Pearlite martensite

High-alloy multiphase steels - Ferritic/pearlitic-martensitic steels I 235... 

High-alloy multiphase steels Ferritic/pearlitic-martensitic steels Ferritic-austenitic steels/duplex steels... 

Unalloyed and low-alloyed steels/cast steel 193 Unalloyed cast iron and low-alloy cast iron 224 High-alloy cast iron 226 Ferritic chromium steels with < 13% Cr 228 Ferritic chromium steels with >13% Cr 229 High-alloy multiphase steels 235 Ferritic/pearlitic-martensitic steels 235 Ferritic-austenitic steels/duplex steels 235 Austenitic CrNi steels 237 Austenitic CrNiMo(N) steels 239 Austenitic CrNiMoCu(N) steels 244 Nickel 260... 

Ferritic chromium steels with < 13 % Cr 320 Ferritic chromium steels with > 13 % Cr 320 High-alloy multiphase steels 320 Ferritic/pearlitic-martensitic steels 320 Ferritic-austenitic steels/duplex steels 320 Austenitic CrNi steels 323 Austenitic CrNiMo(N) steels 323 Austenitic CrNiMoCu(N) steels 323 Nickel-chromium alloys 339 Nickel-chromium-iron alloys (without Mo) 339 Nickel-chromium-molybdenum alloys 339 Nickel-copper alloys 339 Zinc 343 Bibliography 344... 

Fig. 5. Metastable Fe—Ni—Cr "temary"-pliase diagram where C content is 0.1 wt % and for alloys cooled rapidly from 1000°C showing the locations of austenitic, duplex, ferritic, and martensitic stainless steels with respect to the metastable-phase boundaries. For carbon contents higher than 0.1 wt %, martensite lines occur at lower ahoy contents (43). A is duplex stainless steel, eg. Type 329, 327 B, ferritic stainless steels, eg. Type 446 C, 5 ferrite + martensite D, martensitic stainless steels, eg. Type 410 E, ferrite + martensite F, ferrite + pearlite G, high nickel ahoys, eg, ahoy 800 H,...

To make martensite in pure iron it has to be cooled very fast at about 10 °C s h Metals can only be cooled at such large rates if they are in the form of thin foils. How, then, can martensite be made in sizeable pieces of 0.8% carbon steel As we saw in the "Teaching Yourself Phase Diagrams" course, a 0.8% carbon steel is a "eutectoid" steel when it is cooled relatively slowly it transforms by diffusion into pearlite (the eutectoid mixture of a + FejC). The eutectoid reaction can only start when the steel has been cooled below 723°C. The nose of the C-curve occurs at = 525°C (Fig. 8.11), about 175°C lower than the nose temperature of perhaps 700°C for pure iron (Fig. 8.5). Diffusion is much slower at 525°C than it is at 700°C. As a result, a cooling rate of 200°C s misses the nose of the 1% curve and produces martensite. 

Why is 0.8% carbon martensite approximately five times harder than pearlite ... 

The final note is that pearlite and bainite only form from undercooled y. They never form from martensite. The TTT diagram eannot therefore be used to tell us anything about the rate of tempering in martensite. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

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. 

On tempering or annealing martensite, bainite or even pearlite at even higher temperatures (about 970K) a structure consisting of coarse cementite spheroids (readily visible in a light microscope) in a ferrite matrix is obtained. This is the most stable of all ferrite/cementite aggregates, and it is also one of the softest. 

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diifusionless transformations (austenite -> martensite), continuous precipitation (austenite -> pearlite) and discontinuous precipitation (austenite -> bainite and tempering of martensite) are fairly common in other alloy systems. 

Without these advances in hard, strong materials based on abundant, and therefore low-cost iron ore, there could have been no industrial revolution in the nineteenth century. Long bridges, sky-scraper buildings, steamships, railways, and more, needed pearlitic steel (low carbon) for their construction. Efficient steam engines, internal combustion engines, turbines, locomotives, various kinds of machine tools, and the like, became effective only when key components of them could be constructed of martensitic steels (medium carbon). 

Constituent properties of bainite, 23 280 of martensite, 23 280-281 of pearlite, 23 280 of tempered martensite, 23 281-282 Constrained geometry catalysts, 16 81 20 193... 

C < 1.5 Carbon steel 1000 series—ferrite/pearlite 2000 series—ferrite/pearlite or bainite 3000 series—martensitic 4000 series... 

If austenite is cooled slowly toward ambient temperature, the dissolved carbon in excess of 0.022 weight % comes out of solid solution as cementite, either in continuous layers of FeaC (pearlite) or as layers of separated FeaC grains (bainite). In either case, the iron is soft and grainy, as with cast iron. If, on the other hand, the hot austenite is cooled quickly (i.e., quenched), the 7-Fe structure goes over to the a-Fe form without crystallization of the interstitial carbon as cementite, and we obtain a hard but brittle steel known as martensite in which the C atoms are still randomly distributed through the interstices of a strained a-Fe lattice. Martensite is kinetically stable below 150 °C above this temperature, crystallization of FesC occurs in time. 

If martensite is reheated to between 200 and 300 °C for an appropriate time and is then requenched, a partial crystallization of FeaC occurs, and a tough steel, sorbite, with properties intermediate between martensite and pearlite or bainite, is obtained. This process is known as the tempering of steel.12... 

BAINITE. A product of the decomposition of austenite that usually occurs at temperatures between those that produce pearlite and those that produce martensite. Its structure consists of finely divided carbide particles in a matrix of ferrite. See also Austenite. 

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