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Vol.36, No.1, 123 ~ 130, 1998

Title

Effect of the Microstructure of the Failure of the Hardfacing Valve Seat

이병영 , 박상보 Byung Young Lee , Sang Bo Park

Abstract

We have studied the hardfacing valve seat to improve the resistance against failure, using an austenitic heat resisting steel STR35 precipitation-hardened by (Cr·Fe)_(23)C_6 carbides. Failure of the hardfacing valve seat was due to stress concentration at the vicinity of the interface between STR35 and stellite layer. It was thus necessary to avoid weakening the matrix through the valve forming process. In the conventional forming process of the valve head, the grain size of STR35 was about 20-50㎛ but increased remarkably up to 60-80㎛ at the vicinity of the interface region overheated for overlaying stellite. The post welding hardness of the reaction interface depended on the heat treatment conditions prior to welding. The hardness value of the solution treated specimen was very low, HRc 3-5 (matrix HRc 35), and this was associated with the solute-dilution during welding and makes them more susceptible to failure. The danger of wear and failure of the valve seat face could be minimized by reducing the solutedilution rate to a low value with plasma welding method and by producing very stable carbides with annealing below 700℃ prior to welding to prevent the intergranular corrosion and by tempering at 760℃ for 60 minutes after welding to improve: the hardness of substrate STR35.

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