INSPECTION and TESTING of WELDS & Acceptance Standard

Transcription

1 INSPECTION and TESTING of WELDS & BY A.K.BOSE CONSULTANT Topics of Discussion Important Definitions related to Welding Weld Inspection- Before, During and After Weld Inspection Instruments and Gauges Common weld discontinuities-types and its causes Common Destructive testing Common NDT testing its principle, applicable areas and limitations. Classification of discontinuities as per ISO Acceptance standard as per ISO Conclusion. - by Aloke Kr. Bose, Consultant 1

2 Some Definition Related to Weld Inspection Quality-The totality of features and characteristics of a product or service that bear on its ability to satisfy stated and implied needs. Weld Quality-Weld quality is defined as the level of perfection that a weld exhibits. Weld quality pertains to the entire volume of the weld metal that is in a weldment as well as the surface appearance of a weldment. Discontinuity-An interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a defect. Defect-A flaw or flaws that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications. The term designates rejectability. Inspection-Inspection is that quality control action by means of examination, observation or measurement to determine the conformance of material parts, components, system, structures as well as processes and procedures with predetermined quality requirements. Testing -It is defined as the physical performance of operations to determine quantitative measure of certain properties It aim to determine quantity i.e. to discover facts regardless of the implication of result. Acceptance Criteria - The term Acceptance Criteria is used by any standards define the required level of quality Weld acceptance criteria -the weld acceptance criteria by any standard are a list of maximum allowable weld discontinuities/imperfections in the weld. These discontinuities /imperfections can refer to the size (length, width, depth or diameter) of the weld discontinuities/ imperfections or the quantity of the same. When quantity of the weld discontinuities/imperfections is indicated as the acceptance criteria, the allowable quantity is usually specified per unit length of weld. Nine Variables of Welding 1) Joints 2) Base metal 3) Filler metal 4) Position 5) Pre-heat 6) PWHT 7) Shielding Gas 8) Electrical Characteristics 9) Technique - by Aloke Kr. Bose, Consultant 2

3 Inspection Before Welding 1) Application Standard 2) WPS,PQR,WPQ 3) Drawings 4) Material Composition 5) Condition of Material 6) Type of edge preparation,method & finish 7) Consumables 8) Welding process 9) Clearance dimensions,type of backing(if any) 10) Alignment, Tack welds,presets etc. 11) Pre-heat (if any) Inspection During Welding 1) Welding Process Parameter 2) Inter pass Temperature 3) Filler metal/electrode condition 4) Inter pass cleaning 5) Distortion control 6) Flux /Shielding gas flow - by Aloke Kr. Bose, Consultant 3

4 Inspection After Welding 1) Dimensional accuracy 2) Appearance of the weld 3) Post Weld Heat Treatment (if any) 4) Evaluation of internal and surface defects with or without the aid of Destructive/Non-destructive testing. Welding Inspectors Instruments Measuring devices: flexible tape, steel rule Temperature indicating crayons Welding gauges Voltmeter Ammeter Magnifying glass Torch / flash light Gas flow-meter - by Aloke Kr. Bose, Consultant 4

5 Welding Inspectors Gauges 0 IN 1/4 1/2 3/ HI-LO Single Purpose Welding Gauge TWI Multi-purpose Welding Gauge Misalignment Gauges Hi-Lo Gauge Throat Thickness Checking Fillet Weld - by Aloke Kr. Bose, Consultant 5

6 Reinforcement Cap Measurement Measuring the height of the cap Bevel Angle Measurement - by Aloke Kr. Bose, Consultant 6

7 Misalignment Measurement Shown on scale Weld Size Gauge - by Aloke Kr. Bose, Consultant 7

8 Types of Discontinuities The types of welding discontinuities are:- Dimensional Structural Property related(chemical, Mechanical or Metallurgical) Dimensional Dimensional discontinuities refer to the interruptions in continuity of the weld size, shape, and finished dimensions. Incorrect weld size and profile along with distortion are most pronounced dimensional discontinuities that if left unattended can lead to in-service failure by fatigue or overload. Structural Structural discontinuities are those that develop within the weld during welding. They include slag inclusions, porosities, cracks, incomplete fusions, Incomplete penetrations and undercuts. Depending upon their type and extent, single or combination of those discontinuities can lead to an in- service leak or rupture in the facilities, causing an environmental and personal disaster. Property Related The most pronounced of these discontinuities are those related to welding consumables and base metal properties. Use of incorrect chemistry and/or mechanical properties of filler metals and fluxes often leads to premature in-service failure of pipeline welds. Such failures are due to weld metal cracking, high hardness and using metals susceptible to corrosion. Not all weld discontinuities are caused by improper welding conditions or welding consumables. Base metal properties that do not meet the prescribed requirements of chemical composition, mechanical properties, metallurgical structure and surface condition can result in unacceptable weld discontinuities/defects that lead to failure during fabrication or in-service. Weld Discontinuities Dimensional Structural Property Related Misalignment (hi-lo) Undercut Lamellar tearing Underfill Burn-through Laminations and Delimitations Concavity or Convexity Incomplete or Insufficient Penetration Laps and Seams Excessive reinforcement Incomplete Fusion Heat-affected zone microstructure alteration Improper reinforcement Surface irregularity Overlap Unequal fillet leg size Bulbous Contour Inclusions Slag Wagon tracks Tungsten Porosity Uniformly Scattered Cluster Linear Piping Spatter Cracks Longitudinal Transverse Crater Throat, Toe, Root Under bead and Heat-affected zone Hot, Cold or delayed - by Aloke Kr. Bose, Consultant 8

9 Common Weld Discontinuities-Porosity Causes of Porosity Base Metal contaminated Wet, unclean electrodes Insufficient or damp shielding gas Excessive arc length Welding current too high Weld speed too fast Common Weld Discontinuities-Slag Inclusions Causes of Slag Inclusion Insufficient inter-pass cleaning Unsteady travel speed Welding weave too wide Allowing slag to run ahead of the arc Using too large and electrode - by Aloke Kr. Bose, Consultant 9

10 Common Weld Discontinuities-Incomplete Penetration Causes of Incomplete Penetration Too small a root opening Too large a root face Excessive high low Travel speed too fast Welding current too low Electrode diameter too large Excessive arc length Common Weld Discontinuities-Excess Penetration Causes of Excessive Penetration Too large a root opening Too small a root face Welding current too high Travel speed too low Arc length too short - by Aloke Kr. Bose, Consultant 10

11 Common Weld Discontinuities-Incomplete Fusion Causes of Incomplete Fusion Improper electrode angle Travel speed too fast Welding current too low Faulty joint preparation Electrode diameter too large Common Weld Discontinuities-Cracks Causes of Cracks Base metal contaminated Excessive joint restraint Incorrect pre-heat Incorrect filler metal Too fast a cooling rate - by Aloke Kr. Bose, Consultant 11

12 Common Weld Discontinuities-Undercut Causes of Undercut Incorrect electrode manipulation Welding current too high Excessive arc length Travel speed too fast Common Weld Discontinuities-Tungsten Inclusions Causes of Tungsten Inclusions Contact of electrode tip with weld pool Contact of filler metal with tip of electrode Contamination of electrode Exceeding current limit for the electrode Splits or cracks in electrode - by Aloke Kr. Bose, Consultant 12

13 Cracks most unacceptable defects Cracks form in the weld and base metal when localized stresses exceeds the ultimate strength of the material. Cracking may occur at elevated temperature during weld metal solidification or after solidification,whenthe metal solidification has equalisedin temp. Cracks can be classified as either hot cracks develop at elevated temperature during solidification Or cold cracks which are develop after solidification is complete. Hot cracks propagates along the grain boundaries only but cold cracks propagates both along grains boundaries and through grains. Relation between type of loading vis-à-vis weld discontinuties Type of Defect Type of Loading Effect on Joint Porosity & Slag inclusion(isolated type) Porosity & Slag inclusion(isolated type) Porosity & Slag inclusion(arranged in linear fashion along the weld) Static Impact and vibratory load Static and repeated Reduces effective area of x-sec available for loading Weld deposits become less ductile,reduces fatigue life- critical when defects open to surface of the weld May cause total failure of the weld Not acceptable - by Aloke Kr. Bose, Consultant 13

14 Relation between type of loading vis-à-vis weld discontinuties(contd.1) Type of Defect Type of Loading Effect on Joint Cracks Static loads Source of stress concentration and reduction in available area of x-sec.- Not acceptable Lack of Fusion Static and Dynamic 5% of lack of fusion reduces fatigue limit by 23%- Not acceptable Lack of Penetration- Root for single V groove Lack of Penetration- Root for double V groove Static and Dynamic Static and Dynamic Develops failure cracks- Not acceptable The void at the centre becomes a source of weakness- Not acceptable Relation between type of loading vis-à-vis weld discontinuties(contd.2) Type of Defect Type of Loading Effect on Joint Reinforcement Static Not very critical Reinforcement Dynamic Effect fatigue life adversely and it should be machined or handground flush with the parent metal - by Aloke Kr. Bose, Consultant 14

15 Types of Destructive & Non-destructive Testing Destructive Non-destructive Tensile Test Charpy V-Notch Impact Test Hardness Test Fracture Test Bend Test Ultrasonic Test (UT) Magnetic Particle Test (MPT) Liquid Penetrant Test (LPT) Radiography Test (RT) Visual Test (VT) Tensile Test Transverse Tensile Specimen All-Weld Metal Tensile Specimen - by Aloke Kr. Bose, Consultant 15

16 Charpy V-Notch Impact Test Objectives: measuring impact strength in different weld joint areas assessing resistance toward brittle fracture Information to be supplied on the test report: Material type Notch type Specimen size Test temperature Notch location Impact Strength Value Charpy V-Notch Impact Test Specimen Pendulum (striker) - by Aloke Kr. Bose, Consultant 16

17 Brinell Hardness Test Hardened steel ball of given diameter is subjected for a given time to a given load Load divided by area of indentation gives Brinell hardness in kg/mm 2 More suitable for on site hardness testing 30KN Ø=10mm steel ball Fillet Weld Fracture Tests Hammer 2mm Notch - by Aloke Kr. Bose, Consultant 17

18 Fillet Weld Fracture Tests This fracture indicates lack of fusion This fracture has occurred due to saw cut to root Bend Tests Objective of the test: To determine the soundness of the weld zone. Bend testing can also be used to give an assessment of weld zone ductility. There are three ways to perform a bend test: Root bend Face bend Side bend Side bend tests are normally carried out on welds over 12mm in thickness - by Aloke Kr. Bose, Consultant 18

19 UT- Its Principle,Application areas & Limitations Principle:-In this NDT method beams of high frequency sound waves are introduced into the material to detect surface and sub-surface flaws. The sound waves travel through the material with some attenuation of energy and are reflected at interfaces. The reflected beam is detected and analyzed to define the presence and location of any flaw. Application areas:-almost all weld discontinuties can easily be detected by the use of this technique which includes voids, cracks, inclusion etc. It is best suitable to detect planner discontinuties. Limitation:-Manual testing require careful attention by experienced technicians. Parts that are rough, irregular in shape, very thin or not homogenous are difficult to be tested. Ultrasonic Testing (Contd.) - by Aloke Kr. Bose, Consultant 19

20 Ultrasonic Testing (Contd.) MPT- Its Principle,Application areas & Limitations Principle:-It depends on the principle that when the material or part under testing is magnetized, discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. Application areas:-it may be applied to all types of heavy weldments as long as the materials are magnetic.mpt is frequently used to inspect plate edges prior to welding. The purpose of this type of inspection is to detect cracks, laminations, inclusions and segregations. Limitations:-Thin coating of paint and other nonmagnetic covering adversely affect sensitivity of this test. This method will work only on ferromagnetic materials. Demagnetizing following the test is often necessary. - by Aloke Kr. Bose, Consultant 20

21 Magnetic Particle Test (Contd.) RT- Its Principle,Application areas & Limitations Principle:-This method is based on the principle of differential absorption of penetrating electromagnetic radiation of very short wave length. Application areas:-it is extensively used for detecting almost all types of weld defects.it is best for detecting voluminous defects.both ferrous, non-ferrous metals/non-metallic materials and composites can be tested by this method. Limitations:-Compared to other NDT methods, it is expensive.high activity source require heavy shielding for protection of personnel.laminations are impossible to detect with this method. - by Aloke Kr. Bose, Consultant 21

22 Radiography Testing(Contd.) Radiography Testing (Contd.) - by Aloke Kr. Bose, Consultant 22

23 Radiography Testing (Contd.) LPT- Its Principle, Application areas & Limitations Principle Application Areas Limitations In this method the liquid penetrant seep into various types of minute surface openings(as fine as 4 micro inch in width) by capillary action It is used extensively for testing of weld defects of wrought and cast products of both ferrous and nonferrous metals. Surface defects as cracks, porosity etc. on the surface and mainly chip back testing of weldments during multipass welding. The major limitation of this method of testing is that it can detect those discontinuities that are open to surface. It is restricted to non porous metals/nonmetals only. - by Aloke Kr. Bose, Consultant 23

24 Liquid Penetant Test (Contd.) Liquid Penetrant Test (Contd) - by Aloke Kr. Bose, Consultant 24

25 VT- Its Principle,Application areas & Limitations Principle Application Areas Limitations It utilizes the most sensitive organ of our body i.e. eyes for detecting and evaluating surface defects. For detection of following surface weld defects. Cracks Surface Irregularities. Contour Defects and Root Defects It is restricted to exposed or accessible surface and discontinuities of opaque materials only. Classification of Imperfections ( Discontinuities) as per ISO :2007 In order to avoid any confusion, different type of weld imperfections (Discontinuities) are very systemically numbered and defined with explanations and illustrations in ISO As per ISO the numbering system of different weld imperfections (Discontinuities) have been classified into six main groups. Cracks Cavities Solid inclusions Lack of fusion and penetration Imperfect shapes and dimensions Miscellaneous imperfections - by Aloke Kr. Bose, Consultant 25

26 Few Examples of Discontinuities as per ISO Sl. Nos. Classifications Basic Code Nos. Sub Code Nos. (Few Examples) Explanation of Sub cod Nos. 1 Cracks Cavities Solid Inclutions LOFs & IPs Longitudinal Cracks Transverse Cracks Crater Cracks Gas Cavity Shrinkage Cavity Micro-shrinkage Slag Inclusion Flux Inclusion Metallic inclusion Lack of fusion Incomplete Penetrations Spiking 5 ISs and IDs Undercut Excessive Penetration Burn -through 6 Misc. Imperfections Arc Strike Spatter Slag Residue Acceptance standards as per ISO 5817:2003 This standard provides quality levels of imperfections (discontinuities) in fusion-welded joints in all types of steel, nickel, titanium and their alloys. It is applicable to material thickness above 0.5 mm thickness. It covers full penetration butt welds and all fillet welds. As per this standard the quality levels are designated in three different classes, B,C and D. Quality level B corresponds to highest quality requirement on finished weld while D corresponds to the lowest. Requirements as per this standard have been illustrated in the following few tables. - by Aloke Kr. Bose, Consultant 26

27 Acceptance standards as per ISO 5817:2003- Example 1 Acceptance standards as per ISO 5817:2003- Example 2 - by Aloke Kr. Bose, Consultant 27

28 Inspection and Testing of Welds & Acceptance standards as per ISO 5817:2003Example 3 Acceptance standards as per ISO 5817:2003Example 4 - by Aloke Kr. Bose, Consultant 28

29 Inspection and Testing of Welds & Acceptance standards as per ISO 5817:2003Example 5 Acceptance standards as per ISO 5817:2003Example 6 - by Aloke Kr. Bose, Consultant 29

30 Conclusion We know that ISO 9001 standard specifies requirement for a quality management system where an organization needs to demonstrate its ability to provide consistency in there product/services that meets customer s need and regulatory requirement as well as enhance customer s satisfaction with an approach for continual improvement of its processes and product/services. As per Cl. No of ISO 9001 Welding is considered as a special process where result of this process cannot be fully verified by subsequent inspection and testing of the product and processing deficiencies may become apparent only after the product is in use, hence qualitative and quantitative measurement of this process is a must to achieve desired quality assurance. Hence, ISO 3834 defines various approaches to quality requirements for welded fabrication, both in work-shops and on sites and provide guidance for describing the ability of a manufacturer to produce welded constructions of agreed and specified quality. More over this standard has been in force in most of the developed countries for all welding related manufacturing products. In order to adhere to the above objectives an organization should adopt different quality level depending on the criticality of the product as described in ISO 5817 and understand discontinuities classifications as per ISO ISO 9606 which is considered to be less complex than that of ASME Sec.IX may be adopted for preparation of WPS, PQR and WPQ in future which is mandatory as per ISO Thank You for your patient hearing. - by Aloke Kr. Bose, Consultant 30