A Critical Review on Heat Treatment Processes, Microstructural Evolution, and Mechanical Property Enhancement in Engineering Steels and Alloys

Heat treatment is one of the most fundamental metallurgical processes in mechanical engineering, profoundly influencing the microstructure, hardness, toughness, fatigue resistance, and properties of ferrous and non-ferrous alloys. This review paper systematically examines conventional heat treatment processes including annealing, normalising, hardening, tempering, and case hardening, along with advanced thermo-chemical treatments such as nitriding, carburising, and induction hardening. The relationship between process parameters (temperature, soaking time, cooling rate) and the resulting mechanical properties in medium-carbon steels, tool steels, stainless steels, and aluminium alloys is analysed. Phase transformations, Time-Temperature-Transformation (TTT) and Continuous-Cooling-Transformation (CCT) diagrams and their industrial significance are discussed. Recent developments in vacuum heat treatment, plasma nitriding, laser surface hardening, and cryogenic treatment are also reviewed.