SCM435 is a chromium molybdenum alloy structural steel defined in JIS G4053, the Japanese standard for alloy steels for machine structural use. The designation follows the JIS convention: SCM identifies a chromium molybdenum steel, and 435 indicates the nominal chemistry of the group. The actual carbon content is about 0.33 to 0.38 %, so the grade is a medium carbon alloy steel that is intended for use after quenching and tempering rather than in the as supplied condition.
Chromium and molybdenum together give the steel its characteristic behaviour. Chromium increases hardenability and contributes wear resistance, while molybdenum strongly increases hardenability, suppresses temper brittleness and raises high temperature strength. With proper heat treatment the grade reaches a high static strength, good impact toughness and a high fatigue limit, which is why it is specified for components that carry heavy alternating loads.
Chemical Composition
The analysis is controlled within tight ranges to give repeatable hardenability from heat to heat, which matters for parts produced in large batches.
| Element | Typical range, % |
|---|---|
| Carbon | 0.33 to 0.38 |
| Silicon | 0.15 to 0.35 |
| Manganese | 0.60 to 0.90 |
| Chromium | 0.90 to 1.20 |
| Molybdenum | 0.15 to 0.30 |
| Sulfur, phosphorus | Permissible residual levels of 0.030 or lower |
| Nickel, copper | Permissible residual levels of 0.25 and 0.30 or lower |
Mechanical Properties After Heat Treatment
Properties quoted by suppliers are for the quenched and tempered condition on a defined test blank. Representative values for the grade are a tensile strength of 985 MPa or more, a yield strength of 835 MPa or more, elongation of 12 % or more, reduction of area of 45 % or more, Charpy impact energy of 63 J or more and a hardness not exceeding 229 HB in the specified condition.
These figures apply to the standard test section. A larger ruling section cools more slowly during quenching, so the properties obtained at the centre of a heavy part are lower than those measured on a small test blank. The ruling section must therefore be considered when the grade is selected and when final properties are agreed.
Heat Treatment and Processing
The steel is normally quenched and tempered, and its hardenability is high enough that it can be oil quenched in moderate sections. Because the carbon level is medium, the fully hardened condition is difficult to machine, so rough machining is usually done before hardening and finishing after. Welding is not recommended for loaded joints: the carbon and alloy content give a high hardenability, the heat affected zone can become hard and crack sensitive, and the welds would in any case be weaker than the parent metal. Where a welded joint is unavoidable, preheat, controlled heat input and immediate post weld heat treatment are required. The grade retains useful strength at elevated temperature and can be used for parts that operate hot, provided the tempering temperature chosen is comfortably above the service temperature.
Applications and Supply
Typical applications include transmission and engine parts for vehicles, rotors and main shafts of turbine generators, heavily loaded transmission shafts, connecting and drive components, large section machinery parts and high strength fastener stock. Plate and bar are supplied in the annealed, normalized or quenched and tempered condition with certificates covering chemical composition and mechanical properties; the required heat treatment state and hardness range must always be stated on the order.
Frequently Asked Questions
Q: Which standard defines SCM435?
A: JIS G4053, the Japanese standard for alloy steels for machine structural use, which specifies the chemistry and the property requirements for the grade.
Q: What is the carbon content of SCM435?
A: Approximately 0.33 to 0.38 %, which places it among the medium carbon alloy steels and requires a hardening and tempering treatment for loaded service.
Q: Is SCM435 weldable?
A: It is not recommended for welded structural joints because of its high hardenability. If welding is required, preheat, controlled heat input and post weld heat treatment are needed.
Q: What hardness is normally specified?
A: In the standard quenched and tempered condition the hardness is typically not more than 229 HB, but the required range should be agreed according to the component and its ruling section.
Q: Why does the test blank size matter?
A: Because hardenability is limited by cooling rate. A small test blank cools faster than a heavy component, so it shows higher properties than the core of the finished part.
