Cracking analysis and process improvement of pulse preheating flash butt welded joint of dual phase wheel steel

Pulse preheating flash butt welding technology was used to weld dual phase wheel steel SRS590 LW. After butt welding,wheel rim was manufactured by scraping slag,cutting end,expanding,3 steps rolling forming and expanding finishing. In the process of expanding and expanding finishing,weld cracking occurred and the cracking rate was 20%. Analysis of metallographic microstructure,microstructure,precipitates and hardness distribution was carried out on cracked samples of wheel rim welded joint. The results showed that microstructure of the base metal was ferrite + bainite and the grain size was 3 ～ 10 μm. Size of precipitates was 10 ～ 100 nm. The cracking weld was overheated,its grain size became abnormally coarse to 60 ～ 120 μm. Proeutectoid ferrite appeared and precipitates below 20 nm disappeared. The maximum hardness of weld was 338 HV. The fracture was brittle fracture,and the fracture source was shear burr. When the length of the sample was increased 10 mm,the total extension length was reduced 5 mm,the preheating allowance was reduced 2 mm,times of pulse preheating decreased from 19 to 12,welding voltage was reduced from 8 V to 7.6 V,the welding time was shortened from 10 s to 8 s,the electric upsetting time was reduced from 0.8 s to 0.6 s. Then the overheated microstructure in the weld center was eliminated,microstructure in the weld center was mainly acicular ferrite and the grain size was 60 ～ 80 μm. The maximum hardness of the welded joints was reduced to 216 HV and cracking rate of the wheel rim was reduced to less than 2%.