Creep Behavior at the Intercritical HAZ of a 1.25Cr-0.5Mo Steel.

Abstract

Nowadays the preferential creep damage accumulation at the Intercritical HAZ (ICZ) leading to Type IV cracking has been a great concern for various industries. The ultimate failure of the welded components fabricated from ferritic steels often takes place at this particular region. Type IV cracking has been found in almost all the ferritic steel weldments so far, from a conventional 1.25Cr-0.5Mo steel to a modified 9Cr-1Mo steel. However, the mechanism of Type IV cracking has not yet been understood equivocally. In the present work, cross-weld creep behavior of a service exposed 1.25Cr-0.5Mo steel has been examined in order to clear the feature of Type IV damage. The discussion shall be made on the important role of grain boundaries around small grains, which was transformed into austenite during welding, to promote Type IV cracking. The evident feature of grain boundary facets suggests strongly that Type IV cracking is induced by the grain boundary sliding around small grains. Significant impurity segregation, which is expected to accelerate the damage development by stabilizing cavities, was found at grain boundaries.KEY WORDS: Type IV cracking; 1.25Cr-0.5Mo steel; grain boundary sliding; multiaxial stress state; impurity. were allocated cast references of FJA, FJB and FJC.A weld preparation was 60°V and Tungsten Inert Gas (TIG) welding was applied to a root run and the remaining was filled with Manual Metal Arc (MMA) welding. Post Welding Heat Treatment (PWHT) was performed at 700°C for 2 h. Chemical Compositions and Creep EmbrittlementFactor Chemical compositions of sampled casts are shown in Table 1. Parent materials on the flange side, which exhibited the grain boundary damage at HAZ, contain higher tramp elements such as phosphorus (P), tin (Sn), arsenic (As) and antimony (Sb) than pipe parents which showed no grain boundary damage. Takamatsu et al. 4) correlated the impurity contents using the following factor with creep ductility at Coarse Grained HAZ (CGZ) of a 1.25Cr-0.5Mo steel. This factor, termed Creep Embrittlement Factor (C.E.F), was originally proposed by King 5) to assess the susceptibility to stress relief cracking for a 0.5Cr-0.5Mo-0.25V steel.C.E.FϭPϩ3.57Snϩ8.16Sbϩ2.43As ............. (1) where P, Sn, Sb and As are in wt%.Although the threshold value to assess the likeliness of premature cracking at CGZ has not been derived, C.E.F values of the cracked reactors fabricated from a 1.25Cr-0.5Mo steel found in relevant papers 6,7) and those of damaged casts in the present work are higher than 0.15. The fact that higher damage at flange CGZs of FJA and FJC is consistent with higher concentrations of impurities in the material on the flange side as discussed in the following section. Damage Distribution of a Service Exposed WeldsThe grain boundary damage was examined by replication tests although no defect was detected by Wet Fluorecent Magnetic Tests (WFMT) when three welds were sampled. A large difference in damage level was observed among these three casts in spite that they had been closely ...

Grain Boundary Damage Evolution and Rupture Life of Service-exposed 1.25Cr-0.5Mo Steel Welds

“…In the work on Type IV cracking for 0.5Cr-0.5Mo-0.25V steel by Walker et al, 4) it was found that cavitation preferentially took place at fine grains whose grain sizes were smaller than 5 mm. The similar finding was made in the work on 1.25Cr-0.5Mo steel by Fujibayashi et al 2) They found that 90 % of cavities were associated with fine grains which were smaller than 10 mm. It can be considered that fine grains at the ICZ are products of partial transformation during welding.…”

“…From the findings described above, it can be considered that the susceptibility to Type IV cracking is not determined simply by creep strength of base metal. It was pointed out that higher susceptibility could be arisen from higher non-metallic inclusion, 12) higher impurities 2,13) and morphology of fine grains, which became preferential cavity nucleus sites. 2) Further research on factors determining the sensitivity to this phenomenon should be conducted.…”

“…The largest obstacle to examine the behavior of Type IV damage, especially for low alloy ferritic steels, would be long testing duration to produce Type IV cracking experimentally. In the previous work on cross-weld creep behavior of 1.25Cr-0.5Mo steel by the author and co-workers, 1,2) genuine Type IV cracking was not generated when tested without enhanced multiaxiality by application of the vee notch. It is noteworthy that the previously examined weldment, which had been in service for 23 years, had got service-induced creep damage at the ICZ.…”

The Effect of Grain Boundary Cavities on the Tertiary Creep Behavior and Rupture Life of 1.25Cr-0.5Mo Steel Welds

“…Earlier experimental works using a square cross-weld specimen were not necessarily successful since the ultimate failure mostly took place at the parent material on the pipe side in a ductile transgranular manner despite the growth of cavitational damage at the ICZ on the flange side. 6,7) The longest testing time for a square cross-weld specimen was 2 700 h. In order to generate Type IV cracking within reasonable testing duration, the specimen subjected to higher triaxiality has been employed. And it successfully generated Type IV cracking.…”

“…In addition, it was considered that the presence of notches would increase the rupture life of pipe parent due to reduction in the equivalent stress as found by the Finite Element Analysis (FEA). 7) All the creep tests were conducted in air with a constant load machine. Strain measurement for a spirally notched specimen during a test was not made.…”