Hardness refers to ability of a material to resist pressing of a hard object into its surface. It is one of important performance indicators of metal materials. Generally, the higher hardness, the better abrasion resistance. Common hardness indicators are Brinell hardness, Rockwell hardness, and Vickers hardness. According to German standard DIN50150, following is a comparison table of tensile strength and Vickers hardness, Brinell hardness, Rockwell hardness of commonly used steels. Recommended collection!

 

Data in this table comes from German standard DIN50150

With a certain load (generally 3000kg), a hardened steel ball of a certain size (generally 10mm in diameter) is pressed into surface of material and held for a period of time. After unloading, ratio of load to indentation area is Brinell hardness value HB), unit is kgf / mm2 (N / mm2).

 

Difference between HBS and HBW is indenter. HBS indicates that indenter is a hardened steel ball, which is used to determine materials with a Brinell hardness value below 450, such as mild steel, gray cast iron and non-ferrous metals. HBW indicates that indenter is a hard alloy, which is used to measure materials with a Brinell hardness value below 650.
Same test block, when other test conditions are exactly same, two test results are different, HBW value is often greater than HBS value, and there is no quantitative law to follow.
Since 2003, China has adopted international standards equivalently, eliminating steel ball indenters and using hard alloy ball heads. Therefore, HBS is discontinued and all Brinell hardness symbols are represented by HBW. Many times Brinell hardness is only expressed by HB, which means HBW. However, HBS is still seen in literature.
Brinell hardness measurement method is suitable for cast iron, non-ferrous alloys, various annealed, quenched and tempered steels. It is not suitable to measure samples or workpieces that are too hard, too small, too thin, and do not allow large indentations on surface.

When HB> 450 or sample is too small, Brinell hardness test cannot be adopted and Rockwell hardness measurement is used instead. It is a diamond cone with a vertex angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18mm. It is pressed into surface of material under a certain load, hardness of material is obtained from depth of indentation. According to hardness of test material, it is expressed in three different scales.

 

Use range of HRC scale is 20 ~ 70HRC. When hardness value is less than 20HRC, because conical part of indenter is pressed too much, sensitivity is reduced. At this time,HRB scale should be used. When hardness of sample is greater than 67HRC, pressure on the tip of indenter is too large, diamond is easily damaged, life of indenter will be greatly shortened, so HRA scale should generally be used instead.
Rockwell hardness test is simple, fast, and small indentation. It can test surface of finished products, harder and thinner workpieces. Because indentation is small, for materials with uneven structure and hardness, hardness value fluctuates greatly, and accuracy is not as high as Brinell hardness. Rockwell hardness is used to determine hardness of steel, non-ferrous metals, hard alloys, and so on.
At production site, due to limitation of testing equipment, hardness of large quenched parts is often measured using a Brinell hardness tester. If you want to know Rockwell hardness value of workpiece, usual method is to first measure Brinell hardness value, then find out corresponding Rockwell hardness value according to conversion table. This method is obviously a bit tedious. So, is it possible to directly calculate Rockwell hardness of workpiece based on indentation diameter of Brinell hardness tester? Answer is of course yes. According to Brinell hardness and Rockwell hardness conversion table, an empirical formula that is simple to calculate and easy to remember can be summarized: HRC = (479-100D) / 4, where D is measured value of indentation diameter of Φ10mm steel ball indenter pressed on workpiece at 30KN pressure. Error between calculated value and converted value of this formula is in range of 0.5 to -1. This formula is very convenient to use. You may wish to try it.

Diamond square cone indenter with a load within 120kg and a vertex angle of 136 ° was pressed into material surface, surface area of material indentation pit was divided by load value to obtain Vickers hardness HV value (kgf / mm2).

 

Practice has proved that there is an approximately corresponding relationship between various hardness values and strength values of metal materials. Because hardness value is determined by initial plastic deformation resistance and continued plastic deformation resistance, the higher strength of material, the higher plastic deformation resistance, and the higher hardness value.