A Hot Time on the Old Tine Tonight
The video posted online was created to entertain. It showed several welders in protective gear focusing their flames on one tine of the raised fork of a large forklift. The caption and narration said, “How welders heat up their lunch.” After the section of the tine was red-hot, one welder put an ear of corn wrapped in foil on the tine and then carefully unwrapped it to see how it was cooking.
The comments on the video blew up with criticism. “Are you sure it’s worth ruining the structural integrity of the forklift to ‘cook’ lunch?” one asked. “You guys just annealed that fork and now it is weak and no longer tempered,” wrote another.
Along with the comments about the structure of the metal being permanently altered, some remarked wryly that the forklift must be rented, while others lamented the extensive waste of gas to perform such a stunt. However, some noted that the fork seemed to be already distorted and that they were likely heating it to try and straighten it, and probably added the last part about cooking lunch just to be funny. In any event, the critics made a valid observation—heating the metal to that extent almost certainly changed its fundamental makeup.
Ever since ancient metalsmiths discovered forging, heat and metal have had a productive relationship through the ages. Heat-treating metal to change it so that it is harder, more durable, or has some other characteristic is common, being found in a variety of methods such as aging, annealing, case hardening, differential hardening, flame hardening, induction hardening, normalizing, quenching, stress relieving, and tempering.
Once the metal has been formed as intended, however, the improper application of heat can cause damage to the metal that should be avoided wherever possible. This is a cause for concern for welders, who bind different pieces of metal together primarily through heat, as well as metal fabricators and machinists who might use lasers, plasma, and other heat sources to melt through the material. Any time heat is applied to metal for a practical purpose like welding or cutting, the area around that application is also changed to an extent and is referred to as the “heat-affected zone.”
Entering the Danger Zone
The heat-affected zone (HAZ) is a critical region in the vicinity of a welded or heat-affected area in a material, where its microstructure and mechanical properties undergo significant changes due to the heat generated during welding or other thermal processes. Understanding the concept of the heat-affected zone is crucial in various industries, including construction, manufacturing, and aerospace, as it directly influences the structural integrity and performance of welded or otherwise extremely heated components.
When a material undergoes welding, for example, it is subjected to intense localized heat, causing it to experience a range of temperature gradients. The HAZ is the portion of the material that encounters these elevated temperatures without actually melting. The extent and nature of the changes in the HAZ depend on factors such as the welding process, material type, thickness, and the heat input during the welding operation.
The heat-affected zone can be divided into three distinct regions based on the temperature levels experienced:
- Critical Temperature Zone (CTZ): This is the region closest to the weld bead where the temperature is the highest. In this zone, the material may reach temperatures above its critical transformation temperature. As a result, the microstructure undergoes significant changes, such as phase transformations and grain growth, which can affect the material’s mechanical properties.
- Transition Zone (TZ): The area following the CTZ is the transition zone, where the temperature decreases gradually. In this region, some changes in the microstructure may still occur, but they are typically less pronounced compared to the CTZ. The material may experience partial transformations and alterations in grain structure.
- Stress-Relieved Zone (SRZ): Farther away from the weld, the material may return to ambient temperature. This region is known as the stress-relieved zone, where the microstructure stabilizes, and the material regains some of its original properties. However, depending on the material and welding process, there may still be residual stresses present in this zone.
The changes in the heat-affected zone can have profound effects on the mechanical properties of the welded joint. For instance, the hardness, toughness, and ductility of the material may be altered, which can impact the overall performance of the welded structure. Engineers and welders must carefully consider these changes to ensure that the welded components meet the desired specifications and standards.
Just like welding, any high-temperature cutting operation can also cause a heat-affected zone. The hotter and slower the process is, the larger the HAZ is, due to the higher temperature staying in one location longer. If the cutting can be performed faster or at a lower temperature, the size of the HAZ tends to be reduced.
Different cutting processes create different size heat-affected zones. Oxyacetylene cutting creates a larger HAZ because of its slow speed and flame width, while plasma cutting with its faster speed will cause a smaller HAZ, and laser cutting with its pinpoint focus of heat creates the smallest HAZ. For the fabricator who wants to entirely avoid creating a HAZ, utilizing waterjet cutting or good, old-fashioned shearing might be the solution.
Dealing with a HAZ
While it is an unwanted byproduct of heat cutting or welding, a heat-affected zone can’t be avoided. To mitigate the potential negative effects of a HAZ in welding, various welding techniques and post-weld heat treatments (PWHT) can be employed. Preheating the base material, controlling the welding parameters, and applying specific heat treatments can help manage the microstructural changes in the HAZ, thereby improving the overall integrity of the welded joint.
The key to reducing the formation of the HAZ in cutting is speed, regardless of the type of operation. The shorter the time the metal is exposed to the heat source, the smaller the HAZ will be. A well-trained operator who understands how to optimize machine settings will likely achieve a more efficient cut with a smaller HAZ. As in welding, heat-treating metal following an operation by using a method like annealing can help strengthen the material’s molecular bonds.
Utilizing experienced operators who are better trained and using better equipment is the best bet at minimizing the effects of the HAZ in a workpiece, so the wise shop manager will budget accordingly to ensure consistently reliable products.
