🔩 Radiography of metallic products — basic rules of the film technique (castings, forgings, rolled products)

Materials and NDT PN-EN ISO 5579

Overview

The standard gives general rules for industrial X-ray and gamma radiography for flaw detection purposes, using film techniques, applicable to the inspection of metallic products and materials. It does not establish acceptance criteria for imperfections. The introduction states that the standard specifies the basic techniques of radiography so that satisfactory and repeatable results are obtained economically, and that standards for specific applications should comply with these basic rules. The standard is therefore a document superior to the standards for specific applications.

The techniques are divided into two classes: class A (basic) and class B (improved), used when class A might be insufficiently sensitive. Techniques better than class B are possible and may be agreed by the parties by specifying all the relevant test parameters; the choice of technique is agreed by the parties concerned. If, for technical or industrial reasons, one of the conditions of class B cannot be met, for example the type of radiation source or the source-to-object distance, the parties may agree to adopt the condition of class A; the loss of sensitivity is compensated by raising the minimum density to 3.0 or by choosing a better film system class with a minimum density of 2.6, with the other conditions of class B unchanged, in particular the image quality achieved.

The scope of the standard is limited in two ways, which is worth knowing before citing it in documentation. First, it covers only metallic products and materials. Second, since the third edition of 2013 the word "film" has been part of the title, and the standard is valid only for films for non-destructive testing as image detectors and does not apply to digital radiographic detectors. Digital radiography, imaging plates and real-time radioscopy systems require a different reference document, although the physics of the test itself is the same.

The radiographic method is a volumetric method: it allows surface and internal discontinuities to be detected, also in objects of complicated shapes, and allows the size of a discontinuity to be determined accurately. At the same time it is a relatively expensive and labour-intensive method, requires access to two opposite surfaces, and the detectability of planar defects depends strongly on their orientation relative to the direction of radiation, which can be the reason for multiple exposures of the object.

Method principle

The object is irradiated with X-rays or gamma rays. The radiation penetrates the material, and its intensity and the uniformity of its distribution after passing through depend on the thickness and density of the material in the path of the beam. The emerging radiation is recorded on radiographic film; after photochemical processing a radiograph is obtained on which a loss of material gives a higher optical density. The basic geometric quantities are the nominal thickness t, the penetrated thickness w calculated from the nominal thicknesses of all penetrated walls, the object-to-film distance b, the source or focal spot size d, the source-to-object distance f and the source-to-film distance SFD, which is the sum of f and b. Image quality is checked with an image quality indicator according to the ISO 19232 series of standards, which specify wire-type and step/hole-type indicators, image quality classes and the experimental evaluation of image quality values.

Applications

• Testing of castings: shrinkage cavities, gas pores, inclusions, misruns
• Testing of forgings and rolled products
• Inspection of structural components, pipelines and subassemblies of ships and aircraft
• Basis for specific standards, among others radiography of welded joints

Key parameters

Standard

Standard number

PN-EN ISO 5579:2014-02

Title (PL)

Badania nieniszczące — Badania radiograficzne materiałów metalowych z zastosowaniem błony i promieniowania X lub gamma — Zasady podstawowe

Title (EN)

Non-destructive testing — Radiographic testing of metallic materials using film and X- or gamma rays — Basic rules

Step-by-step procedure

1. Personnel qualification

   The tests are carried out by personnel qualified in accordance with ISO 9712 or equivalent, at the level appropriate to the industrial sector concerned.

2. Protection against ionising radiation

   The standard warns that exposure of any part of the body to X- or gamma rays can be highly injurious to health; wherever X-ray equipment or radioactive sources are in use, the appropriate legal requirements must be applied and local, national and international safety precautions strictly observed.

3. Stage of manufacture and surface preparation

   Surface preparation is usually not necessary, but where surface imperfections or coatings hinder the detection of defects, the surface is ground smooth or the coatings are removed. The surface of the object must be cleaned of spatter, excess material and residues of moulding sand. Unless otherwise agreed, radiography is carried out after the final stage of manufacture, for example after grinding or heat treatment.

4. Selection of the technique class

   Agree class A or B with the parties concerned; use class B when class A may be insufficiently sensitive. For a deviation from a class B condition, apply compensation by raising the minimum density to 3.0 or by a better film system class with a density of at least 2.6.

5. Test arrangement and selection of exposure

   Test arrangements are specified by the specific standards for the given application. Depending on the type and geometry of the object, select the radiation source, the method of exposure and the exposure time, and the geometric quantities f, b and SFD.

6. Identification and marking

   Attach to each section of the object symbols whose images will appear on the radiograph outside the area of interest and ensure unambiguous identification of the section. Make a permanent marking of the location of each radiograph; where this is not possible, document the location with accurate sketches or photographs.

7. Film overlap and image quality indicator

   When an area is radiographed with two or more films, ensure sufficient overlap, confirmed by a high-density marker visible on each film. Check image quality with an image quality indicator in accordance with the application standards and ISO 19232-1, -2, -3 and -4; the indicator is placed on the object during the test, so that the quality level is recorded for the given radiograph.

8. Processing, assessment and report

   Develop and fix the films with process control, assess the radiographs on a film viewer and document the result in the test report together with the numerical image quality value.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Warning in the standard: exposure of any part of the body to X- or gamma rays can be highly injurious to health
• Strict application of local, national and international precautions for ionising radiation
• Isotope sources emit radiation continuously, also outside exposure — storage and transport of the equipment as required by regulations
• Fencing off of the zone, signalling, personal dosimeters, radiation protection supervision
• Darkroom chemicals — gloves, ventilation, spent solutions as chemical waste

Frequently asked questions