O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

Nitrocarburizing and cyanidation . These processes, compared with gas carburizing, have an advantage in the rate of saturation. The surface layer is more wear-resistant than with gas carburizing due to the presence of nitrogen and a fine-grained structure. The structure of the surface layers after cyanidation is distinguished by the presence of a carbonitride zone. Nitrocarburizing is carried out in a gaseous environment, the depth of the hardened layer is (0.15...1.0) mm, the hardness after hardening is HRC52–60.
Cyanidation is carried out in molten salts, the depth of the cyanidated layer is (0.1 ... 1.6) mm.
Boridingused mainly for medium-carbon unalloyed steels, is carried out in a solid, liquid or gaseous medium. The surface hardness of steel after boriding reaches HV 1400…1500, which ensures high wear resistance. The depth of the borated layer is (0.12…0.85) mm.
Sulfidation and sulfocyanation . Sulfiding, usually carried out in salt baths, gives a significant anti-seize effect and reduces the coefficient of friction. Wear resistance increases by (2 ... 5) times.
When sulfocyanidation (simultaneous saturation of surfaces with nitrides and sulfides) is achieved, both the extreme pressure properties of surfaces and their wear resistance are increased.
20.2. Application of wear-resistant coatings
To increase the wear resistance of products, galvanic coatings are used - chromium plating, remaining, nickel plating.
Chrome plating . Chrome coating applied to the surface of parts by galvanic method with a thickness of (0.1 ... 0.2) mm has a high hardness (HB 1000 ... 1100), a low coefficient of friction, which significantly reduces heat generation during friction. The wear resistance of a hard smooth chromium coating is (5…15) times higher than that of steel parts.
Chrome plating is of two types: hard smooth and porous. A hard, smooth chrome coating has one significant drawback: it is poorly wetted by lubricating oils. To increase the wettability of the coating, they resort to creating pores, depressions and channels in it (porous chromium plating) by anodic etching of a hard smooth chromium coating. The porosity of the coating can be channeled (the cross section of the channels is 0.050.05 mm) or dotted.
Point porosity has a greater oil absorption, so it is used to harden parts operating in particularly difficult conditions (for example, for the upper compression rings of engines). It is characterized by fast running-in, but its wear resistance is somewhat lower than that of chromium plating with channeled porosity. Channeled chrome is often coated on cylinder liners. The wear of porous-chrome-plated liners and piston ends is less than non-chrome-plated ones by (4…7) times, and the wear of mating steel parts also decreases by (3…5) times.
Chrome plating of parts can be carried out with an allowance for subsequent machining (grinding and lapping) or without an allowance "in size".
Chrome-plated parts work well with babbitt, fine-grained cast iron, or soft-to-medium-hardened steels, provided they are lubricated and the pressure is not too high. It is not recommended to chrome parts that work in tandem with titanium alloys.
Chrome plating does not replace hardening and carburizing. Moreover, to increase the wear resistance of hardened and case-hardened steels, they are also chromium-plated.
Chrome plating is also used to restore worn parts.
Hardening , widely used to restore worn surfaces of steel and cast iron parts, unlike chromium plating, is more productive (about 10 ... 15 times), inexpensive, coating thickness reaches 3 mm; hardness of the galvanized surface HV 6000…6500.
Remaining can also be used to create an undercoat before chrome plating.