38CrMoAlA metallographic diagram

38CrMoAlA is an alloy steel that is used in many applications such as axles and heavy forgings, in the automotive and aerospace industries due to its superior strength and durability compared to other steels. It is often used to create high-performance components, such as gears and shafts, that need to withstand extreme heat, stress and wear and tear.

The chemical composition of 38CrMoAlA is quite complex and includes both carbon and alloying elements like chromium, molybdenum and aluminum. This alloying combination provides superior strength, toughness and thermal stability. Additionally, 38CrMoAlA provides higher oxidation and hot corrosion resistance, which helps it withstand extreme temperatures and pressures.

When looking at a micrograph or metallographic image of the alloy steel 38CrMoAlA, a few different structures can be observed that are typical of an alloy steel of this type. The structures or phases shown are ferrite, pearlite, bainite and martensite.

The ferrite phase is the primary component in 38CrMoAlA and provides superior tensile strength and ductility. This phase contains both carbon and a solid solution of other elements, such as chromium and molybdenum.

The pearlite phase is a combination of primary austenite and secondary ferrite and provides a good balance between strength and ductility. This phase is more uniform in composition, with significantly higher levels of carbon and other elements compared to ferrite.

The bainite phase is a combination of primary ferrite and secondary cementite, which is a super hard substance composed of iron and carbon. This phase provides superior wear resistance, making it ideal for components that experience heavy use or require high strength and high wear resistance.

The martensite phase is a combination of primary ferrite and secondary carbide, and provides superior hardness and strength. This phase is one of the most important aspects of 38CrMoAlA, since it gives the alloy superior strength and wear resistance.

Overall, the microstructure of 38CrMoAlA is complex and features a mix of ferrite, pearlite, bainite and martensite. This combination of phases helps the alloy to provide superior strength, toughness and wear resistance. These characteristics, combined with its thermal stability, make 38CrMoAlA suitable for many applications in the automotive and aerospace industries.

Structural steel 38CrMoAlA is a kind of mechanical properties and surface hardness enhanced steel for quenching and tempering. It is used for manufacturing parts with large bit load and thicker cross section, such as crankshaft shaft, steel sleeve, camshaft, crosshead etc..

The microstructure of 38CrMoAlA is shown in figure 1 by optical microscope(OM). The matrix of 38CrMoAlA consists of ferrite, pearlite and distributed carbide (Vanadium Molybdenum, Tungsten-chromium, chromium oxide). The typical microstructure of the 38CrMoAlA steel was observed under OM which shows small ferrite grains and a large number of fine pear-skin particles with small amounts of chromium tungsten and molybdenum vanadium carbides. The large number of fine pear-skin particles indicate that the steel is tempered at low temperature .

The microstructure of 38CrMoAlA measured by scanning electron microscope (SEM) is shown in Figure 2. The 38CrMoAlA steel shows a hierarchical structure with micron ferrite grains, sub-micron primary carbide and nanometer secondary carbide. It can be seen that the sub-micron carbides are well distributed between the ferrite grains, which suggests that an effective quenching temperature is applied to obtain an even distribution of carbide. The secondary carbide formed during tempering process is well distributed between the primary carbide and ferrite grains, further confirming the moderating temperatures used during tempering process.

Figure 3 shows the metallographic image of 38CrMoAlA steel taken by electron backscatter diffraction (EBSD). The high magnification picture of ferrite grain boundary reveals that the grain boundaries have high dislocation density and complex network like structure. The formation of such high dislocation density is due to the low-temperature quenching process. The presence of dislocations and the redistribution of atoms result in the development of different elastic constants within neighboring grains, which is beneficial for the improvement of 38CrMoAlA steels hardening properties.

Finally, Figure 4 shows the quenched and tempered sample 38CrMoAlA steel taken by Auger electron spectroscopy (AES). It can be seen that there is a significant difference in composition between the quenched and tempered sample. Auger electron spectroscopy shows that the tempering sample has higher concentrations of chromium, vanadium, and molybdenum than the quenched sample. This is indicative of increased carbide formation due to tempering, and confirms the microstructural observations made by SEM. This confirms that the moderate-temperature tempering process has effectively increased the surface hardness of the 38CrMoAlA steel.