Why welding crack

There are three common types of hot cracking: segregation, bead shape and crater cracking. Segregation cracking is a type of centerline cracking, which as its name implies occurs through the center of the weld. It results when low melting point elements are concentrated in the center of the weld after it hardens.

It is common when welding metals that have higher sulfur and phosphorous content, ones with zinc plating or galvanized coatings, and on materials that have been painted or primed. There are several options to help prevent this type of cracking: 1 Order steels with low residual phosphorous and sulfur less than 0.

The next type of hot cracking, bead shape cracking typically results from poor part fit-up or joint design. If there is poor fit up, the tendency is to make wider welds with thinner throats. Conversely, poor joint design usually results in a narrow bead width-to-depth ratio.

In both situations, these bead shapes put undue stress on the center of the weld causing it to crack. Welding at too high of voltages can also result in a concave weld bead that is more prone to cracking. Prevent bead shape cracking by: 1 Having good part-fit up and joint design. Instead, use a weld drag angle to create a more convex weld bead. 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.

Why is Stainless Steel Corrosion Resistant? An Introduction to Hydrogen Embrittlement. An Intro to Pipeline Corrosion in Seawater. Follow Connect with us. Sign up. Thank you for subscribing to our newsletter! Connect with us. Weld Cracking. What Does Weld Cracking Mean? Two principal culprits are incorrect travel speed and voltage setting. A small reduction in arc voltage 1 to 1. Reducing the voltage too much, though, can result in a severely convex weld bead contour.

Note that pulse GMAW brings up further considerations that are beyond the scope of this article. If you set voltage too high, the weld pool becomes difficult to control, and this may encourage you to increase travel speed.

This in turn gives you insufficient weld throat depth and weld strength. You probably will get incomplete penetration, lack of fusion, and undercutting—common problems with vertical-down welding. In fact, performing a vertical-down fillet weld with an acceptable weld throat requires masterful weld pool control.

To avoid these problems, slow the travel speed and give the weld time to build an acceptable bead contour. You can waste a lot of time and weld metal depositing an excessively high bead profile. Such a weld is unsightly and almost always unacceptable, mainly because of the weld re-entrant angle to the base metal see Figure 3.

Such bead shapes can have an effect on cracking, especially on cracking that occurs over time. The crack generally is directed down into the base metal, right at the weld toe.

Such a high volume of weld metal creates significant shrinkage forces. When these forces exceed the strength of the weld, cracking ensues. To avoid this problem, try increasing travel speed. You can also take a look at your voltage setting. A small increase in voltage increases electrical pressure, forcing the weld contour down to a more acceptable profile. Undercut defects see Figure 4 reduce the base metal thickness where the base metal meets the filler metal.

This loss of metal interrupts the transfer of stresses from member to member through the weld. If severe, this creates a stress concentration point and has the potential to accumulate and initiate a crack, rapidly. On high-stress joints, acceptable levels of undercut are near zero. Figure 3: Excessively convex weld bead shapes can have an effect on cracking, especially on cracking that occurs over time. Undercut develops because of improper welding techniques and procedure settings.

It usually has no single cause but can come from a range of factors, including incorrect voltage settings, travel speed, and electrode-to-work angle. 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. To control solidification cracking, three principal factors need to be manipulated: weld metal composition; weld solidification pattern; strain on the solidifying weld metal.

Weld metal composition affects solidification cracking since composition dictates the weld metal's freezing range. Weld metals are invariably alloys with a range of freezing temperatures.

Low melting point constituents are rejected by the solidifying dendrites, and result in a thin film of liquid, persisting to low temperatures, and thus some distance from the main pool of liquid metal. This film cannot withstand the contraction strain, and if it cannot be fed successfully from the weld pool, then a crack is formed.

Several elements which increase the risk of solidification cracking have been identified [1]. Generally, these are ones which form a second phase, so that small additions can increase the freezing range of the alloy. Weld bead shape dictates the weld metal solidification pattern and, in turn, is influenced largely by welding parameters.