To study the effect of controlled cooling on microstructures and hardness of medium carbon steel and alloy steel forged components

Abstract

The forging is a well-established near and net shaping process, primarily used to manufacture the components which are subjected to high strength and fluctuating stress conditions. The forged components are usually heat-treated after the forging operations. The components are again heated to meet the heat treatment requirements. In this work ‘Green Process’ of controlled cooling is established as a substitute to isothermal annealing heat treatment as a post-forging process. The objective of this work was to establish post forging controlled cooling of hot forgings of AISI 8620H steel, AISI A387 grade 22 alloys steel and AISI 1045 steel. The components manufactured went through three different cooling rates such as: (A) fast cooling (3.5 °C/s to 3.7 °C/s) (B) medium cooling(1.7 °C/s to 1.9 °C/s) and (C) slow cooling (0.3 °C/s to 0.6 °C/s). Phase transformation was observed at different cooling rates. Microstructures and hardness tests were analyzed to understand the phase transformations the for each cooling condition. As a result of different cooling rates and material composition, the controlled cooling of forgings of AISI 8620H steel, AISI A387 grade 22 alloys steel, and AISI 1045 steel showed finer bainite at fast cooling rate, coarser bainite at medium cooling rate and ferrite and pearlite phases at slow cooling rates. Hardness of forgings was higher at higher (294 BHN to 320 BHN) cooling rate and lower (154 BHN to 167BHN) at slow cooling rate which is similar to annealed forgings. Machinability of slow cooled forgings was superior as compared to the forgings cooled at higher and medium cooling rate. Controlled cooling is a novel route to eliminate post forging heat treatment process. A significant amount of reduction in total processing time by 7 h as well as the direct savings in heating energy, manpower and corresponding equipment.