Optimization of Heat Treatment parameters to facilitate machining of SAE4340 steel without compromise on Mechanical properties

Abstract

SAE 4340 is a medium carbon low alloy steel used in many automobile and aircraft applications because of its high strength and toughness. But its machinability is very low and hence this poses difficulty in manufacturing the parts needed for such applications. Machinability of this material can be improved by adopting suitable tool material like CBN or ceramics. But these tool materials are costly and usually need high speed machines like CNC which are suitable mainly for mass production. Moreover, these inserts are brittle and chip off fast especially when intermittent cuts are involved especially in the rough machining of castings in large numbers. For this reason, it is proposed to improve machinability by adopting suitable heat treatment to the steel without considering the type of tool material being used. This will change the basic property and microstructure of the steel to facilitate machining. The inter-critical heat treatment process is suggested wherein the material is heated between the upper and lower critical temperatures followed by water quenching and suitable tempering. To begin with, the material was normalized to 850°C in order to carry out specimen preparation. The specimens were then subjected to quenching at two different temperatures of 770 and 790°C in the inter-critical range after which tempering was carried out at 580°C.  Tensile strength of around 1100 N/mm², impact strength of around 120J and hardness in the range of 35 to 40 HRC were obtained. Machinability tests were carried out on a centre lathe with lathe tool dynamometer set up using a brazed tip tool at low and high speeds giving a depth of cut of 1mm. The cutting forces were in the range of 60 to 70 kg force indicating good machinability. Thus without compromise in mechanical properties, good machinability was attained.