The differences and conversions of hardness classification for common steel and metallic materials
The most common hardness test method is the static load indentation method, such as Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC), Vickers hardness (HV), rubber and plastic Shore hardness (HA, HD), etc. The values of these hardness tests represent the ability of the material's surface to resist the indentation by a hard object. The popular Rockwell hardness (HL) and Shore hardness (HS) belong to the rebound method hardness test, and their values represent the amount of elastic deformation work of the metal. Therefore, hardness is not a simple physical quantity, but a comprehensive performance indicator reflecting the elasticity, plasticity, strength and toughness of the material.
Steel hardness: The symbol for metal hardness (Hardness) is H. Depending on the different hardness testing methods, the common representations include Brinell (HB), Rockwell (HRC), Vickers (HV), and Leeb (HL) hardness, among which HB and HRC are more frequently used. HB has a wider application range, while HRC is suitable for materials with high surface hardness, such as heat-treated hardness. The difference between the two lies in the different measuring heads of the hardness testers; the measuring head of the Brinell hardness tester is a steel ball, while that of the Rockwell hardness tester is a diamond.
·HV - Applicable for microscopic analysis. Vickers hardness (HV) is determined by pressing a diamond square pyramid indenter with a tip angle of 136° into the material surface under a load of 120kg or less. The Vickers hardness value (HV) is obtained by dividing the surface area of the indentation by the applied load.
·HL Portable hardness tester, convenient for measurement. It measures hardness by striking the surface with a spherical impact head and calculating the ratio of the rebound velocity at 1mm from the sample surface to the impact velocity. The formula is: Leeb hardness HL = 1000 x VB (rebound velocity) / VA (impact velocity).
·Currently, the commonly used portable Leeb hardness testers can convert the measured Leeb hardness (HL) into Brinell (HB), Rockwell (HRC), Vickers (HV), and Shore (HS) hardness. Or, hardness can be directly measured using the Leeb principle with Brinell (HB), Rockwell (HRC), Vickers (HV), Leeb (HL), and Shore (HS). The TH series Leeb hardness testers produced by Times Company have this function and are a beneficial supplement to traditional benchtop hardness testers!
1. HB - Brinell Hardness:
Brinell hardness (HB) is generally used for softer materials, such as non-ferrous metals, steel before heat treatment or after annealing. Rockwell hardness (HRC) is typically used for harder materials, such as steel after heat treatment. Brinell hardness (HB) is determined by applying a certain test load to a hardened steel ball or carbide ball of a specific diameter onto the surface of the material to be tested, maintaining the load for a specified period, and then removing the load. The Brinell hardness value is the quotient of the load divided by the spherical surface area of the indentation. Generally, a certain load (usually 3000 kg) is applied to a steel ball of a certain size (diameter usually 10 mm) onto the surface of the material, maintained for a period of time, and then the load is removed. The ratio of the load to the area of the indentation is the Brinell hardness value (HB), with the unit being kilograms force per square millimeter (N/mm²).
2. HR - Rockwell Hardness
Rockwell hardness (HR-) is determined by the depth of plastic deformation in the indentation. 0.002 millimeters is used as the unit of hardness. When HB > 450 or the sample is too small, Brinell hardness test cannot be used and Rockwell hardness measurement should be adopted. It uses a diamond cone with a 120° apex or a steel ball with a diameter of 1.59 or 3.18 mm. Under a certain load, the hardness of the material is determined by the depth of the indentation on the surface of the tested material. Depending on the hardness of the tested material, three different scales are used to represent it:
HRA: This is the hardness obtained using a 60 kg load and a diamond cone indenter, and is used for materials with extremely high hardness (such as hard alloys, etc.).
HRB: This is the hardness obtained using a 100 kg load and a 1.59 mm hardened steel ball, and is used for materials with lower hardness (such as annealed steel, cast iron, etc.).
HRC: This is the hardness obtained using a 150 kg load and a diamond cone indenter, and is used for materials with very high hardness (such as quenched steel, etc.). Additionally:
(1) HRC means the Rockwell hardness C scale,
(2) HRC and HB are widely used in production,
(3) The applicable range of HRC is HRC 20 - 67, which is equivalent to HB 225 - 650. If the hardness is higher than this range, use the Rockwell hardness A scale HRA. If the hardness is lower than this range, use the Rockwell hardness B scale HRB. The upper limit of Brinell hardness HB is 650, and it cannot be higher than this value.
(4) The indenter of the Rockwell hardness tester with scale C has a 120° apex diamond cone and a fixed test load. The standard in China is 150 kilograms of force. The indenter of the Brinell hardness tester is a hardened steel ball (HBS) or a carbide ball (HBW). The test load varies depending on the diameter of the ball, ranging from 3000 to 31.25 kilograms of force.
(5) The Rockwell hardness indentation is very small, and the measurement value is local. Multiple points should be measured to obtain the average value. It is suitable for finished products and thin sheets and belongs to non-destructive testing. The Brinell hardness indentation is larger, and the measurement value is accurate. It is not suitable for finished products and thin sheets, and is generally not classified as non-destructive testing.
(6) The hardness value of Rockwell hardness is an unnamed number and has no unit. (Therefore, it is incorrect to say that the Rockwell hardness is how many degrees.) The hardness value of Brinell hardness has a unit and has a certain approximate relationship with tensile strength.
(7) Rockwell hardness is displayed directly on the dial or digitally. The operation is convenient, fast, intuitive, and suitable for mass production. Brinell hardness requires measuring the indentation diameter with a microscope and then looking up the table or calculating. The operation is more cumbersome.
(8) Under certain conditions, HB and HRC can be interchanged using the table. The approximate mental calculation formula can be roughly remembered as: 1 HRC ~ 1/10 HB.
3. HV Vickers Hardness
Vickers hardness is determined by applying a certain test force load, using a square conical diamond indenter with an included angle of 136°, to press it into the surface of the material. After a specified period of time, the test force is removed, and the diagonal length of the indentation on the sample surface is measured. Then, the hardness value is calculated according to the formula, which is Vickers hardness.
Vickers hardness is represented as HV. The numerical value preceding the symbol HV indicates the hardness value, and the value following it represents the test force. The standard test duration is 10 to 15 seconds. If the selected duration exceeds this range, the duration should be noted after the force value. For example:
600HV30 indicates that a hardness value of 600 was obtained when a test force of 294.2N (30kg) was applied for 10 to 15 seconds. 600HV30/20 indicates that a hardness value of 600 was obtained when a test force of 294.2N (30kg) was applied for 20 seconds.
Vickers hardness tests are divided into three types of hardness testers based on the test force size: Vickers hardness tester, low-load Vickers hardness tester, and micro Vickers hardness tester (all with a test force of 1000N or less are collectively referred to as micro hardness testers). The appropriate hardness testing equipment should be selected based on factors such as the thickness of the workpiece and the hardness level.
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