Hardness Conversion Chart: HRC HB HV Table (Free PDF)

Then you find the known hardness value and look at the column it’s under. Then you read across the row. Every number show the same level of hardness in the material.

That’s the whole range covered by this table from ASTM E140 Table 1, for hardened steels. The lowest numbers is around 20 HRC where we can start to detect hardening with respect to the C scale. And the highest numbers are around 68 HRC, which is near how hard steel can be without some sort of exotic treatment.

Now if you have a Brinell or Vickers number you will scan down that column until you get close to your number, then scan along the row to see what number matches it. If you’re in-between two rows, then you interpolate between those values.

The day-to-day scale for hardening steel is Rockwell C (HRC), where 150 kgf is applied to a 120-degree diamond cone. The lab-preferred scales are Vickers (HV), using a 136-degree diamond pyramid; it is the most linear and wide-ranging scale. Brinell (HB) in this instance mean testing with a 10 mm ball under 3000 kgf.

After around 650 HB, the ball begins to deform, which is why the column terminates and values at 60 HRC and over are best regarded as a guide only. Rockwell A (HRA) use the same diamond cone at a lower load of 60 kgf and is suitable for case-hardened items that are too thin for a 150 kgf load.

HR15N is the shallowest superficial scale and is used on sheet metal and other very thin parts. Finally, Shore (HS) is the old scleroscope rebound test and you may still see it cited for large machined parts and rolls.

Of the remaining columns, the two tensile strength ones is the most useful and the most abused. Hardness correlates reasonably well with ultimate tensile strength for plain carbon and low alloy steels, so they're good for sanity checks and to sort stock. Roughly 500 times HB in psi or 3.4 times HV in MPa is the rule of thumb; it's right out of the table.

That isn't a replacement for a tensile test, however: different sources have published correlations that differ by some 5 percent. Within any individual alloy, scatter may be as much as 10 percent. The correlation get pretty bad above 55 HRC or so, which is why the columns end while the rest of the table continues.

A ball diameter has nothing directly to do with a pyramid diagonal, which has nothing directly to do with a cone depth, it’s all empirical. There isn’t one formula that ties them together.

ASTM E140 (for non-austenitic steel) lists measured equivalences in tables. Both E140 and ISO 18265 make it clear that converted values are approximations. A converted value may never substitute for a real measurement for acceptance. Expect the converted value to fall within approximately 1-2 points of the direct reading on the desired scale.

In some locations, the ISO 18265 tables differ from the ASTM figures by roughly this same amount; the ISO tables are commonly used in Europe and are based off German data. Test on the scale called out in the contract or drawing.

There are two practical pitfalls with using this table from the user’s perspective. The first is that they are for steel. Other metals such as aluminium, copper alloys, cast iron, and austenitic stainless work-harden and deform different. For these materials, this table will not give accurate results that stays within the stated tolerance; instead, separate tables are published in ASTM E140.

The second is to note the type of ball used on older certificates. Above about 440 HB, HBS (steel ball) and HBW (carbide ball) differs. The steel ball gives a reading a few points lower because it will flatten at greater hardness. Where the two differ, this table uses carbide ball values as current practice does.

The difference means that an old certificate dated in the 60s could of been legitimately out by a few points at the high end of the range compared to this table.

This table is your companion for non-hardened steels. That is to say it includes mild steel, low grade fasteners, structural sections and annealed/normalised stock etc. Find your HRB, Brinell or Vickers number and read across the row.

The working span covered by this table extend from 55, 100 HRB on the Rockwell B scale. This equates approximately to a range of 100, 240 HB. Above 100HRB the B scale lose its accuracy and goes out of range. Because 100HRB is close to 22 to 23HRC, this is where the HRC table come into play.

Below around 55HRB, you're into very soft irons and brasses. In this lower range, steel-based conversions aren't listed.

At 100 kgf, use a 1.588 mm ball for Rockwell B (HRB). This is the quick shop-floor test at this end of the range. At 3000 kgf a Brinell (HB) leave a sizeable indent. That’s to average-out the coarse structure of hot rolled plate, forgings and castings. Structural steel is quoted in mill certificates using HB for that reason.

The lab use Vickers (HV) and Knoop (HK). ASTM E140 says that HV and HB are numerically identical in this range. This happy coincidence allows for easy cross checking of certificates.

Some thin parts also gets tested on Rockwell A, which explains why that number appears. Thin sheet gets tested on the HR30T light-load ball scale. 100 kgf would of bulge thin sheet on its back face.

Just like the hardened table, the tensile columns relates to carbon steel, which is the most reliable in this range. For plain carbon and low alloy steel there is an old rule of approximately 500 psi per Brinell point which is accurate to around 5 to 10 percent. That equates to 3.45 MPa per HB.

Hot rolled mild steel at around 135 HB or 74 HRB have a tensile strength of approximately 460 MPa. This puts it right in the ballpark of common A36 or S275 material.

The correlation is still based off steel. It's not one you would apply to brass, aluminium or austenitic stainless. These materials fall into this range of hardness but totally break the relationship.

Now, what about when you read HRB values above 100? This is where people gets confused often. If you’re reading 110 HRB or even something like 105, these values are out of range for the scale. In this area, the tiny ball doesn’t sink enough for there to be any resolution. They recommend moving to the C scale (ASTM E18). Anything over 100 HRB is considered questionable and should be repeated on HRC.

The same applies in reverse. Below 20 on HRC is also suspect. Those are the upper rows overlapping into that range. Here’s where you’ll want to convert it to HRB. It’s the right table.

All these matches are based on observation. They're on non-austenitic steel. The conversion is approximate and depends on the material; see warnings in ASTM E140 and ISO 18265. Expect plus-or-minus a point or two scatter different than a direct read.

A rougher surface, or a surface that is cold worked or decarburised, will have greater scatter. For acceptance work, test on the scale the specification names.

The table above has each test listed on a row. On that row, you see what they press into the metal and how hard they do it. You also see what they actualy measure and where the scale excels.

What all this boils down to is why the conversion tables can only be approximate. The tests vary by measuring things like diameter, depth, or diagonal. In addition, they is subject to forces that can differs from one another by up to 200,000 times.

Each scale has its area bounded by the columns of load and indenter. Hardness that won’t eat up a 150 kgf (HRC) diamond cone takes some material. So begins its use at about 20 HRC.

A 1.588 mm ball at 100 kgf (HRB) needs material soft enough to mark, so it tops out near 100 HRB. The Brinell test use a 10 mm ball with a 3000 kgf load. It leaves an indent in millimetres. On a casting this averages out any coarse grain, but on a finished part it tear up the surface.

Since Vickers uses the same geometry but ranges from grams to 120 kgf loads, it covers all bases. That’s why HV is the reference scale. The ASTM E140 and ISO 18265 tables centers around it.

Accuracy and speed come from what you’re measuring. In roughly 10 seconds, Rockwell displays your depth directly either on-screen or on a dial. It has no need for optics. That’s why Rockwell is used to manage production floors.

For Vickers and Brinell, you have to optically measure the indent. This takes longer, but it’s more precise and doesn’t depend on how much load is being applied.

Shore and its replacement Leeb measure the bounce off a small mass. They don’t require nearly as much fixturing. On a 5 tonne roll, it’s the only practical choice. But they give you the least accurate number on any conversion table.

Choose your scale based off what you’re practicing. Match the load to the thickness (the indent must be shallow relative to the part, roughly a tenth of thickness for Rockwell).

Match the indenter to the hardness you expect. Diamond is good for hard material, ball for softer material.

Match the scale to the paperwork. Verify HRC on a certificate that states it’s HRC. Don’t change it into a Vickers reading!

Those superficial scales is really just there for thin material. They use the same indenter, smaller loads, and shallower indents.

Historically, the scales are named for their load. HV30 is 30 kgf Vickers.

Today's ASTM and ISO specifications use newtons for force. The numbers didn't change. What changed was the units on the piece of paper. So a chart from today and one from the 70s both describes the same tests. You should of checked that.

The table converts abstract numbers to something you've felt in your hands. Each row is labeled with the name of a common part and its hardness number from a datasheet or spec sheet. It also shows the corresponding value on the other scales, based off the two conversion tables at the top of this page.

Use it both ways. Use it to understand what 45 HRC feels like. Or use it to guess at how hard some unknown part might be by matching it up with known parts.

You’ll notice the fastener rows specify the exact value as per ISO 898-1. Classes 8.8 has to be between 22 to 32 HRC (23 to 34 HRC above M16). Classes 10.9 is between 32 to 39 and classes 12.9 is between 39 to 44.

Which is where the class markings come into it down at the bench. You can drill out an 8.8 bolt with a good HSS bit. A 12.9 socket screw will go-glaze an ordinary bit, you want something like cobalt or carbide.

Tensile strength is also well-correlated across the classes. Class 8.8 is about 800 MPa tensile. The hardened table above show that 25 HRC gives nearly that exact value.

A practical takeaway from the rows of file tests is that a newly produced file is approximately 62 to 66 HRC. Up until around 55 HRC a file will bite into whatever it touches and hence filing is the oldest form of hardness testing.

Around 60 the file begins to skid across with a glassy sensation on such hard items as blades and bearing races. This is shown by commercial hardness test file sets made of graded files at 40, 45, 50, 55, 60 and 65 HRC. You shouldn't of put an HSS drill onto something if a file doesn't touch it.

What the table doesn't say. What it says is the Rockwell and Vickers equivalents of parts marked HB (mild steel, white cast iron). They are also conversions with their accompanying uncertainty of up to 2 points.

The row about nitrided surfaces reminds us: hardness is sometimes skin deep. It's easy to read 65 HRC on a nitrided or carburised component but the core might only be 30 HRC. Test Brinell punches right through the case to something in between, while HV tests reads the hard case.

If a figure confuses you, then ask what test, what load, and how deep this hardness goes.

These are typical published numbers, they are not guarantees. Every single one of these will move with heat treatment. An economy bolt, a badly tempered file might be out of band.

A real test and the required value for anything structural beat any table of typical numbers. This one included.