Heat Treatment and Property Control

Learning outcomes

The learner should be able to explain transformation control in steels, compare common thermal treatments, relate time-temperature history to hardness and toughness, recognise non-ferrous solution treatment and ageing, and predict NDT consequences of thermal processing.

5.1 Three variables, not one temperature

Heat treatment is a controlled combination of heating rate, temperature, holding time, atmosphere and cooling rate. Temperature alone never defines a treatment. Section size, prior structure, chemistry and furnace uniformity determine whether the intended transformation occurs through the whole component.

In steel, heating into an appropriate austenite region allows carbon and alloying elements to redistribute. Slow cooling permits diffusion-controlled products such as ferrite and pearlite. Faster cooling can produce bainite or martensite depending on hardenability and section cooling rate. Martensite forms by a diffusionless shear transformation and is usually hard, strong and comparatively brittle in the as-quenched state.

Time-temperature-transformation diagrams describe isothermal transformation after rapid transfer to a holding temperature. Continuous-cooling-transformation diagrams are more representative of many production cooling paths. Neither diagram is universal: composition, prior austenite grain size and austenitising practice matter.

5.2 Common treatments in steels

Full annealing commonly softens, improves machinability and reduces structural non-uniformity through slow furnace cooling. Normalising uses air cooling from an austenitising range and often produces finer, stronger structure than full annealing. Quenching uses sufficiently rapid cooling to obtain a hard structure; the surface usually cools faster than the core, creating stress and possible distortion or cracking.

Tempering reheats quenched steel below the lower critical transformation range to reduce brittleness and adjust hardness and toughness. A quenched-and-tempered condition is therefore not equivalent to as-quenched martensite. Stress relief reduces residual stress at a temperature chosen to limit major phase change, but may still alter strength or toughness in susceptible materials.

Spheroidising forms rounded carbide particles in a ferritic matrix and is useful for machinability or cold forming of high-carbon steels. Case hardening creates a hard surface with a tougher core by carburising, nitriding, carbonitriding, induction hardening or another qualified route. Depth and hardness profile must be specified and verified by the applicable procedure.

5.3 Non-ferrous treatments

Many aluminium, nickel, copper and titanium alloys gain strength by precipitation hardening. Solution treatment dissolves selected constituents at elevated temperature, rapid quenching retains a supersaturated solid solution, and natural or artificial ageing forms fine precipitates that impede dislocation motion. Over-ageing coarsens precipitates and usually reduces peak strength while sometimes improving dimensional stability or corrosion behaviour.

Cold-worked alloys may be annealed to recover ductility through recovery and recrystallisation. Some alloys cannot be strengthened by heat treatment and rely on solid-solution strengthening, work hardening or both. Temper designations are alloy-system specific and must be read from controlled material data.

5.4 Heat-affected zones and residual stress

Welding imposes a steep thermal cycle without fully melting the heat-affected zone. Different distances from the fusion boundary experience grain growth, phase transformation, tempering, sensitisation or precipitation changes. Cooling can form hard crack-susceptible regions in some steels. Conversely, welding can over-age or locally soften precipitation-hardened alloys.

Non-uniform expansion and contraction leave residual stress. High tensile residual stress can assist fatigue, stress-corrosion cracking or brittle fracture. Post-weld heat treatment may reduce stress and modify hardness, but incorrect heating can introduce new distortion, oxidation, cracking or property loss.

5.5 NDT connection and practical

Heat treatment can change permeability, electrical conductivity, ultrasonic attenuation and wave velocity. A hard case or decarburised surface can influence electromagnetic response. Quench cracks often start at stress raisers or section changes and may be surface breaking. Grinding after hardening can add thermal checks.

Practical: Compare a treatment chart, furnace record and hardness traverse for a sample component. Mark heating, hold and cooling stages. Identify which microstructural claims are supported and which require metallography or additional testing. Select NDT for surface cracking and internal quench damage. Stop if item identity or thermocouple traceability is missing.

Common errors and misconceptions

Several common errors recur in this topic: calling every furnace cycle annealing; assuming stress relief leaves all mechanical properties unchanged; treating hardness as a complete measure of toughness; applying a steel heat-treatment explanation to precipitation-hardened aluminium; and ignoring the different cooling rates of surface and core.

Chapter summary

Heat treatment controls microstructure through time, temperature and cooling history. Diffusional and diffusionless transformations produce different combinations of strength, hardness and toughness. Thermal gradients also create residual stress, distortion and cracking. NDT planning should consider the exact treatment and material family.