tailieunhanh - Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2007) Episode 10

Tham khảo tài liệu 'steel heat treatment - metallurgy and technologies 2nd ed - g. totten (crc_ 2007) episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | When a piece of steel is austenitized and cooled at various rates as can occur due to sectional thickness changes various structures can result. The structure of the austenite phase has the smallest volume and the untempered martensite phase has the largest phase. If there are mixed phases any residual austenite will transform to martensite over time or with the application of heat. This will cause a dimensional change in the steel. With the diffusion of nitrogen into the steel surface a volume change will occur which means a size change in the form of growth. The amount of growth that will take place will be determined by the thickness of the formed case. The thickness of the formed compound layer will also contribute to the amount of growth. With gas nitriding and considering nitriding steel the thickness of the compound layer is generally 10 of the total case thickness. Do not be confused by this to mean the effective case. It is the total case. With the ion nitride procedure the thickness of the compound layer can be controlled by the gas ratios selected for the process which ultimately means the growth can be controlled more effectively. There will always be a growth no matter what process method is chosen. The growth will also be uniform in all directions. Another method of ensuring dimensional stability is to subject the steel to a cryogenic treatment followed by a final temper followed by the final machine and then the nitride procedure. The cryogenic treatment will ensure a complete phase change which means any residual retained austenite will be transformed to untempered martensite. This means that no further phase transformation will occur and will thus ensure dimensional stability of the part. FERRITIC NITROCARBURIZING INTRODUCTION FNC is a low-temperature process that is processed in the ferrite region of the iron-carbon equilibrium diagram at a process temperature of approximately 580 C 1075 F . The objective of the process is to form both .