Integrated experimental modeling, optimization and microstructural correlation of heat treated AISI 4340 steel machinability during dry turning

This study investigates the dry turning machinability of heat treated AISI 4340 steel by correlating heat treatment condition, hardness, microstructure, tool life, surface roughness and tool wear behavior. Specimens were annealed, normalized and SAE 40 oil quenched after austenitizing at 900, 925 and 950°C for 1, 1.5 and 2 h. Turning experiments were carried out using a Taguchi L27 orthogonal array by varying heat treatment temperature, soaking time, cutting speed, feed rate and depth of cut. The responses were analyzed using ANOVA, regression modeling, response surface multi response optimization and confirmation assessment. Results showed that annealed specimens resulted in the highest tool life due to their softer ferrite-pearlite matrix, whereas oil-quenched specimens showed the lowest tool life because of hard martensitic phase. Heat treatment temperature was the dominant factor for tool life, while feed rate was the dominant factor for surface roughness (Ra). Linear models predicted tool life reliably, whereas surface roughness required interaction or quadratic consideration because of chip formation, built up edge tendency and tool wear evolution. SEM observations of the heat treated microstructures and the worn tool analysis support the heat treatment, microstructure, machinability relationship. The study provides a practical framework for selecting heat treatment and turning parameters for improved machinability of high hardenability AISI 4340 steel.