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Austenitic Stainless Steel
Description of material
APFI/SI is a Chromium-Nickel –Silicon austenitic stainless steel with good heat resisting properties such as scaling and creep at elevated temperature and a suitable resistance to carburizing and reducing atmospheres better than type 304/316 series steels.
This grade is designed for service in high temperature environments, where it offers very good corrosion and creep resistance. It’s widely use in engineering and automotive industries, the chemical industry, petrol-chemical and refinery plants.
Argon Oxygen Decarburization
The optimum resistance is obtained after annealing and rapid quenching. In continuous service, APFI/SI offers a good scaling resistance up to temperature 1130°C and about a hundred degrees lower in the case of intermittent service. APFI/SI is liable to suffer embrittlement after working at temperatures between 600° and 930°C. It has good resistance to carburizing and oxidizing environments. Where a better resistance to oxidation is required, APFIS, with its lower Silicon content, may be a valid alternative, as it is less susceptible to sigma phase embrittlement than APFI/SI. However, both APFIS and APFI/SI have a better high temperature corrosion resistance than type 304 /309 series steels but it should be pointed out that the 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 temperature, 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 transformations such as the formation of sigma phase and other intermetallics which would result in a strong reduction of both corrosion resistance and toughness, particularly at low or room temperatures. This embrittlement is could jeopardize any eventual operations of maintenance after service. 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.
APFI/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 to obtain a soft strain hardened structure after cold deformation. This could result in a rapid die wear. Cold working doesn’t increase so much its magnetic permeability if compared to type 304/L and similar grades. Strong cold deformation will require an annealing to reduce the structure hardness and restore the ductility.
APFI/SI has the typical machinability of austenitic structures of high Carbon and Nickel grades and some difficulties could happen in drilling, turning, threading and milling processes due to its capacity to cold work harden. 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.
APFI/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 the usual 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 corrosion resistance. In the case of autogenous welding processes, there could be some risk of hot cracking in the fused zone due to a solidification mode from primary ferrite to primary austenite. No preheating is normally necessary. APFI/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.
APFI/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 temperature at the top end of the range during the extrusion and forging process, could generate internal bursts. Small forgings should be cooled rapidly in air or water quenched. However, the best corrosion resistance is obtained by annealing followed by rapid cooling.