Washers — no thread, no property class, and a stacking problem
Plain washers have no property class — only ISO 7089 hardness classes. Why flat blanks shadow each other in a barrel, why thin sections need low-force hardness testing, and when a hardened conical washer needs relief baking.
At a glance
| Thread / dimensional standard | ISO 7089 / DIN 6796 · no thread |
|---|---|
| Tolerance class | No thread — nothing in ISO 965 or ISO 1502 applies. |
| Barrel window | nominal 3 – 24 mm, thickness 0.5 – 4 mm |
| Rack / basket | large-series and thin blanks that nest |
| Strength specification | no property class — 100/200/300 HV classes |
| ISO 4042 relief bake | Depends on the hardness class |
| Reviewed | 2026-08-04 |
A washer has no thread to gauge, no property class to certify and, in the plain versions, no strength requirement at all. That makes it the easiest part on this list to specify and one of the harder ones to plate evenly — because flat blanks stack face to face and shield exactly the surface that carries the joint load.
Why this shape is processed the way it is
Flat blanked discs are the classic barrel problem. They stack, and a stacked pair shields the faces in contact, so the deposit comes out thinnest precisely where the bearing face needs it. Thin washers below about a millimetre also cone slightly when they are hardened, and a coned washer will not sit flat under a head — it rocks, it fretts, and it loses preload. Load size, rotation and dwell decide coverage far more than chemistry does.
Grades we run in this shape
Where these parts go
Threads, tolerances and dimensional standards
There is no thread here, so nothing in ISO 965 or ISO 1502 applies. The controlling dimensions are the inside diameter, outside diameter, thickness and flatness, taken from ISO 7089 for product grade A, ISO 7090 for chamfered, ISO 7091 for grade C, ISO 7093-1 for the large series and ISO 10673 for washers used with tapping screws. Spring and conical types come from DIN 127, DIN 6796 and DIN 6798, and disc springs from DIN 2093 — a genuinely different family with a load curve, not a flat spacer.
Size band and handling route
Nominal sizes from 3 to 24 mm and thicknesses of roughly half a millimetre to four millimetres cover the normal barrel window. The size that matters for handling is not the diameter but the thickness-to-diameter ratio: thin large-diameter blanks nest hardest, and those are the ones that go into a spaced load or on a rack even when their nominal size says barrel.
Strength specification
Plain washers have no property class — ISO 898 simply does not apply to them, and a drawing that asks for a "class 8.8 washer" is asking for something that does not exist. What ISO 7089 and ISO 7091 do specify is a hardness class: 100 HV, 140 HV, 200 HV or 300 HV, which is a material and heat-treatment requirement rather than a load rating. The exceptions are the sprung types. Conical spring washers to DIN 6796 and disc springs to DIN 2093 are hardened, preset and specified by load at a stated compressed height, which puts them closer to a spring than to a spacer.
No property class exists for this part type. Anything on a drawing that reads like one is a specification error worth fixing before the order runs.
Hardness: ISO 7089 / ISO 7091 hardness classes 100 HV · 140 HV · 200 HV · 300 HV; DIN 2093 disc springs ≈ 42–52 HRC
Coating thickness and what limits it
With no thread allowance to respect, washers are the one part type on this list where coating thickness is limited by the surface rather than by a fit, so the heavier ASTM B633 service conditions are all realistic. The constraint moves entirely to uniformity: what matters is not the average thickness on a lot but the minimum on the bearing face, and the two can differ by a factor of two on a badly loaded barrel. Specify where the measurement is taken, and the number stops being an argument.
- ASTM B633 SC 2 — Fe/Zn 8
- SC 3 — Fe/Zn 13
- SC 4 — Fe/Zn 25
Hydrogen embrittlement relief
This is where the answer genuinely splits, which is why the summary above says so rather than giving one verdict. A 200 HV plain washer is nowhere near the ISO 4042 threshold of about 390 HV, so a relief bake is neither required nor useful. A 300 HV class washer is still below it. A hardened conical washer to DIN 6796 or a disc spring to DIN 2093 is far above it, is loaded in service, and behaves like a spring: if it is electroplated it must be relieved as soon as possible and normally within 4 hours — and there is a good argument for not electroplating it at all.
Hydrogen de-embrittlement bake checkerHow this part fails
- Thin patch on the bearing face
Two washers travelled through the barrel stuck face to face. The outside faces measure to specification and the shielded faces do not, so the lot passes an average thickness check and fails in service on the side that matters.
- Coning after hardening
A thin blank distorts into a shallow dish during quenching. It no longer seats flat, the load path moves to the rim, and preload is lost the first time the joint is torqued.
- Loss of load on a conical washer
A DIN 6796 or DIN 2093 part exposed to a temperature above its original temper relaxes: the free height drops and the load at working height goes with it. A de-embrittlement bake chosen for a bolt, applied to a hardened washer, does exactly this.
- Brinelled bearing face
A soft washer under a hardened bolt head indents on first tightening. The joint loses preload immediately and the washer is often blamed for a corrosion failure that started as a mechanical one.
What to check on the lot
- 1Hardness with a test force the section can carryThin parts need low-force Vickers to ISO 6507-1 or superficial Rockwell HR15N to ISO 6508-1. A standard HRC indent on a one-millimetre washer reads the anvil underneath as much as the part, and the number that comes back is meaningless.
- 2Thickness and flatness before coatingMeasure the blank, not just the finished part. Coning and thickness variation introduced in heat treatment are invisible under a coating and are the usual reason a washer will not seat.
- 3Coating thickness on the bearing faceISO 3497 readings taken on the load-carrying face specifically, with the sampling position written into the specification. This is the single most useful change a washer drawing can make.
- 4Salt spray by lot, on the right faceISO 9227 exposure with the acceptance criterion naming the surface. A washer that fails from the shielded face is a loading problem, not a chemistry problem, and only a face-specific criterion will show it.
Test methods
Standards cited on this page
Questions engineers ask
What property class should we specify for a washer?+
None — there is no property class for plain washers, and ISO 898 does not cover them. Specify the hardness class from ISO 7089 or ISO 7091 instead, together with the material and the coating. For sprung types, specify the standard (DIN 6796 or DIN 2093) and the load at working height, because those parts are graded by load, not by hardness alone.
Can washers take a thicker zinc coating than bolts?+
Yes, and this is one of the few places where the answer is that simple. A washer has no thread whose pitch diameter can grow, so the heavier ASTM B633 service conditions are all available. The limit is coverage, not clearance — which is why the specification should say where the thickness is measured rather than just how thick.
Why did our spring washers go soft after plating?+
Almost certainly the de-embrittlement bake. A hardened conical washer was tempered at a specific temperature during manufacture; a relief bake at or above that temperature relaxes it and the load at working height drops. Hardened sprung washers need the bake temperature checked against their temper, or a coating route that does not need a bake at all.
We reply to quote requests within 24 hours.
Guidance based on published standards and general fastener metallurgy. Every figure is a typical range that depends on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. Coating designations describe what a drawing can call for, not a result guaranteed on your parts without testing.