Martensitic Stainless Steels
The martensitic types contain as characterizing element almost exclusively chromium, from the lowest level that ensures the corrosion resistance of the material (10%) up to a maximum of 18%. Accordingly they have the advantage of being less expensive stainless steels; however, they are less resistant to corrosion. The carbon content can be up to 1.20% and allows to contrast with its high austenitic effect, the ferritic effect of chromium. Therefore, as the majority of types of steel, the state diagram of these types has α magnetic structure at low temperature, and γ non-magnetic at high temperatures, making it possible for the heat treatment of tempering. Following the heat hardening treatment it will get an hardness that will be all the higher, the higher the C content; the characteristics of toughness will obviously be limited, but it will be possible to recover a large part with a subsequent treatment of tempering. In fact, with a tempering at 200°C-300°C (distension) there is an appreciable recovery of toughness without losing almost nothing in hardness. With a tempering at 600°C there is a significant increase in toughness accompanied by a consequent reduction of hardness which for many applications is still quite valid and desired. The martensitic stainless steels are not suitable for use at temperatures below ambient as for most of them the transition temperature is above 0°C. Thus already at this temperature it becomes fragile and below zero lose almost all of their toughness and ductility. In general it can be said that these steels are discrete properties at low temperatures, fully comparable with structural steels and quenched and tempered in the usual uses of mechanical constructions. For use at higher temperature than the environment these steels in general, a tensile strength and yield strength higher than that of ferritic and austenitic types up to a temperature of about 500°C.
As regards the hot creep, martensitic types have better characteristics than carbon steels despite being comparable to the ferritic stainless. It is not used more than 600 ° C both to the lower resistance to hot oxidation that for the uselessness to perform operations of quenching and tempering to then have the same characteristics of ferritic stainless steel. The corrosion resistance of these types is lower than that presented by other steels, although much higher than that of the carbon steel. Typically resist unpolluted rural atmospheres, fresh water and chemicals of low aggression non-oxidizing or reducing agents, such as weakly alkaline solutions and diluted organic acids. There were no appreciable changes in corrosion resistance after tempering of relaxation, whereas in the field of tempering 400-500 ° C it decreases greatly because there is the formation of chromium carbides, depleting the corrosion resistance.
Although historically it was not the first type of martensitic steel used, and even if it is not that of the most wide consumption, for convenience it is considered as the reference type for the series which is illustrating. It can be used both in the hardened which in the hardened and tempered at 600°C. It is the typical steel for the blades from the turbine and for any particular subject to high voltages.
Suction hood wall
It is the kind of wider utilization of this series, available in products of any shape. It is equally economical of the foregoing type which has the same possibility to heat treatment with the same mechanical properties and corrosion resistance. Among the many specific uses include: pump shafts, bushings, bolts, hoppers, cutlery, tools, machinery for mining, screws, valves, parts for jet engines.
It is the prototype of martensitic stainless steels. It began to use it for cutlery and even today this is its main purpose as the type AISI 410 is preferred for its greater ability to cut and seal the wire, due to a carbon content approximately double. It is used in the conditions of hardened and stretched at 200-300°C; in this state it can easily exceed 50 HRC with breaking loads of about 160 kg / mm2, and still an elongation of 10%, with properties of corrosion resistance equal to those dell’AISl 410 in the same conditions of treatment. Other typical uses are surgical instruments, valves, special highly resistant to wear, tools, cutting blades, forms for glass.
AISI 440 A
The high C content gives this type, after hardening and tempering of detente, an exceptional hardness (55 HRC), a breaking load of 180 kg / mm2 with an elongation of 5%. As for the type AISI 420, it is advisable to avoid tempering treatments above 250°C. The highest percentage of Cr (17%) is added to compensate for the loss of this element in the matrix during the formation of carbides. For this type of steel exists the class B and C that contain a C content higher and higher, while Class F contains a percentage of sulfur which increases workability to machine tools. The high hardness conferred by the martensitic matrix and carbides make it particularly suitable in cases where you require high resistance to wear, valves, cutlery, surgical instruments.