Carbonitriding

According to different media, carbonitriding is divided into solid, liquid and gas three kind. Solid carbonitriding is similar to solid Carburizing, and 30% 40% yellow blood salt 10% ammonium carbonate and 50% 60% charcoal are often used as infiltrating agents. This method has low production efficiency and poor working conditions and is rarely used at present. Liquid carbonitriding uses cyanide as raw material, has a long history, and its quality is easily controlled. However, cyanide is highly toxic and expensive, and its use is limited. Gas carbonitriding is the fastest growing.

According to the co-osmotic temperature, carbonitriding is generally divided into low temperature (500 ~ 560 °C), medium temperature (780 ~ 850 °C) and high temperature (880 ~ 950 °C). The former is dominated by nitriding and is now defined as nitrocarburizing. The latter two are mainly carburizing. The custom of carbonitriding, mainly refers to medium temperature gas nitrocarburizing.

The mechanical properties of carbonitrided parts are related to the concentration of carbon and nitrogen on the surface of the permeation layer, the depth of the penetration layer and the concentration gradient.

The carbon-nitrogen concentration of the co-permeation layer must be strictly controlled, and the content is too low to obtain high strength, hardness, and ideal residual stress, which affects wear resistance and fatigue strength. On the contrary, not only a large number of heterogeneous bulky carbonitrides appear in the surface layer, but also the brittleness increases, and the amount of retained austenite after quenching increases dramatically, affecting the surface hardness and fatigue strength.

The best carbon and nitrogen concentrations were respectively 0.70~ 0.95% C and 0.25~ 0.40% N. For a small number of alloy steel parts that work under high contact stress, when more uniformly distributed carbon-nitrogen particles are required on the surface, the surface carbon content can reach 1.20-1.50% or even 2-3%, and the nitrogen content is still 0.50% or less.

The depth of the osmotic layer should be compatible with the service conditions of the workpiece and the steel composition. When the carbon content of the heart part is relatively high or the load carrying capacity of the workpiece is low, such as the textile machine steel ring and the 40Cr steel automobile gear, the penetrating layer should be thinner, often more than 0.50 mm. Some of the original carburized parts were changed to carbonitriding, and the depth requirement could be appropriately thinned.

The concentration gradient in the osmotic layer should be as flat as possible to ensure that the depth of the layer is well combined with the heart, preventing the delamination of the osmotic layer.