Significance of the [sigma] phase for the erosive

The work presents a possibility of increasing the tribological properties of the ferritic-austenitic cast steel (the so called duplex cast steel) of GX2CrNiMoCu25-6-3-3 grade by taking an advantage of the mechanism of precipitation strengthening with the [sigma] phase arisen due to the eutectoidal ferrite decomposition. The examinations have been held using specimens cut out of both a massive casting (an impeller of about one-ton mass) and a thin-walled casting (of the wall thickness about 2 mm). The huge impeller has been made of cast steel containing 0.09% of carbon (i.e. exceeding the demands of the current standard), while the thin-walled casting has been of cast steel with extremely low carbon content (0.024%) due to applying the input material of a great purity. Employing the moulding material with alkaline phenolic binder hardened with CO_2 and using the zirconia coating has allowed for achieving about 20% fraction of [sigma] phase in the as-cast state of the casting. An increase in [sigma] phase fraction in the structure of duplex cast steel results in improving the abrasive wear resistance of the material. It should be stressed that despite the high fraction of the sigma phase in the material, the crack-free castings have been obtained. An addition of copper has made possible producing the high-quality thin-walled (as for cast steel) castings of smooth surfaces. No defects (including shrinkage porosity) have been found.