Carbonitride Dissolution and Austenite Grain Growth in a High Cr Ferritic Heat-resistant Steel

Abstract

Dissolution of carbonitrides and growth of austenite grain in X12CrMoWVNbN10-1-1 steel under various austenitizing conditions were investigated. The experimental results showed that the carbonitrides existing in after-forging heat treated state were mainly Cr23C6 and small amounts of NbN and Cr2N. However, Cr23C6 and Cr2N precipitates were dissolved completely after austenitization at 1 070°C for 60 min or at 1 200°C for 15 min, only the NbN particles left. The state of dissolution of NbN particles was also studied in detail as a function of austenitization holding time (tA) at different temperatures. Initially, the NbN particles dissolved with increasing tA at 1 070°C. When tA reached over 360 min, the amount of NbN remained approximately constant. On the contrary, during the process of austenitization at 1 010°C, no NbN particles dissolved into matrix. Starting from the fine and uniform grains, abnormal grain growth was observed after austenitizing at 1 010°C for 960 min or at 1 070°C for 180 min due to the heterogeneous distribution of NbN particles. The plot of grain size against tA indicated that the NbN particle with mean diameter of 117 nm and volume fraction of 3.1×10 -4 would be sufficient to inhibit the austenite grain size effectively. Finally, the EBSD measurements demonstrated that the high energy grain boundaries whose misorientation range is 20-45° might be responsible for abnormal grain growth.

Microstructure evolution and mechanical properties of X12CrMoWVNbN10-1-1 steel during quenching and tempering process

“…It was reported that in this steel more than half of these precipitates were not dissolved after austenitization at 1080 • C for 16 h [16]. These dispersed particles exerted a strong pinning force on the growth of austenite grains [9]. In addition to these Nb-rich MN particles, the microstructure of the specimen A also exhibited the need-like Fe-rich M 3 C precipitates.…”

“…1). The carbonitrides present in the asreceived microstructure were mainly Cr-rich M 23 C 6 and a small amount of Nb-rich MN and Cr-rich M 2 N [9]. The heat treatments were performed in an electric resistance furnace.…”

“…The samples were ground and a b 50 um 25 um The polished samples were etched with a mixture of 10 g iron trichloride, 15 mL hydrochloric acid and 50 mL ethanol for 10 s. The method for displaying the prior austenite grain is in Ref. [9]. The linear intercept method [12] in optical micrographs was employed to estimate the average austenite grain size.…”

“…The final microstructure of the steels typically consists of tempered martensite, undissolved and/or re-precipitated carbonitrides [6]. The austenitizing temperature employed during heat treatment determines the partitioning of carbon or nitrogen and alloying elements between the austenite and carbonitrides, with an increase in temperature leading to increased carbonitrides dissolution, higher dissolved alloy element contents and grain growth [7][8][9]. Once the steels have been austenitized, cooling to below the martensite transformation range facilitates the formation of martensite.…”

Effect of Quenching Rates on Microstructure and Impact Ductile of X12CrMoWVNbN10-1-1 Steel

“…8) Additionally, the relationship between dissolution of precipitates and the austenite grain growth of this steel had also been studied. 9) Once the steels have been austenitized, quenching to below the martensite transformation range facilitates the formation of martensite due to its high Cr content. For the large high/middle pressure rotor forging (e.g.…”