Carbide Hardness and Strength: HRA, HV and Transverse Rupture

Two carbide parts can arrive with the same hardness certificate and behave as if they came from different suppliers. A carbide hardness HRA reading quoted on its own tells a buyer less than it appears to, because hardness, transverse rupture strength and the measurements that sit behind them describe different properties of the same part, and the part is not the same as the joint that holds it. Reading them together is what makes a grade sheet usable.

What Each Measure Describes

Each number describes a different property.

Hardness describes resistance to surface indentation, transverse rupture strength describes resistance to bending, and density and magnetic response describe what the material is made of; none substitutes for another.

The four values that appear on a carbide specification answer four different questions, and the confusion in procurement usually comes from treating them as alternative measures of quality. Hardness, quoted on the HRA scale for cemented carbides, is a surface property: it predicts how quickly a wear scar advances. Transverse rupture strength is a bulk property measured by breaking a test bar, and it predicts how much bending load a section can take before it fractures, which is a different matter from resisting impacts at an edge.

Density and magnetic measurements are indirect but highly informative. Density confirms that the carbide and binder proportions are as specified; magnetic response, often reported as coercive force or magnetic saturation, responds to the binder content and to the state of the carbide skeleton. Between them they separate grades that a hardness reading would show as identical.

What each value on a carbide specification measures, and where it stops being useful.

Value	What it measures	Obtained by	Limitation
HRA hardness	Resistance to surface indentation and abrasion	Indentation test on a prepared surface	Describes a sample, not a batch; says nothing about toughness
HV hardness	Hardness at smaller scale, used for thin layers and coatings	Vickers indentation under low load	Highly sensitive to surface preparation and load selected
Transverse rupture strength	Resistance to bending fracture of a section	Three-point bending of a test bar	Test-bar geometry affects the result; not a direct wear prediction
Density	Correct proportion of carbide and binder	Mass and volume measurement	Does not detect grain size or porosity
Coercive force	State of the binder and carbide skeleton	Magnetic measurement	Interpretation depends on the grade family

The hardness scales used for steels cannot simply be applied to cemented carbide, and the measurement side of this is the work of the national measurement bodies such as NIST and the material and test standards issued by ASTM.

How Hardness Is Measured on Carbide

By indentation, on a prepared surface, as a sample.

Hardness is measured by pressing a diamond indenter into a prepared surface and reading the size of the impression, repeated across a sample rather than taken once.

On cemented carbide the conventional scale is Rockwell A, reported as HRA. The indenter and load are chosen so that the impression stays within the material rather than cracking it, and the surface has to be flat and clean: a ground finish gives a different reading from a sintered skin, and a curved or rough area makes the result meaningless. This is why a hardness figure should always be accompanied by the method, the surface condition and the number of readings taken.

Vickers hardness (HV) is used where the volume of material is small — a coating, a thin section, or a specific feature of a part — because the impression is far smaller and the load can be reduced. Its sensitivity is also its weakness: at low loads, surface preparation and the exact load chosen change the result substantially, so HV figures are only comparable when the conditions match. That distinction matters in wear parts, where a grade may be quoted in HRA while a coating on the same part is quoted in HV.

Matters become more complex around the edge of a part, which is where wear actually happens. Hardness measured on the flat face of an insert describes the material, not the tip condition after brazing and finishing, so the useful verification is a combination of a material measurement and an inspection of the finished assembly. Material engineering reference material, including the resources published by ASM International, covers how these measurements are interpreted across grades.

Reading a Grade Sheet Correctly

A grade sheet is a specification, not a certificate, and it is written for a family of products. The values in it are typical for that family, at a stated binder content and grain size, and they tell a buyer what the material is intended to do rather than what any single part has been measured as doing.

Four habits make a grade sheet useful. First, read hardness, binder content and grain size as a set, because each one is qualified by the others. Second, note whether the strength figure is a typical value for a standard test bar or a minimum guaranteed for production parts; those are different claims. Third, check what the sheet does not state: the joining process the grade is qualified for, the tolerances held on the part, and whether the properties are verified on every batch or only on a type test.

Fourth, compare like with like. A hardness figure from one supplier is comparable with another only if the scale, method and surface condition are the same, and a strength figure only if the test geometry is the same. Where a supplier publishes grade data at the material level, as the cemented carbide grades from Hyperion Materials & Technologies do, that data is a starting point for the conversation rather than a description of the delivered part.

What the Numbers Do Not Cover

They do not describe the part or the joint.

Hardness and strength figures say nothing about porosity, residual stress, the condition of the braze joint or the dimensional accuracy of the pocket the insert sits in.

The most expensive failures in wear parts usually happen outside the properties a grade sheet covers. Porosity or a binder-rich region in the wrong place becomes a crack initiation site. Residual stress from pressing or from the joining cycle can leave a part that meets its hardness specification and still cracks in service. A pocket machined outside tolerance leaves an insert unsupported or standing proud, and the resulting failure looks like a carbide defect.

Geometry is the other blind spot. Two parts of identical grade perform differently if the edge angle, tip radius or insert spacing differs, because geometry controls the stress the material sees. Grade selection and geometry design are therefore one decision, and a grade change cannot compensate for a geometry that loads the edge in the wrong way.

Finally, the numbers describe the material as supplied, not as worn. Hardness does not stay constant across a working surface: the tip that has been ground, heated and run for a season is not the same material as the face that was measured in the factory. This is why service records, rather than certificates, are the better guide to the next specification.

Verifying a Delivered Batch

Verification on a delivered batch is a sampling exercise, not a single measurement. A practical set for a wear-part batch combines hardness readings taken to a defined method on prepared parts, a density check, a magnetic measurement that responds to binder content, and a visual and dimensional inspection of the finished items.

The results only become useful when they are linked to the batch. Lot-level traceability, with inspection results archived against each batch, means a claim can be investigated against a record rather than an impression; the arrangement used at SENTHAI is described on the quality control page. Where an assembly is supplied rather than a loose insert, sample inspection should extend to the joint, because that is where an out-of-specification condition tends to appear first.

For a buyer, three points are worth agreeing before the first order: the test method and the sampling rate, which property is guaranteed rather than typical, and what record travels with the delivery. All three are commercial as well as technical matters, and the applicable terms are stated in the written quotation or order documents. Material suppliers set out the same ground in their own technical documentation, for example the materials engineering resources published by Sandvik.

Cost of Over-Specifying

Specifying a harder grade than the duty requires has a visible price and an invisible cost. The visible part is the price of the part itself: finer powder, tighter process windows and more inspection all cost money. The invisible part is the service life, because a harder grade that begins to chip at the tip can fail sooner than the softer grade it replaced.

Over-specification usually enters through a specification document copied from another machine or another district. Values that were appropriate for a heavy machine on abrasive surfacing can be transferred to a light machine on shoulders, where the impact duty is different and the abrasion lower. The result is a purchase that pays for properties the duty never uses.

The correction is not to buy the cheapest grade but to match the specification to the recorded duty. Manufacturers and distributors publish ranges for this purpose, for example the tooling and wear-part ranges from Kennametal, and published research on carbide structure and properties, such as the peer-reviewed literature indexed at MDPI, shows why a single figure is a poor proxy for performance. Where a fleet has failed-part records, those records are a better input than any catalogue value.

Questions to Put to a Supplier

The questions below are the ones that separate a specification from a description. They are worth asking once, in writing, and keeping with the order documents.

• Method: which hardness scale, which instrument, what surface preparation and how many readings per batch?
• Guarantee: which values are guaranteed and which are typical, and what happens if a batch falls outside them?
• Structure: is porosity or grain growth checked, and is a cross-section result retained with the batch record?
• Joining: which grades in the range are qualified for the brazing or welding process used on the assembly?
• Traceability: what identification accompanies a delivery, and can it be tied to an inspection record?
• Documentation: what is supplied as standard with an order and what can be agreed for a programme?

The definitions behind these answers are collected in the carbide grades reference on this site. A supplier who can answer all six is describing a manufacturing process; one who can answer only the first is quoting a material.