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Austenitic Stainless Steel
Description of material
V2018MN is a low-carbon austenitic stainless steel with higher Molybdenum and Nickel contents than type 316 series steels. It offers a very high resistance to pitting and crevice corrosion together with a good resistance to general corrosion and stress corrosion cracking. In addition, its low Carbon content avoids intergranular corrosion after welding processes or in situations of slow heating/ cooling.
V2018MN is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, offshore plant, heat exchangers, storage tanks, paper and pulping bleach equipment, oil & gas production, rural applications, many products used in chemical processes, sterilizing solutions, and parts working in corrosive environments where type 316 grades do not provide a sufficient corrosion resistance. In particular, this grade, offering a very good resistance in sea water, is widely used in naval applications.
Argon Oxygen Decarburization
V2018MN is resistant to fresh water, many organic chemicals and inorganic compounds, and atmospheric corrosion. It’s very high Chromium –Nickel and Nitrogen contents, provide a high pitting and crevice corrosion resistance both at ambient and warm water temperature. In addition, these high contents help to improve stress corrosion cracking resistance. However, it is important to note that this kind of corrosion increases fast in the case of high stress generated by welding or cold deforming processes and/or in chloride rich environments. This situation must be well evaluated, because highly strain hardened structures strongly increase the risk of this kind of corrosion. 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.
V2018MN has very high cold working hardening factor compared to type 316 series steels, mainly due to its high Nitrogen content, and consequently some cold deformation processes such as cold heading and cold up-setting could result in cracking. Nevertheless, not too severe cold deforming processes can be carried out paying attention on how much the strain hardening will raise the Rp 0,2 % and Hardness values and, consequently, increasing the susceptibility to corrosion induced cracking in certain environments containing H-Sulphide and Chlorides.
V2018MN has a poor machinability when compared to austenitic grades mainly due to its high alloyed composition and the Nitrogen is content able to promote a significant strain hardening of the austenitic structure when the tool works on the surface of the piece. In addition, V2018MN is not usually supplied in the micro-resulphured condition and this strongly decreases the chip-ability. 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. In any case, fair performance can be obtained when employing the correct machining parameters when using multi - spindle and automatic screw machines.
V2018MN requires some different welding process evaluations when compared to standard austenitic grades because its highly alloyed chemical balance is prone to cause significant variations in composition of some elements such as Molybdenum in high energy autogenous welding unless a post welding full annealing and quenching is applicable. This heat treatment improves the resistance corrosion of the weld. It should be underlined that minor repairs and patches should also use filler metals. In fact, very high alloyed filler metals or Ni-alloy restore, or improve, the composition, allowing welded parts to be used in the as-welded condition. Correct welding practices such as right heat inputs, suitable filler metals, inert shielding gas and cleanliness before/after welding must be followed to obtain best results in terms of corrosion resistance and avoiding the possibility of hot cracking in the fused zone, due to a solidification mode from primary ferrite to primary austenite.
V2018MN is harder than standard austenitic grades at high temperature. This should not be used as a reason to increase the forging temperature, because this can result in internal bursts, cracking and heavy scale formation as well. Avoid overheating or reaching the upper limit of forging temperature and reheating as often as is necessary, is strongly recommended. Small pieces, rolled rings or bars could be either air or fast quenched after forging because slow or improper cooling rates generate the precipitation of deleterious intermetallic phases, causing a considerable reduction of corrosion resistance and toughness. However, in the case of insufficient fast cooling, these intermetallic phases could occur, in the centre of large forged pieces: this situation therefore doesn’t jeopardize their corrosion resistance because these intermetallic phases do not emerge on the surface.