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Austenitic Stainless Steel
Description of material
MVAPMLD2 is a low-carbon austenitic stainless steel with more Nickel and Molybdenum than the type 316L series .It offers good general, pitting and crevice corrosion resistance as well as good intergranular corrosion resistance after welding processes. This grade is a Ca-treated stainless steel obtained by a special steel making practice whose aim is a calculated and precise control of non-metallic inclusions which are characterized by particular morphology and lubricating properties which are able to improve machinability. This results in a lower tool wear and higher chip-breaking due to the formation of a layer between the tool and the chip.
MVAPMLD2 is suitable for the fabrication of many products such as flanges, valves, bolting, pumps shafts, food /beverages industry equipment , heat exchangers, storage tanks, many organic chemicals and parts working in medium corrosive environments where type 304L /type 316L grades do not offer sufficient corrosion resistance.
Argon Oxygen Decarburization
MVAPMLD2 is resistant to fresh water, many organic chemicals and inorganic compounds , atmospheric corrosion, paper production equipment, marine environments, rural applications, many products used in Chemical processes and sterilizing solutions. In sea water, this grade is more resistance to pitting than type 304/304L grades, such as MVAISL and similar. However, pitting and crevice corrosion may occur in environments if the chloride concentrations, pH and temperature are at determinate levels. As with other standard austenitic grades, MVAPMLD2 suffers from stress corrosion cracking about forty /fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. Strain hardened structures increase the risk of stress corrosion cracking. It should be noted that this grade, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint, and passivated for optimum resistance to corrosion. MVAPMLD2 should not be used for parts of urea plant and some pharmaceutical plants. In these particular cases and similar, non Ca-treated grades with a lower machinability (such as APMLD2) and lower or no ferrite content must be chosen.
MVAPMLD2 is readily fabricated by cold working such as cold drawing and bending and allows cold heading thanks to its higher Nickel and low Carbon contents. Its structure after cold deformation is less hard than MVAISL and MVAPML. If heavy cold heading performance is required, Valbruna produces other grades for this purpose whose chemical balance provides the highest cold deformability and the lowest CWHF. These grades have a poor machinability due to a low Sulfur content.
MVAPMLD2 is not a Free Machining grade (FM) but rather an alloy with enhanced machinability characteristics. In terms of machinability, MVAPMLD2 stands in an intermediate position between standard and Free Machining grades but cannot compete with FM (such as AISI 303, EN 1.4305). MVAPMLD2 offers a lot of advantages in terms of productivity when compared to standard grades. The best performance are obtained when employing the correct machining parameters when using multi - spindle and automatic screw machines. However, Machinists should know that Austenitic grades are different from Ferritic and Alloy steels and require more rigid and powerful machines in addition to the correct choice of tools, coating and cutting fluids. The Austenite structure is prone to transform into α’Martensite caused by strain hardening of the tool on the surface of the work piece. Even if MVAPMLD2 has a hardening factor lower than MVAISL / MVAPML, the knowledge of this behavior must be correctly considered when a piece requires two or several cutting steps to be finished. The layer of α’Martensite is very hard and, if the subsequent turning or milling processes work on this hardened layer, a rapid tool wear could happen. The tool must work under this layer. MVAPMLD2 shouldn’t be used for high polish-ability or mirror finishing processes.
MVAPMLD2 has a different behavior when compared to standard grades of similar alloy composition due to its special steel making process because its Calcium –treated process influences the surface tension of liquid and the regular morphology and geometry of the fused (weld) zone. Using the correct filler may reduce or overcome the difficulties of geometry. MVAPMLD2 has a special chemical composition which should help to avoid solidification cracks in the fused-zone of autogenous welds. Nevertheless, high energy density autogenous welds require an evaluation of the Creq/Nieq ratio because a higher Ni content may result in a change in solidification mode from primary ferrite to primary austenite. This could increase the solidification cracking susceptibility. This kind of welding requires a particular care and techniques in the case of a fully austenitic structure. In solid state joining such as Friction Welding, MVAPMLD2 may not provide a quality bond line.
MVAPMLD2 is not specifically designed for hot working and is usually supplied as cold finished round, hexagonal, flat and square bars for machining processes. If a suitable level of machinability is desired on forged pieces (as flanges, rolled rings, closed - die forgings) a special grade can be supplied. However, when open die forging of large ingots and shapes, MVAPMLD2 has a good hot plasticity if suitable soaking and the right forging temperature are applied. No preheating is required. Small forgings can be cooled rapidly in air or water.