Types of Crack in Welding

Abstract

Whether the result of poor parts fit-up, rapid cooling or a variety of possible contaminants—from the atmosphere, base material or filler metal—weld cracking carries with it significant consequences for any welding operation. Not only does this defect adversely affect the integrity of the finished weldment, but it also requires significant time and money to rectify. In a best-case scenario, a welding operator must remove the weld crack by carbon arc gouging (or other means) and repair the weld, while in other instances the welded part must be completely rejected and scrapped.

In critical applications, companies can also incur significant costs for inspecting welds that may ultimately be rejected if a crack is present. Once reworked, these parts require a second round of testing that inevitably increases the company’s cost for this portion of the operation. In the case of products that need to be post-weld heat treated (PWHT), hours or even days must pass between the completion of the rework and a new evaluation, adding to a company’s overall downtime and lost productivity. For companies that must meet contract terms, such delays can even result in fines for missed deadlines.

Fortunately, weld cracking doesn’t have to be a complete enigma—or a total drain on a company’s productivity or profit. Understanding the basic types of weld cracks and their causes can go far toward preventing them in the first place.

Hot weld cracking occurs at high temperatures—generally over 1000 degrees Fahrenheit (538 degrees Celsius)—and the defect appears more or less immediately (though not always visibly) upon solidification of the weld. Hot cracking is almost always appears in the longitudinal direction of the weld bead itself, or directly adjacent to it. Hot cracking can also appear intermittently along the weld’s length.

There are two main types of hot cracking: centerline and crater, with the first being further divided into either segregation or bead shape cracking.

As indicated, centerline cracking occurs through the center of the weld, and in the case of segregation cracking, is the result of elements with low melting points being rejected to the center of the weld upon solidification. Materials, such as free machining steels (due to their high sulfur and phosphorous content), are especially susceptible to segregation cracking as are materials with zinc plating, galvanized coatings or those covered with paints or primers. Certain alloys found in filler metals may also be responsible for the problem. For example, boron, which is added to many filler metals to help refine grain structure can, in excess, cause segregation cracking.

The second type of centerline cracking—bead shape cracking—occurs as the result of poor part fit-up or a poor joint design. To compensate for these conditions, welding operators often create a wide weld bead with a thinner throat, which makes the finished weld weaker and puts undue stress on the centerline, in turn causing a crack. Welding at high voltages, especially with an electrode or welding wire that creates a particularly fluid weld pool can also cause the resulting weld bead to be more concave. This again reduces the throat thickness and weld strength, making it more prone to bead shape cracking. Similar results can occur from the presence of shallow craters as well.

Crater cracking occurs when the welding operator stops welding prior to finishing a pass on a weld joint, leaving a wide, thin depression at the end. It can also appear in areas that have been tack welded when the corresponding weld passes does not meet fully against the tacks. In both instances, using a backfill technique—backing up slightly to fill in the area at the end of the weld—can help prevent the problem by adding greater thickness to the crater.

For all the aforementioned reasons, careful filler metal and base material selection, as well as good parts fit up, proper joint design and thorough welding techniques are all essential to avoiding hot weld cracking.

Keywords: Crack, welding, Hot Cracking, post-weld heat treated