Back to basics on heat-treatment of steel, underlying metallurgy

An ongoing discussion of the topic will begin with the Iron-Carbon phase diagram.

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In this article, and future articles, we will discuss the basics of the heat-treatment of steel and the metallurgical reason behind it. We start off with the Iron-Carbon phase diagram.

While many investigators examined the microstructure effects of temperature and steel, it wasn’t until Chernov in 1868 established that there are critical temperatures at which transformations occur in steel [1], and these temperatures vary as the content of carbon is changed [2]. Sir William Chandler Roberts-Austen published his first Fe-C diagram in 1897 [3]. He continued to improve his diagram until he published his final improvement to the Fe-C diagram in 1899 [4]. This was further modified by Roozeboom [5] to incorporate thermodynamic considerations. This is considered to be the first true phase diagram for the Fe-C system since it complies with the Gibbs phase rule [6]. This is shown in Figure 1.

Figure 1: Fe-C phase diagram published by Roberts-Austen and Roozeboom [4], [5].

While the phase fields are not labeled entirely accurately, it is a close approximation of the accepted phase diagram (Figure 2).

Figure 2: Modern Fe-C phase diagram [7]

In its most basic form, the Fe-C phase diagram shows the equilibrium phases present as a function of temperature and carbon content. For most heat-treating applications, the focus is on carbon contents less than 1.2 percent and temperatures less than 1,100°C. In this small portion of the overall phase diagram, there are two phases present: Austenite (stable above 727°C), and Ferrite (stable below 912°C). Cementite, Fe3C, is also a stable phase, but the equilibrium concentration of carbon is approximately 6.7 percent. There are three mixed phase fields: Austenite (γ) + Ferrite (α); Austenite + Fe3C; and Ferrite + Fe3C. It is these fields that are important for heat treating, and their combinations result in the microstructures developed during heat treatment.

Austenite (γ) is a solid solution of carbon in iron, with a 2.14 percent carbon maximum solubility in iron. Crystallographically, it is a face-centered cubic structure (Figure 3). In this crystal structure, each iron atom occupies the corner of a cube, and the center of each face. Carbon would occupy the spaces between the iron atoms, since much smaller. These spaces are called interstitial spaces. Austenite is non-magnetic.

Figure 3: Crystal structure of Austenite. It is classified as a face-centered cubic structure.

Ferrite (α) is a solid solution of carbon in iron at temperatures below 912°C. It is a body-centered cubic structure, where the iron atoms occupy the center of the cube, as well as the corners of the cube. Again, carbon would occupy the interstitial spaces. The maximum solubility of carbon in ferrite is 0.022 percent. Ferrite transforms to austenite at 912°C. Ferrite is magnetic below 768°C. (Figure 4)

Figure 4: Crystal structure of ferrite. It is a body-centered cubic structure.

Cementite (or iron carbide) is an intermetallic compound that’s very hard and brittle. It is 6.7 percent carbon and 93.3 percent iron by weight. Because of its hard and brittle nature, it is often considered to be a ceramic. It has a crystal structure that is orthorhombic, and is a complicated structure. (Figure 5)

Figure 5: Crystal structure of Iron Carbide, Fe3C [8]. Iron atoms are shown in blue, and carbon atoms are shown in black.

There are three classifications of steel based on the carbon content:

  • Eutectic steels with a carbon content of approximately 0.78 percent carbon;
  • Hypoeutectic steels containing less than 0.78 percent carbon; and
  • Hypereutectic steels with greater than 0.78 percent carbon (up to about 1.5-2 percent C).

When slowly cooled from the austenitizing temperature, each of these three types of steel will have a different microstructure.

Eutectoid steels. In these steels, at the carbon content of 0.78 percent C austenite transforms directly from austenite to ferrite and cementite at 727°C. Depending on the cooling rate, the cementite can form either as spheres in a matrix of ferrite (extremely slow cooling), or as a lamellar mixture of ferrite and cementite. This lamellar mixture of ferrite and cementite is also known as pearlite. 

Pearlite is a layered structure of alternating layers of ferrite and cementite. This microstructure was originally observed by Henry Clifton Sorby [9] and called Sorbite. He described it as having a “pearly appearance.” Later, this microstructure was name pearlite by Henry Howe [10]. This structure is shown in Figure 6.

Figure 6: Microstructure of pearlite. Structure consists of alternating layers of ferrite (white) and cementite (dark).

Hypoeutectoid steels. Consider a hypoeutectic steel in the austenite region of the phase diagram. It is slowly cooled until it reaches the ferrite + austenite phase field. Ferrite will start to precipitate and grow at the prior austenite grain boundaries. As the steel further slowly cools, it will cross the ferrite and cementite phase boundary. As this point, the remaining austenite will transform to pearlite and ferrite. The microstructure will consist of proeutectoid ferrite at the prior austenite grain boundaries and the lamella structure of pearlite, consisting of ferrite and iron carbide (Figure 7).

Figure 7: Microstructure of a slow cooled hypoeutectoid steel, resulting in a microstructure of proeutectoid ferrite at prior austenite grain boundaries, with pearlite present on the interior of the grains.

Hypereutectoid steels. In a hypereutectic steel, when cooling through the austenite field, it encounters the austenite + cementite two phase field. Proeutectoid cementite forms at the prior austenite grain boundaries. As it crosses the ferrite and cementite two phase field, the residual austenite transforms to ferrite and cementite in the interior of the prior austenite grains (Figure 8).

Figure 8: Microstructure of a slow cooled hypereutectic plain carbon steel. Microstructure consists of cementite at the prior austenite grain boundaries, with pearlite at the interior of the grains.

A schematic of the different sequences of transformation are shown in Figure 9.

Figure 9: Schematic representation of the sequence of slow cooling for two types of steels: (a) Hypoeutectoid and (b) Hypereutectoid steels [11].

In describing the phase diagram, there are several temperatures that are used not only for the Fe-Fe3C phase diagram, but are also used in actual heat-treating practice. These critical temperatures and their definition are shown in Table 1.

Table 1: Critical temperatures from the Fe-Fe3C phase diagram, and the critical temperatures of non-equilibrium structures present in steel.

In addition to the phases present, and the mixtures of phases in the Fe-Fe3C phase diagram, there are additional non-equilibrium phases that are important to the heat treater. These are martensite and Bainite. Each of these phases will be discussed in later articles.

Conclusions

In this short article, an explanation of the basic Iron-Carbon phase diagram is provided, with details on how microstructure evolves. [12] This diagram is the initial introduction to the heat treatment of steel. 

References

  1. D. K. Chernov, “Critical review of papers by Lavrov and Kalakutskii on steel and steel cannons,” Zapiski Russkogo Tekhnicheskogo Obshchestva, p. 399, 1868.
  2. D. K. Chernov, Zhurnal Russkogo Metallurgicheskogo Obshchestva, Vols. 3-4 Part I, p. 189, 1916.
  3. W. Roberts-Austen, Proc. Inst. Mech. Eng., 1897.
  4. W. C. Roberts-Austen, Proc. Inst. Mech. Eng., 1899.
  5. B. Roozeboom, L. Physik. Chem., vol. 30, pp. 385-413, 1899.
  6. J. W. Gibbs, “On the Equilibrium of Heterogeneous Substances,” Transactions of the Connecticut Academy of Arts and Sciences, vol. 3, pp. 108-248; 343-524, 1877-1878.
  7. W. D. Callister, Materials Science and Engineering: An Introduction, Hoboken, NJ: Wiley & Sons, 2007.
  8. Orci. [Online]. Available: https://commons.wikimedia.org/w/index.php?curid=7495713.
  9. H. C. Sorby, “On the Microstructural Structure of Iron and Steel,” J. Iron and Steel Institute, vol. 1, pp. 255-288, May 1887.
  10. H. M. Howe, The Metallography of Steels and Cast Iron, New York: McGraw-Hill, 1917.
  11. ASM International, “Introduction to Steel Heat Treatment,” in Steel Heat Treating Fundamentals and Processes, vol. 4A, J. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM International, 2013, pp. 3-25.
  12. “History of the Development of Iron-Carbon Diagrams,” Metal Science and Heat Treatment, vol. 10, no. 5, pp. 344-350, 1968.