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Martensitic Stainless Steel
Description of material
VAL2AH is Chromium martensitic stainless steel with a Carbon content capable of suitable hardness after heat treatment together with a moderate corrosion resistance. This popular grade is provided for different International Norms that allow some variations of Carbon and Chromium contents compared to EN (VAL2A/UK) or a composition substantially the same (VAL2A/UK2).
VAL2AH has been designed for turbine blade manufacturing thanks to its exact chemical balance and its special steel making operating practice. This is the main application of this steel grade. In addition, it is used in those applications where high hardness and corrosion resistance are indispensable such as dental and surgical instruments, kitchen knives parts of brakes, valve seats, plastic molds, parts of pumps, pump shafts, wear resistant devices, gauges, conveyor chains, guides, instruments and table cutlery. This grade after hardening and tempering provides a hardness lower than VAL2B and VAL2DE.
VAL2AH 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 environments containing Chloride, should be avoided. VAL2AH has good corrosion resistance in mild environments such as fresh water, industrial and rural atmospheres, petroleum products, gasoline fuel oil and alcohol. This grade could be supplied in the micro-resulphured condition but in the case of mirror finishing, such as table cutlery, special purposes and turbine blades, this condition must be avoided. In the case of high polishability requirements, a special steel making process and/or heat treatments 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 is suitable for cold forming operations such as cold heading or up-setting. It should be pointed out that VAL2AH is prone to surface decarburization: a protective atmosphere should be considered in heat treatments of finished pieces.
Because turbine blades are the main application of this grade, VAL2AH is supplied with a low Sulphur content. However, in the annealed condition and in the high temperature tempered after hardening condition it offers a suitable and acceptable 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 for 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. In solid state joining such as Friction Welding, VAL2AH 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 reached room temperature and, then, immediately, heat treated. A right and suitable heat treatment of pieces after forging processes creates a structure with little or no retained austenite avoiding stress delayed cracks.
VAL2AH should be left to cool down to room temperature after hardening in order to allow the complete transformation of martensite and the need for a double tempering after hardening should be evaluated. Double tempering is not normally used in cutlery production where a single one should be sufficient. For turbine blade heat treatment, the tempering temperature has to be chosen in order to offer the best properties, avoiding those ranges of temperatures and cooling rates able to cause a significant reduction in toughness and corrosion resistance.