From phase diagrams to process control: A unified CALPHAD-based decision framework for heat treatment design, microstructure prediction, and industrial quality assurance

Heat treatment manipulates metal microstructure through controlled thermal cycles, yet in industrial practice it is too often specified as a recipe—temperature, time, and quenchant—without explicit connection to transformation kinetics, section size constraints, or the verification protocols that distinguish success from latent failure. This review reframes heat treatment as a microstructure-first, evidence-bounded process-design discipline. The central contribution is HT-PASS, a Process Assurance Sheet linking alloy grade, prior condition, section size, thermal cycle, quench severity, tempering parameter, predicted microstructure, distortion risk, and verification standard into a single auditable document. Five supporting frameworks are introduced: Q-TEMPER, a three-state decision envelope balancing hardness, toughness, retained austenite, distortion, and cracking risk; the TTT/CCT Route Engine, connecting transformation diagrams to process selection with explicit cooling-rate constraints; HT-DEFECT, a root-cause ledger for quench cracking, soft spots, decarburization, distortion, and hydrogen embrittlement; the Heat Treatment Balance Sheet; and the Dynamic Microstructure-Property Optimization and Phase Transformation Index, a CALPHAD-driven digital twin parameter that unifies thermodynamic driving forces, diffusion fluxes, hardenability cooling rates, and grain refinement strengthening to determine optimal cycle termination. The review integrates thermodynamic and kinetic fundamentals from binary phase diagrams through multi-component CALPHAD, solid-state phase transformations, hardenability, surface engineering, process simulation, and non-ferrous heat treatment. Ten industrial case studies validated against published data demonstrate the framework deciding real engineering questions.