Detecting overheated cracks requires a combination of nondestructive testing (NDT) and metallographic analysis to accurately identify irreversible damage characteristics such as intergranular cracks and grain boundary oxidation, ensuring that potential safety hazards are detected in a timely manner.
1. Nondestructive Testing (NDT): Rapid screening of surface and internal cracks
|
Methods |
Applicability |
Detection Characteristics |
|
Penetrating Testing (PT) |
Detects surface-opening cracks. |
Uses fluorescence or colorants to reveal micron-sized cracks, suitable for complex structural areas. |
|
Magnetic Particle Testing (MT) |
Ferromagnetic materials (such as H13 steel) |
Magnetic particles are adsorbed at the crack location under a magnetic field to form a visible indicator; high sensitivity. |
|
Ultrasonic Testing (UT) |
Detects internally buried cracks. |
Can locate intergranular propagation cracks in welds and heat-affected zones; strong depth resolution. |
Recommended combination: PT + UT, covering both surface and internal defects for efficient initial screening.
2. Metallographic Microscopic Analysis (Most Reliable Method)
Sampling Location: Take cross-sectional samples from the weld and heat-affected zone (HAZ);
Sample Preparation Process: Grinding → Polishing → Etching (4% nitric acid alcohol);
Observational Features:
Intergranular Cracks: Cracks extend along grain boundaries, exhibiting a "candy-like" or "stone-like fracture" appearance;
Grain Boundary Oxide Network: Black oxides distributed along grain boundaries, indicating irreversible damage;
Remelting Balls: Rounded traces of liquid phase solidification, suggesting localized melting.
Judgment Criteria: Rating according to GB/T 6394 standard; the presence of any of the above features confirms an overheated crack.
3. Auxiliary Verification Methods
Scanning Electron Microscopy (SEM): Shows typical intergranular fracture morphology, no dimples, and oxide particles attached;
Energy Dispersive Spectroscopy (EDS): Detects enrichment of elements such as O, Cr, and Fe at grain boundaries, confirming the presence of oxides;
Hardness Testing: Large hardness fluctuations in the heat-affected zone reflect uneven microstructure and localized softening.
Special Reminder: Once a crack is detected, it indicates that the material has lost its load-bearing capacity and should be immediately discontinued and replaced.
4. Suggested Actual Inspection Process
Preliminary Screening: Inspect the entire weld surface using PT or MT;
In-depth 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 it was caused by overheating.
Practical Tips: High-risk molds should establish a "pre-welding prohibition review + post-welding mandatory inspection" mechanism to prevent operation with defects.
