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Hardness Tests and Hardness Number Conversions (SAE J417-2018)
SAE J417-2018 provides approximate hardness conversion values between Vickers, Brinell, Rockwell, Rockwell Superficial, and Shore (Scleroscope) tests, along with approximate tensile strength conversions for steels. This stabilized standard is a mature resource for engineers working with homogeneous constructional alloy and tool steels. While the tables offer valuable guidance, exact conversions are impossible due to the inevitable influence of size, mass, composition, and heat treatment.
Understanding Hardness Conversion in Steels 🛠️
Conversion tables in SAE J417 are based on extensive tests on carbon and alloy steels, mostly in the heat-treated condition. They provide approximate relationships for common hardness scales. As an illustration, the following approximate values show how scales relate for several hardness levels, but always refer to the standard for full tables.
| Vickers HV | Brinell HB | Rockwell C HRC | Tensile Strength (MPa) |
|---|---|---|---|
| 245 | 230 | 21 | 800 |
| 305 | 280 | 29 | 950 |
| 390 | 360 | 38 | 1250 |
| 490 | 450 | 45 | 1550 |
| 610 | 560 | 55 | 1900 |
Note: Values are for illustration and represent approximate conversions. The standard includes more detailed tables and specifies conditions for use.
⚠️ Critical limitation: Conversion tables are not applicable to case-hardened, coated, or decarburized surfaces. For non-homogeneous materials, develop specific relationships for each composition and heat treatment.
Testing Methods and Best Practices
Each hardness test method—Vickers, Brinell, Rockwell, and Shore—has specific requirements for accurate results. Common guidelines include:
- Specimen thickness: Must be at least 10 times the depth of the indentation to avoid anvil effect.
- Surface preparation: Test surfaces should be flat, smooth, and free of scratches or ridges. Rockwell Superficial and Vickers tests require especially smooth surfaces.
- Brinell ball selection: Use a steel ball for hardness <450 HB and a tungsten carbide ball for hardness <630 HB.
- Conversions to shallow tests: Converting from Brinell to Rockwell Superficial or Vickers should only be done on materials with uniform hardness to a depth at least 10 times the indentation depth.
- Reporting: Rockwell values are reported to the nearest whole number despite tables showing tenths for exact relationships.
💡 Best practice: For accurate conversions, use the tables only on flat surfaces and follow the specific test procedures and precautions from ASTM E10 (Brinell), ASTM E18 (Rockwell), ASTM E92 (Vickers). The standard also warns that values in parentheses are beyond practical range and should not be used for specifications.
📐 Engineering Design Insights and Common Mistakes
Key insights from the standard include:
- The conversion tables are reliable for constructional alloy steels and tool steels in as-forged, annealed, normalized, quenched and tempered conditions, provided homogeneity.
- High manganese steel, 18% chromium—8% nickel austenitic steel, nickel-base alloys, and cold-worked steels may not follow the relationships with the same accuracy.
- For case-hardened parts, follow recommendations in SAE J417 and ASTM E18 for proper testing.
Common mistakes to avoid:
- Expecting exact conversions – always treat tables as approximate.
- Applying conversion to non-homogeneous materials like surface-hardened or coated parts.
- Using values in parentheses for specification purposes.
- Neglecting thickness requirements (at least 10× indentation depth).
- Using the wrong ball type for Brinell testing in high-hardness ranges.
Frequently Asked Questions
How do I convert hardness numbers between Vickers, Brinell, Rockwell, and Shore scales for steels?
Use the conversion tables in SAE J417, but note they are approximate and based on homogeneous carbon and alloy steels. Always follow test standards (ASTM E10, E18, E92) for accurate results and apply conversions only under the conditions specified.
Can hardness conversions be used for case-hardened or coated surfaces?
No. Conversions from Brinell to shallow impression tests (Rockwell Superficial, Vickers) should not be applied to surface-hardened, coated, or decarburized surfaces unless specific relationships are developed for that condition and material.
What specimen thickness is required to avoid anvil effect?
The specimen should be at least 10 times the depth of the indentation to avoid the anvil effect that can distort readings. Thinner samples may require different test scales or reduced loads.
What are the limitations of hardness conversion tables?
Exact conversions are impossible due to influences of composition, heat treatment, and part geometry. The tables are reliable for constructional alloy steels and tool steels in common heat treatment conditions, but may not apply equally to austenitic steels, nickel-base alloys, or cold-worked materials. Always verify with direct testing when precision is critical.
