Alloy steel, tool steel and mould steel are bought on mechanical properties and heat treatment behaviour rather than on corrosion resistance, and the specification that matters most is often the one that is easiest to leave out: the heat treatment condition and the hardenability. This guide covers the main groups and the verification steps that prevent a costly mismatch.
The main groups
Quenched and tempered structural alloy steel
4140 and its variants are the classic chromium molybdenum grades used for shafts, gears, bolts and general high strength parts. 4340 adds nickel for deeper hardenability, so thicker sections can be through hardened. 4130 chromoly tubing is the standard for aircraft frames, bicycle frames and motorsport structures. EN24T is the equivalent European designation. These grades are supplied in annealed, normalised, or quenched and tempered condition, and the delivered condition determines what you can machine and what strength you obtain.
Carburising and gear steel
8620 and 20CrMnTi are case carburising grades: a hard wear resistant surface is produced over a tough core. They are the standard for gears, shafts and transmission parts, and the specification includes case depth and core hardness, not just the grade name.
Mould steel
P20 is the standard pre hardened mould steel for plastic injection moulds. 718 and NAK80 are higher hardness, better polishability grades used for demanding optical surfaces and engineering plastics. S136 is a corrosion resistant mould steel for plastic with corrosive additives or for medical moulds. H13 is a hot work tool steel for die casting, extrusion and forging dies, supplied pre hardened or for hardening by the buyer.
Cold work, hot work and shock tool steel
D2, A2 and O1 are cold work tool steels: D2 for high wear resistance, A2 for a balance of wear and toughness, O1 for oil hardening and general toolroom use. S7 is a shock resisting grade for chipping hammers and punches. M2 and T15 are high speed steels for cutting tools and drills. The choice is a trade off between wear resistance, toughness and hot hardness.
Ultra high strength and maraging
300M and maraging 250 and 300 grades reach very high strength levels with good toughness, used in aerospace landing gear, tooling and high stress components. Maraging steels achieve strength through ageing rather than quenching, giving low distortion and good machinability before ageing.
Specification axes that separate grades
Hardenability rather than hardness alone
Two steels of the same grade can harden differently through section if their grain size and alloy content differ. Specify the required hardness at a defined depth for a defined section, or reference a hardenability band, rather than only specifying surface hardness.
Cleanliness and microstructure
Non metallic inclusions, segregation and banding degrade fatigue life. For critical parts, specify the cleanliness level and inspection method, since two mills can meet the chemistry and still deliver different fatigue performance.
Decarburisation and surface condition
A decarburised layer is softer than the core and ruins a hardened surface. Specify the maximum allowable decarburisation depth and verify it.
Heat treatment condition and response
Specify whether the material is annealed, normalised or pre hardened, and the hardness range in that condition. For tool steel, specify the intended hardening temperature range and the required hardness after hardening and tempering, so the mill can confirm the material suits your process.
Incoming verification checklist
- Verify chemistry by independent analysis on each heat, focusing on the alloying elements that set hardenability.
- Check hardness across the section, not only at the surface, to detect hardenability or heat treatment problems.
- Inspect for decarburisation on the surface of hardened or heat treated material.
- Confirm microstructure by metallography for critical parts, including carbide distribution in tool steel.
- Measure dimensions and straightness for bar and plate, since distortion in a pre hardened mould plate is expensive to correct.
- For ultrasonic or cleanliness requirements, confirm the inspection standard and obtain the report.
Common sourcing mistakes
- Ordering by grade name without specifying the heat treatment condition, then finding the delivered hardness wrong for machining or for service.
- Assuming the same grade from two mills will harden identically, when hardenability bands differ.
- Ignoring cleanliness requirements for fatigue critical parts.
- Overlooking decarburisation, then discovering the hardened case is not where it should be.
- Specifying tool steel without agreeing the hardening range, then finding it unsuitable for the intended process.
Sourcing alloy and tool steel from Hunan and China
China produces the full range of alloy, tool, mould and high speed steels, supported by integrated special steel mills and a mature tooling industry. Hunan hosts advanced steel and hard material suppliers within its advanced materials base, serving both domestic equipment makers and export customers. LiiFoo maintains verified supplier profiles in the advanced steel materials cluster covering alloy, tool and special steel.
For grade matched quotations, mill certificates and sample material, see the cluster above or contact our sourcing team.
发表回复