What a hardness tester is
A hardness test measures the resistance of a material to being pressed into by a harder body, and reports the result on a scale. The measurement is a proxy: in metals it correlates broadly with tensile strength, it is far quicker than a tensile test, and in most forms it leaves only a small mark. That is why it is so widely used as an incoming inspection and as a check after heat treatment.
The method matters more than the instrument. A hardness number without its scale is meaningless, because Rockwell, Brinell, Vickers and rebound methods press differently, measure differently and produce figures that are not directly comparable. The same piece of steel will report one figure on a bench tester, a different one on a portable rebound device, and a third on a Vickers machine - and all three can be correct on their own terms.
The practical selection problem is therefore which method suits the part: its size, whether it can be brought to a bench, whether the surface can be prepared, whether a visible mark is acceptable, and what the governing specification asks for. This page works through those, and through the question of where a portable instrument is and is not a valid substitute for a bench machine.
How a hardness tester measures
A bench test presses a defined indenter into the surface under a defined load and measures the result. In the Rockwell method the depth of penetration is read directly, and the scale letter encodes both the indenter and the load - which is why the reading is immediate and why the scale must always be quoted with it. In Brinell and Vickers the size of the impression left behind is measured optically and converted, which is slower and leaves a larger mark but is far less sensitive to surface condition.
The thickness of the specimen relative to the depth of the impression is what most often invalidates a reading. If there is not enough material beneath the indenter, the anvil supports part of the load and the number comes out high - which is a false pass rather than an obvious error. The minimum thickness rule attached to each method exists for this reason, and it is checked against the drawing rather than assumed.
Portable rebound testers work on a different principle: a defined body is struck against the surface and its rebound velocity is compared with the impact velocity. This is genuinely useful on large parts that cannot be brought to a machine, but the result depends on the mass and rigidity of the part, so a reading taken on a thin plate is not comparable with one taken on a heavy casting. The instrument converts its rebound value into other scales mathematically, and those conversions carry limits that have to be respected.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Test method and scale | The bench methods with their scale letters, and the portable rebound method | Which scale the governing specification names, and whether the mark left behind is acceptable on the part. |
| Indenter type | Diamond cone, hardened ball, diamond pyramid, and the rebound impact body | What the instrument can measure. Some indenter and load pairings suit hard materials and some suit soft ones, and the scale letter encodes that pairing. |
| Test load | Fixed for a given scale, or selectable over a range | Which scales the instrument can cover, and whether one machine is enough for more than one specification. |
| Bench or portable format | A fixed machine with a stage and anvil, or a hand-held rebound device | Whether the part can be brought to an instrument at all, and whether the reading has to be taken in situ. |
| Specimen support and minimum thickness | The anvil arrangement, and the smallest section the method permits | Whether the reading is valid at all. An under-supported specimen reads high and passes a part that should fail. |
| Surface preparation requirement | The finish the method expects, and how much preparation is carried out on the part | Whether preparation time has to be allowed for, and whether the preparation itself is acceptable on a finished component. |
| Reading and data handling | Direct display, and whether readings can be stored, averaged or exported | Whether a batch record has to be produced, and whether individual readings or a statistical result are wanted. |
| Verification accessories | Reference blocks matched to the scale in use | Whether the instrument can be shown to be reading correctly, which most quality systems require at a defined interval. |
How to choose a hardness tester
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| Material family | Metals, plastics and elastomers use different scales, and no single instrument covers all three | State the material and the specification that governs the test. |
| Scale named in the specification | The number is only meaningful with its scale, and the required scale fixes the instrument | Quote the scale and the acceptance band from the drawing or the standard. |
| Part size and mass | A heavy casting cannot be taken to a bench machine, and a light part cannot be tested reliably by rebound | Give the largest and smallest parts, with approximate mass and section thickness. |
| Access to the test surface | The indenter has to reach the surface square to it | Describe the geometry, and whether a special anvil or support is available. |
| Whether a mark is acceptable | Some methods leave a visible impression, which rules them out on a finished surface | State whether the part is to be used or sold after testing. |
| Thickness at the test point | Below the minimum for the method the reading is invalid rather than merely approximate | Give the wall or section thickness at the point to be tested. |
| Record and verification requirement | Traceable reference blocks and recorded readings may be imposed by the customer’s quality system | State whether certificates or batch records are required, and at what interval the instrument is verified. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a hardness tester usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a hardness tester |
| Tensile testing machines | Pull a prepared specimen to failure to give strength, elongation and modulus directly. The test is destructive and needs a machined specimen, and it is much slower - but it measures the property itself rather than a proxy for it. Where a design calculation or a certification demands a tensile figure, hardness cannot be substituted. |
| Compression testing machines | Apply load to crush or deform a specimen, which is the relevant test for concrete, packaging and elastomeric parts. It answers how much load the part carries, not how hard its surface is. |
| Impact testing machines | Measure the energy absorbed when a notched specimen is broken, which is about toughness rather than resistance to indentation. Two steels of equal hardness can differ markedly in impact behaviour, and a hardness test would not reveal that difference. |
| Coating thickness gauges | Measure the thickness of a layer on a surface rather than a property of the substrate. On a surface-treated part both readings are often wanted, but they answer different questions and neither reports the other. |
| Surface roughness testers | Report the texture of a surface in profile terms, which governs friction, sealing and appearance. Surface finish also affects a hardness reading, which is why the two instruments so often share an inspection bench. |
| Material testing machines | A general heading for machines that apply load to a specimen, covering tension, compression, bending and fatigue in one frame. Where a laboratory needs several of those, a universal machine covers them; hardness remains a separate, quicker and usually non-destructive check. |
Hardness and toughness are the pair most often treated as interchangeable. They are not: a surface treatment can raise hardness substantially while leaving the material’s toughness where it was, or lowering it.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- Material, and the specification or standard that governs the test
- Required scale and acceptance band
- Largest and smallest parts, with approximate mass
- Section or wall thickness at the test point
- Whether the test surface is accessible, and its geometry
- Whether a visible mark is acceptable after testing
- Whether readings must be recorded for a batch
- Whether reference blocks or verification accessories are required
- Quantity, destination port and required delivery date
