Teniferation

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Teniferating is a chemical / physical process to increase the hardness of steel materials .

Procedure

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The objects intended for treatment are immersed in a nitriding bath with controlled cyanide - cyanate content for between a few minutes and a few hours, depending on the steel composition and shape . The standard treatment time is 90 minutes at a temperature of 580 ° C. The Tenifer treatment is carried out on the finished, heat-treated and machined workpiece. The last tempering temperature should be above the Tenifer treatment temperature of 580 ° C, so that no undesirable structural changes occur. Before treatment, the workpieces should be preheated slowly and as homogeneously as possible (so that no tension occurs) to around 350 ° C. This is followed by immersion in the Tenifer bath at 580 ° C. After the maximum treatment time has been reached, the workpieces are quenched in water, oil or a polymer .

A subsequent post- oxidation has also proven useful (Tenifer-Q process). Here the workpiece is immersed directly from the Tenifer bath in an AB1 bath at around 380 ° C and held for 10–15 minutes, thereby oxidizing the surface (black color) and at the same time neutralizing adhering cyanide (detoxification reaction). The adhering cyanide is converted into non-toxic sodium carbonate. This is followed by quenching in the water.

The Tenifer QPQ process is an extension. QPQ stands for quenching - polishing - quenching . After the Tenifer treatment, the parts are oxidized in an oxidation bath (AB bath) and then washed. In practice, glass bead blasting is used as a substitute for polishing. The parts are then oxidized again in the AB1 bath for about 30 minutes, washed and dried. The oxidizing cooling after the Tenifer treatment fills the pores with magnetite . As a rule, a uniform oxidation layer of 2 to 4 µm is achieved.

properties

Dimensional change

Since there is no structural change in the base material during the Tenifer treatment and the thermal stresses are very low as a result of slow heating and cooling, no significant changes in dimensions are to be expected. Only the compressive stresses occurring in the thin edge zone as a result of the nitride or carbo-nitride formation lead to minor dimensional changes, which are, however, only of the order of magnitude of 2-4  µm and are limited to the edge zone. However, larger parts can show a certain amount of distortion due to tension in the surface.

Link layer

The connecting layer is created when the nitrogen is stored. If nitrogen is stored in the edge area, the solubility is exceeded. A homogeneous, well-adhering connection layer is formed by precipitating iron nitrides. Hardnesses of over 1000 HV can arise in the connecting layer.

Surface hardness

The surface hardness increases dramatically with the Tenifer treatment. The hardness in the nitrided zone and in the base material was measured with a small load tester according to Vickers (diamond pyramid) with a test load of 50 g. The maximum measurable hardness was 1150 HV 0.05 kg / mm². (Vickers limit values ​​are 3 for lead to 1500 for ceramics). With unalloyed steels (S235JR2, C15, C45) the surface hardness is approx. 450 HV, with quenched and tempered steels around 600–800 HV. In the case of nitriding steels or high-alloy tool steels, the surface hardness can be over 1000 HV. Nitride-forming alloying elements such as B. chrome , aluminum , nickel etc. increase the surface hardness after the Tenifer treatment.

Wear resistance

Due to the extensive binding of the iron and the alloy elements on the surface, the connection zone takes on a non-metallic character. This creates a non-metal / metal pairing during sliding processes, which prevents welding and sticking and thus increases wear resistance. A side effect is the low coefficient of friction that occurs with these pairings.

Stiffness

The considerable increase in surface hardness during the Tenifer treatment also results in greater dimensional stability. This results in greater security against bending or warping.

Corrosion resistance

Due to the nitrogen-rich connection or diffusion zone, the material surface is given a certain protection against corrosive attack. Post-oxidation (Tenifer-Q process) makes it possible to achieve excellent corrosion resistance. This method has therefore proven itself in the automotive industry. Workpieces treated with Tenifer® QPQ can achieve a service life of 500 hours in the salt spray test (for comparison: chrome-plated, 20 µm, approx. 80 hours, nickel-plated, 20 µm, approx. 100 hours).

Resilience

Tenifer layers are extremely tough. Even after loading (compression tests) beyond the yield point (limit elastic range / plastic range) into the plastic range (permanent change in shape), no damage could be seen on the surface; The prerequisite is a slight change in shape.

Other properties

The sliding properties and wear resistance are retained up to temperatures of 500 ° C and for a short time above. Since the connection zone is a poorer heat conductor than the base material, it does not heat up as quickly as it would without teniferation.

Tenifer® is a registered trademark of HEF Durferrit .

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