Using various mixed gases of CO
2-O
2, CO
2-N
2 and N
2-H
2 systems, gas-shielded metal arc welding AISI Type 304 stainless steel was studied as compared with A or A-O
2 shielded metal arc (MIG) of welding.
The experimental results were as follows :
(1) With reduction of oxidizing property, i.e. decrease of oxygen in CO
2-O
2 system and increase of nitrogen in CO
2-N
2 system, manganese and silicon contents of weld metal increased gradually, but molybdenum and nickel contents remained almost constant. Chromium content also was unchangeable above 70% CO
2 of CO
2-O
2 system.
(2) Carbon content decreased as increase of oxygen in CO
2-O
2 system.
(3) With increase of hydrogen content in N2-H2 system, manganese, silicon and molybdenum contents increased gradually, chromium content remained unvaried, and nickel content showed a decreasing tendency.
(4) A remarkable porosity was recognized in the radiograph of the weld metal obtained using the supplied gas above 60-80% N
2 in CO
2 -N
2 system, but not in that of the weld metal obtained gas containing even 100% O
2 in CO
2-O
2 system.
(5) Microscopic cavities were sometimes recognized in the weld metal obtained using N
2 _H
2 and A-O
2 systems.
(6) According to the results of corrosion tests of single pass weld metal with Strauss and 10% HF+3% NaF reagents, the corrosion resistance of weld metals by CO
2-O
2 or CO
2 N
2 systems up to 30% O
2 or N
2 is better than or equal to those of weld metals by A or A-O
2 arc welding and base metal.
(7) In tension tests of joints obtained by CO
2, CO
2+28% O2 and CO
2+30% N
2, all specimens broke at base metal, but the joint specimens by A at fusion line.
(8) All weld metals by supplied gases described in (7) were always excellent in tensile strength compared with the base metal, but were inferior to the latter in elongasion, reduction of area and corrosion resistance.
(9) Intercrystalline-corrosion and intercrystalline-corrosion bend tests showed that the effect of heat treatment or heating cycle was more serious than that of selection of process or shielding gas in welding stainless steel. The intercrystalline-corrosion cracking was sensitized by annealing at 600°C, decreased remarkably by that at 950°C and was prevented completely by solution treatment at 1050°C.
(10) When the CO
2 N
2 system was adopted, the nitrogen content of weld metal increased and the amount of ferrite of weld metal decreased with addition of nitrogen in supplied gas.
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