If you are designing a drive shaft, a hydraulic piston rod or a high-strength bolt, you have most likely chosen a quenched and tempered steel (Vergütungsstahl, Q&T steel). This group provides a combination of high strength and toughness through quenching and tempering; but the sequence of CNC machining and heat treatment carries practical details the designer needs to know in terms of distortion and cost. Following our articles on free-cutting steels and stainless steels, this article gives the contract shop's point of view.
The Most Widely Used Q&T Steels
| Material (EN 10083) | W.-Nr. | Typical +QT strength | Use |
|---|---|---|---|
| C45 / C45E | 1.0503 / 1.1191 | 630–780 MPa (Ø≤16) | Shafts, pins, general machine parts |
| 42CrMo4 | 1.7225 | 1000–1200 MPa | Highly loaded shafts, gears, bolts (10.9/12.9) |
| 34CrNiMo6 | 1.6582 | 1100–1300 MPa | Large sections, impact loads; crankshafts, axles |
| 25CrMo4 | 1.7218 | 700–850 MPa | Weldable Q&T steel, pressure components |
| 16MnCr5 | 1.7131 | Case-hardening steel | Hard surface, tough core: gears, cams |
Delivery Condition Decision: +N, +QT, +A
The same steel is bought in different conditions, and this determines the whole process:
- +N (normalised) or +A (annealed): Easy to machine (machinability index ~60–70% for C45); hardening is needed after machining; heat-treatment distortion must be accounted for.
- +QT (quenched and tempered): Arrives as bar at 900–1200 MPa; machined directly to finished size, no heat-treatment step. Machining is 20–40% slower and tool life shorter, but the risk of distortion and a second operation disappears.
Practical rule in contract manufacturing: if the part geometry is simple and tolerances moderate, direct machining from +QT bar is the cheapest and fastest route. If the part is complex, has large section changes or requires hardness above 40 HRC, the chain rough machine → heat treat → finish/grind is needed. This is why it is critical whether the drawing says "42CrMo4 +QT" or "42CrMo4, to be hardened to 40±2 HRC"; the two are very different prices.
Machinability and Tooling Strategy
- C45 +N: Close to the reference steel; high cutting speed, long tool life. Surface finish can be "smeary" in the annealed condition; the +QT condition gives a better surface.
- 42CrMo4 +QT (~300 HB): Cutting speed falls 30–40% versus C45; coated carbide and rigid clamping are essential. Long slender shafts need tailstock and steady support.
- 34CrNiMo6 +QT: Harder to machine due to high nickel content; cutting forces are high and chip control becomes difficult.
- Machining in the hardened state (>45 HRC): Hard turning with CBN tools is possible; as an alternative to grinding it is economical for tight-tolerance shafts. For surface-finish expectations see our surface roughness article.
Distortion After Heat Treatment and Grinding Allowance
Shape change during quenching is unavoidable; long shafts bend, thin-walled parts go oval. The practical approach:
- In rough machining, leave a grinding/finishing allowance of 0.3–0.5 mm on diameter and 0.5–1 mm on length.
- Shafts with a large L/Ø ratio may need straightening after heat treatment.
- Surfaces with IT6 or tighter tolerances are ground after heat treatment or finished by hard turning.
- Threads are cut before hardening and usually protected from it (local hardening, or a post-heat-treatment draw on the thread zone).
The tolerance decision determines the heat-treatment chain; our tolerance selection article discusses what is needed on which surface.
Surface-Hardening Options
| Method | Suitable steel | Surface hardness | Depth | Note |
|---|---|---|---|---|
| Induction hardening | C45, 42CrMo4 | 55–60 HRC | 1–5 mm | Local, fast, suited to series production |
| Gas nitriding | 42CrMo4, 31CrMoV9 | 600–900 HV | 0.2–0.6 mm | Low distortion, can be applied after finishing |
| Case hardening | 16MnCr5, 20MnCr5 | 58–62 HRC | 0.5–2 mm | Gears and cams; grinding required |
| Nitrocarburising (salt bath) | C45, free-cutting steels | ~500–700 HV | ≤0.03 mm compound layer | Wear + corrosion, inexpensive |
Material Certificate and Traceability
For Q&T steels the EN 10204 3.1 certificate gives the chemical analysis and mechanical values (Rm, Re, A, KV). For +QT bar the certificate values are directly the part's values; if heat treatment is done at the shop, a heat-treatment report (furnace record, measured hardness) should also be requested. For automotive and energy customers, hardness measurement points are marked on the drawing.
Practical Checklist for the Buyer
Before sending the RFQ and drawing for Q&T steel parts to the supplier, check these points:
- Is the EN 10083 grade and the delivery condition (+N, +QT) stated? If an old DIN or AISI equivalent is used, has the EN equivalent been added?
- Are the hardness requirement (HB/HRC), its tolerance and the measurement location specified? Is hardness required only at the surface (induction, nitriding) or through the section?
- Has the heat-treatment sequence been clarified: rough machine → heat treat → finish, or direct machining from +QT material?
- Are the surfaces to be ground after heat treatment and their tolerances marked?
- Are the material certificate (3.1) and hardness report on the document list?
- If there are mechanical-property requirements (Rm, Rp0.2, impact), are the specification and sampling location given?
- If welding is involved, is there a carbon-equivalent and preheating note? (Welding 42CrMo4 requires special precautions.)
Completing these seven items ensures the supplier prices the correct process chain and the first sample is not rejected for hardness or dimensions. In Turkey heat-treatment plants are common in the industrial zones and machining shops work with them routinely; hardness is usually measured in the shop and added to the report. For the general framework of surface treatments see also our material certificate article.
Frequently Asked Questions
Can I machine 42CrMo4 +QT directly and use it without heat treatment?
Yes; +QT bar is already at 1000–1200 MPa. It is sufficient for most shafts and pins and is the most economical route.
Are tolerances held after heat treatment?
IT7 and wider tolerances can be held with careful allowances; for IT6 and tighter, grinding or hard turning is planned.
How much does choosing 42CrMo4 instead of C45 raise the price?
Raw material is 15–25% and machining 20–40% more expensive; on small parts the total effect is around 15–30%.
How should the hardness tolerance be written?
Target and range (e.g. 40±2 HRC), measurement location and method (HRC/HV) should be given on the drawing; the phrase "to be hardened" alone is insufficient.
Is 42CrMoS4 or 34CrMo4 accepted instead of 42CrMo4?
42CrMoS4 contains added sulphur and has better machinability; its mechanical values are almost identical. 34CrMo4 has lower carbon and lower hardenability. If an alternative is acceptable, it must be stated clearly on the drawing.
Does hardness vary from batch to batch in +QT material?
For EN 10083 +QT, mechanical properties are given as ranges by diameter band; hardness may vary by 20–40 HB. For parts needing a narrow hardness tolerance, a dedicated heat treatment for the customer is safer.
Conclusion
With Q&T steels the right decision concerns the delivery condition and heat-treatment sequence more than the material itself: direct machining from +QT bar is the most economical route for most parts, while complex, tight-tolerance parts need the rough machining–hardening–finishing chain. Send your drawing and hardness requirement via our quote form; we propose the material condition and heat-treatment option together with the quote. You can also look at our CNC turning scope.
