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Corrosion Resistant Alloys
Description of material
VAL4529 is a low-carbon austenitic stainless steel with higher Molybdenum, Nickel and Nitrogen contents than grade AN4. It offers a very high resistance to pitting and crevice corrosion together with a very good resistance to general corrosion and stress corrosion cracking.
VAL4529 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 and AN4 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.
VAL4529 is resistant to fresh water, many organic chemicals and inorganic compounds, and atmospheric corrosion. Its very high Chromium, Nickel and Nitrogen contents, provide a high pitting and crevice corrosion resistance both at ambient and warm water temperatures. 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 has good resistance in diluted sulphuric acids. 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.
VAL4529 has very high cold working hardening factor compared to type AN4 and 316 series steels, mainly due to its high Nitrogen and Molybdenum 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. A final annealing should be carried out to avoid this risk and, in case of strong cold reduction, an intermediate annealing should be considered.
VAL4529 has a poor machinability when compared to austenitic grades mainly due to its high alloyed composition. Nitrogen and Molybdenum contents promote a significant strain hardening of the austenitic structure when the tool works on the surface of the piece and some difficulties could happen in drilling, turning, threading and milling processes due to its capacity to cold work harden and low chip-ability. Operators should know that this grade requires more rigid and powerful machines; in addition it does not have the low work hardening factor of 316 austenitic grades and the knowledge of this behavior must be correctly considered when a piece requires two or several cutting steps to be finished. The cold worked layer caused by the cutting tool is 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. Some improvement could be obtained by dissipating heat using an appropriate and large amount of cutting fluids and tools with a correct edge geometry with suitable chip breaker. This is particularly important when using multi-spindle and automatic screw machines.
VAL4529 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 corrosion resistance 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. The weld discoloration should be removed by acid pickling or, at least, by mechanical pickling (shot blasting) if were impossible to perform the first one.
VAL4529 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. The choice of hot working temperature and process parameters must always evaluate both the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top of the range during the hot forming process, could generate structural loss of cohesion or internal bursts. Small pieces, rolled rings or bars should be either air or fast quenched after forging because slow or improper cooling rates could generate the precipitation of deleterious intermetallic phases, causing a considerable reduction of corrosion resistance and toughness. It is useful to point out that VAL4529 is less prone to promote intermetallic phases than a similar grade with close chemical composition such as V2018MN. In any case, with an 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. In any case, an annealing after forging is suggested. A slow cooling must be avoided because this could reduce or jeopardize the corrosion resistance of this grade.