Heat treatment is a process that alters the internal structure of metallic materials through heating, holding, and cooling to achieve desired properties. During this process, due to uneven temperature changes, structural transformations, and constraints within the material, heat treatment stress is generated, which significantly impacts material performance, dimensional stability, and subsequent processing.
Heat treatment stress refers to internal stress generated by uneven thermal expansion/contraction, uncoordinated volume changes during structural transformations, and external constraints (e.g., molds, fixtures) or internal constraints (e.g., property differences in different regions) within the material.
Its essence is the mutual force between atoms or grains when atomic arrangement or macroscopic volume changes are hindered, manifesting as a tendency toward elastic or plastic deformation.
The generation of heat treatment stress is primarily related to two core processes:
When materials are heated or cooled, uneven temperature changes in different parts of the workpiece (e.g., surface vs. core, thin vs. thick walls) lead to varying degrees of volume expansion or contraction:
- Heating phase: The surface heats up and expands first, while the core remains cooler with slower expansion. The surface is constrained by the core, generating compressive stress, and the core is stretched by the surface, generating tensile stress.
- Cooling phase: The surface cools and contracts first, while the core remains hotter with slower contraction. The surface is constrained by the core, generating tensile stress, and the core is compressed by the surface, generating compressive stress.
Faster cooling rates (e.g., quenching) create larger temperature gradients, intensifying thermal stress.
During solid-state phase transformations (e.g., austenite to martensite or pearlite), different structures have varying specific volumes (e.g., martensite has a larger specific volume than austenite). Asynchronous phase changes across the workpiece generate structural stress:
- For example, during quenching, the surface first undergoes austenite→martensite transformation (volume expansion), while the core remains austenitic. The surface expansion is constrained by the core, generating compressive stress. When the core later expands during phase transformation, the surface—already transformed and possibly hardened—constrains the core, leading to tensile stress in the core and additional tensile stress in the surface.
Larger differences in transformation rate and extent (e.g., concentrated martensite formation on the surface during quenching) increase structural stress.
- External constraints: Fixation by clamps or contact with molds restricts free expansion/contraction, exacerbating stress.
- Internal constraints: Complex workpiece structures (e.g., grooves, sharp corners) or uneven material composition cause property differences between regions, amplifying stress concentration.
Based on the generation stage and existence state, heat treatment stress is categorized into three types:
Stress dynamically present during heating, holding, or cooling, changing with temperature or phase transformation. Examples include:
- Thermal stress from temperature gradients during heating;
- Instantaneous structural stress from volume changes during cooling phase transformations.
If transient stress exceeds the material’s yield strength at that temperature, plastic deformation occurs; exceeding fracture strength leads to immediate cracking (e.g., quenching cracks).
Stress remaining in the workpiece after cooling to room temperature, the residual stress after partial release (e.g., plastic deformation) of transient stress. Its distribution depends on heat treatment processes:
- Quenched workpieces typically have residual compressive stress in the surface (due to martensite expansion constrained by the core) and possible residual tensile stress in the core;
- Annealing or tempering reduces residual stress, but improper processes may cause new stress accumulation.
- Thermal stress: Stress from uneven thermal expansion/contraction alone, unrelated to structural transformation (e.g., in pure metals or non-transforming alloys).
- Structural stress: Stress from volume changes during phase transformations alone, unrelated to temperature gradients (e.g., phase transformation stress under ideal uniform temperature).
In practice, thermal and structural stress often coexist, collectively forming heat treatment stress.
Heat treatment stress (especially residual stress) has multiple impacts on material performance, processing, and application, with both adverse and beneficial effects when regulated.
- Residual stress exceeding the material’s yield strength causes plastic deformation (e.g., bending, warping, dimensional deviations);
- Excessive residual stress (especially surface tensile stress) may directly lead to cracking (e.g., "delayed cracking" if tempering is delayed after quenching).
Example: High-carbon steel may crack along grain boundaries if not tempered after quenching due to surface tensile stress.
- Residual stress gradually releases during subsequent processing or use (e.g., cutting, welding, temperature changes), causing secondary deformation and affecting precision parts (e.g., bearings, molds).
Example: Unrelieved residual stress in precision gears may cause tooth profile deviations after long-term use due to stress release.
- Residual tensile stress reduces fatigue strength (cracks easily initiate at stress concentration points under cyclic loading);
- Excessive internal stress may increase brittleness and reduce impact toughness.
- Uneven residual stress distribution causes inconsistent deformation during cutting (e.g., warping of thin-walled parts after machining);
- Stress concentration regions may develop grinding cracks during grinding or polishing.
Not all residual stress is adverse; proper processes can utilize it to improve performance:
- Surface residual compressive stress enhances fatigue strength (e.g., carburized and quenched gears with surface compressive stress have longer service life);
- Prestressing (e.g., retaining appropriate compressive stress in springs after quenching and tempering) improves deformation resistance.
To mitigate adverse effects, stress generation must be controlled through process optimization, and residual stress eliminated via subsequent treatments:
- Control heating/cooling rates: Use stepwise heating (slow temperature rise) or graded cooling (e.g., isothermal quenching) to reduce temperature gradients;
- Optimize workpiece structure: Avoid sharp corners or uneven wall thickness to minimize stress concentration;
- Select appropriate media: Use oil cooling (slower than water) during quenching to reduce thermal stress;
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