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Austenitic Stainless Steel
Description of material
NTR60 is a Chromium –Nickel –Manganese -Silicon austenitic stainless steel strengthened by additions of Nitrogen and Carbon. This grade provides a good corrosion resistance similar to type 304/316 series steels, a higher strength and better high temperature oxidation.
NTR60 is used where both wear and galling resistance are indispensable demands. It is suitable for the fabrication of many products such as valves, bolts & nuts , pump shafts, chains, fittings, fastenings, wear rings, food industry equipment , parts working in the corrosive environments typical of petrochemical and chemical equipment, offshore oil production systems and several applications where galling is the main concern at cryogenic and elevated temperatures.
Argon Oxygen Decarburization
NTR60 is resistant to fresh water, several organic chemicals and inorganic compounds, atmospheric corrosion, marine environments, many products used in chemical processing. In sea water, this grade is more resistant to pitting than type 304 /316 series steels. 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, NTR60 suffers from stress corrosion cracking about forty/fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. This risk is strongly reduced in the case of material in the fully annealed condition. This grade has a good resistance to intergranular corrosion provided that suitable fast cooling is performed during quenching, avoiding the typical temperature range of carbides precipitation on the grain boundaries. It should be noted that NTR60, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint, and passivated for optimum resistance to corrosion.
NTR60 is readily fabricated by cold working operations such as cold drawing and bending, but should only be used for a moderate amount of cold heading and cold upsetting 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.
NTR60 has the typical machinability of austenitic structures strengthened by Nitrogen with high Carbon and 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 NTR60 requires more rigid and powerful machines, in addition to the correct choice of tools, coating carbides and cutting fluids.
NTR60 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. Suitable filler metals and 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 and in order to maintain wear properties. 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. No preheating is normally necessary but a postweld annealing should be done because this heat treatment restores its intergranular corrosion resistance. In the case of high energy autogenous welding processes such as LBW and, particularly, in EBW-HV evaluations should be done concerning a probable outgassing, especially in the case of a Nitrogen content at the top of the range.
NTR60 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 temperature at the top end of the range during the extrusion and forging process, could generate internal bursts. No preheating of small/medium sizes is required but very large ingots could require a slow heating from room temperature up to the forging temperature and reheated as necessary.