This case study illustrates material selection for a different mode of severe wear associated with high-speed engines. A light truck engine rated at 300 hp at an engine speed of 5000 rpm was converted from gasoline to CNG fuel. The clean complete combustion achieved by natural gas and alcohol-base fuels leaves minimal combustion product to ease metal-to-metal sliding wear between the valve and insert, resulting in excessive wear. With valve speeds in excess of 4000 rpm, there is also less time for new oxide film to form on the contact surfaces to reduce wear. Alternate fuels also create a hotter combustion environment reducing the strength of the contacting materials, their resistance to deformation, and removal of material by adhesion.
The original powder metal inserts exhibit severe wear after brief testing and cobalt-base common trade material inserts are tested based on positive experiences in larger engines. Wear problems have generally been resolved in larger moderate speed off-highway and power generation engines by using expensive cobalt-base common trade material inserts and faced valves, but these engines operate at engine speeds of 1200 to 2400 rpm. It was learned that the cobalt-base common trade material provides limited relief in high-speed engines using light valve designs that readily flex. They are generally best used in moderate- to low-speed applications with thicker low flexing valves. In these applications, most wear is associated with normal contact rather than sliding wear. Smaller high-speed engine designs often cause the cobalt-base common trade material to experience accelerated wear from a combination of contact fatigue and abrasion from the removed material. The worn surface often appears as a finely polished surface. The cobalt-base common trade material wear surface from the test shows tiny surface voids related to contact fatigue.
Recession wear is greatly reduced by using iron-base W77T6-P®, a material having superior sliding wear resistance, and often with tool steel containing lubricant. The graph below shows the wear rates of the truck engine test for the cobalt-base common trade material and W77T6-P® exhaust inserts coupled with common material faced valves. The cobalt-base common trade material combination experienced about three times as much wear as W77T6-P® after 200 hours of testing.
The depth of recession is approximately 0.58 mm for the cobalt-base common trade material combination and 0.18 mm for the W77T6-P® coupling. The W77T6-P® wear surface shows a fine horizontal wear pattern, but appears to be less polished showing the coarse texture related to the carbide structure. The unique carbide blend of W77T6-P® improves wear resistance and imparts strength to the base material. The composition also provides higher thermal conductivity amounting to about a 30 percent improvement over the cobalt-base common trade material. This reduces the temperature of the valve and amount of flex sliding, and consequently reduces wear.