The Impact of Weld Metal Creep Strength on the Overall Creep Strength of 9% Cr Steel Weldments

Abstract

In this work, three joints of a X11CrMoWVNb9-1-1 (P911) pipe were welded with three filler metals by conventional arc welding. The filler metals varied in creep strength level, so that one over-matched, one undermatched and one matched the creep strength of the P911 grade pipe base material. The long-term objective of this work was to study the influence of weld metal creep strength on the overall creep behavior of the welded joints and their failure mechanism. Uni-axial creep tests at 600°C and stresses ranging from 70 to 150 MPa were performed on cross-weld samples of all three welds. A total creep testing time of more than 470,000 hours was accumulated. The longest running sample achieved a time-to-rupture of more than 45,000 hours.Creep testing revealed that the use of under-matching weld metal led to premature fracture in the weld metal at higher stress levels. Compared to under-matching weld metal, the use of matching and over-matching filler material increased the time-to-rupture at high stress levels by 75% and 33% at lowest stress levels. At typical component stresses below 100 MPa, all samples failed in the grain-refined heat-affected zone by characteristic Type IV failure. For investigations of the failure modes, cross-sections of fractured samples were investigated by optical light microscopy, scanning electron microscopy and electron backscatter diffraction. The mechanism of weld metal creep failures and Type IV creep failures are discussed in detail.

Microstructural features, mechanical properties and high temperature failures of ferritic to ferritic dissimilar welds

“…Strength of filler metal : A strong filler metal can provide additional constraint on the weakened, CDZ and prolong failure. A similar effect has been noted in materials susceptible to heat-affected zone (HAZ) failure, such as 9%Cr CSEF steels as reported in [32–34]…”

“… PWHT : Higher temperatures and/or longer durations at a given PWHT temperature can accelerate dissolution of carbides and provide the necessary free carbon for migration in service. Strength of filler metal : A strong filler metal can provide additional constraint on the weakened, CDZ and prolong failure. A similar effect has been noted in materials susceptible to heat-affected zone (HAZ) failure, such as 9%Cr CSEF steels as reported in [32–34] Combinations of base metal(s) and filler metal : In cases where the parent or filler material is susceptible to carbon migration, the presence of deleterious elements may increase susceptibility to damage. Design : DMWs are often designed with poor consideration of the application. In many cases, where DMWs are required and especially in piping, materials of dissimilar thicknesses are machined to match the inside diameter dimension without consideration of the service stresses.…”

“…(iv) Strength of filler metal: A strong filler metal can provide additional constraint on the weakened, CDZ and prolong failure. A similar effect has been noted in materials susceptible to heataffected zone (HAZ) failure, such as 9%Cr CSEF steels as reported in [32][33][34] (v) Combinations of base metal(s) and filler metal: In cases where the parent or filler material is susceptible to carbon migration, the presence of deleterious elements may increase susceptibility to damage. (vi) Design: DMWs are often designed with poor consideration of the application.…”