The limiting temperature and time for diffusion welding of 18-8 stainless steel to mild steel were investigated under the conditions of no insert metal, 1kgf/mm2
torr vacuum degree and the buff polishing surface, in order to obtain a perfectly welded joint. The effects of the vacuum degree and the surface roughness were also examined under the conditions shown above. And the fractographic features of imperfect welded joints were observed by means of a scanning electron microscope.
The results obtained are summarized as follows:
(1) Under the above conditions, the limiting temperature and time for perfect welding were approximately 800°C for 7 minutes.
(2) The weldability of the joint was reduced as the vacuum degree became lower, and the rate of perfect welding in 0.6∼2.4×10-4
torr was about 40per cent. In the case of 10-3
order, perfect welding was impossible.
(3) When the surface roughness was over Rmax
1.2μm for SUS 304 and 2.4μm for mild steel, perfect welding was difficult.
(4) The fractographic features of fractured bonded interface of imperfect welding were as follows:
(a) Mild steel-mild steel welding showed transgranular ductile dimples and cleavage fractures.
(b) SUS 304-SUS 304 welding showed only ductile dimples.
(c) In mild steel-SUS 304 welding, mild steel was similar to mild steel-mild steel welding, and SUS 304 showed small ductile dimples with precipitated Cr carbide particles, and besides intergranular fracture with carbide precipitation occurred in 1000°C welding.