When S45C runs out of hardenability — typically above 50 mm cross-section — SCM440 is the next steel to reach for. Standardized under JIS G4051, SCM440 adds chromium and molybdenum to a medium-carbon base, dramatically extending the depth to which the steel can be hardened by quenching. The result is a steel capable of delivering 930 MPa or higher tensile strength uniformly through sections up to 150 mm — making it the workhorse of heavy-duty shaft, gear, and fastener applications worldwide.
Reading the SCM440 Designation
The prefix “SCM” identifies the steel as a chromium-molybdenum alloy type within JIS G4051. The number “440” encodes the alloy series (4) and approximate mean carbon content (40 → 0.40 % C). Compare SCM435 (C ≈ 0.35 %) and SCM445 (C ≈ 0.45 %) in the same family: only the carbon level shifts while Cr and Mo additions remain the same.
Chemical Composition — SCM440, AISI 4140, and DIN 42CrMo4
| Element | SCM440 (JIS G4051) | 4140 (AISI / SAE) | 42CrMo4 (DIN EN 10083-3) |
|---|---|---|---|
| C (%) | 0.38 – 0.43 | 0.38 – 0.43 | 0.38 – 0.45 |
| Si (%) | 0.15 – 0.35 | 0.15 – 0.35 | ≤ 0.40 |
| Mn (%) | 0.60 – 0.85 | 0.75 – 1.00 | 0.60 – 0.90 |
| Cr (%) | 0.90 – 1.20 | 0.80 – 1.10 | 0.90 – 1.20 |
| Mo (%) | 0.15 – 0.30 | 0.15 – 0.25 | 0.15 – 0.30 |
| P (%) | ≤ 0.030 | ≤ 0.035 | ≤ 0.025 |
| S (%) | ≤ 0.030 | ≤ 0.040 | ≤ 0.025 |
Mechanical Properties
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Annealed | ~750 | ~600 | ~20 | ≤ 229HBW |
| Q&T (temper 620 °C, standard) | ≥ 930 | ≥ 785 | ≥ 12 | 277 – 321HBW |
| Q&T (temper 530 °C, higher strength) | ≥ 1080 | ≥ 930 | ≥ 10 | 321 – 375HBW |
| Q&T (temper 480 °C, max strength) | ~1200 | ~1050 | ≥ 9 | ~360HBW |
Heat Treatment
① Annealing
Full anneal: 830–860 °C, furnace cool to ≤ 650 °C at ≤ 15 °C/h, then air cool. Used to soften the steel for machining or cold forming. Delivers ≤ 229HBW — most SCM440 bar stock arrives in the annealed condition for heavy-stock removal machining before final heat treatment.
② Quenching and Tempering (Q&T)
Austenitize at 830–880 °C, soak adequately for section size, quench in oil (water quench risks distortion and cracking in complex sections). Temper immediately — letting the part cool to room temperature before tempering significantly increases cracking risk. Recommended temper range: 530–650 °C depending on target strength level.
③ Stress Relieving (after machining)
For precision components, a post-machining stress relief at 550–600 °C (below the temper temperature) can be applied to minimize distortion during service. This step is optional but standard practice in gear and spindle manufacturing.
Hardenability — Why SCM440 Outperforms S45C for Large Sections
Hardenability describes how deeply a steel can be hardened by quenching — not how hard the surface gets, but how far the hardness extends into the cross-section. It is governed primarily by alloying elements, with Cr and Mo being among the most effective additions.
| Steel | Max reliable section (oil quench) | Core hardness at max section | Key alloying |
|---|---|---|---|
| S45C | ~50 mm dia. | Drops below 25HRC | Carbon only |
| SCM435 | ~100 mm dia. | ~35HRC (core) | Cr + Mo |
| SCM440 | ~150 mm dia. | ~38HRC (core) | Cr + Mo |
| SNCM439 (Ni-Cr-Mo) | > 200 mm dia. | ~40HRC (core) | Ni + Cr + Mo |
In practical terms: if you specify S45C Q&T for a 100 mm diameter shaft, the core may achieve only 130–150HBW — well below the Q&T specification. The same shaft in SCM440, oil quenched and tempered, will show 277–321HBW uniformly from surface to core.
Common Applications
Any shaft exceeding ~50 mm diameter that requires Q&T properties through its full cross-section. Machine tool spindles, rolling mill shafts, and propeller shafts are typical examples.
The combination of high fatigue strength, toughness, and hardenability makes SCM440 standard for automotive and industrial crankshaft forgings. Journal surfaces are often induction hardened after Q&T.
Q&T SCM440 meets the mechanical requirements of ISO 898-1 property class 10.9 (min. 1040 MPa tensile) and 12.9 (min. 1220 MPa tensile) depending on tempering level. Widely used in structural and automotive fastener applications.
Where S45C induction hardening is insufficient for the load cycle — large module gears, heavily loaded helical gears — SCM440 provides the core toughness and fatigue strength needed under combined bending and contact stress.
For lower-cost tooling applications where tool steel grades (SKD, SKH) are over-specified, SCM440 Q&T provides adequate hardness (~321HBW) with better machinability and lower material cost.
SCM440 (4140) is used globally in pressure-containing components requiring ≥ 930 MPa tensile at elevated temperatures, where carbon steel would temper-soften during service.
Machinability and Weldability
Machinability
In the annealed condition (≤ 229HBW), SCM440 machines comfortably — machinability index approximately 55–65 % relative to B1112. As hardness rises after Q&T (277–321HBW), cutting parameters must be reduced accordingly. Best practice is to rough-machine in the annealed condition, heat-treat, then finish-machine to final dimension with carbide tooling.
Weldability
SCM440 has a carbon equivalent of approximately 0.70–0.80 %, placing it firmly in the “difficult to weld” category. Welding is possible but requires 200–300 °C preheat, controlled interpass temperature, low-hydrogen consumables, and mandatory PWHT (550–620 °C). In most structural applications, bolting or mechanical joining is preferred over welding.
Practical Trouble Spots
SCM440 vs. S45C — When to Switch
| Criterion | Use S45C | Use SCM440 |
|---|---|---|
| Cross-section diameter | ≤ 50 mm | > 50 mm |
| Q&T tensile requirement | ≤ 780 MPa | > 780 MPa (or 930 MPa+) |
| Surface hardening only | Induction hardening ✅ | Induction hardening ✅ |
| Material cost | Lower | +30 – 50 % |
| Machinability (annealed) | Slightly easier | Similar (≤ 229HBW) |
| Fatigue / impact requirements | Moderate | High or critical |
| Welding in production | Conditional (preheat) | Difficult — avoid |
Summary
- SCM440 (JIS G4051) is a Cr-Mo alloy steel equivalent to AISI 4140 and DIN 42CrMo4, with C: 0.38–0.43 %, Cr: 0.90–1.20 %, Mo: 0.15–0.30 %.
- Q&T properties: ≥ 930 MPa tensile (620 °C temper), up to ≥ 1080 MPa (530 °C temper); hardenability effective to ~150 mm cross-section.
- Critical handling rule: temper within 1–2 hours of quenching, before the part drops below 80 °C, to prevent quench cracking.
- Choose SCM440 over S45C when: cross-section > 50 mm, tensile requirement > 780 MPa, or high fatigue/impact loading governs.
- Do not over-specify: for ≤ 50 mm lightly loaded shafts, S45C Q&T is fully adequate at significantly lower cost.
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