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Ferritic Stainless Steel
Description of material
X17M is a ferritic free machining stainless steel grade with similar properties to X17L , but with better pitting corrosion resistance due to its Molybdenum content, allowing applications in mild chloride corrosive environments and it offers a high resistance to stress corrosion cracking.
Typical applications of X17M are devices in contact with low chloride content fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, some marine environments, as well as many products used in chemical processes, rural applications and sterilizing solutions, pump shafts, electromagnetic devices, sensors in automotive applications, electromagnetic switches and relays, sensors and fuel pumps. X17M is also used in some automotive applications such as exhaust systems environments but, as with most of the ferritic stainless steel grades, suffers from sub-zero embrittlement in cryogenic applications.
X17M has a good resistance in mildly corrosive environments such as crude oil, gasoline, alcohol, some beverages and atmospheres. In addition, it offers a good resistance to intergranular corrosion resistance after an indispensable annealing in order to diffuse back the Chromium into the depleted zones close to grain boundaries, restoring its corrosion resistance. It’s useful to remember that X17M, as with all the ferritic grades, offers a better stress corrosion cracking resistance than Austenitic grades in addition to a good pitting and crevice corrosion 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.
This grade is suitable for cold heading, up-setting, bending, cold drawing and general cold forming.
X17M has a good machinability typical of all Ferritic machined grades micro – resulphured. Nevertheless, it should be pointed out that X17M is not a free machining grade such as type 430F. Productivity gain depends on the type of machines used, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces produced. Its structure influences the surface finish (roughness) and the chip morphology. Within certain limits, a little bit harder structure typical of annealing + cold finishing (such as cold drawing) offers advantages in some machine operations and better surface roughness.
It should be noted that this grade, as for every kind of ferritic structures, produces large grain structures in FZ and HAZ caused by high temperatures of welding with some risk of embrittlement at room or lower temperatures. Austenitic fillers, in order to increase the ductility of the weld, don’t solve the problems in HAZ unless a PWHT is applied. A PWHT restores the ductility in the improbable case of Martensite formation and diffuses back the Chromium into the depleted zones close to grain boundaries restoring its corrosion resistance. Autogenous high energy welding shouldn’t form any martensitic structure in the fused zone thanks to the grade’s low content of Carbon, and Nitrogen content, but a PWHT must be always be carried out To avoid a ductility decrease in the weld, shielding gases such as Hydrogen and Nitrogen must not be used. Argon and Helium are the preferred choices. In any case, all welding procedures should apply and maintain low heat input processes.
X17M has a very good hot plasticity thanks to a wide range of possible forging temperatures. Avoid overheating and long soaking since this could cause grain growth. The last final steps of forging must be carried out at a lower temperature range with a suitable reduction in order to obtain a structure with fine and uniform grains. Blooms and ingots may require a suitable preheating to avoid cracks and an air cooling after forging. Overheating must always be avoided in order to reduce the risk of internal bursts and a structure with large coarse grains. An annealing after forging should be always be performed in order to restore both corrosion resistance and enhance the ductility.