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Austenitic Stainless Steel
Description of material
NTR50 is a Chromium –Nickel –Manganese austenitic stainless steel strengthened by an addition of Nitrogen. This grade provides a very good corrosion resistance together with high strength in marine and other environments where a greater resistance corrosion than type 316/317 series stainless steels are required.
NTR50 is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, chains, fittings, food /beverages industry equipment , storage tanks, and parts working in corrosive environments such as chemical processing, paper and urea production , rural applications, subsea and deep-water offshore oil production systems and several marine applications. In the case of boat shaft fabrication, Marinox 22/22HS should be the best choice in terms of corrosion resistance in sea water and high mechanical properties.
Argon Oxygen Decarburization
NTR50 is resistant to fresh water, several organic chemicals and inorganic compounds, atmospheric corrosion, marine environments, and sterilizing solutions. In sea water, this grade is more resistant to pitting than type 304/316/317 series steels and similar (see Marinox22/22HS). However, pitting and crevice corrosion may occur in environments where the chloride concentrations, pH and temperature are at determinate levels. As with other standard austenitic grades, NTR50 suffers from stress corrosion cracking about forty/fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. This grade has a good resistance to intergranular corrosion but very strain hardened structures increase the risk of stress corrosion cracking. This risk is strongly reduced in case of material in the fully annealed condition. It should be noted that NTR50, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint, and passivated for optimum resistance to corrosion.
NTR50 is readily fabricated by cold working such as cold drawing and bending, but should only be used for a moderate amount of cold heading, because its chemical balance does not allow it to obtain a soft strain hardened structure after cold deformation, due to a high CWHF (Cold Working Hardening Factor). This could result in a rapid die wear. Cold working doesn’t increase so much its magnetic permeability as compared to type 316/L and similar steels.
NTR50 has the typical machinability of austenitic structures strengthened by Nitrogen and some difficulties could happen in drilling, turning, threading and milling processes due to its capacity to cold work harden. Machining parameters should consider that this grade work hardens more than other typical austenitic grades and operators should know that NTR50 requires more rigid and powerful machines, in addition to the correct choice of tools, coating carbides and cutting fluids.
NTR50 can be welded by using any one of welding process employed with typical austenitic grades but requires some different welding process evaluations when compared to these ones. Correct welding practices such as right heat inputs, inert shielding gas and cleanliness before/after welding must be followed to obtain best results in terms of corrosion resistance. In the case of autogenous welding processes, there could be some risk of hot cracking in the fused zone due to a solidification mode from primary ferrite to primary austenite. In this case, special filler metals assure avoiding hot cracking in this zone. No preheating or post welding are normally necessary but an annealing at higher temperature should be done if the weld works in very aggressive environments because this heat treatment improves its resistance corrosion. Pay attention on the high energy autogenous welding processes such as LBW and, particularly, in EBW-HV process because of the high risk of outgassing.
NTR50 has a good hot plasticity and is suitable for processing by hot extrusion or by upsetting with electric resistance heating. However, overheating must be always avoided. The choice of hot working temperature and process parameters must always evaluate the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at top end of the range during the extrusion and forging process, could generate internal bursts. With the right choice of a low temperature of forging or rolling, together with an appropriate reduction, NTR50 warrants high mechanical properties, together with high resistance to stress corrosion cracking. This operative practice substitutes and/or avoids the use of cold deformation in order to obtain high Rp0,2 and Rm with low or no risk of SCC in certain environments.