Hit it again, harder

When finishing parts, shops may want to hit them with their best shot. While commercial shot peening equipment looks and is used like the small abrasive blasting machines most shops use to clean, remove slag, scale and corrosion, or provide texture on parts, the process has other purposes: to significantly improve part performance and life.

In the process, metallic, glass or ceramic shot impacts a surface to plastically deform it. Essentially, each particle functions as a tiny ball-peen hammer. A peened surface spreads plastically, changing its mechanical properties.

Multiaxis, robotic, closed-loop process control shot peening of an aircraft engine component. Image courtesy of Progressive Technologies.

Shot peening is widely used on highly stressed parts to remove detrimental tensile surface stresses and replace them with compressive stresses that resist operating forces and cyclic stresses. It is one of the easiest, least expensive and safest part finishing processes, and accommodates almost any part geometry and work material.

With proper peening, heavily stressed parts can withstand higher cyclic stresses, reducing the potential for fatigue fracture and extending part life. In turn, the higher fatigue strength may allow specifications of slightly lighter parts, which saves material and transportation costs for conventional parts and reduces jet fuel consumption in aircraft applications.

Repeated cycles of twisting, bending or flexing a part cause fatigue failures if loads are too high. Fatigue cracks originate from surface flaws, but peening closes them; in some instances, it actually welds them together, thus eliminating them as immediate sources of cracks and failures.

Shot Peening Applications

The most common users of shot peening are producers of automotive transmission gears, highly stressed coil and leaf springs, welded joints and turbine blades. Peening is particularly useful on turbine blades because they operate continuously under large centrifugal forces that try to tear the blades apart. Chemical plants use peening to reduce stress-corrosion cracking in piping and vessels subject to stress corrosion. Fastener makers use it to reduce fretting corrosion. Shot peening is also used on aluminum die-cast transmission housings and gearboxes to close pores, preventing lubricant loss through the porous walls.

Shot peening effectiveness depends upon several factors. Longer peening time increases surface hardness and compressive stresses—up to a point. Shot size, shape, density, hardness and velocity are major factors in how the shot impacts the surface and in the effects of the peening process. Distance of the nozzle from the part affects particle velocity at impact.

Courtesy of Electronics

Coil springs are one of the common parts that are shot peened to increase their life.

The impact of shot size and velocity that each particle delivers to the part is expressed by the equation E=1/2mv2, where E is the energy produced when an individual shot particle hits, m is the mass (weight) of the shot particle, and v is the velocity at which it hits the part. Most of that energy stretches the metal at the point of impact beyond its elastic limit so the part stores the energy of the impact as plastic deformation at and just below the part surface. That plastic deformation causes compressive stresses.

Aerospace Materials Specification 2431 describes seven types of peening media: cast iron shot, cast steel shot, carbon and stainless steel cut wire, peening balls, glass shot and ceramic shot. Shot media must be as hard or harder than the workpiece. Peening needs smooth-edged (normally almost round) particles to provide uniform plastic deformation.

Cast steel shot varies in hardness from 45 to 52 HRC, so it is used for parts softer than 45 HRC (a 55 to 62 HRC version of cast shot is also available). Carbon steel cut wire lasts longer than cast materials, so it is more economical in many instances. Stainless steel cut wire is used where carbon steel would leave corrosion-causing particles.

Typical shot blasting media comes in standard sizes (Table 1). Peening balls vary in size from 1⁄8 ” to ½ “. Cut steel wire shot is defined by cut wire numbers from 20 to 62, with a CW20 corresponding to a 0.020 “-dia. wire, each about 0.020 ” long. Glass and ceramic beads are chemically inert and use different SAE, Mil Spec and company size numbering systems. Glass beads are available from 0.0015 ” to 0.0661 ” in diameter. The smaller sizes allow the shot to reach into small crevices and root sections. Ceramic shot ranges from 0.008 ” to 0.046 ” in diameter and lasts longer than glass. Detailed information about glass beads is available on the Media Blast & Abrasive Inc. Web site: www.mediablast.com/glass-bead-conversion.asp.

After shot has been used once, it begins to break down into smaller particles. Because used shot is not of uniform size, it must be continuously sieved to remove undersized particles.

Shot Blasting Equipment

Tyler Cotton, president of Blast Abrade Inc., Elyria, Ohio, said small, simple shot blasting equipment costs about $30,000, while a larger, more automated machine can cost up to $250,000. Heavy-duty, large-scale, high-production units may cost several million dollars. All machines need special screening classifiers and air-wash separators to remove undersize media and dust from the shot operating mix.

The cost of shot media varies. The least expensive is glass-bead shot, which costs less than 40 cents/lb. Cast steel shot typically sells for 35 to 45 cents/lb., depending on the size grade and order quantity. Hard cast steel shot may have a process and price premium of 6 to10 percent. Carbon steel cut wire shot costs about $1.50/lb., while stainless steel cut wire shot costs about $3.50/lb.

Table 1: Cast steel shot sizes and equivalent cut wire shot sizes.