Hardness Conversion Chart for Steel and Fasteners

The drawing specifies Rockwell C. The mill certificate reports Brinell. Your incoming inspection lab runs Vickers. Nobody made a mistake, and nobody can release the lot.

That standoff stops assembly lines. It forces sort-and-containment work, delays PPAP submissions and lets non-conforming hardware reach finished product. A hardness conversion chart for fastener specification closes the gap, but only when your team knows what the chart proves and what it does not.

Hardness Conversion Chart for Steel (Rockwell C, Vickers, Brinell)

Find the value you have in any column and read across. The chart covers hardened carbon and alloy steel, which is what most bolts, screws and pins are made from. It does not apply to austenitic stainless, aluminum or brass: those need their own tables.

Non-austenitic carbon and alloy steels, from ASTM E140-02 Table 1. Brinell values in brackets fall outside the range ASTM E10 recommends for Brinell testing and are for comparison only. Approximate tensile strength is from the Buehler/Wilson conversion chart (based on ASTM A370), whose hardness values match ASTM E140 at every point we checked. A dash means no published value.

Hardness Converter

Enter a value on any scale to convert it. The converter uses the same ASTM E140 values as the chart, interpolating between rows.

Rockwell B Conversion Chart for Softer Steel

Softer steels, from ASTM E140-02 Table 2 (Brinell with a 10 mm steel ball). A dash means no published value.

Hardness Ranges for Common Bolt Grades and Property Classes

These are the hardness bands the standards themselves set. A bolt outside its band fails the specification regardless of what a conversion says.

ISO 898-1:2013 Table 3 core hardness. SAE J429 specifies Rockwell hardness only, so Vickers and Brinell are left blank rather than converted; values confirmed by two distributor references (Triangle Fastener, Fastenal).

Start with the governing documents. ASTM E140 publishes 10 separate conversion tables, one per material family: non-austenitic steels in the Rockwell C range, the same steels in the Rockwell B range, nickel alloys, cartridge brass, austenitic stainless plate, austenitic stainless sheet, copper, alloyed white iron, wrought aluminum and a Leeb table. The same standard states that “it is not possible to state confidence limits for the errors in using a conversion chart.” ASTM’s own guidance is to convert specification limits and avoid converting test data whenever the original scale is available.

Overseas mills work from the international equivalent. EN ISO 18265:2013 spreads its conversion tables across Annexes A to F and covers seven material groups: unalloyed and low alloy steels and cast iron, steels for quenching and tempering, steels for cold working, high speed steels, tool steels, hardmetals, and non-ferrous metals and alloys. It reprints sections of ASTM E140 with permission, and it carries the same warning. Converted values are “only directly applicable to the exact material tested,” and for every other material they are an indicator only.

Everything below builds on those two documents. Conversions are material specific, and they carry error that nobody can bound.

 

Top Hardness Testing Methods for Fasteners: Rockwell, Brinell and Vickers

Hardness drives wear resistance, tensile strength and ductility. ASTM, DIN and ISO specifications each call out a test method based on material, part size and application. Know the three methods before you try to reconcile a supplier report.

Rockwell Hardness Testing

Rockwell measures indentation depth under a major load against a preliminary preload. Fastener prints use Rockwell C (HRC) for hardened high-strength steel and Rockwell B (HRB) for softer metals and unhardened machine screws.

The test reads depth rather than surface area, so operators run it fast and without optical measurement. It leaves a small indentation and works well on finished parts at the receiving dock. ASTM E18 calls Rockwell an empirical indentation test whose results may correlate to tensile strength, wear resistance and ductility. Empirical, not derived. That distinction drives every problem in the next section.

Brinell Hardness Testing

Brinell presses a hardened steel or tungsten carbide ball into the part under heavy load, then measures the diameter of the indentation. The HBW scale suits large fasteners, castings and forgings with coarse or uneven grain structure.

The wide indentation averages hardness across a larger volume of material. That makes Brinell the working reference for heavy structural bolts in construction and heavy equipment, where a single point reading would mislead.

Vickers Hardness Testing

Vickers uses a diamond pyramid indenter and calculates hardness from the optical measurement of the indentation area. One indenter covers every metal at every strength level, which gives HV a continuous range from very soft to very hard.

That continuity makes Vickers the right call for microhardness work: small C-class components, miniature aerospace fasteners, case depth checks and surface hardening verification.

Where Hardness Conversions Break Down

Conversion is never a clean one-to-one translation. Eight failure points show up repeatedly when procurement and quality teams map specifications across a global supply base.

The Tables Are Material Specific

ASTM E140 states that conversion values hold only for the material each table covers, because alloy, grain structure and heat treatment all shift the relationship. Table 1 covers homogeneous non-austenitic carbon, alloy and tool steels. Table 5 covers annealed austenitic stainless plate and maps Brinell to Rockwell B only.

Pull a number from the wrong table and it is not an approximation. It is a different material’s answer to a different question.

ASTM E140 and ISO 18265 Do Not Line Up Across a Global Supply Base

US prints call out ASTM E140. European and Asian mills convert under EN ISO 18265. The two documents share text, but they sort material differently: E140 runs 10 tables by alloy family, while ISO 18265 splits its tables across six annexes covering quenching and tempering steels, cold-working steels, high speed steels, tool steels, hardmetals and non-ferrous alloys.

A supplier converting a Vickers reading under one annex and a buyer checking it against a different E140 table reach two verdicts on the same lot. Name the standard and the table on the purchase order, not just the scale.

Coatings and Case Hardening Skew the Reading

Zinc plating, specialized coatings and case hardening create a surface that behaves nothing like the core. A standard Rockwell C indenter drives through a thin hardened case and reports a low number. A microhardness Vickers test reads the case itself and reports a high one. Both results are correct on their own terms.

ASTM E18 makes the point plainly: testing at a specific location on a part may not represent the physical characteristics of the whole part. Decarburization compounds the problem, leaving a soft carbon-depleted skin over hard core material after heat treatment.

Stainless Steel Weakens the Hardness to Tensile Link

Hardness works as a tensile proxy for quenched and tempered carbon and alloy steels. That is the exact case ASTM E140 Table 1 was built around, and it is why incoming inspection leans on hardness in the first place.

Austenitic stainless behaves differently. Cold heading work-hardens the material and raises tensile strength without moving surface hardness in proportion. Convert anyway and you will overstate what a cold-formed stainless fastener actually carries.

Test Location Decides the Answer

A hardness value describes the spot the indenter touched. Test a hex head, a thread flank and a shank on the same bolt and you can log three different numbers. Specify where the test happens, not just the scale and the range.

Suppliers Report on Whatever Scale Their Lab Owns

A Rockwell tester on a production floor and a Vickers microhardness rig in a lab produce different data for the same lot. Overseas suppliers frequently report in HV because that is what their equipment reads.

Fix the reporting scale in the purchase order and the argument disappears before the container ships.

Generic Metal Hardness Conversion Calculators Fail Audits

Free online conversion calculators rarely state which table, which material family or which test conditions produced their output. The number looks authoritative in a spreadsheet and collapses the moment a customer auditor asks for the source.

Quality systems have to show the provenance of every reported value. Convert from the published table in ASTM E140 or EN ISO 18265, then record the standard and the table number in the inspection record alongside the value.

Inch Grades and Metric Classes Carry Separate Hardness Bands

SAE J429 governs inch-series grades. ISO 898-1 governs metric property classes such as 8.8, Class 10.9 and Class 12.9. Each system sets its own hardness limits against its own tensile requirements, so a Grade 8 bolt and a Class 10.9 bolt are not interchangeable on the strength of a converted number.

Yield strength is a separate problem. Conversion tables produce estimates of ultimate tensile strength only. Yield comes from a tensile test, and no hardness chart substitutes for one.

How Hardness Drives Fastener Failure

Specify hardness correctly and the joint holds. Overly hard fasteners turn brittle and crack. Soft fasteners relax under clamp load and lose preload. Both failures start on the print, not on the line.

Hydrogen embrittlement is the sharper risk. Electrolytic plating baths put free hydrogen against the steel, and it diffuses into the lattice with no external sign. The NASA Fastener Design Manual reports that embrittlement problems increase as fastener strength increases, and that most plating specifications require plated carbon steel fasteners to be baked for at least 23 hours at 375 degrees F, within 2 hours after plating, for embrittlement relief. A verified hardness value tells you whether a part needs that bake.

Galling sits at the other end of the range. Thread surfaces in A2 and A4 stainless cold-weld under installation pressure and friction, then seize before the joint reaches clamp load. Building a hardness differential between bolt and nut is standard practice for reducing that risk, though lubrication and driving speed carry just as much weight.

Correlating Hardness to Grade and Tensile Strength

A hardness conversion chart confirms that a fastener meets the mechanical requirement its grade carries. The confirmed value tracks the load the part holds before it deforms permanently. EN ISO 18265 is blunt about the limit of that inference: tensile strength estimates are the least reliable values its conversion tables produce.

Structural bolts still have to land inside a hardness band to perform under vibration and cyclic load. Metric Class 10.9 and Class 12.9 fasteners need tight heat treatment control to hold high tensile strength while keeping enough ductility to stretch instead of snap. Our structural fasteners ship with the documentation that proves the grade.

Hardness also feeds installation. Once the verified value and the grade behind it are settled, run the numbers through our torque and preload calculator so assembly torque matches what the fastener can take. The rest of the engineering tools sit in our calculators and converters library.

Best Practices for Engineered Fastener Specification

Put the scale and the test location on the drawing next to the range. A hardness band with no scale and no location forces every supplier in the chain to guess, and they will each guess differently.

Ask for raw test data on the specified scale. If a certificate arrives already converted, request the original readings and the table that produced the conversion. ASTM gives the same instruction: convert limits, not data.

Match the method to the part. Brinell for large forgings and coarse grain structures. Rockwell C for hardened high-strength steel. Vickers for thin cases, coatings and small C-class components. A method mismatch produces a number that passes review and still misses the real property.

Then qualify the supplier, not just the shipment. Procurement teams sourcing traceable, spec-compliant bolts and screws should confirm that the partner tests and documents mechanical properties before parts leave the plant. Component Solutions Group works under ISO 9001:2015 and AS9120 Rev B, is ITAR registered, and its A1 Fastener automotive division is IATF 16949 certified; and our quality team supplies material test reports and compliance documentation on request.

Matching Hardness Specification to Your Fastener Print

Balance the tensile requirement against embrittlement risk. Weigh the base metal, the service environment and the test method the engineering drawing mandates. Decide which standard governs the conversion, ASTM E140 or EN ISO 18265, and write that decision on the print so a mill in Ohio and a mill in Taiwan answer the same question the same way.

Release still belongs to lot testing on the specified scale. Pick a partner who delivers traceable documentation with every critical part rather than a converted number and a promise.

Component Solutions Group secures OEM supply chains with verified quality, technical engineering support and global sourcing through the Bufab network. To tighten your fastener specification process and stop repeat quality failures, submit a request for quote to our OEM team.

Frequently Asked Questions (FAQs)

Is there a universal hardness conversion calculator for metals?

No. Conversion is empirical and material specific, so no single formula covers every metal. ASTM E140 publishes 10 separate tables by material family, and EN ISO 18265 spreads its tables across Annexes A to F by material group. ASTM states that confidence limits for conversion error cannot be established, which is why a generic online calculator that never names its source table does not hold up in an audit.

Can I use hardness to determine a fastener’s tensile strength?

Hardness gives a workable estimate of ultimate tensile strength for quenched and tempered carbon and alloy steel fasteners. EN ISO 18265 warns that tensile estimates are the least reliable values its tables produce, and the correlation weakens further for work-hardened austenitic stainless and non-ferrous metals. Yield strength never comes from a conversion. It comes from a tensile test.

What is the difference between Rockwell C and Rockwell B?

Rockwell C uses a diamond cone indenter and a heavy major load for hardened high-strength steels. Rockwell B uses a steel ball indenter and a lighter load, which suits softer metals and unhardened fasteners. ASTM E140 publishes a separate conversion table for each range.

How does case hardening affect a hardness test?

Case hardening puts a hard skin over a softer core. A standard Rockwell C indenter penetrates the thin case and reports a low reading for the part. Microhardness methods such as Vickers measure the case accurately, which is why the specification has to name the method.

What is the standard hardness for Grade 8 and Class 10.9 bolts?

Grade 8 is an inch-series grade under SAE J429. Class 10.9 is a metric property class under ISO 898-1. SAE J429 sets Grade 8 at Rockwell C 33 to 39. ISO 898-1 sets class 10.9 at Rockwell C 32 to 39 (320 to 380 HV). The bands are close but each is tied to its own tensile and proof-load requirements, so read the band off the governing standard. Component Solutions Group ships material test reports that state the as-tested scale and the measured value, which is what incoming inspection needs to release the lot.

How many conversion tables does ASTM E140 publish and how are they organized?

ASTM E140 publishes 10 separate conversion tables, one per material family: non-austenitic steels in the Rockwell C range, the same steels in the Rockwell B range, nickel alloys, cartridge brass, austenitic stainless plate, austenitic stainless sheet, copper, alloyed white iron, wrought aluminum, and a Leeb table.