Microstructure pearlite

Figures 11.2-11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straightforward when yiron cools below 914°C a grains nucleate at y grain boundaries and the microstructure transforms to a. If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the microstructure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the ystarts to transform as soon as the alloy enters the a+ yfield (Fig. 11.4). "Primary" a nucleates at y grain boundaries and grows as the steel is cooled from A3...

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. 

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

Fig. 20.49 Schematic illustration of some of the ferritic/pearlitic microstructures observed in hypo-eutectoid steels after various heat treatments...

Microstructure ferrite/acicular ferrite + small quantity pearlite Yield stress high... 

Figure 5.29. Fe-rich region of the Fe C phase diagram. Stable Fe-C (graphite) diagram solid lines metastable Fe-Fe3C diagram dashed lines. The following current names are used ferrite (solid solution in aFe), austenite (solid solution in 7Fe) and cementite (Fe3C compound). Pearlite is the name given to the two-phase microstructure which originates from the eutectoid reaction ...

Figure 2.13 Microstructures obtained by varying thermal treatments in cast irons (Gf = graphite flakes = graphite rosettes G = graphite nodules P = pearlite). From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Lithium-Structure Compatibility. One of the critical chemistry problems of HYLIFE is the compatibility of structural alloys with the molten liquid of the jet array. Two candidate liquid metals are lithium and Pbg3Lij 7. High-Z metal (such as lead from target debris) will enter the liquid metal and may affect the compatibility. The structural alloy selected in the HYLIFE study is Cr-1 Mo, a ferritic steel. The carbides usually present in this steel are M3C (cementite) and M2C, where M is primarily Fe. Both of these carbides are unstable in lithium. M3C is usually present as platelets within pearlite, the eutectoid structure in pearlitic steel. The most common microstructure for the 2 4 Cr-1 Mo steel is large grains of ferrite with small islands of pearlite. M2C is present as a fine spray of precipitate within large ferrite grains. Lithium... 

Figure 9. Microstructure of 2 V4 Cr-1 Mo steel. Key 0, ferrite grain peppered with M C , pearlite (platelets of alternating M,C and ferrite) and K, ghost boundary of prior austenite grain boundary.

Fig. 10.7. Lamellar pearlite microstructure of a hot rolled steel bar. Micrograph 354 from Metals Handbook (ASM 1972). This figure is at a maguificatiou of 2000 x.

Carbon steels are the most widely used materials of construction. Unalloyed carbon steels typically contain nominal amounts of manganese, silicon, phosphorus, and sulfur. They are normally supplied with a pearlitic-ferritic microstructure (see Figure 21.3) produced by air cooling a hot-formed product (e.g., hot-rolled plate) or by a normalizing heat treatment. They are available as either killed carbon steel or plain carbon steel. 

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). 

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