Pearlite/bainite microstructure

Bainite. In a given steel, bainite microstructures are generally found to be both harder and tougher than pearlite, 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 pearlite of the same hardness (33). 

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 ... 

Briefly cite the differences among pearlite, bainite, and spheroidite relative to microstructure and mechanical properties. 

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. 

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

To the right of the 99% line and below the nose lies bainite. Bainite can be thought as the extreme limit of fine pearlite. The cementite is in the form of very fine needles or platelets, so fine that they can only be seen by an electron microscope. Bainitic iron is stronger and more ductile than pearlitic iron because of the fine scale of its microstructure. 

Bainite forms isothermally as an intermediate transformation product above but below the temperature of eutectoid decomposition in ferrous alloys. This product is a mixture of ferrite and carbide, which is distinctly different from pearlite (a eutectoid product), and is termed bainite in honor of E. C. Bain who first observed such microstructures in collaboration with Davenport (Davenport and Bain, 1930). Bainite also forms during continuous cooling. Bainite transformations occur not only in ferrous alloys but also in non-ferrous... 

Briefly describe the microstructure for each of the following microconstituents that are found in steel alloys fine pearlite, coarse pearlite, spheroidite, bainite, martensite, and tempered martensite. 

We now discuss the mechanical behavior of iron-carbon alloys having the microstructures discussed heretofore—namely, fine and coarse pearlite, spheroidite, bainite, and martensite. For all but martensite, two phases are present (ferrite and cementite), and so an opportunity is provided to explore several mechanical property-microstructure relationships that exist for these alloys. 

The hardness profiles in Figure 11.15 are indicative of the influence of cooling rate on the microstructure. At the quenched end, where the quenching rate is approximately 600°C/s (1100°F/s), 100% martensite is present for all five alloys. For cooling rates less than about 70°C/s (125°F/s) or Jominy distances greater than about 6.4 mm (jin.), the microstructure of the 1040 steel is predominantly pearlitic, with some proeutectoid ferrite. However, the microstructures of the four alloy steels consist primarily of a mixture of martensite and bainite bainite content increases with decreasing cooling rate. 

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