The Causes of Fissure Formation in Cooling of Steel
When steel passes through the molten state, it cools and solidifies and forms a solid steel. As the cooling proceeds, a variety of internal stresses and strains are created due to uneven contraction, which can lead to fissures appearing along the steel structure. Fissures are the result of inhomogeneous cooling, which in turn results from uneven heating or from various imperfections in the steel or from the steel being confused or bad. It is also possible for certain metals with different thermal expansion coefficients to undergo different cooling rates when welded together, creating an environment for differential stress to occur.
The most common cause of fissure formation in cooling steel is from the rapid cooling of the steel from an elevated temperature. This is often caused by the introduction of water or other mediums (such as oil) which are used to cool the steel in order to hasten the process. The speed of cooling is too fast to allow the internal stress to dissipate slowly, resulting in high temperature gradients and internal tensions that can produce fissures. Additionally, weldments may experience higher than expected temperatures as a result of inadequate preheating, which can also lead to fissures.
The characteristics of the steel itself is a significant factor in producing fissures. The steel’s shape, size, composition, grain structure, and thermomechanical properties are all integral in its behaviour when undergoing cooling. The basic structure of pearlite or ferrite applied in its grains gives it the capability to resist internal stresses, by allowing energy released from contraction to be absorbed. If this basic structure is altered or replaced by a alloy for instance, the cooling steel can be particularly more susceptible to suffering fissures.
Finally, impurities in the steel can lead to weakness in the steel and can contribute to the onset of fissures. The presence of impurities weakens the metallurgical bond between the elements of the steel, leading to accelerated cooling. This accelerates the internal stresses which form cracks.
In summary, fissures resulting from rapid cooling of steel can occur due to numerous reasons. This can include the use of a medium to cool the steel, inadequately preheated welds, the presence of certain impurity elements or an defect in the steel. Furthermore, certain modifications or changes in the structure of the steel can cause the structure of the grain to become weakened and susceptible to fissures. In order to avoid fissure formation in cooling steel, it is essential to take precautionary measures that ensure sufficient heated surface area and preheating, as well as the selection of materials that have suitable properties to resist internal stresses during cooling. Additionally, properly designed and installed steel cooling systems must be used and maintained according to maintenance instructions.
When following these guidelines and strict quality management regulations, fissure formation in cooling of steel should be minimal.
