Welding Imperfections - Cracks

Imperfection

Any deviation from the ideal weld

Defect

An unacceptable imperfection

Classification of imperfections according to BS EN ISO 6520-1

This standard classifies the geometric imperfections in case of fusion welding, dividing them into six groups:

Cracks

Cavities

Solid inclusions

Lack of fusion and penetration

Imperfect shape and dimensions

Miscellaneous imperfections

It is important that an imperfection is correctly identified thus allowing for the cause to be identified and actions taken to prevent further occurrence.

CRACKS

Definition

An imperfection produced by a local rupture in the solid state, which may arise from the effect of cooling or 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

Transverse

Radiating (cracks radiating from a common point)

Crater

Branching (a group of connected cracks originating from a common crack)

These cracks can be situated in the:

Weld metal

HAZ

Parent metal

Exception: Crater cracks are found only in the weld metal.

Depending on their nature, these cracks can be:

Hot (i.e. solidification cracks liquation cracks)

Precipitation induced (i.e. reheat cracks, present in creep resisting steels).

Cold (i.e. hydrogen induced cracks).

Lamellar tearing

HOT CRACKS

Depending on their location and mode of occurrence, hot cracks can be:

Solidification cracks:

Occur in the weld metal (usually along the centerline of the weld) as a result of the solidification process

Liquation cracks:

Occur in the coarse grain HAZ, in the near vicinity of the fusion line as a result of heating the material to an elevated temperature, high enough to produce liquation of the low melting point constituents placed on grain boundaries.

SOLIDIFICATION CRACKS

Generally, solidification cracking can occur when:

The weld metal has a high carbon or impurity (sulphur etc) element content.

The depth-to-width ratio of the solidifying weld bead is large (deep and narrow).

Disruption of the heat flow condition occurs, e.g. stop/start condition the cracks can be wide and open to the surface like shrinkage voids or subsurface 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 sulphur and phosphorus. These elements 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 a 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 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 CRAKS

Hydrogen induced cracking occurs primarily in the grain-coarsened region of the HAZ, and is also known as cold, delayed 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 arrests

A combination of four factors is necessary to cause HAZ hydrogen cracking:

Hydrogen level > 15ml/100g of weld metal deposited

Stress > 0.5 of the yield stress

Temperature < 300°C

Susceptible microstructure > 400Hv hardness

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 inter pass 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- instead of single-run technique (eliminates susceptible microstructures by means of self-tempering effect, reduce the hydrogen content by allowing hydrogen to effuse from the weld area).

Use a temper bead or hot pass technique (same effect as above).

Use austenitic or nickel filler (avoid susceptible microstructure formation and allow hydrogen diffusion out of critical areas).

Use dry shielding gases (reduce hydrogen content).

Clean rust from joint (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 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 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 clean steel with guaranteed through-thickness properties (Z grade).

A combination of joint design, restraint control and welding sequence to minimise the risk of cracking