Hardenability of steels: alloying effect

Known, that the active alloying elements of steel, such as chromium and molybdenum, form carbides in it. It means, that these elements will tend to enter the carbide part of pearlite and bainite when they are formed from austenite.

Diffusion of carbon during the decomposition of austenite: from 0,8 % to 0,02 % and 6,7 %

When some volume of austenite is converted to pearlite or bainite in common carbon steels, then carbon atoms must be rearranged from a uniform distribution, which they have in austenite. The volume already converted from austenite may not contain carbon at all (0,02 %) in the ferritic region and be 6,7 % carbon in cementite area. This redistribution of atoms occurs due to diffusion.

Harder diffusion - slower decomposition of austenite

Likewise, in the transformation of austenite alloy steel, the alloying atoms, eg, chromium and manganese, should also redistribute from a uniform distribution in austenite to a high content in carbides and low in ferrite. However, diffusion redistribution for alloying elements is much more difficult., for carbon. The thing is, that their diffusion coefficient is much lower, than carbon. Therefore, the presence of alloying elements in steel makes it difficult to form pearlite and bainite.. Accordingly, the curves of the onset of pearlite and bainitic transformations on the diagrams of the transformation of austenite - isothermal and continuous - will shift to the right, in more, so to speak, later times.

All alloying additives in steel, except cobalt, shift the curves of the onset of ferrite formation, perlite and bainite on the isothermal transformation diagrams to the right.

The effect of nickel on the hardenability of steel

However, it is known, what, eg, nickel is quite an inactive element, and also slow down the rate of formation of pearlite and bainite. In this case, the reason lies in the influence of nickel on the phase diagram. It just can't be explained. However, the end result is easy to remember.: almost all alloying elements in steel slow down the decomposition of austenite to form ferrite, perlite or bainite.

How chromium slows down the transformation of austenite

The figure below shows a comparison of the isothermal transformation diagrams of austenite for two American steels. – carbon steel 1060 and alloy steel 5160 (analogues of our steels 60G and 50HGA) - with different chromium content. You can say, what steel 5160 Is the same steel 1060, but with the addition 0,8 % chrome.

stal-1060stal-5160Figure - Diagrams of isothermal transformation for steels 1060 and 5160.
Alloying steel 5160 chrome shifts the nose of the transformation curves to the right.
(A – austenite, F – ferrite, FROM – cementite)

Figure shows, that such a low chromium content has a significant effect on the position of the austenite transformation onset curves in the isothermal transformation diagram. Even though, what is the grain size in steel 5160 turned out to be smaller, than steel 1060, the nose of the isothermal transformation diagram for steel 5160 shifted to the right by about 5 seconds, and in steel 1060 - just for 0,5 seconds.

Effect on hardenability of the grain size of steel

The effect of grain on the hardenability of steel is due to the fact, that the decomposition of austenite always begins at the boundaries of its grains. Grain boundary area, naturally, depends on grain size. Large grain size will reduce the overall grain boundary area per unit volume. This leads to a shift in the curves of the beginning of the transformation - an increase in the delay in the beginning of the transformation - and, thereby, increases the hardenability of steel. That is why the position of the curves in the isothermal transformation diagram depends on the grain size of austenite. For the same reason, the austenite grain size is always indicated on the isothermal transformation diagrams..

Source:
John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007