Steel grade guide

SCM440 (42CrMo4 / AISI 4140) — hardness, tempering and crack sensitivity

How SCM440 differs from SCM435 in practice: more carbon, more strength, more quench-crack sensitivity, and the tempering range to keep away from on Cr–Mo steels.

Grade data

SCM440 — Grade data
JIS designation	SCM440
Closest equivalents	AISI 4140 / DIN 42CrMo4
Carbon (%)	0.38–0.43
Steel family	Alloy steel (Cr / Cr–Mo)
Heat-treatment route	Quench and temper
Typical surface hardness (HRC)	50–58 HRC
Typical hardness specified in service (HRC)	28–48 HRC
Hardenable by heat treatment	Yes

Hardness values are typical industry ranges for the small sections we process, and depend on the part, the section size, the quench and the governing specification. They are not a guarantee and do not replace your drawing or standard.

Reviewed: 2026-08-04

Hardness by grade — reference table Related service: Heat Treatment

What it is

SCM440 is SCM435 with more carbon. That buys higher attainable strength and better wear resistance, and it costs quench-crack tolerance — the same corner radius that survives in SCM435 is less forgiving here. It is the grade of choice where a pin or a bolt has to be hard and strong in a section that a plain carbon steel could not harden through.

How it is heat treated

Quench and temper from an oil quench, tempered to the specified class or hardness. Higher carbon means the as-quenched structure carries more stress, so geometry matters: sharp corners, cross-drillings and abrupt section changes are where the cracks start, and a stress-relief before hardening is cheap insurance on machined parts.

Plating and hydrogen embrittlement

At the strength levels this grade is normally used at, the part is above the ISO 4042 trigger and electroplated parts should be baked as soon as possible after plating. The higher the delivered strength, the longer the bake the specification will call for, and the less tolerance there is for delay.

ISO 4042 de-embrittlement bake

Indicated after electroplating

ISO 4042 calls for embrittlement-relief baking on electroplated fasteners at property class 10.9 and above, or at roughly 390 HV (≈ 40 HRC) and above. Many customer specifications set a lower trigger — the drawing decides, not this page.

Hydrogen de-embrittlement bake planner

Typical ISO 898-1 property classes

10.9 12.9

When to choose it

Choose it for high-tensile bolts, dowel and clevis pins and small shafts in fastener assemblies where 10.9 or 12.9 is required in a section SCM435 would find marginal, or where wear as well as strength is in the requirement.

When not to choose it

Do not choose it when 8.8 will do, and do not choose it for thin cold-formed screws — the extra carbon makes cold heading harder and gains you nothing. Avoid slow cooling through the Cr–Mo temper-embrittlement range after a high temper; the recovery for a batch that has been embrittled that way is scrap.

Parts we run in this grade

Bolts Pins Nuts By Part →

What tends to go wrong

Quench crackingStraight or slightly curved cracks that appear after quenching — under the head fillet, along the shank, across a head recess, at a thread root, or radially at the corner of a nut. They are open and sharp-tipped, and after plating they often show as a dark line that bleeds. Cracked parts may ring dull instead of clean, and a cracked lot normally shows several pieces cracked in the same geometric position.

Hydrogen embrittlement — delayed fracture after platingHeads pop off or bolts split longitudinally hours to days after plating — often 2 to 48 hours after tightening, sometimes weeks later on the shelf. The fracture is brittle with no necking, usually starting under the head fillet, at the first engaged thread or at a thread root. The same lot passed hardness and tensile before plating, there is no overload evidence on the joint, and the failures cluster in the hardest parts of the lot.

Over-tempering — strength lost in the tempering stageHardness and tensile strength are uniformly below the class while the parts are otherwise sound — no scatter, no cracks, no soft patches. Bolts stretch and yield rather than break, joints lose preload, and a tensile test gives a fully ductile fracture with heavy necking. A deep blue-grey oxide film on bare parts can be a hint of a hot temper, but colour is never evidence on its own.

Distortion after heat treatment — bend, ovality, pitch driftParts that were straight and in tolerance before the furnace no longer are. Long screws bow, thin shanks bend, nuts and washers go out-of-round or dish, flatness and perpendicularity fail, and threads that gauged correctly now fail the GO gauge because pitch diameter and helix have moved. Distortion is systematic — the whole lot leans the same way — where handling damage is random.

Frequently asked questions

SCM435 or SCM440?+

SCM435 for cold-formed high-tensile fasteners where formability and crack tolerance matter; SCM440 when the section is heavier, or when hardness and wear are part of the requirement. The hardness ranges in the tables overlap — the difference in practice is hardenability and crack sensitivity, not the headline number.

Why did our SCM440 parts crack in the quench?+

On this grade the usual causes are geometry and delay, not the furnace: sharp internal corners, unblended section changes, cross-holes near a thread root, or parts held too long before tempering. Send the drawing with the crack location and it is normally visible in the geometry.

Other grades we run

S20C S45C SCM435 SCr440 SWCH10B21 SWCH10B33 SUP9 SK85 (SK5)SUJ2 SKS3 SUS420J2 SUS304 All steel grades →

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