Steel grade guide

SCM435 (34CrMo4 / AISI 4135) — hardness, quench and temper, and ISO 4042 baking

Why SCM435 is the standard class 10.9 and 12.9 fastener steel, how minimum tempering temperature protects toughness, and why de-embrittlement baking after zinc plating is not optional on this grade.

Grade data

SCM435 — Grade data
JIS designation	SCM435
Closest equivalents	AISI 4135 / DIN 34CrMo4
Carbon (%)	0.33–0.38
Steel family	Alloy steel (Cr / Cr–Mo)
Heat-treatment route	Quench and temper
Typical surface hardness (HRC)	48–55 HRC
Typical hardness specified in service (HRC)	33–44 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

SCM435 is a chromium–molybdenum alloy steel and the default choice for high-tensile fasteners. The chromium buys hardenability so a bolt hardens through its section rather than only at the surface; the molybdenum buys temper resistance, so the part still holds strength after the high temper that the property class demands.

How it is heat treated

Quench and temper, normally from an oil quench. ISO 898-1 sets a minimum tempering temperature for the high classes — 425 °C for 10.9 and 380 °C for 12.9 — and that minimum is the toughness of the finished bolt, not a formality. Tempering higher lowers hardness; tempering below the minimum to hit a hardness number is how a batch passes a hardness check and fails in service.

Plating and hydrogen embrittlement

This is the grade where hydrogen embrittlement actually bites. Class 10.9 and 12.9 are above the ISO 4042 trigger, so electroplated parts must be baked, and the bake has to start quickly after plating — a bake run the next shift is far less effective. Where the customer will accept it, a non-electrolytic zinc-flake system removes the hydrogen source instead of managing it.

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 says 10.9 or 12.9, when a socket screw or high-tensile bolt has to be uniform through the section, or when the part will be tempered high and still has to hold strength.

When not to choose it

Do not choose it when the drawing only needs 8.8 — you pay for alloy and you inherit an embrittlement risk you did not need; a boron cold-heading grade does 8.8 more cheaply. It is also the wrong answer for small self-tapping screws, where the requirement is a hard case on a soft core, not through-hardening.

Parts we run in this grade

Bolts Screws Nuts Anchor Bolts By Part →

What tends to go wrong

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.

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.

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.

Frequently asked questions

How many HRC can SCM435 reach?+

Two different numbers matter and the grade table above separates them: the hardness reachable as-quenched, and the hardness actually specified in service after tempering to a property class. The service figure is the one your drawing will call out. Both are typical ranges for small sections and depend on the part and the specification.

SCM435 or 10B21 for a class 8.8 bolt?+

For 8.8 in a cold-formed size, the boron grade is normally the better engineering and commercial answer. SCM435 earns its cost at 10.9 and 12.9, where boron steel runs out of hardenability and temper resistance.

Does an SCM435 bolt have to be baked after zinc plating?+

At class 10.9 or 12.9, yes — it is above the ISO 4042 trigger. The parameters and the maximum delay between plating and baking come from the governing specification; we run de-embrittlement baking in house with logged time and temperature.

Other grades we run

S20C S45C SCM440 SCr440 SWCH10B21 SWCH10B33 SUP9 SK85 (SK5)SUJ2 SKS3 SUS420J2 SUS304 All steel grades →

Send the drawing and we will review it