，题目是：Surface Hardening (I)
Surface hardening is a type of heat treatment process used to alter the properties of the surface layer or layers of a part. The portion of the part which is altered during this process is the exterior or boundary of the part and the layer below, which may be composed of a different material if a coating is used. Surface hardening techniques can be used to increase the hardness and wear resistance of the part’s surface layer, as well as improve the part’s corrosion resistance and fatigue strength. Common surface hardening methods include carburizing, nitriding, and tempering.
Carburizing is a process where carbon atoms are introduced into the outer surface of a part through the diffusion of carbon in super-heated gases, liquids, and solids. Heat and pressure are applied, which increases the part’s surface hardness, wear resistance, and ductility. This process is beneficial for parts with complex geometry as it can be used in all alloy grades, including low-carbon steels, stainless steels and cast irons.
Nitriding is a process, where nitrogen atoms are introduced into alloys, typically steels, to create a harder outer surface layer. Nitrogen atoms diffuse into the metal surface at a high temperature, typically above 500°F (260°C). Nitriding enhances a part’s wear resistance and fatigue strength, and slightly increases its corrosion resistance. The process does not increase the strength, but it can increase surface hardness, often significantly.
Tempering is a process of heating a part at a low temperature, typically between 300 and 1300°F (150 to 705°C). Depending on the composition of the part, tempering can cause the surface of the part to harden and become brittle. This process is beneficial for parts which are soft and easily deformed, as it increases strength, toughness and wear resistance.
Surface hardening enables the introduction of higher levels of hardness, strength and wear resistance to the surface of a part. These processes are beneficial for parts which require enhanced performance and protection from wear, forceful impacts, high temperatures and corrosive agents. Although these processes offer great benefits, they need to be performed by an experienced technician to ensure the part is not damaged when hardened and the desired results are achieved.
