Crack growth in welded turbine materials at elevated temperatures

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Abstract
Due to thermal gradients and load changes steam‐turbine casings suffer combined creep‐fatigue loading. Creep and fatigue portions depend on service conditions. Special interest is focused on the behaviour of creep‐fatigue cracks, originating from potential defects due to construction and manufacturing weldments, on residual lifetime assessment. The project deals with the crack‐growth behaviour at 530°C and r.t. of welded and unwelded heat‐resistant cast steels: 1% CrMoV‐cast (GS‐17 CrMoV 5 11, two melts: low and high V‐content) and 12% CrMoV‐cast (G‐X 22 CrMoV 12 1). The crack planes of CT‐specimens investigated, taken from manual arc‐welded plates, were situated in the heat‐affected zone (HAZ). The fatigue, creep and combined creep‐fatigue crack‐growth behaviour was investigated by means of low‐frequency (0.5 and 0.05 Hz) tests, holding‐time (20 min) tests and constant load tests up to about 5 000 h.Metallographic investigations show that in the case of both 1% and 12% CrMoV‐cast weldments the preferred crack path lies in the outest, i.e., the high tempered and not α/γ transformed region of the HAZ. A strong increase on the crack‐growth rate at 530°C is exerted by dominant creep portions. In the case of the 1% CrMoV‐cast this acceleration becomes important at stress intensity ranges higher than about 25 MPa√m with some higher sensitivity of the HAZ. For the 12% CrMoV‐cast creep dominance begins at about 50 MPa√m. No differences for the HAZ and the base material were found for this material.

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