Ferritic steels brittle fracture

The austenitic steels can be used at very low temperatures (low-alloy ferritic steels containing 9% nickel down to -196°C) without the risk of brittle fracture [23]. 

VI.4. The first method is included to cover the approach which seeks to ensure that, whatever the loading conditions required to cause failure, such a failure will always involve extensive plasticity and/or ductile tearing, and unstable brittle fracture will not occur in any circumstances. The second is addressed to provide consistency with the generally accepted practice for evaluating ferritic steels. The third provides a method for evaluating brittle fracture that is suitable for a wide range of materials. It... 

Nickel is known to improve both toughness and strength as measured by impact and room temperature tension stress [1968Jol]. For a particular heat treatment, nickel refines the ferrite grain size as eompared with that of a plain carbon steel of similar eomposition. This grain refinement would produee an improvement of the yield strength and a better resistance to brittle fracture. 

Elastic-plastic fracture toughness test brittle fracture in ferritic steels... 

Other modeling developments include the work of EricksonKirk and EricksonKirk (2006a) on a so-called upper-shelf fracture toughness master curve, and by EricksonKirk and EricksonKirk (2006b) on linking of the Wallin Master Curve in the transition region and the EricksonKirk upper-shelf master curve in a model that predicts the temperature dependence of ferritic steels for both brittle and ductile behaviour. A more detailed discussion of the Master Curve concept and development is available in Planman and Server (2012). 

The irradiation embrittlement of ferritic steel is a special manifestation of low-temperature brittleness. A transition from ductility to brittle failure with decreasing temperature is observed for many metals and alloys, but primarily for those with the body-centred cubic (bcc) crystal structure. This phenomenon was first explained via Ioffe s well-known scheme (Figure 4.18). In this scheme, the nature of brittle failure is determined by the ratio between the temperature-dependent flow stress (yield stress CTy) and the temperature-independent brittle fracture stress (cTbf), which characterise the material s resistance to the cleavage. 

Brittle fracture. It is generally known that ferritic steels are subject to the phenomenon of brittle fracture. Although the likelihood of brittle fracture in the HRE-2 pre.ssure vessel was known to be small, an investigation was made [33] in order to determine the consequences of such an accident as a result of the pressure rise and missile damage. 

If a ferritic RPV steel (body-centered-cubic structure) specimen experiences brittle cleavage fracture prior to the full development of a resistance curve, the /-integral value at the onset of fracture, / , is used to calculate an equivalent stress intensity factor, Kj, shown in Table 10.1 and Eq. 10.4. The specimen types used to measure Kj are essentially the same as those shown in Rg. 10.1. The Kj parameter is also used to determine the parameter To, the temperature at which the median fracture toughness (Kq) of a minimum specified number of IT specimens (25.4 mm thick) is 100 MPa Vm. This parameter. To, defines the fracture toughness transition temperature using the Master Curve concept developed by Wallin (1984). The Master Curve concept has been further developed as a consensus test standard in ASTM E 1921 (ASTM, 2013f) and is described in greater detail below. 

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