Fine pearlite

Coarse pearlite Fine pearlite Upper Bainite... 

Flgwe 5.9 Effect of drawing or swaging operation on the strength of pearhtic steel wires. Both coarse and fine pearlite show a linear relationship between yield strength and exp (6 4) where e is the strain (after Embury and Fisher, 1966). 

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). 

To summarize, the relative hardness of the various phases discussed thus far (Brinell hardness values in parentheses) martensite (300-700) > tempered martensite (300-450) > bainite ca. 400) > fine pearlite (100-300) > coarse pearlite (100-220) > spheroidite (90-180). The hardness and brittleness of cementite is much greater than ferrite, whereas the latter has significantly greater ductility. 

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. 

In curve D, the sample is quenched and held until 50% is converted to fine pearlite. Then the temperature is lowered rapidly into the bainite region and held until the rest of the sample is converted to bainite. As before, these structures are retained as the material is quenched to room temperature. 

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. 

Photomicrographs of (fl) coarse pearlite and (h) fine pearlite. 3000X. 

ConcBpt Check 10.3 Make a copy of the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure 10.22) and then sketch and label on this diagram a time-temperature path that will produce 100% fine pearlite. 

The layer thickness of each of the ferrite and cementite phases in the microstructure also influences the mechanical behavior of the material. Fine pearlite is harder and... 

Coarse pearhte is more ductile than fine pearlite, as illustrated in Figure 10.30h, which plots percentage reduction in area versus carbon concentration for both microstructure types. This behavior results from the greater restriction to plastic deformation of the fine pearlite. 

Fine pearlite a-Ferrite + Fe3C Alternating layers of a-ferrite and FesC that are relatively thin Harder and stronger than coarse pearlite, but not as ductile as coarse pearlite... 

Bainite a-Ferrite + Fe3C Very fine and elongated particles of FesC in an a-ferrite matrix Harder and stronger than fine pearlite less hard than martensite more ductile than martensite... 

Coarse and fine pearlite—the alternating a-ferrite and cementite layers are thinner for fine than for coarse pearlite. Coarse pearlite forms at higher temperatures (isothermally) and for slower cooling rates (continuous cooling). 

Bainite—this has a very fine structure that is composed of a ferrite matrix and elongated cementite particles. It forms at lower temperatures/higher cooling rates than fine pearlite. 

Fine pearlite is harder, stronger, and more brittle than coarse pearlite. 

On the basis of diffusion considerations, explain why fine pearlite forms for the moderate cooling of austenite through the eutectoid temperature, whereas coarse pearlite is the product for relatively slow cooling rates. 

Briefly explain why fine pearlite is harder and stronger than coarse pearlite, which in turn is harder and stronger than spheroidite. 

D5 Is it possible to produce an iron-carbon alloy that has a minimum tensile strength of 620 MPa (90,000 psi) and a minimum ductility of 50% RA If so, what will be its composition and microstructure (coarse and fine pearlites and spheroidite are alternatives) If this is not possible, explain why. 

FE On the basis of the accompanying isothermal O transformation diagram for a 0.45 wt% C iron-carbon alloy, which heat treatment could be used to isothermally convert a microstructm-e that consists of proeutectoid ferrite and fine pearlite into one that is composed of proeutectoid ferrite and martensite ... 

---