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Galling, sometimes known as cold welding, is a form of severe adhesive wear which can occur when two metals are in relative motion and under sufficient load to permit the transfer of material. Lubrication in improves galling resistance. A solid lubrication system such as a PTFE coating gives better galling resistance than greases. Altering the surface characteristics by nitriding or chromium plating also improves wear and galling resistance.
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Stainless steels can be readily cut using laser methods. The properties of the steel, including magnetic permeability, should not be affected by laser cutting. Any distortion experienced during or after cutting should only be due to the relief of ‘self restraint’ on cutting and not due to any changes in the material from the cutting method.
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Austenitic stainless steels are usually described as non-magnetic, with a relative magnetic permeability of around 1.0. Permeabilities above 1.0 are associated with the amount of ferrite or martensite phases present in the austenitic steel. These depend on the precise chemical composition and the effects of cold working and heat treatment.
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Ferritic, martensitic and duplex stainless steels are usually classified as magnetic. The effect of different microstructures on magnetic permeability is explained.
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The magnetic attraction of ferromagnetic ferritic martensitic and duplex stainless steels is compared to that of the low magnetic permeability austenitic types. Hard and soft ferromagnetic types are compared. The paramagnetic austenitics have relative permeabilities around 1 and can be classed as non-magnetic. The affect of steel composition and degree of cold work on the magnetic permeabilities of austenitic types is discussed, with reference to the nitrogen grades 304LN, (1.4311), and 316LN, (1.4406), and the high nickel grade 310, (1.4845). Austenitic castings grades and solidified weld metal areas have higher ferrite levels than wrought, parent materials and so have higher magnetic permeability levels.
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Stainless steels are widely used for their good oxidation resistance at elevated temperatures. Although other forms of attack, such as sulphidation and carburisation need to be considered in certain applications, oxidation is of primary importance. Oxidation resistance is dependent, primarily, on the chromium content of the steel. The strength of the steel at the intended service temperature is also important when selecting stainless steels for high temperature service.
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Mechanical properties of stainless steels to BS EN 10217-7 for welded stainless steel tubes for pressure purposes.
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Austenitic stainless steels work-harden significantly during cold working. During cold deformation some martensite is formed, which makes the steel slightly ferro-magnetic. The work hardening coefficient, ‘n’, and the anisotropy index or strain ratio ‘r’ are outlined.
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Stainless steels are alloys and therefore do not melt and freeze at a fixed temperature, as do metallic elements, but over a temperature range, depending on the chemical composition of the steel. Melting range does not directly affect the creep strength or oxidation resistance of individual stainless steels. A table of melting ranges for some of the common stainless steel grades is shown.
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The NACE MR0175 standard for sulfide, (sulphide), stress corrosion cracking resistant metallic materials for oilfield equipment is now also available as an ISO standard 15156. This article summarises the requirements for all types of stainless steel austenitic , ferritic, duplex, martensitic and precipitation hardening stainless steels which are covered by the standard. Mill softened material will normally be found to comply with the requirements.
