🔩 Radiographic testing (RT) of welded joints with film — test technique classes A and B, image quality indicators

Materials and NDT PN-EN ISO 17636-1

Overview

The standard specifies techniques of radiographic testing of fusion-welded joints in metallic materials, carried out with industrial film techniques, so that satisfactory and repeatable results are obtained. It applies to joints of plates and pipes, where "pipe" also covers other cylindrical bodies: tubes, penstocks, boiler drums and pressure vessels. The standard does not specify acceptance levels for indications visible on radiographs — these are given in the ISO 10675 series. If the contracting parties apply lower test criteria, the quality achieved may be significantly lower than when the standard is strictly applied.

The scope of the standard is broader than that of the former Polish standard PN-M-69770: it covers various metals (copper, steel, aluminium, titanium and their alloys), steel up to a thickness of 150 mm, X-rays from X-ray tubes up to 1000 kV and the isotopes Yb 169, Tm 170, Se 75, Ir 192 and Co 60, as well as X-ray sources with energies from 1 MeV to over 12 MeV. The recommended films are the equivalents of the former type II films, i.e. D5 (class C4) and D7 (class C5) according to ISO 11699-1; the maximum fog density is 0.3.

The techniques are divided into two test classes: class A (basic techniques) and class B (improved techniques), used when class A is insufficiently sensitive. The choice of class is agreed by the contracting parties. If, for technical or industrial reasons, one of the conditions of class B cannot be met, for example the type of source or the source-to-object distance, a condition of class A may be adopted with the agreement of the parties, and the loss of sensitivity compensated by raising the minimum optical density to 3.0 or by choosing a better film system class with a minimum density of 2.6. Personnel must be certified in radiographic testing according to ISO 9712 or an equivalent recognised scheme, at the level appropriate to the sector.

Method principle

X- or gamma radiation penetrates the joint under test and is attenuated according to the law of absorption, depending on the thickness and density of the material in the path of the beam. A discontinuity filled with gas or slag attenuates the beam less than solid metal, so a larger dose of energy reaches the film at that point and after photochemical processing an area of higher optical density forms; high-density inclusions, for example tungsten, give spots lighter than their surroundings. The latent image forms in an emulsion containing silver halides and becomes visible after developing and fixing. The area of interest is the weld together with the heat-affected zone on both sides — the heat-affected zone is taken as a band the width of the weld face on each side, but not less than 10 mm. Image quality is confirmed with a wire-type or step/hole-type image quality indicator (IQI): the measure of sensitivity is the number of the thinnest visible wire, a wire being considered visible when a continuous length of at least 10 mm is clearly visible in an area of uniform optical density.

Applications

• Acceptance testing of butt joints of plates and pipelines, also of nozzle welds and fillet welds
• Inspection of pressure vessels, boiler drums and penstocks
• Testing of circumferential welded joints of large-diameter gas pipelines
• Testing of tanks, loading arms and pipelines in technical inspection laboratories
• Permanent documentation of joint quality — the radiograph as verifiable evidence for the customer

Key parameters

Standard

Standard number

PN-EN ISO 17636-1:2023-02

Title (PL)

Badania nieniszczące spoin — Badania radiograficzne — Część 1: Techniki promieniowania X i gamma z błoną

Title (EN)

Non-destructive testing of welds — Radiographic testing — Part 1: X- and gamma-ray techniques with film

Step-by-step procedure

1. Radiation protection

   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 used, the appropriate health and safety requirements and local, national and international regulations apply.

2. Stage of manufacture and surface preparation

   Surface preparation is usually not necessary, but where surface imperfections or coatings might hinder the detection of defects, the surface is ground smooth or the coatings are removed. Unless otherwise agreed, radiography is carried out after the final stage of manufacture, for example after grinding or heat treatment.

3. Selection of class and test arrangement

   Agree the technique class (A or B) and select the exposure arrangement from those in the standard: single wall, double wall with a single image, double wall with a double image (elliptical and perpendicular technique), arrangements for nozzles and fillet welds. Place the films as close to the object as possible. The minimum number of radiographs for a circumferential joint is given in Annex A of the standard.

4. Marking and identification

   Attach to each section of the object symbols whose images are to 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 on the object (zero point, direction, identification, measure); where the material or service conditions do not allow this, document the location with accurate sketches or photographs. When the radiograph does not show the weld, place high-density markers on both sides of it, outside the area of interest.

5. Positioning the image quality indicator

   Place the wire-type or step/hole-type indicator on the source side, in the centre of the area of interest, on the parent metal next to the weld, in close contact with the surface and at a location of uniform thickness. Position the wires perpendicular to the weld so that at least 10 mm of wire length lies in an area of uniform optical density. When the indicator has to be placed on the film side, the letter F is placed next to it and this is noted in the report. When radiographing pipes with the source in the centre, it is recommended to distribute at least three indicators evenly around the circumference.

6. Exposure and film overlap

   Select the voltage or source, the source-to-object distance and the exposure time for the penetrated thickness. When an area is radiographed with two or more films, ensure sufficient overlap, confirmed by a high-density marker visible on each film. Multi-film techniques are not used to shorten the exposure time on sections of uniform thickness.

7. Processing and assessment of image quality

   Develop and fix the films with process control according to ISO 11699-2. Measure the optical density with a densitometer: minimum 2.0 for class A and 2.3 for class B in the area of interest. Read the number of the thinnest visible wire or the smallest visible hole and compare it with the minimum values from Annex B; with a step/hole-type indicator having two holes of the same diameter, both must be visible. Give the indicator value obtained in the report together with the type of indicator.

8. Interpretation and report

   Assess the radiograph on a film viewer, classify the visible discontinuities and refer them to the acceptance levels of the ISO 10675 series (levels 1, 2, 3 correspond to the quality levels B, C and D of joints according to ISO 5817). In the report give the technique class, the source, the exposure parameters, the type and value of the image quality indicator, the optical density and the result of the assessment.

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
• Fencing off and marking of the controlled area, signalling of source operation, personal dosimeters and dose monitoring
• Isotope sources radiate continuously — transport container, records, storage in accordance with radiation protection regulations; an X-ray tube radiates only during operation
• Licensing and training of the operator and supervision by a radiation protection officer
• Darkroom chemicals (developer, fixer) — gloves, ventilation, collection of spent solutions as chemical waste

Frequently asked questions