Precipitated Phases of Weld Metal

Introduction

Welding is a common way of joining two pieces of metal, but welding is not always a good process for steel and other metals. The thermal properties and chemical properties of welded metal joints can change dramatically depending on the process and the alloy being welded. In some cases, welded metals can be prone to corrosion and fatigue, as well as oxidation and embrittlement. This is why it is important to understand the effects of welding on the metallurgy of a welded joint.

Background

When welding two metals together, the heat and the pressure combine to cause a reaction between the two elements.When joining two similar metals, the reaction is often simple. For example, when joining two pieces of steel, the heat causes the joint to become metallurgically bonded. In some cases, the heat may cause a phase change in the structure of the material, leading to a new set of physical and chemical properties. In other cases, the heat may cause a change in the crystalline structure of the material, leading to a new set of properties.

When welding dissimilar metals, the heat and pressure can cause the two alloys to react in unexpected ways. For example, when welding two pieces of carbon steel to aluminum, the heat and pressure can cause the aluminum to oxidize and form an oxide coating on the surface. This oxide coating can inhibit the welding process, and can lead to weakness in the joint if not removed. In addition, the heat and pressure can cause atoms of one element to migrate into the other, leading to changes in the composition of the alloy and further weakening the joint.

Composition of welded joint

The composition of a welded joint is an important consideration when determining the properties of a welded joint. The composition of a welded joint is affected by the alloy content of the parent metal and the type of welding process used. For example, when welding two pieces of mild steel, the composition of the joint is typically very similar to the composition of the parent metal. However, differences in the composition can occur, particularly when welding with higher temperatures.

The composition of a welded joint is also affected by the weld bead. The weld bead is formed during the welding process and it is important to understand how it affects the overall composition of a welded joint. The weld bead can contain scrap materials and contaminants that may not have been present in the parent metal, and these can affect the alloy content and properties of the welded joint.

Welded metals are also prone to microstructural changes due to the thermal and mechanical stresses involved in the welding process. This is why it is important to understand the microstructure of a welded joint before welding.

Conclusion

Welding is a common way of joining two pieces of metal, but the effects of welding on the metallurgy of the welded joint must be understood to ensure that the welded joint is strong and durable. The composition of the welded joint, the weld bead and the microstructure of the welded joint are all affected by the welding process. It is important to analyze the joint in order to determine how the welded joint will react to the conditions it is subjected to. With this understanding, the welded joint can be designed and fabricated to have the desired properties.

Metal Corrosion and welds

Metal corrosion occurs when a metal surface is exposed to air, moisture, and other elements. All metals are affected by corrosion, though some are more resistant than others. Common metals that are vulnerable to corrosion include iron, copper, aluminum, and steel. Various forms of corrosion can be found on metal surfaces, such as rust, pitting, general corrosion, and crevice corrosion.

Welds, which join two pieces of metal or alloys together, also experience corrosion. There are two types of weld corrosion to be aware of: atmospheric weld corrosion, which occurs in typical everyday conditions, and galvanic corrosion, which occurs when two metals of different potentials are in contact with a conductive electrolyte. Traditional methods of corrosion control do not address the corrosion of welds, and accordingly they are often left unprotected, making them susceptible to corrosion.

Even weldments that are undercoated are vulnerable to corrosion around the welded area due to the release of electrode elements such as iron and manganese, which can cause contamination in the entire structure of the weldmetal. The phenomenon is known as arc welding fume corrosion and it can be used to predict the behavior of a weld under the equilibrium of the electrode elements.

To protect welds from corrosion, it is important to execute the weld correctly and correctly select the filler material. Protective coatings and weld sealants can also be used to protect against corrosion. Weld sealants are generally made of epoxy-based polymers and are designed to fill in crevices that may lead to corrosion in a welded joint. These types of sealants should be applied during or after the welding process to protect a weld from corrosion.