Tufftride® (Ferritic Nitrocarburising)
Tufftride® is a salt bath process which can be carried out to most ferrous materials. It improves component quality by retarding wear, increasing fatigue strength, resisting corrosion, increasing hardness, enhancing appearance and achieving specific properties.
Tufftride® salt bath nitrocarburising is usually carried out between 580ºC and 610ºC, with treatment times in the order of 30 minutes to 3 hours. In some cases, salt bath nitrocarburising can be used as an alternative to other surface zone processes such as case hardening or hard chromium plating with equal or improved effectiveness and greater economy.
Properties of Tufftride®
The Tufftride® process produces a thin outer iron nitride compound layer, with a deeper nitrogen diffusion layer underneath. The compound layer of composition 7%-9% nitrogen and approximately 1% carbon is formed to a depth of 10µ – 20µ during average nitrocarburising times. Due to its composition, the layer does not possess metallic properties, thus making it particularly resistant to wear, seizure, corrosion, hot cracks and is durable almost to the temperature at which it was formed. It also produces surface micro hardness values from 400Hv up to 1500Hv, this is dependent upon material used.
The compound layer is also responsible for improved corrosion resistance which is further enhanced by a special quench – Tufftride® Q. Subject to component shape, form and treatment given, corrosion resistance in excess of 200 hours salt spray is possible.
Typical materials that can be treated:-
The base material affects the composition structure and properties of both the compound layer and diffusion layer. Most significantly, the hardness is affected where high alloy steels are seen to show the greater values, although this is not the only factor responsible for the improved wear and fatigue properties.
The formation of the nitride-rich compound layer acts to reduce the weld-ability of the surface; hence improving the frictional wear properties. Nitrogen in the diffusion layer inhibits the formation of cracks, so increasing the fatigue resistance.
Identified as a surface treatment, it is used to improve the service life of steel, sintered iron and cast iron components.
Tufftride® Improving properties
The two layers compound and diffusion, contribute to improved wear, corrosion resistance and fatigue properties and because processing temperatures are below critical transformation temperatures for steel, distortion is extremely low. Very often treatments whose aims are to increase one or two of these properties fail to achieve the third. This improvement of all three properties is achievable from a single heat treatment.
Nitrocarburised components are well known for their excellent sliding and running properties plus higher wear resistance and fatigue strength, i.e. ability to withstand bending and torsional stresses. Everyday use has shown that nitrocarburised components have a longer service life than other chemical and thermal surface treatments.
Benefits of process modifications
Modifications to the base Tufftride® process have seen additional benefits:
The Tufftride® TF1-AB1 process where components are subjected to intermediate cooling offers the following advantages: further improvement in corrosion resistance, better sliding properties and wear resistance, improved size and shape retention compared with cooling in water or oil, less danger of cracking when cooling sensitive parts/materials and improved surface appearance.
Tufftride® QPQ means quench, polish, quench and consists of salt bath nitrocarburising followed by oxidative cooling, mechanical polishing, lapping or vibratory finishing and brief re-immersion into the oxidising salt melt. The end result gives unalloyed and low alloyed steels, a lustrous black finish with corrosion resistance which is superior in many cases to surfaces developed by galvanic or chemical processes.
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