Weld Metal Additive Manufacturing for Grade 91

Although Grade 91 steel has decades of successful service history in the power generation and petrochemical industries, the most troubling aspect of grade 91 steel structures is the softening of the heat-affected zone (HAZ) in the base metal caused by welding. This softened area within the HAZ of the grade 91 weld joint is commonly referred to as “Type IV” zone. Changes to base metal chemical composition, changes to filler metal composition and welding procedures, and changes to joint design have helped, but the issue persists and cannot be completely removed.

A novel approach is to replace the traditional base metal with base metal additively manufactured (AM) from filler metals for improved performance where failures are most likely to occur. The current work explores the feasibility of this approach for components that will be subjected to services at designed high temperatures and stresses.

This paper will present how the inherent reheating of weld metal in multiple-pass welding does not result in substantially softened areas like those encountered in the Type IV zones of traditional P91 base metals, thus effectively lowering the risk of premature creep failure for a part comprised of weld metal. Properties of additively manufactured weld metal using ASME SFA “–B91” consumables will be presented. The -B91 weld metal alloy system is designed to develop properties after post-weld heat treatment (PWHT) in the lower critical range. Nearly all of the service experience with –B91 weld metal is in that PWHT condition, thus allowing correlations to be made with historical all-weld-metal properties. Factors influencing the quality of AM built weld metals and their properties will also be examined. Benefits and flexibility in procurement and field construction from this new technique will be identified. Finally, in-service data and application cases will be explored.