Crack

An imperfection produced by a local rupture in the solid state, which may arise from the effect cooling of stresses. Cracks are more significant than other types of imperfection, as their geometry produces a very large stress concentration at the crack tip, making them more likely to cause fracture.
 Types of crack :
     -  Longitudinal crack
     - Transverse crack
     - Radiating crack
     - Crater cracks
     - Branching crack


These crack can be situated in the weld metal, in the HAZ, in the parent metal, Exception Crater crack are found only in the weld metal.
Depending on their nature these cracks can be :

• hot cracks (i.e. solidification cracks liquation cracks)
• cold crack ( i.e . Hydrogen induces crack)
• Precipitation induces crack (i.e. reheat cracks, present in creep resisting steels)
• Lamellar tearing

Hot Crack

Depending on the situation and mode of occurrence, hot cracks can be :

• Solidification cracks : Occur in the weld metal ( usually along the center line of the weld) as a result of the solidification process.
• Liquation Cracks : Occur in the coarse grain HAZ, in the near vicinity on the fusion as a result of heating the material to an elevated temperature, high enough to produce liquation on the low melting point constituents placed on grain boundaries.

Solidification Crack

Solidification crack can occur when :

• The weld metal has high carbon or impurity (sulfur etc) element content
• The depth to width ratio of the solidifying weld bead is large (deep & narrow)
• Disruption of the heat flow condition occur, e.g stop/start condition

The crack can be wide and open to the surface like shrinkage voids or sub-surface and possibly narrow.

        Solidification cracking is most likely to occur in compositions which result in a wide freezing temperature range. In steels this is commonly created by a higher than normal content of carbon and impurity elements such as sulfur and phosphorus. These element segregate during solidification, so that intergranular liquid films remain after the bulk of the weld has solidified. The thermal shrinkage of the cooling weld bead can cause these to rupture and form crack.

It is important that the welding fabricator does not weld on or near metal surfaces covered with scale or which have been contaminated with oil or grease. Scale can have a high sulphur content, and oil and grease can supply both carbon and sulphur. Contamination with low melting point metals such as copper, tin, lead, and zinc should also be avoided

Hydrogen Induced Cracks

Hydrogen induced cracking occurs primarily in the grain-coarsened region of the HAZ, and is also known as cold cracking, delayed cracking or underbead / toe cracking. Underbead cracking lies parallel to the fusion boundary, and its path is usually a combination of intergranular and transgranular cracking. The direction of the principal residual tensile stress can, for toe cracks, cause the crack path to grow progressively away from the fusion boundary towards a region of lower sensitivity to hydrogen cracking. When this happens the crack growth rate decreases and eventually arrest.
A combination of three factors is necessary to cause HAZ hydrogen cracking.

In addition, the weld must cool down to near normal ambient temperature where the effect of hydrogen is at its maximum. If any one factor is not satisfied, cracking is prevented. Therefore cracking can be avoided through control of one or more of these factors.

• Apply preheat (to slow down the cooling rate and thus avoid the formation of susceptible microstructures)
• Maintain a specific interpass temperature (same effect as preheat)
• Post heat on completion of welding (to reduce the hydrogen content by allowing hydrogen to effuse from the weld area
• Apply PWHT ( to reduce residual stress and eliminate susceptible microstructures)
• Reduce weld metal hydrogen by proper selection of welding process / consumable (e.g. use TIG welding instead MMA, use basic covered electrodes instead cellulose ones)
• Use multi-run instead single-run technique (eliminate susceptible microstructures by means self of tempering effect, reduce the hydrogen content by allowing hydrogen to effuse from the weld area)
• Use temper bead or hot pass technique
• Use austenitic or nickel filler (avoid susceptible microstructures formation and allow hydrogen diffusion out critical areas)
• Use dry shielding gases (reduce hydrogen content)
• Clean joint from the rust (avoid hydrogen contamination from moisture present in the rust)
• Reduce residual stress
• Blend the weld profile (reduce stress concentration at the toes of the weld)

Lamellar Tearing

Lamellar Tearing occurs only in rolled steel product (primary plates) and its main distinguishing feature is that the cracking  has terraced appearance

Cracking occurs in joint where :

• A thermal contraction strain occurs in the trough thickness direction of steel plate
• non metallic inclusion are present as very thin plates, with their principal planes parallel to the plate surface

Two main option are available to control the problem in welded joints liable to lamellar tearing :

• Use clean steel with guaranteed through-thickness properties (Z_grade)
• A combination of joint design, restrain control and welding sequence to minimize the risk of cracking.