Penetrant Inspection Process
The Liquid Penetrant Inspection method is applicable for inspecting material or components when attempting to locate flaws open to the surface of mostly non-porous material.
How to Penetration Test & NDT Methods
These basic steps are uniform for the most part, but vary mostly in how each is completed, based on the particular technique being used. It is important for the inspector to know the tolerances and how to maintain the control of them i.e. using a timer for all dwell times and eye adaptation requirements.
- Pre-clean article to be inspected
- Apply penetrant to test part
- Allow correct dwell time to elapse
- Remove excess penetrant from part surface
- Dry, if applicable
- Apply developer and allow to dwell
- Inspect component
- Post-clean part
Penetrant System Identification Table
Type | Fluorescent Dye | Visible Dye | Method | Water Wash | Post Emulsified Lipophilic | Solvent Removable | Post Emulsified Hydrophilic | Form Developer | Aqueous (soluble) | Aqueous (suspendable) | Non-aqueous(solvent suspended) | Sensitivity
|1. Type (identifies the type of dye used)|
|I. Fluorescent Dye||II. Visible Dye||III. Dual (Contains Both Visible and Fluorescent Dyes)|
|2. Method (identifies the method used for excess penetrant removal)|
|A. Water Wash||
B. Post Emulsified (Lipophilic)
|C. Solvent Removable||
D. Post Emulsified (Hydrophilic)
|3. Form (identifies the developer form used)|
|(a) Dry||(b) Aqueous (soluble)||(c) Aqueous (suspendable)||(d) Non-aqueous (solvent suspended)|
|4. Sensitivity ½ Ultra low, 1= low, 2= medium, 3= high, 4 and up are ultra-high|
Pre-cleaning is most assuredly the pivotal stage of penetrant inspection processing, even though it is the first step that must be completed. This may or may not be obvious, but is nevertheless true. If the component under examination is not adequately pre-cleaned, and contaminants remain on the surface of the test part, the penetrant inspection medium will not have an opportunity to enter the flaw, rendering the process useless. This is not only true when contaminants remain within the flaw, but also when surface contaminants do not allow the component to remain wet with penetrant for the entire dwell time, or impact the properties of the penetrant inspection material. Some routinely selected pre-cleaning methods are listed below.
- Solvent cleaning
- Steam cleaning
- Vapor degreasing
- Detergent cleaning
- Chemical stripping
- Chemical etching
- Ultrasonic cleaning
- Alkaline baths
Penetrant Inspection Methods
Penetrant Application—After the part has been cleaned, dried, and is within the specified temperature range, the penetrant is applied to the surface to be examined so that the entire part or area under examination is completely covered with penetrant. Application methods include dipping, brushing, flooding, or spraying.
The penetrant methods are broken down into four types:
A water washable penetrant material is almost self-explanatory. The penetrant has a built-in emulsifying agent allowing it to be easily washed off with a coarse water spray.
Post emulsified, lipophilic (oil based) penetrants are highly penetrating materials that do not have a built-in emulsifying agent. With this process, the penetrant is removed by applying the emulsifier to the part’s surface after the penetrant dwell time has elapsed.
The solvent removable process when in visible dye form is the most portable liquid penetrant system available.
This process is very similar to the lipophilic process, except that the emulsifier is a water-soluble material, similar to detergent that can even be diluted with water for different applications. Where the lipophilic emulsifier can only be applied by dipping or pouring, the hydrophilic emulsifier, may be applied by dipping, pouring or spraying
A general rule for developers is the longer they stay on a part the more difficult they are to take off. For this reason it is common practice to fully post clean the part just after the completion of the fluorescent inspection penetrant testing process. Three basic types of developers are commonly used in liquid penetrant processing, with one of the groups having two sub-types. The types are: (1) dry developer, (2) wet, non-aqueous, and (3) wet aqueous which can be further categorized into water suspendable and water soluble [heading type=6]Dry Developer Form A[/heading]A dry developer is essentially a white powder that is applied either via a dusting chamber or dip station. The dusting chamber is very efficient because it takes very little developer powder to develop many parts, and the possibility of contamination is reduced to a much smaller volume of developer. [heading type=6]Wet Non-Aqueous Developer Form D[/heading] The wet non-aqueous developer is most widely used with the solvent removable, visible dye process, and is considered to be most sensitive of the developers used today. The developer has the biggest single effect on test sensitivity, and a non-aqueous developer will in many instances produce an increase of one full step in process sensitivity. [heading type=6]Wet Aqueous Developers[/heading] Wet aqueous developers are powders that are mixed in water to form a developer bath of a specific concentration or density. They are available in both soluble and suspendable forms.
[/one_half_last] [heading type=1]Dwell Times[/heading] Each step in the penetrant process has specific dwell times to insure valid results regardless of the application. Adherence is critical when determining the penetrant, emulsifier, or developer dwell time for the application at hand and governing standards. [heading type=6]Pre-Cleaners[/heading] When using solvent-type pre-cleaners, sufficient evaporation time must be allowed to insure that suspect flaws are not contaminated with the cleaning solvent. [heading type=6]Penetrant Dwell Time[/heading] The dwell time is chosen based on the type of penetrant used, the material being tested, the flaws that are suspect, and even specific test conditions such as temperature. [heading type=6]Emulsifier Dwell Time[/heading] The emulsifier dwell time is extremely critical, especially when attempting to locate shallow or very small flaws. [heading type=6]Developer Dwell Time[/heading] Developer dwell time vary between the different developer forms. In most cases, the main difference is the maximum dwell time allowed for inspection before the part must be reprocessed. [heading type=1]Calibration and Verification[/heading] There are a number of basic calibration checks that should be performed to insure consistency with penetrant processing. In some instances, this may be as simple as insuring that the materials have been chosen correctly for the given task, such as with self-contained spray can materials where contamination is not an issue. On the other end of the spectrum, a number of in-process calibration and verification checks may be needed to insure that components are processed within the required test parameters. [heading type=1]Indications[/heading] Once the parts or components requiring inspection have been processed and developed, the challenging task of interpretation and evaluation begins. Also, indications are the general term used to define sites that trap and retain liquid penetrant that require close scrutiny for disposition. [heading type=1]Discontinuities[/heading] [heading type=6]Inherent Flaws[/heading] [heading type=6](a) Inherent Cast Flaws | (b) Porosity | (c) Cold Shuts | (d) Hot Tears | (e) Inherent Wrought Flaws[/heading] [heading type=1]Process Discontinuities[/heading] As may be obvious for the category, process flaws are generated or produced through additional processing of materials or components. [heading type=1]Welding:[/heading] Common discontinuities formed in welding are: [list type=”icon-circle”]
- lack of fusion
- incomplete penetration
- slag inclusions
- tungsten inclusions
[/list] [heading type=1]Service Discontinuities[/heading] This particular group of discontinuity lends itself well to the Liquid Penetrant Inspection method. This is due to the fact that most service flaws are open to the surface of the component or material under evaluation. [heading type=1]Evaluation[/heading] A procedure and an acceptance standard are two different things and should be noted as such on the inspection report. A written penetrant procedure is a document a technician follows to perform the inspection and an acceptance standard or accept reject criteria is what is utilized to determine if a relevant indication is rejectable. [heading type=1]Accept Reject[/heading] Is the Final phase of our process