Analysis of Precipitation Phase of Alloy1.4957 Heat-Resistant Steel for Gasoline Turbocharger

Abstract

Recently, strengthened environmental regulations have required the downsizing of gasoline engines, and as a result, demand for gasoline turbochargers has rapidly increased. The vane of a turbocharger controls the flow of gases toward the turbine, and it is manufactured by powder metallurgy due to its complex shape. Gasoline engines have a high exhaust gas temperature (~1000oC), and thus, Alloy1.4957 (GX15CrNiCo21-20-20) containing large amounts of Cr, Ni, and Co is used. In this study, Alloy1.4957 powders were sintered by hot isostatic pressing (HIP), and then homogenized and thermally exposed to exhaust gas temperatures. Then, a microstructural analysis was conducted to observe the changes that occurred for each process. M6X carbonitride containing Si, called Cr3Ni2SiX, was observed to be the main precipitate phase in this alloy. In general, it is known that Cr3Ni2SiX is only rarely observed in heat-resistant steel. However, in Alloy1.4957, a large amount of Cr3Ni2SiX was precipitated or dissolved depending on the process, and this is probably due to the high Si and N content of Alloy1.4957. In addition to Cr3Ni2SiX, Cr23C6 and NbX were observed. Cr23C6 was dissolved during the homogenization process, but NbX, which has high thermal stability, retained a fine size during the homogenization process and provided a nucleation site for Cr3Ni2SiX during thermal exposure.