In the 11th century, every combatant carried a sword. Swords were thick and heavy to prevent breakage, making them unwieldy in battle. Those who were wealthy could buy a lightweight, virtually indestructible sword blade from Toledo, Spain. Over the ages the secret of their manufacture was not revealed until the 1970s, when X-ray diffraction detected an induced compressive stress layer on the surface. Further examination revealed subsurface patterns made by repeated blows with a ball-peen hammer. In the 1940s it was discovered how to cast large quantities of uniform steel shot—0.062 in. or less in diameter—that could be used as an abrasive to descale steel. Engineers at General Motors discovered that the process involved cold working and “peening” the surface, thereby providing a permanent compressive stressing of the part.
Shot peening is a cold-working process. The surface under treatment is bombarded with small spherical pieces called shot. As each shot hits it is, in effect, a minute ball-peen hammer dimpling the surface. When the dimples are created the surface fibers yield in tension, the principle being that under the dimple is a hemisphere of highly stressed cold-worked material in compression. Cracks cannot initiate or propagate in compressively stressed zones. The layer created improves the load carrying capacity by increasing the tooth bending fatigue strength. Other benefits include reducing surface fatigue and, in slowing the subsurface crack propagation, pitting is also reduced. The total amount of stress may be increased exponentially. However, the internal stresses cannot break through the compressive barrier created from the process.
The potential benefits for gears vary with the type of gear and its ultimate use. Current standards state that the process should only apply to carburized and case-hardened gears, although gears that are through hardened, induction hardened, and made from ADI can also benefit from shot peening. Applications have a wide range, from automotive, heavy vehicles, and marine to mining and power tools.
ISO gives values based on material quality of 0 percent for ML, 10 percent for MQ, and 5 percent for ME. A proposal from the U.K. to ISO suggested as a general guide high quality gears gained the least, perhaps 15 percent in fatigue strength; gears of lower quality based on machining/heat treatment/material gain most—perhaps 25 percent. Lloyds of London’s Register of Shipping allows an increase in tooth loading for both wear and strength of up to 20 percent with controlled shot-peened gears. Det Norske Veritas marine standard for quality gears (no grinding notches, significant surface decarburization) states that “we add 20 percent to the fatigue limits.” NASA has also conducted extensive test programs to verify the benefits for shot peening gears.
As a general rule, the hardness of the shot should be at least as hard as the gear tooth material. The authority Roy Kern, writing in Heat Treating on solutions to gear failures by use of shot peening provides an example on pinions of 61 to 60 R.C and shot “about 45 to 50 R.C… this was not supposed to work but it did. I have seen at least one other carburized part failing by fatigue, and peening by soft shot solved the problem.” Kern also added “the long-life fatigue strength was improved several fold.”
Calibration of the peening intensity of the shot stream is a function of the shot size, material, hardness, velocity, and impingement angle. J.O. Almen, of General Motors, developed the approved method using one of three standard Almen strips. These SAE1070 spring steel strips are bombarded for a set time period, which results in curvature of the strip. The arc height is measured using an Almen gauge. Control of the depth of compressive stress is very important because the engineer must take other factors into consideration, such as wear, surface finish, and design loads, which result in a tensile stress lower than the level of compressive residual stresses.
One of the latest developments is the use of lasers to generate a shock wave that creates compression on the gear surface. The compressive stresses are much deeper than those achieved by shot peening, providing even more protection against fatigue and corrosion cracking. The process is more expensive and takes longer, and it can be used in conjunction with shot peening. The use of shot peening is growing more popular as engineers realize that increased strength can be obtained without increasing the size of the gear.