Since hardness and tensile strength have a certain conversion relationship, and other mechanical properties are related to tensile strength, hardness has a certain relationship with other mechanical properties. Practice has proved that the relationship between Brinell hardness (HB) and tensile strength (σb) is σb≈1/3HB, and the relationship between bending fatigue limit (σ-1) and tensile strength (σb) is σ-3. ≈ 1/2σb, thus the following approximate relationship exists between σ-1 and HB: σ-1≈1/6HB In addition, for medium and low-strength steel, the following empirical relationship is obtained: Carbon steel σ-1=12HC+ 122 High-strength alloy steel σ-1=8.7(1+1.35ψ)HRC (ψ is the surface shrinkage ratio) There is a general straight line between the fatigue limit number and the static strength. In some data, the more specific bending fatigue limit and tensile strength of some materials are given, for example, σ-1=0.35σb+12.2 for carbon steel and σ-1=0.25σb for gray cast iron. +2; σ-1=(0.25-0.4) σb for aluminum; σ-1=(0.3-0.4) σb for single-phase brass. Combining these relationships with the conversion data of HB and σb given by "the relationship between ferrous metal hardness and tensile strength" and "the relationship between hardness and tensile strength of non-ferrous metals", it is not difficult to obtain σ-1 and HB. The data is converted, that is, the bending fatigue limit (σ-1) is inferred from the Brinell hardness (HB). The relationship between fatigue limit and hardness under other stresses can also be derived from the bending fatigue limit (σ-1). The conversion has the following formula: Compressive fatigue σ-1P=0.85σ-1 (steel) σ-1P=0.65σ-1 (cast steel) torsional fatigue τ-1P=0.85σ-1 (steel and light alloy) τ-1P=0.8σ-1 (cast iron) It is also proved that for general carbon steel, when the hardness is HRC40～45, it has the most Good fatigue strength, but based on complete quenching and tempering, which is just the upper limit of the above-mentioned application of σ-1 and HRC. The hardness increases again and the fatigue limit decreases. In addition, hardness has a certain relationship with wear resistance, abrasion resistance, machinability and the like. Under normal circumstances, if other conditions are the same, the higher the hardness value, the better the wear resistance (or abrasion resistance), such as gages, cutting tools and grinding balls. When the hardness is high in the HB range, the machinability is optimal, and too high or too low will deteriorate the machinability.