To understand this unusual expansion/contraction phenomenon, we need to look at the actual arrangement of atoms within the metal. In general, metals tend to form very symmetrical, densely packed crystal structures. Two of the most common types of regularly repeating crystal lattice structures are known as “body-centered cubic” (BCC) and “face-centered cubic” (FCC), using nomenclature many of you may remember from the past.
Metals such as iron (and thus carbon steels) have a unique property called “polymorphism”, which, simply put, means that the metal can exist in alternate crystal forms, depending on the temperature and pressure being applied to it. The alpha-iron phase in 1018 steel is BCC at room temperature, but changes to gamma-iron (austenite), which has an FCC crystal structure, when heated to just above 1300F (710C), as shown in Fig. 3 below.
While changing from a BCC to an FCC crystal structure during heating, two things occur that cause shrinkage of the metal lattice structure: (1) heat is absorbed during the phase change, and (2) the packing of atoms becomes more efficient in FCC, allowing more iron atoms to fit in a given space than in the BCC alignment. Once the phase change has been completed, further heating will once again cause the steel to expand. The opposite reactions occur when the steel is cooled.
These seemingly insignificant “reversals” in thermal expansion and contraction curves for alloys such as 1018 steel can lead to major distortion problems (and even scrapped parts) if these changes are not taken into account during the furnace brazing cycle! For example, suppose you were brazing 1018 steel to an Inconel® 600 component. As you heated the assembly above approximately 1250F (675C), the 1018 steel would begin to shrink, as it started to realign its internal structure from BCC to FCC, while the Inconel would continue to expand almost linearly during that same time. As long as the parts were free to move relative to each other, you might not notice a problem. However, suppose the parts were tack-welded prior to brazing. When the assembly exceeds about 1250F (675C) and the Inconel continues to expand while the 1018 steel shrinks, either the tack-welds may break apart (losing part alignment), or the 1018 tubing may be forced to yield (stretch). This could then result in the tube buckling (distortion) at brazing temperature and also upon cooling.