Many of our heat-treatment processes require us to choose the proper austenitizing temperature so that subsequent transformation will produce the desired microstructure and properties. This choice is influenced by a number of factors, including the presence of various alloying elements and their effect on the composition, grain size and transformation of the austenite. Let’s learn more.
Classification of Alloying Elements
The ability of alloying elements to promote the formation of a particular phase or to stabilize that phase is well known.[1-4] Alloying elements (the most influential of which are shown in bold) are broadly classified as:
• Austenite formers (e.g., C, Ni, Mn, N, Cu, Co, Zn, Au)
• Austenite stabilizers (e.g., Ni, Mn, Co, Pt)
• Ferrite formers (e.g., Cr, Si, Mo, W, Al, B, Ta, Ti, V, Zr, Nb)
• Ferrite stabilizers (e.g., Si, Mo, Cr, Al, Be, P, Ti, Mo, V)
• Carbide formers (e.g., Cr, W, Mo, V, Ti, Nb, Ta, Zr)
• Carbide stabilizers (e.g., Cr, Mn, Si, W, Mo, V, Hf, Ti, Ta, Zr)
• Nitride formers (e.g., Al, Cr, Ti, Mo, V, Nb, Ta, Zr)
It should also be noted that the influence of carbide stabilizers is dependent on the presence of other alloying elements in steel. Austenite stabilizers lower the eutectoid temperature, thereby expanding the austenite phase field (i.e., temperature range over which austenite is stable). By contrast, ferrite formers raise the eutectoid temperature, thereby shrinking the austenite phase field.
In addition, alloying elements affect such heat-treatment issues as the temperature of martensite formation, formation of pearlite and bainite during isothermal transformation and resistance to tempering.
Alloying Effects
As additions to steel, certain alloying elements are known to have very specific effects (Table 1). These same elements influence the size of the austenite phase field (Figs. 1-5; note: Figs. 3-5 are only online). In these figures, the field shown is for pure austenite formed at equilibrium. In using them, it is important to note that compositions lying to the right of the triangles will be largely austenite, with increasing amounts of carbide (or possibly graphite in the case of high-silicon steels). Below the areas of pure austenite (i.e., at lower temperature), a three-phase zone exists in which ferrite, austenite and carbide will be present even at equilibrium. To the left of the austenite field, austenite with more or less ferrite is found.[2]
Alloying elements also confer certain desirable characteristics for heat treatment (Table 2). For example, all alloying elements (with the possible exception of Co) lower the martensite start (Ms) temperature, with carbon having the greatest influence. In those steels over 0.5% C, the martensite finish (Mf) temperature is typically below room temperature, implying that retained austenite will form on hardening. Similarly, these same alloying elements delay the formation of ferrite and cementite (i.e., the nose of the TTT diagram is shifted to the right), and some elements affect the bainite transformation more than the pearlite transformation. In addition, the pearlite-bainite transformation is delayed slightly at carbon contents over 1% (helpful, for example, in carburizing and hardening of tool steels).
Schaeffler-Delong Diagrams
The Schaeffler diagram (Fig. 6) and the Schaeffler-Delong diagram (Fig. 7, online) can be used to predict the influence of microstructural transformation by three or more alloying elements.[5] In these diagrams, the austenite formers are arranged along the y-axis and the ferrite formers along the x-axis. The original Schaeffler diagram involved only Ni and Cr, but the Delong version includes other elements and gives them coefficients that reduce them to their Ni or Cr equivalents. A “nickel equivalent” is calculated for the austenite-stabilizing elements (Equation 1) and a “chromium equivalent” for the ferrite-stabilizing elements (Equation 2). These diagrams are particularly useful for predicting ferrite levels in austenitic stainless steel welds.
The nickel and chromium equivalents can be determined as follows:
1) Ni (equivalent) = Ni + (30 x C) + (0.5 x Mn) + (30 x N)
2) Cr (equivalent) = Cr + Mo + (1.5 x Si) + (0.5 x Nb)
Conclusion
In order to select the right austenitizing temperature or make minor adjustments to published data, the heat treater needs to understand the influence of alloying elements on the austenite phase field. The austenitizing temperature is influenced not only by the presence of alloying elements and their relative amounts but also by how they interact with one another. Selecting an improper austenitizing temperature (too high or too low for the given steel) can result in an undesirable microstructure and failure to achieve desired properties.
References
1. Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014.
2. Bain, Edgar, Alloying Elements in Steel, 2nd Edition, ASM International, 1961.
3. Thelning, Karl-Erik, Steel and Its Heat Treatment, Bofors Handbook, Butterworths, 1975.
4. Bullens, D. K., Steel and Its Heat Treatment, Volume II – Engineering and Special Purpose Steels, John Wiley & Sons, Inc., 1939.
5. Key to Metals (www.keytometals.com)
6. KVA Stainless (www.kvastainless.com)
7. British Stainless Steel Association (www.bssa.org.uk)
