The key to distinguishing overheat cracks from other weld cracks lies in their metallographic characteristics and formation mechanisms. Overheat cracks exhibit a "candy-like" morphology that extends along grain boundaries, accompanied by irreversible damage such as grain boundary oxidation and remelting spheres. Other weld cracks, on the other hand, mostly extend through grains and are related to hydrogen embrittlement or stress concentration.
1. Microscopic Morphology Comparison (Metallographic Analysis)
Characteristics: Overheated Cracks, Other Welding Cracks (e.g., Cold Cracks, Hot Cracks)
Crack Path: Extends along grain boundaries, exhibiting a "candy-like" or "stone-like fracture" appearance. Mostly transgranular extension, capable of cutting through grains.
Grain Boundary State: Black oxide network at grain boundaries, obvious remelting spheres. Clear grain boundaries but no oxidation, possibly micropores.
Fracture Morphology: No plastic deformation, grayish-white granular, no metallic luster. Cold cracks show dimples, hot cracks exhibit dendritic characteristics.
Key Identification Point: If the crack is observed in metallography to strictly follow the grain boundary + oxide + remelting sphere pattern, it can be identified as an overheated crack.
2. Timing and Driving Factors
Stage: Overheating Cracks vs. Other Welding Cracks
Occurrence Time: Occurs during welding cooling process, without external load. Cold cracks may be delayed for hours to days.
Main Causes: Overheating leading to grain boundary oxidation and localized melting. * Hydrogen embrittlement, residual stress, solidification shrinkage.
Temperature Conditions: Exceeded the material's overheating temperature (H13 steel > 1150°C). Usually occurs within the normal welding temperature range.
Example: Welding H13 steel after holding at 1200°C easily produces overheating cracks; while cracks in thick parts welded under normal processes are mostly hydrogen-induced cold cracks.
3. Auxiliary Detection Methods for Verification
Scanning Electron Microscopy (SEM):
Overheating Cracks: Intergranular fracture, surface covered with oxide particles;
Cold Cracks: Cleavage or dimple fracture, no oxide coverage.
Energy Dispersive Spectroscopy (EDS): Oxide, Cr, and Fe oxide enrichment can be detected at the grain boundaries of overheated cracks; other cracks show no such elemental anomalies.
Non-destructive Testing (PT/UT): Both methods can detect cracks, but cannot distinguish their type; metallographic confirmation is required.
Important Reminder: Visual inspection or NDS alone cannot accurately distinguish cracks; metallographic analysis is essential.
4. Suggested Judgment Process:
Initial Screening: Use penetrant testing (PT) to locate the crack.
Sampling and Analysis: Cut a cross-sectional sample from the crack area for metallographic sample preparation.
Feature Identification: Observe for intergranular propagation, grain boundary oxidation, and remelting spheres.
Comprehensive Judgment: Make a final judgment based on the heating history (whether overheating occurred).
