In this blog post, we’ll examine the principles behind the formation of martensite based on the iron-carbon phase diagram.
People often think of iron as a very hard material. However, 100% pure iron is very soft and easily deformed, making it unsuitable as a structural material for buildings or machinery. To make iron hard, other elements must be added to it, and it must undergo appropriate heat treatment. Generally, the most commonly added element is carbon. The reason iron’s properties change is that the arrangement of iron atoms changes, and this arrangement varies depending on temperature and the amount of carbon added. In materials engineering, this atomic arrangement is called the microstructure, and regions that are distinctly different from other parts due to differences in microstructure are called phases. Iron becomes hard because the microstructure formed by iron and carbon changes; among the various microstructures, the hardest is martensite. This article examines how martensite is formed. The reason martensite is hard is a separate topic and will not be covered here.
To understand the principles behind the formation of martensite, one must first understand the phase equilibrium diagram. In the iron-carbon phase equilibrium diagram, the horizontal axis represents the concentration of carbon added to iron, and the vertical axis represents temperature. The symbols α, γ, δ, and “Liquid” shown in the figure represent the phases that can exist most stably under the given conditions. It is not necessary to know the names and characteristics of each phase here. What is important is that the regions where α (ferrite) and γ (austenite) can exist stably differ significantly. This is the key reason why martensite forms. The region where α can exist stably is relatively narrow, whereas γ can exist stably over a much wider region than α. This difference is closely related to the added carbon.
Both α and γ refer to phases of iron and do not refer to carbon itself. So where does the added carbon exist? When carbon is added to iron, carbon atoms—being much smaller than iron atoms—enter the empty spaces between the iron atoms and occupy those spaces. This is similar to placing sand between large pebbles, where the sand fills the empty spaces between the pebbles. The reason the amount of carbon that can exist in the α and γ phases differs is that the arrangement of iron atoms is different, resulting in different sizes of empty spaces. As the carbon concentration increases and reaches saturation, carbon can no longer exist between the iron atoms, and cementite (Fe₃C), which is bonded to iron, becomes the most stable form. Therefore, as shown in the iron-carbon phase diagram, cementite forms alongside both α and γ phases when the carbon concentration exceeds a certain threshold.
Now, let’s consider the following scenario. What happens when iron containing approximately 0.5% carbon is cooled from about 900°C to around 200°C? Looking solely at the phase diagram, it appears that α and cementite—the most stable phases—will ultimately form. However, there is a crucial factor that must be considered here: the cooling rate. For α and cementite to form during the cooling process down to 200°C, carbon atoms that were originally between iron atoms within the γ phase must migrate to bond with iron and form stable cementite. This is because the α phase can hold much less carbon than the γ phase. However, if the cooling rate is very fast, the γ phase transforms into the α phase before the carbon atoms have had time to diffuse sufficiently. As a result, the α phase becomes supersaturated with carbon. The transformation from γ to martensite illustrates this phenomenon.
Martensite is a structure in which carbon is forcibly retained within the α phase. Therefore, it is not a thermodynamically stable phase. This is also why martensite is not shown on the iron-carbon phase diagram, which depicts only stable phases. In other words, martensite is a microstructure that forms because the amounts of carbon that γ and α can contain differ.
If sufficient energy is supplied, the carbon remaining within martensite will continue to diffuse, attempting to form the stable phases α and cementite. However, at low temperatures, the energy required for diffusion is not available, so the martensite structure is maintained. A phase that is not completely stable but persists under certain conditions is called a metastable phase. In actual steel manufacturing, such metastable phases occur much more frequently than equilibrium states. Consequently, separate data to describe metastable phases became necessary. Upon reflection, the reason metastable phases form is that the variable “time”—which is not included in equilibrium diagrams—comes into play. Time-Temperature-Transformation (TTT) diagrams, plotted at a constant carbon concentration with time on the horizontal axis and temperature on the vertical axis, illustrate the formation process of metastable phases as a function of cooling rate. Pearlite and bainite, along with martensite, are representative microstructures of steel, but their formation mechanisms and properties differ.
On the Time-Temperature-Transformation (TTT) diagram, cooling curves 1 through 4 represent the process of cooling γ at approximately 800°C at different rates. The lines labeled Ms, M50, and M90 represent, respectively, the temperature at which martensite begins to form and the states where approximately 50% and 90% of martensite has formed. Here, the “s” in Ms stands for “start.” Additionally, the curves in the center of the graph indicate the conditions under which pearlite and bainite form. For martensite to form, the cooling curve must reach the region below Ms. Therefore, martensite forms along curves 1, 2, and 3, but it does not form along curve 4, where the material first completely transforms into pearlite during cooling before reaching Ms. Furthermore, since the cooling paths below Ms differ for curves 1, 2, and 3, the final amount of martensite formed also varies. Generally, as the martensite content increases, strength and hardness increase, but ductility decreases.
Iron remains one of the most widely used structural materials today due to its excellent strength and the ability to adjust its various properties. However, while martensite is the hardest microstructure in iron, it is also highly brittle, making it prone to fracture. Therefore, in actual heat treatment processes, the rate of martensite formation is adjusted according to the required mechanical properties, and various data—such as time-temperature-transformation diagrams and continuous cooling transformation (CCT) diagrams—are utilized for this purpose. These data are based on a deep understanding of iron’s phase transformations, and research to develop steel materials with even better performance continues to this day.
