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Austenitic Stainless Steel
Description of material
APMTH1 is an austenitic stainless steel, Titanium-stabilized, and also with added Molybdenum, which has good general and pitting corrosion resistance as well as good intergranular corrosion resistance after welding processes. This grade has been designed in order to offer a good hot plasticity (forge-ability) thanks to an exact chemical balance of elements such as Carbon, Nitrogen, Titanium and Molybdenum.
APMTH1 is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, food/beverages industry equipment, storage tanks, and parts working in medium corrosive environments including applications at elevated temperatures. This grade is chosen in the case of welding processes and in applications where intermittent heating up to 870 C° can happen.
APMTH1 is resistant to fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, marine environments, many products used in chemical processes, paper production equipment, and 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, APMTH1 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.
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 APMTH1 is not micro-resulphured like grades such as APMT/DE and this strongly reduces its chip breaking ability.
Its Titanium carbides do not precipitate on the grain boundaries after slow cooling following welding. No preheating or post welding are normally necessary. However, an annealing after welding should be done if the weld works in very aggressive environments and a post welding stabilization should be done in the case of high temperature service. APMTH1 has a special chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds. In the case of filler metal welding, a filler with a matching composition of APMTH1 is recommended to maintain weld steel properties.
APMTH1 is specifically designed for hot working and is 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, APMTH1 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.