Shot peening is one of the most critical surface enhancement processes in modern manufacturing. Unlike grinding, polishing, or buffing — which remove material to improve surface finish — shot peening intentionally modifies the subsurface metallurgy of a component by inducing compressive residual stress. This compressive stress layer is the single most effective means of improving fatigue life, resisting stress corrosion cracking, and preventing fretting fatigue in metal components subjected to cyclic loading. The choice of peening media — steel shot versus ceramic shot versus glass bead — directly determines the intensity, coverage, surface finish, and process economics of the operation. This guide provides a comprehensive engineering analysis of shot peening media types, specifications, process variables, and selection criteria.
This guide is written for manufacturing engineers responsible for process specification, quality engineers overseeing peening compliance, metallurgists evaluating fatigue performance, and procurement professionals sourcing peening media. It assumes familiarity with basic metallurgy and surface finishing concepts. For a broader introduction to mass finishing media, see our Mass Finishing Media Guide.
What Is Shot Peening?
Shot peening is a cold-working surface treatment in which small spherical, cylindrical, or conditionally angular particles — collectively called "shot" — are propelled at high velocity against the surface of a metal workpiece. Each particle that strikes the surface creates a tiny indentation, or "dimple," plastically deforming the surface layer. The surrounding material resists this deformation, and the result is a layer of compressive residual stress that extends from the surface to a depth typically between 0.05 mm and 0.50 mm, depending on process parameters.
The Physics of Compressive Residual Stress
When a shot particle impacts a metal surface at high velocity (typically 20–100 m/s), the kinetic energy of the particle is transferred to the workpiece as plastic deformation. The surface layer yields in compression under the contact force. Because the subsurface material remains elastic, it constrains the plastically deformed surface layer as it tries to recover. This constraint is what produces residual compressive stress in the near-surface region, balanced by a smaller tensile residual stress in the deeper material.
The compressive stress profile has three characteristic parameters:
- Peak compressive stress — typically 50–80% of the material's yield strength, occurring at a depth of 0.02–0.10 mm below the surface.
- Depth of compressive layer — the distance from the surface to the point where residual stress transitions from compressive to tensile, typically 0.10–0.50 mm.
- Surface compressive stress — the stress value at the very surface, which is typically lower than the peak but still significantly compressive.
The presence of this compressive layer is what makes shot peening so effective at improving fatigue performance. Fatigue cracks initiate at the surface where tensile stress is highest during cyclic loading. By converting the surface from a tensile-dominated state (after machining, grinding, or heat treatment) to a compressive state, the crack initiation threshold is raised, and the effective stress range seen by the material is reduced. Documented fatigue life improvements range from 2x to over 10x, depending on the material and loading conditions.
Shot peening typically extends fatigue life by 200–1,000% for high-strength steels, 100–500% for aluminum alloys, and 50–300% for titanium alloys. The improvement is most dramatic for parts with stress concentrators like fillets, threads, notches, and spline roots.
Peening vs. Blast Cleaning: A Critical Distinction
It is essential to distinguish shot peening from blast cleaning. Both processes involve propelling particles at a surface, but their objectives, process controls, and quality requirements are fundamentally different:
| Parameter | Shot Peening | Blast Cleaning |
|---|---|---|
| Primary Objective | Induce compressive residual stress | Remove contamination, scale, or coatings |
| Media Shape | Strictly spherical (roundness ≥ 90%) | Angular or spherical (no shape requirement) |
| Media Specification | AMS 2430, AMS 2432, SAE J444 | SAE J199, SSPC-SP standards |
| Process Control | Almen strip intensity, 100%+ coverage | Visual inspection, surface profile |
| Hardness Tolerance | Tightly controlled (±2 HRC) | Not critical |
| Documentation | Full traceability required (AMS 2430/2432) | Minimal documentation |
Using blast-cleaning media for shot peening is one of the most common and dangerous process errors in the industry. Angular particles create stress risers instead of compressive stress, dramatically reducing fatigue life rather than improving it. This is why AMS specifications require strict media qualification and traceability.
Shot Peening Process Fundamentals
Understanding the shot peening process requires grasping several interrelated variables that together determine the peening intensity and coverage achieved on the workpiece. These variables form the "peening equation" — a set of parameters that must be controlled and documented for every peening operation, especially in aerospace and automotive safety-critical applications.
The Four Core Process Variables
Every shot peening process is defined by four primary variables. Changing any one of these changes the peening intensity, which must be re-verified with Almen strip testing:
1. Media Size and Type
The shot diameter, material, hardness, and density directly determine the energy delivered per impact and the dimple size. Larger shot produces deeper compressive layers; harder shot produces higher peak stress but also more surface roughening.
2. Media Velocity
The kinetic energy per particle is proportional to mass × velocity squared. In air-blast systems, velocity is controlled by air pressure (typically 20–80 psi / 1.4–5.5 bar). In wheel-blast systems, it is controlled by wheel speed and blade geometry.
3. Impact Angle
Maximum energy transfer occurs at 90 degrees (normal incidence). Angles below 45 degrees produce a glancing impact that creates less compressive stress and more surface shearing. Most peening specifications require impact angles of 75–90 degrees.
4. Exposure Time
The total peening time determines coverage — the percentage of the surface that has been impacted at least once. Full coverage (100%) is the minimum requirement for fatigue-critical parts, typically achieved at 98% coverage based on visual or peening-scanner inspection.
The Saturation Curve
Peening intensity is not linearly proportional to exposure time. When shot first impacts a virgin surface, each impact produces maximum plastic deformation. As coverage increases and the surface becomes increasingly work-hardened and textured, subsequent impacts produce less additional deformation per impact. This relationship is described by the saturation curve.
The peening saturation point is defined per SAE J443 as the exposure time at which the Almen arc height increases by no more than 10% when the exposure time is doubled. The intensity specification (e.g., "0.012A" meaning 0.012 inches of arc on an A-scale Almen strip) is always defined at saturation. Peening beyond saturation produces diminishing returns — additional compressive stress gain is minimal, while surface roughening, surface damage, and over-peening risk increase.
Peening well beyond saturation (typically >200% coverage) can cause micro-cracking, surface spalling, and a reduction in fatigue life due to excessive work-hardening and surface damage. The SAE J2441 and AMS 2430 standards specify maximum coverage limits for critical applications. Always verify that your process achieves saturation and does not significantly exceed it.
Shot Peening Media Types
Four primary media types are used in shot peening, each with distinct characteristics that make it suitable for specific applications. The selection depends on the workpiece material, required intensity, surface finish requirements, and applicable specifications.
Hardened Steel Shot
The most widely used peening medium. High-density (7.4–7.9 g/cm³), spherical cast or conditioned steel shot with hardness of 55–65 HRC. Available in SAE J444 sizes from S70 to S660. Produces deep compressive layers and high peak stress. Used for ferrous alloys, titanium, and high-strength aluminum.
Stainless Steel Shot
Austenitic or martensitic stainless shot (300 or 400 series). Prevents iron contamination on stainless and non-ferrous parts. Hardness 40–55 HRC (austenitic) or 45–58 HRC (martensitic). Used in medical, food, and nuclear applications where ferrous contamination is unacceptable.
Ceramic Shot (Zirconia)
Zirconia-toughened alumina or pure zirconia spherical beads. Density 3.5–6.0 g/cm³, Mohs hardness 8–9. Used for peening thin sections, aluminum, and titanium where steel shot would cause excessive mass or contamination. Specified under AMS 2430 Type 2 for ceramic media.
Glass Bead
Soda-lime or borosilicate glass beads. Density 2.4–2.6 g/cm³, Mohs hardness 5.5–6.5. Produces very low-intensity peening with minimal surface roughening. Used for cosmetic peening, thin-walled components, and where a matte finish is desired. Conforms to AMS 2430 Type 3.
| Property | Hardened Steel Shot | Stainless Steel Shot | Ceramic (Zirconia) Shot | Glass Bead |
|---|---|---|---|---|
| Density (g/cm³) | 7.4–7.9 | 7.6–8.0 | 3.5–6.0 | 2.4–2.6 |
| Hardness | 55–65 HRC | 40–58 HRC | 8–9 Mohs (1200–1500 HV) | 5.5–6.5 Mohs (550 HV) |
| Typical Size Range | 0.18–2.40 mm (S70–S660) | 0.15–2.00 mm (CW–S660 equiv.) | 0.05–1.50 mm (B20–B505) | 0.02–0.85 mm (BT-7 to BT-36) |
| Media Life (cycles) | 3,000–8,000 | 2,000–5,000 | 1,000–3,000 | 500–1,500 |
| Max Compressive Depth | 0.30–0.50 mm | 0.25–0.45 mm | 0.15–0.35 mm | 0.05–0.15 mm |
| Surface Roughening | High (Ra 1.0–3.5 µm) | Moderate–High | Low–Moderate (Ra 0.5–2.0 µm) | Very Low (Ra 0.3–1.0 µm) |
| Contamination Risk | Ferrous transfer to Al/Ti | Minimal (passivation layer) | None (non-metallic) | None (non-metallic) |
| AMS 2430 Type | Type 1 (cast steel) | Type 1 (stainless variant) | Type 2 (ceramic) | Type 3 (glass) |
| Cost per kg (USD) | $1.50–$4.00 | $6.00–$15.00 | $8.00–$25.00 | $2.50–$8.00 |
| Primary Use | General-purpose peening of steel parts | Peening stainless, non-ferrous parts | Peening Al, Ti, thin sections | Cosmetic/low-intensity peening |
Steel Media for Shot Peening
Hardened steel shot is the workhorse of the shot peening industry, accounting for an estimated 70–80% of all peening operations worldwide. Its high density, high hardness, and excellent media life make it the most cost-effective choice for the majority of steel, cast iron, and heavy-section non-ferrous applications.
Manufacturing and Conditioning
Cast steel shot is manufactured by atomizing molten steel through a high-pressure water or air stream, producing spherical droplets that solidify as they fall through a cooling tower. The raw cast shot is then quenched and tempered to achieve the target hardness of 55–65 HRC (Rockwell C scale). This heat treatment creates a martensitic microstructure throughout the shot particle, giving it the toughness and hardness needed to survive thousands of impact cycles without fracturing.
"Conditioned" steel shot refers to cast shot that has been run through a peening machine (or a dedicated conditioning chamber) for a period of time before being put into production. This conditioning process fractures and removes any weak, cracked, or non-spherical particles, leaving only sound, round shot that meets the roundness specification of ≥ 90% (per SAE J444). Conditioning is mandatory for AMS 2430 compliance.
SAE J444 Size Designations
Steel shot for peening is classified by the SAE J444 standard, which designates sizes from S70 (finest) to S660 (coarsest). The number refers to the nominal shot diameter in thousandths of an inch. The table below shows the standard sizes used in shot peening:
| SAE J444 Designation | Nominal Diameter (mm) | Nominal Diameter (in) | All Pass Sieve (mm) | Max 10% On Sieve (mm) | Typical Peening Intensity | Typical Application |
|---|---|---|---|---|---|---|
| S70 | 0.18 | 0.007 | 0.180 | 0.150 | 0.005A–0.008A | Thin sheet, delicate components |
| S110 | 0.28 | 0.011 | 0.300 | 0.212 | 0.008A–0.012A | Gears, springs (light) |
| S170 | 0.43 | 0.017 | 0.425 | 0.300 | 0.012A–0.018A | Automotive gears, shafts |
| S230 | 0.58 | 0.023 | 0.600 | 0.425 | 0.015A–0.022A | Springs, axles, structural parts |
| S280 | 0.71 | 0.028 | 0.710 | 0.600 | 0.020A–0.028A | Heavy springs, turbine blades |
| S330 | 0.84 | 0.033 | 0.850 | 0.710 | 0.025A–0.035A | Landing gear, large forgings |
| S390 | 0.99 | 0.039 | 1.000 | 0.850 | 0.030A–0.040A | Large structural components |
| S460 | 1.17 | 0.046 | 1.180 | 1.000 | 0.035A–0.045A (C-strip) | Heavy forgings, descaling + peening |
| S550 | 1.40 | 0.055 | 1.400 | 1.180 | 0.040A–0.050A (C-strip) | Heavy structural peening |
| S660 | 1.68 | 0.066 | 1.700 | 1.400 | 0.045A+ (C-strip) | Very large forgings, heavy descaling |
As a rule of thumb, shot diameter should not exceed one-third of the fillet radius or one-half of the minimum feature width being peened. Larger shot produces deeper compressive layers but rougher surfaces. Smaller shot produces smoother finishes and lower intensities. For detailed sizing calculations, use our Process Calculators.
Hardness Specifications
Steel shot hardness is critical to peening performance. AMS 2430 requires shot hardness to be within ±2 HRC of the specified value. The most common hardness ranges are:
- 55–62 HRC — Standard peening shot for general applications. Provides good balance of energy transfer, media life, and workpiece surface integrity.
- 60–67 HRC — High-hardness shot for peening very hard workpiece materials (>50 HRC). The shot must be harder than the workpiece to ensure plastic deformation of the workpiece rather than the shot.
- 40–50 HRC — Lower-hardness (softer) shot used for peening softer materials like aluminum and brass, where high-hardness shot would cause excessive surface damage.
The hardness relationship between shot and workpiece is fundamental: the shot must be at least slightly harder than the workpiece surface to produce effective plastic deformation. If the workpiece is harder than the shot, the shot deforms rather than the workpiece, and no compressive stress is induced. This is why high-hardness steel shot (60–67 HRC) is required for peening hardened steel gears (58–62 HRC) and carburized components.
Ceramic Media in Shot Peening
While steel shot dominates the peening industry, ceramic shot — particularly zirconia-based media — has carved out important niches where its unique properties offer advantages over steel. Ceramic shot is specified under AMS 2430 Type 2 and is increasingly used in aerospace, medical, and electronics manufacturing.
Zirconia Shot: Properties and Advantages
Zirconia (ZrO₂) shot is the most common ceramic peening medium. It is manufactured as solid spherical beads with a density of 5.5–6.0 g/cm³ — significantly lower than steel (7.4 g/cm³) but much higher than glass (2.5 g/cm³). This intermediate density makes it ideal for peening applications where steel shot is too aggressive or too heavy, and glass bead is too light to produce adequate compressive stress.
Key properties of zirconia peening shot include:
- High hardness — 1200–1500 HV (Vickers), equivalent to 8–9 on the Mohs scale. Harder than all steel shot, allowing effective peening of very hard workpiece materials.
- Non-metallic — eliminates ferrous contamination on aluminum, titanium, and stainless components. This is critical for aerospace and medical applications where iron transfer can initiate corrosion or interfere with subsequent anodizing or passivation processes.
- Lower density — at 5.5–6.0 g/cm³, zirconia shot produces less impact energy per particle at the same velocity as steel shot. This allows peening of thin sections and delicate geometries without risk of distortion or excessive cold work.
- Smooth surface — zirconia beads have an inherently smooth, glossy surface that produces minimal surface roughening, preserving the dimensional integrity of precision components.
- Chemical inertness — zirconia is chemically inert and will not react with workpiece materials, process compounds, or cleaning solutions.
Alumina Shot
Alumina (Al₂O₃) shot, with a density of 3.5–3.9 g/cm³ and hardness of 1100–1400 HV, is also used in specialized peening applications. Its lower density compared to zirconia makes it suitable for very delicate peening operations on thin-walled aluminum and titanium components. Alumina shot is more brittle than zirconia, resulting in shorter media life and more frequent replacement. It is typically used where the very lowest peening intensity is required and where the smoother surface finish of zirconia is not justified by its higher cost.
When to Choose Ceramic Shot Over Steel Shot
Choose Ceramic Shot When:
- Peening aluminum, magnesium, or titanium where ferrous contamination must be eliminated
- Workpiece walls are thin (< 1.5 mm) and steel shot would cause distortion
- Surface roughness must be minimized (Ra < 1.0 µm after peening)
- The part will subsequently be anodized, passivated, or coated, and iron transfer would interfere
- The workpiece material is harder than available steel shot (> 65 HRC)
- Medical implant peening requires complete absence of metallic contamination
Choose Steel Shot When:
- Maximum compressive depth is required (> 0.30 mm)
- Workpiece is steel or cast iron (no contamination concern)
- Media life and cost per part are primary economic drivers
- High-intensity peening (0.020A+) is required
- Heavy section thickness tolerates the higher impact energy
- No non-metallic media specification is required
Steel Shot vs. Ceramic Shot: Detailed Comparison
The decision between steel and ceramic shot for peening is one of the most consequential choices in process design. The following analysis compares the two media types across the dimensions that matter most to manufacturing engineers.
Energy Transfer and Compressive Stress
At equal velocity, steel shot delivers more kinetic energy per impact than ceramic shot due to its higher density (7.4 vs. 5.5 g/cm³ for zirconia). This means steel shot produces deeper compressive layers and higher peak stress. To achieve equivalent intensity with ceramic shot, the process must either use larger diameter shot, higher velocity, or longer exposure time. However, the higher hardness of ceramic shot (1200–1500 HV vs. 600–750 HV for hardened steel) means that on very hard workpiece materials, ceramic shot can induce compressive stress where steel shot would simply deform on impact without deforming the workpiece.
Surface Finish
Ceramic shot produces a significantly smoother peened surface than steel shot. A zirconia-peened surface typically measures Ra 0.5–1.5 µm, compared to Ra 1.0–3.5 µm for steel-shot-peened surfaces of the same material and intensity. This is because ceramic shot has a smooth, non-porous surface that creates clean dimples without the micro-tearing that occurs with steel shot impact. For parts where surface finish after peening is specified (common in aerospace and medical), ceramic shot may eliminate the need for a post-peening polishing operation.
Media Life and Economics
Steel shot has a longer media life than ceramic shot — typically 3,000–8,000 impact cycles for conditioned cast steel shot versus 1,000–3,000 cycles for zirconia shot. This is because steel shot is tougher (able to absorb impact energy through elastic and plastic deformation), while ceramic shot is harder but more brittle and fractures more readily upon impact. However, ceramic shot's higher cost per kg ($8–$25 vs. $1.50–$4.00 for steel) means the total cost difference is substantial. For cost-sensitive high-volume automotive applications, steel shot remains the dominant choice. For low-volume, high-value aerospace and medical applications, the premium for ceramic shot is justified by contamination avoidance and surface finish benefits.
For peening 10,000 aluminum aerospace brackets per month: Steel shot (S170, $2.50/kg) requires 15 kg/month media replacement at 8,000-hour life = $37.50/month media cost, but requires post-peening cleaning to remove ferrous contamination ($200/month). Ceramic shot (Z170, $15/kg) requires 25 kg/month replacement at 3,000-hour life = $375/month media cost, but eliminates contamination cleaning. Total monthly cost: $237.50 (steel) vs. $375 (ceramic). The 58% premium for ceramic is justified by improved surface finish (eliminating a $300/month polishing step), making ceramic the more economical choice.
Dimensional and Distortion Effects
The higher impact energy of steel shot can cause dimensional changes and distortion in thin-section parts. A typical steel-shot-peened surface experiences a 0.01–0.05 mm dimensional growth (peening introduces compressive stress that causes the surface to expand). For thin sections (< 2 mm), this can cause warping. Ceramic shot, with its lower mass and energy per impact, produces less dimensional change (0.005–0.02 mm) and is preferred for thin-walled components like sheet metal structures, thin-walled tubes, and aerospace skin panels.
Shot Peening Standards and Specifications
Shot peening for aerospace, automotive, and defense applications is governed by a framework of industry standards that define media requirements, process control, quality verification, and documentation. Compliance with these standards is mandatory for safety-critical components and is enforced through customer quality system requirements (e.g., AS9100, IATF 16949, NADCAP).
AMS 2430: Shot Peening (General Specification)
AMS 2430 is the foundational aerospace material specification for shot peening. It defines the requirements for peening processes used on aerospace components including:
- Media qualification — shot must meet SAE J444 size requirements and be tested for hardness, shape (roundness), and freedom from cracks. Media must be conditioned before first use.
- Media types — Type 1 (cast steel, hardened), Type 2 (ceramic), Type 3 (glass bead), Type 4 (conditioned cut wire shot), and Type 5 (stainless steel).
- Process control — intensity must be verified using Almen strips per SAE J443. Coverage must be 100% minimum unless otherwise specified.
- Documentation — full traceability of media lot, intensity verification, and machine setup parameters for each production lot.
- Re-qualification — media must be inspected at specified intervals for size distribution, shape degradation, and contamination. Out-of-spec media must be replaced.
AMS 2432: Shot Peening (Computer-Controlled)
AMS 2432 extends AMS 2430 requirements specifically for computer-numerically-controlled (CNC) peening machines. It adds requirements for:
- Automated nozzle or part manipulation with positional feedback
- Real-time monitoring and recording of air pressure, media flow rate, and nozzle position
- Alarm and interlock systems that halt the process if any parameter deviates from specification
- Electronic records of every peened part with full process parameter traceability
- Nozzle distance and angle verification systems
AMS 2432 is typically required for critical aerospace components like turbine blade roots, landing gear, and primary structural components where process repeatability and traceability are paramount. Many aerospace OEMs require AMS 2432 for all peening of flight-critical parts.
SAE J444: Cast Shot and Grit Size Specifications
SAE J444 defines the size designations, sieve analysis requirements, and minimum roundness values for cast steel shot and grit used in peening and blast cleaning. It specifies the allowable size distribution for each shot designation (e.g., S170) in terms of the percentage of shot that must pass through or be retained on specific sieve sizes. This standard is referenced by AMS 2430 and is the primary reference for shot size selection and incoming inspection.
SAE J2441: Shot Peening Coverage Measurement
SAE J2441 defines the methods for measuring and verifying peening coverage — the percentage of the workpiece surface that has been impacted by shot. Coverage is a critical quality parameter because incomplete coverage leaves areas of tensile residual stress that can initiate fatigue cracks. The standard defines:
- Visual coverage — inspection under magnification (10x–20x) to assess the percentage of surface showing peening impressions.
- Dyescan / tracer methods — application of a fluorescent dye before peening and UV inspection after peening. Remaining fluorescence indicates unpeened areas.
- Peening scanners — automated optical or tactile scanning devices that measure dimple density and coverage.
- Full coverage (100%) — defined as 98% coverage when measured by the methods above, with remaining 2% distributed (no adjacent unpeened areas).
- Over-coverage — exposure beyond 100% coverage, expressed as a percentage (e.g., 150% coverage = 1.5× the time to reach 100% coverage).
Never assume that meeting one specification satisfies all customer requirements. Many aerospace primes (Boeing, Airbus, Lockheed Martin, GE) have company-specific peening specifications (e.g., BAC 5730, AIPI 02-02-007, PS-series) that supplement or modify AMS requirements. Always verify which specifications apply to your specific part number and customer before commencing production.
Almen Strip Testing and Intensity Measurement
The Almen strip test is the universal method for measuring and verifying shot peening intensity. Developed by John Almen in the 1930s, this test provides a repeatable, quantifiable measure of the energy transferred to the workpiece surface by the peening process.
How Almen Strips Work
An Almen strip is a flat, spring-steel test coupon of precisely controlled dimensions and hardness. Three types are specified in SAE J442:
| Almen Strip Type | Thickness | Dimensions (mm) | Hardness (HRC) | Typical Intensity Range | Used For |
|---|---|---|---|---|---|
| N (Thin) | 0.79 mm (0.031 in) | 76 × 19 × 0.79 | 44–50 HRC | 0.004N – 0.010N | Low-intensity peening (glass bead, fine ceramic) |
| A (Standard) | 1.29 mm (0.051 in) | 76 × 19 × 1.29 | 44–50 HRC | 0.006A – 0.030A | General-purpose peening (steel shot S110–S330) |
| C (Thick) | 2.39 mm (0.094 in) | 76 × 19 × 2.39 | 44–50 HRC | 0.015C – 0.045C | High-intensity peening (steel shot S390–S660) |
The test procedure is as follows: an Almen strip is placed in an Almen block holder, which exposes a known area of the strip to the shot stream. The strip is peened for a series of increasing exposure times. After each exposure, the strip is removed and its arc height is measured using an Almen gage — a precision dial indicator that measures the curvature induced in the strip by the compressive stress on its peened surface.
The arc height values are plotted against exposure time to produce a saturation curve. The intensity is defined as the arc height at the saturation point — the exposure time where doubling the time produces less than 10% increase in arc height. Intensity is reported as "0.0xx A" (inches of arc on an A-scale strip), "0.0xx N" (N-scale), or "0.0xx C" (C-scale).
Almen Strip Testing Procedure
Type N for low-intensity (< 0.006A), Type A for standard peening (0.006A–0.030A), Type C for high-intensity (> 0.030A). The strip type must match the expected intensity range.
The block is mounted on a test fixture at the same distance and angle as the workpiece surface during production peening. Four retaining bolts hold the strip flat against the block face.
Start with a short time (e.g., 2–4 seconds for air-blast, 5–10 seconds for wheel-blast). Remove the strip and measure arc height on the Almen gage.
Use fresh strips for each exposure time. Typical sequence: 2, 4, 8, 16, 32, 64 seconds (doubling each time). Record arc height for each.
Plot arc height (y-axis) vs. exposure time (x-axis). Identify the saturation point where doubling exposure time yields less than 10% increase in arc height.
The arc height at saturation must fall within the specified intensity tolerance (typically ±0.001A of the target). If out of tolerance, adjust process parameters (pressure, media flow, etc.) and repeat.
Always run Almen strips at the start of each production shift, after any media addition or change, after any machine adjustment, and after media life limits are reached. Document all results with media lot number, machine settings, date, time, and operator. For AMS 2430/2432 compliance, these records must be retained for a minimum of 3 years.
Coverage Measurement and Verification
Coverage — the percentage of the workpiece surface that has been impacted by shot — is as critical as intensity to the effectiveness of shot peening. Even with correct intensity, incomplete coverage leaves unpeened areas where the surface retains tensile residual stress from prior machining, grinding, or heat treatment. These tensile areas become preferential fatigue crack initiation sites, potentially negating the benefit of peening altogether.
Visual Coverage Inspection
The simplest coverage verification method is visual inspection under 10x–20x magnification. Peened surfaces have a distinctive matte, textured appearance with visible dimples. Unpeened areas retain the original surface finish (machined, ground, or polished). An experienced inspector can estimate coverage to approximately ±5%. Full coverage (100%) is defined as 98% coverage — all but isolated, non-adjacent unpeened spots.
Fluorescent Tracer Method (Dyescan)
The fluorescent tracer method provides a more objective coverage measurement. A fluorescent dye is applied to the workpiece surface before peening. After peening, the part is inspected under UV light. Areas where shot has impacted will have removed the dye; remaining fluorescence indicates unpeened areas. The percentage of non-fluorescent area gives the coverage value. This method is specified in SAE J2441 and is commonly used in aerospace peening.
Automated Peening Scanners
Modern peening facilities increasingly use automated optical scanning systems that project structured light or laser patterns onto the peened surface and analyze the reflected pattern to measure dimple density and coverage. These systems provide objective, repeatable measurements and can scan complex geometries. They are particularly valuable for high-volume production where manual inspection would be impractical and for AMS 2432 computer-controlled peening where coverage data must be recorded for every part.
Coverage and Over-Peening
While 100% coverage is the minimum requirement, excessive coverage beyond what is needed is not beneficial. Over-peening (typically defined as coverage exceeding 200%) can cause surface damage including micro-cracking, overlapping flaps, and work-hardening embrittlement. Most aerospace specifications limit maximum coverage to 200%, and some critical applications specify a narrower range (e.g., 100–150%). Coverage should be controlled through exposure time, which is calculated from the saturation curve determined during Almen strip testing.
Process Parameters
Beyond the four core variables (media, velocity, angle, time), several secondary parameters affect peening quality and must be controlled for consistent, specification-compliant results.
Shot Velocity and Air Pressure
In air-blast peening systems, shot velocity is primarily controlled by air pressure. The relationship between air pressure and shot velocity is approximately:
| Air Pressure (bar / psi) | Approximate Shot Velocity (m/s) | Kinetic Energy per S230 Shot (mJ) | Typical Intensity (A-strip) |
|---|---|---|---|
| 1.4 bar / 20 psi | 25–35 | 0.13–0.25 | 0.006A–0.010A |
| 2.8 bar / 40 psi | 40–55 | 0.33–0.62 | 0.010A–0.015A |
| 4.1 bar / 60 psi | 55–75 | 0.62–1.16 | 0.015A–0.020A |
| 5.5 bar / 80 psi | 70–95 | 1.01–1.86 | 0.020A–0.025A |
| 6.9 bar / 100 psi | 85–110 | 1.50–2.51 | 0.025A–0.030A |
Wheel-blast systems achieve velocity through centrifugal force — the wheel speed (typically 1,500–3,500 RPM) and blade design determine shot velocity, which ranges from 60 to 100 m/s. Wheel-blast systems cannot achieve the fine velocity control of air-blast systems and are typically used for higher-intensity, higher-volume peening.
Impact Angle
Impact angle significantly affects the energy transfer efficiency and the resulting compressive stress. At 90 degrees (normal incidence), maximum energy is transferred as plastic deformation. As the angle decreases, a larger portion of the impact energy goes into surface shearing rather than compression, reducing effective intensity. The relationship is approximately:
- 90 degrees — 100% effective energy transfer (reference)
- 75 degrees — ~95% effective
- 60 degrees — ~80% effective
- 45 degrees — ~60% effective
- 30 degrees — ~30% effective (minimal peening, mostly surface shearing)
Most peening specifications require impact angles of 75–90 degrees for critical areas. For complex geometries where 90-degree impact is impossible (e.g., spline roots, thread valleys), nozzles are positioned at the best achievable angle, and exposure time is increased to compensate for reduced efficiency. Multi-nozzle setups are often used to ensure adequate coverage and angle on all surfaces.
Media Flow Rate
Media flow rate affects the number of impacts per unit area per unit time, which determines how quickly saturation is reached. Too high a flow rate can cause shot-to-shot collisions in the air stream, reducing effective velocity and creating inconsistent intensity. Too low a flow rate extends cycle time unnecessarily. Typical flow rates for air-blast systems are 2–10 kg/min; for wheel-blast systems, 50–200 kg/min.
Equipment Types
Shot peening equipment falls into three primary categories, each suited to different part sizes, production volumes, and intensity requirements.
Air-Blast Peening
Uses compressed air to accelerate shot through a nozzle. Offers precise control of velocity (via air pressure) and targeting (via nozzle position). Ideal for complex geometries, selective peening, and low-to-medium volume production. Can use suction-feed (low pressure, media drawn into air stream) or direct-pressure (media in pressure vessel, forced into air stream) systems.
Wheel-Blast Peening
Uses a high-speed rotating wheel with blades to centrifugally accelerate shot. Handles very high media flow rates (200+ kg/min) and is ideal for high-volume peening of large surfaces. Less precise than air-blast but more energy-efficient and faster. Common in automotive and heavy machinery applications.
Vibratory Peening
Uses vibratory motion of media in a tub or bowl to peen part surfaces. Produces lower intensity than impact peening but can process complex geometries and internal surfaces simultaneously. Emerging technology with growing acceptance for moderate-fatigue applications. See our Mass Finishing Media Guide for details.
Specialized Equipment Configurations
Beyond these three categories, specialized configurations address specific needs:
- CNC robotic peening — multi-axis robots with peening nozzles for complex aerospace geometries. Conforms to AMS 2432.
- Satellite/table machines — rotating tables with multiple part fixtures for batch peening of medium-volume parts.
- Continuous-flow machines — in-line conveyors for high-volume peening of uniform parts (e.g., leaf springs, coil springs).
- Spinner-hanger machines — parts mounted on rotating hooks that spin during peening for uniform coverage.
- Internal peening lance — specialized nozzles for peening internal bores, tubes, and cylinder walls.
Shot Peening Applications
Shot peening is applied across virtually every industry where metal fatigue is a design-limiting factor. The following applications represent the most common and technically demanding peening operations.
Aerospace Springs and Landing Gear
Helical compression springs, leaf springs, and torsion springs in landing gear are among the most fatigue-critical components in aircraft. These springs undergo millions of load cycles during service and their failure can be catastrophic. Shot peening of landing gear springs is mandatory under AMS 2430/2432, typically using S170 or S230 conditioned steel shot at 0.012A–0.018A intensity. The peening process adds 0.15–0.25 mm of compressive depth, extending fatigue life by 300–500% compared to unpeened springs.
Gear and Spline Peening
Gear teeth are subject to high cyclic bending stresses at the root fillet — the transition radius between the tooth flank and the gear body. This fillet is the primary fatigue initiation site for gear failure. Shot peening of gear tooth roots is standard practice in automotive transmissions, helicopter gearboxes, and industrial power transmission. Typical process: S110 or S170 shot, 0.010A–0.015A intensity, with masked or selectively targeted nozzles to peen only the root fillet region without affecting the tooth flank profile. Documented fatigue life improvements of 200–400% are typical for peened gear roots.
Turbine Blades and Engine Components
Gas turbine engine components — including blade roots, disk rims, and compressor blades — operate under extreme cyclic loading at elevated temperatures. Shot peening of these components is specified by engine OEMs (GE, Rolls-Royce, Pratt & Whitney, Safran) using proprietary specifications that extend AMS 2430/2432 requirements. Ceramic (zirconia) shot is increasingly used for titanium and nickel-alloy components to avoid ferrous contamination and surface embedding. Typical process parameters: Z170–Z280 ceramic shot, 0.008A–0.015A intensity, CNC-controlled per AMS 2432.
Automotive Powertrain
The automotive industry applies shot peening to crankshafts, connecting rods, transmission gears, coil springs, and torsion bars. Automotive peening is typically done with wheel-blast equipment for high throughput. Typical process: S230 or S280 steel shot, 0.015A–0.022A intensity, on continuous-flow or satellite-table machines. While automotive peening is often less rigorously controlled than aerospace peening (many OEMs do not require full AMS 2430 compliance for non-safety-critical parts), the fatigue benefits are nonetheless substantial — typically 150–300% life improvement on peened components.
Additional Peening Applications
- Oil & Gas — drill pipe threads, subs, and collars
- Medical implants — hip stems, bone plates, dental implants (improves osseo-integration)
- Defense — artillery components, weapon mechanisms
- Marine — propeller shafts, rudder components
- General manufacturing — shafts, pins, linkages under cyclic load
Stress Corrosion Cracking Prevention
Beyond fatigue, shot peening is highly effective at preventing stress corrosion cracking (SCC) and hydrogen embrittlement. The compressive stress layer acts as a barrier to crack initiation in corrosive environments. Peening is specified for stainless steel components in marine, chemical processing, and nuclear applications where SCC is a primary failure mode.
Shot Quality and Inspection
The quality of peening shot — its shape, hardness, size distribution, and condition — directly determines the effectiveness and consistency of the peening process. AMS 2430 requires that peening media be inspected at defined intervals and replaced when out of specification. The following inspection requirements are critical for specification-compliant peening.
Shape (Roundness)
Peening shot must be spherical. Angular or broken particles create stress risers rather than beneficial compressive stress. SAE J444 requires that at least 80% of shot particles meet a roundness specification of 0.80 (aspect ratio). Most peening-grade conditioned shot achieves 90% or higher roundness. Roundness is inspected microscopically by projecting shot particles onto a screen and measuring the minimum and maximum diameters of each particle. The roundness ratio (min/max diameter) must exceed the specified threshold for the lot to be accepted.
Hardness
Shot hardness must be within ±2 HRC of the specification value. Incoming inspection typically involves mounting a sample of shot in epoxy, polishing a cross-section, and performing Vickers or Rockwell micro-hardness testing on the cross-sectioned particles. Hardness is also monitored during production — as shot is used, it work-hardens and its hardness distribution changes. Media that has drifted more than 3 HRC from specification must be replaced.
Size Distribution
Shot size is verified by sieve analysis per SAE J444. A sample of shot is passed through a stack of standard test sieves, and the weight retained on each sieve is measured. The size distribution must fall within the SAE J444 limits for the designated shot size (e.g., S170). Key acceptance criteria:
- Maximum 10% retained on the "all pass" sieve (largest opening)
- Maximum 10% passing through the "maximum 10% on" sieve
- At least 70% within the nominal size range
During production, shot breaks down and the size distribution shifts toward smaller particles. When the proportion of undersized shot exceeds the SAE J444 limit, the media must be either screened (remove fines and add new shot) or replaced entirely. Media screening is typically performed daily or per shift in high-production operations.
Freedom from Contamination
Peening media must be free from non-conforming materials including broken glass, foreign metal particles, sand, and debris. Visual inspection of a media sample under magnification is typically performed at each media addition. For aerospace applications, media contamination requires immediate process stoppage, full investigation, and re-qualification of the process before resuming production.
Peening Media Life and Replacement Criteria
Peening media has a finite life. As shot particles impact the workpiece surface thousands of times, they gradually fracture, deform, and wear. The broken fragments are smaller, angular, and potentially harmful — they can create stress risers instead of compressive stress. Controlling media life and replacing media at the correct interval is critical to maintaining peening quality.
Media Life Metrics
Media life is tracked by several metrics depending on the application and specification requirements:
- Operating hours — total machine operating hours since media was last replaced or topped up. Simplest metric but does not account for production rate.
- Media throughput — total weight of parts peened. Better metric because it correlates with actual media usage. Typical limits: 500–2,000 kg of parts per kg of steel shot; 100–500 kg of parts per kg of ceramic shot.
- Media wear rate — weight loss of media per hour of operation, measured by periodic sampling. Steel shot wear rate: 0.5–2.0% per hour. Ceramic shot: 1.0–3.0% per hour.
- Size distribution shift — periodic sieve analysis to track the shift toward smaller sizes. When fines exceed the SAE J444 limit, media must be screened or replaced.
Replacement Criteria
Media must be replaced when any of the following conditions are met:
- Size distribution falls outside SAE J444 limits after screening
- Hardness has drifted more than 3 HRC from specification
- Roundness falls below 80% (non-spherical particles exceed 20%)
- Contamination detected that cannot be removed by screening
- Operating hours or throughput limits reached (per specification)
- Almen intensity cannot be maintained within tolerance after parameter adjustment
For AMS 2430/2432 compliance, media replacement events must be documented with the reason for replacement, media lot number, operating hours since installation, and the results of the final media inspection. New media must be conditioned (run through the machine for a specified period, or run against a deflector plate) before being used on production parts, to remove fragile particles and verify size distribution.
Safety Considerations
Shot peening involves high-velocity projectiles, abrasive dust, compressed air or high-speed rotating equipment, and potentially hazardous materials. Comprehensive safety procedures are essential.
Personnel Safety
- Eye protection — safety glasses with side shields are minimum; face shields required for machine loading and media handling.
- Hearing protection — air-blast and wheel-blast machines typically generate 90–105 dB. OSHA-compliant hearing protection is mandatory.
- Respiratory protection — dust from media breakdown and workpiece material removal requires dust extraction. N95 or P100 respirators may be required for media handling and machine cleaning.
- Hand protection — cut-resistant gloves for media handling. Steel and ceramic shot edges from broken particles are sharp.
- Machine guarding — interlocked doors and light curtains must prevent operation when chambers are open. Wheel-blast machines must have full enclosure.
Environmental Controls
- Dust collection — all peening machines must be connected to dust collectors rated for the media and workpiece material. Steel and ceramic dust is respirable and must be filtered to meet OSHA PEL (5 mg/m³ for respirable iron oxide).
- Noise control — machine enclosures with sound-absorbing liners. Peening rooms with acoustic treatment.
- Media waste disposal — spent shot containing workpiece material residues may be classified as hazardous waste. Verify local environmental regulations for disposal requirements.
- Compressed air safety — air-blast systems operate at pressures up to 100 psi. Hoses, fittings, and pressure vessels must be inspected and certified per OSHA and ASME requirements.
For more information on media selection across all mass finishing processes, see our Media Selection Handbook. For comparison of ceramic and steel media properties, visit our Ceramic vs Steel Media Comparison Guide. For industry-specific applications, see our Aerospace Industry and Automotive Industry pages.