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Modern Physical Metallurgy and Materials Engineering Part 11

Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Strengthening and toughening 291 fracture. In this respect the coarser high-temperature products of steel such as pearlite and upper bainite have inferior fracture characteristics compared with the finer lower bainite and martensite products. The fact that coarse carbides promote cleavage while fine carbides lead to ductile behaviour has already been discussed. 8.4.5 Hydrogen embrittlement of steels It is well known that ferritic and martensitic steels are severely embrittled by small amounts of hydrogen. The hydrogen may be introduced during melting and retained during the solidification of massive steel castings. Plating operations e.g. Cd plating of steel for aircraft parts may also lead to hydrogen embrittlement. Hydrogen can also be introduced during acid pickling or welding or by exposure to H2S atmospheres. The chief characteristics of hydrogen embrittlement are its 1 strain-rate sensitivity 2 temperaturedependence and 3 susceptibility to produce delayed fracture see Figure 8.34 . Unlike normal brittle fracture hydrogen embrittlement is enhanced by slow strain-rates and consequently notched-impact tests have little significance in detecting this type of embrittlement. Moreover the phenomenon is not more common at low temperatures but is most severe in some intermediate temperature range around room temperature i.e. 100 C to 100 C . These effects have been taken to indicate that hydrogen must be present in the material and must have a high mobility in order to cause embrittlement in polycrystalline aggregates. A commonly held concept of hydrogen embrittlement is that monatomic hydrogen precipitates at internal voids or cracks as molecular hydrogen so that as the pressure builds up it produces fracture. Alternatively it has been proposed that the critical factor is the segregation of hydrogen under applied stress to regions of triaxial stress just ahead of the tip of the crack and when a critical hydrogen concentration is obtained a small crack grows and links