tailieunhanh - The Science and Technology of Materials in Automotive Engines Part 9

Tham khảo tài liệu 'the science and technology of materials in automotive engines part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 188 Science and technology of materials in automotive engines which requires alloying elements such as Al Cr Mo V and or Ti. Al gives high hardness Cr increases the thickness of the nitrided layer and Mo suppresses temper embrittlement even if the part is heated for a long time during nitriding . Productivity is low so that this treatment is used only for special purposes at present. Conversely nitrocarburizing is widely used for mass-produced parts. Nitrocarburizing Nitrocarburizing is another case-hardening process and is also known as ferritic-nitrocarburizing or It is a modified nitriding process in which a gas containing carbon is added to the ammonia atmosphere. Steels held at high temperatures in this gaseous atmosphere absorb carbon and nitrogen simultaneously at a temperature below A1 around 560 C in the ferrite region of the phase diagram. The shorter time period as well as the lower temperature gives a shallow case depth typically about mm. The amounts of nitrogen and carbon in the layer are adjustable within certain limits. Gas nitrocarburizing is suitable for mass-produced N and C are diffused under an atmosphere of 50 NH3 and 50 RX gas a transformed gas of propane and butane . Heat treatment at around 560 C results in a hard surface containing The hardness can be adjusted by changing the time of treatment from 15 minutes up to 6 hours. It is normally implemented in a tunnel-type furnace where parts enter at one side and exit on the opposite side but a batch type furnace may also be used. Figure shows the hardness distribution of a Cr-Mo steel JIS-SCM435 that has undergone gas nitrocarburizing. Depth from the surface mm Section hardness distribution of gas nitrocarburized Cr-Mo steel SCM435. The quench-tempered sample is nitrocarburized for 3 h at 570 C followed by oil cooling. The crankshaft 189 Carburizing is implemented in the austenite region at around 900 C and distortion22 during heating and .