The surface quenching and tempering heat treatment of straight seam welded pipes is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness, and effective hardened layer depth. Hardness testing can be done with a Vickers hardness tester, a Rockwell, or a surface Rockwell hardness tester. When the surface heat treatment hardened layer is thick, the Rockwell hardness tester can also be used. When the thickness of the heat treatment hardened layer is between 0.4 and 0.8mm, the HRA scale can be used. When the thickness of the hardened layer exceeds 0.8mm, the HRC scale can be used.


If the parts require high local hardness, local quenching heat treatment can be carried out by induction heating. Such straight seam welded pipes usually need to mark the location of local quenching heat treatment and the local hardness value on the drawing. The hardness testing of straight seam welded pipes should be carried out within the area. The hardness testing instrument can use a Rockwell hardness tester to test the HRC hardness value. If the heat treatment hardened layer is shallow, a surface Rockwell hardness tester can be used to test the HRN hardness value.


The three hardness values of Vickers, Rockwell, and Superficial Rockwell can be easily converted to each other and converted into hardness values required by standards, drawings, or users. The corresponding conversion tables are given in the international standard ISO, the American standard ASTM, and the Chinese standard GB/T.