More restrictive exhaust emissions laws for diesel engines have driven changes in engine design including the need for high-pressure electronic fuel injection systems. Engines built according to the new designs use higher combustion pressures, higher operating temperatures and less lubrication than previous designs. Components of the new designs, including valve seat inserts (VSI), have experienced significantly higher wear rates. Exhaust and intake valve seat inserts and valves, for example, must be able to withstand a high number of valve impact events and combustion events with minimal wear (e.g., abrasive, adhesive and corrosive wear). This has motivated a shift in materials selection toward materials that offer improved wear resistance relative to the valve seat insert materials that have traditionally been used by the diesel industry.
Another emerging trend in diesel engine development is the use of EGR (exhaust gas recirculation). With EGR, exhaust gas is partially routed back into the intake air stream to reduce nitric oxide (NOx) content in exhaust emissions. The use of EGR in diesel engines can change engine combustion characteristics and consequently the valve/VSI working environment. Accordingly, there is a need for lower cost exhaust valve seat inserts having suitable metallurgical and mechanical properties for use in diesel engines using EGR.
Also, because exhaust gas contains compounds of nitrogen, sulfur, chlorine, and other elements that potentially can form acids, the need for improved corrosion resistance for alloys used in intake and exhaust valve seat insert applications is increased for diesel engines using EGR. Acid can attack valve seat inserts and valves leading to premature engine failure.
There is a need for improved iron-based alloys for valve seat inserts that exhibit adequate hardness, as well as corrosion and wear resistance suitable for use in, for example, intake and exhaust valve seat insert applications.