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Austenitic Stainless Steel
Description of material
APFR/SI is a high Chromium-Nickel austenitic stainless steel with excellent corrosion resistance at high temperatures. Its Silicon content makes it more suited for intermittent and higher temperature applications compared to APFR.
This grade is designed to offer excellent corrosion resistance in high temperature service as well as good creep characteristics. It is widely used in the engineering and automotive industries, furnaces, the chemical industry, high temperature applications, petrol-chemical and refinery plants.
Argon Oxygen Decarburization
The optimum resistance is obtained after annealing and rapid quenching. In continuous service, APFR/SI offers a good scaling resistance up to temperatures of 1150°C and about a hundred degrees lower in the case of intermittent service. It has good resistance to oxidizing environments up to 1150°C and up to 980 °C in the case of carburizing and high Sulphur environments. It should be pointed out that composition and steel making fabricating process of these grades is optimized to obtain best performance at high temperatures. This means that at low or room temperatures, corrosion resistance could not be as good as the typical austenitic grades and this behavior must be well considered in the case of the formation of stagnating low PH condensate products. A further evaluation should concern the consequences of high operating temperatures able to cause local structural transformation such as the formation of sigma phase and other intermetallics resulting in a strong reduction of corrosion resistance. APFR, with a lower Silicon content, is less susceptible to sigma phase embrittlement than APFR/SI. 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.
APFR/SI is readily fabricated by cold working operations such as cold drawing and bending, but should only be used for a moderate amount of cold heading or cold up-setting because its chemical balance does not allow it to obtain a soft strain hardened structure after cold deformation. This could result in a rapid die and tool wear. Heavy cold deformation will require an annealing to reduce the structure hardness and restore the ductility.
APFR/SI has the typical machinability of austenitic structures of high Carbon and Nickel which are not micro resulphured grades and difficulties could happen in drilling, turning, threading and milling processes due to its capacity to cold work harden in addition to a low level of chip-ability. Machining parameters should consider that this grade work hardens more than other typical austenitic grades and requires more rigid and powerful machines, in addition to the correct choice of tools, coating carbides and cutting fluids.
APFR/SI can be welded by using any one of welding process employed with typical austenitic grades, but requires some different welding process evaluations when compared to these ones. Correct welding practices such as right heat inputs, inert shielding gas and cleanliness before/after welding must be followed to obtain best results in terms of resistance corrosion. In the case of high energy autogenous welding processes, there could be a risk of hot cracking in the fused zone due to a solidification mode from primary ferrite to primary austenite. No preheating is normally necessary. APFR/SI is not a low Carbon grade, therefore, a PWHT annealing at a high temperature should be done because this heat treatment improves its intergranular corrosion resistance.
APFR/SI has a good hot plasticity and is suitable for processing by hot extrusion or by upsetting with electric resistance heating. However, overheating must always be avoided. The choice of hot working temperature and process parameters must always evaluate the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top end of the range during the extrusion and forging process, could generate internal bursts. Small forgings can be cooled rapidly in air or water quenched. However, the best corrosion resistance is obtained by annealing followed by rapid cooling.