SK85 / SK5 (AISI 1084–1086) — high-carbon steel for washers and flat spring parts
How SK85 is hardened and tempered or austempered for spring washers and flat parts, why distortion and quench cracking dominate, and what finish suits a part at this hardness.
Grade data
| JIS designation | SK85 (SK5) |
|---|---|
| Closest equivalents | AISI 1084–1086 / DIN C85 |
| Carbon (%) | 0.80–0.90 |
| Steel family | Tool / high-carbon steel |
| Heat-treatment route | Quench and temper + Austempering |
| Typical surface hardness (HRC) | 58–64 HRC |
| Typical hardness specified in service (HRC) | 45–60 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:
What it is
SK85, still widely called SK5, is a high-carbon tool and spring steel. There is far more carbon here than in any fastener grade, so it reaches high hardness easily and it is correspondingly brittle at that hardness. It appears in our work as spring washers, lock washers, retaining rings, shims and small flat spring parts.
How it is heat treated
Harden and temper, or austemper. At this carbon level the as-quenched structure is highly stressed, so tempering is not optional and the temper defines whether the part is a spring or a piece of glass. For thin flat parts, austempering is often chosen because it gives usable toughness with far less distortion than a quench-and-temper cycle.
Plating and hydrogen embrittlement
A part at this hardness is above the ISO 4042 trigger. Where the finish is negotiable, phosphate with oil or a zinc-flake system avoids the electrolytic hydrogen source; where zinc electroplating is specified, prompt de-embrittlement baking belongs in the process.
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 plannerTypical ISO 898-1 property classes
When to choose it
Choose it for flat spring parts, spring and lock washers and retaining rings, where hardness, springback and wear matter and the part is thin enough to harden through.
When not to choose it
Do not choose it for threaded fasteners in tension or for parts that see impact — at working hardness it has very little ductility. Do not choose it where distortion tolerance is tight without agreeing the route first, and do not expect any corrosion resistance from the grade itself.
What tends to go wrong
Frequently asked questions
Why do thin SK85 parts come out bowed?+
Because a thin section cools unevenly and the transformation itself changes volume. Fixturing, quench route and austempering instead of quench and temper are the levers; on a flat part it is normally cheaper to change the route than to straighten afterwards.
What hardness should we specify?+
That is a drawing question, and the answer follows the function: springback and fatigue for a spring washer, wear for a retaining ring. The table above gives the typical ranges we see on small sections — the specification, not the grade, sets the number.