How to Find Surface Defects on Non-Ferromagnetic Materials

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How to Find Surface Defects on Non-Ferromagnetic Materials 

How to find surface defects on non-ferromagnetic materialsOne of the main methods for carrying out this procedure is to use dye penetrant testing. 

Dye Penetrant Testing can be used on most austenitic products. These include weldings, castings and forgings. The majority of stainless steel products are austenitic and are for the most part, corrosion resistant. More often than not, dye penetrant testing will be used because it is efficient and cost effective.  

However, some martensitic and duplex stainless steel products are magnetic and so a different type of testing may need to be used. To decide what type of test is needed, the type of material should be determined. Here we’ll explain all the different types of testing that can be used to find surface defects on non-ferromagnetic materials.  

Different Types of Testing to Detect Surface Defects 

Usually, design codes or operational conditions will dictate the material selection between the different grades of stainless steel. If there is any uncertainty, then the safest choice is to use Dye Penetrant testing.  

Dye Penetrant Testing

Dye penetrant testing, or Liquid Penetrant Inspection is a cost-effective method of testing used to identify surface breaking flaws such as cracks, porosity, laps, seams and other surface irregularities. 

The advantage of this is that it can be used with both ferrous and non-ferrous materials. It can also be used on all non-porous materials whether they are metal, plastic or ceramic.  

It is a portable means of testing and is usefully used on sites. It requires no power or water and is therefore a convenient non-destructive testing method for inspecting surface discontinuities. 

This technique uses capillary action. This is when fluid with low surface tension penetrates into clean and dry surface breaking flaws. Application of the penetrant is usually by dipping, spraying, or brushing.  

One the right amount of penetration time has been allowed, any excess penetrant is removed. A developer is then applied to draw penetrant from flaws, making indications visible.  

The advantages of dye penetrant testing are:  

  • It’s a simple process 
  • It highlights small surface breaking defects 
  • Cost effective 
  • Very few limitations on materials 
  • Can allow for objects of many different sizes to be inspected.  
  • Portable  

The disadvantages of dye penetrant testing are: 

  • Critical to pre-clean anything that needs testing 
  • There is no depth sizing 
  • Only relatively non-porous surfaces can be inspected 
  • Access to the surface under the test is required  
  • The finish of the component’s surface might affect sensitivity of the inspection.  
  • Components will need to be cleaned afterwards 

However, if stainless steel is tested and found to be magnetic, then there are two other types of testing that be used to detect surface defects instead of dye penetrant testing. These are: 

  • Magnetic Particle Examination 
  • Eddy Current Testing 

Which is chosen is dependent on a range of factors, and each has their own advantages and disadvantages.  

Magnetic Particle Examination

This is a very sensitive methods of testing. It gives instant results and is easily transportable. It can therefore be used on and off a site. Its main disadvantage is that is uses flammable liquid to test, and therefore any metals that have paint on them, must have it removed before testing can take place.  

Magnetic Particle Examination’s main principle is to magnetise the part under test. Once this has occurred either a wet suspension or dry ferrous particles will be applied.  If there is a surface or subsurface flaw in the part, magnetic flux will leak, as air does not support as much magnetic field per unit volume as metal does. If a flux leakage is present, ferrous particles will be attracted to it and they will build up and form an indication.  

The advantages of Magnetic Particle Examination 

  • It’s an uncomplicated procedure 
  • It gives an almost immediate indications of any defects 
  • It is cost effective  
  • Allows for the inspection of large and small objects 

The disadvantages of Magnetic Particle Examination  

  • It can often only be used on ferromagnetic materials 
  • There is surface preparation needed  
  • It requires electricity 
  • There is no depth sizing 
  • Clean up of components is required afterward 
  • It can’t be used on components with thick paint or coatings  

Eddy Current Testing  

This type of testing doesn’t require any solvent, and neither does it require an external power source. It can be used on painted items. It is considered to be a cleaner process and doesn’t leave any residue behind.  

It is a technique used to detect surface and subsurface defects via electromagnetic induction. In the simplest of eddy current tests, a coiled copper wire is ‘excited’ with an alternating electrical current. This will produce a magnetic field around it. Once the coil approaches a conductive material, currents that are opposed to the ones in the coil are induced in the material, and these are known as eddy currents. Defects in the conductive material can disturb the path of eddy currents. This creates a local magnetic field, changing the balance of the system. This can then be quantified by measuring the variations of impedance in the coil. 

The advantages of Eddy Current Testing 

  • It is sensitive to surface and subsurface defects 
  • It is capable of detecting surface flaws through multi layered structures 
  • It can detect through thin non-conductive surface coatings, like paint 
  • Doesn’t need much in the way of pre cleaning components  
  • Test equipment is usually portable and light 

The disadvantages of Eddy Current Testing 

  • It can be affected by variations in magnetic permeability  
  • It is only effective on materials that are conductive  
  • It is operator dependent 

Format NDT For Non-Destructive Testing  

All of these well established non-destructive testing methods are used by Format NDT to detect for surface flaws and defects on non-ferromagnetic metals as part of our full range of services, which we provide to a wide range of customers. 

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