Cracks are imperfection formed in the solid state of welding or metal.
In particular, cracks are caused by a local rupture, as a result of cooling effect or stresses. The geometry of cracks produces a huge stress concentration at the crack tip, as a result, making them more likely to cause a fracture.
This is why cracks are more significant than other types of imperfection.
Type of Cracks
There are at least 5 types of cracks situated either in the weld metal, heat affected zone (H.A.Z.) or parent metal.
- Longitudinal cracks
- Transverse cracks
- Radiating cracks – cracks radiating from a common point
- Crater cracks
- Branching cracks – a group of connected cracks originating from a common crack
Depending on their nature, these cracks can exist in a form of:
- Hot Cracks
- Precipitation induced Cracks – reheat cracks present in creep resisting steels
- Cold Cracks – hydrogen induced cracks
- Lamellar tearing
Depending on their location and mode of occurrence, hot cracks can either be Solidification cracks or Liquidation cracks.
Usually, exist along the center-line of a weld metal as a result of the solidification process.
They generally occur when the weld metal’s content is high in Carbon or there is Sulfur or Phosphorus in it that causes impurity.
These elements segregate during solidification, leaving inter-granular liquid films to remain after the weld solidified.
Therefore, it is important to make sure that Welding is not performed on or near metal surfaces that are covered with scale or contaminated with oil or grease.
A scale can have a high sulfur content and oil and grease can supply both carbon and sulfur.
Solidification cracks can also occur when the weld bead is deep and narrow due to a large depth-to-width ratio.
The disruption of heat flow, for example, the stop/start condition in welding activity, can also trigger Solidification crack.
As a result, due to thermal shrinkage, the cooling weld bead will be ruptured and form a crack.
Occur in the coarse grain H.A.Z., liquidation cracks reside in the near vicinity of the fusion line.
This is as a result of heating the material to an elevated temperature, high enough to produce liquidation of the low melting point constituents placed on grain boundaries
Precipitation Induced Cracks
These are reheat cracks present in crack resisting steels.
Cold Cracks – Hydrogen Induced Cracks
Hydrogen-induced cracks occur primarily in the grain coarsened region of the H.A.Z.
They are also known as cold, delayed or under-bead / toe cracking.
It lies parallel to the fusion boundary and its path is usually a combination of inter and trans-granular cracking.
The direction of the principal residual tensile stress in toe cracks can cause the crack path to grow progressively away from the fusion boundary.
Eventually, the cracks move towards a region of lower sensitivity to hydrogen cracking.
When this happens, the crack growth rate decreases and eventually arrests.
Four factors are necessary to cause H.A.Z hydrogen cracking:
- Hydrogen level > 15ml / 100g of weld metal deposited
- Stress > 0.5 of the yield stresses
- Temperature < 300°C
- Susceptible micro-structure > 400HV hardness
If any one factor is not satisfied, cracking is prevented, so can be avoided through control of one or more factors:
- Apply preheat to slow down the cooling rate thus avoiding the formation of susceptible micro-structures.
- Maintain a specific inter-pass temperature (same effect as preheat).
- Post-heat on completed of weld to reduce the hydrogen content by allowing hydrogen to diffuse from the weld area.
- Apply PWHT to reduce residual stress and eliminate susceptible micro-structures.
- Reduce weld metal hydrogen by proper selection of welding process / consumable (e.g. use TIG welding instead of MMA, basic covered electrodes instead of cellulose).
- Use a multi-run instead of a single run technique to eliminate susceptible micro-structures by the self-tempering effect,
- Reduce hydrogen content by allowing hydrogen to diffuse from the weld area.
- Utilize a temper bead or hot pass technique (same effect as above).
- Use austenitic or nickel filler to avoid susceptible microstructure formation and allow hydrogen to diffuse out of critical areas.
- Apply dry shielding gases to reduce hydrogen content.
- Clean rust from joint to avoid hydrogen contamination from moisture present in the rust.
- Reduce residual stress.
- Blend the weld profile to reduce stress concentration at the toes of the weld.
Lamellar tearing occurs only in rolled steel products (primarily plates) and its main distinguishing feature is that the cracking has a terraced appearance.
Cracking occurs in joints where:
- A thermal contraction strain occurs in the through-thickness direction of the steel plate.
- Non-metallic inclusions are present as very thin platelets, with their principal planes parallel to the plate surface.
Contraction strain imposed on the planar non-metallic inclusions results in progressive decohesion to form the roughly rectangular holes which are the horizontal parts of the cracking, parallel to the plate surface.
With further strain the vertical parts of the cracking are produced, generally by ductile shear cracking. These two stages create the terraced appearance of these cracks.
Two main options are available to control the problem in welded joints liable to lamellar tearing:
- Use a clean steel with guaranteed through-thickness properties (Z grade).
- A combination of joint design, restraint control and welding sequence to minimize the risk of cracking.
- American Welding Society – Welding Inspection Guide
- CSWIP – Welding Inspection Guide
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