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Austenitic Stainless Steel
Description of material
APMH is a Carbon - Cr –Ni austenitic stainless steel with good general, pitting, and crevice corrosion resistance. Moreover, it offers both low and high temperature corrosion resistance. This grade has been designed in order to offer a good hot plasticity thanks to an exact chemical balance of its elements.
APMH is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, food /beverages industry equipment, storage tanks, heat exchangers and parts working in medium corrosive environments, including applications at high temperatures.
APMH is resistant to fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, marine environments, many products used in chemical processes, paper production equipment, rural applications and sterilizing solutions. In sea water, this grade is more resistant to pitting than type 304/304L steels. However, pitting and crevice corrosion may occur in environments if the chloride concentrations, pH and temperature are at determinate levels. As with other standard austenitic grades, APMH suffers from stress corrosion cracking about forty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. 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.
APMH is readily fabricated by cold working operations such as cold drawing and bending and allows a moderate cold heading due to its Carbon and Nitrogen contents. Its structure, after cold deformation, is a little harder than AISL and APML.
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 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. The structure of APMH is not micro - resulphured unlike grades such as APMTDE, and this strongly reduces its chip breaking ability.
APMH has a special chemical composition which should help to avoid solidification cracks in the fused-zone of autogenous welds. Nevertheless, high energy density autogenous welds require an evaluation of the Creq/Nieq ratio because a higher Carbon content may result in a change in solidification mode from primary ferrite to primary austenite. This could increase the solidification cracking susceptibility. This kind of welding requires a particular care and technics in the case of a fully austenitic structure. APMH can be welded without PWHT but due to its higher Carbon content a precipitation of Cr-Carbide on the grain boundaries may happen. Therefore, in the case of aggressive environments or where there is a risk of stress corrosion, a PWHT should be considered. In the case of filler metal welding, a filler with a matching composition of APMH or over-alloyed is recommended to maintain weld steel properties. In solid state joining such as Friction Welding, APMH provides a quality bond line.
APMH is also specifically designed for hot working and are usually supplied as billets, blooms, or ingots. No preheating is required. In Primary hot transformation processes, a high temperature homogenization of large ingots and dynamic recrystallization parameters should be rightly evaluated. In the case of open die forging of large ingots and shapes, APMH offers a good hot plasticity if a suitable soaking and a right temperature are applied. In Secondary hot transformation processes, such as extrusion, rolling or close die forging, temperatures, strain and strain rate should be well considered because they influence the properties of the austenitic structure. Suitable strain in terms of section reduction ( for instance: 15-30%) at a lower range of hot working temperatures is recommended in order to obtain a fine grain austenitic structure which is very important for mechanical , fatigue and corrosion resistance properties and makes it easier for ultrasonic testing to detect small indications as required by several International Norms. Small forgings can be cooled rapidly in air or water.