What Happens to Steel During Heat Treatment? Part One: Phase Transformations
April 9, 2007
If we were traveling from one city to another, we would need to input our destination into a GPS locating device, use an Internet website such as MapQuest® to plot our route or simply pull out a map to help us find our way. The Iron-Carbon, or perhaps more accurately the Iron-Iron Carbide phase diagram (Fig. 1), is nothing more than a roadmap for heat treaters. A phase diagram simply helps us predict what will happen to the internal structure of the steel. Let’s learn more.
Heat treating of steel involves phase transformations. These can be diffusion-dependent transformations such as the formation of pearlite and diffusionless transformations such as when martensite forms. Let’s begin by reviewing the phases identified in Figure 1 – namely ferrite, austenite, cementite and pearlite.
At room temperature, the most stable form of iron is ferrite, also known as a-iron (“alpha” iron), which has a body-centered cubic (bcc) crystal structure. Ferrite is a fairly soft material that can dissolve only a very small amount of carbon – no more than 0.021% at 1670°F (910 °C) and only 0.008% at room temperature. Ferrite is the phase that exists below the upper critical temperature of a steel with less than 0.80% carbon.
The formation of austenite or g-iron (“gamma” iron) begins when steel is heated above its lower critical temperature, the Ac1 (A1) line shown on the diagram. The structure is fully austenitic above the Ac3 (A3) or Accm line. Austenite has a face-centered cubic (fcc) crystal structure and can contain up to 2.03% carbon at 2110°F (1154°C) or approximately 100 times greater than the maximum limit for ferrite. Carbon strengthens steel and gives it the ability to be hardened by heat treatment.
As carbon-rich austenite cools, the mixture attempts to revert back to the ferrite phase, resulting in some areas having an excess of carbon. One way for carbon to leave austenite is for a phase called cementite to precipitate out – leaving behind iron that is pure enough to take the form of ferrite – creating a cementite-ferrite mixture (Fig. 2). Cementite, or iron carbide, is a hard, brittle intermetallic compound consisting of iron and carbon at 6.7% (at this percentage, every fourth atom is a carbon atom). Its chemical symbol is Fe3C. Carbon in the form of graphite is the stable phase – the phase having minimum free energy. Thus cementite is metastable – it simply forms before graphite can develop and will decompose into graphite given the right conditions.
The formation of pearlite involves the mixture of two phases, iron and iron carbide, and is a growth-dominated process requiring a combination of both high temperatures and long times. Pearlite is not a true phase. The iron takes the crystalline form of ferrite while the iron carbide takes the crystalline form of cementite, and the resulting structure consists of alternating lamellar bands, or plates. The distance between these plates as well as their thickness is dependent on the cooling rate of the material. Fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness. A fully pearlitic structure occurs at approximately 0.8% carbon, the eutectoid point on the phase diagram.
Under certain cooling conditions, another phase known as bainite can form. The formation of bainite is dominated by the nucleation process, resulting in a fine dispersion of ferrite and cementite, and is typified by intermediate hardness and good toughness. Bainite forms as needles or plates depending on the temperature of the transformation. Pearlite and bainite transformations compete with one another, and once a portion of the steel microstructure has transformed to one or the other, it is not possible to transform between them without reheating to form austenite. Bainite will normally not form on continuous cooling to room temperature because all of the austenite will have transformed to pearlite by the time the bainite transformation is ready to begin. For this reason it is not shown on the phase diagram. Fast cooling followed by interrupted quenching and an isothermal hold above the martensite start (Ms) temperature is required to form bainite.
Martensite is usually considered to be a grain structure, not a phase. For this reason, martensite is not shown on the phase diagram of the iron-carbon system. Martensite is characterized by an angular needle-like or plate-like structure. Alloys with less than 0.6% carbon form lath martensite. Alloys with more than 1.0% carbon form plate martensite, and alloys between 0.6 to 1.0% carbon form mixtures of the two in varying degrees.
The hardness of martensite is dependent on its carbon content. You might be surprised to know that pure martensite is soft and ductile. As a result of heating and rapid cooling, however, we form martensite with interstitially dissolved carbon in a supersaturated state, resulting in very high hardness – about four to five times the strength of ferrite. A minimum of 0.4% carbon is needed to form martensite. When austenite is quenched, the carbon is "frozen" in place as the crystal structure changes. Since the carbon atoms are too large to fit in the interstitial vacancies, a distortion of the crystal structure occurs, creating a body-centered tetragonal (bct) crystal structure.
The formation of martensite involves nucleation and growth, but the mechanism is not controlled by diffusion. Rather, it is primarily a shear process similar to twinning. Martensite formation also produces some dilatation. While the nucleation of martensite is thermally activated, the growth process is not – the glide of transformation dislocations shears the material, forming thin plates. These plates grow along specific crystallographic directions until either they encounter an austenite grain boundary, another martensite plate or until enough martensite has formed causing the growth reaction to cease. Because martensite plates form at approximately the speed of sound – much too fast for carbon to diffuse out of the austenite – martensite and austenite have identical chemical compositions. The martensite phase is metastable and wants to decompose into ferrite and cementite on heating.
By raising the temperature to a few hundred degrees, martensite will decompose into bainite. The decomposition of martensite starts by forming bainite at the boundaries of the martensite needles. If the temperature is high enough, this bainite can coarsen into lower (fine) pearlite and eventually upper (coarse) pearlite.
Mechanical properties are affected by which phase(s) are present in the microstructure. Hardness (Fig. 3) is the measurement with which heat treaters most often relate. However, one must be careful. For example, increasing cementite concentration will lead to an increase in hardness but with a resultant loss of ductility.
Elongation is a measure of ductility (Fig. 4). We see that the ductile behavior decreases in a rather linear fashion with increasing carbon content. Once the matrix contains only a small percentage of ferrite, the ductility approaches zero. Impact energy is a fairly direct measure of brittleness. Low energy means the material is brittle and will fracture easily. We see impact values decrease steeply and then taper off around 0.5% carbon. For hypereutectoid steels (those having greater than 0.80% carbon), the fracture energy is relatively constant and quite low. Essentially, the fracture toughness for these steels is determined by the fracture properties of cementite. IH
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: austenite, martensite, bainite, diffusion, pearlite