Welding is the process of joining two or more materials (mostly metals) using intense heat, such that they fuse together to become one homogeneous component. The art of welding can be traced all the way back to the Bronze Age, where pressure welding was used to fuse metal joints together to construct gold boxes and other artifacts. Today, welding is used to construct automobiles, buildings, airplanes, pipelines and oil rigs, to name a few.
At first glance, welding may not seem to have a direct correlation to corrosion in metals. However, applying intense heat to metals using improper techniques can result in defects that encourage corrosion formation. (For an introduction to this subject, see Causes and Prevention of Corrosion on Welded Joints.) One such defect is hot shortness.
What is Hot Shortness?
Hot shortness, also known as hot cracking, hot tearing, or solidification cracking, is a welding related defect where the metal begins to crack along its grain boundaries as the weld metal cools and solidifies. To understand the mechanisms involved in hot shorting, let us briefly explain what grain boundaries are.
As molten metal solidifies, crystals begin to form independently of each other. Each crystal is randomly oriented and distributed throughout the metal. These crystals form what is called the grains of the solid metal. The grain size has a significant influence on the physical properties of the metal.
The interface where grains contact each other is known as the grain boundary, and they are known to have a major effect on the overall behavior of the hardened metal. Unfortunately, grain boundaries are often preferred locations for the onset of localized corrosion.
Metal impurities, which tend to segregate at the grain boundaries, typically possess melting points lower than the metal itself. As the weld cools, the metal, which has a higher solidification temperature, hardens first while the grain boundaries remain in a semisolid state. The residual stresses imposed on the weakened grain boundaries due to the hardening of the surrounding areas cause them to crack and pop open.
Types of Corrosion Caused by Welding and Hot Shortness
The cracks caused by hot shortness not only weakens the overall strength of the metal, but they also serve as ideal locations for the formation of corrosion-related defects such as stress corrosion cracking, corrosion fatigue and crevice corrosion. Here we shall discuss how the cracks formed by hot shortness contribute to each of these types of localized corrosion.
- Stress corrosion cracking (SCC) – This type of corrosion is characterized by the formation of cracks along the grain boundaries of the metal while the bulk of the surface remains unaffected. This occurs because the grain boundaries represent a path of high corrosion susceptibility. The segregation of impurities at the boundaries makes it difficult for passivation to occur, rendering it less corrosion resistant than the rest of the surface. (Learn more about SCC in the article What Causes Stress Corrosion Cracking In Pipelines?)
Hot shortness further lowers the repassivation capabilities of the grain boundaries, making the metal even more vulnerable to stress corrosion cracking. The tensile stresses induced by the cooling of the surrounding areas advances the opening up of the cracks, allowing easier diffusion of corrosion products away from the crack tip.
Tests conducted by Thomas Böllinghaus and Horst Herold in their publication “Hot Cracking Phenomena in Welds,” shows that longitudinal hot cracks in particular represented an initiation site for subsequent stress corrosion cracking. Meanwhile, cracks perpendicular to the boundary did not seem to influence this type of corrosion as much.
- Corrosion fatigue – Corrosion fatigue is a type of failure caused by the combined action of alternating or cycling stresses, and a corrosive environment. As the metal goes through alternating stress cycles, the passive film ruptures, causing an accelerated growth of corrosion.
The presence of concurrent cracking mechanisms such as hot shortness can contribute to the overall fatigue crack growth rate.
- Crevice corrosion – Crevice corrosion is a type of highly localized corrosion that takes place at, or next to, a gap or crevice on a metallic surface. The rest of the metal, however, remains uncorroded. The cracks resulting from hot shortness create ideal conditions for crevice corrosion to occur.
As water (the electrolyte) enters the crack it can become stagnated. In other words, the gap is just wide enough for water to get into but too small for it to flow out (this is one of the critical components for crevice corrosion). Because there is no significant movement of fluid out of the crack, oxygen diffusion gets restricted to the crevice. A series of complex chemical reactions then occur where the fluid trapped in the crack becomes acidic and begins to break down the passive layer. With the substrate unprotected, corrosion is free to propagate within the cracked areas.
How to Prevent Hot Shortness
The key to preventing hot shortness is understanding the mechanisms that cause cracks to appear in the first place. Some of the solutions to help prevent hot cracks are:
- Proper material selection – Selecting steels with low residual phosphorous or sulfur content can help minimize the chances of impurities in the grain boundaries.
- Reduce heat during welding – Lowering the amount of heat during welding lessens the amount of time for segregation to occur at the grain boundaries.
- Reduce contamination of the weld pool – Elements like sulfur and copper make the weld composition prone to cracking as the low melting compounds become entrapped in the final weld. These impurities can be removed from the weld pool by ensuring that the weld joint is free from oil and grease and ensuring the cleanliness of welding consumables.
- Use appropriate filler metal – Welding filler metal containing manganese combines with the sulfur impurities in the grain boundary to form manganese sulfide. Manganese sulfide has a much higher melting point than iron sulfide and is more resistant to cracking.
The relationship between hot shortness and corrosion lies in the weaknesses formed when hot cracks develop. The cracks create an environment that is conducive to specific types of localized corrosion.
Paying particular attention to the composition of filler and parent metal and to the welding temperatures can go a long way to preventing hot cracks and eliminating the types of corrosion they can cause.