| Brazing Fundamentals In our last issue, we began to explore the effects that thermal expansion of metals has on joint strength and quality. We will continue that discussion in this edition. We mentioned last time that all metals expand as they are heated. Occasionally, we encounter strange occurrences that seem to challenge this fact. This is the case with certain alloys such as 1018 carbon steel, 4130alloy steel and even 17-4PH. When 1018 steel is heated, it actually begins to contract or shrink (rather than expand) from around 1300° F (710° C) until it reaches about 1500° F (815° C); at which point it begins to expand again. (See Figure 2, p.2). 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 lattices structures are (see Figure 5a'): body-centered cubic (BCC) and face-centered cubic (FCC).
Some metals such as iron (and thus, 1018 steel) 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 in 1018 steel is BCC at room temperature, but changes to gamma-iron (FCC) when heated to just above 1300° F/710°C (see Figure 5b(2). 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 (see Figure 6(3), 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. However, once the phase change has been completed, further heating will cause the steel to once again 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 will lead to major distortion problems (and even scrapped parts) if these changes are not taken into account! For example, suppose you were brazing 1018 steel to Inconel* 600. As you heated the assembly above 1400°F (760°C), the Inconel would continue to expand while the steel contracted (shrunk). 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 1 400°F (760°C) 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 stretch. This would then result in the tube buckling (distortion) at brazing temperature and also upon cooling. To prevent this situation from becoming a problem when you are brazing assemblies that contain polymorphic materials, we recommend the following: (1) thermocouple the assemblies adequately, so that you can accurately monitor the temperature of the different base metals involved, as well as of the thinnest and thickest cross-sections; (2) include a built-in "hold" into your programmed furnace cycle at the beginning of the transition zones on both heating and cooling, in order to allow all parts to achieve thermal equilibrium before proceeding further; and (3) move through the transition zones slowly, monitoring your thermocouples for thermal equilibrium. This may add extra time to your furnace cycle, yet it can make the difference between producing good parts, or scrap! In our next issue of Nicrobraz News, we will take a closer look at the use of thermocouples - which kind you should use, how it should be connected to the part, where it should be placed in the furnace, etc.
*Inconel is a trademark of Inco Alloys International, Inc. |
