Defects in Weld Metals | Industries | Metallurgy

The ideal weld should be such that adequate fusion exists between the filler metal and edge preparation together with good penetration. Slight reinforcement should also exist above the two welded parts.

A wide is subjected to two classes of defects:

(a) Those due to faulty technique on the part of operator such as lack of fusion, lack of penetration, over-penetration, slag lines.

These are basically the faults of the welder or choice of process: 

i. Insufficient Fusion:

It is usually caused by insufficient heat and too fast a travel of torch/electrode.

ii. Slag Inclusions:

Inclusions may be caused by contamination of the weld metal by atmosphere. Slag inclusions in arc welding are generally derived from electrode coating materials or fluxes employed in welding operations. In multilayer welding operations, failure to remove the slag between layers will result in slag inclusion in these zones. Other causes may be wide weaving, long arc length, less electrode angle, slow speed, too large electrode.

iii. Inadequate Penetration:

It exists in groove welds when the deposited metal and base metal are not fused at the root of the weld. This condition occurs when the root fence has not reached fusion temperature along its entire depth. Inadequate penetration may also be caused by weld metal bridging from one member to the other.

The condition may be caused by areas of base metal above the root reaching a molten condition before the correct temperature is obtained at the root. Inadequate penetration may lead to cracking. It is also caused by incorrect edge preparation or using wrong diameter of filler rod/electrode.

iv. Incomplete Fusion:

This is defined as the failure to fuse together adjacent layers of weld metal or adjacent weld metal and base metal. This condition may be caused by failure to raise the temperature of base metal to its melting point or failure to remove the oxides or other foreign material.

v. Over Penetration or Undercutting:

It is the melting or burning away of the base metal at the toe of the weld. On multilayer welds it may also occur at the puncture of a layer with the wall of a groove. Undercutting in a fillet weld reduces the cross- section of the member and acts as a stress raiser. Undercuts are generally due to excessive weaving speed. Big electrodes and incorrect electrode angle also cause it.

(b) Those resulting from fundamental difficulties in a welding operation such as cracking and porosity. Although faulty technique on the part of operator can also cause these defects, their incidence may be inherent in the process.

Porosity:

Porosity is caused by the entrapment of gas bubbles by the freezing dendrites during the cooling of the weld pool and, therefore, will occur essentially only in weld metal. This may be caused by excessive welding heats. Porosity may be scattered uniformly throughout the entire weld length, or isolated in small groups or concentrated at the root.

Porosity scattered along the entire weld length may be due to impurities (S or P) in parent metal, presence of rust, grease or dirt on surface of parent material, excessive moisture in covering on electrodes, improper arc length and excessive current, high welding speed, freezing of weld puddle before gases are able to escape.

Gas may be evolved from the melt for two reasons and the first of these is a chemical reaction such as between ferrous oxide and carbon which liberates CO (FeO + C Fe + CO). The second reason is super saturation of gases, for example, hydrogen and nitrogen in the melt.

This is typical in bare wire welds in which nitrogen is picked up from the atmosphere. Porosity may also arise in atomic hydrogen process in which considerable quantities of hydrogen may be absorbed by the weld pool if an incorrect technique is employed.

A high sulphur content can induce porosity, probably due to the delaying effects which low melting compounds may have on the nucleation of gas bubbles, by coating the sharp profiles of the freezing dent rites on which the bubbles form more readily.

Spatter:

Spatter refers to small particles or globules of metal scattered around the vicinity of the weld along its length.

It occurs due to:

(i) Arc blow making the arc uncontrollable.

(ii) Use of too long arc or too high arc voltage.

(iii) Use of an excessive current.

(iv) Bubbles of gas becoming entrapped in molten glob­ules of metal and expanding with great violence.

Cracking in Weld Metal:

Cracking is the most serious defect in a weld since it can lead to premature failure particularly in a dynamically loaded component. Cracking may occur due to incorrect welding technique or using a filler metal having a different rate of contraction compared to that of parent metal. Cracking may occur at two stages during the cooling of the weld metal.

1. At a high temperature just below the freezing point, known as hot cracking.

2. At root temperature or a little above, known as cold cracking.

(i) Hot Cracking (Carbon and Low Alloy Steels):

Hot cracking is influenced by the sulphur and carbon con­tent of mild steel weld metals. Sulphur tends to form a low melting point compound with iron, namely iron sulphide (FeS), which can form a low melting eutectic of Fe/FeS and segregate to form a network at the grain boundaries of the steel, and it remains liquid after the metal has frozen.

There is, therefore, no cohesion between the grains and the weld metal which may tear apart under contractional stresses. This difficulty is overcome by ensuring that sufficient man­ganese is present. If the Mn : S ratio is appreciable (greater than 10), then, manganese sulphide is formed in preference to iron sulphide and since the former solidifies at a tempera­ture slightly above that of steel, the inclusions appear as discrete spheres and as such are relatively harmless. The sulphur content of wire for electrode is limited to 0.04% max and that of parent steel to 0.05 or 0.06% maximum.

Generally values are considerably below this especially in the core wire. High carbon content in weld metal also tends to promote hot cracking and should be kept below 0.15%. These remarks apply equally to the low alloy steel weld metals.

(ii) Cold Cracking (Carbon and Low Alloy Steels):

This occurs at a relatively low temperature (considerably below that at which steel solidifies), and in ferritic transformable steels it is due to stresses arising from the following factors, usually in association with a susceptible microstructure typical of which is martensite.

a. Volume changes accompanying transformation from austenite to ferrite / pearlite / bainite / martensite.

b. Cooling Rate:

Dependent upon size and temperature of weldment and heat input of welding process, a sudden drop of temperature is sufficient to produce stresses of significant values.

c. Hydrogen:

Introduced by the electrode or its coating as chemically combined water, damp coatings or rusty and wet workpieces.

The transformation product is related to the alloy content of a steel and to its cooling rate. Mild steel being unalloyed has a high transformation temperature so that residual stresses arising from transformation are low, in consequence it is not normally susceptible to this form of cracking.

(iii) Cracking in the Heat Affected Zone:

The above remarks have been concerned with the weld metal itself but apply equally to the heat affected zone. This is zone of parent metal which has its structure altered but not completely melted by the heat of the welding process, and into which hydrogen may diffuse from the weld metal.

(iv) Centre Line Cracks:

These often occur in single pass concave fillet welds due to incorrect relationship between the size of weld and thickness of parent metal.

(v) Under Bead Cracks:

These occur while welding a hardenable base metal, excessive joint restraint, and presence of hydrogen.

(v) Weld Craters:

These are recessions in weld surface caused by solidification of molten weld puddle after arc has extinguished.

(vi) Gaps from Incomplete Fusion:

These could occur between weld metal and base metal or between weld beads in multipass welding due to excessive travel speed, big electrode size, low current, poor joint penetration.

(vii) Overlapping:

It is the protrusion of weld metal beyond the toe or root of weld and could occur due to insufficient travel speed which permits the weld puddle to move ahead of electrode, or incorrect electrode angle which causes the force of arc to push molten metal over un fused portion of base metal.

Generally the elements (metallic) or otherwise which should be considered in connection with carbon and low alloy steels are as follows viz., C, Mn, P, S, Si, Ni, Cu, Cr, Mb, Al, Zr, O₂, H₂ and nitrogen.

In general the above mentioned elements exert their influence on the microstructure and physical properties across the welded joint by:

(1) Solid solution strengthening of ferrite;

(2) Formation of carbides;

(3) Formation of intermetallic compounds;

(4) Oxidation or dexidation of the melt;

(5) Increasing or decreasing the hardenability of the heat affected zone;

(6) Grain size control;

(7) Segregation phenomena; and

(8) Raising or lowering the transition temperature (that is, the temperature at which a material ceases to display ductile properties and becomes brittle).

(viii) Hot Cracks in Austenitic Stainless Steels:

Stainless steel is generally welded by autogenous gas tungsten arc welding process. It is believed that as welded delta ferrite content is responsible factor for hot cracking in austenitic stainless steel.

If room temperature ferrite content is high, then cracking susceptibility is reduced because:

(i) Ferrite has more solubility for impurities than austenite, thereby reducing the amount of harmful low melting interdendritic liquid eutectic,

(ii) Ferrite promotes a duplex structure which is inherently finer than a single phase austenitic structure, thus providing more interfacial or grain boundary area for the distribution of harmful impurities.

Further experiments indicate that room temperature ferrite content is not responsible for hot cracking during welding of austenitic stainless steel, but the solidification behaviour has more predominating effect. Weld is said to be more crack resistant when the primary solidification product is delta ferrite and more crack susceptible when the primary solidification product is austenite.

The room temperature ferrite is not true indicator of the primary solidification phase as a considerable solid state transformation takes place through diffusion, converting one primary phase into another. It is also observed that primary solidification behaviour is controlled by the composition, solidification rate and also welding process and parameters.

Experiments on welding of austenitic stainless steel by GTAW process reveal that if welding speed exceeds a certain value, then a fully austenitic region is obtained along the weld centre lines which are susceptible to cracks due to its extremely poor ductility.

Such cracks can be avoided by ensuring that stresses during welding due to joint geometry as well as external restraints are minimum, reducing welding speed to make weld metal more duplex in nature, ensuring that impurity level of the base metal is less than 200 ppm, selecting base metal having higher Cr eq/Ni eq equivalency ratio (which determines whether metal solidifies as ferrite or as austenite) so that austenite formation does not take place even at high solidification rate.

Corrosion Resistance of Welded Joints:

In general no special attack is experienced at or adjacent to welds. However, since the weld metal may differ chemically or metallurgically from the base metal, corrosion of one or other may occur in certain media and under certain conditions.

There is less likelihood of such special attack when the weld metal is of the same composition as the base metal particularly if the welded parts are annealed or given a suitable heat treatment after welding. It is important to obtain welds that are free from pinholes, trapped slag or other mechanical defects. Such imperfections may well provide a starting point for attack by corrosion.

The presence of residual flux may cause corrosion. When flux is necessary, joints should be designed to permit the removal of surplus flux. Entrapment of flux is a primary cause of weld defects and promotes corrosion.

Many types of fluxes are corrosive to the metals or alloys with which they are used. Therefore, it is important in most cases that residual flux be removed from the metal surface immediately after the welding operation.

Highly localised residual tensile stresses remaining in a structure following its fabrication by welding may produces stress corrosion cracking in media which would corrode the unstressed metal only slightly, if at all.