Estimation of Type IV Cracking Tendency in Power Plant Steels

J. A. Francis, W. Mazur and H. K. D. H. Bhadeshia


The cracking of welded joints is usually classified according to the position of the crack; Type I and Type II modes occur within the weld metal, the former confined to the weld metal whereas the latter may grow out of the weld into the plate; Type III cracking occurs in the coarse grained region of the heat-affected zone. Type IV cracking is a feature of welded joints in creep-resistant steels. It is associated with an enhanced rate of creep void formation in the fine grained and intercritically annealed heat-affected zones of the weld, leading to premature failure when compared with creep tests on the unwelded steel. These zones contain coarse carbide particles, leading to a reduction in creep strength; the particles also help nucleate voids. In a cross-weld test, the weakened Type IV region is sandwiched between the stronger base plate and coarse-grained heat affected zone. The resulting accumulation of creep damage in the Type IV region causes the premature failure.

Type IV cracking is prominent in the stronger 9-12 wt% chromium steels. Since the problem arises from the heterogeneous microstructure of the weld heat-affected zone, it can be eliminated by a reaustenitisation and tempering heat treatment. Unfortunately, this rarely is a practical option. Instead, components have to be designed allowing for a reduction Δσ in the creep strength (or equivalent reduction in creep life) due to Type IV cracking.

The magnitude of Δσ is known to depend on the chemical composition, heat treatment and the state of stress. It therefore has to be estimated experimentally for each application. This clearly is a limiting factor in the design process, whether it involves alloy development or welding. The purpose of the work presented here was to develop a method which allows for the estimation of Type IV limited creep rupture life, as a function of the steel composition and heat treatment.

ISIJ International Vol. 44, 2004, pp. 1966-1968.

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