Investigations on hot deformation behaviors and abnormal variation mechanisms of flow stress at elevated temperature for
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Xudong Xu MCC Capital Engineering & Research Incorporation Limited, Beijing 100176, China (Received 11 December 2014; accepted 31 March 2015)
The hot deformation behavior of X45CrSi93 valve steel was investigated by a series of compression and tensile tests by means of the Gleeble-1500 simulator and microstructural analyses. The experimental results show that the flow stress decreases with the increasing temperature between 850 and 900 °C followed by an abnormal increase with the increasing temperature between 900 and 1000 °C under the compressive conditions. A normal decrease of the flow stress is continued with the increasing temperature above 1000 °C. Meantime, the tensile specimen conducted at 1000 °C shows double necking effect. Further microstructural analyses show that the phase transition from a-ferrite to austenite and the solution strengthening caused by carbide dissolution are the main reasons for abnormal variation of flow stress for X45CrSi93. The negative temperature gradient in the tensile specimen results in the symmetrical microstructure and then the double necking phenomenon.
I. INTRODUCTION
Valve steel is an important part of heat-resistant steels. It is mainly applied for intake and exhaust valves of combustion engines.1 Valves are among the most important components of engines and regulate both the intake of air for the combustion process and the discharge of the exhaust gases generated in the combustion chamber. Usually, intake valves and exhaust valves are made of martensitic and austenitic steels, respectively.2 Valves are complexly loaded structural parts in combustion engines and subjected to high temperature, high pressure, thermal shocks, friction with valve base, and severe hightemperature corrosion condition all the time.3,4 Furthermore, exhaust valves are the ones most likely to burn because they run hotter than the intake ones which makes them much more vulnerable to erosion and burning than the intakes.5 Taking Otto and Diesel engines as an example, the operation temperature can reach 550 °C and 700–800 °C, even 900 °C under some more severe conditions in the intake and exhaust valves during service time. Furthermore, the gases and fuel vapors cause the operational environment to be extremely oxidant, which makes the valves expose to a severe high-temperature corrosion condition.6 Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.98 J. Mater. Res., Vol. 30, No. 10, May 28, 2015
Consequently, valve steels should characterize with good high-temperature stability, wear resistance, fatigue strength, and oxidation/corrosion resistance at elevated temperatures.7,8 X45CrSi93 as martensitic valve steel investigated in this study exhibits excellent hardening capacity, hightemperature stability, oxidation/corrosion resistance.1 X45CrSi93 valve steel is mainly used for intake valves in internal combustion engines. The hot deformation behavior of metals and alloys is important to designers for metal-forming processe
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