Shot peening is a cold working process that is designed to strengthen the surface of a material, to make it more resistant to tensile stress.
In shot peening, a surface is bombarded with small spheres (usually steel shot, glass beads, or ceramic beads) at high velocity.
When each piece of shot impacts the surface, it causes a minuscule dimple. Like a ball-peen hammer, these dimples cause the surface to expand. At the molecular level, beneath the surface, the material pushes back in resistance. This creates compressive stress. Like Sir Issac Newton said, for every action, there is an equal and opposite reaction. In this fashion, penning an object makes it stronger and less likely to develop surface cracks from metal fatigue or external processes.
By peening the surface of an object, a series of overlapping dimples are formed. These can be thousands or even millions of infinitesimally small dimples that create a pattern of compressive stress. The result is a much stronger part or object that is more resistant to tensile stress.
Compressive and Tensile Stress
Stress is the measure of internal forces between the molecules of an object. Tensile stress is the measure of forces that pull on the molecules of an object, causing it to expand. Compressive stress is the opposite of tensile stress.
Many types of manufacturing processes create tensile stress, such as heat treatment, grinding, milling, turning, bending, welding, or forming. Shot peening is used as a deterrent to the tensile stress that parts and components go through in industries like aerospace, automotive, and energy.
Examples of components that undergo shot peening are crankshafts, camshafts, connecting rods, engine valve springs, compressor and turbine disks, landing gear, coil springs, rock drills, ring and pinion gears, turbine blades, and aircraft wheels.
History of Shot Peening
Early blacksmiths would often use a ball-peen hammer to manually peen the curved side of a leaf spring.
Modern shot peening became a practice in both the United States and Germany in the 1930s. In 1945, an inventor named John Almen patented a way to measure the intensity of a peening process. Today we still use these devices, called the Almen strip and the Almen Gage.
Modern Shot Peening: The Almen Strip
The Almen Strip is a thin strip of carbon steel SAE 1070 with a hardness of about 45 on the Rockwell scale. The strip is fasted to a flat block with four screws, and one side of the strip is peened. By this process, compressive stress is placed into the surface of the strip. When the strip is detached from the block, the strip bows into an arc, since the thin metal cannot resist the compressive stress. The Almen Gage is then used to measure the arc of the strip.
Here’s where it becomes useful. By peening the Almen strip for varying times, manufacturers can create a saturation curve. This is a graph where arc height (or displacement) is the Y-axis on the graph, and the peening time is the X-axis. The intensity of the shot stream is evaluated onto the “best fit line”. You can find this by looking at the point where the peening time doubles, and there is a 10% or less increase in arc height.
By utilizing the same peening process, manufacturers can get standardized results from their shot peening.
Almen Strips come in three varieties: A type (for steel shot or wire), N type (for ceramic or glass beads), and C type (thicker material, and more rarely used).