DISTORTION IN CARBONITRIDING AND NITRO-CARBURIZING HEAT TREATMENT PROCESS

 Hardening of a material is to be done to increase the strength and wear properties.

Case Depth = Distance from Surface to Hardness Limit

Distortion is the major concern in heat treatment process and it is the unexpected or inconsistent change in size and shape of the part.

Size distortion is the result of changes in the metallurgical structure that accompany phase transformations. In Heat treatment, the volume expansion and contraction associated with the crystal structure results in change to the volume and linear dimensions of the part such as elongation, shrinkage, thickening, and thinning. Shape distortion originate from the applied stresses that exceeds the elastic limit of the material and results change the geometrical form of the part, which can include twisting, bending, and non symmetrical dimensions.

Carbonitriding and Nitro-carburizing are both thermo-chemical diffusion process, which utilize the diffusion of carbon and nitrogen to increase the hardness and wear resistance at the surface. Although, both the processes takes place at different temperatures and lead to distortion in the part. However, the main advantage of nitro-carburizing is the minimal distortion associated with the process due to the absence of the phase transformation from austenite to martensite.

In gas nitro-carburizing, the part is heated at the temperature around 570 degrees Celsius, mixed gases with the main ingredient of ammonia gas is added which creates carbides and nitrides to form a compound layer on the surface of the material and resulting in a diffusion layer underneath. This formative nitride layer improves mechanical properties such as wear resistance and strength of the steel part.

Gas nitro-carburizing process can accommodate the trend towards lightening more compact parts from the popularization of electric vehicles and it is considered more eco-friendly heat treatment method that is why it continues to increase and is adapted by the manufacturing industries.

In carbonitriding, parts are heated in a sealed chamber into the austenitic range around 850 degrees celsius before nitrogen and carbon are added. The part is heated into the austenitic range of temperature, a phase change in the steel’s crystal structure takes place which allows carbon and nitrogen atoms to diffuse into the part.

Experimental Results

Conclusion: The Nitro-carburizing process can be the perfect replacement of carbonitriding heat treatment when low carbon steel(SAE 1010, SAE 1018, IS513 etc) is subjected to the light load applications, close fitting assemblies where wear resistance is required at part surface. Nitro-carburizing of low carbon steel gives you less dimensional changes in size and shape as compared to carbonitriding. The minimal changes associated with nitrocarburizing process lays in lower temperature and lack of phase transformation from austenite to martensite.