The detection of welding cracks in hot runners mainly relies on non-destructive testing (NDT) techniques, combined with macroscopic and microscopic analysis methods, to ensure that cracks are detected and accurately identified in a timely manner.
1. Non-destructive Testing (NDT) Methods
(1) Penetrant Testing (PT)
Applicable to: Surface-opening cracks; simple operation and low cost.
Principle: Fluorescent or colored penetrants are used to penetrate the crack; after cleaning, a developer is applied to make the defect visible.
Advantages: Can detect micron-sized cracks; suitable for various metallic materials.
Limitations: Limited to surface defects; cannot detect internal cracks.
Commonly used for preliminary screening after welding, especially suitable for surface inspection of complex hot runner components.
(2) Magnetic Particle Testing (MT)
Applicable to: Surface and near-surface cracks in ferromagnetic materials (such as carbon steel and low alloy steel).
Principle: The weld area is magnetized, and a leakage magnetic field is generated at the crack to attract magnetic particles, forming visible magnetic traces. Advantages: High sensitivity, capable of detecting microcracks smaller than 0.1 mm.
Note: The weld seam needs to be ground smooth to avoid signal interference.
In copper molten metal crack detection, MT can effectively identify microcracks caused by copper penetration due to differences in magnetic permeability.
(3) Ultrasonic Testing (UT)
Applicable to: Internal cracks, deep-buried defects, with a penetration depth of over 1 meter.
Principle: High-frequency sound waves propagate in the material and are reflected when they encounter discontinuous interfaces such as cracks. The location and size of the defect are determined by analyzing the echoes.
Advantages: Can quantitatively assess crack length and depth, suitable for thick-walled hot runner systems.
Advanced technologies: Phased array ultrasonic testing (PAUT) and time-of-flight diffraction (TOFD) can improve detection accuracy and visualization.
PAUT+TOFD combined detection can effectively identify copper molten metal cracks in V-grooves, with typical characteristics in its "clump-like" image and diffraction signal. (4) Radiographic Testing (RT)
Applicable to: Volumetric defects (such as porosity and slag inclusions), with a low detection rate for cracks.
Principle: X/γ rays penetrate the weld, and film or digital detectors image the weld. Defective areas appear as dark zones due to absorption differences.
Limitations: Cracks parallel to the X-ray direction are difficult to detect; copper and iron have similar densities, and molten copper cracks in V-grooves are easily missed.
RT has limited detection capabilities for heat-affected zone (HAZ) cracks and transverse cracks, requiring combination with other methods.
2. Macroscopic and Microscopic Analysis
(1) Macroscopic Visual Inspection
Use a magnifying glass or microscope system to observe the weld surface and identify obvious cracks, crater cracks, or network cracks.
Dyeing agents can be used to improve visibility.
The KEYENCE microscope system can be used for high-magnification observation of solder cracks and porosity, achieving full-frame focusing and three-dimensional imaging.
(2) Metallographic Analysis
Sampling and Cutting: Samples are cut from suspected crack areas and polished and etched.
Microscopic Observation: Grain boundary cracking and intergranular/transgranular morphology are observed using optical or scanning electron microscopy (SEM) to determine the crack type (hot crack, cold crack, reheat crack).
Energy Dispersive Spectroscopy (EDS): Elemental segregation (e.g., Cu, S) in the crack area is detected to aid in determining the cause.
In the case of molten copper cracks, metallographic analysis showed cracks in samples M6-M15, and EDS confirmed a significantly high copper content.
(3) Fracture Surface Analysis
Observe the fracture surface morphology:
Fatigue Fracture: Beach-like streaks, shell-like patterns;
Brittle Fracture: Smooth, crystalline;
Overload Fracture: Obvious plastic deformation.
Fracture surface classification and failure attribution are performed in accordance with GB/T 19620 standard. 3. Comprehensive Inspection Process Recommendations
|
Steps |
Methods |
Purpose |
|
1 |
Visual inspection + PT/MT |
Initial screening for surface cracks |
|
2. |
UT (PAUT/TOFD) |
Detection of internal and deeply buried cracks |
|
3. |
RT (auxiliary) |
Inspection of coexisting defects such as porosity and inclusions |
|
4. |
Metallographic/SEM/EDS |
Qualitative analysis of crack origin |
|
5. |
Fracture surface analysis |
Determination of fracture mode and its impact on service environment |
