Analyses the effect of molten zone on the BF production, discusses the controlling of reasonable molten zone.

Crack propagation conducts alternatively between the weld zone and the partially molten zone, and is not strictly to rupture at W/F boundary surface.

By strictly clearing up weld zone, improving welding material's purity, strengthening molten pool's shield and controlling energy input etc measures, weld zone's hot crack and blowhole were prevented.