Three Types of Hardness in Stainless Steel Pipes

14 Jul,2019

Three Types of Hardness in Stainless Steel Pipes

Stainless steel pipe is a hollow, long, round steel product that is widely used across various industries. Its hardness is an important property to understand; this article briefly introduces three common hardness grades.

　　1. Rockwell hardness

　　Like the Brinell hardness test, the Rockwell hardness test for stainless steel pipes is also an indentation‑based method. The key difference is that it measures the depth of the indentation. The Rockwell test is widely used today; among steel pipe standards, its HRC scale ranks second only to the Brinell HB scale. Rockwell hardness is suitable for assessing metals ranging from very soft to very hard, addressing some of the limitations of the Brinell method. It is simpler than Brinell and allows hardness values to be read directly from the instrument’s dial. However, because the indentation is small, the resulting hardness values are less accurate than those obtained with the Brinell method.

　　2. Brinell Hardness

　　In stainless steel pipe standards, Brinell hardness is the most widely used; material hardness is typically expressed in terms of the indentation diameter, which is both intuitive and convenient. However, this method is not suitable for steel pipes that are either very hard or very thin.

　　3. Vickers Hardness

　　The Vickers hardness test for stainless steel tubes is also an indentation method, suitable for measuring the hardness of very thin metal materials and surface layers. It combines the key advantages of the Brinell and Rockwell methods while addressing their principal drawbacks; however, it is less convenient than the Rockwell method, and the Vickers method is seldom used in stainless steel tube standards.

　　4. Hardness Testing

　　For annealed stainless steel tubes with an inner diameter of 6.0 mm or greater and a wall thickness of 13 mm or less, the W‑B75 Webster hardness tester can be used. This instrument provides rapid and convenient testing, making it well suited for quick, nondestructive pass/fail inspections of stainless steel tubing. For stainless steel tubes with an inner diameter exceeding 30 mm and a wall thickness greater than 1.2 mm, a Rockwell hardness tester is employed to measure HRB or HRC hardness. For tubes with an inner diameter greater than 30 mm but a wall thickness less than 1.2 mm, a superficial Rockwell hardness tester is used to determine HRT or HRN hardness. For tubes with an inner diameter between 4.8 mm and 30 mm, a tube‑specific Rockwell hardness tester is utilized to measure HR15T hardness. Additionally, when the inner diameter exceeds 26 mm, Rockwell or superficial Rockwell hardness testers may also be applied to assess the hardness of the tube’s inner wall.

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