Preventing Weld Failures

Weld failures are an expensive and inconvenient part of any welding operation. They are also an unfortunate reality at some point or another. And whether cracking, cold lap, slag inclusions or some other defect is to blame for the weld failure, the results are the same: delays in production, downtime for rework and cost for labor that isn’t contributing to the overall throughput.

Not surprisingly, weld failures are often the result of simple oversights, many of which can be avoided with proper training and close attention. Here are four ways that welding operators can protect against weld failures and help maintain the productivity and profitability of a company’s welding operation.

MATCH FILLER METAL AND BASE MATERIAL STRENGTHS
Selecting the appropriate filler metal strength is an important way to help minimize the risk of weld failures. In most cases, matching the tensile or yield strength of the filler metal to that of the base material works best. The strengths should be as close as possible and selected according to the application’s design requirements.

Welding operators who are welding lower strength steel to higher strength steel should always remember to match the filler metal strength to that of the lower strength steel. Doing so allows for greater ductility in the weld metal, reduces stress and helps mitigate the risk of cracking. For certain fillet welds, or when welding on an application requiring only partial joint penetration (PJP), it sometimes is helpful to undermatch the strength of the filler metal to the base material since it can minimize the residual stresses in the finished weld.

Welding operators should always consult and follow their welding procedures for proper filler metal requirements.

PREHEAT MATERIALS CORRECTLY
High-strength steels and materials with high carbon or high alloys tend to be susceptible to weld failures due to cracking. They are less ductile, and more affected by stresses along the base metal and the finished weld during the cooling process. To prevent such problems, it is critical to preheat these materials according to the temperature indicated in the welding procedure. Adequate preheating helps ensure that a uniform heat soak has occurred throughout the base material. This slows the cooling rate, which reduces shrinkage stresses in the weldment.

UTILIZE PROPER FIT-UP AND JOINT DESIGN
Proper part fit-up and good joint design can both help prevent weld failures. Unnecessarily wide joints often cause the welding operator to compensate by creating a wider weld bead to fuse the metal together. This can lead to a thin, weak weld throat and create stress on the center of the weld. The result is quite often a condition called bead-shape cracking. This is a specific type of hot cracking that appears immediately upon the weld cooling and must be reworked by the welding operator.

When possible, the joint should be designed so that the welding operator has easy access to the root. This access better allows for proper bead depth to width ratio. A good range for that ratio is to make the depth 0.5:1 to 2:1 the size of the width.

USE LOW HYDROGEN FILLER METALS
When welding ferritic (or iron-based) steels, using low-hydrogen filler metals can often reduce weld failures caused by hydrogen-induced cracking (or cold cracking). This type of weld failure results from residual stress in the base material along the weld and the presence of hydrogen and typically occurs within hours to days after the weld has cooled. It is more prevalent in thicker materials, which tend to create areas of high restraint and often serve as a heat sink that leads to fast cooling rates. These fast cooling rates, in turn, offer the ideal condition for hydrogen to gather and add to the residual stresses in the weld. High-strength steels and applications with constrained joints are prone to similar weld failure via cold cracking.

Look for filler metals with an H4 or H8 designator to help prevent weld failures associated with cold cracking. These filler metals have less than 4 or 8 ml of hydrogen per 100 g of weld metal, respectively. In certain cases, using filler metals with a basic slag system can also help reduce the risk of weld failures from cold cracking. These filler metals typically contain high levels of hydrogen scavengers, including fluoride, sodium and calcium that can combine with hydrogen to remove it from a cooling weld.

Paying close attention to these four recommendations, as well as carefully following all requirements set forth by the given welding procedure is critical to gaining good welding performance and avoiding weld failures. Welding operators should always follow proper procedures for equipment setup, as well as filler metal storage, too. Clean, dry filler metals and the right welding parameters contribute meaningfully to welding success.

Whenever there is a question about an application, welding operators should always err on the side of caution and consult with a trusted welding distributor, equipment manufacturer or filler metal manufacturer for recommendations.