PROBLEM TO BE SOLVED: To provide a heat-treatment method by which, when applying a softening heat-treatment to a duplex stainless steel piece produced by a hot-forging or a hot-rolling or as cast, the occurrence of residual stress is restrained to such an extent that machining can be sufficiently performed, and also the internal crack can be prevented.;SOLUTION: The solid duplex stainless steel piece having 300-500 mm diameter or 300-500 mm length between the facing surfaces, is cooled till 920-1,000°C (T1) on the outer surface temperature of the steel piece at ≤150°C/hr (2.5°C/min) cooling speed V1 after soaking at the temperature of ≥1,050°C, and further, cooled from the above temperature till at least 300°C (T2) at 30-40°C/min cooling speed V2. The above cooling till 920-1,000°C can be performed in a heat-treatment furnace or while holding the temperature with a heat-insulating material etc.;COPYRIGHT: (C)2009,JPO&INPIT
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