Finally, to prevent crater cracks — cracking that occurs if you stop welding prior to completing a pass on a weld joint — use a backfill technique to provide greater thickness to the crater.
This technique entails backing up slightly to fill in the area at the end of the weld prior to extinguishing the arc. Having good parts fit-up can prevent crater cracks as well.
Cold cracking also called hydrogen-induced or heat-affected zone HAZ cracking occurs at temperatures below deg F deg C and does not appear until hours after the weld cools. In some cases, it may not appear for days.
Most cold cracking begins in the base material as opposed to the weld itself and passes transversely into the weld. It is particularly common in thick materials, which are prone to rapid cooling due to the large heat sink, and results from induced residual stresses in the base material and the presence of diffusible hydrogen in the weld. Cold cracking can also occur in materials with high carbon or alloy content, as these are also higher in strength and can be less ductile.
Preventing cold cracking depends primarily on reducing residual stress and hydrogen in the weld. Hydrogen can enter the weld through moisture in the atmosphere, filler metals, shielding gas or from that retained in rust and mill scale on the base material. It can also enter from hydrocarbons in coatings, oils and lubricants on the material. Back stepping the weld is one way to help prevent cold cracking and it serves as a type of stress relieving.
To perform this technique: 1 Weld in one direction for a short length. The welders are all good welders and these same welders have done lots of other jobs at our factory with very good results and continue to get good results on subsequent jobs. Does anyone have a similar experience that can shed some light?
Your first line of your article mentions a truck axle. We have been seeing a lot of axle failures and the axle company in central MO says they use a robot weld and it is not there fault. All failures have been the weld.
All of the re-welded welds hold but they clam no responsibility and they note that they know what they doing. By the looks of all failures they are clueless about welding. There are cracks on the top and the root of the weld is incomplete. The fusion to the axle is little to none. Again they state that it meets there expectations.
What do we do? A manufacturer that claims no responsibility for weld failures without proper investigation certainly sounds a bit fishy. Assuming that the axles are being used per the manufacturers specifications not exceeding limits or being misused , they should not fail. We can learn a lot simply by looking at the failed part. If it is evident that there is lack of penetration at the root then it is NOT a good weld.
The strength of the weld is determined by the effective throat, which extends form the face of the weld all the way to the point of deepest penetration. If a weld size is called out it assume there will be root penetration. The trapped gases create a bubble-filled weld that becomes weak and can with time collapse. This welding imperfection is the groove formation at the weld toe, reducing the cross-sectional thickness of the base metal. The result is the weakened weld and workpiece.
It can also appear between adjoining weld beads. This creates a gap in the joint that is not filled with molten metal. Slag inclusion is one of the welding defects that are usually easily visible in the weld. Slag is a vitreous material that occurs as a byproduct of stick welding , flux-cored arc welding and submerged arc welding. If the electrode favors one member more and the travel speed is too fast, the arc will naturally melt the member as part of the fusion process, but the high travel speed will not allow the melting electrode to fill in the washed-out area, resulting in an unacceptable weld.
To prevent these defects, make every effort to maintain proper voltage levels. For the constant-voltage processes nonpulsed GMAW and flux-cored arc welding , the voltage stays fairly constant and can be adjusted manually. If you increase the arc length, you increase arc voltage. Be sure to maintain a correct electrode angle, and try decreasing travel speeds to allow weld deposition to do its job.
Overlap see Figure 4 , or cold lap, is more serious than you might think. A classic example of a stress riser, such overlap opens the door for cracking if stress accumulates to unacceptable levels. Overlap is a common fault when you have to weld blind. This ensures that the weld stresses flow evenly and, most important, stops those detrimental cracks from forming.
Figure 4: Undercut defects reduce the base metal thickness where the base metal meets the filler metal. Callahan, FL Email Phil Evans. Read more from this issue.
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