軟鋼のろう付強度におよぼす表面状態の影響(第5報)

Abstract

Among the factors affecting the tensile strength of mild steel butt joint brazed with Ag-Cu alloy, it is usually considered that the following are very important for joint strength:(1) Chemical composition of filler metal.(2) Chemical composition of base metal.(3) Joint clearance.(4) Brazing temperature and time.(5) Surface preparation of base metal.Of these factors, there are some investigations made hitherto, regarding the factors (1), (2), (3), (4). However, fundamental investigations of surface preparation of base metal which affects the joint strength are very few.Especially, there are no studies and development as to effect of rouhgness of base metal surface on joint strength. This paper presents some experimental evidences that roughness of base metal surface affects the butt joint strength of meld steel brazed with BAg-8 filler metal. To obtain some experimental data as to the effects of surface preparations, specimens of mild steel (ss4l) were prepared according to the design of Fig. 3. The two abutting surfaces of each specimen were identically finished by emery paper No. 60 120, 240, 400, 600. These specimens were then brazed with BAg-8 filler metal using a resistance furnace operated in hydrogen atmosphere, and machined according to design of Fig. 4.Tensile test of these specimens gave results shown as follows:(1) Tensile strength of specimens polished by emery No. 120 was very high, its value being 43.5 kg/mm2. But the specimens polished by emery No. 600 showed 35.2 kg/mm2.(2) In general, these results indicated that tensile strength of specimens polished by large grain size emery powder was higher than that of ones polished by small grain size emery powder. This difference depends on the actual interface area of filler metal and base metal. Strength of specimen polished by emery paper No. 120 is extremaly larger than that of one polished by emery paper No. 600.(3) Most of specimens were failured at filler metal zone, but a few of them were failured in base materials.(4) Usually, as regards the joint strength of mild steel, clearance of base metal is very important, but this experiment showed that the main factor affecting the joint strength is not joint clearance of base metal but roughness of base metal.Also, studies on spreadabilty of filler metal on base metal with varied roughness were conducted. To obtain some experimental data, mild steel plates (SS41 50φ × 1.6t) polished by emery paper No. 60. 120. 240, 400, 600 and BAg-8 filler metal (wt=0.1 g) were prepared. This experiment was carried out using a resistance furnace and operating it in a hydrogen atmosphere.As the result, spreadabilty of filler metal on the specimens polished by emery paper No. 60 was ex-cellent and that of one polished by No. 120 was second. However, there was a little difference between No. 60 and No. 120 in the spreadabilty of filler metal.