Cracks In Welding

[Patrice Mieczkowski]

Asindicated, 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.

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.

Hydrogen HAZards: A Closer Look at Cold Cracking

Unlike hot cracking, cold weld cracking occurs at temperatures well below 600 degrees Fahrenheit (316 degrees Celsius) and does not appear until hours, even days, after the weld cools. Also, most cold cracking begins in the base metal instead of the weldment itself and passes transversely into the weld as it progresses. Cold cracking is often referred to as hydrogen-induced and/or heat-affected zone (HAZ) cracking.

Broadly speaking, cold cracking occurs in the HAZ as the result of residual stresses from the base material restraining the weld, along with the presence of diffusible hydrogen. Cold cracking is particularly prevalent in thick materials, as they tend to create areas of high restraint and can serve as a heat sink that leads to fast cooling rates. Such rapid cooling cause the microstructure in the HAZ to form a new crystalline microstructure called martensite. While very hard, martensite is also very brittle and lacks ductility. Martensite provides a location for diffusible hydrogen to coalesce, which in turn creates residual stresses that build in the HAZ. Once these residual stresses reach a critical level, cold cracking occurs.

Materials prone to cold cracking include those with high carbon and/or high alloy levels and which are therefore also higher in strength. Such materials, especially thicker ones, are generally less ductile and tend to shrink after welding, which causes additional residual stresses that lead to cracking.

Techniques like back stepping can help prevent cold cracking. To perform this technique, the welding operator welds in one direction for a short length, returns to prior to the beginning of the last weld and repeats the weld pass, stopping at the start of the first weld. In essence, the heat of the subsequent weld pass serves as a type of stress relieving. The best defense against cold cracking, however, is proper pre- and post-weld heat treatments, along with general practices that minimize the exposure to hydrogen sources (these will be discussed in detail in the following section).

Pre-heating the base metal prior to welding slows down the cooling rate, allowing more time for hydrogen to diffuse from the weld, and it also forms a more ductile, less hard microstructure often referred to as pearlite. Pearlite forms a the expense of martensite and is much less susceptible to the damaging effects of hydrogen.

Similarly holding the finished weld at a given temperature for period of time (via a process like induction heating) slows the cooling process, allowing hydrogen to diffuse more readily and limiting the chance of cold weld cracking.PWHT reduces the propensity for cold cracking by both relieving residual stresses and driving the diffusible hydrogen from the weldment. Once released, hydrogen cannot naturally diffuse back into the weldment. Note that rapid heating and cooling rates during PWHT generate thermal gradients that can increase residual stresses and lead to cracking just before the full benefits of PWHT are realized.

Other Best Defenses: Selecting and Storing Filler Metals

In addition to the aforementioned techniques, proper filler metal selection and storage can also help prevent costly weld cracking, especially cold cracking.

In particular, using filler metals that feature a low hydrogen designator, such as H4 or H8, are a good defense against cold cracking. These designators indicate that the product offers low diffusible hydrogen levels, or in these examples, less than 4 or 8 mg of hydrogen per 100 g weld metal.

Filler metals with a basic slag system can similarly mitigate the risk of cracking, as they have a high volume of hydrogen scavengers (fluoride, sodium and calcium, for example) that are able to combine with hydrogen, removing it from the solidifying weld. Note, despite having large amounts of hydrogen scavengers, products with a basic slag system typically cannot be used for out-of-position welding. They also tend to have more challenging operating characteristics than those with an acidic slag system.

Filler metals should be stored in a dry area and remain in the packaging in which they were received from the filler metal manufacturer until ready for use. Ideally, that packaging should be heat- and/or vacuum-sealed to block moisture from reaching the product. The storage area should also be similar in temperature to the environment in which the welding will take place; storing the filler metal in a cold area and moving it to a hot one can lead to condensation, increasing the chance of hydrogen pickup in the weld. If a storage area of similar temperature is not available, allowing the filler metal to acclimate to the temperature of the welding environment before opening the package can help minimize the risk of condensation and subsequent chances of cracking.

Unfortunately, even the best filler metals and the most appropriate storage practices are not guaranteed defenses against cold cracking. Hydrogen can still be absorbed into the weld via the atmosphere, especially in high humidity areas, or by contaminants on the base metal, particularly rust, mill scale, oils, lubricants and primers. Defective gas lines or connections can also cause shielding gases to have a high dew point and therefore, greater amounts of hydrogen. For all of these reasons, care must be taken throughout the welding process to avoid these additional hydrogen sources.

One of the primary objectives of any weld fabrication is to prevent weld defects, especially cracks. Cracks are the most severe of all weld defects and are unacceptable in most circumstances. Rework robs the company of precious time and material (that is, money), so prevention is the primary concern.

A discontinuity is a weld fault that may or may not be serious enough to cause a rejection. Whether or not it violates code specifications will depend on further examination by a competent person against code requirements or in-house quality assurance specifications. If the fault violates either of these two, it becomes a defect. Defects require repair, but discontinuities do not. Violationsof customer requirements often fall under the discontinuity rule and the weld will have to be repaired.

The responsibilities of both welder and supervisor affect weld quality. The welder is responsible for the defect when it is due to his or her skill level or weld deposition technique. Weld characteristics like incomplete fusion, excessively concave or convex bead contours, and improper weld size all can come from poor welding technique, improper travel speed, poor electrode manipulation,incorrect weld parameter settings, as well as failure to notify supervision of a problem with the job at hand.

Supervisors must ensure welders have the tools necessary to do an effective job. They must maintain a shop safety program in compliance with OSHA regulations. They also should, among other things, ensure welders are using the correct base and filler metal; have proper weld procedure testing; work with adequate and functional welding equipment; receive effective and meaningful welder training;and work with properly designed, accessible weld joints.

If no design alternative exists, managers must plan for potential weld errors. If an unacceptable weld defect occurs, can a worker get a grinder into the joint to remove the bad weld? If so, how will the weld be repaired? A welder or supervisor can answer all these questions, but the best solution often requires input from customers and product designers.

The length of the weld deposit also is highly stressed, so that crater crack can very easily travel back through the entire length of the weld centerline. This is a common problem in aluminum and some tool and die steels. The remedy is simple: Fill the crater to its full cross section (the same as the weld size) before the weld is finished. You can accomplish this with various methods. You maypause for two or three seconds at the end of the weld before stopping the arc; or you may choose to backstep (reverse direction of travel) for about 0.5 inch at the end of the bead.

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.

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. For constant-current processes, GTAW, and SMAW, voltage varies with the arc length. If you increase the arc length, you increase arc voltage. Be sure to maintain a correctelectrode angle, and try decreasing travel speeds to allow weld deposition to do its job.

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