Tests for a Japan based Industrial Member, the JGC Corporation, have confirmed the benefits of a two stage heat treatment on disbonding resistance.
JGC Corporation had placed an order for the fabrication of a number of heavy wall pressure vessels in 2 1/ 4Cr-1Mo steel for hydrogen service at high temperature and pressure with the Dutch company Rotterdamsche Droogdok Maatschappij BV (RDM). RDM proposed to apply an internal single layer austenitic stainless steel cladding to the vessels. This would be deposited by submerged-arc or electroslag welding and was designed to have type 347 composition instead of the conventional two layer 309L/347 cladding.
Because of concern over the resistance of the cladding to in-service hydrogen-induced disbonding, the company contracted TWI to conduct an investigation.
Clad samples with a single post-weld heat treatment (30hr at 690◦C) were compared with samples subjected to a second, lower temperature heat treatment (5hr at 600◦C). TWI investigators applied a standard hydrogen charging procedure to all the samples. They then used ultrasonic (C-scan) examination to determine the extent of any disbonding after an incubation period at room temperature.
The C-scan surveys showed that the areas of disbonding after two stage heat treatment were roughly half those following a single stage treatment.
It is clear that a lower temperature second heat treatment reduces disbonding susceptibility. This heat treatment leads to a lower peak hardness in the cladding/base metal interface region where disbonding occurs. This is believed to result from a reduction in the amount of residual, untempered martensite in the interface.
'Disbonding is controlled by hydrogen level, microstructure and applied stress', explains TWI's Mike Gittos. 'We believe that the beneficial effect of the second heat treatment comes primarily from the microstructural changes giving lower interface hardness. There may also be some reduction in residual stresses resulting from thermal expansion mismatch.'