Non-destructive materials testing

Radiographic testing or X-ray testing is used to examine components and weld seams for volumetric defects such as cracks, foreign matter, pores or slag entrapments. As most of us know from medical radiography, radiation is transmitted from an X-ray source on one side through the object to be tested. The rays are received by a radiographic film on the remote side, resulting in an image with different degrees of black and white, depending on the amount of radiation received. Radiography is an imaging technique that has been used for many years across the entire machinery and plant engineering industry. For documentation purposes a test report is drawn up, including relevant films.

This test can either be performed in our X-ray bunkers in Duisburg, Oberhausen, Koblenz, Frankfurt and Großostheim, but also elsewhere using mobile equipment. mtl has six fully equipped laboratory vans for mobile operations. Each of these vans has an in-built dark room so that the radiographic film can be developed, analyzed and documented on site. As radiation source we use electric X-ray tubes up to 300 kV and isotope testing gear SE75 and IR192. If weld seams on heat exchangers (tube-to-tubesheet welds) are to be examined we also have a special X-ray camera (RB1) that produces images using the X-ray reflection method.

The experience of our testing staff in handling the radiation sources, applying standards and regulations and interpreting and analyzing the test results is of critical importance in this test method as well.

We also offer mobile digital radiography as an option or additional service.

mtl is an accredited service provider of digital radiography. Our service portfolio also includes digital radiography (DR/CR) as an alternative to conventional radiographic testing. Imaging plate systems (CR) or detectors (DR) are used instead of films. The equipment can either be used as a stationary system or – as it is installed in a laboratory van – can be moved flexibly to the required site. This method is suitable for corrosion tests, weld seam tests and wall thickness measurements on pipes.

Digital radiography (CR/DR) has the following advantages:

• Imaging plate systems have a higher dynamic range than films
• Less exposure to radiation due to shorter exposure times
• Immediate results after reading the image plate
• Improved detection of faults and inhomogeneities
• Environmentally friendly as there is no need for film processing
• Improved image analysis due to digital filters and enhanced image
• processing
• High detail resolution due to magnification options
• Electronic, digital archiving
• Images are available immediately via e-mail

Imaging plate systems are ideally suited for tangential projection radiography to measure the wall thickness of isolated pipes. There is no need to remove insulation from pipes, unlike measuring wall thicknesses by ultrasound. Residual wall thickness can be determined directly on the image during analysis at the PC, and discontinuities such as defects in the weld seams shown or damage caused by corrosion can also be detected and documented.

Testing CFRP/GFRP materials or light-metal components is another example where digital radiography – whether mobile or stationary – has distinct advantages regarding the issue of how fast and how easy the X-ray images can be analyzed.

For data transmission or archiving purposes it is also possible to digitize conventional films at a later stage with a laser film scanner.

Digital archiving of X-ray films

X-ray films are still archived in physical form. Our film digitizer is our response to the request for digital storage and fast availability. This system scans the films and stores the data in DICONDE format. This file format conforms to ASTM standards and can be read by other systems. In addition, DICONDE always maintains the original file. On request, other file formats can be selected as well.

mtl’s digitization system conforms to EN 14096 and meets the requirements for class DS film digitizers.

Quality classes for film digitizers are defined by EN 14096-2 as follows:

DS: The enhanced technique, which performs the digitization with an insignificant reduction of signal-to-noise ratio and spatial resolution. Application field: digital archiving of X-ray films (digital storage)

DB: The enhanced technique, which permits some reduction of image quality. Application field: digital analysis of X-ray films (original films have to be archived)

DA: The basic technique, which permits some reduction of image quality and further reduced spatial resolution. Application field: digital analysis of X-ray films (original films have to be archived)

Ultrasonic testing is a volumetric inspection because, ideally, the entire component volume is tested for defects and inhomogeneities. Based on the acoustic reflection or the conduction of sound in the component, this is the preferred test method for sound-conductive materials ultrasonic testing. Flaws, boundaries or other discontinuities reflect the sound or prevent its transmission, which can be analyzed by our test inspectors. This test method requires a very advanced expertise and routine. The applicable standards and the sometimes extremely difficult component geometries are very challenging, and our people are perfectly able to meet these challenges.

TOFD and phased array are new test methods applied by mtl. These methods are medical imaging techniques that have been adapted for use in many applications as an alternative to radiographic testing. One specific application of phased array imaging is testing surfaces such as tank wallings for corrosion damage.

For internal inspections (wall thicknesses and detection of defects) of pipes on heating boilers, air coolers, feedwater heaters and heat exchangers an internal rotating inspection system (IRIS) is used. This system produces images of entire pipes made of ferrous and non-ferrous materials. Test results can also be analyzed off-line.

For documentation purposes, usually a test report is drawn up. The imaging techniques allow for documenting flaws as images.

In addition to conventional testing jobs, ultrasonic testing is especially suited for automated testing using complex test arrangements, e.g. for pipes, axles and wheels.

Eddy current testing is a method to detect surface cracks and is becoming more and more popular with our customers. We use it in case of electrically conductive materials to detect inhomogeneities, surface cracks or near-surface cracks. The main advantages of this method are its high sensitivity, even with microscopically small defects, its sometimes high testing speed and the fact that no testing agents are needed. The test can be performed automatically and is also suited for small parts. In some pipe testing cases we can use eddy current testing instead of a pressure test.

A test defect is used to balance the test units. All defects larger than the test defect will be detected reliably during the test. This test method requires people with very advanced expertise and routine and the applicable standards and the sometimes extremely difficult component geometries are very challenging. For documentation purposes, usually a test report is drawn up. Defects in the component can then be verified with a CT scanner or by preparing a metallographic specimen.

Penetrant testing is an inspection method to check surface-breaking defects and can be used to detect exposed defects such as cracks or porosity in components. We use this method on metallic materials as well as on plastic and non-porous materials.

During the test, a test fluid (the penetrant) is applied to the test component. Due to capillary action the fluid penetrates into any kind of surface-breaking discontinuities. The component is then thoroughly cleaned – e.g. with a special cleaning agent – and a developer is applied to the component. This developer produces a capillary counteraction so that the fluid that has penetrated voids and cracks becomes visible. This test is also known as red-white test because the red penetrant stands out against the white, chalk-like developer at the defect locations.

The quality of test performance and the experience of test inspectors are critical for the quality of the test results. There is no doubt that our test inspectors meet both requirements. For documentation purposes, usually a test report is drawn up. This test method also permits photo documentation.

Magnetic particle testing, also known as fluxing, is a method to check surface-breaking defects and is the most widely used testing technique for the magnetizable materials iron and steel. This test reveals cracks or discontinuities at the surface as well as near-surface defects. To perform the test, the test component is magnetized, either on a stationary test bench or, in case of mobile tests, with a hand-held yoke or electric conductors/coils produced specifically for this purpose. Magnetization means that a testing agent is applied which contains magnetizable particles. These particles settle at the defect locations which cross the lines of electric flux and produce a stray field (magnetic poles). The test can be performed at daylight or in the dark with a fluorescent testing agent. With fluorescent testing agents the contrast is stronger so that discontinuities are easier detected.

As with penetrant testing, test preparation, cleaning of components and selection of appropriate test parameters are critical and have a great influence on the top quality of our test results. For documentation purposes, usually a test report is drawn up. This test method permits photo documentation.

Visual testing is the first and oldest non-destructive testing method ever. Components are checked for defects using the human eye. Of course, simply looking at the test components is not enough. The observer’s eye must be trained – as is the case with our staff. Customer’s requirements, rules and regulations as well as standards define the type of defects; defect sizes and ambient conditions such as the lighting intensity have a great influence on the test. Therefore, we are able to detect and classify corrosion, production defects and workmanship defects very quickly.

mtl is able to perform inspections of the insides of pipes and vessels by means of endoscopes, video endoscopes with light guides that are up to 8 meters long and are equipped with movable tips, pipe cameras, tank cameras or mirrors. For documentation purposes test reports are drawn up and photos/videos are made.

Usually, the first assessment or inspection of structures, vessels or subassemblies as well is a purely visual test. After that our experienced inspectors arrange for additional tests to be performed on critical areas.

Our inspectors with their extensive experience ensure top quality test results.

For the most part, hardness testing does not belong to non-destructive test methods. Nevertheless, it is used on many sites immediately in combination with non-destructive testing. This is why our inspectors are also able to perform these tests as well directly on site. Depending on the material and the test method hardness testing leaves a small indentation on the material.

The ultrasonic contact impedance method (UCI) is the only non-destructive method and works according to the rebound principle. With this method hardness is automatically converted to the different hardness numbers.

Portable hardness test units are mainly used for measuring weld seams to determine the hardness profile of the weld seam, the heat affected zone and the basic material. Hardness testing allows conclusions to be drawn about material characteristics such as tensile strength, resistance to wear or machinability.

Common procedures are hardness tests according to Rockwell, Brinell and Vickers. For documentation purposes, usually a test report is drawn up.

Positive material identification testing is used to clearly identify and distinguish materials. This test method is mainly used for incoming goods inspection, on construction sites or for factory acceptance tests. There are two PMI test methods:

X-ray fluorescence analysis (XRF analysis):

For the analysis and identification of alloys we use handheld XRF spectrometers from the SPECTRO xSORT family. These devices are based on energy dispersive X-ray fluorescent technology and use an X-ray tube as X-ray source. XRF analysis is a non-destructive method that can also measure chips, wire, small objects and samples with extremely difficult geometries. It is not possible to determine carbon content with this method.

Optical emission spectrometry (OES):

A light spectrum is emitted from a source to an optical system where it is split into its spectral components which are then used to determine the concentration of the chemical elements. This method can also be used to determine the approximate carbon content of a material. In most cases portable units are sufficiently precise to yield meaningful results. For analysis purposes we recommend using stationary units in our partner laboratory.

Leak testing is used to detect leaks in pressurized systems. mtl’s service portfolio includes Helium leak testing and vacuum leak testing.

Helium leak testing is one of the most sensitive test methods and can show even very small leakages.

The systems to be tested, e.g. valves, subassemblies, pipes, vessels or tank floors, are pressurized or evacuated. This procedure is monitored using a mass spectrometer which shows leakages.

This test is performed using air or low-molecular gases, which increases detection sensitivity and improves leak location.

To perform internal inspections of pipes and tanks we use our special cameras. The tank camera is a pan-tilt-camera with a 40 x zoom, an in-built high-performance lighting unit and a working length of up to 15 meters.

For pipes with diameters between 80 mm and 1,000 mm a pipe inspection camera is used. This camera has a 360 degree endless rotating camera head with 10 x optical zoom and two cameras with LED lighting units (side view and front view). The maximum work length is 30 meters.

Inaccessible areas can be tested by means of endoscopes, video endoscopes or mirrors. mtl has four video endoscopes, one tank camera and one pipe camera.

Non-destructive testing of coatings by means of high voltage or conductivity measurement helps to detect even the smallest discontinuities. Wherever conductive and non-conductive layers are combined voids and discontinuities are reliably detected by means of porosity detection or conductivity measurement. This inspection is typically performed on coated and enameled vessels, reactors, rubber coatings, for porosity detection on the inside coating of pipes and for valve inspection.

The high-voltage pulse porosity detectors are indispensable instruments for this inspection. Coating defects cause a flash-over resulting in visual and acoustic signals from the test instrument. The devices are very efficient and ensure a reliable test at accessible locations while the coating material is exposed to only minimal stress.

The second method is conductivity measurement. For this test the test tank is filled with a conductive fluid and the conductivity between the fluid and the outer wall is measured with a probe. Conductivity gives information about coating defects. mtl’s service portfolio includes both test methods.