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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 martensitic type 400. In the hardened and tempered condition, this grade shows a structure made up of tempered martensite + stable austenite. The quantity and distribution of these phases strictly depends on the chemical balance. This is the reason why some different chemical compositions of this grade (such as X134M / X134M/3 ) could be supplied, whose choice depends on the final use of finished product. In the as quenched condition, high values of Rm and HB are obtained that reach their maximum after tempering at about 400°C. Increasing temperature beyond 450°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 doesn’t transform into martensite after cooling. On the contrary, temperatures above A1 ( e.g. 620°C ) a part of 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 be cause quenching cracks and stress corrosion.
Every use where high mechanical properties together with high values of impact ( toughness) and corrosion resistance are necessary, such as compressors, pumps, turbines, valves in the oil and gas industries, mining and refrigerator industries, pump shafts, axles , suspensions, bolting, separation technologies, homogenizers and centrifuges. This grade offers good corrosion resistance for sour gas applications in the oil gas equipment and devices.
EAF + AOD
Any amounts of untempered martensite must be avoided. 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 and high values of Kv impact in cryogenic environments. It’s important to point out that the surface of every kind of stainless steel should be free of contaminants, heat tint, and scale and passivated for optimum resistance corrosion.
Due to high resistance and hardness even in tempered condition, this grade is not suitable for cold forming operations such as cold heading.
It is not as good as typical martensitic 400 grades at the same hardness. A slight micro - resulphuring could reduce the gap. Depending on final specific use, a stress relieving after 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 in order to soften the martensite in heat affected and fused zones. Double tempering could be necessary. X134/EL has an exact chemical balance designed for filler wire, bare stainless steel welding electrodes and rods for welding of semi-martensitic and martensitic stainless steel. In addition, X134/EL could be used when higher toughness were a necessary requirement.
Large forgings and ingots require a suitable preheating to avoid cracks. Avoid overheating and improper cooling. Overheating could generate internal bursts and/or promote the formation of ferrite stringers. Improper cooling could result in stress delayed cracks. Large sections should be tempered after forging or slow cooled in the furnace.
Depending on thickness, geometry and required mechanical properties of parts, this grade could be air or oil hardened. Tempering temperatures have to be chosen in order to offer the best properties of final product. In certain cases, a double tempering could be required.