hardness test

Core Principle

Common Test Methods and Applicable Scenarios

I. Common Methods for Metals and Hard Materials

1. Brinell Hardness (HB)

• Principle: A cemented carbide ball (or steel ball) with a diameter of 1–10 mm is pressed into the material surface under a high load (250–3000 kgf). After holding for a specified time, the indentation diameter is measured, and the hardness value is calculated using a formula (unit: HBW, for cemented carbide ball).

• Applicable Materials: Metals with lower hardness (e.g., low-carbon steel, cast iron, copper alloys, aluminum alloys), or materials with relatively rough surfaces (accommodating larger indentations).

• Advantages: Large indentation, highly representative result (reflects the overall hardness of the material);

• Disadvantages: The indentation damages the sample, unsuitable for thin materials or precision parts.

• Standards: GB/T 231.1, ISO 6506-1, ASTM E10.

2. Rockwell Hardness (HR)

• Principle: A diamond cone (120° apex angle) or a steel ball (1.588 mm diameter) is used as the indenter. A preliminary load (10 kgf) is applied first, followed by a main load (60/100/150 kgf). After unloading, the hardness value is calculated from the difference in indentation depth.

• Characteristics: Uses multiple "scales" corresponding to different indenters and loads, covering different hardness ranges:

• HRC: Diamond cone + 150 kgf, suitable for high-hardness materials (e.g., quenched steel, tool steel, hardness 20–70 HRC);

• HRB: Steel ball + 100 kgf, suitable for medium-to-low hardness materials (e.g., mild steel, copper alloys, 20–100 HRB);

• HRA: Diamond cone + 60 kgf, suitable for ultra-hard materials (e.g., cemented carbide, 20–88 HRA).

• Advantages: Small indentation (minimal sample damage), fast testing speed, suitable for batch inspection;

• Disadvantages: Small indentation yields lower representativeness (multiple measurements with averaging are needed).

• Standards: GB/T 230.1, ISO 6508-1, ASTM E18.

3. Vickers Hardness (HV)

• Principle: A square-based diamond pyramid (136° apex angle) is used as the indenter, pressed into the material under a low load (1–120 kgf). The diagonal length of the indentation is measured, and the hardness value is calculated using a formula (unit: HV).

• Applicable Materials: Thin materials (e.g., sheet metal, coatings), high-hardness materials (e.g., ceramics, cemented carbide), precision parts (e.g., gear tooth surfaces).

• Advantages: Wide load range (can test materials from extremely soft to extremely hard), high precision, results under different loads are directly comparable (no scale differences);

• Disadvantages: Low testing efficiency (requires diagonal measurement, suited for precision laboratory testing).

• Standards: GB/T 4340.1, ISO 6507-1, ASTM E92.

4. Leeb Hardness (HL)

• Principle: An impact body (with a diamond-tipped indenter) strikes the material surface at a fixed velocity. The rebound velocity is measured to calculate the hardness value (HL), which can be converted to other hardness values such as HB, HR, etc.

• Applicable Scenarios: On-site testing of large workpieces (e.g., machine tool guideways, pressure vessels) without sampling (portable).

• Standards: GB/T 17394, ISO 16859, ASTM A956.

II. Common Methods for Non-Metallic Materials

1. Shore Hardness (HS)

• Principle: A specifically shaped indenter pin (conical or flat) is pressed into the material under spring force. Hardness is determined based on penetration depth, using two common scales:

• Shore A (HA): Suitable for soft materials (e.g., rubber, foam, soft plastics, hardness 0–100 HA);

• Shore D (HD): Suitable for hard materials (e.g., rigid plastics, nylon, PVC, hardness 0–100 HD).

• Advantages: Simple to operate, portable, suitable for on-site rapid inspection;

• Standards: GB/T 2411, ISO 868, ASTM D2240.

2. Knoop Hardness (HK)

• Principle: An elongated rhombic diamond indenter (long/short diagonal ratio ≈7:1) is pressed into the material under a low load, and the long diagonal length is measured to calculate hardness (unit: HK).

• Applicable Materials: Extremely hard and extremely thin materials (e.g., ceramics, glass, metal foils, coatings), with shallow indentation (minimal sample damage).

• Standards: GB/T 18449.1, ISO 4545, ASTM E384.

Key Influencing Factors

• Indenter Type: Steel balls (soft materials) and diamond (hard materials) must be matched to the material hardness (to prevent indenter deformation);

• Load Magnitude: Excessive load may penetrate through thin materials, while insufficient load yields indistinct indentations (select based on material thickness);

• Sample Condition: The surface must be flat and free of oxide layers/oil contamination (otherwise abnormal indentations may result);

• Dwell Time: The load holding time (typically 10–30 s) affects the adequacy of plastic deformation (especially for high-temperature materials or plastics).

Significance of Testing

• Performance Evaluation: Hardness is positively correlated with material strength (e.g., tensile strength) and wear resistance (e.g., high-hardness steel is more wear-resistant);

• Quality Control: Verify heat treatment effectiveness (e.g., HRC value of quenched steel must meet requirements) and material consistency (hardness deviation of parts within the same batch must fall within range);

• Material Selection Reference: Select materials with matching hardness based on application scenarios (e.g., gears require high hardness for wear resistance, gaskets require low hardness for sealing).