Steels continued martensitic

ConcBpt Check 10.4 Briefly describe the simplest continuous cooling heat treatment procedure that would be used to convert a 4340 steel from (martensite + bainite) into (ferrite + pearlite). 

We saw in Chapter 8 that, if we cool eutectoid y to 500°C at about 200°C s , we will miss the nose of the C-curve. If we continue to cool below 280°C the unstable y will begin to transform to martensite. At 220°C half the y will have transformed to martensite. And at -50°C the steel will have become completely martensitic. Flypoeutectoid and hypereutectoid steels can be quenched to give martensite in exactly the same way (although, as Fig. 11.8 shows, their C-curves are slightly different). 

Provided, of course, that we continue to cool the steel down to the martensite finish temperature. 

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. 

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. 

Austenitic steels are produced as castings, ingots of all sizes and as continuously cast billets and slabs. The other types of stainless and heat resistant materials mentioned in table 4.1 are cast predominantly as ingots of a moderate size, although martensitic and ferritic-austenitic steels are also commonly used as castings. 

The martensite transformation of stainless steels under stress at low temperature has been extensively studied [ ]. The purpose of this investigation has been to obtain systematic information on this phase change in an alloy (18 Ni, 10 Cr, C < 0.03%, quenched) under continuous constant stress at 20° and 77 °K. 

Bainite is a crystalline constituent that can be formed during heat treatment of steel by isothermal transformation or continuous cooling in the temperature range between those of perlite and martensite. Here, iron diffusion is no longer possible, while... 

Fig. 3.1-113 Continuous-cooling-transformation (CCT) diagram for a 4130 grade low-alloy steel. Acs and Ac signify the temperatures of the y/ y+a) and eutectoid reation, respectively. A - austenite, F - ferrite, B - bainite, P - pearlite, M - martensite. The cooling rate is measured at 705 °C. The calculated critical cooling rate is 143 K/s [1.80]...

Type 416 stainless steel is a low-carbon-class martensitic alloy, a free-machining variation of type 410 stainless steel. The chemical composition is shown in Table 9.4. It has a maximum continuous operating temperature of 1250°F (675°C) and an intermittent maximum operating temperature of 400°F (760°C). 

Figure 6.1 (a) Schematic microstmcture of a tempered martensitic steel, for example, steel grade 91 [14], Low-angle boundaries are drawn in dashed lines (in practice disorientation angle less than 5 degrees) while the other boundaries are shown by continuous lines (block boundaries, packet boundaries, former austenitic boundaries) (b) Initial microstmcture (TEM) [18],... 

Grade 91 steels and their equivalents, along with some other conventional ferritic-martensitic steels, are implemented in the major structural design codes in the world, such as the ASME Code, RCC-MRx, and the JSME Code. These codes are continuously improving their provisions to further meet the requirements of Generation IV projects. The current major issue on ferritic-martensitic steels application is the extension of time-dependent allowable stresses to 500,000 h. In conjunction with this, provisions on items that involve time-dependent material properties such as weldment... 

STEELS Processing Diffusion V Recrystaiiization V Isothermal transformation tiiagrams, continuous-cooling transformalion tiiagrams heat treating tor tempereil martensite Heat treatment of steels ... 

For the continuous cooling of a steel alloy, there exists a critical quenching rate, which represents the minimum rate of quenching that produces a totally martensitic structure. This critical cooling rate, when included on the continuous transformation diagram, just misses the nose at which the pearlite transformation begins, as illustrated in... 

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