The welding that is done does not always run smoothly.
Often, some welding flaws occur. It could be due to procedural errors, technical errors, or due to poor preparation.
One of the flaws that often occur in welding is a welding crack.
So, to prevent this defect from happening, we need to know what welding crack is, its types, what causes it, and how to prevent it.
Let’s check the following information.
Understanding Welding Crack
Cracks are one of the most common welding defects.
This crack can occur in welding either on the base metal, weld face, or root face.
There are several types of weld defects; the most common are hot and cold cracks.
A hot crack occurs when the temperature is above 400 degrees Fahrenheit, while a cold crack occurs after welding is complete.
The types of cracks that occur are differentiated based on the location of their emergence and the formation process.
There are at least 8 types of weld cracks, as listed in the information below.
1. Cold Crack
Cold cracking occurs due to residual stresses, which cause catastrophic damage.
These cracks occur in steel due to martensite formation when the weld metal is cold.
In addition, cold cracks can occur due to the presence of hydrogen in the microstructure, which causes brittleness.
Limiting heat input can be done by doing gradual welding to reduce the residual stress.
2. Hot Crack
This crack is often called a compaction crack. Hot cracks occur in the weld fusion area and can appear on all types of metal.
This often happens when the weld still has a temperature above 500 degrees Celsius.
Causes of this defect include the incorrect selection of electrodes, or it could be because the welding object is steel which contains quite high carbon.
The degree of stiffness of the objects being welded can also cause these cracks.
Heat distribution when the welding process is unbalanced. Cooling too fast.
3. Crater Crack
The cracks that form in the weld crater are due to the incomplete filling of the crater when turning off the arc so that the outer side of the crater cools faster than the weld crater, and overvoltage occurs.
4. Crack Down The Bead
Cracks that appear below the weld bead, namely in the heat-affected zone, appear on low alloy and high alloy steels.
One of the causes of the appearance of these cracks is the emergence of internal stresses, which arise due to contraction between the base metal and the weld metal or stresses due to the microstructure.
5. Longitudinal Crack
This crack forms lengthwise on the weld bead.
These cracks can occur on the surface or extend from the roots to the surface.
These cracks are generally caused by shrinkage stress, especially in the last strip.
The first weld bead that is too small can also cause this welding defect.
6. Root Crack
This crack is a crack that forms at the initial bead of the root of the weld. The main cause of its appearance is due to hydrogen brittleness.
7. Cracked Heel in Welding
This crack occurs due to the presence of water vapor in the weld area. This defect can be avoided by proper preheating and joint arrangement.
8. Transverse Crack
This crack appears in a direction intersecting with the welding direction.
The most common cause of defective transverse cracking is due to the longitudinal shrinkage stress in a weld metal with low elasticity.
Causes of Cracks
From the description above, here are some of the causes of cracks in welding in general for all types of welding cracks, as follows.
- Incorrect or inappropriate choice of electrode type.
- The workpiece is made of high carbon steel.
- The distribution of heat on the parts being welded is not balanced.
- Cooling after welding too fast.
- The workpiece being welded is too stiff.
Hot cracking generally occurs when the metal cools after freezing and occurs because of the stresses that arise.
It can also be caused by shrinkage and the toughness of steel, whose toughness drops at temperatures below freezing temperature.
Other cracks are the cracks along the weld ridges, cracks beside the welds, and cracks extending outside the ridges.
How to Prevent Cracks in Welding?
To avoid the occurrence of weld cracks in hot areas, or make efforts to overcome them so that cracks do not occur in the weld, including:
- Before welding, clean the area of water, rust, dust, oil, and organic matter, which can be a source of hydrogen.
- Cool slowly after welding.
- Free the seam from stickiness.
- Conduct preliminary heating before starting welding.
- Using the right electrode, in this case, as much as possible using an electrode with a flux with a low hydrogen content.
How to Remove or Reduce Hydrogen?
As was already mentioned, steel’s susceptibility to cracking will depend on how much hydrogen is present.
Here are several steps you can take to cut hydrogen content down or out entirely.
- Use a flux with a high carbonate content to produce carbon dioxide gas, which can lower the partial pressure of hydrogen in the electric arc and reduce hydrogen diffusion.
- Provide initial heating to the metal to be welded at a temperature of 50 – 200 degrees Celsius. This is useful for reducing the cooling rate.
- Provide final heating or after welding at a temperature of 200 – 300 degrees Celsius. Useful for reducing residual stress and reducing unstable phases at room temperature.
- Use welding wire that contains very low hydrogen.
- Heat the electrode before use to reduce the content of water or hydrogen-carrying organic matter.
- Use CO2 or other shielding gas.
Meanwhile, factors that increase hydrogen in molten weld metal are Water and organic matter contained in the flux. Oil, organic matter, water in cavities, and metal surfaces to be welded.
The humidity of the atmosphere where welding is carried out. Oil, organic matter, and water on the welding wire.
The amount of diffused hydrogen in the weld metal positively correlates with the air humidity where the welding is done.
In that instance, the weld metal will have a high hydrogen content if the welding crack is done in a moist environment.
This may lead to a tendency for cracks to occur. Thus, avoid welding while it is raining or just raining. Work on welding in an area with mostly dry air.