Thursday, August 16, 2012

Welding and Nondestructive Examination


Welding and Nondestructive Examination

Introduction
The purpose of this guide is to provide Inspectors with supplemental information on the subjects of welding and nondestructive examination (NDE).

Welding Controls
This section of the guide is provided to outline the elements that should be addressed in an effective weld control program.

Welding Qualifications
General. Section IX of the ASME Code relates to the qualification of welders and welding operators and the procedures they employ in welding according to the ASME Code. Other section of the ASME Code, such as Section I or Section VIII, and this Code state the conditions under which Section IX requirements are mandatory and give additional requirements.

Welding procedure specification. The purpose of a welding procedure specification (WPS) and procedure qualification record (PQR) is to determine that the weldment proposed for construction is capable of providing the required properties for its intended application. Each welding process has associated items (called variables) which have an effect upon the welding operation. The WPS lists the variables, both essential and nonessential, and the acceptable ranges of these variables when using the WPS. The WPS provides direction for the welder. The PQR lists what was used in qualifying the WPS and the test results. The Inspector should review the appropriate WPS and PQR to assure that the welding to be performed is within the limits of essential and nonessential variables prescribed in the ASME Code.

Performance qualification. The performance qualification test is intended to determine the ability of welders or welding operators to deposit sound weld metal and to operate the welding equipment in accordance with a qualified WPS. The performance qualification testing shall be documented by the welder performance qualification (WPQ). The Inspector should determine that the welder or welding operator is currently qualified. The Inspector may require requalification when there is specific reason to question the welder’s or welding operator’s ability to make welds that meet the WPS and the requirements of this Code.

Identification of Welders and Welding Operators
The Inspector should check to assure that each qualified welder and welding operator has been assigned an identifying number, letter or symbol.

Welding Materials Control
General. The Inspector should check to assure that the repair organization has an effective control program for purchasing, receiving inspection, and storage and handling of welding materials. In general, welding materials should conform to one of the specifications in Section II of the ASME Code, Part C – Welding Rods, Electrodes and Filler Materials. Many of the material specifications in Section II, Part C, have an appendix which provides useful information about welding materials. For example, the appendix to Specification SFA-5.1 is entitled, Guide to Classification of Carbon Steel Covered Arc Welding Electrodes. The Appendix reviews classification designations and tests, electrode conditioning, and a description and intended use of electrodes. The Inspector should review this appendix and other similar appendices to obtain useful knowledge about welding materials.

Storage and Handling of Welding Material
The purpose of storage and handling program is to control the potential for moisture (and thus hydrogen) absorption in welding materials. Under certain conditions, hydrogen can lead to “cold” or “delayed” cracking of the welded joint. An effective storage and handling program for welding materials should address the following areas as appropriate:

  1. Instructions for storage, rebaking, and amount of time of exposure to atmosphere of covered, flux cored and bare electrodes, and flux used in submerged arc welding;
  2. Provisions for heated storage ovens convenient to the work area;
  3. Provisions for return of unused electrodes to heated storage ovens;
  4. Reuse of flux.
 Next table is contains examples of typical storage and drying conditions for some commonly used covered arc welding electrodes. However, the supplier of the welding material should be consulted for the exact drying conditions because of the wide variety of inherent different from one manufacturer to another.

TYPICAL STORAGE AND DRYING CONDITIONS FOR
COVERED ARC WELDING ELECTRODES1








STORAGE CONDITIONS2




AWS Classifications
Ambient Air
Holding Ovens
Drying3
E6010, E6011
Ambient temperture
Not recommended
Not recommended
E6012, E6013, E6020, E6022, E6027, E7014, E7024
80±20oF (30±10oC) 50% max relative humidity
20oF (10oC) to 40oF (20oC) Above ambient temperature
275±25oF (135±15oC)      1 hour at temperature
E7015, E7016, E7018, E7028, E7048
80±20oF (30±10oC) 50% max relative humidity
50oF (30oC) to 250oF (140oC) Above ambient temperature
475±25oF (245±15oC)       2 hour at temperature
Note :
1 This table applies to carbon steel covered electrodes only. Refer to the manufacturer's recommendation for    highner strength, low alloy electrodes.
2 After removal from manufacturer's packaging
3 Because of inherent differences in manufacture, the suppliers of these electrodes should be consulted for the exact drying conditions.

Information for this table obtained from Specification for Carbon Steel Arc Welding Electrodes SFA-5.1, Table A1




























Inspection of Welded Joints
Fit-Up Inspection. The Inspector should schedule his inspections to check the fit-up of parts to be welded. The dimensions and shape of the edges to be joined should be such as to permit complete fusion and, where required, complete joint penetration. The placement of tack welds or clamps should be reviewed to assure that alignment tolerances of the applicable section of the ASME Code are maintained.

Completed Welds
Completed welds should be inspected for appearance and unsatisfactory conditions such as cracks, excessive reinforcement and excessive undercutting.

Nondestructive Examination
General. Section V of the ASME Code relates to the requirements and methods for nondestructive examination which are requirements to the extent they are specifically referenced and required by other ASME Code sections. These nondestructive examination methods are intended to detect surface and internal discontinuities in materials, welds, and fabricated parts and components. They include radiographic examination, ultrasonic examination, liquid penetrant examination, magnetic particle examination, eddy current examination, visual examination, leak testing and acoustic emission examination. In the foregoing methods, the skill, experience and integrity of the personnel performing these examinations are essential to obtain meaningful results. The Inspector should review the methods and procedures to be employed to assure compliance with the requirements of this Code.

Radiographic Examination (RT)
This method is commonly used to examine for surface and subsurface discontinuities. The use of this method may be restricted due to the configuration of the welded joint or the limitations of the radiographic equipment. Radiography will not give an indication of the depth of discontinuity unless special procedures are used.

The technique used for radiography depends largely on the equipment used and what experience has shown will produce the best results. It is not the function of the Inspector to indicate the procedure to be followed, provided the procedure and films satisfy all requirements of the applicable section of the ASME Code.

The interpretation of radiographic films of welds requires not only a knowledge of welding and welding discontinuities, but also the exercise of good judgement as to whether the discontinuities are actually defects. Where a difference of opinion exists, rather than cut out the weld, a reexamination of the suspect area using more sensitive techniques may indicate whether or not the weld is sound. Surface irregularities may show on the radiograph. In such cases, the film or tracing may be placed on the weld, the area in question located on the surface and the necessary correction made.

Ultrasonic Examination (UT)
This method will provide indications of surface and subsurface discontinuities, the depth of which can be determined by the use of the proper technique. Since normally there is no record of the results other than electronic indicators on a screen, the skill, experience and integrity of the personnel performing the test are of prime importance.

Ultrasonic examination for thickness determination may be made using either the resonance or pulse-echo technique.

Liquid Penetrant Examination (PT)
The liquid penetrant method is used to detect discontinuities which are open to the surface of the material being examined. This method may be used on both ferrous and non ferrous materials. Liquid penetrant examination may be used for the detection of surface discontinuities such as cracks, seams, laps, cold shuts, laminations and porosity.

Magnetic Particle Examination (MT)
The magnetic particle method can be used only on ferromagnetic materials to reveal surface discontinuities and to a limited degree, those located below the surface. The sensitivity of this method decreases rapidly with depth below the surface being examined and, therefore, it is used primarily to examine for surface discontinuities.

Eddy Current Examination (ET) of Tubular Products
The eddy current method is used to detect discontinuities in pipe and tubing by subjecting the material to a strong external magnetic field. A test coil which induces eddy currents in the material is used for this purpose. If a discontinuity exists, there will be variations in the eddy currents produced and these will be indicated by a signal in the test apparatus.

Visual Examination (VT)
Visual examination is generally used to determine such things as the surface condition of the part, reinforcement and undercutting of welds, alignment of mating surface, shape or evidence of leaking. Methods employed may be either (a) direct visual examination, (b) remote visual examination, or (c) translucent visual examination using artificial directional lighting.

Leak Testing (LT)
The leak tests are performed using gas and bubble formatting testing, vacuum testing, the halogen diode detector, the helium mass spectrometer reverse probe (sniffer), the helium mass spectrometer hood methods and the pressure change test.

Acoustic Emission Examination (AE)
Acoustic emission examination is used to identify areas of a vessel in which structurally significant surface and subsurface discontinuities are located. The method is used in conjunction with a pressure test or on-line pressurization, and assesses the structural intergrity of the entire vessel unit.

Other methods of nondestructive examination, in particular ultrasonic, are used following an acoustic emission examination to precisely located and characterize discontinuities that have been identified.

Qualification of Nondestructive Examination (NDE) Personnel
Nondestructive examination personnel shall be qualified in accordance with the requirements of the referencing ASME Code section. The qualification records of such personnel shall be available to the Inspector.

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