Effect of Heat Treatment on Quenching of Steel
Heat treating steel is an important process that adjusts its properties based on the intended application. The process of hardening steel, called quenching, involves heating the steel to a specific temperature,insuring uniform temperature of through the cross section, and immediately cooling in oil, water or polymer. The rate of cooling, known as quench rate, depends upon the type of steel grade and the intended application and it is a key factor for achieving the desired properties. This paper will discuss the role of heat treatment and quenching on the properties and performance of steel by reviewing the literature on the process parameters in quenching.
Heat treatment of steel begins with controlling the temperature to which the metal is heated. Commonly, it is heated to a temperature slightly above the critical point (Critical Temperature). The goal is to achieve a uniform cross-sectional temperature for the entire shape before quenching. Depending on the type of steel, this Critical Temperature may range from 800°C to 1100°C. After increasing the steel’s temperature above the critical point, it is held at the temperature for an appropriate length of time to ensure that it passes through the austenitic phase and reaches the desired uniform temperature. This process is called “soaking”. The soak time will depend upon the thickness and grade of steel, but it is typically between one and five minutes depending on steel type. Once uniform temperature is achieved in all the phases, the steel is immediately quenched via immersion in oil, water or polymer.
The quenching medium should be selected carefully in order to achieve the appropriate quench rate. Mineral oil is one of the most widely used quenchants and it is self-cooling, meaning that it can be maintained at fixed temperatures between 40 and 70°C. Water may also be used but it can be more difficult to work with due to its very fast cooling rate and the need for active cooling equipment. Polymers, such as polyurethane, are also gaining popularity due to their excellent quenching characteristics and their ability to be easily manipulated to achieve various quench rates.
In order to achieve optimal quenching, the temperature of the quenching medium should closely match the starting temperature of the steel. The quenchant temperature should be slightly lower than the steel temperature in order to start the heat extraction process from the steel part and accelerate the cooling. The rate of cooling is known as the quench rate. The ideal quench rate is determined by the type of steel and its intended application. Lower carbon steels typically require slower cooling rates in order to achieve the desired strength and toughness, while higher carbon steels require higher cooling rates to achieve their desired properties. Quenching rates of 5-20°C/s are generally considered acceptable for typical steels and applications.
The quench rate also affects part distortion. Slower quench rates yield more uniform cooling, while faster rates lead to greater distortion. Distortion should be minimized in order to reduce the cost of machining parts to size.
Quenching can also be used to increase surface hardness. Lower carbon steels are often used in parts where mainly wear resistance is required and quenching is used to increase the surface hardness of the steel parts. Surface hardness can be increased using high quench rates and special quench agents, such as quench hardeners or polymer quenchants. These materials can be used to increase the surface hardness of the part from a Rockwell C scale of 20-30 up to 50 or even up to 66.
In conclusion, heat treatment and quenching of steel is an important process for obtaining desired properties for a part. Heat treatment involves heating the steel above the critical temperature and then quenching it in oil, water or polymer. The quenching temperature, quenching medium, and quenching rate are important parameters that should be carefully selected in order to achieve the desired properties. Quenching can be used to increase surface hardness, particularly in lower carbon steels, and to minimize distortion.
