Effect On The Formation Of Pearlite And Bainite During The Isothermal Transformation

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Effect on the formation of pearlite and bainite during the isothermal transformation


All alloying elements except Co delay the formation of ferrite and cementite. It is very difficult to formulate any general rules regarding the influence exerted by the various alloying elements. However, it has definitely been found that some elements affect the bainite transformation more than the pearlite transformation, while other elements act in the opposite manner.
Certain elements will, paradoxically, accelerate the transformations if their concentration increases beyond a certain limiting value, this limit been affected by other alloying elements present. For case-hardening and tool steels the time taken to initiate the pearlite-bainite transformation is reduced as the carbon content exceeds about 1%. For tool steels and constructional steels Si-concentrations of 1,5% and above have been found to promote pearlite formation.
As a general principle it may be stated that by increasing the concentration of one alloying element by some few percent and the basic carbon content being kept about 0,50%, only a relatively small retardation of the transformation rates is noticed. For plain carbon steels a successive increase in C from 0,30% to 1% produces but a negligible effect. It is only in conjunction with several alloying elements that a more noticeable effect is produced.
The diagram in Figure 3, applicable to steel W 1 (l% C) will serve as a basis for this discussion. The shortest transformation time for this steel is less than 1/8th second. Note that the time scale is logarithmic; hence there is no zero time. As has been mentioned previously, both pearlite and bainite form simultaneously in this steel at about 550°C. Since the curves overlap it is customary to draw only one curve. With increasing contents of certain alloying elements, however, the noses of the pearlite and bainite curves will separate.
The structures shown in Figure ...