Low Magnification Organization of AISI/SAE 4140 and AISI/SAE 52100
Low-magnification organization of AISI/SAE 4140 (chromium-molybdenum-tungsten alloy steel) and AISI/SAE 52100 (chrome steel) after heat treatment—including quenching and tempering—was studied to define their microstructures and associated mechanical characteristics. To determine the effects of quenching and tempering, AISI/SAE 4140 steels are treated with quench rates of 5, 50, and 1000 degrees Celsius per second (5, 50, and 1000 K s⁻¹), and AISI/SAE 52100 steels are treated with quench rates of 5, 30, and 100 degrees Celsius per second (5, 30, and 100 K s⁻¹). Low-magnification optical microscopy was used to determine the grain size, grain boundary structure, and former shape of the ferrite grains. Scanning electron microscopy (SEM) revealed that both types of steel were eutectoid in annealed condition and proeutectoid ferrite in a tempering temperature range of 650 to 850 degrees Celsius (650-850 °C). The quench rate of 5 K s⁻¹ is found to produce the largest grain size and the lowest hardness. The quench rate of 1000 K s⁻¹ is found to produce the smallest grain size and the highest hardness. The hardness of AISI/SAE 4140 and AISI/SAE 52100 increases with increasing tempering temperature in the range of 650-850 °C.
The initial microstructure of AISI/SAE 4140 steel after annealing is characterized by relatively long ferrite grains of irregular shape surrounded by irregular mushy zone from the heavy quenching operation. X-ray diffraction (XRD) patterns reveal that the alloy is composed of ferrite, carbides, carbide islands, and untempered martensite at the quenched condition. After quenching, the hardness of the steel increased significantly due to the presence of untempered martensite and carbides. It was found that the hardness of the steel decreased with increasing tempering temperature as the precipitation of secondary phases occurs as the tempering temperature increases. XRD patterns show that the tempered steel consists of tempered martensite, ferrite, and carbides.
The initial microstructure of AISI/SAE 52100 steel after annealing is characterized by long, sub-rectangular ferrite grains surrounded by a broad heat-affected zone. The XRD pattern reveals the presence of ferrite, austenite, and proeutectoid carbon at the quenched condition. It was observed that the hardness of the steel increases with increasing quench rate. The quench rate of 100 K s⁻¹ is found to yield the highest hardness due to the presence of untempered martensite at the quench condition. It was found that the hardness of the steel decreases with increasing tempering temperature as the precipitation of secondary phases occurs as the tempering temperature increases. XRD patterns show that the tempered steel consists of tempered martensite, ferrite, and austenite.
In conclusion, the hardness of AISI/SAE 4140 and AISI/SAE 52100 steels after heat treatment—including quenching and tempering—increases with increasing quench rate and tempering temperature. The hardness of AISI/SAE 4140 and AISI/SAE 52100 steels can be significantly changed by the quenching and tempering processes. The quench rate of 5 K s⁻¹ and tempering temperature range of 650-850 °C are found to yield the optimal hardness of the steels. Low-magnification optical and scanning electron microscopy show the microstructural changes associated with the quenching and tempering processes.
