Spring shot blasting strengthening process
Shot blasting strengthening (also known as shot peening strengthening) of spring shot blasting strengthening machines is a key surface treatment process, aiming to significantly enhance the fatigue life and stress corrosion resistance of springs. Its effect is not achieved through "feeling", but is controlled and verified through a series of strict and quantifiable technical indicators.
The core indicators of spring shot blasting strengthening can be classified into three major categories: process control indicators, result verification indicators and performance guarantee indicators.
I. Process Control Indicators of Shot Blasting Strengthening Machine (Ensuring Correct Process Parameters)
These indicators must be monitored and recorded in real time during the shot blasting process.
Shot blasting strength (Almen strength)
Definition: This is the most crucial process control parameter. Measure using standardized Almen test pieces (usually type A or Type N). Fix the test piece at the same position as the spring. After receiving shot blasting, the test piece will arch towards the shot blasting surface. Measure the arch height with an arc height gauge.
The arc height value when the saturation point is reached (i.e., when the shot blasting time is doubled, the increase in arc height does not exceed 10%) is the shot blasting intensity. Unit: Millimeter (mm) or inch (inch), for example 0.35mmA.
Meaning: Too low strength, insufficient strengthening effect; Excessive strength may damage the surface or cause dimensional changes.
Coverage rate
Definition: The percentage of the area of surface craters after strengthening to the total area. 100% coverage means that the surface is completely and evenly covered by craters.
Indicator: Spring strengthening typically requires a coverage rate of ≥200% (i.e., twice the time required to achieve 100% coverage). This is to ensure that all areas, especially the microscopic depressions, are fully treated.
Inspection method: Compare with the standard spectrum under a microscope, or use professional image analysis software.
The material, size and shape of the projectile
Material: Commonly used are cast steel shot, stainless steel shot and ceramic shot. It depends on the spring material (for example, ceramic balls are commonly used for high-stress valve springs).
Size: It must be strictly controlled. For example, Φ0.3mm, Φ0.5mm, etc. Small-sized pellets can produce shallower but denser compressive stress layers, making them suitable for fine surfaces. Large-sized pellets can generate deeper compressive stress layers.
Shape: It is required to be spherical, without sharp edges or corners, and without damage. Broken projectiles can scratch the surface of the spring and must be removed through a sorting system.
Shot blasting time
The required processing time under the determined strength and coverage requirements. It needs to be determined through process tests.
Ii. Verification Indicators of Shot Blasting Strengthening Machine Results (Direct Measurement of Strengthening Effect)
These indicators need to be measured on the strengthened springs or representative specimens.
Surface compressive stress and stress layer depth
Definition: The magnitude of residual compressive stress introduced by shot blasting on the material surface and its distribution along the depth direction.
Measurement methods: X-ray diffraction (XRD), drilling method, etc. This is the most direct and scientific verification method.
Indicator
Maximum compressive stress value: It usually reaches its peak at a certain point beneath the surface (such as -800 MPa to -1200 MPa).
Compressive stress layer depth: The depth at which the residual compressive stress decays to zero. For high-performance springs, it is required to have a sufficiently deep effective stress layer (for example, >0.1mm).
Surface roughness
Definition: Shot blasting slightly alters the surface morphology.
Indicator: Ra value (Arithmetic Mean Deviation of Contour). The Ra value will increase after shot blasting, but it must be controlled within the allowable range (for example, Ra ≤ 3.2 μm). Excessively high roughness can become a new source of fatigue and counteract the strengthening effect.
Surface integrity
Definition: Visual or microscopic inspection to ensure that the surface is free from excessive shot blasting (causing "orange peel" or micro-cracks), shot embedding, and corrosion points caused by exposed fresh metal.
Iii. Performance Assurance Indicators of Shot Blasting Strengthening Machine (Final Verification of Strengthening Effect)
This is the ultimate verification of the shot blasting strengthening process.
Fatigue life improvement rate
Definition: It is evaluated by comparing the fatigue test results of the springs before and after reinforcement on the bench.
Indicator: Generally, it is required that the fatigue life (the number of cycles to fracture) be increased by several times or even an order of magnitude or more. For instance, the lifespan of a spring without shot blasting is 500,000 times, and it is required to reach over 2 million times after shot blasting. This is the core performance metric that users care about the most.
Relaxation resistance/ability to resist stress relaxation
Definition: The ability of a spring to maintain its elastic force under long-term loading. Shot blasting can significantly improve this performance.
Test method: Maintain for a certain period of time under specified temperature and load, and measure the loss rate of the load (or height). The loss rate after shot blasting should be much lower than that of the unshot blasted parts.
In short, spring shot blasting strengthening is not merely a simple "hit" process, but a sophisticated engineering system composed of precise parameter control, scientific measurement verification and final fatigue testing. Non-professional places or individuals attempting shot blasting operations on gas cylinders.