The effect of cooling medium characteristics and cooling rate on the hardenability of steel
Hardening of steel, also known as quenching, is an important process used to manufacture products with improved mechanical properties. In the hardening process, heat treatment is performed by heating the steel to a certain temperature then cooling it quickly. The cooling rate and the characteristics of the cooling medium are crucial for determining the success of the hardening process. In this article, the effect of cooling medium characteristics and cooling rate on the hardenability of steel will be discussed.
1. Cooling Medium Characteristics
Still used in steel hardening today, quenching with oil is the traditional method of cooling. Oil quenching can result in improved mechanical properties of steels due to its moderate cooling rate, ranging between 40 to 70 degrees Celsius per second. Oils are usually chosen because of their wide operating temperature range (40 to 400 degrees Celsius) and good lubricity. This provides protection against abrasive wear of the surface layers due to expansion and contraction of the heated steel.
On the other hand, water is a more commonly used cooling medium for hardening steel due to its high cooling rate. As water can reach up to 520 degrees Celsius per second, it is suitable for cooling steels which require a higher cooling rate such as high carbon steels. Despite its high cooling rate, water also possess its other advantages such as good availability and low cost.
2. Cooling Rate
The cooling rate is another factor that must be taken into account when hardening steel. A faster cooling rate will result in higher strength and hardness of the steel, whereas a slower cooling rate will result in lower strength and hardness of the steel. The optimal cooling rate depends on the type of steel being used. For example, a higher cooling rate is required for high carbon steels and a slower cooling rate is suitable for more ductile steels due to their greater sensitivity to hardening.
3. Hardenability
Hardenability is an important measure to determine the success of hardening steel. It is defined as the ability of a given steel grade to harden in depth following a specific quenching treatment. It can be assessed by measuring the hardness of the steel after hardening. It is essential to know the hardenability of steel before hardening in order to achieve the optimal mechanical properties.
It is evident that the cooling medium characteristics and cooling rate have a significant effect on the resulting hardenability of steel. A higher cooling rate is required to achieve higher hardness whereas a slower cooling rate is suitable for more ductile steels. In addition, the characteristics of the cooling medium should be taken into consideration to help improve mechanical properties. The use of an oil quenching medium can improve hardness and abrasion resistance due to its lubricity. On the other hand, water is a more commonly used cooling medium thanks to its high cooling rate.
