The work here offers a comprehensive analysis of the effect of tempering temperature on the microstructure, hardness, and machinability of AISI D3 tool steel under dry end milling operations. The key findings, which are closely intertwined with each other, are summarized as below:
1. Microstructural evolution: The study confirmed that the microstructure significantly alters with tempering temperature. The as-received annealed structure, which was a soft ferritic matrix plus spheroidized carbides, was transformed by quenching into a hard, brittle martensite-retained austenite microstructure. Tempering resulted in decomposition of the structure at a later stage. Temperature-dependent behavior of carbides was one important observation: as the tempering temperature
increased from 450 °C to 650 °C, carbide particles underwent massive merging and aggregation. At 650 °C, this produced a homogeneous microdistribution of very fine particles within the tempered martensitic
matrix, a substantial influence on mechanical behavior and machinability.
2. Hardness and mechanical properties: The microstructural modifications were directly indicated in the hardness measurements. Hardness was substantially increased with quenching from 29.1 HRc in the asreceived state to a maximum of 61.2 HRc after tempering and quenching at 250 °C. As was anticipated, there was a tempering softening phenomenon with declining hardness to 48.3 HRc at the highest tempering temperature of 650 °C. This indicates a clear inverse correlation between the tempering temperature and hardness for the steel in question.
3. Elemental uniformity: EDS microanalysis confirmed high uniformity in the elemental composition under all conditions of heat treatment. The carbides (zone α) were chromium-rich in all instances, and the matrix (zone β) was primarily iron. This guarantees that the variation in
mechanical and machinability properties is due to morphological and distributional variations of the phases (martensite, austenite, carbides) and not because of any significant change in the overall chemical composition.
4. Chip formation as a diagnostic of machinability: Chip formation dynamics was a primary diagnostic tool, providing a visual bridge between microstructure and macroscopic machining behavior.
● As-received state: The ferrite matrix’s high ductility caused it to form continuous, extended chips, which created process control difficulty and surface damage.
● Quenched and tempered at 250 °C: The martensitic matrix with submicrocarbides, which were hard and brittle, caused unstable adiabatic shear banding. It created very segmented, saw-toothed chips with high dynamic loads and severe plastic deformation of tool wear, the worst machinability.
● Quenched & tempered 450 °C: This condition exhibited hybrid behavior, creating semi-continuous chips with serrated sharpness. While improved compared to the 250 °C state, the machinability was still in-between and not consistent due to severe abrasive and plastic deformation of tool wear.
● Quenched & tempered at 650 °C: This condition produced the optimal chip morphology: long, unbroken chips with minimal deformation. This stable plastic shear and easy breakage along the coarsened carbides exhibited low, easy removal of chips, and low wear on the tool.
5. Material machinability: The material was determined to be more difficult to machine at tempering temperatures of 250 °C and 450 °C. At 650 °C, the material was determined to be easier to machine with a hardness of 48.3 HRc. With a cutting speed (Vc) of 60 m/min, fz = 0.05 mm/tooth, ap = 8 mm, and ae = 0.3 mm, a very high milling time of t = 216 min was achieved. This means that increased tempering temperatures minimize machining difficulties by softening the material’s hardness and promoting favorable chip formation, as demonstrated by the strong correlation between lower hardness, stable chip morphology, and significantly longer tool life.