Sensitisation of Two 11–12 % Chromium Type EN 1.4003 Ferritic Stainless Steels During Continuous Cooling after Welding

Abstract

The susceptibility to sensitisation of two type EN 1.4003 ferritic stainless steels containing 11–12% chromium during continuous cooling after welding was investigated during the course of this project. These steels are designed to transform partially to austenite in the high temperature heat-affected zone (HTHAZ) adjacent to the fusion line during cooling, with the austenite subsequently transforming to martensite below the Ms temperature. The investigation was prompted by a number of in-service failures of fillet welds attributed to stress corrosion cracking caused by sensitisation. These failures were associated with fast welding speeds and fillet weld overlap, implying that low heat inputs promote sensitisation in these alloys. Two steel grades, designated A and B, were welded using a range of heat inputs (from 0.03 kJ/mm to 0.45 kJ/mm) and welding speeds (from 2.36 mm/s to 33.3 mm/s). Steel B has a higher austenite potential than steel A. Rosenthal’s heat flow model was used to relate the heat input and welding speed to the cooling rate, and to demonstrate the influence of welding parameters on the martensite content of the HTHAZ. A decrease in heat input increases the cooling rate after welding. At low heat input levels, austenite nucleation in the HTHAZ was suppressed due to the fast cooling rate and almost fully ferritic microstructures were observed. Higher heat inputs resulted in slower cooling rates with a higher fraction of grain boundary martensite in the HTHAZ. Chromium-rich carbides were identified using a 10 % oxalic acid etch (ASTM 763–93, Practice W), whereas chromium depletion associated with these carbides was confirmed with a potentiostatic etch at 0 Vsce in 0.5M H2SO4. Both steel grades were found to be sensitised when lower heat inputs and faster cooling rates resulted in a continuous network of ferrite-ferrite grain boundaries in the HTHAZ. An increase in heat input reduced the cooling rate and increased the amount of martensite in the HTHAZ. The ferrite-martensite phase boundaries were generally not sensitised. The results suggest that if enough austenite is allowed to form in the HTHAZ during cooling, the austenite acts as a carbon sink and absorbs excess carbon. This prevents supersaturation of the ferrite phase and subsequent carbide precipitation that could lead to chromium depletion and sensitisation of the ferrite-ferrite grain boundaries. Reducing the welding speed and increasing the heat input during welding resulted in satisfactory welds. As a result of its higher austenite potential, grade B retained martensite in the HTHAZ down to lower heat input levels than grade A, and therefore appeared to be less sensitive to the cooling rate.