TWI Industrial Member Report Summary 534/1995
A M Barnes
The modified 9%Cr 1%Mo steels (ASTM A387 Grade 91) are now finding significant application within the power generation industry both in construction projects and in the life extension programmes of existing plant. Their enhanced high temperature properties, achieved through the controlled addition of Nb, V and N to the standard 9%Cr composition, allow cost and weight savings to be realised as well as improved efficiency through higher allowable operating temperatures. These good elevated temperature properties, together with a high resistance to hot hydrogen attack, are attracting significant attention from other industry sectors, for example the petrochemical industry. There are, however, uncertainties in consumable design for these materials to ensure optimum weldment properties in terms of both toughness and creep strength and these have, to date, limited the wide exploitation of these steels.
Earlier studies at TWI investigated the relative weld metal hydrogen cracking susceptibilities of the standard and modified 9%Cr 1%Mo grades and assessed the effect of Nb on toughness after PWHT. The first phase of the present programme showed that the fracture toughness was significantly improved by the addition of -1%Ni, with associated reduction in the levels of Nb, Si and N. This report presents the results of the second stage of the investigation which has employed both commercial and experimental manual metal arc (MMA) consumables to study the effect of weld metal composition and PWHT on microstructural development and the ensuing hardness, toughness and creep rupture properties of the deposits.
- To determine guidelines for formulating better weld metals for modified 9%Cr 1%Mo steel.