Investigation of Nitrogen Transfer during Low-Pressure Carbonitriding Processing

Monday, September 14, 2009 - 2:40 PM

Investigation of Nitrogen Transfer during Low-Pressure Carbonitriding Processing

A. Goldsteinas, Ipsen, Inc., Cherry Valley, IL

Low pressure carbonitriding is a modified form of low pressure carburizing that uses acetylene and ammonia to impart a hard, wear-resistant case, generally measuring between 0.003 and 0.030” (0.075 and 075 mm) deep.

As is the case with low pressure carburizing cycles, carbonitriding also consists of several pairs of boost and diffusion segments. Nitrogen cannot be absorbed into a parts surface simultaneously. For this reason, carbonitriding adds a twist – this process uses ammonia in the one or more of the final diffusion segments. The best results are received by reducing a parts temperature to the hardening temperature, adding ammonia, and then gas quenching.

When compared to carburizing, the microstructure of carbonitrided parts have increased nitrogen content, higher temperature resistance, increased hardenability, and improved wear resistance. The results from several carbonitriding cycles with varying time, temperature and pressure as well as several carbonitriding applications will be presented.

Summary: Low pressure carbonitriding is a special case hardening process and modified form of low pressure carburizing. This process uses acetylene and ammonia as reactive gases. It is primarily used to impart a hard, wear-resistant case, generally measuring between 0.003 and 0.030” (0.075 and 0.750 mm) deep. Low pressure carburizing cycles consist of several pairs of boost and diffusion segments. However, because carbon and nitrogen cannot be absorbed into a parts surface simultaneously, the carbonitriding process uses ammonia in the diffusion segment or segments. During those segments, acetylene has been evacuated from the furnace and allows the nitrogen to diffuse into the parts surface. In most cases, it is sufficient to add ammonia to the final diffusion segment of the carbonitriding process. The best results are received by reducing the temperature in the last diffusion segment to hardening temperature, adding ammonia, and then gas quenching. When compared to carburizing, the microstructure of carbonitrided parts have increased nitrogen content, higher temperature resistance, increased hardenability, and improved wear resistance. The results from several carbonitriding cycles with varying time, temperature and pressure as well as several carbonitriding applications will be presented.