Hardness Conversion Chart
Trojan Special Fasteners Ltd can provide material certificates on request for goods; all will have chemical composition data and some will come with mechanical data. Please note material certificates must be requested at the quotation stage.
What is hardness?
Strength and hardness are not the same and one should be careful when determining what kind of fastener you need.
Hardness is defined as a measure of a material's resistance to localised plastic deformation. Plastic deformation is an engineer's way of saying that the deformation is permanent and the material won't return to its original shape, for example, an indentation or scratch on the surface won't 'heal'. In plain English this means the harder a material is, the more difficult it is to scratch, mark or penetrate.
For example, butter is more easily cut/deformed/penetrated by a steel knife than a piece of tungsten. The tungsten won't even be marked by the butter knife because tungsten is very hard. This is also a reason why tungsten is often added as an alloying element in tool steels because it helps to reduce wear at high temperatures and difficult cutting conditions.
What is Indentation Hardness?
Indentation hardness testing refers to the resistance to plastic deformation when metal is subject to a continuous load.
Metals commonly used to make nuts are usually tested using Indentation Hardness tests such as Brinell, Vickers or Rockwell.
It has been shown that there is an approximately linear correlation between indentation hardness and yield strength as well as, more importantly, tensile strength for ferrous (non-austenitic) hypo-euctoid (less than 0.8% carbon) steels. This means that it is relatively cost-effective and easy to perform non-destructive Rockwell testing of metals.
When choosing a nut it is a good idea to know what your ultimate application is, as this will determine the hardness or tensile strength of the nut.
Table of Hardness Figures
Please note that this table provides approximate equivalents only and should not be relied upon for accuracy. This is because data on it cannot be generated mathematically for a diverse range of materials with different chemical compositions. Hardness conversions are no substitute for direct measurements. This information must be used with caution and only in accordance with the principles of conversions.
Method: Linear interpolation between table values.
Colour Guide: 🟢 Green = Exact value from standard table | ⚪ White = Interpolated | 🔴 Red = Value not available for this scale
Note: Values are approximate conversions for guidance only. For critical applications, direct testing is required.
Hardness Conversion Table
BS EN ISO 18265:2013 - Metallic materials
Frequently Asked Questions (FAQ) – Hardness Conversion for Steel
1. What is the purpose of this hardness conversion table?
This table provides approximate equivalent values between different hardness scales (e.g., Rockwell C, Brinell, Vickers) and tensile strength for metallic materials, primarily non-austenitic steels. It is based on BS EN ISO 18265:2013, the international standard for converting hardness values. The table allows engineers, quality control personnel, and fastener specifiers to estimate mechanical properties when direct testing on the required scale is not possible.
2. Why are there empty cells or "—" in the table?
Empty cells indicate that a particular hardness scale is not applicable or not defined for that specific value range in the ISO standard. For example, Rockwell B (HRB) is only valid for softer materials (typically below HRC 30). Attempting to convert HRB values into the high hardness range (e.g., above HRC 40) is not supported by the standard and would be inaccurate. The table only shows values where the standard provides a defined relationship.
3. Can I use this table for stainless steel, cast iron, or non-ferrous metals?
This table is most accurate for non-austenitic steels (carbon steels, alloy steels, and tool steels). For austenitic stainless steels, cast irons, aluminum, copper, or nickel alloys, the hardness relationships differ significantly. For those materials, you should refer to the specific annexes within ISO 18265 or ASTM E140, which provide separate conversion tables for each material family.
4. Are these conversion values exact or just approximations?
They are approximate equivalents only. Hardness conversion is not a mathematically perfect relationship because different hardness tests measure different physical phenomena (depth of indentation vs. width of indentation vs. rebound). The values in this table are the best available standardised estimates, but they should never be used as a substitute for direct measurement on the actual material in question, especially for critical applications or compliance testing.
5. Why do some hardness values have one decimal place and others are whole numbers?
The precision reflects the original standard. Scales like Vickers (HV) and Brinell (HB) are typically reported to the nearest whole number or half-point, while Rockwell scales (HRC, HRA, HRB) are often quoted to one decimal place due to the higher resolution of the testing machine. This table preserves the format given in ISO 18265.
6. How do I convert a value that falls between two rows in the table?
Use linear interpolation. For example, if you need the Brinell hardness for a tensile strength of 1000 MPa, locate the nearest values above and below in the "MPa" column (e.g., 995 MPa and 1030 MPa), find their corresponding HB values (295 and 304), and interpolate between them. Most engineering calculators and spreadsheet software can perform this automatically. The table is structured to allow accurate linear interpolation between all adjacent rows.
7. Why is there a relationship between hardness and tensile strength for steel?
For non-austenitic hypo-eutectoid steels (carbon content below 0.8%), there is an approximately linear correlation between indentation hardness (Brinell or Vickers) and ultimate tensile strength (UTS). This relationship exists because both properties are governed by the material's resistance to plastic deformation. The standard provides estimated tensile strength values based on hardness, but these are conservative approximations and should not replace direct tensile testing for safety-critical components.
8. What is the difference between Rockwell C (HRC) and Rockwell B (HRB)?
Rockwell C uses a diamond cone indenter with a 150 kgf load and is designed for hard materials (typically above HRC 20). Rockwell B uses a 1/16-inch steel ball indenter with a 100 kgf load and is used for softer materials (typically below HRB 100). The two scales overlap only in a narrow range, and converting between them is not recommended. This table clearly separates the valid ranges for each scale.
9. Can I use this table to certify fastener mechanical properties?
No. This table is for estimation and reference only. Material certifications (e.g., EN 10204 3.1 certificates) require direct testing of the actual material using the specified standard test method (e.g., ISO 6506 for Brinell, ISO 6508 for Rockwell). Hardness conversion should never be used as a substitute for the required test, particularly for safety-critical fasteners like nuts and bolts. Always request the appropriate direct measurement from your supplier.
10. What standards apply to hardness testing of steel fasteners?
For fasteners, common standards include:
- ISO 898-1 for property classes of carbon steel bolts and screws (uses HRC or HV)
- ISO 898-2 for nuts (uses HRC or HB)
- ASTM F606 for mechanical testing of fasteners
- ISO 6506 (Brinell), ISO 6507 (Vickers), and ISO 6508 (Rockwell) for the test methods themselves.
This conversion table is a supplement to, not a replacement for, those standards.
11. What does "non-austenitic hypo-eutectoid steel" mean in plain English?
It means ordinary carbon steels and low-alloy steels that are not stainless steel (austenitic) and have a carbon content less than 0.8%. This includes most structural steels, fastener steels (grades 4.6, 5.6, 8.8, 10.9, 12.9), and tool steels. For these materials, the hardness-tensile relationship is well understood and reliable.
12. How accurate are the tensile strength estimates in this table?
The tensile strength values are estimated from hardness using empirical relationships in ISO 18265. For non-austenitic steels, the typical accuracy is within ±10-15%. However, actual tensile strength depends on factors like heat treatment, microstructure, and specimen geometry. Always perform direct tensile testing for final verification, especially for high-strength fasteners (grade 10.9 and above).
13. Why does this table start at 80 HV (approximately 255 MPa tensile) and go up to 940 HV?
This range covers the majority of engineering steels used in fasteners, tools, and structural applications. Below 80 HV, materials are typically non-ferrous or very low-strength steels that follow different conversion relationships. Above 940 HV, materials become very hard and brittle (e.g., case-hardened or tool steels), and the conversion relationships become less reliable. For extreme hardness values, consult the full ISO 18265 standard.
14. Can I copy this table into my own quality management system or website?
You may use the data for internal reference or on your own website, provided you clearly state the source as "Based on BS EN ISO 18265:2013" and include a disclaimer that values are approximate and not a substitute for direct testing. The standard itself is copyrighted, but the tabulated values are considered factual data. For professional or commercial use, purchasing a copy of the official standard from ISO or BSI is recommended.
15. Where can I learn more about hardness testing and conversion?
- ISO 18265:2013 – Metallic materials – Conversion of hardness values
- ASTM E140 – Standard Hardness Conversion Tables for Metals
- ASTM E18 – Rockwell Hardness Standard Test Method
- ISO 6508-1 – Rockwell hardness test method
For fastener-specific guidance, refer to ISO 898 (mechanical properties of fasteners) or consult with your supplier's quality assurance team.
Disclaimer: This table and the accompanying information are provided for guidance only. Hardness conversion is an approximation and should never be used as the sole basis for material acceptance, rejection, or design decisions. Always perform direct testing in accordance with the relevant product standard. Trojan Special Fasteners Ltd accepts no liability for errors, omissions, or decisions made based on this converted data.