tailieunhanh - Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2007) 6

Tham khảo tài liệu 'steel heat treatment - metallurgy and technologies 2nd ed - g. totten (crc_ 2007) 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | When comparing the curves for the start of transformation in CCT and IT diagrams for the same heat and steel grade Figure and Figure we found that in the CCT diagram the curves are slightly shifted to longer times and lower temperatures. For example in the IT diagram of Figure the shortest time to start the transformation of ferrite is 16 s at 650 C 1200 F and the corresponding time for bainite is 9 s at 480 C 900 F . In the CCT diagram of Figure however the shortest transformation start time for ferrite is 32 s at 620 C 1150 F and the corresponding time for bainite is 20 s at 380 C 716 F . This indicates that in CCT processes the transformation starts later than in IT processes. The basis of this phenomenon is related to the incubation time and can be found in a hypothesis of Scheil 6 . It should also be noted that with higher austenitizing temperature the curves denoting the start of transformation of a particular phase can be shifted to longer times. Figure shows the CCT diagram of DIN 16MnCr5 steel after austenitizing at 870 C 1600 F a and after austenitizing at 1050 C 1922 F b . In the latter case the regions of ferrite and pearlite are shifted to longer times. It is necessary therefore when using a CCT diagram to ascertain that the austenitizing temperature used to develop the CCT diagram corresponds to the austenitizing temperature of the parts treated. A CCT diagram is developed in the following way. Many small specimens . 4-mm diameter X 2 mm for high cooling rates and diameter X 15 mm for medium and low cooling rates are austenitized and cooled within a dilatometer with different cooling rates. Start and finish of transformation of relevant phases with each cooling curve are recorded and these points are connected to obtain the regions of transformation for the relevant phases see Figure . Therefore a CCT diagram can be read only in the way in which it was developed . along the cooling curves. As can be seen from .