Phase
Diagram for Alloy 38CrMoAl (Carburized and Heat Treated)
Alloy 38CrMoAl is one of the most widely used alloy steels in various industries. This material has unique properties, high strength, and exceptional corrosion resistance. Common applications include construction of machinery parts, pistons, gears, and engine components.
The alloy steel 38CrMoAl has two main elements, namely; 38 per cent chromium [Cr] and 0.20 per cent molybdenum [Mo]. Alloying elements such as niobium (Nb), vanadium (V), and tungsten (W) are also added to improve the properties of the alloy. The alloy’s high-temperature properties are determined by the iron-carbon phase diagram.
The iron-carbon phase diagram shows the relationship between the carbon content and the iron-rich phase. The relationship between carbon and iron-rich phase is dependent upon the alloys composition and the temperature of the material.
At temperatures below 727°C, the iron-carbon phase diagram shows a pro-eutectoid domain. In this domain, the initial Fe-C diagram is typical for low-alloy steel and shows an austenite phase and a low-carbon ferrite phase.
At temperatures above 727°C, Fe-C diagram is different from that of low-alloy steels. In this domain, there exist three distinct phases, austenite, ferrite, and martensite. The Fe-C diagram for 38CrMoAl alloy steel above 727°C is shown in Fig. 1.
Fig. 1 The Fe-C diagram for 38CrMoAl alloy steel above 727°C
In the Fe-C diagram, austenite is the primary phase and occupies the majority of the diagram area. When the alloy is alloyed with niobium and molybdenum, the austenite phase is highly stable and resistant to transformation. As the carbon content of the alloy increases, the amount of carbon solubility in austenite increases. At higher temperatures, the carbon saturation level of austenite is much higher than that of ferrite or martensite. This, in turn, results in a higher percentage of austenite in the alloy.
When the alloy is heated to a temperature of 727°C, the austenite-ferrite transformation begins. At this temperature, the ferritic phase is more stable than the austenitic phase, and the ferrite constituent starts to dominate the Fe-C diagram. As the temperature is further increased, the austenite-martensite transformation occurs. At a temperature of 960°C, austenite is completely replaced by martensite. At this temperature, the Fe-C diagram of 38CrMoAl is comprised of mostly martensite.
Due to its high strength, 38CrMoAl alloy steel is most commonly used in the carburizing processes and subsequent heat-treatment of components and parts. In the heat-treatment processes, the alloy is heated to a temperature of 900°C for carburizing and then cooled to a temperatureof about 500°C for tempering. The Fe-C diagram for the carburized and heat-treated 38CrMoAl alloy is shown in Fig. 2.
Fig. 2 The Fe-C diagram for carburized and heat-treated 38CrMoAl alloy
In the Fe-C diagram in Fig. 2, the carburized and heat-treated 38CrMoAl alloy exhibits three distinct phases, namely, ferrite, austenite, and martensite. At 900°C, the ferrite phase is the predominant phase and increases as the carbon content of the alloy increases. At 500°C, the austenite phase is the most dominant and increases with the increase of carbon content. At 500°C carburizing temperature, the volume fraction of austenite is higher compared to ferrite and martensite. The Fe-C diagram for carburized and heat-treated 38CrMoAl also shows that with increasing carbon content, the volume fraction of austenite decreases with a concomitant increase in ferrite and martensite.
In conclusion, the phase diagram of 38CrMoAl alloy steel is complex and affected by alloy composition and temperature. The iron-carbon phase diagram for the alloy shows clearly the relationship between carbon content and iron-rich phases. As the temperature is increased, the ferritic phase is replaced by austenite and martensite. Moreover, the phase diagram for the carburized and heat-treated alloy indicates that the relative amount of austenite decreases with increasing temperature.
