Stress relieving
Residual-stress reduction, material-condition limits, machining sequence and verification of thermal and mechanical treatments.
Tags: processes, heat-treatment, residual-stress
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Uddeholm's Formvar stress-tempering guidance, revised August 19, 2024, calls for checking hardened-tool hardness and using a temperature 50°F below the lowest prior temper, a difference of 27.8°C. Confirm the prior thermal record and shape-specific cooling. Hardness checks and residual-stress measurements answer different acceptance questions. Formvar guidelines, one-page producer document
Thermal stress relief can reduce machining distortion or alter the stress state in service. Specify the required dimensions, material condition or stress evidence. Bodycote: stress relieving
What changes, and what should remain
Residual stress exists without an externally applied load. It is not equivalent to hardness. In recovery-type thermal treatments, internal defects rearrange and residual stresses decrease while strength and ductility can remain largely unchanged. Recrystallization instead produces new grains in previously cold-worked material and substantially changes its mechanical properties. Thus, stress relieving and recrystallization annealing are different objectives, even though both involve heating. ASM International, Heat Treating subject guide, ©2015, pp. 1–2
Tempering of hardened steel can also relieve stresses, but it deliberately establishes mechanical properties following hardening. A subsequent stress-relief operation must respect that previous treatment. The boundaries between these operations depend on material and condition; “below the transformation temperature” alone does not establish that an existing temper or cold-worked strength will be preserved. ASM guide, p. 2
For process selection, identify the alloy, product form, supplied condition, welding or cold-work history, previous heat treatments and final property requirements. Treat a missing prior tempering record as missing process information rather than selecting a temperature from a generic steel table.
Thermal relief and the machining sequence
Uddeholm explains that rough machining introduces thermal and mechanical stresses. At an elevated temperature, reduced yield strength permits stress redistribution through plastic deformation; movement during relief is therefore compatible with its intended function. Its tool-steel guide recommends rough machining, stress relieving and semi-finish machining before hardening, allowing resulting distortion to be corrected before the final stages. The recommendation concerns tool manufacture, not every alloy or finished assembly. Uddeholm, Heat treatment of tool steels, Edition 10, 04.2018, printed p. 6
The same guide identifies temperature gradients as a source of thermal stress and calls for slow cooling after its stress-relief treatment. A furnace setpoint by itself is consequently an incomplete instruction: the supplier needs the section geometry, load arrangement, heating and cooling requirements, and a defined basis for the hold. Removing material after treatment can still redistribute the remaining stresses. Uddeholm guide, printed pp. 6 and 11–12
Bodycote generally places stress relief after rough machining and before final grinding or polishing, and offers protective atmosphere or vacuum where oxidation matters. Its general guidance for already hardened and tempered parts uses a treatment temperature around 50°C below the previous tempering temperature to avoid affecting hardness. This is supplier guidance requiring material-specific confirmation, not a universal offset or permission to reheat any finished part. Bodycote: application and materials
Named example: stress tempering a Formvar tool
Uddeholm’s Formvar Hot Work Tool Steel Heat Treatment Guidelines, revised August 19, 2024, separately describes stress tempering hardened tools after EDM, welding with its identified QRO 90 or DIEVAR TIG rods, or during preventive maintenance. It requires checking tool hardness and specifies 50°F below the lowest tempering temperature. That temperature difference is approximately 27.8°C, not 50°C. Do not combine this instruction with Bodycote’s different general offset. Formvar guidelines, one-page producer document
The Formvar document also differentiates cooling for simple and complex shapes and warns that an unprotected atmosphere oxidizes the tool. It presents general product guidance rather than a warranty of specific properties or fitness. For an actual tool, agree the applicable revision, previous lowest temper, treatment purpose, surface condition and dimensional allowance with the heat treater. A hardness check establishes tool condition; it does not directly quantify the remaining residual stress. Formvar guidelines
Mechanical stress relief is a distinct route
Stress relief need not mean a furnace cycle. Los Alamos researchers measured rolled 7050 aluminum plate before and after stress relief by stretching. Their documented specimens were approximately 75 mm thick, with the stress-relieved condition identified as 7050-T7451 and stretching of 1.5–3% in the rolling direction. The laboratory reports about a tenfold reduction in residual stress in that comparison, while still measuring nonzero stresses afterward. LANL experiment summary, updated October 6, 2003; associated 2002 paper
The associated research explains why maintaining the favorable properties of these 7000-series plates constrains thermal relief and why remaining stresses can still distort machined components. This supports selecting the correct supplied product condition; it does not prescribe stretching a finished component or transferring the result to other alloys, sections or loading directions. Prime and Hill, Residual Stress, Stress Relief, and Inhomogeneity in Aluminum Plate, 2002, primary manuscript
Verification and an original calculation
Specify dimensional inspection and material-property checks separately from residual-stress measurement. ASTM E837-25e1, listed active and updated February 27, 2026, covers hole-drilling measurement of near-surface, in-plane stresses in isotropic, linearly elastic material. It cannot establish the deep interior stress state, and the hole causes localized damage. A result needs its location, direction, depth interval and method applicability. ASTM E837-25e1 scope
For another measurement route, ISO 21432:2019, Edition 1, December 2019, confirmed in 2025, addresses neutron diffraction in polycrystalline materials. Its scope includes sampled volume, measurement directions, stress-free references and uncertainty. Neither standard supplies a universal permissible residual-stress value for a component. ISO 21432 publication record and scope
Illustrative comparison, not a treatment prediction: assume comparable measurements give +240 MPa before treatment and +80 MPa afterward in the same specified direction, location and depth interval. The tensile-stress reduction is:
(240 − 80) / 240 × 100 = 66.7%
The remaining result is +80 MPa tensile, not zero. An assumed acceptance limit of +100 MPa would be satisfied by the nominal value, but an uncertainty or decision rule could change acceptance. The comparison says nothing about an unmeasured interior region, another direction, fatigue life or distortion after later machining. Unchanged hardness cannot substitute for this stress measurement.
Information to put on the order
Provide the material certificate and prior condition, the operation that created the stresses, the next manufacturing steps, and the dimensions or properties that must survive. Agree the treatment specification and revision, load and temperature-record requirements, oxidation protection, inspection stage and any residual-stress acceptance method. Record whether dimensional checks occur restrained or free, and leave an agreed correction allowance before final sizing. This turns “stress relieve” into a reviewable manufacturing operation without promising a distortion-free part.