Table of Content
- Overview of a Typical LPI Process
- Methods of Fluorescent Dye Inspection
- Steps in the Water Washable Method
- Sensitivity Levels
- Determining an Appropriate Sensitivity Level
- Benefits of Water-Washable Fluorescent Penetrants
- The Importance of Liquid Penetrant Inspection
- Key Takeaways
- FAQs
Water-washable fluorescent penetrants have evolved significantly over the years compared to early penetrants that required complex removal processes. Such penetrants can be removed with water alone, simplifying the dye penetrant inspection process and reducing the need for additional chemical handling.
This innovation has made liquid penetrant inspection more accessible and safer for operators while maintaining high standards of defect detection, in the Non-destructive Testing industry.
Overview of a Typical LPI Process
The ASTM E1417: Standard Practice for Liquid Penetrant Testing provides a general procedure for LPI which includes the following steps:
1. Preparing the workpiece: Cleanliness of the test surface is mandatory. It must be dry and free from any dirt, grease, paint, or other materials which might interfere with the inspection.
2. Applying the penetrant: Dipping, brushing, or spraying can help apply Liquid Penetrant Testing to the test surface.
3. Dwell Time: Penetrant needs adequate time to enter any discontinuities. The dwell time depends on the penetrant, developer, and defects.
4. Excess Penetrant: The leftover penetrant must be removed conserving the penetrant that entered the defects. Depending on the type of remover method, the part may need to be dried in an oven after the excess penetrant is removed.
5. Developer Layer: A thin, even layer of developer is applied to draw out the penetrant in the discontinuity to the surface to form indications. The development time will vary for different penetrants, developers, and flaws.
6. Inspection: Parts are assessed under adequate lighting conditions for indications and the appropriate steps to interpret and document any findings.
7. Post-inspection: Parts are cleaned after the inspection to remove any residual developer and penetrant.
Read More: Procedure for Liquid Penetrant Testing
Methods of Fluorescent Dye Inspection
The different methods of fluorescent dye inspection include:
- Washable Method
- Post-Emulsifiable Method, Lipophilic
- Post-Emulsifiable Method, Hydrophilic Selection of Materials
Steps in the Water Washable Method
The water washable method in Liquid Penetrant Inspection is straightforward and effective, especially when using products like Magnaflux's AMS 2644 Type 1 water washable penetrant. The steps involved in the detection of surface discontinuities using fluorescent dye penetrant testing procedure include:
1. Preparing the surface: The part must be carefully cleaned to remove excess oil, grease, or contaminants.
2. Penetrant Application: Apply the water-washable fluorescent penetrant to the test surface. The part must be dipped into a penetrant, poured, or sprayed. The entire test area must be sufficiently coated.
3. Dwell Time: The penetrant is left to dwell on the surface, typically 10 to 30 minutes, as in the process instructions.
4. Excess Penetrant Removal: As per ASTM standards the excess penetrant is rinsed off using water at a specified pressure and temperature. The penetrant must be carefully removed without disturbing the penetrant that has entered the discontinuities. A black-light helps ensure all excess penetrant is removed.
5. Drying: Place the part in a forced-air dryer to remove excess water. The drying process should be monitored to avoid drying out the penetrant in the discontinuities. The goal is to only dry off excess water.
6. Developer Application: Apply a developer, such as ZP-4B dry developer powder, to the part. The developer helps eliminate the fluorescent penetrant from any discontinuities, improving visibility. The developer is left to dwell for a specified time.
7. Inspection: The part is inspected under ultraviolet light. The fluorescent penetrant in the discontinuities will glow, making any defects visible. Findings are then documented.
8. Post-Cleaning: After inspection, the part is cleaned to remove any remaining developer and penetrant. This final step ensures the part is free from inspection residues and ready for further use or processing.
Water-washable penetrants like Magnaflux's AMS 2644 Type 1, are key to NDT penetrant testing, providing a reliable means to detect and evaluate surface-breaking defects.
Sensitivity Levels
Sensitivity levels are a classification system for fluorescent liquid penetrants outlined in AMS 2644 Inspection Material, Penetrant specification. Sensitivity levels are not applied to visible penetrants.
The distinct sensitivity levels are as follows:
- Ultra-low Sensitivity
- Low Sensitivity
- Medium Sensitivity
- High Sensitivity
- Ultra-high Sensitivity
Determining an Appropriate Sensitivity Level
The primary consideration when deciding on the sensitivity level of the penetrant to use in liquid penetrant inspection is the guidance provided by governing specifications. These specifications outline the penetrant sensitivity level required for the process, particularly for critical safety parts. The standard operating procedures and specifications will dictate the appropriate penetrant sensitivity.
If the penetrant sensitivity level is not specified in a procedure, actual parts using various penetrants are strongly recommended for evaluation. For Fluorescent Dye Penetrant Testing, a lower-sensitivity penetrant is effective on rough surfaces, while a higher-sensitivity penetrant is better suited for highly machined surfaces.
Per AMS 2644, the different penetrants, methods, developer forms etc. are thoroughly classified to aid process selection. The classification is as follows:
1. Penetrant Types
- Fluorescent dye
- Visible dye
2. Penetrant Methods
- Water washable
- Post emulsifiable, lipophilic
- Solvent removable
- Post emulsifiable, hydrophilic
3. Developer Forms
- Dry powder
- Water soluble
- Water suspendable
- Nonaqueous Type 1 fluorescent
- Nonaqueous Type 2 visible dye
Evaluating penetrants on actual parts is crucial to ensure proper sensitivity with acceptable background fluorescence, thereby detecting the typical defects found on the parts.
Testing the penetrant on actual parts can also confirm correct processing parameters, such as dwell and rinse time. It is common to use multiple penetrant sensitivity levels within a facility due to various parts, processes, and customer specifications.
Benefits of Water-Washable Fluorescent Penetrants
The benefits of water-washable fluorescent penetrants include:
- Ease of Use: Water-washable fluorescent penetrants can be easily removed with water, simplifying the Penetrant Testing Procedure.
- Efficiency: The method allows for rapid processing and inspection of parts as it is quick and efficient.
- Versatility: Water-washable fluorescent penetrants can be used on multiple materials, including non-ferrous metals such as aluminium and titanium. This versatility makes them suitable for various industries, from aerospace to automotive.
- Defect Detection: These penetrants provide excellent Surface Crack and Defect Detection capabilities. The fluorescent dye enhances Visual Inspection under ultraviolet light.
- Environmentally Friendly: Water-washable penetrants are generally more environmentally friendly against solvent-based alternatives. The reduced use of harsh chemicals minimises environmental impact and improves workplace safety.
The Importance of Liquid Penetrant Inspection
Fluorescent liquid penetrant testing is critical in NDT for Surface Crack and Defect Detection. Using washable fluorescent penetrants enhances visual inspection by making even the smallest defects visible under ultraviolet light.
Washable Fluorescent Penetrant inspection provides many merits to the industry, some of which include:
1. Comprehensive Surface Crack Detection: This method identifies defects open to the surface, including surface cracks, fractures, and other discontinuities, which might result from fatigue, impact, or manufacturing processes.
2. Enhanced Defect Detection: Fluorescent penetrants provide high sensitivity, enabling the detection of fine defects.
3. Versatility in Material Inspection: Liquid Penetrant Inspection suits various materials, including non-ferrous metals like aluminium and titanium, which cannot be inspected using magnetic methods.
4. Efficiency in Visual Inspection: The fluorescent dyes used in penetrant testing significantly elevate defect visibility under UV light.
5. Non-Destructive Nature: This method does not cause any test changes to the test subjects or its ability to operate effectively, making it ideal for components that require ongoing maintenance and evaluation.
Industries can ensure high quality and performance standards by incorporating fluorescent liquid penetrant testing into the NDT process.
Key Takeaways
- Liquid penetrant inspection does not require the test subjects to be porous or extremely rough. This includes metallic, non-metallic, magnetic, non-magnetic, conductive, and non-conductive materials. Parts of almost any size and dimension can be inspected.
- Liquid penetrant inspection is a fast and easy surface inspection method. Large quantities of parts or materials can be inspected quickly.
- Sensitivity levels are a classification system specifically for fluorescent liquid penetrants. Higher sensitivity penetrants help detect smaller cracks and defects.
FAQs
1. What makes water-washable fluorescent penetrants environmentally friendly?
A: Water-washable fluorescent penetrants are more environmentally friendly because they can be removed with water alone, reducing the need for harsh chemicals. This lowers environmental impact and improves workplace safety by limiting exposure to potentially hazardous substances.
2. How does the sensitivity level affect the detection of defects in liquid penetrant inspection?
A: Sensitivity levels classify fluorescent liquid penetrants based on their ability to detect fine defects. Higher sensitivity levels are suitable for detecting smaller cracks and defects, especially on highly machined surfaces, while lower sensitivity levels are effective on rougher surfaces.
References:
1. Helling
2. Mehdi Mehdizadeh
3. IndiaMart
4. RNDT
5. Asseteng