refers to the fasteners with strong bearing capacity, good on or above grade8.8in GB/T3098.1-2010. The fasteners are widely used in automotive, machinery manufacturing, chemical industry and aviation and so on. However, comparing with international advanced level, the research and production on high strength fasteners in China is relatively backward at present. Particularly, as domestic product cannot meet requirements, high strength bolts are still imported from overseas. So the research on high strength bolt(42CrMo) is great significance to the national economy development.This paper focus on the study on hot deformation behavior of 42 CrMo bolts(A sample with Ni, B sample without Ni).Based on the thermal expansion curve at different cooling rate and phase transition temperature of 42 CrMo bolts, making the CCT curve of the steel was made. It was achieved that calculating the temperature curve of 42 CrMo bolts in furnace by using mathematical model, and optimizing heating system. After rolling, the microstructures of samples were analyzed, and mechanical and fatigue properties were tested.There sults show that: The rheological stress of 42 CrMo bolts decreased with the increase of deformation temperature, and increased with the increase of strain rate during hot deformation. The rolling temperature of 42 CrMo bolts is between 850 with 1100 °C, and strain rate is 0.1s-1. Low strain rate is conducive to hot working of 42 CrMo bolts.With the increment of cooling rate, 42 CrMo bolts microstructure evolves gradually from ferrite + pearlite + bainite mixed structure to martensitephase. In order to obtain ferrite + pearlite + bainitehybrid microstructure, slowcooling method was adopted in the rolling process, and the cooling rate was 0.1 /s.The 42 CrMo bolts were heatedusing preheat- heating- soaking heating mode, and the total time of heating is more than 300 minutes. Beginning rolling temperature is 1050 ~ 1100 ℃, finishing temperature is 750 ~ 850 ℃.The 42 CrMo bolts were rolled 6 times, the first two times for roughing, the last four times for finish rolling. The microstructure of 42 CrMo bolts rolled consist of ferrite + pearlite + bainitehybrid structure, the ferrite contentof A sample is 12.1% higher than that of B sample, ferrite hardness is 8.92% higher than that of B sample.After slow cooling, A sample has the materials performance of 1060 MPayield strength, 1130 MPatensile strength, 15%break elongation,47%section shrinkage, low temperature toughness of 44 J and fatigue limit of 412.5 MPa, B sample with the materials performance of 980 MPa yield strength, 1080 MPa tensile strength, 5%break elongation, 47% section shrinkage rate, low temperature toughness of 46 J and fatigue limit of 397.5 MPa...