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Before we can answer this question, we must provide the answer to what toughness is in a metal. Toughness is often confused with strength but the differences are quite significant. Generally speaking, strength is related to how much force the metal can support, while in the science of metallurgy,听toughness听is the ability of a material to absorb energy and plastically deform without fracturing. Good toughness, therefore, is necessary to resist the initiation and retard the growth of cracks over time.

For example,听brittle听materials such as ceramics, that are strong but with limited ductility are not tough; glass is a good example of a brittle material that lacks toughness. A metal that can be bent without breaking is tougher than a metal that will break rather than bend. Think of a 鈥渃ar body hood鈥� which is bent into extraordinary shapes without failure. 听

We can produce a tough steel (or other metal) at the steel mill by chemical seeding (alloying), by heat treatment and by mechanical work for metallurgical grain size control. Such a steel can be designed to gives us good toughness, amongst other properties. However, when we come to weld it how to we maintain the toughness in the joint, both in the weld metal and in the heat affected zone (HAZ)?

One of the other properties of a steel is that once its toughness is established at a certain temperature, then that toughness will lessen as the temperature of operation lessens. A steel that is tough at room temperature may fracture in a brittle mode under the right loading conditions at temperatures seen in some areas of Canada and in the Far North.

The basic method for defining toughness is the Charpy V-Notch test (CVN) and, in this process, toughness is defined in 鈥渏oules鈥� at the 鈥渢emperature of test鈥�. There are other, more complex methods but the CVN test is cheap, is a good Quality Control (QC) method, and is universally accepted. A charpy toughness curve for a basic steel is depicted in Figure 1 where it can be seen how temperature affects the toughness.

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Figure 1. A Standard CVN curve for a steel which depicts the transition from ductile (essentially tough) to brittle behavior as the temperature drops

The Charpy test, is destructive, and determines the toughness or impact strength of the metal using fast loading conditions that are scripted by a swinging a heavy pendulum targeted at the test sample. The specimen itself is notched, a sharp defect simulation, and the test is performed at a selected temperature(s).听

If the design of a fabricated end product is such that we need a toughness of 27 joules at -20 C, how do we ensure that the steel, ordered and reported at this toughness, will achieve this in the welded joint.听

With regard to the welding consumables, most filler metal manufacturers formulate, classify, and produce filler metals according to strict standards. In North America these are the Canadian Standards CSA W 48 鈥淔iller Metals and Allied Materials for Metal Arc Welding鈥� and the American Welding Society (AWS) A5 鈥淔iller Metal Specifications鈥�. These specifications provide minimum impact toughness requirements for each filler metal classification, as well as the method by which these electrodes should be tested.

If we now have a steel with a known toughness and have a selected filler metal with deposited toughness properties that are also known, how about the toughness that manifests itself in the Heat Affected Zone (HAZ). This is the area of the base metal that gets heat treated during the weld cycle, and is located next to the fusion boundary of the weld as shown in Figure 2.

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Figure 2. Areas of the Weld Zone at the Fusion Boundary

The width of the HAZ, and its metallurgy, will vary according to the heating and cooling conditions seen by the parent plate during welding. In order to get information on the HAZ toughness it will be necessary to take CVN samples from the HAZ with the tip of the notch sampling the various microstructures across the width of the HAZ.听

The CVN tests will normally be performed as part of a welding procedure qualification or PQR. This can be done in several ways and several locations in the weld zone depending on design and the specification of record. For example, CSA W 47.1 provides guidelines in its Annex E which stipulates the location of CVN samples and the notch position for both the weld metal and HAZ.听

If we obtain satisfactory toughness in the HAZ and weld metal, as shown in the samples taken from the PQR, we must then repeat the proven welding procedure during practice in order to guarantee that the toughness will be maintained. Weld procedure Specifications (WPS) or Weld Procedure Data Sheets (WPDS) must then stipulate the controls required for the 鈥渟ite weld鈥� to achieve the toughness properties attained in the laboratory tested PQR. These will come in the selection of the filler, the precise preheat and interpass to be used and controls on the heat input (voltage, amperage and travel speed) of the welding process.听

It will then become the responsibility of both the welder and the supervisory staff to ensure that the procedure is being followed to the letter, so that the equipment being fabricated will maintain its integrity while underload. The defined toughness, baked into the fabrication by diligently following the welding procedure, makes it resistant to crack propagation under the design loading and, therefore, reduces the probability of damage to the environment or to the safety of the general public at large.