Hardness and hardness testing of seamless steel tubes

2021-11-12
Hardness is one of the most commonly used indicators to evaluate the mechanical properties of seamless steel tubes. The main purpose of the hardness test of the seamless steel pipe is to measure the applicability of the seamless steel pipe, or the effect of the special hardening or softening treatment of the seamless steel pipe for the purpose of use.
 
The hardness of the seamless steel pipe represents the comprehensive performance of various physical quantities such as elasticity, plasticity, strength, toughness, wear resistance and so on reflected by the seamless steel pipe under the action of a certain indenter and test force. Because the hardness test can reflect the difference in performance of seamless steel pipes under different chemical compositions, organizational structures and heat treatment conditions, the hardness test is widely used to test the mechanical properties of seamless steel pipes.
 
 
Commonly used hardness index of seamless steel pipe
 
Seamless steel pipe is commonly used in the three hardness indicators Brinell, Rockwell, Vickers to measure its hardness.
1) Brinell hardness
In the seamless steel pipe standard, Brinell hardness is the most widely used, and the indentation diameter is often used to express the hardness of the material, which is intuitive and convenient. However, it is not suitable for steel pipes with harder or thinner steel materials.
2) Rockwell hardness
Seamless Rockwell hardness test is the same as Brinell hardness test. It is an indentation test method. The difference is that it measures the depth of the indentation. The Rockwell hardness test is a widely used method at present, and HRC is used in the steel pipe standard only after the Brinell hardness HB. Rockwell hardness can be applied to the determination of very soft to extremely hard metal materials, it makes up for the lack of the Brinell method, is simpler than the Brinell method, can read the hardness value directly from the dial of the hardness machine. However, due to its small indentation, the hardness value is not as accurate as the Brookfield method.
3) Vickers hardness
The Vickers hardness test of seamless steel tubes is also an indentation test method that can be used to determine the hardness of very thin metallic materials and surface layers. It has the main advantages of the Brinell and Rockwell methods, and overcomes their basic disadvantages, but it is not as easy as the Rockwell method. The Vickers method is rarely used in steel pipe standards.
 
 
Hardness test method for seamless steel pipe
 
The hardness test of stainless steel should take into account its mechanical properties, which is related to the performance and quality of processing such as deformation, stamping, and cutting performed with stainless steel as raw material. Therefore, all the seamless steel tubes are tested for mechanical properties. There are two main types of testing methods for mechanical properties. One is tensile testing, and the other is hardness testing.
 
Tensile test is made of seamless steel pipe into a sample, in the tensile tester to pull the sample to fracture, and then determine one or several mechanical properties, usually only measured tensile strength, yield strength, elongation after fracture and rate of reduction in area. Tensile test is the most basic test method for mechanical properties of metal materials. Almost all metal materials, as long as the mechanical properties are required, have specified tensile tests. Especially for those materials whose shape is not convenient for hardness testing, the tensile test becomes the only means of testing the mechanical properties.
 
The hardness test is to determine the hardness of a material by slowly pressing a hard indenter into the surface of the specimen under specified conditions and then testing the indentation depth or dimension. Hardness testing is the simplest, fastest, and easiest to implement method for testing the mechanical properties of materials. The hardness test is non-destructive and there is an approximate conversion between the material hardness value and the tensile strength value. The hardness value of the material can be converted into the tensile strength value, which has great practical significance.
 
Since the tensile test is inconvenient for testing and is easily converted from hardness to strength, more and more people only test the hardness of the material and test its strength less. In particular, due to the continuous advancement and innovation of the hardness tester manufacturing technology, some materials that could not be directly tested for hardness, such as seamless steel pipes, stainless steel plates, and stainless steel bands, are now possible to directly test the hardness. Therefore, there is a tendency for hardness tests to gradually replace tensile tests.
 
Most of the national standards for stainless steel materials specify both tensile tests and hardness tests. For materials that are not convenient for hardness testing, such as seamless steel tubes, only tensile tests are specified. In the stainless steel standard, three hardness test methods are generally specified, namely, HB, HRB (or HRC) and HV hardness values. It is stipulated that only one of the three hardness values ​​can be measured. In particular, the company's newly developed portable surface Rockwell hardness tester and Rockwell hardness tester can perform rapid and accurate hardness test on stainless steel plates as thin as 0.05mm, stainless steel bands, and seamless steel pipes as fine as 4.8mm. Makes it difficult to solve problems in the past.

 
Seamless steel pipe hardness testing tool
 
The seamless steel tube with an inner diameter of 6.0 mm or more and an annealed seamless steel tube with a wall thickness of 13 mm or less can be tested with the W-B75 Vickers hardness tester. It is very fast and easy to test, and is suitable for the seamlessness of seamless steel tubes. Qualified inspection. Seamless steel tubes with an inner diameter greater than 30mm and a wall thickness greater than 1.2mm are used for the seamless steel tubes. Rockwell hardness testers are used to test the hardness of HRB and HRC. Seamless steel tubes with an inner diameter greater than 30 mm and a wall thickness of less than 1.2 mm in a seamless steel tube are tested with a Rockwell hardness tester to test the HRT or HRN hardness. The inner diameter of the seamless steel pipe is less than 0mm, greater than 4.8mm, using a Rockwell hardness tester for pipe, test HR15T hardness. When the inner diameter of the seamless steel pipe is greater than 26mm, Rockwell or Rockwell hardness tester can also be used to test the hardness of the inner wall of the pipe.
 
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