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Description of material
APMRU is a low Carbon Cr-Ni-Mo-Cu austenitic stainless steel with good pitting resistance as well as good intergranular corrosion resistance after welding processes. This grade has been designed in order to offer a good cold deforming and cold head ability thanks an exact chemical balance of elements such as Carbon, Nitrogen, Nickel and Copper. This grade has substantially the same cold head- ability of AISRU.
APMRU is suitable for the fabrication of many products such as flanges, valves, screws, bolting, pumps shafts, food /beverages industry equipment, storage tanks, many organic chemicals and parts working in mild to medium corrosive environments. This grade is to be considered when an improved pitting and crevice corrosion resistance is required, together with good cold head – ability.
Argon Oxygen Decarburization
APMRU is resistant to fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, rural applications and sterilizing solutions where the chloride level is low. Pitting and crevice corrosion may occur in chloride environments only if concentration, pH and temperature are at determinate levels. As with other standard austenitic grades, and even if this grade has a lower cold hardening capacity, APMRU suffers from stress corrosion cracking about forty / fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. Very Strain hardened structures increase the risk of stress corrosion cracking. Its intergranular corrosion resistance is very good. 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.
APMRU offers the same very good head ability and cold deforming of AISRU but has a better pitting and crevice corrosion thanks to its Molybdenum content. This grade has good polish-ability or mirror finishing properties but it’s should be pointed out that these properties improve with a fine grain structure, harder than a fully annealed one. APMRU is usually supplied in wire and wire rods with different conditions such as: (1) annealed+ pickled (2) annealed + pickled and special mill coating (3) annealed + cold drawn with some kind of mill coating and ready to be entered in the header (4) annealed +cold drawn + protective atmosphere annealed + mill coated ready for cold heading with or without sizing operations before entering in the header. It should be noted that, such as with all hot rolled wire rod surfaces, conditions (1) and (2) offer a higher surface roughness compared to other ones.
APMRU has a poor machinability due to a low Sulfur content and produces tough and stringy chips. The best performances are obtained when employing the correct machining parameters while using multi - spindle and automatic screw machines. However, Austenitic grades are different from Ferritic and Alloy steels and require more rigid and powerful machines, in addition to the correct choice of tools, coatings and cutting fluids. The roughness of machined parts improves in the case of a harder structure than the fully annealed one.
APMRU has a special chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds due to a suitable Ferrite balance. Neither preheating nor post welding heat treatment is required. Nevertheless, in the case of aggressive environments, a post welding heat treatment is suggested. If welded or cold headed parts have to be heat treated, an annealing at 1020-1050°C and fast cooling should be done. But annealing with a long soaking between 500-850 °C must be avoided because the structure may suffer a certain level of embrittlement.
APMRU is usually supplied in wire or wire rod for cold heading processes. This grade can be easily hot headed, no preheating is required, and forging temperatures are those typical of austenitic stainless steel. Small forging must be rapid quenched in air or water cooled. In any case, slow cooling or heat treatments with long soaking in the temperature range between 500 - 850° C must be avoided.