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Welding Procedures (overview)

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The cost impact of not being able to control the quality of welding and repair rates can be substantial. This will and can cost the contractor considerable delay in the completion of the project. Thereafter invoking contractual penalties. Resulting in the loss of profit margins and later arbitration.
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Approvals prior to the commencement of welding operations.
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Welding Procedure Specifications.
Procedure Qualification Reports.
Welder Certification Certificates.
Welding Consumables.
NDT Company.
NDT Operators.
NDT Procedures.
Radiation Safety.
Post Weld Heat Treatment Company.
Post Weld Heat Treatment Procedure.
Welding Inspection Personnel.
Welding Repair Procedure.
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Welding Defects Defined
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DEFECT CAUSE
Pipe off set mismatch Pipe misalignment
Lack of root penetration Welding technique
Insufficient root fill Welding technique
Excessive penetration Welding technique
External undercut Excessive amps/volts
Internal undercut Excessive amps/volts
Internal concavity Welding technique
Root burn through Welding technique
Lack of root penetration Weld joint set up
Interpass slag inclusions Weld technique, grinding, cleaning
Elongated slag inclusions Weld technique, grinding, cleaning
lack of side wall fusion Weld technique, amps/volts to low
Interpass cold lap Weld technique
Scattered porosity Weld technique
Cluster porosity Weld technique, insufficient wind cover
Root pass aligned porosity Weld technique, insufficient wind cover
Transverse crack Insufficient wind cover, lack of pre-heating & post weld heat treatment of weld joint. Material problem.
Longitudinal crack Insufficient wind cover, lack of pre-heating & post weld heat treatment of weld joint. Material problem.
Longitudinal root crack Insufficient wind cover, lack of pre-heating & post weld heat treatment of weld joint. Material problem.
Tungsten inclusions Weld technique
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Welding Process Metology
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Welding Procedures to avoid hydrogen induced cracking.
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To control cracking when completing the welding procedure the following factors must be considered.
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- Combined thickness of the material to be welded
- Carbon equivalent values
- Hydrogen scales
- Welding arc energy
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Codes and Standards That are most Commonly Used on Construction Projects
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Codes and Standards That are most Commonly Used on Construction Projects
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ASME Boiler and Pressure Vessel Code
Clause

Power boilers
Pressure vessels
Pressure vessels
Heating Boilers
Nondestructive Ex
Welding & brazing
qualifications
Section

I
VIII, DIVISION 1
VIII, DIVISION 2
IV
V
IX
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American Pipeline Institute

API

API

AWS

ANSI
1104

650

D1.1
Welding Pipelines & related facilities

Welded Steel Tanks for Oil Storage

American Welding Society

American National Standards Institute
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B31.1 Power Piping
B31.2 Industrial Gas & Air Piping
B31.3 Petrolium Refinery Piping
B31.4 Oil Transportation Piping
B31.5 Refrigeration Piping
B31.6 Chemical Industrial Piping
B31.7 Nuclear Power Piping
B31.8 Gas Transmission & Distribution Piping Systems

Method Statement

Welding of Piping and Structural Components

 

1) Welders shall only work within the limit of their qualification range.

2) Welding supervisor/welders shall ensure that "Welding Procedure Specification" (WPS) selection is taken from the approved matrix for each specific line class/application accompanying each WPS.

3) Withdraw consumables only from designated welding stores and endorse initials on the "Welding Consumables Distribution Form" maintained by the issue clerk.

4) Withdraw only sufficient quantities of welding consumable for a four (4) hour period of work.

5) All low-hydrogen welding electrodes shall be maintained in heated "portable rod caddies". The lid of the caddie should be closed following the withdrawal of each withdrawal electrodes.

6) Low-hydrogen electrodes shall only be conveyed to the job site in heated "portable rod caddies".

7) At shift change, all "rod caddies" must be returned to the originating welding store for checking. All issued and unused low-hydrogen electrodes shall be scrapped.

8) The technical requirements of the "Welding Procedure Specification" (WPS) must be followed at all times.

9) Pre-heat and interpass temperatures shall be monitored using "Tempil Stiks" or calibrated "digital" temperature by pyrometers. greater than the nominal wall thickness. ii) For flanges, use the thickness corresponding to two (2) higher wall thickness ranges greater than the nominal wall thickness.

10) Pre-heat values shall be taken calculated and WPS assuming a value of 0.42 CE (Carbon Equivalent). Wall thickness values shall be selected as follows:


a) For pipe-pipe, use the actual wall thickness.

b) For pipe-to-fitting or fitting-to-fitting.

c) For fittings (other than flanges), use the next higher wall thickness range a sufficient period of time to ensure an Oxygen content of less tan 0.5%. When available, "purge monitoring devices" shall be used.

d) Backpurge shall be maintained for a minimum of four (4) passes. Pipe bungs (purge dams) shall be left in place until completion of the joint.

e) The end of partly used filler wire shall be snipped off prior to use.

f) Maintain the tip of the filler rod within the gas shroud during welding to avoid contamination.
   g) Interpass temperatures for these materials is critical. The welder shall check the temperature prior to the next pass. All austenitic stainless welds on this project will be tested for "Ferrite" content therefore, cleanliness during fabrication and monitoring of interpass temperatures will have a direct effect on test results.

h) Only approved "iron-free" cutting and grinding disks shall be used.

i) Only approved marking materials (sulphur, chlorine free etc.) are to be used.

j) All material handling devices and equipment shall be adequately protected/lined, to prevent the possibility of carbon "pick-up" (i.e. workbench, transport trucks, supports etc.).

k) Wherever possible, tools shall be colour coded to prevent cross-use with other material types.

11) Pre-head shall be applied through the full thickness of the joint and checked from the opposite side wherever possible.

12) For repair welds, the pre-heating temperature shall be 500 C (1220 F) above that used for the original weld. Maximum pre-heat for repair welding is 1500 C (3020 F).

13) approved WPS. Internal misalignment for butt joints shall not exceed 1.5 mm (1/16'').

14) Deposit the root pass and six (6) successive passes or 1/3 of the weld volume prior to interruption (allowing to cool to ambient temperature).

15) Buttering (build-up) is permitted as follows:

a) Buttering shall not exceed the lesser of 10 mm or 1/3 base metal thickness.

b) If buttering will exceed 10 mm or 1/3 base metal thickness then this shall be witnessed by the customer and the area shall be tested by PT/MT following completion of buttering but before final welding of the joint.

16) Backwelding is permissible for all applications, so long as the same electrodes and process is used as for the fill pass.

17) The following points shall be observed when fabricating/welding stainless and non-ferrous materials:

a) Weld preps and filler materials shall be degreased using an appropriate solvent.

b) Weld prep surfaces shall be buffered using flapper wheels.

c) Argon hoses shall be checked for any loose connections or leakage etc.

d) Fit up geometry shall be in accordance with the applicable, approved WPS.

e) Bridge tacks to be used. Avoid tacking directly on to the adjacent pipe wall. Bridge tack within the fusion faces of the joint wherever possible.

f) Prior to welding, the backpurge shall be set up in accordance with the approved WPS for a sufficient period of time to ensure an Oxygen content of less tan 0.5%. When available, "purge monitoring devices" shall be used.

g) Backpurge shall be maintained for a minimum of four (4) passes. Pipe bungs (purge dams) shall be left in place until completion of the joint.

h) The end of partly used filler wire shall be snipped off prior to use.

i) Maintain the tip of the filler rod within the gas shroud during welding to avoid contamination.

j) Interpass temperatures for these materials is critical. The welder shall check the temperature prior to the next pass. All austenitic stainless welds on this project will be tested for "Ferrite" content therefore, cleanliness during fabrication and monitoring of interpass temperatures will have a direct effect on test results.

k) Only approved "iron-free" cutting and grinding disks shall be used.

l) Only approved marking materials (sulphur, chlorine free etc.) are to be used.

m) All material handling devices and equipment shall be adequately protected/lined, to prevent the possibility of carbon "pick-up" (i.e. workbench, transport trucks, supports etc.).

n) Wherever possible, tools shall be colour-coded to prevent cross-use with other material types.

   
NDT Technique Materials applicable Detection capability Depth Sizing Orientation Evaluation Access problem Remote Detection Automated detection
Liquid penetrant All Surface No No Yes No No
Ultrasonic All Volumetric Yes Yes Limited Yes Yes
Radiography All Volumetric Yes Yes Yes No Yes
Magnetic Particle Magnetic Surface, near-surface No No Yes No No
Magnetic Fluxleakage Magnetic Surface, near-surface Yes Yes No Yes Yes
Eddy current Conducting Surface, near-surface Yes yes Yes Yes yes
Acoustic Emission All Volumetric Yes No No Yes Yes
Thermo-graphy All Surface, near-surface No Yes No Yes Yes
Visual All Surface No No Limited Yes Yes
XRD Conducting Suface Stresses Yes No Yes No No
Potential drop Conducting Surface Yes No Yes No Yes

 

Engineering and Construction Codes and Standards

 

ASME IX - American Society of Mechanical Engineers
ASME IX - B 31.3 PROCESS PIPING
ASME IX - B 31.1 POWER PIPING

Material (P numbers) will assist the Welding Engineer to complete the Welding Procedure Specification prior to the mechanical testing of the Procedure Qualification Report.

P-numbers Material Composition
1 Carbon Steel
3 Up to 1/2% Cr and up to 1/2% Mo
4 1 to 2% Cr, 1% Mo Alloy Steel
5A 2 to 3% Cr, 1% Mo Alloy Steel
5B 5 to 10% Cr, 1% Mo Alloy Steel
5C All 5A and 5B Materials heat treated to 85ksi+
6 Martensitic Stainless Steel
7 Ferritic Stainless Steel
8 Austenitic Stainless Steel
9 2 to 5% Ni Alloy Steel
10 Mn-V, Cr-V, 9%Ni, High Cr Alloy Steels
11 Low Alloy Steel, Quenched and Tempered to 95ksi+
21 1.2% Mg or Mn alloy Aluminium
22 1.2% Mn, 2.5% Mg, 0.25% Cu Aluminium
23 1.3% Mg, 0.7% Si, 0.25% Cr Aluminium
25 1.5% Mg, 0.8% Mn, 0.15 Cr Aluminium
31 Copper
32 Admirally, Naval, Aluminium brass, Muntz Metals
33 Cu-Si Alloys
34 Cu-Ni Alloys
41 Nickel
51 Titanium
61 Zirconnium

 

Welding Defects

Cold Lap

Cold Lap is a condition where the weld filler metal does not properly with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into the base material without bonding

Porosity

Porosity is the result of "gas entrapment" in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called "wormhole porosity" All porosity is a void in the material and it will have a higher radiographic density than the surrounding area.

Cluster Porosity

Cluster Porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into gas when heated and becomes trapped in the weld during the welding process. Cluster porosity appears just like regular porosity in the radiograph but he indications will be grouped close together.

Slag inclusion

Slag inclusions are non metallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joints areas are indicative of slag inclusions.

Incomplete Penetration (IP) or lack of penetration (LOP)

Incomplete Penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well defined, straight edges that follows the land or root face down the center of the weldment.

Incomplete fusion

Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on radiograph is usually appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area.

Internal Concavity or Suck Back

Internal concavity or suck back is a condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to a lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

Internal or Root Undercut

Internal or Root Undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of ht weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

External or Crown Undercut

External or Crown Undercut is an erosion of the base metal next to the crown on the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Offset or Mismatch

Offset or Mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land area.

Inadequate Weld Reinforcement

Inadequate Weld Reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base metal. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be higher (darker) than the image density of the surrounding base material.

Excess Weld Reinforcement

Excess Weld Reinforcement is an area of a weld that has a weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

Cracks

Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks can appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

Discontinuities in TIG Welds

Tungsten Inclusion

Tungsten inclusion: Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radio graphically, tungsten is more dense than aluminum or steel, therefore it shows up as a lighter area with a distinct outline on the radiograph

Oxide inclusions

Oxide Inclusions are usually visible on the surface of material being welded (especially aluminum). Oxide inclusions are less dense than the surrounding material, and, therefore appear as dark irregularly shaped discontinuities in the radiograph

Discontinuities in Gas Metal Arc Welds (GMAW)

Whiskers are short lengths of weld electrode wire, visible on the top or bottom su6rface of the weld or contained within the weld. On a radiograph they appear as light, "wire like" indications.

Burn Through

Burn Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumps of metal sag through the weld, creating a thick globular condition on the back of the weld. These globs of metal are referred to as icicles. On the radiograph, burn through appears as dark sports, which are often surrounded by light globular areas (icicles).

Hands on Quality Control Weld Defects

     
     
     
     
     
     
   

 

PIPING MATERIAL COLOUR CODE

Piping Material Colour Code

 

WELDING METHOD STATEMENT

FLOW CHART OF PIPING PROCESS IN THE FABRICATION SHOP

DESIGN CODE ANSI B 31.3 & ASME SECTION IX

Welding Procedures

Control of Hydrogen Removal in 11/4 Cr. ½ Mo low alloy steel for the prevention of weld cracking

The above specification was reviewed and a comment made advising a 60 minute minimum soak at 300-400C on completion or partial completion of weldments in Cr. Mo. Material. The Specification tabulated 15minutes for materials 10inch SCH 80 and below (15mm and below) and 30 minutes for 12 inch SCH 80 and above (above15mm to 40 mm.).
The following notes are intended to give some additional information and should be taken into account.

The presence of hydrogen depends on a number of factors as follows:

  1. Joint cleanliness (removal of surface contaminants, scale, rust oil,grease,paint etc)
  2. Presence of moisture in the fluxes used in welding (controlled by baking and the use of low hydrogen consumables, typically 5ml hydrogen/100 gms weld metal maximum. Control and issue of electrodes is also crucial after baking.
  3. Preheat controls (adequate in Chiyoda's specification)
  4. Microstructure susceptibility (high in this alloy)
  5. Stress levels (greater in thicker joints).
  6. Temperature limits on cooling. (the greatest risk occurs when near ambient temperatures are reached and cracking can occur several hours or sometimes up to three days after welding is completed.

Most Fabricators/Contractors are aware of these causes and take measures to reduce the risk where possible. The common additional technique to remove/reduce hydrogen levels is by introducing the post weld heating cycle. Cracking is unlikely to occur when adequate soak times above 250C are used immediately on completion or partial completion of welds. Removal rates are dependant on soak time and temperature. The table overleaf gives some details for the amount of hydrogen removal based on different soak times and temperatures for the SMAW process using low hydrogen electrodes, stored, baked, issued to the welder and controlled in accordance with the manufacturer's recommendations.

Material
Thickness

Soak Temperature
(Centigrade)

Soak Time
(Minutes)

% Original
Hydrogen Remaining

7mm 300 60 0
7mm 300 15 20
15mm 300 60 20
15mm 300 15 70
15mm 400 15 65
30mm 300 60 75
40mm 300 30 90
40mm 400 30 85

 

These figures were calculated by the Welding Institute who emphasised that although the technique for calculation is considered conservative they have been shown to produce satisfactory results over a two year period. They take account of the potential hydrogen which is a laboratory measurement of the moisture or hydrogen of any consumable and a diffusivity (of hydrogen) factor for Cr/Mo material of 4 x 10-5 cms/sec.

Assuming low hydrogen electrodes are used and taking account of the potential hydrogen factor, the average deposited weld metal hydrogen would be 5ml hydrogen/100gms weld metal. Hydrogen cracking has been encountered in forgings as low as 2-5 ml hydrogen /100 gms weld metal. Based on all information above i.e Chiyoda's experience and the Welding Institute (conservative) recommendations our advice is as follows: