Development of X40CrSiMo10-2 steel series for heavy-duty in hot environment of marine diesel engines

Different X40CrSiMo10-2 Steel series are developed to serve as a heavy-duty exhaust valve for marine diesel engines. The alloys contain 0.35-0.50 % Carbon combined with 8.5-10.0 % Chrome, 2.0-2.5 % Silicon and 1.0-1.3 % Molybdenum. The steel series were processed in an induction melting furnace and cast as round cylinders with 105 mm mean diameter, and 360 mm height. Melt steel was bottom poured with 27 mm average diameter runner. Each Cast ingot was weighting 26.5 Kg. The thermodynamic calculations ensured formation of strong-stable Cr-Mo carbides containing Cr23C6 and Mo2C at the specified working temperatures. Cr enhances oxidation resistance of the alloy, while Si besides Cr are working to secure a ferrite phase matrix at room temperature and up to Ac1(855 oC). On the other hand, Mo beside the strengthening effect, it is fighting against coagulation of carbides within the matrix during working in the hot environment. A microstructure of the as – cast condition exhibits coarse ferrite grains due to slow rate of cooling during solidification in the sand mold. Cr and Mo- carbides are precipitated as thin layers enveloping the solidified ferrite grains, while the majority of carbides are precipitated as clusters scattered over the grains. On heating the steel to a temperature higher than Ac1, considerable portion of the ferrite matrix is transformed to austenite phase and the scattered carbides are then dissoluted. Multi-step hot forging is a complementary process, where the matrix would be finer with homogeneously distributed embedded carbides. Forging would be carried out by 5- successive controlled steps to obtain 50 mm diameter round bars.