Reading the lines
| Item on the spec | What it generally means | What to ask |
|---|---|---|
| Brinell (often written HB or HBW) | A hard ball is pressed into the surface and the size of the impression is measured | Which ball and load, and where on the part was it tested? |
| Rockwell (HR plus a letter, such as HRC) | An indenter is pressed under a set load and the depth of penetration is read | Which Rockwell scale, and what test location? |
| A number with no method | Not interpretable | Ask for the method and the standard |
| A range such as “min X” | A lower limit, not a typical value | Is it a minimum, a target or an average? |
| Surface vs core | Hardness can differ near the surface and in the interior after heat treatment | Where was it measured? |
Why scales are not interchangeable
Hardness scales measure different things, and a number without its scale is not a specification.
- Each method loads the material differently, so numbers are not equal across scales.
- Conversion tables exist but are approximations and depend on the material.
- For a quote comparison, ask each supplier for the value in the same scale, with the test method.
What hardness does and does not say
Hardness predicts wear resistance within a family of steels, and says nothing about toughness.
- It is related to resistance to indentation and often to abrasive wear.
- It does not describe toughness or how the edge fails on impact.
- A very high reading with poor toughness can chip, as discussed in our hardness-versus-toughness article.
What to put in a purchase spec
A hardness line is only useful if it names the scale, the method and the location.
- The scale and method, with the standard named by the supplier.
- The location and number of tests.
- Whether the figure is a minimum, a range or a typical value.
A short email you can send
Ask the supplier: what hardness scale and method do you use for this edge, at which location, how many tests per lot, and can you send the report for the last lot? Ask the same of every supplier you compare. Clear answers tell you the supplier measures; vague ones tell you that comparison on this point is not yet possible and that a sample test of your own may be needed.
Boundary: general background only; no test standard was read and no hardness values or conversions are given. Use the supplier’s documented test method.
Further reading on this site.
Steel Grades for Plow Edges: Hardness, Toughness and Weldability Trade-Offs ↗AASHTO and DIN Bolt Patterns for Plow Edges: Reading the Standards ↗Carbide snow plow blades ↗Prop 65 and Carbide Products: Why a Warning Label May Appear on California Shipments ↗What a Standard Carbide-Blade Specification Covers: A Quote Checklist ↗AQL Sampling for Incoming Edges: How to Choose a Sample Size ↗
