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Austenitic Stainless Steel
Description of material
AU188ZU is a free machining austenitic stainless steel with a special Copper and Sulphur balance. It’s widely used in applications where some machining operations, such drilling, thread rolling and machining of cold headed or strain hardened parts, is the most important factor of choice in terms of cost-saving in the production of pieces obtained by automated screw machines.
AU188ZU is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, pins, rings, screws, nuts, beverage industry equipment, many organic chemicals and parts working in mild corrosive environments. AU188ZU is used when a moderate cold deforming is required and , in terms of machinability, is a valid substitute for the MAXIVAL type 303 series, particularly when the structure of these types is not suitable for certain products. AU188ZU is not recommended for applications in pressure vessels.
Argon Oxygen Decarburization
AU188ZU 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 other standard austenitic grades, AU188ZU suffers from stress corrosion cracking about forty degrees (C°) above room temperature and above certain stress and halogens concentration. Strain hardened structures increase the risk of stress corrosion cracking even if this grade, thanks to its lower cold working hardening factor due to its Copper content, may reduce this risk. 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.
AU188ZU may be used for moderate cold deformations, cold heading or upsetting, while maintaining a good machinability. It should be pointed out that Sulphur always maintains its embrittlement capacity even if this is attenuated by Copper effect on the cold plasticity. However, in case of heavy cold deformation, other austenitic grades for this purpose, such as AISRUH series, should be used. However these grades have a poor machinability due to the low Sulfur content.
AU188ZU allows good performance in screw machining thanks to the optimum balance between Copper and Sulphur effects on both the austenitic structure and Mn-Sulphides. Productivity gain depends on type of machines, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces produced. Even if the Copper effect reduces the strain hardening of Austenite, it necessary to underline that 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. AU188ZU allows better results than standard type 303 group steels in operations such as drilling and where a cold deformation is required, such as thread rolling. Machining of strain hardened structures works better with this grade.
AU188ZU is not recommended for welding because its Sulphur content may generate porosity and cracks in weld zone. Nevertheless, if welding process were required, AU188ZU should normally have 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. In any case, an evaluation of the Creq/Nieq ratio of the every single heat must be done in order to verify if solidification of primary ferrite is ensured. A welding process with a suitable austenitic filler could help to 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 is above 0,03%. In solid state joining such as Friction Welding, AU188ZU provides a very poor quality bond line or no joining.
AU188ZU is not specifically designed for hot working and is usually supplied as cold finished round, hexagonal, flat 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. Anyway, for this grade, overheating and high strain rates must be avoided. No preheating is required. Small forgings can be cooled rapidly in air or water.