For structural parts subject to high loads (especially tension, compression, and shear forces), steel with good hardenability should be chosen; for shaft-type parts subjected to bending and torsional stresses, due to the large stress on the surface and less on the core, low hardenability steel can be used. Weldments generally do not use high hardenability steel, as it can easily lead to quenching structures in the weld and heat-affected zones, causing workpiece deformation and cracking. The hardenability of steel is significant in enhancing the mechanical properties of parts and realizing the potential of the steel. The hardenability of steel refers to its ability to achieve high hardness during quench hardening under ideal conditions, primarily depending on the carbon content in the steel. The higher the carbon content, the better the hardenability. Hardenability and hardenability are two distinct concepts. A steel with good hardenability does not necessarily have good hardenability; conversely, a steel with good hardenability does not necessarily have good hardenability. For example, although carbon tool steel has high hardness after quenching (good hardenability), its hardenability is low; whereas certain low-alloy steels, although not as hard after quenching, have good hardenability. Tempering is performed at temperatures above 500°C, aiming to give the workpiece a good combination of strength, plasticity, and toughness. The resulting structure is spheroidal pearlite with a hardness of 200~350HBW. It is mainly suitable for various important load-bearing structural components, such as connecting rods, bolts, gears, and shaft-type parts. The heat treatment process of quenching followed by high-temperature tempering is called case hardening. The case hardening process for spheroidal cast iron pipes not only serves as the final heat treatment for certain critical parts, such as shafts, gears, connecting rods, and bolts, but also as the preliminary heat treatment for precision parts like lead screws, gauges, and molds, to achieve a uniform and fine structure, reducing deformation during final heat treatment. In cases where tensile strength and hardness are roughly similar, the plasticity and toughness of case-hardened steel are significantly higher than those of normalized steel. This is because the structure of the steel after case hardening is spheroidal pearlite, with carbides in granular form, whereas the structure obtained by normalizing is lamellar spheroidal pearlite.