COATINGS AND HARDFACING ALLOYS FOR CORROSION AND WEAR-RESISTANCE IN DIESEL-ENGINES

The potential for surface engineering solutions to increase the corros ion and wear resistance of diesel valves is examined with regard to op eration in residual fuel oil combustion products. The environment in w hich the exhaust valve operates is discussed. The corrosion test is us ed to sort the 27 coating treatments and 16 hardfacing alloys examined . Hardfacing materials showing the greatest potential are also evaluat ed in hot wear tests. As reference materials, En 52 steel, Nimonic 80A , and Stellite 6 are included. Corrosion tests over the temperature ra nge 500-700 degrees C have shown that the surface treatments can be ra nked in terms of their composition and expected metal surface temperat ure. Good corrosion resistance to residual fuel oil combustion product s reflects primarily the content of the alloy, the base metal (Fe, Ni, or Co) being of secondary importance. Refractory metals (Ti, Nb, Ta, Mo, and W) and other minor alloying additions do not significantly alt er the corrosion behaviour, unless they are present in large quantitie s. Hot wear tests (650 degrees C, 80 MN m(-2) contact pressure) of can didate hardfacing alloys have also demonstrated the importance of the Cr content of the alloy. This probably results from the ease of format ion of a protective glaze, lowering the coefficient of friction and th erefore the wear rate. Additions of refractory metals were beneficial. Since optimum wear resistant alloys contained high levels of Mo and W , they were poor in terms of corrosion resistance. It is therefore nec essary to balance wear resistance against corrosion resistance in the selection of materials for residual fuel oil service. Two coating syst ems, vacuum plasma sprayed Ni-50Cr and a chromised diffusion treatment appeared to have potential as valve surface treatments for improved c orrosion resistance; and two hardfacing alloys, Tristelle TS2 and Colm onoy 8, offered balanced behaviour as seat materials with acceptable w ear resistance and improved corrosion performance. these materials hav e been evaluated in the engine tests of three different diesel engine manufactures.