SAE 1020 steel is one of the most widely used low-carbon steels in industry. Its relatively low cost, strength, and ductility make it attractive for many applications. In order to optimize SAE 1020 steel for those applications, heat treatment and carburizing processes are applied.
Carburizing is a process involving the diffusion of carbon into the surface of a metal. It is primarily used for steel parts that require a high surface hardness and wear resistance. The SAE 1020 steel is especially well suited for carburizing. The carbon diffuses into the metal greatly increasing its surface hardness while leaving the core of the metal unaffected.
Carburizing heat treatment involves the slow heating of the metal, generally at no more than 10°F per hour, to a temperature just above its critical temperature. This temperature range is known as the austenitizing temperature. Once the part is heated to this temperature, it is maintained at that level for a predetermined amount of time. The time of treatment depends on the depth of the treated steel. The longer the part is maintained at the austenitizing temperature, the deeper the hardness penetrates. In the case of SAE 1020 steel, the common austenitizing temperature range is between 855°F and 885°F.
Following the austenitizing cycle, the metal is quenched to a predetermined temperature. This temperature is determined by the desired core hardness of the SAE 1020 steel. Once the metal has been quenched to the desired hardness, it is then generally reheated and held at a lower temperature known as the tempering temperature. This tempering temperature determines the level of hardness and wear resistance of the steel. In the case of SAE 1020 steel, the tempering temperature is usually between 350°F and 400°F.
In addition to the above process, there are also several other process parameters that affect the mechanical properties of SAE 1020 steel. Such parameters include cooling rate, carbon concentration, and carbon penetration. Cooling rate is the rate at which the SAE 1020 steel is cooled from its austenitizing temperature to its tempering temperature. Carbon concentration is the amount of carbon that remains in the surface of the steel after the carburizing and quenching processes. And, carbon penetration is the depth at which the hardened layer penetrates into the steel.
In order to determine the expected properties of low-alloy steels such as SAE 1020 steel after carburizing, and other heat treatments, optical metallography techniques can be used. Such techniques allow for the assessment of the microstructure of the steel through the use of etching solutions. This can be done through a metallographic analysis of the steel following the heat treatment. In the case of SAE 1020 steel, it is generally found that the microstructure of the steel becomes a mix of ferrite, pearlite and martensite after carburizing and heat treatment. Typically, an optimal combination of hardness, tensile strength and ductility are achieved at a maximum carbon concentration of 0.15%.
Overall, carburizing of SAE 1020 steel is a common heat treatment to optimize the steel for various applications. The carburizing of SAE 1020 steel involves a slow heating of the steel to its austenitizing temperature and then quenching the steel in order to achieve a desired level of hardness. Optimal properties are generally achieved when the carbon concentration of the steel is between 0.10% and 0.15%. Metallograph analysis of the SAE 1020 steel reveals that the steel typically has a microstructure composed of ferrite, pearlite, and martensite.
