Grain Refinement and Residual Stress Removal — Two Different Treatments, Two Different Purposes
Normalizing and stress relief are both sub-critical heat treatments — both performed at temperatures below the austenitising range, both used as preparatory or conditioning treatments rather than as the primary hardening process. But they address different metallurgical problems and should not be confused with each other or used interchangeably.
Normalizing
Normalizing heats steel above the upper critical temperature (Ac3) — into the full austenite range — and then cools it in still air rather than quenching. The cycle produces a fine, uniform pearlitic or bainitic microstructure throughout the section, replacing the coarse or non-uniform grain structure that may be present in as-cast or as-forged material.
Why it is applied to castings: As-cast steel has a dendritic solidification structure with compositional segregation and a coarse, non-uniform grain size that varies through the section depending on local cooling rate during solidification. This structure has lower toughness and more variable mechanical properties than a normalised structure. Normalising eliminates the as-cast structure, producing a uniform grain size appropriate to the composition and a predictable mechanical property base. For structural steel castings — crusher frames, conveyor structural components, housing bodies — normalising is frequently specified as the standard delivery condition because it provides a reliable, reproducible property base without the higher cost of quench-and-temper treatment.
Why it is applied to forgings: Forging at elevated temperature produces grain refinement, but if the forging finishing temperature is too high or the cooling after forging is non-uniform, a mixed or coarse grain structure may result. Normalising after forging homogenises the grain structure and eliminates any localised coarsening from high-temperature forging. For forgings that will subsequently be quench-and-tempered to achieve specific mechanical properties, normalising is sometimes applied first to ensure a uniform starting structure that responds consistently to the quench-and-temper cycle.
Properties after normalising: Tensile strength and hardness are lower than quench-and-tempered equivalents but higher than annealed condition. Toughness is good. Machinability is generally better than as-cast or as-forged condition. For many structural castings and forgings where high hardness is not required, normalising to a specified tensile strength and hardness range is the appropriate and cost-effective heat treatment.
Stress Relief
Stress relief heats the component to a temperature well below the lower critical temperature (Ac1) — typically 550–650°C for carbon and low-alloy steels — holds at temperature, and cools slowly. The cycle does not change the microstructure or the bulk mechanical properties of the steel. Its purpose is to reduce the magnitude of residual stresses locked into the component from prior manufacturing operations — welding, casting, rough machining, or cold working — without affecting hardness or strength.
Residual stresses in castings: Large castings cool non-uniformly after solidification. The outer surfaces cool faster than the interior, creating a thermal gradient that introduces residual stress — typically compressive at the surface and tensile in the interior. These stresses are not visible and do not necessarily cause immediate problems, but they can cause dimensional instability during subsequent machining (the component distorts as material containing residual stress is removed) and can contribute to fatigue crack initiation in service. Stress relief before precision machining reduces this dimensional instability, improving the predictability of the machining outcome.
Residual stresses in weldments: Welding introduces high localised residual stresses at and near the weld heat-affected zone. Post-weld stress relief (PWSR) reduces these stresses, improving the fatigue performance of the welded joint and reducing the risk of stress corrosion cracking in aggressive environments. For welded structural components in mining applications — conveyor frames, crusher frames, structural supports — PWSR is frequently specified, particularly for components that will see cyclic loading in service.
Stress relief does not change hardness: A common misunderstanding is that stress relief softens hardened components. At the temperatures used for stress relief of carbon and low-alloy steels (below Ac1), the martensitic microstructure of a quench-and-tempered component is not significantly affected. Hardness loss from a correctly executed stress relief cycle is typically 0–2 HRC, which is within measurement uncertainty. Stress relief can be applied after quench-and-temper to reduce residual stresses from the quench without materially affecting the achieved properties.
Process Application and Records
Both normalising and stress relief require controlled heating to the specified temperature, adequate hold time for the section thickness, and controlled cooling. For stress relief in particular, heating and cooling rates are specified to prevent the introduction of new thermal stresses during the cycle — defeating the purpose of the treatment. Temperature uniformity within the furnace and verification that the component reaches the target temperature through its full section are the key process control parameters.
Time-temperature records for each cycle are retained as part of batch documentation. For components where normalising is a delivery condition requirement and mechanical property certification is required, test bars processed in the same batch are tested and results supplied with the component documentation.
For heat treatment specifications or to discuss normalising and stress relief requirements, contact our engineering team. See also: Heat Treatment overview · Quenching & Tempering.