Detecting cracks after welding overheated materials requires a combination of non-destructive testing (NDT) and metallographic analysis to identify typical defects such as intergranular cracks and heat-affected zone (HAZ) deterioration, ensuring timely detection of potential safety hazards.
1. Non-Destructive Testing (NDT): Rapidly Locating Surface and Near-Surface Cracks
|
Methods |
Applicability |
Detection Principle |
|
Penetrating Testing (PT) |
Detects surface-opening cracks. |
Uses fluorescent or colored penetrants to reveal microcracks invisible to the naked eye, particularly suitable for complex geometric areas. |
|
Magnetic Particle Testing (MT) |
Applicable to ferromagnetic materials. |
Under an applied magnetic field, a leakage magnetic field is generated at the crack, attracting magnetic powder and forming a visible indicator. |
|
Ultrasonic Testing (UT) |
Detects internally buried cracks. |
Utilizes high-frequency sound wave reflection to determine the location and size of defects, detecting intergranular propagation within the weld. |
Recommended Combination: PT + UT, covering both surface and internal defects, low cost, high efficiency.
2. Metallographic Microscopic Analysis (Most Reliable Method)
Sampling Requirements: Cross-sectional samples are cut from the weld and heat-affected zone (HAZ), and then ground, polished, and etched with 4% nitric acid alcohol.
Observational Characteristics:
Intergranular Cracks: Cracks extend strictly along grain boundaries, exhibiting a "candy-like" or "stone-like" morphology.
Grain Boundary Oxide Network: Black oxides are distributed along grain boundaries, indicating irreversible damage.
Remelting Balls: Rounded traces of liquid phase solidification, indicating localized melting.
Judgment Criteria: Rating according to GB/T 6394. The presence of any of the above characteristics confirms overheating and weld failure.
3. Macroscopic and Fracture Surface Analysis for Verification
Macroscopic Observation: Spontaneous cracking occurs during post-weld cooling or early service life, without plastic deformation;
Scanning Electron Microscopy (SEM) of the Fracture Surface: Shows typical intergranular fracture morphology, without dimples, with oxide particles attached;
Hardness Testing: Large hardness fluctuations in the heat-affected zone, with localized softened areas coexisting with hard phases, reflecting uneven microstructure.
Special Reminder: Once intergranular cracks are detected, it indicates that the material has lost its load-bearing capacity and should be immediately discontinued and replaced.
4. Recommended Actual Inspection Procedure
Preliminary Screening: Perform full weld surface inspection using PT or MT;
Deep Flaw Detection: Use UT to inspect internal defects in critical areas;
Sampling Verification: Take samples from suspected areas for metallographic analysis to confirm the nature of the crack;
Comprehensive Assessment: Combine service history and temperature control records to determine whether the welding failure is caused by overheating.
Practical tip: For high-risk molds, it is recommended to establish a "pre-welding prohibition review + post-welding mandatory inspection" mechanism to prevent working with damaged molds.
