Causes of Distortion and Residual Stresses in Heat Treatment19 July 2016
The principal objective of this discussion is to determine the causes of distortion and residual stresses in heat treatment. That’s something of a mouthful to read out, but these attribute-skewing stress factors must be accounted for if an alloy-strengthened component is to retain dimensional and mechanical viability during its passage through a material-torturing heat treatment cycle.
Determining Causal Factors
Distortion on an alloy-hardened scale is caused by tensile and elastic deformation. A thermally active segment of the operation injects stress into one part of the object while releasing compressive tension on another section. A stress gradient forms as these competing forces fight for dominance. Stress relieving techniques neutralize such adverse effects, but, again, these methods clash with the inherited properties of the metal part as it expands and contracts.
Accounting for Microcrystalline Variations
Passage through a machining and forming station breeds uneven mechanical forces, but the shop eliminates such negative events by employing a series of stress relieving methodologies. Conversely, thermal duress occurs when the metal reaches its phase transformation point. The distortion is induced by non-uniform heating, perhaps due to a poorly configured furnace, but the structural properties of the metal also play a role. Impurities cause distortions, as do the disparate elements that form the alloy. Even the volume and geometrical complexity of the worked product influence this distortion quotient, with outer material surfaces cooling faster than the internal volume.
Heat Treatment Distortion is Unavoidable
When these structure-weakening effects are left untended, cracks may develop over time. The product is potentially dimensionally out-of-tolerance, mechanically substandard, and not finished. A diligent shop offsets distortion and residual stresses in heat treatment processing by knowing how these competing forces are generated. The facility then uses post heat treating technology to refine the part and prevent these stress-induced forces from being locked inside the product. Again, and this is worth repeating, slight effects cannot be avoided when the part is subjected to such phase transformative temperature extremes. For example, even the phase conversion of an alloy’s austenite state to its martensite form incurs volumetric change due to the fact that the latter alloy form is incrementally larger than the initial phase-transforming form.
Preheat treatment and thermal uniformity throughout the furnace does reduce material stress, but the superior option is to always incorporate a strong finishing station, a section of the facility dedicated to freeing these residual forces and straightening the component until it conforms to any and all designated design specs.
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