An Intriduction To Fracture Modes , Welding Defects,Causes And Remedies

An Intriduction To Fracture Modes , Welding Defects,Causes And Remedies


Introduction :
Welding is one of the most important techniques in the fabrication industries to join metals in different geometries and sizes with cost-effective and reliable assembly. There are several types of welding processes used in the petrochemical industry that have been around for many decades and new methods developed in recent years. Basically, these processes vary in setup, essential variables and non-essential variables, such as shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), etc. Each welding process has characteristics that affect its quality performance and the soundness of the weld. For example, a weld acceptable for one application, such as for a tank, may not meet the acceptance criteria for pressure vessels per applicable international codes. Welding imperfections such as cracks, porosity, lack of fusion, incomplete penetration, and spatter could be due to various causes, such as poor workmanship, design issues, incorrect material, improper weld procedure specifications, and/or an unfavorable environment. The impact of each defect varies from acceptable to not acceptable, and must be either repaired or cut-out.

Types of Welding 

1. Gas welding 

2. Arc welding 

3. Thermal and Chemical welding 

4. Resistance welding 

5. Newer welding techniques (electron beam,laser)

Definition of Welding Defects 

The defects are those unwanted irregularities or discontinuities produced during the welding of a metal which is beyond the accepted standardized limits and hence can eventually cause the metal to fail.It arises due to many factors like

1. Poor environmental conditions 

2. Incorrect welding parameters or applications 

3. Incorrect combination of the filler metal with the base metal 

It affects the weldability of the metals and hence decreases the chances of the metal to withstand a certain amount of load,hence not making the welded metal suitable for its satisfactory needs for which it was being designed

Classification of welding defects

Before analyzing each and every defect,these defects are classified into two groups

I) External or visual defects are those defects which occur externally at the surface of the weld and can be observed 

1. cracking 

2. porosity 

3. undercut 
4. underfill

5. overlap 

6. spatter 
7- weld decay

II) Internal or hiddendefects are those that occur internally and cannot be observed

1. slag inclusion 

2. lack of fusion

3. incomplete penetration 

4. shrinkage cavities

FIG 1

1- Cracking 

A.Cracking Probably one of the most problematic welding defects is cracking figure 2.Cracks may develop on the interior of a weld or at the surface and along many directions.It may also appear on the areas subjected to high temperatures.It destroys the shape and design of the weld and also makes it distorted. Its hard to observe when cracks develop internally and hence can make the weld less efficient.These defects cannot be overlooked and must be corrected as soon as possible.

FIG 2

Causes
1. Cracking generally takes place due to poor designing of the weld..
2. Prolonged exposure to contraction stresses.
3. Poor metallic properties of the base metal.
4. Due to unequal heating and during thermal shrinkage of the metals.
5. Electrodes having high hydrogen content.
Remedies
1. Avoiding instant cooling in order to prevent rapid shrinkage.
2. Equal and Pre-heating from time to time.
3. Electrodes having low hydrogen content.
4. Proper and symmetric designing of the material.
5. Keeping the surface clean before welding.
6. Reducing the gaps in order to prevent cracking during solidification.
7. Using materials having low impurity levels.

Cracks presents themselves in two major types:
Hot Cracks– These cracks occur during the welding or during crystallization where temperature can be as high as 10000-degrees Celsius.
Cold Cracks– Cold cracks occur after completion of the welding process or during the solidification process. They are normally visible after several hours or even several days after welding.

Cracks can be classified by shape figure 3 (longitudinal, transverse or branched) and position (HAZ, base metal, centreline, crater). An inspector will rarely classify a crack as a particular type
(i.e. fatigue, HICC or SCC) because in most cases it will not be possible to determine the precise cause of a crack until an examination is carried out. The shape and position are facts, anything else is supposition.

FIG 3

1-1 Hydrogen-induced cold cracking (HICC)

HICC may occur in the HAZ of all hardenable steels (i.e. C, C/Mn) or in the weld metal of high strength low alloy (HSLA) steels that are microalloyed with small amounts of titanium, vanadium or niobium (typically <0.05%).

hydrogen breaks down at increased temperatures into atomic hydrogen (which has a small atomic size) and escapes to the atmosphere through the steel microstructure. When the temperature reduces to below around 300 ⁰C the hydrogen starts reforming to the hydrogen element and will no longer

be able to escape from the material. As the H2 reforms it may build up an internal pressure stress within the material structure itself.

FIG 4

1-2 Solidification cracking

Solidification cracking figure 5 (also known as centreline cracking or hot cracking) is a fracture that occurs in the weld metal of ferritic steels with a high sulphur or phosphorus content or in joints with a large depth/width ratio.

FIG 5

1-3 Reheat cracking

Reheat cracking occurs primarily in the HAZ of thick-section high strength low alloy (HSLA) steels, 300 series stainless steels and nickel-based alloys. During PWHT or elevated service temperatures intergranular cracking can occur due to stress relaxation in coarse grained regions under high

restraint or residual stresses. The failure normally initiates at a stress concentration such as a notch or change in crosssection.

Reheat cracking can be avoided by:

. adequate preheating to reduce the stress levels in the HAZ;

. using joint designs that require less restraint during welding in thick sections;

. removing stress concentrations caused by sharp changes in cross-section, such as sharp undercut,        mechanical damage and poorly blended weld toes.

FIG 6

1-4 Lamellar tearing

Lamellar tearing occurs mainly in thick-section T-joints and closed corner joints in carbon and carbon manganese steels with a high sulphur content and/or high levels of restraint. It does not occur in cast or forged steels; only in rolled plate. It has the appearance of a ‘steplike’ crack (Fig. 7) and occurs in wrought (rolled) plates due to a combination of:

. contraction stresses from the cooling weld acting through the parent plate thickness plus

. poor through-thickness ductility due to impurities in the steel (such as sulphides, sulphur, microinclusions and small laminations).

FIG 7

Standard acceptability There is no acceptability for cracks in welding.

2-Porosity

Porosity It is that type of defect in which the welded area has small groups of voids or gas bubbles trapped inside.They may appear spherical in shape like a small colony of cavities.

Causes

. Loss of gas shield.

. Damp electrodes or fluxes.

. Arc length too large.

. Damaged electrode flux.

. Presence of excess moisture content, grease, and oil.

Remedies

1. Selecting electrodes with a proper coating.

2. Maintaining the arc length.

3. Proper cleaning and decreasing of the moisture content at the surface.

FIG 8

Standard acceptability

3-Undercut

UndercutDuring welding,the material has to go through high temperatures and residual stresses due to which it has a tendency to undergo a structural damage.Undercuts are those defects that develop at the welding zone when the base metal melts away forming a depression like groove or a notch and hence can be seen when there is a deep penetration. These defects reduce thestrength of the welding joint.It is mainly an arc welding defect.

Causes

. excessive amps/volts;
. excessive travel speed;
. incorrect electrode angle;
. incorrect welding technique;
. electrode too large.

. Accumulation of rust at the surface of the welding joint.

Remedies

1. Cleaning of the base material after welding.

2. Avoiding exposure to moisture and hence not allowing the metal to rest.

3. Proper arc and electrode settings.

4. Using a smaller electrode.

FIG 9

Standard acceptability Undercuts are not allowed more thanB 0.8mm diameter and must not exceed to more than 4-5% depth from the base material.

4-Underfill

Underfill (Fig. 10) is the term given to a joint that has not been completely filled to the parent metal surface but the edges of the joint have been fused. 

Causes 
. too small an electrode being used;
. too few weld runs;
. poor welder technique.

FIG 10

Standard acceptability

The fillet leg dimension shall not under run the momiral fQ.let size by more than 1/16 inch. For flange to web jolts the undersize condition may not be within two flange thicknesses of the weld end. 

Groove welds may be underfilled by 5 percent. or 1/32-inch, whichever is greater. 

5- Overlap 

Overlap This kind of defect usually occurs due to poor welding procedures and parameters.Overlapping or the over flow of the welded metal takes place beyond its welded joint or toe.

FIG 11

Causes

1. Improper angles of electrode and wrong travel velocities of the arc.

2. Poor welding techniques,procedures and applications.

Remedies

1. Grinding off the excess metal spill can make the welded joint smooth and repair the overlapping.

2. Using proper and correct welding techniques.

Standard acceptability Overlapping of the welded joint cannot be accepted.

6- Spatter

Spatter is molten globules of consumable electrode that are ejected from the weld and quench quickly wherever they land on the weldment. They can therefore cause cracking on susceptible materials so they should be removed and then the area tested with PT or MT. Other problems caused by spatter

include prevention of UT (because UT needs a smooth surface for the probes), unwanted retention of penetrant during PT and problems with paint retention. 

Causes

. excessive current;

. damp electrodes;

. surface contamination from oil, paint, moisture or grease;

. incorrect wire feed speed during MAG welding.

. Large arc length during arc welding process.

Remedies

1. Scrapping or cleaning of the product by hair brush or by
    washing after welding.
2. Proper use of welding equipment.
3. Using correct welding current and arc length.

FIG 12

Standard acceptability spatters are not accepted in welded products as spatters are easier to remove as well.

7-Weld decay

Weld decay is a form of intergranular corrosion that occurs in the HAZ of unstabilised stainless steels. Within the temperature range of around 600—850 ⁰C chromium comes out of solution  to join with free carbon and form chromium carbides. The chromium was in the grain to help

prevent corrosion so corrosion can now occur where it has been depleted. Once this chromium depletion occurs the depleted area is said to be sensitised (meaning it is susceptible to corrosion) and will corrode in the presence of an electrolyte. The critical region is usually in the HAZ parallel to the weld toes (Fig.13) and once the area is sensitised, corrosion can lead to rapid failure.

Weld decay can be avoided by:

. Using low carbon grade stainless steels.

. Using stabilised stainless steels instead of unstabilised grades.

. Quench cooling.

. Keeping heat inputs and interpass temperatures low.

. Solution heat treatment after welding.

FIG 13

8- Slag inclusion

Slag inclusion When impurities,fluxes or many other particles and droplets which can be metallic or sand are entrapped inside the welded zone, inclusions occur which makes the welded metal

brittle.It may be present internally, on the surface and across turns.This defect greatly affects the structural design of the metal and affects its weldability and toughness thereby making it more

susceptible to fractures

Causes

. inadequate cleaning of slag originating from the welding flux;

. inadequate removal of silica inclusions in ferritic steels during MAG or TIG welding;

. touching the tungsten to the weld pool during TIG welding;

. the melting of the copper contact tube into the weld pool during MIG/MAG welding.

. Improper cleaning of the surface.

Remedies

1. Reducing rapid cooling and solidification

2. Maintaining welding speed.

3. Grinding and cleaning of the weld.

Standard acceptability Inclusions should exceed more than 3.2mm in diameter approximately after every 5 inches of weld.

FIG 14

9- Lack of fusion

Lack of fusion (Fig. 15) is weld metal not correctly fused to the parent material or the previous weld bead. 

Causes 

. incorrect joint preparation (narrow root gap, large root face);

. incorrect welding parameters (current too low);

. poor welder technique (incorrect electrode tilt or slope angles);

. magnetic arc blow;

. poor surface cleaning.

Remedies

1. Proper cleaning and positioning of the bead.

2. Maintaining the speed and welding current.

3. Proper cleaning and procedures of welding.

4. Re-welding can also avoid such type of errors.

Standard acceptability Incomplete fusion is not accepted and hence re-welding must be done.

FIG 15

10-Incomplete root penetration

Incomplete penetration usually occurs when the molten metal does not extend to the appropriate depth of the root hence leaving behind an incompletely filled groove.It may also lead to the propagation of cracks

Causes

1. Improper filling of the root gap.

2. When welding metal deposition is low.

3. When electrode diameter is large.

4. Welding done at low temperatures.

Remedies

1. Proper filling of the molten metal.

2. Proper supply of heat.

3. Appropriate functioning and proper diameter of the electrodes.

Standard acceptability Weld metal should not have any type of penetration.

FIG 16

11-Shrinkage cavities

Shrinkage cavities The last and final defect for this paper. Shrinkage cavities are formed during solidification of the welding metal.The welding metal shrinks during solidification.Allowances

must be provided for this defect.

Causes

 It occurs many times during solidification of themetal.

Remedies

. Deposition of filler metal must be done that can compensate foVr the loss of metal during shrinking.

Standard acceptability Shrinking of the metals must be compensated as it may not be able to provide a proper fitting or joining.

12-Root concavity

Root concavity (Fig. 17) is a groove in the root of a butt weld, but with both edges correctly fused. It is sometimes referred to as ‘suck back’. 

Causes

. the root face or root gap too large;

. excessive purge pressure being applied when welding using the TIG process;

. excessive root bead grinding before the application of the second weld pass.

Remedies

. Increase heat input. Reduce root gap. 

. Reduce backing gas pressure. 

. Reduce current for overhead welds.

Standard acceptability some welding codes limit root concavity to 1/32" or about .8mm
others allow for 1/16" 1.3mm

FIG 17

13- Excessive root penetration

A protruding penetration bead is classed as excess penetration because it is excess to requirements and does not contribute to the weld strength. If the level of root penetration is in excess of the design code acceptance criteria it is then classed as excessive penetration (Fig. 18). Do not confuse excess penetration (which may be acceptable to code requirements) with excessive penetration (which by definition is not acceptable to code requirements). 

Causes 

• Incorrect welding parameters. 

• Weld energy input too high. 

• Travel speed too low. 

• Size and type of electrode and welding position. 

• Incorrect assembly or preparation e.g. edge preparation too thin to support the weld underbead,           excessive root gaps, gaps in the abutments of a backing bar system, irregularities in the weld                bead  support system when “single-sided” mechanised welding is used. 

• Poor welder technique.

Remedies

• Verification of weld parameters by qualified testing/welder. 

• Adjusting the welding conditions. 

• Better weld preparation at the root of the joint. 

• Use of permanent or temporary backing bars. 

• Attention to the fit-up of backing systems in single-sided welding.

Standard acceptability When viewing radiographs and there is excessive penetration and the acceptance criteria says maximum of 2mm protrusion.

FIG 18

14- Arc strike

An arc strike (or stray flash) is accidental arcing on to the parent material. This can lead to cracking on crack-sensitive materials due to the fast quenching of the arc strike, causing localised hardened regions. These hardened regions are susceptible to brittle fracture. They can also cause stress

concentrations leading to in-service failures such as fatigue fractures. Arc strikes caused by poorly insulated cables or loose earth clamps may introduce copper or other dissimilar materials into the weldment, causing liquation cracking or other contamination problems. Arc strikes on susceptible

materials require removal and PT or MT to ensure no cracking is present.

FIG 19

15-Magnetic arc blow

Magnetic arc blow is an uncontrolled deflection of the welding arc due to magnetism. This causes defects such as lack of root fusion or lack of sidewall fusion. 

Causes :

. deflection of the arc by the Earth’s magnetic field (can occur in pipelines);

. poor position of the current return cable (the magnetic field surrounding the welding arc interacts        with the current flow in the material to the current return cable and is sufficient to deflect the arc);

. residual magnetism in the material causing distortion of the magnetic field produced by the arc            current.

Remedies

. welding towards or away from the clamp;

. using a.c. instead of d.c.;

. demagnetising the steel before welding.

FIG 20