Steel grade guide

SUP9 (55Cr3 / AISI 5155) — spring steel heat treatment, decarburization and plating risk

How SUP9 springs are hardened and tempered or austempered, why surface decarburization destroys fatigue life, and why zinc flake is usually the safer finish on a stressed spring.

Grade data

SUP9 — Grade data
JIS designation	SUP9
Closest equivalents	AISI 5155 / DIN 55Cr3
Carbon (%)	0.52–0.60
Steel family	Spring steel
Heat-treatment route	Quench and temper + Austempering
Typical surface hardness (HRC)	50–58 HRC
Typical hardness specified in service (HRC)	40–50 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

SUP9 is a chromium spring steel: enough carbon for high hardness, enough chromium to harden through a wire or a washer section. It is bought for elastic behaviour under repeated load, which changes what "good heat treatment" means — a spring is judged on fatigue life and set resistance, not on a single hardness reading.

How it is heat treated

Two routes are used. Quench and temper is the conventional one. Austempering, holding above the martensite start temperature to form bainite instead, gives better toughness at similar hardness and much less distortion, which is why flat and formed spring parts often take that route. Which one applies is a function of the part and the drawing.

Plating and hydrogen embrittlement

Springs are the highest hydrogen-embrittlement risk population there is: high hardness, high residual tensile stress, and a service load that never lets up. Where the customer allows it, a non-electrolytic zinc-flake finish is the safer route. If the part must be electroplated, it is above the ISO 4042 trigger, the bake must start as soon as possible after plating, and the specification sets the parameters.

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

Not applicable

When to choose it

Choose it for compression, torsion and leaf springs, spring washers and lock washers where elastic performance and fatigue life carry the part.

When not to choose it

Do not choose it for threaded fasteners carrying static tension — a spring steel at spring hardness has very little tolerance for a notch at a thread root. And do not accept a spring with surface decarburization, however good the core hardness looks: fatigue cracks start at the surface, which is exactly where the carbon went.

Parts we run in this grade

Springs Washers By Part →

What tends to go wrong

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.

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.

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

Why is decarburization such a problem on springs?+

Because a spring fails from the surface. A decarburized skin is softer and weaker than the material underneath it, so it becomes the fatigue-crack initiation site while the core hardness test still passes. The drawing should limit decarburization depth, and it should be measured.

Can we zinc plate a hardened spring?+

It can be done, and it carries real delayed-fracture risk. If the customer will accept zinc flake or another non-electrolytic system, that is the safer answer. If electroplating is mandatory, treat prompt de-embrittlement baking as part of the process, not an option.

Other grades we run

S20C S45C SCM435 SCM440 SCr440 SWCH10B21 SWCH10B33 SK85 (SK5)SUJ2 SKS3 SUS420J2 SUS304 All steel grades →

Send the drawing and we will review it