Heat treatment for machined parts: when steel needs to get harder

Guide · Heat Treatment Heat treatment for machined parts: when steel needs to get harder Machining shapes a part; heat treatment changes what it is made of. Annealing, stress relieving, quenching and tempering, age hardening and case hardening each trade one property for another. This guide explains when to heat-treat, when not to, and the DFM traps that follow. Why heat treatment exists Machining only removes material. Heat treatment changes the material’s internal structure — its hardness, strength, ductility, residual stress. The same 4140 steel bar can be soft enough to machine easily or hard enough to survive a gear tooth, depending on how it is heated and cooled. The art is choosing the treatment that delivers the property the part needs without breaking something else. The four properties heat treatment buys Heat treatment is a transaction: you pay furnace time and distortion risk to buy one of four properties. Name the one you need before you pick a process. Hardness. Resistance to indentation and scratching, read as HRC, HB, or HV — the number on the drawing. Wear resistance. Hardness is the proxy; wear life on a sliding or rolling face is what you pay for, and case hardening delivers it on a soft core. Strength. Quench and temper and precipitation hardening raise yield and tensile strength without adding mass. Stress relief. The odd one out: it buys nothing harder, only a part that stays put by removing residual stress. The five treatments buyers meet Annealing. Heated and slowly cooled to soften, relieve stress and improve machinability. Used on raw stock or between operations. Stress relieving. A low-temperature bake that removes residual stress from machining or welding, preventing distortion later. Cheap insurance before a tight-tolerance finish pass. Quench and temper. Heated to austenitize, rapidly quenched (hard but brittle), then tempered to trade a little hardness for toughness. The workhorse for tool and structural steel. Age (precipitation) hardening. Used on 17-4PH and other PH stainless: solution-treated, then aged to reach high strength. Lets you machine the soft condition, then harden the finished part. Case hardening. Carburizing or nitriding adds a hard surface skin over a tough core — for wear surfaces that must also absorb shock. Heat-treatment processes at a glance Six families cover nearly every treatment, each with a purpose and a typical hardness. ProcessWhat it doesTypical steelsTypical hardness AnnealingSoftens and relieves stress for machining4140, 4340, tool steel bar< 220 HB, soft NormalizingRefines grain, evens structureLow-carbon structural steel, castingsSlightly above annealed Quench + temperHigh hardness balanced against toughness4140, 4340, 1045, O1, D230–60 HRC CarburizingHard wear skin over a tough core8620, 9310, 20MnCr5, 16MnCr558–62 HRC case NitridingThin hard case, low distortion4140, 4340, Nitralloy 135M, H1360–70 HRC, thin case Precipitation hardeningHigh strength plus corrosion resistance17-4PH, 15-5PH, 7075 aluminum17-4PH up to ~44 HRC Carburizing and nitriding both build a hard skin but differ: carburizing gives a deeper, tougher case that needs a quench; nitriding gives a thinner, harder case with almost no distortion. If a drawing says only “case harden,” ask which. Annealing, quench and temper, and case hardening The five treatments above are not interchangeable. Each sits at a different point on the hardness-toughness-cost curve, and picking the wrong one is how parts get scrapped. Here is what each actually does on the floor. Annealing Annealing heats the steel into the soft austenite range, holds it, then cools it slowly — usually inside the furnace, still wrapped in its own heat. The slow cool lets the structure relax completely, so the part comes out soft, dimensionally stable, and easy to cut. Shops anneal raw bar stock before a heavy machining campaign, and anneal again between operations when deep cuts have built up locked-in stress. The trade is blunt: an annealed part is the opposite of wear-resistant. You anneal to machine, not to use. Quench and temper This is the route to real hardness. The part is austenitized at high temperature, then quenched — plunged into oil, water, or polymer so fast that the structure locks into hard, brittle martensite. On its own martensite is too brittle to trust, so the part is tempered: reheated to a moderate temperature and held, trading a few points of hardness for a large gain in toughness. The tempering temperature sets the final balance — a low temper keeps it hard and keen for tooling, a high temper makes it tough and resilient for structural parts. The quench is also where most distortion is born, which is why grind stock and stress relief matter so much. Case hardening: carburizing and nitriding Case hardening builds a hard skin on a soft, shock-absorbing core. Carburizing packs carbon into the surface at high temperature, then quenches to harden that thin shell. Nitriding diffuses nitrogen at a lower temperature and adds almost no distortion, which is why it is favored for finished, tight-tolerance parts. Both give you a wear face that will not spall under load — exactly what gear teeth, shafts, and slides need. The cost is process complexity and a furnace cycle that usually leaves the building. Age (precipitation) hardening of 17-4PH 17-4PH is solution-treated soft, machined in that soft state, then aged — held at temperature so fine precipitates form and lock the matrix. The result is a stainless part with strength near some tool steels and genuine corrosion resistance. The appeal is the sequence: you do the hard cutting when the metal is easy, then the oven does the hardening. That is why 17-4PH shows up so often in parts that are both corrosion-sensitive and load-bearing. None of these treatments changes the bulk chemistry except case hardening, which adds carbon or nitrogen at the surface. Everything else rearranges the internal structure without adding alloy. That matters to a buyer because the base grade you pick still sets the ceiling — heat treatment tunes within that ceiling, it does not invent new properties. Choose the wrong stock and no furnace will save it. Hardness scales: HRC, HB, and HV A hardness callout means nothing without the scale. Three cover nearly everything: Rockwell C (HRC). The default for hardened steel: a diamond cone pressed in and read directly. A cutting edge sits near 60 HRC, a structural 4140 part around 30–40 HRC. Brinell (HB). A tungsten-carbide ball makes a wider indent, read by diameter. Best for softer steels and castings, and it converts roughly to tensile strength. Vickers (HV). A diamond pyramid that scales from micro to macro; its tiny indent suits thin cases and coatings, which is why a micro-Vickers tester verifies case depth. Do not compare numbers across scales: a “60” means 60 HRC, not 60 HB. The ASM Handbook is the reference for conversions. How heat treatment is done at Nex-G Nex-G runs CNC machining and electronics assembly in Dongguan, and heat treatment is handled on a hybrid model that keeps most low-temperature work in-house and sends only what needs a furnace to qualified partners. The setup is worth understanding before you plan the process route: Two CHW-2 hot-air ovens, rated to 500°C (room temperature to 500°C, adjustable). These cover low-temperature stress relieving and aging — for example, aluminum alloys are stress-relieved around 190°C to stabilize dimensions before the finish pass. Full hardening, carburizing, nitriding, vacuum and bright treatments are done through qualified subcontractors operating inside the Nex-G quality system (ISO 9001 / IATF 16949 / ISO 14001), with hardness verification and lot-level traceability. Honest boundary: the 500°C ceiling cannot reach titanium Grade 5 stress relief, which needs roughly 788°C. Titanium parts are stress-relieved by the subcontractor, not in the Dongguan ovens. One quality record. Running the low-temperature work in-house keeps it on the same paperwork as the machining, so a stress-relieved aluminum bracket and its CNC-turned sibling share one inspection record under ISO 9001 / IATF 16949. A worked process orderFor a carburized part the flow is: incoming inspection → CNC turning → CNC milling (two passes) → QC → post-processing → carburizing (subcontracted) → cleaning → outgoing inspection → packing. Soft machining and final inspection stay in-house; only the furnace step leaves the building. Nex-G capability: carburizing in production Case hardening is not a one-off here; it runs inside a real automotive program. Nex-G operates a 6,800 m² plant in Dongguan Hengli — 100+ staff, founded 2006 — with 80+ CNC machines from Mazak, Brother, TSUGAMI, and Sodick. Carburized parts run the standard route: soft machining on the CNCs, furnace through a qualified partner, finish grind, one quality record. Milling and turning hold ±0.005 mm; grinding and wire-EDM hold ±0.002 mm. SPC targets a Cpk of 1.67 on critical dimensions and 1.33 standard — and the line holds critical characteristics to Cpk ≥ 1.67. The site is certified to ISO 9001, IATF 16949 (design excluded under clause 8.3 — we build to your print, we do not redesign it), and ISO 14001, with URS audit coverage through 2027. A carburized part arrives with case depth, hardness, and tolerance on one inspection record — MOQ of 1 for prototypes, and 3-day sample, 7-day small-batch, or 30-day production lead times. The machinability-vs-hardness sequence The biggest DFM point: you usually machine first, then harden. A part is roughed and even finish-machined in the soft condition, then heat-treated to final hardness — because machining a 40 HRC part is slow and tool-killing, and machining a 17-4PH part after aging is miserable. The sequence is: machine soft → heat-treat → light finish grind. A drawing that specifies hardness before machining is costing you tool life and time. Machining 17-4PH. Where heat treatment sits in the sequence A part that will be hardened runs like this: Rough machine soft, leaving grind stock — typically 0.2–0.5 mm on faces to be ground. Stress-relieve if roughing was heavy. Semi-finish close to size, still soft. Heat-treat to final hardness; the part will move from stress, the quench, and the phase change. Finish-grind (or hard-turn, or hone) back to tolerance. Grind stock is the budget you give the oven; a drawing that leaves none but demands a tight tolerance after hardening is designed to be scrapped. Carburized parts follow the same rule: leave stock, carburize, then grind. Stress-relieving timing: when to bake Stress relieving is the cheapest insurance in the book, but only if you bake at the right moment. A low-temperature cycle in the in-house 500°C ovens costs a day and a tray of oven space; skipping it can cost a scrapped, finish-machined part. Bake: After roughing heavy stock. Deep cuts leave locked-in stress; a stress-relief bake before the finish pass lets the part move while you still have stock to remove. Between operations on long, thin, or section-changing parts, so stress does not accumulate across stages. Before a tight-tolerance finish or before carburizing/nitriding, where residual stress would otherwise surface as warpage. On welded parts and anywhere the thickness changes sharply, because that is where the stress gradient — and the distortion — is steepest. The earlier you relieve, the less you fight distortion later. Treat stress relief as a scheduled step, not a rescue after the part has already moved. Distortion: the tax heat treatment charges Heat and quench warp parts. Thin, asymmetric or finely toleranced features can move beyond spec during treatment, forcing a straightening or a finish-grind allowance. The DFM responses: Leave grind stock. Rough-machine, heat-treat, then finish-grind to tolerance. Avoid thin asymmetric features. They distort most; thicken or symmetrize. Stress-relieve between operations. Removes the stress that would otherwise show up as warpage later. Dimensional change: how much a part moves Two things happen in the furnace. Distortion is a change of shape — a plate cups, a shaft bows. Dimensional change is a change of size — the whole part grows or shrinks a small, predictable amount. You fight the first with geometry and stress relief; you absorb the second with grind stock. The size change comes from the phase change: martensite is slightly less dense, so a quenched part grows — roughly 4% by volume, more with higher carbon. Carburizing adds growth as the surface takes on carbon; nitriding is gentlest, but tight bores must still allow for it. The rule: assume every treated dimension moves a few tenths of a millimeter. A part toleranced to ±0.005 mm that is quenched with no grind allowance is not a drawing; it is a wish. DFM rules for heat treatment Heat treatment punishes geometry that machining forgives: Keep section thickness uniform. Thin webs next to thick bosses quench at different rates, and the uneven contraction bends and cracks parts. Blend transitions with generous radii. Design out quench cracking. Sharp corners, keyways, and blind holes are stress raisers that martensite tears open, and high-carbon and alloy steels crack first. Add fillets, and use an oil or polymer quench for crack-prone grades. Mask what must not harden. Carburizing and nitriding treat the entire surface unless stopped; threads to tap later and faces machined after hardening must be masked with copper plate or stop-off paint — say so on the drawing. Specify the case depth. Write the depth and its tolerance, and do not ask for more case than the load needs. Hardness verification and distortion control You should never accept a hardness claim you cannot see. At Nex-G every treated lot is checked on an HVS-1000A micro-Vickers hardness tester, and the measured value is written into the inspection report — not left as a verbal promise. Ask for that report; it is the only proof the oven did its job. Distortion is controlled upstream, not fixed after the part comes out warped: Leave grind stock on critical faces so a post-treatment grind recovers tolerance. Keep walls and ribs thick and symmetric so the quench cools them evenly. Stress-relieve on the way so the part reaches the furnace already relaxed. Prefer nitriding over quench-and-temper for finished, low-distortion parts where the design allows — nitriding adds almost no warpage. The goal is a part that leaves the oven already close to print, needing only a light grind. That is cheaper and more repeatable than rescuing a distorted one. When to heat-treat vs pick a harder material If you need…Consider High hardness + toughnessQuench & temper 4140 / 4340 Corrosion + high strengthAge-harden 17-4PH Wear surface on a soft coreCase hardening Just easier machiningAnneal, or pick a free-machining grade No treatment at allSpecify a grade that is already hard enough as-supplied Questions to ask a shop about heat treatment Do you heat-treat in-house or subcontract? Subcontracting adds a handoff, a lead-time week, and a traceability gap. How do you control distortion? Look for grind stock and stress-relief in the process plan. What hardness do you guarantee, and how verified? A hardness test report, not a promise. Buyer's guide: material selection and cost Heat treatment is not free, and the cost is rarely just the oven. Budget for it in the schedule, not only the quote: In-house low-temp work is cheap. Stress relief and aging run in the same 500°C ovens already in the building, so they add little beyond a day of lead time. Full hardening, carburizing, or nitriding cost more than the furnace. A subcontract handoff means extra lead time (often a week or more), freight, and a traceability step. Price the calendar, not just the part. Sometimes the cheaper answer is stock. If a grade already supplied at the needed hardness does the job — pre-hardened 4140, a PH stainless, or a hard aluminum — you skip treatment entirely and avoid the distortion tax. Tight tolerances after hardening cost more. The same tolerance held in the soft state is cheaper than the same tolerance forced after heat treatment, because the latter demands a finish grind. Loosen non-critical dimensions where you can. Ask for the treatment and its verification together, so the number on the report matches the number on the drawing — and ask whether a different stock grade would meet the spec for less. As a rule of thumb, plan a subcontracted hardening step as a separate calendar week, not a same-week add-on. A part that needs soft machining, furnace hardening, and a finish grind can span three weeks of elapsed time even when the spindle hours are modest. If the schedule is the constraint, a pre-hardened or PH grade that skips the furnace is often the faster path — sometimes faster than the cheaper-looking treated option. Treat lead time and cost as one decision, not two. Frequently asked questions What is heat treatment in machining?Heating and cooling a part to change its internal structure — hardness, strength, ductility, residual stress. Machining shapes the part; heat treatment changes what it is made of. Do you machine before or after heat treatment?Usually both: rough and often finish-machine in the soft condition, then heat-treat, then a light finish grind. Machining a hardened part is slow and tool-killing, so you avoid it where you can. Does heat treatment cause distortion?Yes — quench and temper especially can warp thin or asymmetric features. The standard defenses are leaving grind stock, stress-relieving between operations, and avoiding thin asymmetric geometry. What is age hardening of 17-4PH?A precipitation-hardening stainless that is machined soft, then aged to reach very high strength. It lets you machine the easy condition and harden the finished part. Can you heat-treat in-house?We heat-treat through qualified partners under our quality system, with hardness verification and traceability — so the treatment does not become a handoff risk or a documentation gap. When should I avoid heat treatment?When a grade already supplied at the needed hardness does the job, or when the part is simple enough that the distortion risk and added lead time are not worth it. Sometimes picking the right stock is cheaper than treating it. What temperature is used for stress relieving?Low-temperature stress relief for steel sits well below the hardening range, while aluminum stress relief runs around 190°C. Nex-G’s in-house CHW-2 ovens are rated to 500°C, which covers these low-temperature cycles and some alloy aging. Can you carburize or nitride in-house?Carburizing and nitriding are done through qualified subcontractors operating inside the Nex-G quality system (ISO 9001 / IATF 16949 / ISO 14001). The soft machining, QC, and final inspection stay in-house, so only the furnace step leaves the building. How is hardness verified?Every treated lot is checked on an HVS-1000A micro-Vickers hardness tester, and the measured value is documented in the inspection report. You should receive that report, not a verbal guarantee. Does heat treatment add lead time?Low-temperature stress relief is fast and done in-house on the 500°C ovens. Full hardening or case hardening through a subcontractor typically adds a week or more to the schedule, so plan the furnace step into the build, not after it. Treatment selection quick reference You need…Choose Softer, easier machiningAnneal or a free-machining grade Remove machining stressStress relieving High hardness + toughnessQuench & temper Corrosion + high strengthAge-harden 17-4PH Wear surface, tough coreCase hardening When a grade already supplied at the needed hardness does the job, skip treatment — it saves lead time and distortion risk. Treatment earns its place when the stock alone cannot meet the spec. Choosing the stock grade. Not sure if your part needs heat treatment?Send the drawing and the load to [email protected] — we will recommend the treatment (or the stock grade) that meets the spec for the lowest cost and risk.Request a quote Related articlesStainless Steel Machining GuideThe Complete Guide to Material SelectionThe Complete Guide to CNC Machining in China