Internal Structures of Metals Used in Welding Industry
What a metal looks like inside, at the atomic scale, has a great influence on its reaction to heat applied to it during the process of welding and cutting. The arrangement and location of the atoms is so small they cannot be seen even through the world's most powerful microscopes, yet these arrangements are responsible for most of its material properties. Strength is the main material property compromised from the heat of welding, but there are others as discussed in previous articles.
Materials can exist in four different states of matter. Solids, liquids, gases and plasmas are the four states of matter that materials on this earth can exist as. Metals are made from crystalline structures that are definite in pattern and shape. Each metal has its own crystal structure, but most metals are made from four predominant types. Body Centered Cubic (BCC), Face Centered Cubic (FCC), Hexagonal Close Packed (HCP) and Body Centered Tetragonal (BCT). Each metal has its favored type, but some materials, like carbon steel, fluctuate between different structures based on temperature and alloy content.
For example, at room temperature, mild steel is a BCC structure, but if raised above 1333F, the structure changes to FCC, and actually loses its magnetic properties. I have seen some wonderful science experiments that show this transformation in a macro scale, easily seen by the naked eye.
It is this change between crystal structures and phases, that allows carbon and alloy steels to respond to a wide range of heat treatment operations, including softening or annealing, hardening, quenching & tempering, stress relief and normalizing.
It is these changes in internal structure, brought about heat from the welding or cutting process, that causes changes to the mechanical properties of the metal we see and can measure. Some metals do not respond to heat treatment, for example, such as the 5XXX series aluminum alloys, but this does not mean their internal structure does not change.
All metals are made up of atoms, captured in the crystal structure of the materials. The atoms are in constant vibration from a number of factors, one being the amount of available heat, and other things like mechanical forces. The electrons orbiting the center of the atoms never stop.
The atomic forces of the atoms are what keep order in the crystal structure, and repel forces that try to disturb or relocate the atoms. As we add heat and energy to the metal, the atoms begin to vibrate rapidly, eventually the vibration in their crystal structure becomes too great, and the orderly structure disappears. In our welding hood, we see the metal change from solid to liquid. The liquid puddle is just a grouping of all these atoms that have broken free from their crystal structure bonds. Just as the heat caused them to break apart, removing the heat causes order, in the form of the crystal structure, to reoccur.
As the puddle cools, solidification occurs. When the crystal structure reforms, no atom returns to its original location. Depending upon how the atoms choose to relocate, has a great influence of the change to mechanical properties we see after a weld cools. It is this reassembly of the atoms into their orderly crystal structure that leads to distortion and warp age as well.
The rate at which the cooling occurs, in combination with the alloy content, also influences the grains and their size in the metal. Metals with coarse grains tend to have lower ductility and strength. On the other hand, fine grained materials have better ductility and strength. The rate of cooling, and alloy elements will influence the final grain size we get. Grains of metal are visible using a microscope. The process of metallography allows us to see the grains themselves, and measure their size.
Just like we see with our own eyes, excess heat is not our friend in most welding and fab operations. Being able to understand what is happening at an atomic scale tends to improve our overall knowledge and understanding that the importance of process selection becomes. Using the correct process, levels of heat input, and fabrication methods will give us the best quality welded product possible. In conclusion, heat causes stress in the crystal structure, and effects the size that grains form. Controlling heat input is a critical step when selecting the equipment and process to perform any weld.
Check out the Longevity website (www.longevity-inc.com) or YouTube channel (www.youtube.com/longevitywelding) for more details and information about equipment for different welding processes. Longevity has the right machine for your exact application, so take a look and choose what is the best fit for your materials, product and needs.