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Tham khảo tài liệu 'steel heat treatment - metallurgy and technologies 2nd ed - g. totten (crc_ 2007) 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 4.1.2 Eutectoid Composition and Temperature The aforementioned division of alloying elements into groups according to their influence on allotropic transformations in alloys of the iron-alloying element system allows one to predict to some extent the effect of the elements on the critical points of carbon steel. For example considering the diagram lines that correspond to the transition of Fe from one allotropic form to another it can be expected that the elements extending the g-phase range Ni group will lower the a g iron transition point Ac3 while the elements narrowing the g-phase range Cr group will elevate that point. A similar effect of the elements is observed to a certain extent in the pearlite transformation Ac1 as in this case too an allotropic change of iron takes place Fea transforms to Feg. Figure 4.5 illustrates the influence of the most important alloying elements on the position of the critical point Ac1. As is seen the elements narrowing the g-phase range do raise the critical point Ac1 while the elements broadening the g-phase range lower it. It should be noted that in the case of Cr group elements one observes a known relationship between the limiting concentration necessary to close the g-phase range in iron-alloying element alloys and the degree of elevation of the point Ac1. The lower the concentration of the element at which the g-phase range is closed the more abrupt the rise of the critical point Ac1. If a steel simultaneously contains two or more alloying elements that influence its critical points in the same direction the critical points usually lower or elevate to a greater extent than would be the case if only one of the elements exerted its influence. But here the result cannot be presented by a simple dependence. If a carbon steel contains alloying elements that have an opposite effect on the position of the critical points during heating the influence of the elements shows up differently depending on their quantitative ratio. Table
