Manufacturers and industrialists who work with metal have many things to worry about. They must select the correct alloy for the job and make sure that it possesses all of the required properties for the particular application. They must reduce waste, whether in terms of time, materials, or costs, and they need to ensure the finished product meets all relevant standards. Of course, those of you who work with aluminum and stainless-steel sheet or plate know one more thing must be added to that list as well: stress relieving.
What do we mean by metal stress?
People hear the word stress and often assume it refers to a general anxiety, something we all have to deal with in our everyday lives. That’s obviously a very different kind of stress than what we mean in metallurgy. In mechanics, stress is a measure of the internal forces that particles within a continuous material exert on each other. When stress occurs, we evaluate its severity by what is referred to as strain, which is the measure of the deformation of the metal.
As an example of what we are talking about, imagine a solid vertical bar that is supporting a weight. Stress is a way to measure the downward force of each particle within the bar as it pushes on the particles immediately below it. When writing out a scientific description of stress, it is generally represented using a lowercase Greek letter sigma (σ).
When working with any type of metal, but in particular with stainless steel and aluminum sheets and plates, the amount of stress that the material will encounter is an important consideration. Any time a metal is subjected to an applied force or load, stress will occur. Manufacturers need to know not only how much stress the material can endure, but how it will react to different levels of stress.
What different kinds of stress are common when working with aluminum and stainless steel?
Even during the processing state, steel and aluminum undergo stress. Cutting, machining, and welding, for example, all create stress. It’s important to anticipate what forms of stress a material will be faced with and to make all necessary preparations to ensure the metal is able to withstand the stress in a satisfactory manner.
One of the main types of stress is known as residual stress. These are stresses that remain in a solid material after the original cause of the stress has been taken away. They can be the result of various mechanisms, including inelastic deformations, temperature gradients, or structural changes. For example, heat from welding can induce localized expansion, which might impact the molten metal or the placement of the parts that are being welded together.
Residual stress can have a positive or negative impact on the material, depending on the desired effect. It can influence how the alloy will be affected by corrosion or metal fatigue and can also change a metal’s breaking strength. As a positive example, laser peening imparts beneficial compressive stresses in metal turbine fan blades.
When we refer to a material’s tensile strength, this indicates how well it can withstand tension, which is a form of structural stress. Tension is stress that is placed on a material by pulling from either end, as opposed to force applied by pushing or some other means. Tensile strength is defined as the capacity of a material to withstand the elongating forces.
Another form of stress is known as thermal stress. It is created by a change in temperature to a material that results in a fracture or plastic deformation. Thermal stress can be caused by temperature gradients, thermal expansion or contraction, or thermal shocks. Metal will expand when its temperature is raised and contract as the temperature decreases. Every material has a thermal expansion coefficient, which is a measurement of how the changes in temperature will affect the material. In general, the greater the change in temperature, the higher the level of stress.
What is metal stress relieving?
Stress relieving is a form heat treating, also known as annealing, that describes a variety of processes designed to alter the physical, and perhaps even the chemical properties, of a metal in order to increase its ductility and reduce its hardness, so that it is more workable. The process usually involves heating the material to a temperature at which the yield strength has become so low that the internal stresses can relieve themselves. This will generally mean subjecting the metal to high temperatures for a period of time, but not so high that there will be a change of phase in steel.
At the molecular level, the heat-treating process causes atoms to migrate in the crystal lattice, so that the number of dislocations goes down. This leads to changes in ductility and hardness. During the cooling process, recrystallization occurs. Following this process, further hot or cold working can alter the metal’s structure. This means that heat treatments can be used to achieve desired properties, a very useful tool for manufacturers.
What are some considerations when stress relieving plate and sheet?
There are a number of considerations you have to make when stress relieving plate or sheet, whether you’re dealing with aluminum or stainless steel. For example, when working with steel or iron, the temperatures will range from 1000 to 1300F. The temperature needs to be below whatever point the austenite phase begins to form.
Another consideration, especially important when dealing with sheet or steel, is that an open fire furnace can cause surface oxidation. This might mean discoloration at lower temperatures, or a fine scale when working with sub-critical temperatures. Furthermore, if the final surface finish is of importance, it’s possible to avoid oxidation through the use of a nitrogen atmosphere, or with a vacuum.
I know this is a lot of information, and in reality, it’s only scratching the surface when it comes to stress relieving. Fortunately, at Clinton Aluminum and Stainless Steel, our specialists fully understand the heat-treating process and how it affects various alloys of plate and sheet. We pride ourselves on building strong relationships with our clients and helping them to succeed.
Contact us today to learn more about our many plate and sheet options.