TWI Industrial Member Report Summary 509/1995
A M Barnes
The use of modified 9%Cr 1%Mo steels (ASTM A387 Grade 91) has significantly increased in the power generation industry, and they are also finding application in the petrochemical industry. The additions of Nb, V and N to a conventional 9%Cr 1%Mo base composition confers enhanced high temperature strength, which, in combination with the enhanced oxidation resistance relative to lower alloyed grades, provides benefits to cost, efficiency and section size. Uncertainties in consumable design to achieve optimum weldment properties in terms of creep strength and toughness, have to date, limited the exploitation of these steels. In earlier studies at TWI the relative weld metal hydrogen cracking susceptibility of the modified and standard 9%Cr 1%Mo steels was examined. A preliminary assessment was made of the effect of welding parameters and Nb content on the weld metal properties and microstructural development. This latter study, under the specific conditions employed, indicated little effect of Nb on toughness when assessed by Charpy testing, but CTOD testing showed a detrimental effect of increased Nb after extended PWHT. This report presents the results of the first stage of an investigation using commercial and experimental manual metal arc (MMA) consumables to study the effect of weld metal composition on microstructural development and on the resulting hardness and toughness of the deposits.