Stress Relieving
Also: stress relief, stress relieve, thermal stress relief, post-weld heat treatment, PWHT, stress relief anneal, subcritical anneal
Stress relieving heats a part below its transformation range and cools it slowly, so welds and machined parts stop moving, with little change in hardness.
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Welding, heavy machining, cold forming and flame cutting leave residual stress locked in a part. When material is later cut away, or the part warms up in service, that stress rebalances and the part bows, twists or cracks. Stress relieving heats the part to a temperature where the metal yields slightly under its own internal stress, typically 550-675 °C (1025-1250 °F) for carbon and alloy steel, holds it, commonly one hour per 25 mm (1") of thickness with a one-hour minimum, and cools it slowly and evenly so no new stress is introduced.
The temperature stays below the lower critical temperature, so the microstructure does not transform and there is no quench. For a quenched-and-tempered part it also stays below the last tempering temperature, commonly by about 30 °C (50 °F), so hardness is not lost. Stress relief after rough machining and before finish machining is the standard way to keep large plates, frames and long parts flat and in tolerance. Stress relief after welding, called post-weld heat treatment (PWHT), reduces movement in later machining, lowers the risk of cracking and stress corrosion, and is mandatory under pressure-vessel and piping codes above certain thicknesses.
Temperatures depend on the alloy. Tool steels are stress relieved at about 650-675 °C (1200-1250 °F) in the annealed state after rough machining, or below the tempering temperature once hardened. Austenitic stainless is a compromise: at or below about 425 °C (800 °F) only part of the stress is relieved, while 815-900 °C (1500-1650 °F) relieves most of it but can sensitize grades that are not low-carbon (L) or stabilized. Aluminum in heat-treated tempers can only be taken to about its aging temperature, so mill-stretched plate (T651, T7351) is used instead. Titanium is stress relieved at about 480-650 °C (900-1200 °F) in vacuum or argon, and gray iron castings at about 540-595 °C (1000-1100 °F).
At a glance
| Typical tolerances | Hardness is essentially unchanged when the cycle stays below the tempering or aging temperature: expect no more than about 1-2 HRC loss on a quenched-and-tempered part and a few points Brinell on annealed or normalized steel. There is no case depth. The part can move during the cycle itself, because the stress it releases shows up as distortion, sometimes by more than the finished tolerance, so leave finish stock and machine critical features afterwards. Done in the right place in the routing, stress relief lets finish machining hold flatness and position that the unrelieved part would lose as soon as it came off the fixture. |
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| Size limits | Car-bottom and box furnaces at commercial heat treaters take weldments from bench size to several meters long and loads of tens of tonnes at the larger shops. The largest weldments and pressure vessels are stress relieved on site with electric resistance blankets or temporary furnaces, and local PWHT of pipe welds is done with heating bands. Vacuum furnaces handle precision parts and titanium within about 600 x 600 x 900 mm (24 x 24 x 36") to 900 x 900 x 1,200 mm (36 x 36 x 48"). |
| Surface finish | In air or a lightly protected car-bottom furnace, steel comes out with a thin, tight oxide, far less than the scale from hardening temperatures, which is usually blasted off before paint. Vacuum stress relief leaves parts bright, which matters for finished machined parts and for titanium. Stainless may show heat tint that is removed by pickling or passivation. |
| Lead time | Commercial stress relief in 2-5 business days. Large weldments are scheduled around car-bottom furnace loads and may take 1-2 weeks. Code PWHT adds time for thermocouple placement, charts and records; on-site PWHT is scheduled with the crew. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon and structural steels (A36, A572, 1018) in weldments and flame-cut plate; alloy steels (4140, 4340) after rough machining, below their tempering temperature if already hardened; tool steels (A2, D2, H13) between roughing and hardening; austenitic stainless (304, 316) weldments, preferably in the L grades; titanium and nickel alloys (Ti-6Al-4V, Inconel 718) after welding, heavy machining or 3D printing; and cast iron and steel castings before machining. Heat-treated aluminum (6061-T6, 7075-T6) and PH stainless lose strength if taken above their aging temperature, so they get only a low-temperature relief or none.
What drives the cost
- Size and weight: car-bottom furnace space is priced by the load, and heavy weldments need long heat-up and cool-down
- Section thickness: hold time scales with the thickest section, commonly one hour per 25 mm (1")
- Controlled heating and cooling rates required by code, which keep the furnace tied up
- Atmosphere: vacuum or argon for finished parts and titanium costs more than air
- Fixturing to support long or thin parts so they do not sag at temperature
- Code documentation: thermocouple placement, time-temperature charts and certification
When to use it
- After rough machining and before finish machining, on large plates, frames, long shafts and thin-walled parts held to tight flatness or position
- After welding, on weldments that will be machined or must hold alignment
- When a code or customer specification requires post-weld heat treatment
- Before hardening tool steel parts with heavy material removal, to reduce distortion and cracking in the quench
- On metal 3D-printed parts before they are cut from the build plate
When not to
- Parts that are quenched and tempered after machining anyway: the hardening cycle removes roughing stress
- Heat-treated aluminum above its aging temperature, or cold-worked parts whose strength comes from the cold work
- Non-L austenitic stainless held in the 425-815 °C (800-1500 °F) range, where it sensitizes and loses intergranular corrosion resistance
- Small, stiff parts with light material removal, where residual stress will not move the part measurably
Design tips
- Put the stress relief in the routing on the drawing or PO: "stress relieve after rough machining" or "PWHT per the applicable code", with the temperature range if it matters.
- Leave finish stock, often 1-3 mm (0.040-0.120") per side, on critical surfaces of weldments and heavily machined plates before stress relief.
- On hardened parts, state the tempering temperature or ask for stress relief at least 30 °C (50 °F) below it.
- Specify L grades (304L, 316L) for stainless weldments that will be stress relieved at high temperature.
- Ask for the part to be supported in the furnace and cooled at a controlled rate, so the relief does not put new stress in.
- For aluminum, buy stress-relieved plate (T651, T7351) and machine symmetrically rather than relying on thermal stress relief.
Frequently asked questions
- What temperature is stress relieving done at?
- For carbon and low-alloy steel, typically 550-675 °C (1025-1250 °F), held about one hour per 25 mm (1") of thickness. Hardened steel is stress relieved below its tempering temperature. Titanium runs about 480-650 °C (900-1200 °F), gray iron 540-595 °C (1000-1100 °F), and heat-treated aluminum only up to about its aging temperature.
- Does stress relieving reduce hardness?
- Not noticeably if it stays below the last tempering or aging temperature, which is the rule. Stress relieving a hardened part above its tempering temperature is really a second temper and will lower hardness.
- Should I stress relieve before finish machining?
- Yes, for large plates, weldments, long shafts and parts where heavy pocketing removes much of the material. Rough machine, stress relieve, then finish machine, so the stress comes out before the final cuts rather than after them.
- Do welded parts need stress relieving?
- Not always. Pressure vessels and piping need post-weld heat treatment above the thickness limits in their codes (ASME Section VIII, ASME B31.3), and weldments that will be precision machined benefit from it. Light structural weldments usually do not need it; AWS D1.1 gives the temperatures and hold times when it is specified.
