QA/QC Tools to Ensure the Quality of Duplex Stainless Steel Components

ABSTRACT It has long been understood that the heat treatment of duplex and superduplex stainless steels is critical to obtain the optimum structure and the desired properties. Over the last twenty years there have been a number of cases where inadequately heat treated components have been delivered by the manufacturer and then subsequently identified as defective further down the supply chain. In some cases the problem was identified and resolved prior to fabrication and installation, while in others fittings have leaked in service due to poor microstructure from incorrect heat treatment. Common to all these cases is that the cast and batch production test certificate indicated that the goods met specification requirements in all respects. Hence the similitude between cast and batch specific test pieces and the production parts has been called into question. The use of additional testing when specifying these alloys is common but there is no agreement on what these tests should be. There has been extensive discussion on how best to test individual components non-destructively to detect unsatisfactory material. Some have suggested that magnetic measurement of the ferrite content is adequate, whilst others believe the test to be insufficiently discerning, resulting in too many good parts falsely being identified as “suspect” and causing unnecessary remedial action. The present paper describes the procurement specifications used by the authors’ company to ensure adequate properties in service. The paper addresses the strengths and limitations of magnetic ferrite measurements and shows how the readings are affected by manufacturing route, product form, surface roughness and radius of curvature. The paper goes on to show how the test can be used to identify material that may contain sigma phase and that in-situ metallography is then required on these suspect areas to either release the part or condemn the part to remedial heat treatment.