“The role of “aluminum” in steel

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I. Effects of aluminum on the microstructure and heat treatment of steel ① Aluminum has a strong affinity for oxygen and nitrogen, and is used as a deoxidizing and denitriding agent in steelmaking. ② Aluminum strongly reduces the austenite region in steel. ③ Aluminum has a low affinity for carbon, and aluminum carbides generally do not form in steel. Aluminum strongly promotes the graphitization of carbon, while the addition of strong magnetizing elements such as chromium, titanium, vanadium, and niobium can suppress aluminum’s graphitizing effect. ④ Aluminum refines the primary grains of steel and raises the temperature at which the steel’s grains become coarser; however, when the amount of aluminum dissolved in the steel exceeds a certain level, the austenite grains tend to grow and become coarser instead. ⑤ Aluminum raises the martensite transformation temperature of steel and reduces the amount of residual austenite after quenching; its effect in this regard is opposite to that of other alloying elements other than cobalt. II. The effect of aluminum on the mechanical properties of steel: ① Aluminum reduces steel’s sensitivity to notches, decreases or eliminates aging phenomena in steel, particularly lowers the temperature at which steel transitions from ductile to brittle behavior, thereby improving its toughness at low temperatures. ② Aluminum provides significant solid solution strengthening, and high-alumina steels have the advantage of high specific strength. Ferritic iron-aluminum alloys have high-temperature strength and fatigue strength that exceed those of Cr13 steel, but they exhibit low room-temperature plasticity and toughness, and are difficult to work through cold deformation. ③ Austenitic Fe-Al-Mn steels possess excellent comprehensive properties. III. Effects of aluminum on the physical, chemical, and mechanical properties of steel ① Adding aluminum to ferrochrome alloys can reduce their resistivity temperature coefficient, making them suitable as materials for electric heating elements. ② Aluminum and silicon have similar effects in reducing core losses in transformer steel. ③ When the aluminum content reaches a certain level, it causes passivation on the surface of the steel, granting it corrosion resistance in oxidizing acids and improving its resistance to hydrogen sulfide. Aluminum is detrimental to the corrosion resistance of steel in chlorine and chloride atmospheres. ④ After nitriding, an aluminum nitride layer forms on the surface of aluminum-containing steel, which improves hardness and fatigue strength as well as wear resistance. ⑤ Adding aluminum as an alloying element to steel can significantly improve its oxidation resistance. Aluminizing or alloying the surface of steel can improve its oxidation resistance and corrosion resistance, and it can be used in the manufacture of solar water heaters and similar products. ⑥ Aluminum has an adverse effect on hot workability, weldability, and machinability. IV. Applications of aluminum in steel ① In ordinary steel, aluminum mainly serves to remove oxygen and control grain size. ② As one of the main alloying elements, aluminum is widely used in various special alloys, including nitrided steels, stainless and acid-resistant steels, heat-resistant steels that do not scale, electric heating alloys, as well as hard and soft magnetic alloys.