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Martensitic Stainless Steel
Description of material
This LC- Martensitic or Soft-Martensitic stainless steel exhibits high tensile properties combined with excellent ductility and better corrosion resistance than the common type 400 series martensitics. In the hardened and tempered condition, this grade shows a structure made up of tempered martensite + stable austenite and small amount of ferrite. The quantity and distribution of these phases strictly depends on chemical balance. In the as quenched condition, high values of Rm and HB are obtained, that reach their maximum after tempering at about 450°C. Increasing the temperature beyond 500°C, the formation of finely dispersed stable austenite begins, reaching its maximum at about 610° C. This stable austenite causes a favorable situation in terms of toughness even at low temperature and it doesn’t transform into martensite after cooling. On the contrary, temperatures above A1 ( e.g. 620°C ) some of the austenite becomes unstable, and changes into martensite, after cooling from temperature below Ms. This behavior requires further tempering to avoid a stress situation that could cause quenching cracks and stress corrosion.
Some typical application areas are water turbine parts, propeller shafting, shafting, piston rods and equipment for hydro power stations, compressors, pumps, turbines, valves in the oil and gas industries, mining and refrigerators industries, separation technologies, homogenizers and centrifuges.
Argon Oxygen Decarburization
Any amount of untempered martensite must be avoided. The best resistance to either intergranular and stress corrosion is obtained with a large amount of stable austenite. This besides warrants a good resistance to H-embrittlement, best corrosion fatigue properties, in case of boat shafts, and high values of Kv impact, useful in cryogenic environments. It should be noted that for this grade, as for every kind of stainless steel, surfaces should be free of contaminants and scale, and passivated for optimum resistance corrosion.
Due to the high tensile strength and hardness, even in the tempered condition, this grade is not suitable for cold forming operations such as cold heading.
Machinability is not as good as typical martensitic 400 grades at the same hardness. A slight micro - resulphuring could reduce the gap. Depending on the final specific use, a stress relieving after a heavy machining process could be useful to avoid deformation of parts.
Preheating of small sections is not always necessary, but must be done in case of both large cross section differences and large welds with several interpasses. Post welding heat treatment is mandatory due the transformation of martensite in heat affected and fused zones.
Large forgings and ingots require a suitable preheating to avoid cracks. Large sections should be tempered after forging or slow cooled in furnace.