C45 (1.0503) is an unalloyed medium-carbon steel and 42CrMo4 (1.7225) is a chromium-molybdenum alloy steel; both are quenched and tempered grades for shafts, gears and similar machine parts. At 16 to 40 mm diameter the specified tensile strength is 650 to 800 MPa for C45 and 1000 to 1200 MPa for 42CrMo4, and 42CrMo4 also keeps its hardness much deeper into a bar.
Which standards cover C45 and 42CrMo4 today?
The European standards EN 10083-2 (non-alloy steels) and EN 10083-3 (alloy steels), which older drawings and mill data sheets still cite, are withdrawn. DIN Media lists DIN EN 10083-2:2006-10 as replaced by DIN EN ISO 683-1:2018-09 and DIN EN 10083-3:2007-01 as replaced by DIN EN ISO 683-2:2018-09. Both replacements are marked current. Status as of 2026-10-30; we found no draft revision of either part.
The strength values in this article come from mill data sheets dated 2016 and 2017 that still name EN 10083. Swiss Steel Group's 2023 data sheets give the same figures at 16 to 40 mm. For a safety-relevant part, read the tables of the current EN ISO 683 part before you rely on a value.
How do C45 and 42CrMo4 differ in composition and use?
C45 relies on its carbon content for hardening. 42CrMo4 has a similar carbon content plus 0.90 to 1.20 % chromium and 0.15 to 0.30 % molybdenum, which is what lets it harden through larger sections.
Table 1. C45 and 42CrMo4 compared, melt analysis in mass percent
| C45 (1.0503) | 42CrMo4 (1.7225) | |
|---|---|---|
| Type | Non-alloy quenching and tempering steel | Cr-Mo alloy quenching and tempering steel |
| Carbon | 0.42–0.50 | 0.38–0.45 |
| Manganese | 0.50–0.80 | 0.60–0.90 |
| Chromium | max 0.40 | 0.90–1.20 |
| Molybdenum | max 0.10 | 0.15–0.30 |
| US designation listed by mills | SAE/AISI 1045 | SAE 4140 |
| Parts named in the data sheets | Axles, shafts, racks, gears under relatively low load | Transmission shafts, gears, tooling, expansion screws |
Source: Saarstahl C45 sheet; Swiss Steel Group C45E/C45R and 42CrMo(S)4 sheets, 2023; thyssenkrupp Werkstoffdatenblatt 42CrMo4, 2017. US designations are equivalents, not identical grades.
Mills list C45E (1.1191) and C45R (1.1201) as the quality designations; 1.0503 is the number Saarstahl gives for C45. Name the full designation on the drawing.
What strength do they reach after quenching and tempering?
Minimum strength falls as the section gets larger, because the core of a thick bar does not harden as fully as its surface.
Table 2. Quenched and tempered (+QT) strength by diameter, minimum Rp0.2 and range of Rm, MPa
| Diameter d, mm | C45 Rp0.2 | C45 Rm | 42CrMo4 Rp0.2 | 42CrMo4 Rm |
|---|---|---|---|---|
| up to 16 | 490 | 700–850 | 900 | 1100–1300 |
| over 16 to 40 | 430 | 650–800 | 750 | 1000–1200 |
| over 40 to 100 | 370 | 630–780 | 650 | 900–1100 |
| over 100 to 160 | not tabulated | not tabulated | 550 | 800–950 |
Source: Saarstahl C45 sheet (diameter ranges to 100 mm); thyssenkrupp Werkstoffdatenblatt 42CrMo4, 2017 (ranges to 250 mm). Bright drawn bar to EN 10277 has its own table; Swiss Steel Group gives a minimum elongation of 12 % for 42CrMo4 at 16 to 40 mm there.
Hardenability explains the gap. Swiss Steel Group's end-quench (Jominy) charts show C45E falling to about 20 to 30 HRC at 15 mm from the quenched end. For 42CrMo4 in the +HH band the chart still reads about 35 HRC at 50 mm. Saarstahl tabulates C45 to 100 mm and 42CrMo4 to 250 mm.
Which delivery hardness and heat treatment temperatures apply?
The delivery condition is a suffix on the designation: +A soft annealed, +S treated for shearability, +QT quenched and tempered. The table gives the hardness limits of the two soft conditions and the guide temperatures a heat treater works within.
Table 3. Delivery hardness and heat treatment guide values
| C45 | 42CrMo4 | |
|---|---|---|
| Soft annealed (+A), maximum hardness | 207 HB | 241 HB |
| Treated for shearability (+S), maximum hardness | 255 HB | 255 HB |
| Normalising, air | 840–880 °C | 850–880 °C |
| Soft annealing, furnace cooling | 680–710 °C | 680–720 °C |
| Hardening | 820–860 °C, water or oil | 820–860 °C, oil or water |
| Tempering, air | 550–660 °C | 540–680 °C |
Source: Saarstahl C45 sheet and Saarstahl 42CrMo4 sheet (both cite DIN EN 10083). thyssenkrupp gives 820–880 °C for hardening and 840–880 °C for normalising 42CrMo4.
These are guide values. The qualified procedure of the heat treater governs, and the tempering temperature sets where in the strength range of Table 2 a part ends up. The certificate should state the procedure actually used.
How do the two steels machine, and when should you machine them?
Thyssenkrupp states that machinability depends strongly on microstructure. The standards limit hardness in the soft conditions (Table 3), and 42CrMo4 is the harder material there. In the quenched and tempered state at 16 to 40 mm its specified Rm lies about 350 MPa above that of C45 at the lower limit (1000 against 650 MPa), which generally means higher cutting forces and tool wear on CNC turning and milling.
Both grades exist with a controlled sulphur range of 0.020 to 0.040 %: C45R and 42CrMoS4, against a maximum of 0.035 % for C45E and 42CrMo4. The sulphur range is part of the designation, so state it instead of leaving the choice to the supplier.
The sequence matters more than the grade. There are two usual routes:
- Machine soft (+A or +S), heat treat, then finish the features that must hold tolerance. Use this when the part must reach its strength at final shape.
- Buy bar in the +QT condition and machine it at strength. Swiss Steel Group supplies peeled quenched and tempered bright bar in Ø 15 to 80 mm, hardened bar by bar, and states that this improves straightness and residual stress compared with conventional hardening. No hardening step follows, so the part cannot move in it.
How much does a part change size when it is hardened?
A part changes size and shape in hardening, and the change is not small enough to ignore on a drawing with fits. A trade article on quench hardening in Thermal Processing (July 2022) separates two effects. Size change is inevitable with the volume change of the phase transformation. Shape change can be significant when different regions of the part transform at different times, and the martensite transformation is the source of most quench stresses and of the residual stress left at room temperature. The same article expects tempering to reduce that residual stress only slightly.
Slender parts and parts with uneven wall thickness cool unevenly, so shape change shows up there first. Three practical rules follow:
- Leave stock on bores, seats, sealing faces and threads, and finish them after heat treatment.
- Agree the allowance per part with the heat treater. It depends on geometry, quenchant and furnace load, and the data sheets we used give no figure.
- Measure after heat treatment and before finishing, so that a moved feature is found while stock remains.
See the CNC machining tolerances handbook for how to set the tolerance of a feature that is finished after hardening.
What should the drawing and the order specify?
- Write the designation with condition, number and dated standard, for example "42CrMo4+QT (1.7225), EN ISO 683-2:2018". An undated or EN 10083 reference now points to a withdrawn text.
- State the diameter or thickness the strength applies to, because the minimum falls with section size (Table 2).
- State which features are finished after heat treatment and whether all other dimensions apply before or after it.
- Name the hardenability band (+H, +HH, +HL) only if the part needs one. Swiss Steel Group says it supplies +HH for 42CrMo(S)4 unless told otherwise.
- Ask for an EN 10204 3.1 certificate that shows the analysis, the heat treatment condition and the mechanical test results.
When is neither grade the right choice?
Welding is the first limit. Thyssenkrupp calls the alloyed quenching and tempering steels difficult to weld by open arc processes: at 0.30 to 0.45 % carbon the heat-affected zone and the weld metal can harden, and preheating, a controlled working temperature and slow cooling are generally needed. The strength of a weld made with low-carbon filler also differs from that of the base steel. Say whether the steel is normalised or quenched and tempered before welding, or move to a weldable grade for welded assemblies.
Corrosion is the second. Neither steel is corrosion resistant, so a part in wet or chemical service needs a coating or a stainless grade, see machining stainless steel. Parts that need a hard, wear-resistant surface over a tough core are normally made from case-hardening steels, which ISO 683-3 covers, not from these two.
Frequently asked questions
- Is C45 the same as AISI 1045?
- They are listed as equivalents by Saarstahl and Swiss Steel Group, not as identical grades. Each standard sets its own composition limits, so the material certificate has to state which standard the bar was tested to.
- Is 42CrMo4 the same as AISI 4140?
- Saarstahl lists SAE 4140H and Swiss Steel Group lists AISI 4137 and 4140 as international designations of 42CrMo4. These are equivalents, not identical grades. Specify the EN designation and standard on the drawing and let the certificate show the actual analysis.
- Has EN 10083 been withdrawn?
- Parts 2 and 3 have. DIN Media lists DIN EN 10083-2:2006-10 as replaced by DIN EN ISO 683-1:2018-09 and DIN EN 10083-3:2007-01 as replaced by DIN EN ISO 683-2:2018-09. Both replacements are listed as current as of 2026-10-30.
- What tensile strength does quenched and tempered 42CrMo4 reach?
- Mill data sheets give a minimum yield strength Rp0.2 of 900 MPa and a tensile strength Rm of 1100 to 1300 MPa up to 16 mm diameter. At 16 to 40 mm the values are 750 MPa and 1000 to 1200 MPa, and at 40 to 100 mm they are 650 MPa and 900 to 1100 MPa.
- Which is easier to machine, C45 or 42CrMo4?
- C45 is the softer steel in every condition in Table 3: soft annealed C45 is limited to 207 HB, soft annealed 42CrMo4 to 241 HB. Both exist with a controlled sulphur range of 0.020 to 0.040 % (C45R, 42CrMoS4), which is the variant to name for volume turning.
- Will a part change size when it is hardened?
- Yes. A trade article on quench hardening states that size change is inevitable with the volume change of the phase transformation, and that shape change can be significant when different regions of the part transform at different times. Finish critical features after heat treatment or agree a stock allowance with the heat treater.
Sources
- DIN EN 10083-2:2006-10, withdrawn, replaced by DIN EN ISO 683-1:2018-09 (DIN Media)
- DIN EN 10083-3:2007-01, withdrawn, replaced by DIN EN ISO 683-2:2018-09 (DIN Media)
- DIN EN ISO 683-1:2018-09, non-alloy steels for quenching and tempering, current (DIN Media)
- DIN EN ISO 683-2:2018-09, alloy steels for quenching and tempering, current (DIN Media)
- Saarstahl C45, material specification sheet, 1.0503 (Saarstahl)
- Saarstahl 42CrMo4 and 42CrMoS4, Werkstoff-Datenblatt, 1.7225 and 1.7227 (Saarstahl)
- Werkstoffdatenblatt 42CrMo4 and 42CrMoS4, MX/TIS-TS_00/01.2017 (thyssenkrupp Materials Services)
- Technical datasheet C45E or C45R +QT +SH, 2023 (Swiss Steel Group)
- Technical datasheet 42CrMo(S)4 +QT +SH, 2023 (Swiss Steel Group)
- Stress evolution during quench hardening of steel, Thermal Processing, 15 July 2022
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