The Effect of Ni on the Formation of Bainite in Fe-Ni Lateritic Steels through Semi-continuous Cooling Method

Abstract

The formation of bainite in steel alloys is affected not only by temperature, holding time, and cooling method but also by steel alloy’s main alloying element. In iron-nickel (Fe-Ni) lateritic steels, the steelmaking process from nickel pig iron (NPI) gives various Ni contents. In this study, five Fe-Ni alloys with various Ni contents were subjected to semi-continuous austempering. The holding temperatures were 535°C (T1) and 360°C (T2). All alloys were held for 1800 s. Both holding temperatures were determined through the average temperatures of bainite start (Bs) and martensite start (Ms) of each alloy. Observations of the microstructures using an optical microscope and Scanning Electron Microscope (SEM) showed the formation of one or more phases in the alloys. These phases are ferrite as the matrix, degenerated pearlite (DP), lamellar pearlite (LP), and plate-like bainite. In Fe-Ni alloys with Ni content of 0.01 to 2.2 wt% Ni, after holding at either T1 and T2, a mixture of DP and LP in the ferrite matrix is more pronounced. Some plate-like bainite is gradually formed in the 3.3% Ni alloy after holding at T1 and T2. As the Ni content increases to as much as 4.5 wt% Ni, pearlite is no longer visible and is replaced by plate-like bainite in the ferrite matrix. These results indicate that the variation of Ni in Fe-Ni alloys with Ni content less than and equal to 4.5 wt% results in different shapes of bainite, which then affects the mechanical properties of the alloy.