What methods are there to test the fatigue performance of stainless steel welded pipes after degreas

The method for testing the fatigue performance of stainless steel welded pipes after degreasing is as follows:

1. Mechanical property testing

Rotary bending fatigue test

Principle: Fix the test sample of the welded pipe and apply alternating bending loads, record the number of fractures (GB/T 4337-2015).

Indicators: Determine the fatigue limit (for example, the fatigue limit of 304 stainless steel after degreasing increased from 200 MPa to 230 MPa). 

Axial tensile fatigue test

Applicable to: Simulation of high-pressure pipeline conditions, application of alternating loads of tension - tension or tension - compression (in accordance with ASTM E466 standard).

Advantage: It can simultaneously monitor the fatigue crack growth rate (da/dN) in the weld zone after degreasing. 

II. Surface and Internal Defect Detection

Fluorescent Penetrant Testing (PT)

Steps: After degreasing, apply fluorescent penetrant and observe surface micro-cracks under ultraviolet light (minimum detectable size 5 μm).

Application: To assess whether degreasing has eliminated sources of surface stress concentration. 

Ultrasonic Testing (UT)

Principle: Detect internal inclusions or incomplete fusion defects (such as tiny air gaps formed by residual oil from degreasing) by measuring changes in sound velocity. 

III. Environmental Fatigue Coordinated Test

Salt Spray - Fatigue Combined Test

Equipment: Place the sample in a salt spray chamber (5% NaCl solution, 35℃), and simultaneously apply cyclic loads.

Indicators: Compare the fatigue life attenuation rate before and after degreasing (the lifespan of the un-degreased part is shortened by 40% in the salt spray environment). 

High-temperature fatigue test

Conditions: Fatigue tests are conducted in an environment ranging from 200 to 500 degrees Celsius (for example, the fatigue limit of 316L welded pipes after degreasing increases by 12% at 300 degrees Celsius). 

IV. Microstructure and Performance Characterization

Scanning Electron Microscopy (SEM) fracture analysis

Observation: Characteristics of the source area, propagation area and instantaneous fracture area of the fatigue fracture, and assessment of the inhibitory effect of degreasing on crack initiation.

Residual stress test

Method: X-ray diffraction (XRD) is used to measure surface residual stress (the fatigue performance is optimal when the compressive stress value after degreasing is ≥ 100 MPa). 

V. Rapid Evaluation Method

Cyclic Corrosion Fatigue Testing Machine: Integrates multi-parameter control of humidity, temperature, and load. It can simulate the fatigue life under actual working conditions within 72 hours (error ≤ ±10%).

Acoustic Emission Monitoring (AE): Real-time capture of acoustic signals during the crack propagation process, locating early damage caused by poor degreasing. 

Detection process suggestions:

After degreasing, perform PT/UT to eliminate surface and internal defects first;

Select representative samples for rotational bending or axial fatigue tests;

Combine SEM fracture analysis and residual stress testing to quantify the improvement in fatigue performance due to degreasing;

Add salt spray - fatigue combined tests for pipes in corrosive environments. 

By using the above methods, the actual impact of degreasing treatment on the fatigue performance of stainless steel pipes can be systematically evaluated, providing data support for process optimization.