The Shrinkage of Steel during Solidification
Solidification of steel has caused problems since steelmaking began. Shrinkage, i.e. the reduction in volume that is often found in steel as it solidifies, can cause significant structural problems and lead to a weakness in the finished product. It is important to understand why shrinkage occurs and how best to prevent and minimise it, if possible.
When metal is heated, it increases in volume due to thermal expansion. Upon cooling and solidification, however, volume can decrease due to a number of causes. One factor that contributes to shrinkage is that atom packing becomes denser as solidification occurs, because atoms “fit” together more closely in their solid form. In addition, shrinkage is caused by the crystalline structure of the material, i.e. the distance between crystal grains. As the grains grow to form a solid, the material occupies less volume. Additionally, the chemical nature of the material also influences shrinkage, as chemical bonds themselves can cause contraction.
These various factors make it difficult to predict the degree of shrinkage in a given steel. The extent of shrinkage may vary depending on the type of steel and the way in which it is solidified, heated and cooled. The primary influence on shrinkage is the solidification process.
In general, two types of solidification are possible: i.e. progressive solidification and full solidification. With progressive solidification, cooling occurs in stages, allowing time for the solidification of a part while the section remains heated throughout. This process can reduce cracking and shrinkage.
In contrast, full solidification occurs when cooling occurs uniformly and rapidly. The rapid cooling leads to shrinkage and cracking, due to thermal stresses. As such, it is often necessary to control the cooling rate of steel during the solidification process in order to avoid shrinkage and cracking.
In addition to controlling the cooling rate of steel during solidification, it is also important to ensure that the steel is heated at a uniform rate. Uneven heating can lead to distortion of the steel as it cools, resulting in uneven shrinkage patterns. Similarly, controlling the composition of the steel can influence the extent and evenness of shrinkage. Alloying elements may affect the thermal and mechanical properties of the steel, and may in some cases, serve to inhibit shrinkage.
Shrinkage can be damaging to the structure, strength and durability of steel, and therefore it is important to understand that the causes of shrinkage. It is also necessary to ensure that the cooling and heating rates, alloy content and solidification technique used in steel fabrication all take into account the potential for shrinkage, and steps taken to prevent it.
