Continuous-cooling transformation 4340 steel

Fig. 18. Continuous-cooling transformation diagram for a Type 4340 alloy steel, with superimposed cooling curves illustrating the manner in which transformation behavior during continuous cooling governs final microstmcture (1). Ae is critical temperature at equiHbrium. Ae is lower critical...

As a first step the steel is placed in one of five grades by means of Table 4.3. These grades are based on welding tests, on studies of the continuous cooling transformation behaviour of steels, and on an empirical formula relating composition to maximum HAZ hardness ... 

Atkins M Atlas of continuous cooling transformation diagrams for engineering steels . British Steel Corporation, Sheffield, 1977. 

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]...

Figure 10.13 Continuous cooling transformation (CCT) diagram of 9Cr-ODS steels.

Figere 10.28 Continuous-cooling transformation diagram for an alloy steel (type 4340) and several superimposed cooling curves demonstrating dependence of the final microstructure of this alloy on the transformations that occur during coohng. 

Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

For a number of applications, particularly those associated with conditions of continuous cooling or heating, equilibrium is clearly never approached and calculations must be modified to take kinetic factors into account. For example, solidification rarely occurs via equilibrium, amorphous phases are formed by a variety of non-equilibrium processing routes and in solid-state transformations in low-alloy steels much work is done to understand time-temperature-transformation diagrams which are non-equilibrium in nature. The next chapter shows how CALPHAD methods can be extended to such cases. 

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... 

Umemoto, M. Horiuchi, K. Tamura, I. (1982). Transformation Kinetics of Bainit during Isothermal Holding and Continuous Cooling. Trans. Iron Steel Inst. Jpn., Vol. 22, 854r-861. 

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... 

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). 

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