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Mechanical Engineers Reference Book 12E Episode 6

Tham khảo tài liệu 'mechanical engineers reference book 12e episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ferrous metals 7 23 heat treatment namely that austenite in transforming through the lines GS SE or PSK develops a numher of grains of the new constituents in each austenite grain thereby refining the grain structure. The mechanical properties of steels consisting of ferrite and pearlite are strongly influenced by the average grain size of ferrite as well as the amount and type of pearlite coarse lamellar fine lamellar etc. . The yield stress varies linearly with the reciprocal of the square root of the grain size. On heating steel through the critical temperatures into the austenitic phase field the behaviour observed on cooling is reversed in the follow ing manner Steel with 0.1 C. On passing through the lower critical temperature which is higher than Ary the pearlite areas first transform to austenite of 0.8 C content. This austenite grows by dissolving the surrounding ferrite grains as the temperature is raised and its carbon content is reduced. However the austenite areas developing from the pearlite consist of numerous crystals so that just above line GS when the structure is W holly austenitic containing 0.1 C it consists of numerous small austenite grains. Heating to higher temperatures in the austenite phase field causes grain growth some grains growing by absorbing smaller ones around them. Eutectoid steel 0.8 C. On heating above the lower critical temperature ACị which coincides with the upper critical temperature Acị at the eutectoid composition theoretically the pearlite should transform to austenite of 0.8 C content. In practice it does so over a temperature range the ferrite lamellae absorbing cementite to form a lower carbon austenite which then dissolves the remaining cementite. Grain growth follows on heating to higher temperatures in the austenite phase field. Hyper-eutectoid steel 1.2 oC. At the eutectoid line the pearlite starts to transform to austenite of 0.8 C content. As the temperature is raised through the austenite plus cementite phase .