Non Destructive inspection (NDi)
NDI comprises a large group of analysis techniques used to get an impression of the quality and condition of a product to be investigated. NDI does not damage the cast piece to be analysed, nor does it change it permanently. So it is a valuable technique that can save time and money in a product evaluation.
The terms non-destructive inspection (NDI), non-destructive testing (NDT) and non-destructive evaluation (NDE) are generally used to describe this technique.
Non-Destructive inspection at CIREX (NDi)
NDI is a commonly used method of analysis in the foundry sector. At CIREX, the techniques mentioned below are performed regularly in our material science laboratory.
Defects on the surface
Defects under the surface
The oldest NDA technique, and at the same time the most underestimated one, is visual inspection. At CIREX, the product and the material that must be analysed are checked carefully for possible casting errors. With this technique, we can only provide an assessment of the piece’s surface.
Visual investigation can be done using simple devices (such as a mirror, magnifying glass or microscope) or with more advanced analysis tools such as bore scopes (rigid and flexible) or video probe systems. These systems are used to look into hollow spaces that cannot be observed directly with the naked eye.
Crack analysis using penetrant (‘Farb eindring verfahren’) is an extremely old technique that relies on the capillary working of fluids. With this type of analysis, products are submerged in a fluorescent fluid, which is drawn into any cracks present. If the cast pieces are then inspected in a dark room under ultraviolet (UV) light, any cracks present in the product will light up brightly.
Cracks, holes or casting errors on the surface of the cast piece can be easily detected in this manner. After this analysis, the products are cleaned and delivered to the customer. At CIREX penetrant analysis is also sometimes called “pentrexen”. It is a fast and cheap NDI technique applicable to mid-size series.
Magnetic analysis is an inspection technique with which errors on or just under the surface (up to a maximum depth of 6 mm) can be detected. With this method, the cast piece is magnetised. In a cast piece without defects, the lines of the magnetic fields are found mainly inside the material. An irregularity can cause the lines of the magnetic fields to occur outside the product. This so-called “leakage field” can be made visible by applying small, easily magnetised iron particles (Fe) to the surface. The small, light iron particles are attracted by the area with the largest density of magnetic field lines: the so-called “leakage field”.
So the iron particles stay in place at the location of any discontinuities in the magnetic field. This makes it relatively simple to detect any surface or internal errors under the surface. Depending on whether the iron components are given a black colour or fluorescent colour, the inspection will take place under conditions of visible light or ultraviolet (UV) light. At CIREX, we work with a fluorescent fluid that lights up in the presence of UV.
This analysis can only be used on ferromagnetic materials. It isn’t usable on stainless steel, 17-4PH, Duplex, Inconel, SiMo, Invar or Ni-Resist.
Radiographic analysis is also called “X-ray analysis”. An X-ray photo is made using radiation that can pass through a product. The penetrating property of this magnetic radiation is caused by its extremely short wavelength (10-9 m). For this analysis, we use two differing radiation sources, the X-ray tube and a radioactive material (gamma rays). The properties of both types of radiation are identical. However, the build-up of energy differs in each type of radiation.
At CIREX, we use the X-ray tube. Using this method, we can detect internal errors such as cracks and inclusions in a cast piece. Differences in the absorption of the radiation form an indication of defects. The results of this radiation can be displayed “live” on a monitor or can be recorded on an analogue or digital photo.
Ultrasonic analysis is the method of material inspection using ultrasound. This inspection technique is used to detect both internal and external errors. In castings, it concerns primarily cracks or porosities that are not visible on the surface of the material.
Ultrasonic inspection techniques use high-frequency sound that is sent into the material to be inspected by means of a probe (transducer). The sound waves are reflected by any defects (such as cracks).
The spectrum of resonances can be detected with a transceiver. Differences in the resonance spectrum are analysed using software. The product is approved or rejected on the basis of this. At CIREX, resonance studies are performed with Quasar measurement equipment.