How to Prevent Weld Delayed Cracking?
Crack free weld is something that you would expect as a welder or an owner of the welded property. But Delayed Cracking or Hydrogen Induced Cracking or Cold Cracking is a very common phenomenon as the aftermath of welding. If the steel material you are working on is very strong, if there is a high level of stress for the residual or mechanical/structural loading, and if there is a source of hydrogen for the moisture in welding filler material or environment & joint contamination, you will experience the delayed cracking. So you need to eliminate and control the factors for preventing the delayed cracking from arising.
Reasons behind the Delayed Cracking:
There are various theories and mechanisms available that explain the reasons for the delayed cracking or Hydrogen Induced Cracking. You will find no single accepted reason for the explanation. Rather, multiple factors play behind the scene.
According to a mass apprehended theory, hydrogen atoms get diffused through the metal and gather at spaces in the metal’s microstructure. With the accumulation of hydrogen atoms, molecular hydrogen (H2) starts to activate and react with carbon forming methane at the points of accumulation. Over time, molecular hydrogen (H2) becomes so large that they diffuse through the metal easily. Gradually, H2 creates mounting pressure to split apart the material. Thus, cracking occurs.
Another popular theory is that hydrogen atoms get accumulated around the carbide inclusions resulting in slow plastic deformation. It creates moving defects or dislocations. If the plastic deformation can no longer take place, the material becomes stiffer and fragile. Finally, the metal loses its ductility and cracking tends to occur.
Whatever is the theory or mechanism, it is definite that delayed cracking happens when all of the following factors act simultaneously:
Presence of Atomic Hydrogen:
At the time of welding, it is natural that some level of hydrogen is present there. It is the result of the moisture or hydrocarbon contamination.
Presence of a Tensile Stress:
If there is lower stress, delayed cracking will not occur. After the weld gets cooled, thermal shrinkage occurs and causes high stress. Stress concentrators like weld defects increase the level of stress.
In case, you weld on a high strength steel structure, the output might not reach your expectation level due to susceptible microstructure that is brittle and hard. Steels with higher strength get hardened because of the presence of high carbon and low alloy that are very chronic to delayed cracking. If you weld on “austenite” stainless steel, it will not crack over time. Because it doesn’t have susceptible microstructure.
Low Enough Temperature:
If the temperature is at a lower level, hydrogen diffuses quite easily and thus creates cracking. Usually, 100 degrees centigrade of temperature is an industry benchmark at which level hydrogen tends not to diffuse. However, in some cases delayed cracking may happen at a temperature slightly higher than the industry average.
Ways to Prevent Delayed Cracking:
Since pre-heating can slow down the cooling rate, it can solidify microstructure in a better way. Through pre-heating, you can expect the desired microstructure of the metal piece that is not prone to cracking. On the other hand, rapid cooling will result in Hydrogen Induced Cracking for sensitive microstructure.
The temperature at which subsequent weld runs get deposited is called inter-pass temperature. Your welder must define the inter-pass temperature for pre-heating the base material at this temperature before placing another weld to pass. This process controls the cooling rate and helps to avoid delayed cracking.
When the welding work is over, you need to apply post-heating for the elimination of remaining hydrogen. If you provide post-weld heating, the metal will give off hydrogen that will reduce the danger of cracking. This post hating also removes harmful residual stresses to get the expected microstructure.
Suitable Arc Process:
You should always choose the arc processes that don’t tempt hydrogen in the weld metal to avoid delayed cracking. If you have to weld the high strength low alloy steel material, experts recommend surface tension technology of MIG welding. You should not use stick welding in this case. Otherwise, cellulosic electrodes will cause to diffuse hydrogen that will lead to severe cracking in the steel material.
Suitable Filler Metal:
Filler metal like cellulosic stick electrodes may contain moisture and other organic compounds that are responsible for delayed cracking. These electrodes with moisture create gas shielding with hydrogen. So, you should always use basic electrodes that use minerals like calcium and magnesium carbonates. These basic electrodes provide low hydrogen shielding and reduce the risk of cracking.
The multi-pass technique through its tempering effect can modulate the microstructure of the steel material for reducing the hydrogen content. It starts with a deposit pass and the next pass tempers the earlier to protect it from delayed cracking. You can apply the hot pass for the same purpose. Hot pass clears the root pass and thus remove the slag content. It also provides heat treatment to the root for protecting it from getting cracked.
Temper Bead Technique:
In cases where post-weld heat treatment is challenging, temper bead method is useful for these. It helps in self-heat treatment to refine the zones of course grain. It ensures improved properties with wanted microstructure. Furthermore, it helps to reduce the residual tensile stresses. These features can reduce the risk of delayed cracking.
Since the cellulosic electrodes may bear moisture and thus the hydrogen, experts suggest that you should use the baked electrodes to reduce the possibility of cracking of your welded material.
Rust And Scale Removal:
Before you start welding, it is always advisable that you clean the joints to remove the rust and scale. Because moisture or grease on the metal usually bears hydrogen, the number one reason for delayed cracking.
For good quality welding, you should exercise the practices like proper cleaning of the joint, pre-heating of joint, use of nickel & austenitic materials and use of multi-pass technique.