Steel grade guide

SCr440 (41Cr4 / AISI 5140) — chromium steel without molybdenum, in practice

What you gain and lose choosing SCr440 over SCM440: similar carbon, lower cost, less hardenability and no molybdenum to hold back temper embrittlement.

Grade data

SCr440 — Grade data
JIS designation	SCr440
Closest equivalents	AISI 5140 / DIN 41Cr4
Carbon (%)	0.38–0.43
Steel family	Alloy steel (Cr / Cr–Mo)
Heat-treatment route	Quench and temper
Typical surface hardness (HRC)	50–57 HRC
Typical hardness specified in service (HRC)	28–45 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

SCr440 is a chromium alloy steel with essentially the carbon of SCM440 but no molybdenum. It hardens and tempers in the same way and costs less, and the difference shows up in two places: hardenability in the heavier sections, and resistance to temper embrittlement, which is what molybdenum is really bought for.

How it is heat treated

Quench and temper, oil quench, tempered to the specified class or hardness. Because there is no molybdenum, cooling rate through the temper-embrittlement band after a high temper matters more than it does on SCM440 — parts should be cooled briskly rather than allowed to sit in the furnace.

Plating and hydrogen embrittlement

Used at the high property classes it is above the ISO 4042 trigger, so electroplated parts are baked, promptly. Nothing about the chromium-only chemistry lowers the embrittlement risk relative to SCM440 — if anything the toughness margin is smaller, so the bake discipline matters more.

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 when the drawing permits a chromium steel, the sections are moderate and cost matters — general high-tensile bolts, nuts and pins that do not need molybdenum-grade temper resistance.

When not to choose it

Do not choose it for heavy sections that need SCM440 hardenability, for service at elevated temperature, or where the process cannot control cooling after a high temper. If the drawing names 42CrMo4 or SCM440, do not substitute it because the carbon looks similar.

Parts we run in this grade

Bolts Nuts Pins 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.

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.

Decarburization — surface carbon loss after heat treatmentHardness is correct in the core but low at the surface — a file bites the thread crest of a bolt that tests in range on the head. Threads gall, strip or round off at nominal torque, self-tapping screws stop cutting, and after plating the finish can look dull or patchy. On a polished and etched section, the thread flank carries a light, ferrite-rich rim that does not darken with the martensite behind it.

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.

Frequently asked questions

Is SCr440 a drop-in substitute for SCM440?+

No. The carbon is comparable but the hardenability and the temper-embrittlement behaviour are not, and on a heavier section the core will not reach the same properties. Substitutions have to be agreed with whoever owns the drawing, in writing.

Why does the tempering step matter more on SCr440?+

Without molybdenum there is less protection against embrittlement when a chromium steel is cooled slowly through the critical band after a high temper. It is a process-control point, not a materials defect — control the cool-down and the grade behaves.

Other grades we run

S20C S45C SCM435 SCM440 SWCH10B21 SWCH10B33 SUP9 SK85 (SK5)SUJ2 SKS3 SUS420J2 SUS304 All steel grades →

Send the drawing and we will review it