Research on Welding for Stainless Steel

Relation between heat treatment and corrosion resistivities

Abstract

The relation between corrosion resistivity and various temperatures of heat treatment on welded parts for austenitic stainless steel was researched in 65 % HNO₃ and 5 % H₂SO₄ at boiling.
The results obtained are as follows:
(1) The acid resistivity of specimen treated at 700°C is much inferior to that at other temperatures, and is followed with specimens treated at 800°C and 600°C. As-welded condition is comparably good and above all that treated at 1, 100°C is most excellent.
(2) Corrosion amount can be easily confirmed by macroscopic appearances and decreasing amount of corrosion, but not apparently by the method of Charpy-impact specimen except of 700°C treatment.
(3) Comparing mother plate with deposited metal, the latter is much superior to the former in corrosion resistivity. From micro-structural researches there are much mote carbide precipitations causing intergranular corrosion in the grain boundaries of mother plate, while intergranular precipitations of carbide are less found in deposited metals by developement of dendritic structure. Moreover, by the heat treatment at high, temperatures the diffusion and the dissolution of carbon in the matrix are caused, and then intergranular attack seems to be eliminated.
(4) Impact value on heat-affected zone is the highest, and gradually decreases on weld metal and next junction parts. Generally Cb bearing electrode is superior and there is no difference among other electrodes.
(5) The organization of corrosion in HNO₃ and H₂SO₄ is not same. In the former grain boundaries containing precipitated carbide are selectively attacked, and in the latter over-all matrix is corroded. Therefore in HNO₃ the differential effects of heat treatment seem to greatly indicate, while not so different in H₂SO₄.
Range of corrosion amount in the former widely covers from 0.16 gr/m²-hr to 62.5 gr/m²-hr, in the latter slightly from 2.7 gr/m²-hr to 7.5 gr/m²-hr