Method For Testing Bolt Hardness

For the carbon steel bolts we commonly use, in addition to necessary performance tests, the hardness standard is also a crucial parameter. So how is bolt hardness tested?

First, the claim that stainless steel bolts do not require hardness testing is inaccurate. While manufacturing stainless steel bolts mainly involves shaping the material, some stainless steel bolts-especially those made of martensitic stainless steel (e.g., 431)-undergo heat treatment to enhance performance, and their hardness changes accordingly. The performance of a finished stainless steel bolt is indeed determined by its material, but for heat-treated stainless steel bolts, hardness testing is necessary as it directly reflects their mechanical properties. Thus, it is incorrect to say that stainless steel bolt hardness never needs to be confirmed; material type alone is not sufficient.

Therefore, the hardness index is not exclusive to carbon steel bolts; some stainless steel bolts also have hardness requirements. Bolts of different grades have different hardness levels. According to national standard parameters:

Grade 4.8 bolts (low-carbon steel, non-heat-treated) have low hardness, usually expressed in Brinell hardness (HB) with a range of 130-180 (HRC is not applicable here, as HRC is used for testing higher hardness).

Grade 6.8 bolts generally use Rockwell hardness B scale (HRB), with a range of 81-96 (HRC is also not applicable).

The next higher grade is the 8.8 grade bolts, which are most commonly used, with a Rockwell hardness C scale (HRC) of 22-32.

The higher 10.9 grade bolts have an HRC of 33-39.

Finally, the 12.9 grade bolts have the highest strength and hardness, with an HRC of 39-44.

Since the shape of a bolt is not a complete flat surface, the hardness values tested at different parts will vary slightly, so the hardness standard refers to the average value.

How exactly is bolt hardness tested?

Let's start with Brinell hardness testing. To measure the Brinell hardness of bolts, steel ball indenters or cemented carbide indenters are used. Brinell hardness is mainly suitable for materials with hardness ≤450 HBW (HBW has replaced HBS in current standards), for which steel ball indenters can be used. For materials with higher hardness, Brinell testing is generally not applicable, and Rockwell hardness testing is used instead. Materials like 12L15 (free-cutting steel) are often tested for Brinell hardness, and some low-strength bolts (e.g., grade 4.8) also require Brinell hardness testing.

The table compares the old and new standards. For bolt products, the most important parameter is Rockwell hardness, i.e., the HRC hardness standard we often refer to. Rockwell hardness testing has been mandatory since 2004, with six additional hardness scales and their applicable ranges added to the original standards: 15N, 30N, 45N, 15T, 30T, and 45T. The old standard specified the use of diamond indenters for Rockwell hardness testing, while the new standard also allows the use of cemented carbide indenters. In addition, the testing environment and temperature changes at the time must be considered; new regulations on archiving test reports have been added, and test results are required to be accurate to 0.5 HR (with specific scales, such as HRC or HRB, specified).