Microstructure of a 1Cr 1Mo 1/4V Steel after Long Service

概要

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Following a previous paper which reported significant deterioration in the notch toughness and high temperature strength and ductility of a 1Cr 1Mo 1/4 V cast steel after service in a steam valve for 74,000 hours at 839° K (566℃), this report studies the microstructure of this service exposed steel, comparing it with that of virgin steel. The service exposed steel contained considerably coarser carbides in the matrix and thick elongated films of M_<23>C_6, type carbides were frequently observed on its grain boundaries. A suitable re-normalizing and tempering treatment resulted in the dissolution of these carbides in austenite and re-precipitation as finer ones, with consequent improvement in the degraded mechanical properties to levels similar to those in virgin steel. No significant grain boundary segregations of impurity elements such as phosphorus, tin etc., were observed in the as service exposed condition. Nor can the embrittlement be attributed to damage due to creep or fatigue during the long service period and it is believed that carbide precipitation and changes in the carbide morphology occurring in service were mainly responsible for the observed embrittlement.
社団法人溶接学会の論文
1985-04-01