It is getting hot outdoors, and what better time to discuss annealing than late July? Making use of heat to increase the ductility of material has been commonplace in the metals industry since blacksmithing in the medieval era. Each of the 150 alloys manufactured at Ulbrich Stainless Steel & Special Metals, Inc. (USSM) responds differently to cold work, but all material has a forming limit under specific forming conditions. Once the forming limit is reached, or maximum strain is induced (please reference my April blog post on mechanical testing), failure will occur. This concept presents a problem when we examine the precision cold rolling process. For instance, assume the goal is to reduce the thickness of annealed UNS 30100 to .0050”. Beginning with a coil that is significantly thicker than .0050”, we may reach the forming limit prior to achieving the desired thickness.The coil will inevitably fracture if we do not soften the material at some intermediate gauge. At USSM, this is accomplished via continuous bright annealing.
Continuous annealing strip involves placing the material in coil form on a mandrel, unwinding the coil, allowing the material to traverse an electric or gas-fired furnace, air quenching, and subsequently winding the material back into coil form. “Bright” annealing indicates that the aforementioned process is performed in a protective atmosphere. The descaling process is nullified since the material is unable to form its oxide in the absence of oxygen. Continuous bright annealing incorporates more precision than other annealing processes, particularly bell annealing, due to the fact that each and every foot of the material is subjected to the same temperature for the same amount of time. In a bell type annealing furnace, the material is annealed in coil form and heat must be transferred from the outside wraps inward; consequently, the outer wraps of the coil are heated more intenselythan the inner wraps for a longer period of time.Continuous bright annealing alsoallows for wide-ranging process control. Temperature, run speed, dewpoint, and atmosphere are all monitored in real-time.
If you are like me, this is the point at which you begin to channel your inner nerd. Most of us are familiar with the concept of using heat to soften, expand, or melt material, but what transformations occur at the microscopic level that result in mechanical property changes? Supplying a metal with thermal energy via the annealing process will promote recovery and recrystallization provided there is significant driving force within the material itself. Driving force is essentially potential energy created by cold work. Grains appear circular and similar in size and shape in their lowest energy state. Grains that are heavily deformed to accommodate dimensional changes will seek to return to their lowest energy arrangement.Annealing provides the thermal energy that makes this transformation possible. In the recovery stage, internal strain energy is relieved. From this point, atomic rearrangement occurs and grains become uniform and equiaxed. This process is known as recrystallization. Any additional time at temperature or increase in temperature once recrystallization occurs will result in grain growth.
These concepts are not easily understood, so perhaps a visual aid will help. The microstructure of the same high-Nickel alloy in various conditions appears in the picture below. Picture “A” depicts the material after cold rolling. Grains appear elongated in the direction of rolling and nonuniform in size and shape. Picture “B” shows uniform, orderly, and equiaxed grains after the annealing process is complete. Picture “C” depicts larger grains that were grown via increasing annealing temperature.
What is the significance, you ask? Continuous annealing process control capabilities at Ulbrich Stainless Steel &Special Metals, Inc. enable us to work to a grain size range of two ASTM grain size (G) numbers. We are capable of creating a microstructure that will facilitate subsequent forming processes such as drawing, coining, and blanking. Our engineering department establishes process routings on an item-by-item basis to ensure that your product undergoes a consistent thermal treatment each time that it is manufactured. In summary, carefully defining annealing process parameters results in a highly consistent product that has been customized to your forming needs. By this method, Ulbrich Stainless Steel & Special Metals, Inc. provides material that excels in performance each and every time a new lot arrives on your loading dock.
For more information contact us: Ulbrich Stainless Steels & Special Metals, Inc.