Development of Engine Valve Materials for Next Generation Higher Efficiency Engines

The growing demand to increase the performance and efficiency of light-, medium-, and heavy-duty engines continues to drive increases in combustion intensities and cylinder pressures, which result in higher exhaust gas temperatures. Thus, there is a critical need for new materials that can meet the performance and cost targets for components such as exhaust valves which are exposed to these higher exhaust gas temperatures. Oak Ridge National Laboratory (ORNL) has developed several lower-cost, high-strength alloys that have the potential to be adopted into intake and/or exhaust valves in the next generation, high-efficiency engines and other high temperature applications. These alloys are covered by two issued patents:1.G. Muralidharan, U. S. Patent No. 9,605,565 B2, “Low-cost Fe--Ni--Cr alloys for high temperature valve applications,” March 28, 2017. 2.G. Muralidharan, U. S. Patent 9,752, 468 B2, “Low-Cost, High-Strength Fe-Ni-Cr Alloys for High Temperature Exhaust Valve Applications, Issued Sep. 5, 2017. The overall scope of this CRADA project was for Oak Ridge National Laboratory to collaborate with Tenneco Powertrain to: 1) better define the properties required for intake and/or exhaust valves for next generation vehicles, 2) fabricate industrial scale heats of alloys down-selected from existing patents, 3) generate critical high temperature property data that will help evaluate the suitability of these new alloys for high temperature intake and/or exhaust valves, and finally to 4) fabricate and evaluate the performance of prototype intake/and or exhaust valves. Results of this work showed that the lower cost alloys developed by Oak Ridge National laboratory maintained their yield and tensile strengths at higher temperatures and hence are promising candidates for applications in exhaust valves that can operate at higher temperatures. Several promising alloys were successfully scaled-up to industrial scale heats. High temperature uniaxial, fully reversed fatigue testing showed long-term stability of alloys for times up to ~2500 hours at 900°C. High temperature fatigue properties of several scaled-up alloys were compared at 900°C using uniaxial fatigue testing. Results from these measurements showed that alloys with lower coarsening rate of γ’ had better fatigue resistance. Rotating beam tests are commonly used by industry to evaluate promising alloys, but there was very little prior data available at 900°C for baseline alloys. In this work, rotating beam tests were performed at 900°C on several new alloys. These results showed a large scatter in fatigue life at 900°C. Hence the need for additional tests was recognized for both baseline commercial alloys and novel prototype alloys to enable identification of the most promising novel alloys. This lack of data and the relatively large scatter prevented down-selection of novel alloys for valve fabrication. Future work will focus on down-selection of one prototype alloy for valve fabrication and testing. In addition, rotating beam fatigue tests for longer times are required to enable comparison with uniaxial fatigue tests and to develop a more robust database for comparison of properties.