Intensive shot peening is a cold working process that bombards the metal surface with a high-speed stream of projectiles to introduce residual compressive stress into the surface layer of components. Through this strengthening process, the fatigue strength of gears can be significantly improved, making them "hard on the outside and tough on the inside".

1. Concept and Basic Principles of Intensive Shot Peening

Intensive shot peening is a cold working process that uses a high-speed stream of projectiles to impact the surface of metal parts, causing plastic deformation, thereby introducing residual compressive stress and work hardening into the surface layer.
It can be imagined as a "gentle" "high-frequency hammering" on the metal surface. Countless tiny projectiles (called shot peening media) impact the workpiece surface like dense raindrops. Each projectile is equivalent to a miniature forging hammer, leaving a tiny pit on the surface. For a pit to form, the material beneath the pit must undergo lateral stretching and plastic flow, while the surrounding unimpacted material tries to prevent this deformation and push it back. This macroscopic mutual constraint ultimately forms a stable and strong residual compressive stress layer on the surface of the part.

Differences from Ordinary Shot Peening/Sand Blasting

Aspect Ordinary Shot Peening/Sand Blasting Intensive Shot Peening
Purpose Mainly for surface cleaning (e.g., rust removal, oxide scale removal), emphasizing material removal. Aimed at changing the mechanical properties of materials, emphasizing surface strengthening.
Parameter Control Relatively loose control over parameters. Extremely strict and quantitative control over parameters such as projectile speed, flow rate, coverage, and intensity; it is a precision process.

Strengthening Effect of Residual Compressive Stress

Fatigue cracks usually originate in areas of the part surface that bear the maximum tensile stress. When a gear is in operation, the bending stress at the tooth root is the largest and is an alternating stress. Intensive shot peening introduces residual compressive stress, causing plastic deformation of the surface material to a depth of 0.1-0.3mm and forming a residual compressive stress layer of up to 800-1000MPa, which offsets the external tensile stress.
This contribution can be understood using a simple formula:
plaintext
σ_total = σ_applied + σ_residual
Where:

σ_total is the actual total stress borne by the material;
σ_applied is the stress generated by the external load during gear operation (including tensile stress and compressive stress);
σ_residual is the residual compressive stress introduced by shot peening (always negative).

When the external load generates tensile stress (positive value), it superimposes with the residual compressive stress (negative value), so the total stress σ_total is significantly reduced, and may even change from tensile stress to compressive stress. This makes it difficult for fatigue cracks to initiate and propagate, thereby greatly improving the bending fatigue strength of the gear.

Grain Refinement and Phase Transformation Strengthening

Intensive shot peening impact increases the dislocation density by more than 1000 times, and the reconstruction of subgrain boundaries reduces the grain size to the micrometer level. At the same time, under the action of intensive shot peening, the retained austenite on the surface is transformed into martensite, and the hardness is increased by 0.5-2HRC. This produces a work hardening effect, further improving the surface hardness and wear resistance.

2. Key Process Parameters of Intensive Shot Peening

Intensive shot peening is by no means a simple "spraying"; its effect depends on a series of precisely controlled parameters.

1) Shot Peening Intensity

Definition: Measured using standard Almen test strips (Type A test strips are commonly used for gear shot peening, with a hardness of HRC 44-50). After shot peening, the test strip will bend toward the shot peened surface. The intensity value is defined as the specific arc height value (usually in mmA or inchA) that the test strip reaches. For example, 0.40mmA means the arc height is 0.40 millimeters.
Importance: It is a measure of process energy and directly determines the depth and magnitude of residual compressive stress. The higher the strength of the gear material, the higher the required shot peening intensity. The typical shot peening intensity range for gears is between 0.35-0.60 mmA.

2) Coverage

Definition: The percentage of the total surface area covered by pits formed by projectile impact. 100% coverage means the entire surface is covered by pits. To ensure sufficient strengthening, coverage is usually required to be no less than 100% (i.e., 200% or even higher). In professional fields, magnifying glasses are often used to observe and compare with standard diagrams for judgment.

3) Projectiles

Material:

Cast steel shot: Most commonly used, low cost, divided into regular hardness (45-52 HRC) and full hardness (55-62 HRC); it is the main choice for shot peening gears with hardness >50 HRC.
Cut wire shot: More rounded and uniform in shape, not easy to break, and has good surface quality; higher cost.
Glass shot: High hardness, light weight, can obtain higher surface compressive stress (but shallow layer), and low surface roughness.
Ceramic shot: High hardness, long service life, stable size, used in high-performance fields.

Shape: Must be spherical to ensure uniform compressive stress, rather than the cutting effect of sharp-angled projectiles.
Size: Diameter is usually between 0.1mm - 1.0mm. Smaller projectiles produce a shallower but smoother strengthened layer; larger projectiles can produce a deeper residual compressive stress layer. It is often selected according to the gear module and tooth root fillet radius, generally not exceeding half of the tooth root fillet curvature radius.

4) Shot Peening Process

The intensive shot peening process usually includes the following steps:

Preliminary Cleaning: Remove oil stains and impurities from the gear surface.
Shot Peening: In a dedicated shot peening machine, strictly control the shot peening time, distance, angle, and projectile flow rate.
Post-Cleaning: Use methods such as air blowing or vibration to remove residual projectile debris.
Inspection: Use Almen test strips to detect intensity, and check coverage under a microscope.

3. Effects of Intensive Shot Peening

Research and practice have proven that intensive shot peening has the following effects:

Improved Bending Fatigue Strength: For carburized and quenched gears, intensive shot peening can increase their bending fatigue limit by more than 30%, and even up to 50%-100%. This means the gear can bear higher loads or have a longer service life.
Inhibited Micro-Pitting: In the tooth surface contact area, the work hardening and residual compressive stress generated by shot peening can effectively inhibit the generation and expansion of micro-pitting.
Enhanced Resistance to Stress Corrosion Cracking: The residual compressive stress layer can effectively block the intrusion of corrosive media and the expansion of cracks.