Reduction of Intergranular Cracking Susceptibility by Precipitation Control in 2.25Cr Heat-Resistant Steels

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I.

INTRODUCTION

THE T/P23 heat-resistant steel (2.25Cr1.5WVNbTi) frequently shows an intergranular cracking in the coarse-grained heat-affected zone (CGHAZ), called the type III cracking or reheat cracking.[1,2] Many studies have supported that the segregation of impurities (especially, phosphorus) to the prior austenite grain boundaries (PAGBs) causes the intergranular cracking by lowering the cohesive grain boundary strength.[3–10] Based on several studies,[11,12] the intergranular cracking initiates first at the GCIs which show the segregation concentration of impurities much higher than the carbide-free PAGBs. The initial cracking at the GCIs is finally followed by the decohesion of the carbide-free PAGBs. Following the recent study,[13] the tungsten-alloyed T/ P23 steel, which is highly susceptible for reheat cracking, shows the higher phosphorus concentrations at the GCIs and the carbide-free PAGBs than the molybdenum-alloyed T/P24 steel. The higher phosphorus concentrations in the T/P23 steel is due to the depletion of the dissolved carbon which can segregate to the GCIs and the PAGBs and so repel the phosphorus from the interfaces. Such a reaction between carbon and phosphorus has been called a repulsive segregation.[14,15] The absence of the repulsive segregation is attributed to the higher driving force for carbide formation of tungsten than molybdenum which accelerated the carbide HYUN JE SUNG, NAM HOE HEO, and SUNG-JOON KIM are with the Graduate Institute of Ferrous Technology, POSTECH, 37673, Pohang, Republic of Korea. Contact e-mail: nhheo@postech. ac.kr Manuscript submitted September 13, 2016. Article published online January 10, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A

reaction at the PAGBs and thus depleted the carbon dissolved in the matrix. Specially, the carbide mainly found is M23C6.[13] It is the purpose of this study to investigate the roles of the vanadium content influencing precipitation reactions on the intergranular cracking susceptibility of the T/P23 heat-resistant steel and to suggest the way how to reduce reheat cracking susceptibility.

II.

EXPERIMENTAL PROCEDURE

Two T/P23 steels, which are mainly different in the vanadium content, were prepared by vacuum induction melting. The ingots were homogenized at 1473 K (1200 °C) for 1 hour and hot-rolled to 12-mm-thick plates. The chemical compositions are shown in Table I. Rectangular bars with a dimension of 11 9 11 9 160 mm3 were machined from the plates in the hot-rolling direction. These bars were given to a heat treatment at 1323 K (1050 °C) for 1 hour which is followed by air cooling and subsequently tempered at 1023 K (750 °C) for 0.5 hour under an argon atmosphere. After the heat treatment, tensile specimens of 8-mm gage length and 7-mm gage diameter were machined from the rectangular bars. The intergranular cracking susceptibility was evaluated using a thermo-mechanical simulator (FDC, THERMECMASTOR-Z) like previous studies.[2,13] The gage part of each specimen was induction-heated to 1573 K (1300 °C) at a heating rate of 25