Skip to main content

Thermal Stress Relief: Temperature Ranges, Soak Times, and Applicable Parts

Thermal stress relief is a sub-critical heat treatment that reduces residual stresses in steel parts without changing microstructure or hardness. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. It is applied to weldments, machined components, cold-formed parts, castings, and any fabricated structure whose service life or dimensional stability would be degraded by the residual stresses left behind by the forming, welding, or machining processes that produced it. This guide covers the temperature ranges, soak time rules, cooling requirements, and part types applicable to thermal stress relief — plus the limits of what it can and cannot accomplish.

What does thermal stress relief actually do to a steel part?​

Thermal stress relief reduces residual stresses present in a steel part by heating the part to a sub-critical temperature (typically 1,000–1,150 °F / 540–620 °C for carbon and low-alloy steels — below the A1 transformation line but above roughly 900 °F, where steel's yield strength drops enough to allow micro-yielding at stress concentration points) and holding at that temperature long enough for creep-driven stress redistribution to occur throughout the part. The mechanism is time-dependent plastic deformation at elevated temperature: regions of the part carrying high residual tensile stress exceed the reduced yield strength at soak temperature and locally yield, while regions in compression accommodate the redistribution. The net effect is a reduction of residual stress magnitude by roughly 70–85% across the cross-section, with the remaining 15–30% representing residual stresses too small to drive continued creep at the soak temperature. Critically, the process does not transform the steel's microstructure — the iron-carbon phases present before the cycle (ferrite, pearlite, martensite, bainite, or any mixture) remain essentially unchanged, and hardness is reduced by only a small amount (typically 5–15 HB for a well-controlled stress relief of quench-and-tempered steel). The steel keeps its as-processed microstructure and mechanical properties; only the residual stress state changes (ASM Handbook, Vol. 4A, ASM International, 2013; Totten, Steel Heat Treatment Handbook, 2nd ed., CRC Press, 2006).

What temperature range is used for stress relief of carbon and low-alloy steel?​

The standard temperature range for stress relief of carbon and low-alloy steel is 1,000–1,150 °F (540–620 °C). The specific temperature chosen within this range depends on the steel grade, prior processing, and the degree of stress reduction required. For annealed or normalized carbon steel (1018, 1020, 1045, A36), 1,100–1,150 °F produces effective stress relief without affecting the pearlitic-ferritic microstructure. For quench-and-tempered alloy steel (4140, 4340 in the heat-treated condition), the stress relief temperature must stay below the original tempering temperature to avoid further softening of the quenched-and-tempered hardness; a common rule is to stress-relieve at least 50 °F below the original tempering temperature. For cold-drawn bar stock or cold-formed parts that were not heat-treated after forming, 1,050–1,100 °F is typical. For parts that require aggressive stress reduction — large heavy-section weldments, critical pressure boundary components — temperatures at the high end of the range (1,150 °F) maximize stress reduction but risk modest creep elongation of the part itself. Published cycles by grade and prior condition appear in ASM Handbook Vol. 4A and in the ASM Heat Treater's Guide (ASM International, 2nd ed., 1995). UTEC Industrial's car-bottom furnace operates throughout this range with the programmable ramp-and-soak control needed to document compliance with code-specified PWHT cycles such as those in ASME Section VIII Div 1 and AWS D1.1.

How long should a part soak at stress relief temperature?​

The standard industry soak rule for thermal stress relief is one hour per inch of section thickness, with a minimum soak of one hour regardless of section size. This provides adequate time for thermal equilibration across the part and for the creep-driven stress redistribution to reach a practical completion. For a 2-inch-thick weldment, the specification is typically: ramp to 1,100 °F at a rate not exceeding 400 °F per hour (to avoid inducing thermal gradient stresses that the stress relief is meant to eliminate), soak 2 hours at temperature, then cool at a rate not exceeding 400 °F per hour to below 600 °F before removing the part to still air. Large heavy-section weldments may require extended soaks (3–6 hours for 6-inch-plus sections) to ensure thermal equilibration and full stress redistribution — the governing rule is that the thickest section controls the soak time. Code-specified PWHT cycles for pressure vessels and structural welding frequently stipulate both minimum soak times and minimum total cycle times above a threshold temperature (typically 600 °F above ambient) — the code requirements generally align with but sometimes exceed the one-hour-per-inch guideline (ASME Section VIII Div 1, UW-40; AWS D1.1, Clause 5.8).

Why does heating and cooling rate matter for stress relief?​

The ramp-up and cool-down rates matter for stress relief because thermal gradients during ramp and cool themselves induce residual stress — the opposite of what stress relief is meant to accomplish. If the outer surface of a thick part heats faster than the core, the outer layer wants to expand while the core remains cool and resists — producing temporary compressive stress at the surface and tensile stress at the core. These transient stresses can exceed the yield strength of the hot outer layer, producing new residual stress as the part cools back to uniform temperature. The standard limit — ramp and cool rates not exceeding 400 °F per hour — is intended to keep through-thickness temperature differentials small enough that transient stresses stay below the yield strength at each location. For very heavy sections (6+ inches thick) or complex geometries, even slower rates (100–200 °F per hour) are often specified by code or by the engineering design authority. For thinner sections (under 1 inch), faster rates (up to 800 °F per hour) are sometimes acceptable because thermal gradients don't develop significantly. Uncontrolled rates — taking a hot part out of the furnace into still air, for example — can introduce more residual stress than the cycle removed, defeating the purpose entirely. UTEC's programmable ramp-and-soak control records the actual temperature profile through the cycle, producing the furnace chart that documents rate compliance for code-specified PWHT work (ASM Handbook, Vol. 4A, ASM International, 2013; ASME Section VIII Div 1).

What types of parts benefit from thermal stress relief?​

Thermal stress relief is specified for part types where residual stress affects service life, dimensional stability, or code compliance. The most common categories: welded fabrications — any steel structure assembled by welding carries residual stress from the weld heat cycle; thick-section weldments (pressure vessels, structural steel assemblies, machine bases, crane bridges) almost always receive stress relief before service. Machined parts with tight tolerances — precision shafts, hydraulic cylinder barrels, machine spindles, and components where in-service dimensional change would cause functional problems benefit from stress relief between rough and finish machining or after final machining. Castings — steel castings cool non-uniformly in the mold, producing substantial residual stresses; stress relief at 1,050–1,150 °F is standard before machining or service. Cold-formed and cold-drawn parts — severe cold forming (deep drawing, cold heading, tube drawing) introduces plastic deformation residual stress that can cause stress-corrosion cracking or distortion over time. Heat-treated parts — quench-and-temper processing leaves a stress state even after tempering; a second stress relief cycle at a lower temperature than the tempering can improve dimensional stability without reducing hardness further. Pressure vessels and piping — code-required by ASME Section VIII Div 1 for specified material-thickness combinations. Thermally cut parts — oxy-fuel or plasma cut edges have hardened, stressed heat-affected zones that benefit from stress relief before machining or service (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).

What are the limits of thermal stress relief?​

Thermal stress relief is not universal. It has important limits. First, it cannot fully eliminate residual stress — the 70–85% reduction is approximate, and parts returning to service always retain some residual stress. For critical applications (fatigue-limited parts), additional treatments (shot peening, post-stress-relief machining) may be required. Second, it does not improve microstructural problems — a weldment with hard, cracked heat-affected zones cannot be "fixed" by stress relief; it needs normalization or full annealing to transform the microstructure. Third, it cannot exceed the temper temperature of previously quenched-and-tempered steel without softening the part. A 4140 part quench-and-tempered to 30 HRC (originally tempered at 1,050 °F) cannot be stress-relieved at 1,100 °F without risking hardness loss. Fourth, it is dimension-sensitive — large parts can experience creep-driven dimensional changes during the soak; pre-machined surfaces, bores, and tight tolerance features may shift by 0.001–0.005 inch over a multi-hour soak. Fifth, it does not restore material that has been damaged by cold working or that contains cracks — pre-existing defects are not healed by the thermal cycle. Finally, stress relief of assemblies containing dissimilar metals, bearings, seals, or pre-machined tight-tolerance features may damage those components — for such cases, vibratory stress relief (VSR) is a common alternative that operates at room temperature. Understanding these limits is essential to specifying stress relief only where it delivers value and not as a default "safety" operation (ASM Handbook, Vol. 4A, ASM International, 2013).

How is a stress relief cycle documented?​

Code-specified stress relief (PWHT under ASME Section VIII Div 1 or AWS D1.1, for example) requires documentation that includes: the furnace identification and its most recent temperature uniformity survey per AMS 2750; the thermocouple placement diagram showing where temperature sensors were located on the load; the actual temperature-versus-time record (furnace chart or data logger output) showing the ramp rate, soak temperature, soak duration, and cool-down rate; the part identification traceable to the heat of steel used; and the inspector's sign-off confirming the cycle met the code-specified parameters. A typical PWHT documentation package contains a cover sheet summarizing the cycle, the raw furnace chart (or digital equivalent), and signed attestations from the heat treater and customer quality inspector. For non-code work, documentation requirements are customer-specific but typically include the temperature record, cycle type, part ID, and equipment ID. UTEC ships every stress relief job with a documentation package recording these elements — the same standard used for code-compliant PWHT work is applied to non-code jobs by default, because buyers who need it later cannot reconstruct it from memory (ASME Section VIII Div 1; AWS D1.1; AMS 2750).

Related Articles

References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASM International. (1995). Heat Treater's Guide: Practices and Procedures for Irons and Steels (2nd ed.). ASM International.
  • Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.
  • Machinery's Handbook (31st ed.). (2020). Industrial Press.
  • ASME Boiler and Pressure Vessel Code, Section VIII Division 1 (current edition). American Society of Mechanical Engineers.
  • AWS D1.1: Structural Welding Code — Steel (current edition). American Welding Society.
  • AMS 2750: Pyrometry. SAE Aerospace.

Need In-House Heat Treating for Heavy Industrial Parts?​

UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.

Request a Quote →

Questions? Call (509) 922-1832 or email sales@utec.co

Last updated on