The formation of cracks during the solidification of a weld
metal, is commonly termed as Solidification cracking. The reason behind that
the hot Weld metal lose the required ductility and may be subjected to
cracking. Since the solidification starts from fusion line to centerline, the
low melting components in composition of molten weld puddle remains liquid in
the center even at low temperature and as a result stresses are developed due
to contraction of remaining solid zone. These stresses are the major cause to
make splitting of a weld bead. Learn What is a Fusion line in Weldment? Normally in Fabrication industries, this problem
is known by three common names; hot, crater and centerline cracking.
Types of Cracking
- Hot cracking – happens when the weldment is at high temperature.
- Crater cracking – the appearance of small cracks at the end of bead.
- Centerline cracking – the appearance of cracks at the centerline of bead.
Causes
Following are the common reasons for above mentioned types
to happen in a weld metal. A weld metal is more subjected to solidification
cracking when three conditions occur simultaneously.
- If the parent metal has chemical composition sensitive to cracking.
- If the welding parameters produce irregular size and shape of weld bead.
- If the weld has unfavorable stress level.
How to Prevent Solidification Cracking?
The prevention requires, the reduction of above mentioned
conditions to an acceptable level. Below are the preventive actions that are
practicable to reduce the trigger causes of cracking.
1. Avoid the Contamination of Weld pool
The contamination of metal (in case of C-Mn/C steels) with
elements like copper and sulphur, makes the weld composition sensitive to
cracking as the entrapment of these constituents lead to the formation low
melting compounds. For example sulphur makes iron sulphide that remains liquid
even at the last stages of solidification.
1.1 Sources
- Contamination of parent metal with oil and grease
- Less refined material delivered by the supplier
- Contaminated welding rods
1.2 Prevention
- Before welding make sure the joint is properly prepared by cleaning and grinding.
- Welding consumables having copper content must be maintained at good conditions.
- Ensure the material testing report of supplied parent material.
2. Avoid the unfavorable Welding Parameters
Improper welding parameters may result in irregular size and
shape of a weld bead. For this a width to depth ratio is an important factor to
determine the soundness of a weld bead which is totally insensitive to cracking.
If the width to depth ratio is less than a specific number*,
the weld bead will be sensitive to cracking.
If the width to depth ratio is greater, the weld bead will
completely insensitive to SC cracking.
*You can find more details about width to depth ratio in
CSWIP 3.1 21.4.3
So the welding parameters must be selected carefully in case
of those welding procedures which are liable to produce beads with SC cracking.
2.2 Prevention
- Use low welding current settings – to produce the shallower bead.
- Slow Travel – to produce the wider bead at comparatively lower pace.
3. Avoid the crater in a weld bead
Crater is a shallow cavity, produced at the end of weld bead. It is
also another trigger to impart cracking in the weldment. In order to avoid
crater, one must follow the below mentioned practices.
In modern TIG welding machine, the function of ‘slope-out’is introduced to help filling the craters. In this case the welding current is
reduced along with the lengthening of arc manually by a welder to produce weld
bead of required width to depth ratio. In case of stick welding the crater is
filled by reversing the direction of weld run at the end of bead.