Penetrant testing is applicable to stainless steel castings, alloy steel castings, carbon steel castings and other steel castings.
1. Basic Principles
This is a detection method that uses the good penetration and capillary action of colored penetrants or fluorescent penetrants into narrow gaps to display the magnified surface exposed defect traces after penetration, cleaning and imaging. Detect the discontinuity of the opening on the surface of the casting by visual inspection or magnifying glass. However, it cannot accurately reflect the nature, shape, size and depth of the discontinuity.
2. Penetrants, cleaning (removal) agents and developers
(1) Penetrants
It is the most critical material in penetrant testing, and its quality directly affects the sensitivity of the test. The penetrant must have excellent wettability to ensure the penetration performance. When the penetrant is mixed with colored dyes or fluorescent dyes, it is beneficial to the imaging of defects. Commonly used penetrants include water-based and oil-based water-washable penetrants, post-emulsified penetrants and spray-canned solvent-based removal penetrants. The former is divided into three types according to the sensitivity of the flaw detection: high, medium and low; the latter is the most commonly used and the most convenient to use. When used in conjunction with a spray-canned solvent-based quick-drying developer, it can obtain a sensitivity similar to that of a fluorescent penetrant. Post-emulsified penetrants are divided into three grades: standard, high and ultra-high. The latter has higher sensitivity. Note that different types of penetrants cannot be mixed. The density, concentration and appearance quality of the penetrants used should also be checked regularly.
(2) Cleaning (removal) agent Water is used as a cleaning agent for water-washable and post-emulsified penetrants. The removers in the spray-canned solvent-based penetrant removal agent are kerosene, alcohol and solvent.
(3) Developer It is another key material in penetrant inspection. Developers are divided into three categories: dry, wet and quick-dry. Dry developers are commonly used in fluorescent penetrant inspection. They have the advantages of showing higher fluorescence brightness than wet methods, longer-term preservation of traces and less prone to changes in display size, but have disadvantages such as environmental pollution. Wet methods are divided into water-suspended and water-soluble types. The former is prone to clumping and sedimentation, while the latter is more uniform and has a higher sensitivity for revealing defects, thus being widely used. Solvent-based quick-drying developers, typically supplied in spray cans, offer excellent penetration, resulting in higher sensitivity. Furthermore, their rapid evaporation reduces diffusion and sharper outlines.
3. Comparison Test Blocks
Comparison test blocks are specially prepared to verify penetrant testing methods and the performance of the testing agent. These test blocks are chrome-plated on aluminum alloy, steel, or copper. The chrome coating on the chrome-plated test blocks exhibits a network of cracks upon pressure application; the aluminum alloy test blocks exhibit quenching cracks. The test blocks are inspected using a reference penetrant (i.e., a batch sample of penetrant) and the results are photographed or replicated. This serves as a standard for verifying the penetrant's effectiveness, defect-revealing ability, and the appropriateness of the testing procedure under identical testing conditions. After each use, the comparison test blocks must be thoroughly cleaned to dissolve any residual reagent, cooled, dried, and stored sealed.
4. Penetrant testing is divided into two types: dye penetrant testing and fluorescent penetrant testing.
Dye penetrant testing uses a colored dye as a color-displaying agent for the penetrant, while fluorescent penetrant testing uses fluorescent powder as a luminescent agent for the penetrant. Dye penetrant testing can be performed indoors or outdoors, while fluorescent penetrant testing uses fluorescence emitted by a blacklight under ultraviolet light to reveal surface defects. Observation must be performed in a darkroom and offers the highest sensitivity. The penetration time, development time, and procedures for each testing method must comply with the operating procedures. The ambient temperature during testing should be between 16°C and 50°C.
5. Preparation and Post-Processing for Penetrant Testing
1) Before testing, the surface roughness of the casting must meet the corresponding quality grade requirements.
2) The surface must be thoroughly cleaned of rust, scale, and any paint, grease, or moisture that would impede penetrant penetration. The cleaning range for local testing extends 25mm.
3) The supplier and buyer specify the inspection locations and the acceptable grades or acceptable grades for each type of defect based on the intended use of the casting. 4) After flaw detection, the casting surface should be cleaned of penetrant and developer, dried, and protected against corrosion if necessary.
6. Features and Scope of Application
1) The method and equipment are simple and adaptable, suitable for a variety of applications.
2) The casting shape is not limited. The surface roughness requirement of the inspected area is related to the casting quality grade and should be agreed upon by both parties.
3) The operation is simple, the detection sensitivity is high, and it is not limited by material properties.
4) Only open surface discontinuities can be detected. The size of the discontinuity displayed does not directly represent the actual size of the material discontinuity. The nature of the discontinuity and the type of discontinuity displayed are approximately related.
5) Some of the flaw detection reagents and equipment contain volatile gases and ultraviolet rays, so appropriate protective measures must be taken.
Post time: Sep-18-2025