Tham khảo tài liệu 'the science and technology of materials in automotive engines part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 138 Science and technology of materials in automotive engines 50 pm Microstructures of JIS-SUH3 showing martensite with dispersed carbide. 50 pm Microstructure of JIS-SUH35 near the valve surface. Polygonal austenite grains with large carbides are observable. The nitride layer of 20 pm thick white layer at the right edge improves wear resistance. three heat treatment stages must be followed firstly solution treatment at 1 100 C secondly quenching and finally age hardening at 750 C. When the alloying element especially C dissolves sufficiently during solution The valve and valve seat 139 treatment the fine carbide precipitates during ageing and increases high-temperature strength. The strength depends on the environmental temperature as shown in Fig. . In the low-temperature range below 500 C martensitic SUH3 is equal to or a little stronger than austenitic SUH 35. However in the high-temperature range SUH35 is stronger. Temperature C High-temperature strength of valve steels SUH3 and SUH35. The martensitic SUH3 is stronger below 500 C. The reason that austenitic heat-resistant steel is stronger above 500 C is due not only to the fine carbide dispersion but also to the slow diffusion rates of elements in the austenite structure FCC .5 The slow diffusion rate of the included elements means that the microstructure generated by heat treatment hardly changes thus maintaining strength at high temperatures. Martensitic steel is hard below 500 C and is used in the mid-temperature range. By contrast austenitic steel is used above 500 C and is an appropriate choice where heat resistance is important. The bonded valve using friction welding Austenitic steel shows excellent strength at high temperatures but unlike martensitic steel quench hardening is impossible due to the lack of martensitic transformation. Nitriding must be used as an additional heat treatment. To obtain high wear resistance at the stem and stem end martensitic steel is bonded to an .
