Hardness is a material’s resistance to being dented, and it is measured by pressing a hard ball or point into the surface under a known load and reading the size of the mark. A file gives a quick ranking, and a Brinell test presses a 10 mm steel ball with 3,000 kg onto steel or 500 kg onto soft metals and divides the load by the dent area, so a 4.0 mm dent under 3,000 kg gives 229. Convert to strength only with a table built by testing your own metal.
Part of Testing and Quality
Hardness testing measures resistance to indentation or scratching, which is quick, nearly non-destructive and closely related to the strength and wear resistance of a metal.
The File Test Is A Ranking, Not A Measurement
The cheapest test needs only a good file. Push it across the surface with firm, even strokes:
- The file skates and leaves no mark: the metal is harder than the file.
- It bites lightly: a little softer.
- It cuts freely: much softer.
A file is itself hardened steel. The same test confirms a hardening quench: Heat Treatment says a properly hardened piece should be glass-hard and a file should skate across it. The Advanced Materials article ties file behaviour to HRC numbers, from materials above 65 HRC to those below 40. Treat that as a rough order only, because files differ and wear, and this article gives no conversion of its own. Keep a reference set: pieces of the same steel in known conditions (annealed, hardened, hardened and tempered) and compare the file’s behaviour on each.
Tempering colours also indicate hardness. The oxide colour on bright steel from straw to blue maps to a temper (see Tempering), and checks that the temper was done.
The Brinell Test: A Ball, A Load, And A Dent
The Advanced Materials article describes a Brinell tester: a hardened steel ball of 10 mm diameter pressed into the surface with a known weight, 3,000 kg for steel and 500 kg for soft metals. The weights are kilograms of force. Then measure the diameter of the dent with a magnifier and ruler. The Brinell number is the load divided by the curved area of the dent:
HB = F / (π × D × h), where h is the depth of the dent and π × D × h is the area of the spherical cap, and h = (D − √(D² − d²)) / 2 for a ball of diameter D and dent diameter d.
Worked example, steel: F = 3,000 kgf, D = 10 mm, d = 4.0 mm. √(100 − 16) = √84 = 9.165. D − 9.165 = 0.835. h = 0.835 / 2 = 0.4174 mm. Area = π × 10 × 0.4174 = 13.11 mm². HB = 3,000 / 13.11 = 229.
Worked example, soft metal: F = 500 kgf, d = 3.0 mm. √(100 − 9) = 9.539. h = (10 − 9.539) / 2 = 0.2303 mm. Area = π × 10 × 0.2303 = 7.24 mm². HB = 500 / 7.24 = 69.
A bigger dent means a softer metal. The hardness unit is load per area, so hardness numbers compare only when taken with the same ball and load.
Rules For A Good Brinell Reading
- Flat, clean, smooth surface. Scale or a crowned surface gives a false dent.
- Hold the load steady for several seconds before releasing; standard practice is 10 to 15 seconds.
- Part thick enough that the back shows no bulge.
- Dent diameter between about a quarter and six-tenths of the ball diameter, which is 2.5 to 6.0 mm for a 10 mm ball. Outside that range, change the load.
- Space the dents well apart and away from edges.
- Measure two diameters at right angles and average. Repeat three times.
Rockwell And Others
A Rockwell tester measures the depth of a dent rather than its width. It applies a small preliminary load, sets a zero, adds the main load and reads the extra depth that remains. It reads fast without measuring a dent. The hard-steel scale (HRC) uses a diamond cone. A depth gauge good to a few micrometres is needed, and the figures in the Advanced Materials article’s HRC (60 or more at a case-hardened surface) come from such a tester. A village can build a Brinell tester more easily than a Rockwell one.
Using Hardness To Predict Strength
Hardness and tensile strength rise together in steels, so a hardness reading can stand in for a pull test. The relationship depends on the steel, so build your own: pull-test and indent several bars of the same steel at different hardnesses (see Tensile Testing), then plot strength against hardness. Do not borrow a table.
Load and flying metal
A 3,000 kgf load is about 29 kN. Whatever applies it (a screw, a lever, a hydraulic jack) must be rated for it and shielded in case a hardened sample cracks or the ball shatters. Use a hardened ball only on metals softer than itself, never on a hardened ball-bearing steel. Wear eye protection.
Common Mistakes
- Reading a dent on a rough surface — an error in d means a big error in HB.
- Using too light a load on hard steel — a dent too small to measure.
- Comparing numbers from different loads — they do not match.
What’s Next
- Learn next: Non-Destructive Testing — checking for cracks hardness will not find
- Combine with: Case Hardening — hardness across a case