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Austenitic Stainless Steel
Description of material
BIOVAL® AU188Z is the most popular austenitic free machining stainless steel and is the one of most machinable of all the Chromium-Nickel austenitic grades. It’s widely used in applications where the machinability is the most important factor of choice in terms of cost-saving in the production of pieces using multi-spindles and screw machines.
BIOVAL® AU188Z is suitable for the fabrication of many products such as flanges, valves, bolting, pumps shafts, pins, rings, screws, nuts, beverages industry equipment, many organic chemicals and parts working in mild corrosive environments. BIOVAL® AU188Z is used when the highest level of machinability, which can be obtained using the MAXIVAL type 303 group of steels, is not necessary, or when that group of steels is not suitable for certain products. BIOVAL® AU188Z is not recommended for applications in pressure vessels.
Argon Oxygen Decarburization
BIOVAL® AU188Z is resistant to fresh water, several 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, BIOVAL® AU188Z suffers from stress corrosion cracking just above room temperature and above certain stress 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.
BIOVAL® AU188Z 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 the AISR series, whose chemical balance provides the highest cold deformability, should be used. However, these grades have a poor machinability due to their low Sulfur content.
BIOVAL® AU188Z has a very good machinability. 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. The Austenite structure is prone to transform in to 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 works on this hardened layer, a rapid tool wear could happen. The tool must work under this layer.
BIOVAL® AU188Z is not suitable for welding because its Sulphur content may generate porosity and cracks in the weld zone. Nevertheless, if a welding process was required, BIOVAL® AU188Z has a chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds due to a correct Ferrite balance but could still be unlikely 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 was above 0,03%.
BIOVAL® AU188Z is not specifically designed for hot working and is usually supplied as cold finished round, hexagonal, flat and square bars for machining processes. However, if open die forging of large ingots and shapes of this grade is required, it has enough plasticity for hot working, if suitable soaking and a correct temperature are applied. No preheating is required. Small forgings can be cooled rapidly in air or water.