Introduction to the shot peening process

Shot peening is applied to components subject to cyclic bending or torsional loads, where fatigue resistance is critical. The most common applications are:
• Automotive and transmissions: gears, pinions and ring gears, cams, crankshafts, connecting rods, torsion bars, diaphragms and clutch discs.
• Springs: coil springs, valve springs and leaf springs.
• Oil and gas: sucker rods, threaded pipe connections and drilling-tool shafts.
• Aerospace: aircraft components and axial compressors.
• Mining and tools: mining components, rock drills, piston rods, pins and chain side bars.

In shot peening the media must be spherical: only a rounded particle produces the hammering effect without damaging the surface. Under that single condition, the most used media are the following.

Cast steel shot (spherical)

• Carbon or stainless steel; with proper heat treatment it combines good hardness and acceptable fracture resistance.
• More economical than cut wire shot.
• Standard sizes per SAE J444.

Conditioned Cut Wire (CCW)

• Made by cutting wire into cylinders and then conditioning it (rounding the edges) to a nearly spherical shape.
• The conditioning grade is key: it ranges from as-cut wire (CW) —cylindrical, with sharp edges— to fully conditioned, virtually spherical. The higher the grade, the more spherical the particle, the longer its life and the better its fatigue performance (usually designated in increasing grades, e.g. G1 → G2 → G3 depending on the manufacturer).
• Excellent hardness with very low breakage → low consumption and constant grain size.
• Standard sizes per SAE J441.

Cast iron shot (spherical and nodular)

• Gray, white and malleable iron; a low initial-cost option.
• Shorter life than steel due to its greater brittleness (breakage is critical in shot peening).
• Nodular iron is used very sparingly: low hardness, poor intensities and leaves graphite residue.

Glass or ceramic beads

• For low intensities and forming of thin sheets.
• Suitable for stainless steel and non-ferrous materials that must not be contaminated with carbon-steel media.

The effectiveness and quality of shot peening depend on the strict control of a set of variables: media size and hardness, impact speed, intensity, impact density (coverage), projection angle and distance, and the media breakage rate.

Media size
• Defines the kinetic energy of the impact. The larger the size, the higher the intensity but the lower the impact density.
• Always choose the smallest size that achieves the desired intensity → faster process and better coverage.
• The diameter must be compatible with the smallest radii of curvature of the part.

Particle hardness
• As long as it is harder than the surface, it does not affect intensity.
• It should be only slightly harder: if too hard, it becomes brittle and breakage increases.
• If softer than the surface, the achieved intensity drops.

Particle speed
• The higher the speed, the greater the kinetic energy and the higher the intensity.
• But excessive speed increases particle fracture, which curbs the real intensity gain.

Projection angle and distance
• The ideal angle is 90° (perpendicular); as it decreases, intensity drops.
• If geometry requires less than 90°, it is offset by increasing media size and/or speed.
• The greater the distance, the lower the intensity: a value is set and kept throughout the process.

Breakage rate
• Only spherical particles should impact; broken ones are removed from the circuit automatically and quickly.
• A high percentage of spherical media must be maintained to sustain the required intensity.

Shot peening intensity is measured with a standardized test: the Almen test (SAE J442). It consists of exposing a thin metal strip (Almen strip) to the particle flow; the hammering compresses the grains on the exposed face and increases its area, while the opposite face keeps the original. That difference bends the strip, and the height of the resulting arc is the measure of intensity. For a saturation impact density, intensity depends on the speed, size and hardness of the particle, and on the projection angle and distance. As a rule, the lowest intensity capable of producing the intended effect is the most efficient and economical.

The Almen strip and the N, A and C ranges
• A standard Almen strip is used, fixed on a base with four support balls.
• Three ranges according to strip thickness: N, A and C (from lowest to highest intensity).
• The result is expressed with the gauge number + the strip letter: e.g. 13 A = intensity 13 on an A-type strip.
• The gauge is graduated in thousandths of an inch (0.025 mm).

Impact density (coverage factor)
• Indicates what percentage of the surface was impacted by the particles.
• Saturation is reached near 100%; with insufficient coverage the intended fatigue improvement is not achieved.
• Coverage grows with exposure time according to Cₙ = 1 − (1 − C₁)ⁿ, where C₁ is the coverage of one cycle and n the number of cycles.
• Since measuring above 98% is imprecise, that value is taken as the saturation reference and coverage is expressed as a multiple of the time needed to reach 98%.

Saturation point
• The arc-height vs. exposure-time curve is plotted at different times.
• The saturation point accepted in industry is reached when, on doubling the exposure time, the arc height increases by less than 10%.

The main application of shot peening is increasing fatigue resistance, but the process also provides other benefits and has usage conditions worth respecting.

Increased fatigue resistance
• Considerably raises the fatigue life of springs, strips, gears, bars and tools (cutters, drills, punches, dies).
• It is the intended effect on any component subject to cyclic bending or torsional loads.

Metal forming (peen forming)
• Allows thin sheets to be formed in a controlled way, without the high residual stresses of cold or hot mechanical forming.
• Widely used in aerospace structural components, such as fuselage skins.

Removal of residual stresses
• Transforms the residual tensile stresses —left by heat treatment, forming or machining— into a uniform compression across the whole surface.

Increased corrosion resistance
• By closing the intergranular spaces and removing tensile stresses, it lowers the system's energy and, with it, the tendency to corrode.

Correct sequence and subsequent processes
Shot peening is always done after heat treatment and grinding, never before (the exception is applications on weld zones). In addition, treated parts must not subsequently undergo mechanical deformation, heating (except very mild), machining or polishing, since the compressed layer is very thin; only a surface passivation is allowed as corrosion protection.

For further reading: SAE J441, J442, J443, J444, J445 and J827, and the SAE HS-84 “Manual on Shot Peening”.