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Martensitic Stainless Steel
Description of material
VAL3S is a high Chromium–Molybdenum martensitic stainless with a Carbon content capable of high hardness after heat treatment together with a fair pitting corrosion resistance better than other C-Cr martensitic grades.
All applications where machinability, high hardness and corrosion resistance are indispensable, such as parts of pump shafts, valves, plastic molds, wear resistant devices, parts of fire arms, nozzles, gauges, conveyor chains, axles, food process equipment, guides and instruments. VAL3S after hardening and tempering provides a hardness higher than VAL1 and VAL2A. This grade is used when a machinability better than VAL3 or of other non-micro-resulphured martensitic steels is required.
VAL3S has its maximum corrosion resistance when in the hardened + low temperature tempered condition and with its maximum hardness. Its use in the annealed condition or any other situation able to reduce the surface hardness and in high Chloride containing environments, should be avoided. VAL3S has good corrosion resistance in mild environments such as fresh water, industrial and rural atmospheres, petroleum products, gasoline fuel oil and alcohol. In addition, VAL3S has a fair pitting corrosion resistance thanks to a high PRE due to its Chromium and Molybdenum contents. In the case of high polishability requirements, a special grade such as VAL3CT processed by a particular steel making process with both an exact chemical balance and heat treatment should be chosen. 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.
In the annealed condition, this grade has a moderate cold forming ability, for example cold heading or up-setting. A higher cold formability could be obtained after a long lasting annealing and very slow cooling in the furnace. It should be pointed out that VAL3S is highly prone to surface decarburization, more than other C-Cr martensitic grades: a protective atmosphere should be considered in the heat treatment of finished pieces.
VAL3S in the annealed condition, and in the high temperature tempered after hardening condition, offers a good machinability. It’s important to know that the productivity gain depends on the type of machines used, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces produced. Grinding and polishing of hardened + tempered material at maximum values of hardness must be carried out with great care in order to avoid the overheating of the surface of the piece resulting in poor corrosion resistance and/or grinding cracks.
This process of martensitic stainless grades is always risky and a special care must be applied in the choice of welding parameters. In any case, if a welding process were required, a preheating is mandatory and the part must be maintained at temperature and followed by immediate annealing or tempering. Fillers of the same composition can be used to obtain mechanical properties close to that of the base metal. Alternatively, austenitic fillers may be used considering an inevitable reduction of these properties. In solid state joining such as Friction Welding, VAL3S provides a quality bond line. When friction welded with different grades, a tempering or annealing of the welded piece must be done in order to soften the martensitic structure of HAZ and bond line.
Blooms and ingots require a suitable preheating to avoid cracks and a slow cooling in the furnace after forging. Overheating must always be avoided in order to reduce the risk of internal bursts. An improper cooling could result in stress cooling cracks. Large forgings and large cross – section shapes should be left to cool until their core reaches room temperature and, then, immediately, heat treated. A right and suitable heat treatment of pieces after the forging process creates a structure with no or little retained austenite, avoiding delayed cracking.
VAL3S should be double tempered after hardening in order to reduce or avoid retained austenite, as well as obtaining high values of hardness.