What is Hot Cracking, Liquation cracking & solidification Cracking?


What is Hot Cracking?

Hot cracking is a general term which is quite often used to describe all types of cracking in metals that have originated in higher temperatures. A simple definition for hot cracking according to the standard EN ISO 17641-1 is:
“Hot Cracks are material separations occurring at high temperatures along the grain boundaries (dendrite boundaries), when the level of strain and the strain rate exceed a certain level. Small cracks, which are visible only at magnifications of more than about 50x, are often described as micro fissures.”
The type of cracking is generally grouped by the temperature range over which they occur. Hot cracks generally develop longitudinally along the weld axis, developing at the centre of the weld, Although, they can also appear across the weld axis influenced by the direction of tensile stress as shown in the below figure.

Typical location of centreline hot cracking
Fig.3. Buried centreline cracking
Buried centreline cracking

What is Solidification Cracking?

Solidification cracking, in which the cracks are formed during solidification from the liquid phase of weld metals. They can sometimes be visualized as dendrites that grow more slowly than the strains being imposed, so that they fail to fuse on the weld Centre line and leave a cavity or centerline crack. They usually extend up to the surface of the weld metal, but sometimes remain subsurface.

Weld solidification cracking occurs during the final stages of solidification when the tensile shrinkage stress accumulates and liquid films still persist along the solidification grain boundaries in the structure. If the imposed shrinkage strain exceeds the inherent ductility of the solidifying weld metal, cracking will occur.

The lack of ductility can result from the presence of liquid metal, micro structural features, orientation (relative to the strain) and in some cases upon the presence of brittle impurities and low melting point (or liquated) films.

Factors affecting solidification cracking phenomenon

There are several factors affecting solidification cracking. The interaction of some of these factors, welding parameters, composition and restraint, are shown in below Figure.

Factors affecting solidification cracking phenomenon

What is Liquation Cracking?

Liquation cracking, in which the cracks are formed in areas of liquation in the HAZ of the parent material or in multipass welds where the weld metal is reheated during the deposition of subsequent weld beads. They are often small (micro fissures) and often subsurface in multi-pass welds.

Hot cracking occurs in the solid-state at temperatures between the solidus and approximately half the melting temperature of the material and may occur either during fabrication or subsequent post weld heat treatment.

Types of Hot cracking are:
1. Ductility-dip cracking
2. Reheat cracking
3. Strain-age cracking
4. Lamellar cracking

Click here to read about Cold Cracking or Hydrogen Cracking.

Reason of Hot Cracking

  1. Strain on the weld pool is too high.
  2. Liquid cannot reach the regions where it is needed due to inadequate supply or blockage/ narrow channels between solidifying grains.
  3. less weld pass thickness in root run.

Prevention of Hot cracking in Welding

  1. Use lower heat input : USing low heat input give rise to the cooling of the deposit weld metal and reduce the time span for the liquidous/ brittle temperature range. It also extend the width-to-depth ratio of weld due to reduced weld deposit, hence minimizing the susceptibility to hot cracking.
  2. Using bigger groove radius: A weld joint with higher groove radius increases the width-to-depth ratio of weld metal, that avoid hot crack. Narrow groove joints are most susceptible for the hot cracking.
  3. Use welding electrode with ferrite-controll: Austenitic stainless steels shall be welded with ferrite content of 3-10% in the austenitic matrix. Weld metal having pure austenitic phases are highly prone to hot cracking. For special applications such as cryogenic temperature service, which require a fully austenitic weld metal, it is recommended to use a weld wire containing low sulfur and phosphorus with higher manganese content.
  4. Deposit thick weld passes: Sometime hot cracking can occurs when welding high restraint weld joints or weld joint with high stress. When welding root run, centreline cracking is observed as small thickness of weld deposit will not be able to withstand the stresses. In such cases, a thicker weld root is beneficial to avoid the hot cracking.

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