Common methods for hardness testing of metallic materials

Essence of hardness: The ability of a material to resist local plastic deformation (indentation, scratching). Different hardness standards cannot be directly equated, and the selection method should be based on the thickness, size, heat treatment state, and application of the workpiece.

1. Press-in method (the most mainstream in industry, divided into three major categories)

1. Brinell hardness (HBW)

Principle: A hard alloy ball indenter is used to apply a test force to press into the test sample, and the indentation diameter is measured to calculate the hardness.

✅ Applicable to: Soft metals: aluminum, copper, cast iron, low-carbon steel, annealed steel; roughcast, coarse-grained materials.

❌ Not applicable: thin specimens, high-hardness quenched steel, and finished machined surfaces.

Marking example: 220 HBW 2.5/187.5

Advantages: Large indentation, strong representativeness; Disadvantages: Cannot test hard materials, cannot test thin layers.

2. Rockwell hardness (HR series, most commonly used in workshops)

Principle: First apply the initial test force + main test force for indentation, and calculate the hardness based on the difference in residual indentation depth. There is no need to measure the indentation size, making the reading quick.

Commonly used scales:

HRC: Diamond cone indenter, suitable for quenched steel, tool steel, and cemented carbide (20~70 HRC); standard equipment for machining and heat treatment.

HRB: 1.588mm steel ball, mild steel, copper, brass (0~100 HRB)

HRA: High-hardness thin layer, hard alloy, surface quenched parts

✅ Advantages: fast speed, small indentation, and direct measurement of finished parts;

❌ Disadvantages: Small indentation, coarse grain size, and discrete material data; thin parts are prone to puncture.

3. Vickers hardness (HV, preferred for thin layers/small parts)

Principle: The diamond square pyramid indenter allows for the selection of a very small test force (microhardness).

Classification:

1) Macro Vickers HV: high load, medium thickness specimen;

2) Micro-Vickers HV (HV0.1, HV0.05, etc.): Micro-load

✅ Applicable to: thin specimens, surface carburized/nitrided layers, coatings, small parts, metallographic specimens, and thin steel strips.

It can measure the hardness of materials ranging from soft aluminum to hard alloys.

Key point: The micro-Vickers method, not HRC, must be used to detect the surface hardened layer.

4.Leeb hardness (HLD)

Principle: The impact body collides with the test sample, and the hardness is converted by measuring the ratio of rebound velocity.

✅ Advantages: Portable, on-site inspection of large items: large molds, rollers, pressure vessels, and giant workpieces that cannot be sampled; no need to prepare flat specimens.

❌ Limitations: Accuracy is lower than that of desktop machines; significantly influenced by the thickness, surface roughness, weight, and curvature of the test sample; suitable only for rough measurements and cannot be used for arbitration.

Quick Guide for Model Selection (for Engineering Applications)

Quenched steel, molds, and cutting tools → HRC

Brass, annealed low carbon steel → HRB / HBW

Castings, blanks, coarse structures → HBW

Carburizing, nitriding, plating, thin steel strip, metallographic specimen → Micro-Vickers HV

On-site inspection of large-scale, unsampleable structural components → Leeb hardness (for reference only)

General points for attention in testing

The surface of the sample must be flat, free of oxide scale and scratches; it is necessary to polish and polish (the microhardness requirement is very high);

The specimen should have sufficient thickness, and the indentation should not penetrate through the specimen;

Maintain sufficient spacing between the two indentations and the edge of the specimen to avoid stress interference;

Hardness conversion can only be done through empirical comparison tables (such as HRC↔HV↔HBW), and cannot replace direct testing. Arbitration tests should preferably adopt the prescribed methods;

The instrument is calibrated regularly using standard hardness blocks.