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Ferritic Stainless Steel
Description of material
X25R is a high Chromium ferritic stainless steel with higher corrosion and heat resisting properties than the typical ferritic grades.
X25R is suitable for high temperature devices and parts requiring resistance to oxidation. Typical uses are combustion chambers, furnaces, nozzles, oil burner parts, tubes, salt bath electrodes and recuperators.
X25R is used for service between 800 and 1150°C with good resistance to reducing sulfurous gases and oxidation in air but it has a poor resistance in nitriding environments. Its scaling temperature is about 1150° C. It should be noted that at these temperatures its strength is low. It is important to emphasize that this grade is a heat resisting steel working at high temperatures and, therefore, its surface must be free of contaminant before using in order to obtain an optimum resistance to corrosion at such temperatures. In addition, this grade suffers with embrittlement when soaked or slow cooled at temperature between 410 to 560°C and sigma phase at 600-700°C.
This grade, as with most of the ferritic stainless steel grades, suffers with room temperature and/or sub-zero embrittlement. This grade is suitable for cold drawing, but a moderate general cold forming could be facilitated by warm working operations.
X25R has a machinability typical of all Ferritic machined grades not micro – resulphured. 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.
It should be noted that this grade, as for every kind of ferritic steel, produces large grain structures in FZ and HAZ caused by high temperatures of welding with some risk of embrittlement at room temperature. Pre-heating could be required in certain situation while a PWHT should be applied in case of the risk of low ductility in the weld deposit or in the case of the exaggerated cold working of components. To avoid a ductility decrease in the weld deposit, 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. Austenitic fillers must be avoided in case of Sulphur or Carburizing high temperature environments. Ferritic fillers are the right choice.
X25R has a good hot plasticity in 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. Large blooms and ingot require a 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 a better corrosion resistance and enhance the ductility.