TWI Industrial Member Report Summary 650/1998
Austenitic alloys with high levels of Cr, Mo and N alloying have excellent corrosion resistance linked with high toughness and ductility to very low temperatures and are easy to fabricate. Recently developed grades with up to 7%Mo, 0.5%N and typically 20-25%Cr have become known as 'superaustenitics', to reflect their high level of corrosion resistance. Autogenous welding of these grades is restricted by loss of corrosion resistance which arises from (i) microsegregation during solidification of Mo and, to a lesser extent, Cr, and (ii) loss of N2 from the molten weld pool. Typically, overalloyed nickel-based filler is used to overcome the problem of reduced autogenous weld area corrosion resistance. However, recent work at TWI and elsewhere has indicated that autogenous welding may be capable of producing weldments with pitting resistance close to that of the parent material, depending on welding conditions and to some extent on composition. Adoption of low heat input, to
The present programme examined the behaviour of welds in a chromium-based austenitic alloy (UNS R20033) made (i) without filler additions, (ii) with matching filler, and (iii) with overalloyed nickel-base filler. Segregation and nitrogen loss behaviour were compared for a range of austenitic materials with varying levels of alloying, to draw conclusions regarding optimisation of the pitting resistance of autogenous welds.
- To quantify and optimise the pitting corrosion resistance of weldments in a chromium-based austenitic alloy with high nitrogen content.
- To quantify segregation and nitrogen loss from weldments in a chromium-based austenitic alloy with high nitrogen content.
- To compare two additions to ASTM G48 ferric chloride pitting corrosion test solution, which are made to prevent precipitation from solution at high temperatures.