Fluorescent Penetrant Material Testing

Fluorescent penetrant material inspection mainly conducts scientific testing and analysis on the fluorescent penetrant sensitivity level determination, fluorescence brightness and contrast evaluation, penetration property and wettability testing, washability and removability inspection, thermal stability and durability analysis, and corrosivity and material compatibility testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Fluorescent penetrant sensitivity level determination: According to the relevant standards (such as ISO 3452 and ASTM E1417), through the use of the test blocks containing the cracks of the known sizes, the defect detection ability of the material is evaluated. Usually divided into five levels, from the ultra-low sensitivity to the ultra-high sensitivity, to determine its applicability to the workpiece testing of the different integrity requirements.

Fluorescence brightness and contrast evaluation: Under the ultraviolet light (black light) of the specific wavelength, the relative brightness of the fluorescence emitted by the penetrant is measured. The high brightness and high contrast can significantly improve the visibility of the defects, and are the core indicators of evaluating the material performance, directly affecting the reliability and efficiency of the testing.

Penetration property and wettability testing: The ability of the penetrant to seep into the surface open defects is tested, including the capillary action strengths, penetration speeds and the wetting and spreading properties on the workpiece surfaces. The good penetration property ensures that the fine cracks can be fully filled, and is the premise of finding the tiny defects.

Washability and removability inspection: The ease and thoroughness of removing the excess penetrants on the surfaces by washing with the water or solvents directly or after applying the emulsifiers are evaluated. The over-washing will cause the penetrant loss in the defects, and the under-washing will cause the excessively strong background fluorescence, interfering with the defect interpretation.

Thermal stability and durability analysis: The retention abilities of the key properties (such as the fluorescence brightness and viscosity) of the penetrant materials after the temperature changes during the storage and use are examined. This is crucial for the process applications in the high-temperature environments or the material effectiveness after the long-term storage.

Corrosivity and material compatibility testing: Whether the penetrants and their matching materials (cleaners and developers) cause the corrosion or harmful effects on the inspected workpiece materials (especially the active metals such as the aluminium alloys and titanium alloys) is verified, ensuring that the testing processes do not damage the workpiece body properties.

Determination of the harmful element contents such as the sulfur and chlorine: For the high-end manufacturing fields such as the aerospace, the contents of the elements such as the sulfur, chlorine and fluorine in the penetrant materials are strictly tested. These elements may cause the stress corrosion of the workpieces in the subsequent high-temperature environments, so their contents need to meet the strict limits (such as the AMS 2644 standard).

Testing Scope

Aerospace engine key components: Including the high-temperature alloy components such as the turbine blades, compressor discs and combustion chamber housings. These components must carry out the fluorescent penetrant testing during the manufacturing and maintenance, to exclude the defects such as the micro cracks and porosity that may cause the catastrophic fatigue fractures.

Pressure-bearing equipment and pipeline welded joints: Applied to the weld surface testing of the pressure vessels, boilers and long-distance pipelines in the petrochemical and power industries. Used to find the surface cracks, lack of fusion and blowholes of the welds and heat-affected zones, and is the important means of guaranteeing the safe operation of the equipment.

Automotive and rail transit parts: Covering the key forged and cast parts such as the engine castings, connecting rods, bogies, axles and gears. Through the testing, ensuring that the parts have no initial surface defects under the alternating loads, preventing the fatigue failures and improving the overall safety and reliability.

Precision castings and powder metallurgy products: For the precision castings with the complex shapes (such as the impellers and pump housings) and powder metallurgy parts, the casting defects such as the surface cold shuts, cracks and porosity are tested. This method can effectively evaluate the integrity and density of the forming processes.

Stainless steel and special alloy workpieces: Suitable for the surface defect testing of the non-ferromagnetic materials such as the austenitic stainless steels, nickel-based alloys and titanium alloys. On these materials where the magnetic particle testing cannot be used, the fluorescent penetrant testing is the preferred surface non-destructive testing method.

Nuclear power facility maintenance and in-service inspection: Used for the periodic inspection of the in-service equipment such as the reactor coolant system pipes, valves and pump bodies of the nuclear power plants. The testing materials need to meet the nuclear-grade standards with the extremely low harmful element contents, to find the stress corrosion cracks and fatigue cracks.

Additive manufacturing (3D printing) parts: As the emerging application field, used to evaluate the surface and near-surface defects of the metal 3D printed parts, such as the interlayer lack of fusion, micro cracks and blowholes, which is of great significance for optimizing the printing processes and verifying the part qualities.

Testing Methods

Water-washable fluorescent penetrant testing method: The water-washable fluorescent penetrants are applied to the workpiece surfaces, and after the penetration, the excess penetrants are directly washed off with the water. This method is simple to operate and has the high efficiency, suitable for the batch workpieces with the rough surfaces or complex shapes, but the sensitivity to the shallow and wide defects is relatively low.

Post-emulsifiable fluorescent penetrant testing method: After the penetration step, a layer of the emulsifiers is applied to make the excess penetrants on the surfaces washable with the water. Divided into the hydrophilic and lipophilic emulsification. This method can provide the higher sensitivity and cleaner backgrounds, especially suitable for detecting the fine fatigue cracks and stress corrosion cracks.

Solvent-removable fluorescent penetrant testing method: The solvent cleaners (usually the acetone or special solvents) and wiping cloths are used to remove the excess penetrants on the surfaces. This method has the good portability and does not require the water sources, especially suitable for the on-site testing, local testing and online testing of the large workpieces.

Dry powder developing and solvent-suspended developing: The dry powder developing evenly sprays the dry developing powders on the cleaned and dried workpiece surfaces, which is simple and fast. The solvent-suspended developing suspends the developing powders in the volatile solvents to form the suspensions for the spraying, which can form the thinner and more uniform developing layers, providing the better defect display resolution.

Hydrophilic emulsification control technology: Through the precise control of the emulsifier concentrations, emulsification times and application methods (such as the dipping and spraying), the fine management of the emulsification processes is realized. The excessively long emulsification times will cause the penetrants in the defects to be excessively washed out (over-emulsification), and is the key point of the quality control of this method.

Black light (UV-A) irradiation and visual inspection: Under the darkroom or low ambient light conditions, the black lights with the wavelengths in the 315-400 nm range and the central wavelength of about 365 nm are used to irradiate the workpieces. The inspectors wear the protective glasses and observe the yellow-green fluorescence emitted at the defects under the UV excitation, and the defect identification, positioning and evaluation are carried out according to the indication characteristics.

UV irradiance and white light ambient light measurement: The dedicated illuminance meters are used to regularly measure the UV irradiances of the black lights on the working surfaces, ensuring that they are not lower than the standard requirements (usually 1000 μW/cm²). At the same time, the white light ambient illuminances of the working areas need to be measured, to avoid the excessively strong white light causing the contrast decrease of the fluorescent indications.

Testing Standards

Domestic Standards:

1. GB/T 18851.1, Non-destructive testing — Penetrant testing — Part 1: General principles

2. GB/T 18851.2, Non-destructive testing — Penetrant testing — Part 2: Testing of penetrant materials

3. GB/T 261-2008, Determination of flash point — Pensky-Martens closed cup method

4. GB/T 265-1988, Petroleum products — Determination of kinematic viscosity and calculation of dynamic viscosity

5. GB/T 3536-2008, Petroleum products — Determination of flash and fire points — Cleveland open cup method

6. GB/T 3143, Liquid chemical products — Determination of colour (Hazen unit — platinum-cobalt scale)

7. GB/T 7533, Determination of crystallizing point for organic chemical products

8. GB/T 6682-2008, Water for analytical laboratory use — Specification and test methods

International Standards:

1. ISO 3452-2, Non-destructive testing — Penetrant testing — Part 2: Testing of penetrant materials

2. ISO 2719, Determination of flash point — Pensky-Martens closed cup method

3. ISO 3696, Water for analytical laboratory use — Specification and test methods

4. ASTM E1417, Standard practice for liquid penetrant testing

5. AMS 2644, Inspection material, penetrant

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Black light (UV lamp) and irradiance meter: The core lighting equipment, providing the UV-A band ultraviolet light needed for exciting the fluorescence. It needs to be equipped with the UV irradiance meters for the regular calibration, ensuring that its output intensities meet the testing standards, and is equipped with the filters to filter out the harmful UV-B/C radiation.

Penetrant automatic spraying and dipping system: Used for the automatic testing units on the production lines, which can realize the automatic and uniform application of the penetrants, emulsifiers, cleaners and developers. The system is usually equipped with the circulation filtration, concentration control and temperature control devices, guaranteeing the process stability and material consistency.

Pretreatment and post-treatment cleaning devices: Including the spray cleaning cabinets, ultrasonic cleaning tanks and vapor degreasing devices, etc. Used to remove the pollutants such as the oil stains, coatings and scales on the workpiece surfaces before the testing, as well as to thoroughly remove the residual penetrant testing materials after the testing.

Drying oven and developing cabinet: The drying ovens are used to quickly and uniformly dry the moisture on the workpiece surfaces before the developing, with the temperatures usually controlled at 60-70°C. The developing cabinets provide the closed and clean environments for applying the dry powder developers or carrying out the evaporation and drying of the solvent-suspended developers.

Fluorescence brightness comparator and standard test blocks: The fluorescence brightness comparators are used for the quantitative or semi-quantitative measurement of the fluorescence brightness. The standard test blocks such as the aluminium alloy quench crack test blocks (type A) and chromium-plated radiation crack test blocks (type B) are used for the routine performance verification, sensitivity level rating and consistency comparison of the different batches of materials.

Industrial endoscope and image recording system: For the areas difficult to directly observe with the naked eyes such as the inner cavities and deep holes, the flexible or rigid industrial endoscopes with the UV light sources need to be used for the inspection. Combined with the high-definition cameras and image processing software, the remote observation, recording and analysis of the defects can be realized.

Ambient light and white light illuminance meter: Used to quantitatively detect the ambient white light intensities of the testing areas, ensuring that they are lower than the standard specified upper limits (usually 20 lux), to avoid the ambient light "drowning" the weak fluorescent indications, guaranteeing that the testing is carried out under the suitable darkroom conditions.

Material characteristic laboratory analysis equipment: Including the viscometers, flash point testers, pH meters and gas chromatography-mass spectrometry instruments (GC-MS), etc. Used for the comprehensive laboratory analysis and quality control of the physical and chemical properties and harmful substance contents of the materials such as the penetrants, emulsifiers and cleaners.