To strengthen welds, use post-weld heat treatment to reduce residual stress, improve toughness by 30%, and limit grain growth.
Altered Microstructure
Welding exerts substantial influence on the microstructure of the metal in the heat-affected zone . It serves to change grains size and distribution uniformity of the alloy components and directly impacts the weld’s strength and durability. Understanding these alterations is essential to appropriate welding and ensuring the safety of the subsequent construction.
Metal alloys, which are utilized in construction, automotive manufacture, and other industrial areas throughout the world, are the classic example of such materials. According to existing research, steel, in particular, it tends to lose up to 20% of its tensile strength due to altered microstructure in many high-carbon systems when steel alloys are welded . The change is directly related to the size of the grains and their distribution. Steel components that have been long exposed to high temperatures predisposing to the growth of grains, demonstrate higher probabilities of failure than their smaller-grain counterparts. Therefore, welded beams in high-rise buildings and welded joints in vehicle chassis are designed taking into account the uneven distribution of grains caused by welding. For maximum efficiency and safety of welds, engineers and technicians must adjust welding regimes, varying the amount of heat input and the speed with which the product is cooled. A lesson learned in this regard is that high strengths of the designed construction details could hardly be entrusted to the fate of the microstructure, and pre-heating and post-heating are routinely applied as well to reduce the likelihood of immediate failure.
Residual Stresses
Welded components have residual stresses due to heated metal, which cools down and contracts. As a result, the uneven cooling causes a physical stress in welded components, which can compromise the structural integrity of the weld. From my point of view, it is especially important in aerospace and maritime construction. For example, a ship’s hull is made from large steel plates welded together. The uneven cooling of these massive welds leaves them under tensile stresses. If the case is unmanaged, they gradually can form a crack and break under the stress of a stormy sea. The study in the shipbuilding industry states that the fatigue life of a weld with residual stress is 60% lower than at without. It puts the safety of the passengers or crew on stake and adds additional maintenance cost. The measures to treat the consequences and reduce the residual stress are needed.
Stress-relief annealing is widely used in these industries. They optically heat the welded component up to a certain temperature and then slowly cool it. This allows the temperature to cool and find a new stress equilibrium that does not affect the component’s steel properties. If the case of aerospace components, the temperature is controlled in a special way. The pieces have to withstand a load of up to 20g and more in some cases. The precise control of the temperatures can improve the fatigue life of a weld by 30% compared to untreated welds. Controlled peening of the components with a minimal effect on their structure is the second method. It is used to introduce compressive residual stresses at the surface of the weld, which counteracts the remaining tensile stress within the metal. For example, car frames are welded to maintain the vehicle’s durability and the passengers’ safety. Laser peening was applied to more successfully introduce additional stresses compared to hammer peening. The wear in components coated with the first method is more than 50% lower than with the second. The fatigue life of the components is also more than 40% higher.
Reduced Hardness and Toughness
The heat-affected zone of a weld experiences reduced hardness and, as the result, the toughness that often causes problems in industries where the mechanical strength plays an important role. The reason for that is that high temperatures lead to the changes in the crystalline structure of metals. They affect the physical properties of materials, decreasing their toughness, hardness, and other characteristics.
An example of this phenomenon may be found in the area of manufacturing of oil pipelines. These products need to nbe extremely sturdy to prevent leakage and the outflow of hazardous substances into the environment. Research shows that the toughness of the HAZ is the pipe steel is about 50% less than that of the base material. This results in the welds being prone to cracking under the impact of the flow pressure and variable temperatures. To improve the conditions of manufacturability and provide a sustainable solution, the industries involved utilize post-weld heat treatment s. They imply heating the welded joints to a specific temperature below the melting point and then proper cooling.
These treatments can help restore from 40 to 50% of the HAZ characteristic due to the process of microstructure refinement. An example of HSLA steel used in the construction of bridges can withstand the LTT and show decrease in toughness by 50%, which makes it too brittle. PWHT may improve the situation and add 30% to the toughness of the material. Another solution to the problem goes through the use of welding consumables with low transformation temperatures. They are designed to produce a specific type of microstructure that will have beneficial physical properties inclined les to a decrease in hardness and toughness. They transform at a lower temperature and, thus, improve the post-weld mechanical characteristics, with the impact strength increasing by 25%.
Grain Growth
Grain growth in the heat-affected zone of a weld is a major concern impacting the mechanical performance of welded structures. Metals are exposed to high temperatures during welding, which results in the grains growing to an abnormally large size. This has the effect of reducing the overall toughness of the material and its strength. Grain growth during welding plays a significant role in the aerospace industry, where strength-to-weight ratio is a critical factor. For instance, in the welding of titanium alloys for aircraft frames, excessive grain growth can reduce the toughness of the heat-affected zone by as much as 40%. Consequently, such welds are more likely to crack and their fatigue life is reduced. A major method used to limit grain growth is the use of controlled cooling after welding. Here, the cooling rate used is controlled in order to prevent excessive grain growth beyond a certain size. The cooling can be carried out by various methods including forced air cooling or water quenching depending on the material and the unique requirements of the weld. These techniques have been shown to cut down grain growth in the heat-affected zones of the welded metals by as much as 30%. Grain refiners can also be added to the weld metal. These refiners facilitate the formation of finer grains during the solidification of the welded material. In the production of high-pressure vessels, titanium carbide grain refiners coated onto the steel electrode have been shown to improve the impact resistance of the steel welds by as much as 20%, which makes the vessels more resilient and less prone to failure during their operational life.
Precipitation of Phases
The precipitation of phases within the heat-affected zone is an important factor that defines the mechanical properties and reliability of the welds. In HAZ, welding leads to the formation of various microstructures, which contain different precipitation forms. New phases, in turn, may significantly affect the hardenability, toughness, and corrosion resistance of the material. In the context of the oil and gas industry, where welding is used to create the pipelines that are intended to transfer fluids under high pressure, the precipitation of phases should be carefully controlled. For example, if stainless steel is used, grains of this material contain chromium carbides, which are not dissolved and are being precipitated along its boundaries. The presence of this form of precipitation may decrease the corrosion uniformity and, thus, the quality of material. For example, in Santor et al., at least 50% of corrosion resistance is lost due to the formation of chromium carbides.
To prevent the precipitation of chromium carbides, usage of the low-carbon alternative to stainless steel or introducing stabilizing elements is recommended. The last alternative is connected with adding a small amount of niobium or titanium, which is intended to bind with the carbon and leave the chromium in solution. This way, the formation of carbides is interrupted, and the quality of the material does not decrease. As is noted by Santor et al., this solution was successful, and corrosion decreases of up to 50% guaranteed the lifespan extension of up to 20 years, while in the case of little to no adjustments, this parameter began to fluctuate during 10-12 years, regarding the characteristics of these types of environments. One more method to avoid the formation of chromium carbides is the post-weld heat treatment.
PWHT consists of the reheating of the weld and the corresponding area to a temperature that is sufficient to redistribute the precipitates that were in the solution or convert them to a less harmful form. In the gas and oil industry, aluminum alloys are widely used as well. However, in the case of such carbohydrates precipitation as Mg2Si and Al3Mg2, PWHT is able to raise the fatigue strength of the parts up to 30%, thus making it more responsive to the environments of aerospace applications.
Susceptibility to Corrosion
Corrosion susceptibility in the heat-affected zone of welds is a substantial issue that might result in the failure of metal constructions and decrease their life span. It is especially relevant in the case of aqueous and industrial environments that allow the attack of sensitive zones, which might not be observed otherwise. In marine welding, which is widely utilized for ships and offshore oil platforms, the issue could be even more severe because the environment is highly corrosive. For instance, in boat-building, the corrosion rate in welded connections is about 30% higher when compared to the entire not heated steel. In such cases, it is possible to lose the majority of the safety margins and reduce the operation time of unit structures to the point that repairs become necessary.
Therefore, industries adopt and currently use systems of cathodic protection that make a given unit a cathode of a cell and anodize the cathode of the protected space elsewhere. However, it is more practical to utilize batteries of specially designed sacrificial anodes, made either from zinc or magnesium. While they are active, the corrosion rate of a unit decreases by 50% of the observed on unprotected units, which allows increasing the life span of marine structures and therefore their safety. The method is widely observed in shipbuilding and platforms for off-shore oil extraction because the long-term benefits and life span advantages of selected units are substantial.
The second method that could be proposed is the utilization of coating that do not corrode in the selected environment and are capable of separating the sensitive zone from the environment completely. There are numerous types of such materials, but the example may be made in the cases of epoxy and poly-urethanes of very high quality. The durability of these materials is drastically higher in comparison to metallic structures, and they were observed to reduce the rate of iron corrosion by up to 70%. It is especially relevant to the coating of vehicles’ undercarriages in the automotive industry, which are sprayed by road salts and other harmful corrosive materials. Nothing degenerative in relation to the vehicle’s structure and appearance happens, even after 8 years or longer, as observed on older cars.