Retort Furnaces for Nitriding

Nitriding is a process for surface hardening of steel in which nitrogen diffuses into the part surface. 

Gas nitriding and nitrocarburizing processes are usually carried out at temperatures of 500 °C to 580 °C and heat treatment times of a few to over a hundred hours in an ammonia atmosphere. As a result, a very hard compound layer of iron and alloy nitrides forms on the part surface. Beneath the compound layer is the diffusion zone, in which nitrogen is dissolved, which increases the fatigue strength of the material. Due to the combination of wear resistance and fatigue strength, nitriding offers advantages for parts that are exposed to high mechanical stresses, such as gear wheels, crankshafts, valves or tools. 

Nitriding can be carried out using various processes, including gas nitriding in an ammonia atmosphere. The Nabertherm retort furnaces of the NRA KN series are designed for gas nitriding processes. 

These furnaces are equipped with a gas-tight retort made of heat-resistant steel (optionally Inconel) to ensure safe operation in an ammonia/hydrogen mixture. 

To ensure good temperature uniformity and gas distribution, the furnaces are equipped with an atmosphere circulation fan. 

For the nitriding gas, the furnaces come with an ammonia connection (NH₃) and an ammonia dissociator for dilution with dissociated ammonia (d-NH₃). Thus, a separate hydrogen connection is not necessary. Nitrogen is used as a safety purge gas. 

The retort is inerted by a vacuum pump before the process starts and after the process end. The exhaust gases are burned in a gas-fired torch, operated with natural gas or propane. For faster process cycles, the furnace can be cooled down with an indirect cooling system. 

The process controls is based on a Siemens PLC with safety monitoring (F-PLC) and an nitriding controller module. A hydrogen sensor is installed to control the nitriding potential KN (optionally also carburizing potential KC with CO₂ connection and oxygen probe for nitrocarburizing). 

The Nabertherm nitriding furnaces NRA KN are characterized by high safety and great variability. Various sizes are available, from compact models for small in-house hardening shops to production furnaces. As an alternative to the NRA KN chamber retort furnaces, Nabertherm also offers pit-type retort furnaces series SRA KN.