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Introduction of heat treatment(2014-02-25)

Carburizing is the most common of the case-hardening methods in use today and can handle a diverse range of part sizes and load configurations . In general, a properly Carburized Helical Gear will be able to handle somewhere between 30-50% more load than a through-hardened Helical Gear. Examples of commonly Carburized Helical Gear steels include SAE grades 1018, 4320, 5120, 8620 and 9310 as well as international grades such as 20MnCr5, 17CrNiMo6, 18CrNiMo7-6 and 20MoCr4.
Carburizing is typically performed in the temperature range of 870-955°C (1600-1750°F), although temperatures approaching 1010°C (1800°F) are used for deep-case work. Carburizing case depths vary over a broad range, typically 0.13-8.25 mm (0.005-0.325 inches).

Carbonitriding is a modification of the carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere in order to add nitrogen to the carburized case as it is being produced. Examples of Helical Gear steels that are commonly carbonitrided include AISI 1018, 1117 and 12L14.

Typically, Carbonitriding  Helical Gear is done at a lower temperature than Carburizing Helical Gear– between 700-900°C (1300-1650°F) – and for a shorter time. Combine this with the fact that nitrogen inhibits the diffusion of carbon and what generally results is a shallower case than is typical for Carburized Helical Gear. A carbonitrided case is usually between 0.075-0.75 mm (0.003-0.030 inches) deep. 

Nitriding is another surface-treatment process that has increasing surface hardness as its objective. One of the appeals of this process is that rapid quenching is not required. Hence, dimensional changes are kept to a minimum. It is not suitable for all Helical Gear applications. One of its limitations is that the extremely high surface-hardness case produced has a more brittle nature than that produced by the carburizing process. Despite this fact, nitriding has proved to be a viable alternative in a number of applications. Examples of commonly Nitrided Helical Gear steels include SAE 4140, 4150, 4340 and Nitralloy 135M.

Nitriding is typically done in the 495-565°C (925-1050°F) temperature range. Case-depth and case-hardness properties vary not only with the duration and type of nitriding being performed but also with steel composition, prior structure and core hardness. Typically, case depths are between 0.20-0.65 mm (0.008-0.025 inches) and take from 10 to 80 hours to produce.  

Nitrocarburizing is a modification of nitriding, not a form of carburizing. In the process, nitrogen and carbon are simultaneously introduced into the steel while it is in a ferritic condition (i.e. at a temperature below that at which austenite begins to form during heating). A very thin “white” or “compound” layer is formed during the process as well as an underlying “diffusion” zone. Like nitriding, rapid quenching is not required. Examples of  Helical Gear steels that are commonly nitrocarburized include SAE grades 4140, 5160, 8620 and certain tool steels.

Nitrocarburizing is normally performed at 550-600°C (1025-1110°F) and can be used to produce a 58 HRC minimum hardness, with this value increasing dependent on the base material. White-layer depths range from 0.0013-0.056 mm (0.00005-0.0022 inches) with diffusion zones from 0.03-0.80 mm (0.0013-0.032 inches) being typical.