Guide
When does stress relieving steel belong on the drawing, and where in the routing?
Stress relieving steel is a subcritical heat treatment: the part is heated to a temperature below the point where the microstructure transforms, held long enough for locked-in stresses to relax as the steel’s yield strength drops at temperature, and cooled slowly so that new thermal stresses are not introduced. Unlike annealing, normalizing or quench-and-temper, it is not meant to change the microstructure or set the hardness; its job is to make the part stop moving, although it does give back some of the strength of cold drawn bar.
By Saola Manufacturing engineering team · Published · Updated
This guide is for the engineer who has already watched a long shaft bow after a keyway was milled, or a welded base plate rock on the surface plate after the pads were faced, and now has to decide whether a stress relief step goes on the drawing, at which point in the routing it goes, and how to word it so an overseas supplier does the same thing every lot. Why cold drawn bar moves is covered in the hot rolled vs cold rolled steel guide, and the aluminum equivalent, stress relief by stretching, in the 6061-T6 vs T651 guide. This page is about writing the requirement.
On this page
Which steel parts actually need a stress relief step?
Add stress relief when a part carries significant residual stress and you are about to remove material unevenly or hold a tight form tolerance over a long length. Welded assemblies that are machined after welding, cold worked bar with heavy one-sided cuts, and flame-cut or heavily straightened plate are the usual cases.
Residual stress comes from three sources on a typical machined steel part: cold work at the mill (drawing, straightening, roller leveling), uneven cooling (welding, flame cutting, quenching), and the machining itself (heavy roughing passes, grinding). It only becomes a dimensional problem when the balance is disturbed, so the useful question is not "is there stress?" but "will this routing release it asymmetrically after the critical surfaces are cut?" Work through four screening questions:
- Is there a weld, and are machined features located off the welded structure? A fabricated base, frame or bracket that gets bored, faced or drilled to tight positional tolerances after welding is the strongest single case for stress relief. Weld shrinkage leaves stresses close to the yield strength of the steel near the joint, and facing a pad can let the whole frame relax into a new shape.
- Is the material removal asymmetric? A keyway, a flat, a deep pocket on one face or a part that is mostly hollowed out from one side releases stress on one side only. Concentric turning of round bar rarely needs it.
- Is the part long or thin relative to its section? Slender shafts, rails, guide bars and thin plates amplify a small stress imbalance into a large straightness or flatness error.
- Is the form tolerance tight relative to the part size? A straightness, flatness or position callout that would be hard to hold even on stress-free stock leaves no room for movement.
If two or more answers are yes, plan for stress relief or for a stress-relieved starting material. If none are, adding the step only adds a furnace cycle, scale and cost without improving the part.
What temperature, hold time and cooling does stress relieving steel need?
Plain carbon and low-alloy steels are typically held well below the lower critical temperature, long enough to soak through the thickest section, then cooled slowly in the furnace. Hardened parts are the exception: they must stay below their last tempering temperature or they lose hardness.
For welded pressure-retaining parts the numbers are not a matter of preference. Per ASME Section VIII Division 1, the postweld heat treatment for carbon steel (P-Number 1 materials) sets a minimum holding temperature, a hold time that scales with the nominal thickness as the code defines it, and maximum heating and cooling rates above 800 °F (425 °C); the table below summarizes those requirements, and the code itself governs. Outside code work, industry guidance for stress relieving machined carbon and low-alloy parts uses the same logic with a little more latitude. Aerospace and defense drawings often reference SAE AMS2759/11, the aerospace specification for stress relief of steel parts, with furnace control under SAE AMS2750 pyrometry; commercial drawings can borrow the structure of those documents even where the full specification is overkill.
Two cautions on the table. The temperatures are metal temperatures, not furnace setpoints, so a thick section needs a soak long enough for the core to arrive. And slow cooling matters as much as the hold: pulling a part out hot and letting one side cool on a cold floor reintroduces the stress you just paid to remove.
| Situation | Metal temperature | Hold | Heating and cooling |
|---|---|---|---|
| Welded carbon steel, P-Number 1, code work | 1100 °F (595 °C) minimum | Up to 2 in nominal thickness: 1 h per in, 15 min minimum; over 2 in: 2 h plus 15 min per additional in | Above 800 °F (425 °C): heat no faster than 400 °F/h divided by thickness in inches (400 °F/h maximum); cool in a closed furnace no faster than 500 °F/h divided by thickness (500 °F/h maximum) |
| Carbon and low-alloy steel, annealed or normalized, before finish machining | 1000–1250 °F (540–675 °C) | About 1 h per in of section, 1 h minimum | Furnace cool to about 600 °F (315 °C), then still air |
| Quenched and tempered alloy steel (for example 4140 prehard) | About 50 °F (28 °C) below the last tempering temperature | About 1 h per in of section | Slow cool; recheck hardness afterward |
| Part to be nitrided after finish machining | Above the planned nitriding temperature, below the tempering temperature | About 1 h per in of section | Slow cool; finish machine after this step |
| Cold drawn bar bought stress relieved | Done at the mill | Per the bar specification | Name the condition on the purchase order |
Where in the routing should the stress relief step go?
Put it after the operations that create or expose the stress and before the operations that create the toleranced surfaces: weld, rough machine, stress relieve, then finish machine.
Stress relieving a finished part defeats the purpose, because the part distorts in the furnace and there is no stock left to correct it. The routings that work look like this:
- Weldment: weld, stress relieve, clean scale, rough and finish machine. If the weldment is large and the machined features are heavy, a second, lighter relief after roughing is sometimes justified.
- Long shaft from cold drawn bar with a keyway: rough turn and rough mill the keyway leaving finish stock, stress relieve, straighten only if the drawing allows it, finish turn and finish mill. Or buy stress-relieved bar and skip the furnace.
- Plate part with deep one-sided pockets: rough both faces and the pockets, unclamp, stress relieve, re-establish datums by skimming the primary face, then finish.
- Hardened part: rough, harden and temper, stress relieve below the tempering temperature if grinding or heavy finish cuts follow, then finish grind.
Leave enough finish stock to absorb the movement. How much depends on the part, which is why a first-article run that measures straightness or flatness both before and after the furnace is worth more than any rule of thumb: it shows whether the relief step is doing anything and whether the finish allowance is adequate. Straightening after stress relief puts cold work back into the part; if the part must not be straightened, say so on the drawing.
Do not confuse a stress relief with the hydrogen embrittlement bake that follows electroplating on hardened steel. The bake is a step after plating that drives out hydrogen; the stress relief is done before plating, and the furnace stress relief described on this page runs far hotter than any bake. High-strength parts that have been heavily machined or ground may also need a pre-plating stress relief and a post-plating bake: ASTM B849 covers the pre-coating stress relief while ASTM B850 covers the post-coating bake. The plating side is explained in the hydrogen embrittlement baking guide.
How should the drawing and purchase order call out stress relief?
State the condition, the position in the routing, the temperature window or governing specification, and the evidence you want back. "Stress relieve" alone is not a requirement a supplier can be held to.
A note that only says "STRESS RELIEVE" lets the shop choose the temperature, the hold, the cooling and the point in the sequence, and the shop will reasonably choose whatever is cheapest, which is often the end of the routing. Write the note so that each of those choices is fixed. Examples that work:
- Weldment, non-code: "STRESS RELIEVE WELDMENT AFTER WELDING AND BEFORE MACHINING; HOLD AT 1100–1200 °F, 1 HR PER INCH OF THICKEST SECTION; FURNACE COOL; SUPPLY FURNACE CHART," where the figures are typical industry guidance for carbon steel and should be adjusted for your grade.
- Pressure-retaining weldment: "POSTWELD HEAT TREAT PER ASME BPVC SECTION VIII DIV 1, UCS-56," which pulls in the code temperature, hold and rate limits without restating them.
- Machined shaft: "STRESS RELIEVE AFTER ROUGH MACHINING. DO NOT STRAIGHTEN AFTER STRESS RELIEF. STRAIGHTNESS PER FEATURE CONTROL FRAME APPLIES AFTER FINISH MACHINING."
- Hardened part: "STRESS RELIEVE AFTER ROUGH GRIND AT A TEMPERATURE BELOW FINAL TEMPER. FINAL HARDNESS PER NOTE 3 TO BE VERIFIED AFTER STRESS RELIEF."
- Material callout instead of a furnace step: "MATERIAL: COLD DRAWN, STRESS RELIEVED BAR," with the grade and bar specification. ASTM A311/A311M covers cold-drawn, stress-relieved carbon steel bars with mechanical property requirements, which gives the purchase order something to cite.
Keep the geometric requirement separate from the process note. The straightness or flatness tolerance belongs in a feature control frame that applies to the finished part, so inspection measures the result rather than checking that a furnace ran. The process note tells the supplier how you expect them to get there; the tolerance tells them what they will be accepted against.
What are the common mistakes with stress relief requirements?
The expensive mistakes are putting the step at the end of the routing, exceeding the tempering temperature on a hardened part, and treating stress relief as a substitute for a stable starting material or a balanced machining sequence.
- Stress relief after finish machining. The part moves in the furnace and there is nothing left to correct. Scale and discoloration land on finished surfaces as well.
- Too hot on quenched and tempered steel. Exceeding the last tempering temperature lowers hardness and strength. If a prehardened bar is stress relieved, the hardness on the drawing must be checked after the furnace, not taken from the mill certificate. The mill test report guide explains what the certificate does and does not tell you.
- Confusing it with annealing or normalizing. Both go above the transformation range and change the microstructure and properties. If you want softer or more uniform steel, call for that treatment by name; if you only want stability, call for stress relief.
- Relying on vibratory stress relief where a code requires thermal treatment. Vibratory methods are used on some large fabrications, but they are not a recognized substitute for code postweld heat treatment, and their effect on a given part should be proven on that part.
- Specifying it everywhere. A blanket stress-relief note on every steel drawing adds a furnace cycle, scale removal and a supplier dependency to parts that never needed it. Reserve it for the cases in the screening questions above.
- Forgetting the plating sequence. On high-strength steel, stress relief belongs before cleaning and plating, and the hydrogen bake after. Reversing them or omitting one leaves the part open to hydrogen embrittlement.
On the supply side, be clear about what a given plant runs. The CNC turning page lists steel bar such as 52100, C1117 and 12L14 and DOM steel tube for turning at our lead CNC plant, and the CNC milling page describes milling as a second step on castings rather than a general billet shop. Heat treatment, including stress relieving, is not listed among the processes on the factory page; welding is done at the lead CNC plant, but a furnace step after welding is not, so it has to be named and arranged. So if a turned steel part needs stress-relieved material, name it on the drawing and RFQ, and ask that the quote state how and where the step is done; for most short turned parts, stress-relieved bar is the simpler route than a furnace step.
How do you verify the stress relief was done, and what should the RFQ say?
Ask for the furnace record and a statement of conformance to the note, and judge the result on the finished-part geometry. There is no simple shop-floor test that proves residual stress is gone.
Residual stress can be measured by X-ray diffraction or hole-drilling methods, but these are specialist techniques suited to qualification, not lot acceptance. For production, the practical evidence is a time-temperature chart from the furnace run with the load identified, a hardness check where the part is hardened, and the dimensional report showing that straightness, flatness or position are within tolerance after finish machining. A certificate of conformance that names the stress relief note, the furnace run and the lot ties those together. On the RFQ, include:
- The grade, bar or plate specification and starting condition (hot rolled, cold drawn, cold drawn and stress relieved, prehardened).
- Whether the part is welded, and the weld code if one applies.
- The stress relief note with its position in the routing, temperature window or governing specification, and cooling method.
- Whether straightening is permitted after the furnace.
- Hardness requirements and when they are to be verified.
- The records you expect with each lot: furnace chart, hardness results, dimensional report.
- Any finish that follows, so the supplier can plan scale removal and the plating sequence.
The machining RFQ guide covers the rest of the package. If the part is still at drawing stage, a first-article run that measures form before and after the furnace is the cheapest way to find out whether the stress relief note is earning its place.
Frequently asked
Does stress relieving reduce the strength of steel?
On annealed or hot rolled carbon steel the change is small. On cold drawn bar it gives back part of the strength gained in drawing, which is why cold drawn, stress relieved bar specifications set their own property requirements. On quenched and tempered steel it lowers hardness if the temperature exceeds the last temper, so keep below it and recheck hardness.
Can I stress relieve a part after it has been zinc plated or painted?
Not at normal stress relief temperatures. Coatings such as zinc plating and powder coat do not survive them, so stress relief belongs before finishing. The low-temperature hydrogen bake after plating is a different step with a different purpose.
Is stress relief needed for stainless steel parts?
Austenitic stainless behaves differently: some temperature ranges cause carbide precipitation and loss of corrosion resistance, so the treatment and the grade need to be chosen together. Do not copy a carbon steel stress relief note onto a stainless drawing; ask a materials engineer or follow the grade producer's guidance.
Will stress relief fix a part that has already warped?
No. It stops further movement but does not pull a part back into shape. If the part has already moved, it needs remachining with remaining stock, or straightening, which reintroduces some stress.
Should I buy stress-relieved bar or add a furnace step?
For turned and lightly milled parts from cold drawn bar, stress-relieved bar is usually simpler and keeps size accuracy. A furnace step after roughing is the better choice for weldments, heavy one-sided machining and hardened parts, where the stress comes from your own processing rather than the mill.
Sources
- BPVC Section VIII, Rules for Construction of Pressure Vessels, Division 1 (paragraph UCS-56, postweld heat treatment of carbon and low-alloy steels), ASME, 2025 edition
- AMS2759/11A, Stress Relief of Steel Parts, SAE International, 2018
- AMS2750, Pyrometry, SAE International, accessed October 2026
- ASTM A311/A311M-04(2020), Cold-Drawn, Stress-Relieved Carbon Steel Bars Subject to Mechanical Property Requirements, ASTM International, reapproved 2020
- ASTM B849-02(2019), Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement, ASTM International, reapproved 2019
- ASTM B850-98(2022), Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, ASTM International, 2022