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Austenitic Stainless Steel
Description of material
MV188HSE is a free machining austenitic Chromium-Nickel stainless steel with a better machinability than the standard type 303 series. This grade is a Ca-treated stainless steel obtained by a special steel making practice, whose aim is a calculated and precise control of non-metallic inclusions, which are characterized by particular morphology and lubricating properties which are able to improve machinability. MV188HSE has a specific chemical balance to obtain better performance in some machining processes thanks to a little bit lower cold work hardening factor. This grade has the same steel making process of MV188ZHS and VPAU188Z and could be a valid substitute in certain kinds of machining. MV188HSE is widely used in applications where the machinability is the most important factor of choice in terms of cost-saving in the production of pieces obtained by multi-spindle and screw machines.
MV188HSE is suitable for the fabrication of many products such as flanges, valves, bolting, pumps shafts, pins, rings, screws, nuts, beverage industry equipment, many organic chemicals and parts working in mild corrosive environments. MV188HSE is not recommended for applications in pressure vessels and shouldn’t be used for high polishability or mirror finishing processes.
MV188HSE is resistant to fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, rural applications and sterilizing solutions where the chloride content is low. It should be well considered that, as with all free machining grades, Sulphur is added to improve machinability and the formation of MnS inclusions could prime points of pitting if exposed to some corrosive environments. Pitting and crevice corrosion may occur in chloride environments if concentration, pH and temperature are at determinate levels. As with other standard austenitic grades, MV188HSE suffers from stress corrosion cracking about forty degrees (C°) above room temperature and above certain stresses and halogens concentration. Strain hardened structures increase the risk of stress corrosion cracking. 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.
MV188HSE should not be used for cold heading or upsetting due to the embrittlement effect of the Sulphur content. Other austenitic grades for this purpose, such as AISR /AISRU series, whose chemical balance provide the highest cold deformability, should be used. These grades have a poor machinability due to the low Sulphur content. In case of moderate cold heading or upsetting, AU188ZU may be used without excessive reduction of machinability or MVAISRU used as an alternative choice. This last grade allows a significant improvement of cold deformability but a reasonable fall in machinability.
MV188HSE has a very good machinability. Productivity gain depends on the type of machines, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces produced. The Austenite structure is prone to transform into α’Martensite caused by strain hardening of the tool on the surface of the machined piece. The knowledge of this behavior must be correctly considered when a piece requires two or several cutting steps to be finished. The layer of α’Martensite is very hard and, if the subsequent turning or milling processes work on this hardened layer, a rapid tool wear could happen. The tool must work under this layer.
MV188HSE is not suitable for welding because its Sulphur content may generate porosity and cracks in the weld zone. Moreover, MV188HSE has a different behavior when compared to standard grades of similar alloy composition due to its special steel making process because its Calcium –treated process, in addition to high Sulphur content, influences the surface tension of liquid and the regular morphology and geometry of the fused (weld) zone. Nevertheless, if welding process were required, MV188HSE has a chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds due to its Ferrite balance but it is still unlikely to avoid porosity and inadequate geometry of the weld. A welding process with a suitable austenitic filler could help to overcome or reduce these problems using low heat inputs. In order to avoid intergranular corrosion , the welded structure should be annealed after welding if the Carbon content of the supplied heat were above 0,03%. In solid state joining such as Friction Welding, MV188HSE provides a poor quality bond line or no joining.
MV188HSE is not specifically designed for hot working and is usually supplied as cold finished round, hexagonal and square bars for machining processes. However, when open die forging of large ingots and shapes of this grade, it has enough plasticity when being hot worked, if suitable soaking and the right temperature are applied. No preheating is required. Small forgings can be cooled rapidly in air or water.