TWI Industrial Member Report Summary 628/1997
By A M Barnes
Modification of a standard 9%Cr 1%Mo composition, through additions of Nb, V and N, has led to the evolution of a steel grade that offers good high temperature strength, oxidation resistance and resistance to hot hydrogen attack. This material is finding increased use within the power generation industry, for example in headers and heat exchanger tubing, and its potential is acknowledged in other industry sectors. Development of consumables began with matching compositions, but this led to weld metals of inferior properties relative to the parent steel. It was found that the levels of C and Nb were critical in the determination of toughness, but that some Nb was required to ensure adequate creep strength. Whilst creep properties are perhaps the major concern for these materials, as they are principally used in high temperature applications, toughness is a consideration if start up/shut down problems are to be avoided, and pressure test requirements satisfied.
Many studies at TWI and elsewhere have shown that modification of the current commercial consumable compositions can lead to significant enhancement of properties, particularly toughness. A variety of elemental effects has been investigated and the present work examined the effects of Mn and N on deposit
microstructure and on both Charpy and CTOD toughness.
To determine the effect of manganese and nitrogen on weld metal hardness and toughness and on microstructural development. the creep properties of the deposits in the present study have yet to be established.