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 ...

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Fig. 1.MHP-s correlation for new and service-exposed 2.25Cr-1Mo tubes.

In general, the ultimate creep failure mode of welds fabricated from low alloy ferritic steels can be considered to be Type IV cracking resultant from coalescence of grain boundary damage. Thus, observation of grain boundary feature using a replication technique has been one of the most common practices for the remnant life assessment of welds operated at high temperatures. The experimental results in the present work and those found in previous works, however, prove that the feature of grain boundary damage highly depends upon testing conditions. Therefore, the relationship between the extent of damage and remaining life would not necessarily be effective for the practical use if it were obtained under the extremely accelerated laboratory conditions.In contrast, strain rate measurement predicted the rupture life with reasonable accuracy independently of the susceptibility to grain boundary cavitaion. The increase in the strain rate due to the presence of creep cavities was not observed in the experiments using heat treated specimens, which had the microstructure expected at the Intercritical HAZ in whole gauge length. This fact suggests the effectiveness of strain rate measurement for the life assessment and the necessity of reconsidering the physical meaning of grain boundary damage.KEY WORDS: Type IV cracking; low alloy ferritic steel; continuum damage mechanics; the Q projection technique; The W method.

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