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Austenitic Stainless Steel
Description of material
AISWT is an austenitic stainless steel developed specifically for high temperature applications containing the carbide formers Tungsten and Titanium. The addition of Tungsten improves its creep strength and other elevated temperature mechanical properties, while the Titanium is a very strong carbide former that precipitates carbides at higher temperatures than those at which chromium carbides will form so there is no Carbon available to react with the Chromium. AISWT offers a good hot plasticity thanks to a good chemical balance of its elements and is widely used in the production of turbine blades, gas turbine and heat resisting products.
AISWT is suitable for elevated temperature components and is chosen in the case of welding processes and in applications where an intermittent heating up to 850 C° may occur. Although this grade may be supplied in the annealed condition or the strain hardened condition, created by warm working, it is worth knowing that in the annealed condition its creep properties are lower than in the strain hardened condition. The difference is more evident at lower temperatures than higher ones.
AISWT has been designed to guarantee high temperature properties. At room temperature, it offers substantially the same corrosion resistance of type 321 series steels ( see AIST and similar ) At high temperatures, this grade shows a good resistance of oxidation up to 850°C or 800° C even in the case of discontinuous service. It should be pointed out that the interaction of the actual atmosphere and service temperature strongly influences the oxidization resistance. 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.
AISWT is easily fabricated by cold working operations such as cold drawing and bending, but should only be used for a moderate amount of cold heading, because its chemical balance does not allow it to obtain a soft strain hardening structure after cold deformation due to a high CWHF (Cold Working Hardening Factor) mainly linked to its high Carbon content. Despite the high Nickel content which should moderate this hardening, this may still result in a quite large amount of die wear.
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 AISWT is not micro-resulphured and this strongly reduces its chip breaking ability.
AISWT offers a good weldability and displays a good intergranular corrosion resistance after slow cooling following welding. No preheating or post welding heat treatment are normally necessary. However, an annealing after welding should be done if a stress relieve of the FZ and HAZ were a concern. A post welding stabilization should be done in the case of high temperature service. In the case of filler metal welding, a filler with a matching composition of AISWT or over-alloyed is recommended to maintain weld steel properties. It’s important to know that any welding process reduces the yield and tensile strength of both the FZ and HAZ of strain hardened structures obtained by warm working. Moreover, it’s recommended to pay attention to the fact that high energy density autogenous welds require an evaluation of the Creq/Nieq ratio, due to the higher Ni content. This may result in a change in solidification mode from primary ferrite to primary austenite increasing solidification cracking susceptibility. This kind of welding process requires a particular care and skill know-how in the case of a fully austenitic structure.
AISWT 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 properly and carefully evaluated. In the case of open die forging of large ingots and shapes, AISWT 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. An annealing heat treatment after forging creates mechanical properties that could be not enough for certain applications. In this case, accurate and right parameters of warm working, such as temperature, reduction and strain rate, would guarantee an adequate increase of the values of yield and tensile strength.