Hardness Test Procedure for Stainless Steel Tubes
1. Specimen and Surface Preparation (Annex A2)
- Location: The test may be conducted on the outside surface, inside surface, or wall cross-section, depending on the product specification and tube geometry.
- Surface Condition: The test surface must be smooth and properly prepared (e.g., grinding or light machining) to ensure an accurate reading. A minimum amount of material may need to be removed to account for surface irregularities or decarburization.
2. Test Equipment and Setup
- Tester: Use a Rockwell hardness testing machine calibrated per ASTM E18.
- Indenter: For the HRB scale, the indenter must be a 1/16-inch (1.588 mm) diameter steel ball.
- Loads: Apply a minor load of 10 kgf, followed by a major load of 100 kgf.
3. Test Execution
- Apply Minor Load: Position the tube specimen on the anvil. Apply the 10 kgf minor load and set the datum (zero position).
- Apply Major Load: Apply the 100 kgf major load smoothly and without impact.
- Dwell Time: Hold the major load for a few seconds (often specified, e.g., 15 seconds) to allow the indentation to stabilize.
- Return to Minor Load: Remove the major load while maintaining the minor load.
4. Reading and Recording
- Measure Hardness: The Rockwell machine automatically measures the net increase in indentation depth and displays the HRB value.
- Record Result: Record the hardness value followed by the scale symbol, e.g., 85 HRB.
5. Special Considerations for Tubes
- Thin Walls: For thin-walled stainless steel tubes, additional support (stiffening) may be required on the inside to prevent deformation under load. Alternatively, a lower load (e.g., 500 kgf for Brinell) might be applicable, or a Superficial Rockwell test may be specified for very thin sections.
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