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Different surface hardnesses and hardness profiles can be produced by adjusting the nitriding parameters. The advantage over other processes such as case hardening is the lower distortion, as there are no transformation processes when cooling from the nitriding temperature. The purity requirements and structural homogeneity ensure uniform mechanical properties, even with large dimensions.
We offer customized nitriding steel for various applications and components. Production takes place in electric arc furnaces, followed by finishing in ladle furnaces. The steel can be cast as ingot casting or continuous bloom casting.
Electro-slag remelting furnaces (ESR) and vacuum arc furnaces (VAC) are available for premium nitriding steel with a high degree of structural homogeneity and purity. ESR removes non-metallic inclusions, while LBV ensures a particularly pure condition.
Our Nitrodur nitriding steel is used in various applications:
The steel offers customized strength ranges, toughness and hot formability. Cold formability and machinability depend on the analysis and microstructure formation and can be optimized by alloying and heat treatment. The quality of Nitrodur is ensured by high process reliability and modern equipment in the areas of melting, secondary metallurgy, continuous casting, hot forming and heat treatment as well as advanced testing facilities.
Nitriding as a method for improving the surface properties of components, particularly with regard to hardness, wear resistance and fatigue strength
Nitriding is an important thermochemical process in which nitrogen is incorporated into the outer layer of a workpiece. Nitrocarburizing involves the incorporation of nitrogen and carbon. Nitrogen, which is not very soluble in the ferritic solid solution, precipitates as a nitride, creating a closed nitride layer. This is followed by a diffusion zone, which hardens the steel matrix.
The temperature for nitriding is typically between 350 °C and 590 °C to enable nitrogen diffusion. The nitriding layer can be up to 500 μm thick and have hardness values of > 1000 HV. As there is no change in volume, the risk of distortion is low. Nitriding steel according to DIN EN 10085:2001-07 is used for such processes.
Nitrogen is transferred by adsorption, absorption and diffusion. The outer compound layer exhibits porosity, while the diffusion layer increases strength and wear resistance. Variation of the conditions and material selection influence the layer structure. The transition from the diffusion layer to the core hardness is smooth.
Tempering temperature during quenching and tempering influences the increase in hardness and tempering resistance. Alloying elements such as chromium, aluminum, molybdenum and vanadium influence the surface hardness. Steels containing aluminum tend to form oxides, which can hinder nitrogen diffusion.
The quality of the nitrided layer depends on the homogeneous, fine-grained basic structure. A tempered condition is usually better than an annealed one. Higher alloy contents increase the surface hardness and residual compressive stresses, but impair nitrogen diffusion and the achievable hardness.
Thermochemical nitriding processes
Gas nitriding and gas nitrocarburizing are thermochemical processes that are carried out at temperatures of 450 °C to 590 °C. Gas nitriding focuses on the release of nitrogen in the surface layer, while gas nitrocarburizing also introduces carbon. Oxi-nitriding uses oxygen to intensify the process. These processes enable a variety of layer structures and hardening depths, are suitable for different component sizes, but require special safety measures due to flammable gases.
Plasma nitriding and nitrocarburizing are carried out in the temperature range from 350 °C to 590 °C. Positive ions hit the components and ensure surface cleaning. The process is then heated up and a pulsed discharge improves process uniformity. A special glow discharge technique enables uniform embroidery to be achieved. This plasma process enables precise layer build-up, minimal dimensional changes and low roughness. However, the placement of the components requires precise arrangement, as the plasma cannot penetrate gaps smaller than 0.6 mm - 0.8 mm.
In salt bath nitrocarburizing, components are immersed in molten salt at 570 °C ≤ T ≤ 590 °C for 30 to 120 minutes and then quenched. This process produces coatings with high resistance to wear and improves fatigue strength. The treatment steps include pre-cleaning, pre-heating, nitrocarburizing, cooling/oxidizing and washing. The process is characterized by interference resistance and enables high batch densities. However, it is not suitable for pure nitriding without C-diffusion. The ratios between compound layer thickness and diffusion depth can only be varied to a limited extent.
We offer the option of subjecting manufactured components to thermochemical heat treatment in our own hardening stores - either by gas nitriding, carbonitriding or nitrocarburizing. Gas nitriding is carried out in the temperature range of short and long-term nitriding for components with dimensions of 900 x 600 x 600 mm. Plasma nitriding is offered for batches of around 1200 x 1900 mm and 800 x 1300 mm. We can process steel with diameters from 5.5 mm to 900 mm Ø.
In addition to engineering steel, we also produce stainless steel and tool steel, which can also be nitrided within certain limits.