Types of Crack in welding and crack prevention

What is a Welding Crack?

Crack is a material separation that does not encompass the entire cross-section of a welding joint, component or material, not yet causing it to fall apart. Cracks are in most cases starting points of a fracture.

Welding Cracks are fracture-type linear imperfections having a sharp tip with a higher length and very small width. Welding cracks can appear in weld deposits, heat-affected zone (HAZ), and base material. They occur once the localized stresses exceed the ultimate tensile strength of the base metal. Initial Cracking usually starts at stress concentrations due to other defects or sharp notches (notches work as stress concentration) in the nearby area. These stresses can be residual stresses caused by the welding or stresses got applied due to service or another external loading. In welding, residual stresses are caused by weld solidification and thermal expansion properties of the metal. In welding cracks, there is little plastic deformation as most of the cracks either occur due to high hardness or Stresses.

Classification of cracks:

  1. Metallurgical caused cracking: Hot crack, Shrinkage Crack
  2. Technologically caused Cracking: Crack due to wrong heat treatment
  3. Load caused cracking: Stress corrosion cracking, Hydrogen cracking

Types of cracks based on size

Cracks based on their size are classified into:

  1. Macro cracks: Visible with naked eye or with magnifying glass
  2. Micro cracks: Visible under metallographic microscope

Further classification can be based on how the cracks propagate, as:

  1. Inter-crystalline crack: Cracks propagating along the grains
  2. Trans-crystalline crack: Cracks propagating through complete microstructure areas.

Types of crack in welding

Various types of cracking are found in welding applications. The cracks types, their appearance, and locations significantly affect the service life of the components especially when they are under cyclic loading applications. As we know cyclic loading can easily create failure of the part if any notches or cracks are present. Each type of cracking found in welding is caused due to particular reasons either single or in combinations. For example, delayed cracking is caused due to presence of hydrogen, stresses, and hard microstructure. So, it’s the conditions that affect the appearance of the cracks in the weldment.

Cold Cracks vs. Hot Cracks

In general, cracks are classified as:

  1. Cold Crack e.g. delayed cracking, fatigue cracks
  2. Hot Crack e.g. Solidification cracking

Hot cracking appears at the solidification stage of the welding. The main reason of hot cracks are lack of ductility at the elevated temperature. Hot cracking is found mostly at the center of the weld as during solidification, this is the area which solidify at the last. These cracks propagate between weld deposit grains or the at interface of the welding.

On the other hand, cold cracks do not appear at the solidification stage but rather they are found after solidification stage is completed after the welding or simply once the welding is completed. Cold crack is also known as hydrogen cracking or delayed cracking as these cracks are generated due to the presence of hydrogen and can appears after several hours of the welding or even after days after welding is done. Cold cracking are found in HAZ, weld metal as well as in the base metal. Delayed cracking propagate through the grains as well as between the grains.

What is Hot Crack and its Types?

Hot cracks appears in the weldment due to various associated phenomena. Hot crack is the type of crack appearance that result in the presence of molten or brittle phases on the grain boundaries at high temperature. They have following types:

  1. Solidification Cracking
  2. Liquation Cracking
  3. Crack due to a reduction of deformability
  4. Centerline cracking

Hot cracks formed in the weld metal are called solidification cracks. They are generated during crystallization of the material from the liquid phase and frequently reach up to the surface of the weld metal forming either center cracking or end crater cracking. The cracks can be either find by visual testing or performing liquid penetration testing or magnetic particle testing.

Longitudinal vs. Transverse Cracks

When a crack is parallel to the weld axis it is called a longitudinal crack regardless of whether it is a centerline crack in weld metal or a toe crack in the base metal heat-affected zone. Longitudinal cracks are shown in the below picture. When welding small length tack weld or small welds between heavy to thin members, longitudinal cracks appears due to high cooling rate and high restraint. in HAZ, longitudinal cracks are caused due to dissolved hydrogen in the weld metal during solidification. Transverse cracks found in submerged arc welds are caused due to high welding travel speed and could be associated with porosity.

Types of weld crack appear perpendicular to the weld axis are called Transverse Crack. Transverse cracks are small in size compare to the longitudinal cracks and mostly within the width of the weld bead as shown in the below picture. Transverse cracks can also propagate into the HAZ and base metal. In high strength materials, transverse cracks can easily develop if preheat and hydrogen level is not controlled and they can appear only in the heat affected zone. Most of such cracks are caused by the longitudinal shrinkage stresses working on low ductile weld metal that is the case in most of high strength materials. Hydrogen cracking or cold cracking can also shows similar pattern of transverse cracking.

Crater Crack

When performing fusion welding processes such as stick welding or MIG or TIG and if the end of the weld is not properly terminated, it can result in starlike network crack formation as shown in the below picture. The crack appearing at the end crater in welding is called Crater Crack or star called as called sometimes. Crater cracks are type of hot crack as cracking occurs during solidification stage. Materials that exhibits high thermal coefficient such as Austenitic stainless steel or aluminum are more prone to crater cracks as heat will be dissipated very fast and there will be high stresses in underfilled crater. The prevention of crater cracks or Crater cracks can be prevented by:

  1. Filling the adequate end crater.
  2. Using back step technique during at the weld end.
  3. Ensure a slightly convex shape at the weld end by welding.
  4. Using short arc.

Toe Cracks

Toe cracks are usually cold cracks as shown in the below picture that appears at the weld toes. They initiate and propagate from the weld toe where shrinkage stresses are concentrated. These types of cracks start from the base metal area approximately. The cracks occurs due to the effect of the welding thermal stresses working on the HAZ. Few Tor cracks initiate due to lack of base metal ductility.

Underbead and Heat-Affected-Zone Cracks

Underbead and heat-affected-zone cracks are generally used interchangeably. Underbead and heat affected-zone cracks are generally cold cracks that form in the heat-affected zone of the base metal. Typical underbead cracks are illustrated in the below Figure. Underbead cracks can occur when three elements are present simultaneously:
(1) Hydrogen
(2) A microstructure of relatively low ductility
(3) High residual stress

Underbred and HAZ cracks can appear in longitudinal and transverse directions. They are found in the heat-affected zone and are not always detectable by visual examination. Underbead cracks are found primarily in fillet welds, but they can also occur in groove welds.

Root Cracks

Root cracks are longitudinal cracks at the weld root or in the root surface. They may be hot or cold cracks. Root cracks are illustrated in the above Figure.

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