The ability of a metal material to resist a hard object's crushing surface is called hardness. According to the test method and the scope of application, the hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, micro hardness and high temperature hardness. For the pipe commonly used are Brinell, Rockwell, Vickers hardness of three

A, Brinell hardness (HB) with a certain diameter of the ball or carbide ball to the specified test force (F) into the sample surface, after a specified retention time to remove the test force, measuring the surface of the sample indentation Diameter (L). Brinell hardness values ??are calculated by dividing the test force by the indentation spherical surface area. Expressed in HBS (steel ball), the unit is N / mm2 (MPa). The formula is: where: F - into the metal sample surface test force, N; D - test ball diameter, mm; d - indentation average diameter, mm. Determination of Brinell hardness is more accurate and reliable, but the general HBS only applies to 450N / mm2 (MPa) the following metal materials, for hard steel or thin plate is not applicable. In the steel pipe standard, Brinell hardness is the most widely used, often with indentation diameter d to represent the hardness of the material, both intuitive and convenient. Example: 120HBS10 / 1000130: indicates a Brinell hardness value of 120 N / mm2 (MPa) measured for 30 seconds (seconds) under a test force of 1000 Kgf (9.807 KN) with a diameter of 10 mm.

B, Rockwell Hardness (HK) Rockwell hardness test with the Brinell hardness test, are indentation test method. The difference is that it is the depth of the indentation measured. That is, under the influence of the initial test force (Fo) and the total test force (F), the indenter (gold steel cone or steel ball) is pressed into the surface of the specimen, and after the specified holding time, The test force is calculated using the measured residual indentation depth increment (e). The value is an unknown number, expressed in symbol HR, the scale used is A, B, C, D, E, F, G, H, K and other 9 scales. Which is commonly used in steel hardness test scale is generally A, B, C, that is, HRA, HRB, HRC. The hardness value is calculated by the following formula: HR = 100-e when tested with A and C scale HR = 130-e where e - residual depth of indentation is incremented, Unit 0.002mm said, that is, when the indenter axial displacement of a unit (0.002mm), that is equivalent to Rockwell hardness change a number. The greater the e value, the lower the hardness of the metal, and vice versa the higher the hardness. The above three scales apply as follows: HRA (diamond cone head) 20-88 HRC (diamond cone head) 20-70 HRB (diameter 1.588mm steel ball head) 20-100 Rockwell hardness test is widely used , Where HRC is used in steel pipe standards after Brinell hardness HB. Rockwell hardness can be applied to the determination of very soft to very hard metal material, which makes up the Buchner method is not, than the Brinell method is simple, can be directly from the hardness of the dial read out the hardness value. However, due to its indentation is small, so the hardness value is not as accurate as the Brinell method.

C, Vickers hardness (HV) Vickers hardness test is also an indentation test method, is a relative angle of 1360 with a positive four pyramidal diamond indenter to the selected test force (F) into the test surface, After the specified holding time to remove the test force, measure the indentation of two diagonal length. Vickers hardness value is the test force divided by the indentation surface area of ??the business, the formula is: where: HV - Vickers hardness symbol, N / mm2 (MPa); F - test force, N; d-- The arithmetic mean of two diagonal lines of indentation, mm. The test force F for Vickers hardness is 6, and the measured hardness F is 5 (49.03), 10 (98.07), 20 (196.1), 30 (294.2), 50 (490.3), 100 (980.7) Kgf (N) The range is 5 ~ 1000HV. Example: 640HV30 / 20 indicates a Vickers hardness value of 640 N / mm2 (MPa) as measured at 30Hgf (294.2N) test force of 20S (sec). Vickers hardness method can be used to determine the thin metal material and surface layer hardness. It has the main advantages of the Brinell and Rockwell methods, and overcomes their basic shortcomings, but it is not as easy as the Rockwell method. Vickers method is rarely used in steel pipe standards.