カーペンター20系鋳鋼品の溶接性の改善

概要

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Carpenter 20 fully austenitic steel (20%Cr-30%Ni-2.5%Mo-3.5%Cu) has superior corrosion resistance.This steel has begun to attract considerable attention as the material of large castings structure in severe corrosive environments.But the weldability of this steel is very poor because of its fully austenitic structure and having much alloying elements as Cu and Ni.Therefore, it used to be impossible to guarantee that Carpenter 20 cast steels production welds which are free from microcracks on the gouged surface or in the weld zone can be made.In order to prevent these microcracks, an investigation was carried out about an influence of alloying elements upon crack, effects of solution heat treatment and welding conditions and the cause of cracking is being described.The experimental results are summarized as follows:(1) Two cracking mechanisms were considered as the cause of welding crack of Carpenter 20.a) Liquation in the grain boundary at temperature close to the solidus.b) Lower ductility zone around 900°C due to carbo-nitride and micro-segregation of impurity elements.(2) The effective countermeasures for prevention of cracks are:a) Decreasing of impurities P, S and C, Nb) Performance of solution heat treatmentc) Decreasing of heat input and lowering interpass temperatureAccording to assembling test of gate valve these countermeasures were proved practical.
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