TWI Industrial Member Report Summary 976/2010
By John Rothwell
The creep resistant steel, X10CrMoVNb9-1, more commonly referred to as ASME/ASTM grade 91, (P91 for pipes), has become a popular choice for steam plant in fossil power stations and oil refineries. The improved high-temperature strength and creep resistance of the parent steel over predecessor alloys, eg ½Cr½Mo¼V, 1CrMo, 2¼Cr1Mo and 12CrMoV (X20) grades, has helped to reduce thicknesses and thus minimise stresses due to thermal cycling.
The poor cross-weld creep strength of welded joints compared with that of the parent material has become a major concern for many operators that have installed materials on the basis of the parent metal qualities without realising the lesser performance of welded joints due to the presence of 'thermal damage', ie microstructural change, in the heat affected zone (HAZ). Information on parent and weld metal toughness performance are widely available. However there is very little information relating to the heat affected zone (HAZ) toughness of this material, which would be of interest particularly where as-welded or 'cold' repairs (without PWHT) are concerned.
The effect of unplanned or unusual heat treatments, or lack of heat treatment, intentionally or not, on the mechanical performance of weldments in P91 is of interest to fabricators and operators. An understanding of how performance is altered is important in order to avoid cracking or failures during start-up or low temperature hydro-testing of plant. It is equally important to understand what the effect will be on the creep performance during extended high-temperature (550-650°C) service.
This report describes an investigation into the effect of various heat treatments, or absence of heat treatment, on the performance of weldments in P91 steel material. Each heat treatment condition is designed to represent a circumstance of possible concern during fabrication.
To evaluate the effect of prior and post-weld heat treatment (PWHT) on the cross-weld creep performance and HAZ toughness of weldments in P91 steel.