Seleção do Sistema de Líquido Penetrante: Considerações e Critérios
Once it has been decided that liquid penetrant inspection will be done, we must select the system. There are many varieties of penetrants, removal systems and developers. There are also different ways of combining these systems. For the vast majority of parts, the conventional water washable LP method is adequate, however, several factors must be considered to select the ideal system. The system should: indicate significant open cracks on the surface of the part not inadvertently affect the material or the part in subsequent service not be so expensive as to be economically unviable not take so long as to affect the production schedule
The material and be tested (test piece)
Many metallic or non-metallic materials can be satisfactorily inspected by conventional visible and fluorescent LP methods, however austenitic stainless steel and titanium alloys (at elevated temperatures) are subject to corrosion. For these metals the penetrant materials must have the sulphur and halogen contents controlled. There are other materials that are impaired by water, for these, a non-aqueous system is indicated. Some non-metallic materials are affected by penetrants or emulsifiers based on certain solvents. For these materials a water-based formulated penetrant is indicated.
What type of penetrant should be used
Generally, liquid penetrant inspection systems are usual for all types of parts, however in services where the part is in contact with liquid oxygen (LOX) or gaseous oxygen (GOX) and in nuclear applications, specific penetrants must be developed.
What is the significant size of defects in the test parts.
Some penetrant processes can detect discontinuities smaller than 1 nanometre in size, but is this extreme sensitivity necessary? The ideal penetrant process is one that indicates surface cracks that may affect part efficiency, while minimising non-relevant indications (such as those caused by surface roughness). To select this ideal system it is necessary to define which cracks are important, which type, size and location affect the part. Checking the history of similar parts is very helpful in making the choice.
Finding the critical parts.
Typically, the vast majority of parts to be checked may be very similar, with only one critical item requiring the maximum sensitivity of the test. Depending on the quantity involved, it may be more cost-effective to install a conventional penetrant system for most parts and send an item with more stringent requirements to a laboratory that provides non-destructive testing commercially (outsourcing). Similarly many companies have their own radiographic equipment for normal applications, but send parts to third party laboratories to perform tests that require very specific equipment.
How many parts should be checked and how often?
After having established the size and type of defect to be found, evaluate the various systems of liquid penetrant inspection that would be appropriate. Consequently the choice here will generally be based on cost factors such as the investment required, the materials used, the need for flushing water treatment or the space required for the test.
Where should the inspection be carried out?
It should be assessed, especially if there are only a small number of parts to be inspected and it is therefore more cost effective to use a laboratory to carry out the testing. Are there commercial facilities readily accessible, or would it cause an unacceptable delay to ship the parts out of the company? How much space is available in the company for the liquid penetrant inspection process, and where is it located? At what point will the part be tested? At the receiving inspection, during the process, during packaging, as the finished product, or at various stages? Are there some items that should be tested in test baskets, or on hooks, or in the field, away from other facilities? For these, portable penetrant kits may be the best solution.
What is the required handling of the items?
What is the size, shape, weight of the part to be tested by the penetrant? Are the parts large, heavy and clumsy, or are they light parts, small parts, or both? Are additional handling systems and techniques required?
Checklist for choosing the appropriate method.
Companies consider the following factors with suppliers of equipment and products for liquid penetrant testing before deciding which penetrant system will be used:
1. material to be tested: aluminium, magnesium, copper alloys, austenitic stainless steel, nickel alloys, titanium, iron, steel, etc.
2. Number of test pieces or test areas to be inspected: per hour, per shift, per day or per week.
3. Size of test objects to be handled: dimensions greater than 2 cm, 10 cm, 50 cm, l m, 2 m, 5 m, 10 m, etc.
4. Weight of the test objects to be handled: l to 300 g, l to 300 g to l kg, l to 3 kg, 3 to 10 kg, 10 to 30 kg, 30 to 100 kg, etc.
5. Place where the inspection is required; receiving, in process along the production line, during assembly, final inspection, in the field during an assembly, providing service or in maintenance departments.
6. Types of defects to be detected: small, deep, large, shallow, clumped or scattered. Porosities, cracks, splices, voids, roughness.
7. Surface condition of parts to be inspected: Cast, forged, machined, bent, polished, rolled, painted, corroded, eroded, scratched, flaky.
8. Conditions that parts will be subjected to after inspection: medical, nuclear, liquid or gaseous oxygen systems; welding, rolling or coating operation; high temperature uses, aerospace, industrial, transportation; inaccessible positions; consumer products; etc.
