The default for hard, accurate steel parts is neither "before" nor "after" — it is both: rough machine, harden, then finish grind the critical features. Below roughly 35 HRC there is a cheaper route with zero quench risk: machine pre-hardened bar directly. This page gives the decision table, the distortion and growth values per process, and the grinding allowances we actually plan with.
Compiled by the EKINSUN engineering team from shop planning values for steel parts. Movement is geometry- and quench-dependent — treat the figures as planning ranges, not guarantees. Last reviewed 18 Aug 2026.
Heat treatment and machining are not a yes/no pair but an ordering problem. Every hard steel part we quote takes one of four routes, and the choice is driven by two numbers: the final hardness, and the tightest tolerance that must survive to the finished part.
| Route | Sequence | Best for | Tolerance outcome |
|---|---|---|---|
| A — no heat treatment | Machine finished | Parts that don't need hardness: brackets, spacers, housings | Full machining tolerance, e.g. ±0.01 mm where specified |
| B — pre-hardened stock | Buy 42CrMo4/4140 at 28–35 HRC → machine finished | Shafts, pins, bolts needing strength without extreme surface hardness | Full machining tolerance, zero quench distortion — our default for strong shafts |
| C — harden between | Rough machine (+ grind stock) → through-harden or carburize → finish grind | HRC 40+ parts with tight fits: arbors, bearing journals, wear parts | Ground features hold ±0.01 mm after hardening; unground features carry quench movement |
| D — nitride last | Machine completely finished → gas nitride | Wear surfaces on finished precision parts, thin parts that would warp in a quench | Growth ~10–30 µm per surface, no quench distortion, no machining afterwards |
Why route B exists: a great deal of "hardened shaft" work does not actually need HRC 55 — it needs a shaft that will not bend or wear quickly, which 42CrMo4 at 28–35 HRC already delivers. Machining that bar directly skips the quench, the distortion, the straightening and the grinding, and the tolerance you order is the tolerance you get. When a buyer's drawing says "hardened" with no number, this is the first question we ask.
| Hardness | Can it be machined? | Process | Practical notes |
|---|---|---|---|
| < 30 HRC | Yes — freely | Normal turning and milling | Annealed or normalized steel; full feature freedom |
| 28 – 35 HRC | Yes — daily work | Turning/milling of pre-hardened bar | 42CrMo4/4140 pre-hard; slightly lower speeds, normal tolerances |
| 35 – 45 HRC | Yes — selectively | Hard turning/milling, CBN or coated carbide | Feature-by-feature decision; threads by thread milling |
| 45 – 65 HRC | Not economically | Grinding (or EDM for forms) | This is grind territory — plan grind stock before hardening |
The reason ordering matters at all is that hardening moves metal. Planning values we use — geometry, section changes and quench severity swing them, which is exactly why tight features are ground after, not gambled through:
| Process | Temperature | Typical dimensional effect | Machining consequence |
|---|---|---|---|
| Through hardening + oil quench | ~830–860 °C | 0.1–0.3% movement; slender shafts can bow visibly | Leave grind stock; straightening is a rework, not a fix — a straightened shaft holds stress |
| Carburizing (case hardening) | 900–950 °C | Largest movement of the common processes + surface growth | Grind stock on all precision features; case depth specified together with stock |
| Induction hardening | Local, surface only | Localized; least global movement of the quench processes | Good for one journal or tooth flank on an otherwise soft part (typical on C45) |
| Gas nitriding | 500–530 °C | Growth ~10–30 µm per treated surface, no quench | Finish-last process — never machine after nitriding, any cut removes the case |
| Stress relieving | 550–650 °C | Negligible size change; releases locked-in stress | Between rough and finish machining on parts that must stay flat or round |
| Precipitation (17-4PH → H900) | ~480 °C | Predictable shrink ≈ 0.05%, very low distortion | Machine near-final, age, then final-cut critical features — the stainless route to HRC 40+ |
Route C only works if the rough-machined part carries enough stock for grinding to clean up the quench movement — and, on carburized parts, not so much that grinding removes the case:
| Feature | Allowance | Note |
|---|---|---|
| Diameters ≤ Ø25 | 0.2–0.3 mm on Ø | Journals, pins, arbor seats |
| Diameters Ø25–80 | 0.3–0.4 mm on Ø | |
| Diameters > Ø80 | 0.4–0.5 mm on Ø | |
| Bores | 0.3–0.5 mm on Ø | Internal grinding is slower — stock kept tight |
| Faces | 0.15–0.25 mm per side | Shoulders and thrust faces |
| Carburized features | Stock ≤ ~⅓ of case depth | Grind more and the hard case is gone — case depth and stock are specified as a pair |
Threads concentrate every problem on this page: they are precise, they cannot be ground economically in small quantities, and they sit at the surface where hardness lives. The working rules: final hardness under ~38 HRC — cut the thread before hardening, it survives with minor growth; above that — thread-mill in pre-hardened material or grind the thread after hardening. And one geometry rule that outranks hardness: a full-depth thread running against a hard shoulder is a thread-milling job, because a die or single-point pass cannot finish the final turns to the shoulder. For non-standard pitches and oversize threads this is daily work in our custom fastener program.
EKINSUN LTD is a custom parts manufacturer in Guangdong, China; heat treatment runs in qualified partner facilities and the machining schedule is planned around it. The material families behave very differently:
| Situation | In the HRC callout? | How EKINSUN handles it |
|---|---|---|
| Final hardness of the part | Yes — that is all it says | Route chosen from hardness + tightest tolerance; route stated on the quote |
| Which dimensions apply before vs after treatment | No — and quench moves them | Quote states that toleranced features are finished after hardening (route C) or that route B/D makes the question moot |
| Who owns distortion | No | Grind stock planned by the table above; bow on slender parts priced as grinding, not "straightening for free" |
| Case depth vs grinding stock | No — two callouts that fight each other | Specified as a pair on carburized parts so the case survives grinding |
| An impossible pairing (e.g. hardened 304) | The callout can demand it; metallurgy refuses | Material substitution (17-4PH / 440C) proposed on the quote, never silently machined |
| A worn hardened part to copy, no drawing | Sample tells hardness only if tested | Reverse engineering with hardness test; wear zones dimensioned from the unworn geometry |
Drawing says "hardened" — with or without the details? Send it as it is. We come back in 12 hours with the route (pre-hard bar, harden-and-grind, or nitride-last), the grind allowances, and the price — from 1 piece. If the callout can't work as written, the quote says so and proposes the fix. Get a quote.
Both, usually. The standard route for hard, accurate parts is rough machine → harden → finish grind the critical features. Machining entirely before hardening only works if the part can tolerate quench distortion; machining entirely after only works below about HRC 38–45. For many shafts the better answer is a third route: pre-hardened 42CrMo4/4140 at 28–35 HRC, machined directly to final size with no quench step and no distortion risk.
Typical planning values: oil-quench through hardening moves dimensions by 0.1–0.3% and can bow slender shafts visibly; carburizing at 900–950 °C causes the largest movement; induction hardening is localized and moves the part least of the quench processes; gas nitriding at 500–530 °C grows each treated surface by roughly 10–30 µm with no other movement; stress relieving changes size negligibly. Exact movement depends on geometry, section changes and quench — which is why features toleranced tighter than about ±0.05 mm are ground after hardening rather than trusted to survive it.
Yes — pre-hardened 42CrMo4/4140 at 28–35 HRC is machined directly to finished size in our daily work, and up to about HRC 38 is routine with the right inserts. From HRC 38–45, hard turning of selected features with CBN tooling is possible; above HRC 45 the practical finishing process is grinding. This is why the quote states the route, not just the part: hardness decides which machines touch it and in which order.
No. 304 and 316 are austenitic stainless steels — they do not respond to quench hardening at all and can only gain hardness slightly through cold work. If a drawing calls for a hardened stainless part, the material has to change: 17-4PH precipitation-hardening stainless reaches around HRC 40–44 at the H900 condition with a predictable ~0.05% shrink and very low distortion, and 440C martensitic stainless quench-hardens to HRC 58+. EKINSUN flags this substitution on the quote instead of silently machining 304.
If the final hardness stays below about HRC 38, threads are cut before hardening and survive it with minor growth. Above that, the honest options are thread milling in pre-hardened material, or grinding the thread after hardening. A full-depth thread running to a hard shoulder is a thread-milling job by geometry — a die or single-point pass cannot finish the last turns against the shoulder.
Planning values on diameter: 0.2–0.3 mm up to Ø25, 0.3–0.4 mm for Ø25–80, 0.4–0.5 mm above Ø80; bores get 0.3–0.5 mm and faces 0.15–0.25 mm per side. Long slender shafts get more, because bow consumes allowance. On carburized parts the allowance is also capped from above — grind off too much and the case itself is removed, so case depth is specified together with the grinding stock.
Slightly and predictably: gas nitriding at 500–530 °C grows each treated surface by roughly 10–30 µm, and because there is no quench, the part does not distort the way through-hardened parts do. That makes nitriding the finish-last process: the part is machined completely, nitrided, and not machined again — any cut after nitriding removes the hard case.
Yes, but only by the grind route: rough machine, harden to HRC 58–62, then finish grind the ±0.01 mm features. No part holds ±0.01 mm through a quench. The drawing should state that final dimensions apply after heat treatment, and EKINSUN quotes it as machine–harden–grind with the ground features listed — MOQ 1, CMM report available.
Send the drawing, sketch or sample — we reply with the route (pre-hard, harden-and-grind, or nitride-last) and the price. MOQ 1, quote in 12 hours.
Hardened shafts, pins, arbors and wear parts — machined, treated and ground in the right order, from 1 piece. Quote in 12 hours.