Microstructure bainite

Bainite, In a given steel, bainite microstructures are generally found to be both harder and tougher than peadite, although less hard than martensite. Bainite properties generally improve as the transformation temperature decreases. Lower bainite compares favorably with tempered martensite at the same hardness and can exceed it in toughness. Upper bainite, on the other hand, may be somewhat deficient in toughness as compared to fine peadite of the same hardness (33). 

Steels for WWER-type RPVs are of low-alloyed type, mostly with bainitic microstructure, and thus their radiation damage nature is, in principle, identical with all other RPV steels, with matrix damage, formation of copper-rich precipitates and solute segregation in grain boundaries. Additionally, in materials with a high nickel content, the existence of a late blooming phase is not excluded. 

Transmission electron image obtained from an unirradiated A508 Gr4N steel. Note the mixed tempered martensite-tempered bainite microstructure. Several M3C and M7C3 carbides are labelled from reference 12. 

The microstructure at position (ii) consisted of grains of ferrite and colonies of pearlite. It was noticed that the pearlite had started to "spheroidise" (see Problem 5.2). The microstructure at position (i) consisted of grains of ferrite and grains of lower bainite in roughly equal proportions. Estimate the temperatures to which the tube been heated at positions (i) and (ii). Explain the reasoning behind your answers. 

Microstructure and Grain size. The carbon steels having relatively low hardenability do not contain martensite or bainite in the cast, rolled, or foiged state. The constituents of the hypoeutectoid steels are therefore ferrite and peadite, and of the hypereutectoid steels, cementite and pearlite. 

X80 pipeline steel microstructure consisted of a polygonal ferrite and bainitic ferrite matrix with martensite/austenite (M/A) constituents distributed along grain boundaries (Fig. 8.13). The aUoy inclusions are Si, A1 oxide. Si-ferric carbide, and Al-Mg-Ca-O mixture. Hydrogen cracks are initiated even in the absence of external stress. Cracks are initiated in the presence of Si and A1 oxide-enriched inclusions [69]. 

Carbon (C). The presence of carbon transforms iron into steel. Carbon is a very basic and essential element of the composition of steel. Carbon imparts strength to the steel by forming the necessary microstructures comprising cementite, pearlite, bainite, and martensite in the required proportions. Increasing carbon content increases the yield strength, hardness, and wear resistance of steel. Very high carbon content is, however, undesirable as it reduces the weldability of the steel. Based on their carbon content, CSs can be classified as follows ... 

The influence of alloying elements (x) upon proeutectoid ferrite/microstructurally defined bainite formation in C-Fe-Mo alloys, where X is Co, Cr, Cu, Mo, Ni, Si, or V, which was examined in terms of the competing influence of the coupled-solute drag effect and the shifting in the paraequilibrium curve are discussed by [2004Aar]. 

Microstructural characteristics, such as grain size and metallurgical phases (lower or upper bainite, ferrite), may influence the sensitivity of radiation damage. 

Low magnification imaging (xlOOOO-20000) can be used to provide information on the general microstructure. For instance Fig. 9.4 from Burke et alP shows a representative micrograph from a low-aUoy forging steel. In this case, the mixed tempered martensite-tempered bainite structure with extensive carbide precipitation is visible. 

Bainite, a plate- or spearhead-shaped product consisting of a ferrite matrix in which carbide particles are dispersed. The bainitic transformation mechanism depends sensitively on alloy composition and the temperature of transformation, yielding essentially two microstructural variants. A somewhat coarser transformation product formed at about 450 is called upper bainite and a finer transformation product formed at about 350 °C is termed lower bainite. 

Subsequent heat treatment of the phases formed is termed annealing with regard to ferrite and bainite, and tempering with regard to martensite. These heat treatments play a major role in optimizing the microstructure to obtain specific properties. Upon subsequent heat treatment the transformation products listed above undergo the following reactions ... 

Quenching of steels containing < 0.08 wt% C such that acicular ferrite or low-carbon bainite is formed. This microstructural state provides an excellent combination of high yield strengths of 275 to 690 MPa, ductility, formability, and weldability. 

Microstructure of Metallic Matrix Phases. Ferritic, pearlitic, austenitic, bainitic (austempered). More details are presented in Fig. 3.1-119 and Table 3.1-79. 

Fig. 8 shows the microstructure of each specimen which corresponds to the mechanical properties at different salt bath temperatures. The bainite structure can be fined with reducing the salt bath temperature. The retained y phase has the contribution to improve the brittleness effect and enhance the tensile reliability. 

A. Abdollah-Zadeh, A. Salemi, and H. Assadi, "Mechanical behavior of CrMo steel with tempered martensite and ferrite-bainite-martensite microstructure". Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 483-84, 2008 pp. 325-328. 

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