The impact of impact toughness
Filler metal whats, whys, and hows that lead to proper weld strength
Choosing a filler metal with the right mechanical and chemical properties for your application is critical. Correct properties not only ensure the proper weld strength, but may also help prevent costly complications. In applications that endure rapid or cyclic loading, experience extreme service temperatures, or are subject to other stressors that threaten the integrity of the weld, using a filler metal that provides the proper impact toughness is especially important.
When choosing filler metals, finding those with the right mechanical and chemical properties for the application is critical. Correct properties not only ensure the proper weld strength, but may also help prevent costly complications. In applications that endure rapid or cyclic loading, experience extreme service temperatures, or are subject to other stressors that threaten the integrity of the weld—seismic activity, for example—using a filler metal that provides the proper impact toughness is especially important. In particular, high-strength, low-alloy materials can benefit from these filler metals, as can some mild steel applications.
Impact Toughness: What It Is, Why It’s Important
Impact toughness is the ability of a weld to permanently deform while absorbing energy before fracturing, specifically when stress is applied rapidly—typically, in under one second. In simpler terms, it’s how much rapid-impact energy a weld can take before it cracks. Impacts can come in the form of natural elements, such as wind or earthquakes, explosions, collisions, and rapid loading.
Filler metals that provide good impact toughness can help minimize the extent and rate of cracking should it occur as the result of an impact or fatigue from cyclic loading. In low-service-temperature applications, such filler metals can also help mitigate the risk of brittle fractures caused by the loss of toughness that steels undergo at subzero temperatures. In some cases, good impact toughness in a weld can even arrest the propagation of a crack, allowing for emergency repairs to occur before catastrophic component failure.
The best-case scenario, however, is that using filler metals with good impact toughness—combined with consistent and appropriate welding procedures—can help prevent cracking altogether. Many codes and specifications require filler metals that are capable of meeting minimum impact toughness for this very reason.
Most filler metal manufacturers formulate, classify, and produce filler metals according to strict standards, such as the American Welding Society (AWS) A5 filler metal specifications. These specifications provide minimum impact toughness requirements for each filler metal classification, as well as the method by which these electrodes should be tested.
You may select filler metal classifications based on the requirements stated in the code or specification for your application. One such example is the AWS D1.8 Structural Welding Code—Seismic Supplement, which was developed to help ensure safety in demand-critical applications, such as structures constructed in seismic areas. Be aware that code or specification requirements may exceed the toughness requirements given in a filler metal specification.
In other cases, engineers may have some discretion over the filler metal’s impact toughness requirements for a given welding procedure. Engineers develop requirements in a way that can account for expected service conditions, as well as any foreseeable misuse or abuse. For example, a bridge engineer may design welds to be able to withstand a boat collision. This structure, of course, is not intended for such an occurrence, but filler metals with high impact toughness can help ensure the ability to safely withstand such unexpected impacts.
How Filler Metals Provide Impact Toughness
Filler metal manufacturers formulate products using a delicate balance of many elements, each of which provides distinct characteristics to both the welding process and the completed weld. These elements can range from deoxidizers that assist welding through mill scale to the addition of chrome to increase corrosion resistance. When designing filler metals to provide good impact toughness, manufacturers may add nickel and eliminate phosphorus and sulfur to obtain the desired properties.
Nickel can improve the properties of weld metal by altering its microstructure in a way that increases strength and offers good resistance to cracking. In some cases, wires with nickel added may have a slightly more sluggish weld puddle or generate additional spatter, but the increased toughness generally offsets such disadvantages.
The presence of phosphorus and sulfur in both the filler and base metals is especially detrimental to weld toughness. The harmful effects of these elements are amplified in single-pass applications where more of the base metal mixes with the filler metal, increasing the chance for weld metal to pick up these elements from the base metal. To ensure good impact toughness, it is best to select both base and filler metals with the lowest possible phosphorus and sulfur content, ideally no more than 0.03 percent.
Comparing AWS classifications is a good starting point when selecting filler metals for impact toughness. E7018-1 shielded metal arc welding (SMAW) electrodes provide improved impact toughness over plain E7018 electrodes. When selecting gas-shielded flux-cored arc welding (FCAW) wires, T-9 electrodes (for example, E71T-9 C/M) are as easy to control during the welding process as T-1 electrodes, but offer improved toughness qualities. T-5 FCAW wires are an even tougher option. They can be a bit more difficult to weld with, but the impact properties they provide are excellent and often make the less-than-optimal operating characteristics worthwhile. These electrodes are typically the first ones recommended in many critical applications and repairs.
When in doubt, consult with a trusted filler metal manufacturer or distributor to determine the best filler metal for each application.
The Charpy V-notch Test: Measuring Impact Toughness in Filler Metals
The Charpy V-notch (CVN) test is one of several tests used to determine impact toughness in filler metals and is considered to be the industry standard. The equipment needed to conduct the test consists of a hammer located at the end of a pendulum (see Figure 1). After the hammer is released from a set height, its force applies a consistent amount of energy to the weld specimen (held inside a special jig) and typically breaks this precision-machined notched specimen as it continues on its fixed path. A computer calculates the CVN impact value by measuring the highest point the pendulum reaches after it contacts the weld.
Impact toughness values are shown in terms of foot-pounds and joules—units of energy absorbed—at a given temperature (for example, 20 ft.-lbs. at -20 degrees F). The temperature at which filler metals are tested for impact toughness varies according to their individual classifications. Most mild steel filler metals undergo testing between zero and -40 degrees F, while some low-alloy products must be tested at temperatures as low as -150 degrees F. Again, steel loses toughness at low temperatures, so it is especially important to consider the test temperature of the filler metal when it will be used in subzero applications.
You should understand that a CVN test is not an exact simulation of how the structure will respond to a real-life impact. Still, the test offers a sufficient comparison for weld and base metal performance, and most welding codes and specifications support the measurements that the test provides.
Other Factors That Affect Impact Toughness
In conjunction with choosing the correct filler metals, the welding procedures (particularly temperature control) and shielding gases you use can affect the impact toughness in a completed weld.
Preheating the base material before welding—and maintaining proper interpass temperatures during welding—can help to slow the cooling rate of both the base and weld metal after welding. A slow cooling rate helps minimize issues with the metal structure on a microscopic level that can affect impact toughness negatively.
Be sure to control the heat input during the welding process, regardless of whether you are welding a single- or multipass application. Like preheat, a high heat input can help slow the cooling rate and help prevent cracks. However, excessive heat input can cause a host of other problems that will eventually minimize toughness, including generating a larger heat-affected zone (HAZ), which is particularly detrimental for quenched and tempered (Q&T) steels. Excessive heat input also can alter the filler metal deposit chemistry, causing certain elements to burn out of the weld and, therefore, decrease its impact toughness.
Postweld heat treatment (PWHT) operations such as stress relieving also can affect a weldment’s impact toughness. Carrying out these operations always should be at the discretion of the engineer, in conjunction with welding code requirements. Likewise, you should consult your filler metal manufacturer before performing PWHT to ensure that the filler metal you’ve chosen will maintain acceptable properties after PWHT is complete.
Finally, always use the manufacturer-recommended shielding gas for a filler metal to obtain the correct impact toughness. Different shielding gas compositions have unique reactions with the weld pool that affect mechanical and chemical properties differently. Filler metal manufacturers are aware of these interactions and carefully adjust formulations to provide optimum impact toughness for intended shielding gases.
Product data sheets provide shielding gas and welding parameter information, so be sure to look them over; each product has an intended operating window for each product to prevent compromising vital filler metal characteristics.
As long as you adhere to correct welding procedures appropriate to the filler metal manufacturer’s recommendations, you should be able to achieve the weld performance that is consistent with the filler metal’s classification – including its impact toughness.
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