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Ferritic Stainless Steel
Description of material
X17Z is the best known free machining grade with high machinability. It is extensively used in the automotive industries, and for pump shafts and soft magnetic components.
Typical applications of X17Z are fresh water pump shafts, electromagnetic devices, fuel injection components, sensors in automotive applications and fuel pumps. This grade is not recommended for pressure vessels. For better magnetic properties, Magival series such as MG1 should be used.
It should be well considered that, as with all free machining grades, Sulphur is added to improve machinability and the formation of MnS inclusions could prime points of pitting if exposed to some corrosive environments. X17Z has a good resistance to mildly corrosive environments such as fresh water, crude oil, gasoline, alcohol, some beverages and atmospheres. 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. It’s useful to remember that the ferritic grades have a better stress corrosion cracking resistance than Austenitic grades.
This grade is not suitable for cold heading and up-setting due to embrittlement effect of the Sulphur content and has not been specifically designed for cold forming. It is usually supplied as cold finished round, hexagonal and square bars for machining processes. This grade cannot be heavy cold deformed.
X17Z has a very good machinability typical of all Ferritic free machined grades. Productivity gain depends on the types 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.
X17Z, as with other free machining grades with a high Sulphur content, is not recommended for welding, because the fused zone tends to form porosity. In any case, low heat inputs and a type 430 filler metal should be used. Austenitic fillers, in order to increase the ductility of weld, don’t solve the problems in HAZ unless a PWHT is applied. Autogenous high energy welding may form a certain amount of Martensite spread in the ferritic structure. A PWHT tempers the Martensite restoring the ductility and diffuses back the Chromium into the depleted zones close to grain boundaries restoring its corrosion resistance. It should be noted that this grade, as for every kind of ferritic steel, produces large grain structures in FZ and HAZ.
X17Z has not been designed for hot working and is usually supplied as cold finished round, hexagonal and square bars for machining processes. It’s important to know that all free machining grades have a poor hot plasticity and this characteristic must be well evaluated in forging processes. 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.