What Does Hot Shortness Mean?
Hot shortness is a type of welding defecting characterized by the cracking of a material along its grain boundaries as the welded area cools and solidifies.
When a molten metal is heated and solidifies, crystals are formed independently of each other. These crystals are distributed throughout the metal and are randomly oriented. These crystals are known as the grains of the solid, and the interfaces between grains are known as grain boundaries. The properties of these grain boundaries heavily influence the overall behavior of solidified metal.
As molten metal re-solidifies, impurities tend to segregate at the grain boundaries. As such, these locations tend to have a lower melting point than the metal itself.
During melting and subsequent re-solidification, the metal, which has a higher solidification temperature, hardens first, while the grain boundaries remain in a semisolid state. The hardening of the metal around the grain boundaries induces a tensile stress on the weakened grain boundaries, causing them to pop open, resulting in cracks. These cracked grain boundaries are preferred sites for several types of corrosion-related defects.
Hot shortness is also known as hot tearing, hot cracking or solidification cracking.
Corrosionpedia Explains Hot Shortness
The types of localized corrosion caused by hot shortness include:
- Stress corrosion cracking - This type of corrosion is characterized by localized corrosion and cracks in the grain boundaries of a material while the rest of the material’s surface remains unaffected. The segregated impurities at these boundaries make passivation of these areas less effective than the bulk material surface. As a result, the grain boundaries are less corrosion resistant and thus are preferred locations for the onset of corrosion.
- Corrosion fatigue – Corrosion fatigue is caused by the combined action of cyclic (alternating) stress in a corrosive environment. As the metal surface experiences alternating tensile and compressive stress, the weakened passive film ruptures, leaving the grain boundaries vulnerable to corrosion. Cracking due to hot shortness serves to contribute to the overall fatigue crack growth rate.
- Crevice corrosion – The cracks caused by hot shortness are ideal sites for collecting water. These gaps are typically big enough for water to enter, but too small for it to flow out. The stagnating water is one of the critical elements for crevice corrosion. Since there is no movement of water in the crack, oxygen diffusion is limited to the crevice. Subsequent chemical reactions take place, causing the fluid in the crack to become acidic. This increased acidity eventually attacks and breaks down the passive film layer. Corrosion is then free to propagate in the cracked areas.
How to Prevent Hot Shortness
The most effective way to avoid hot shortness is to minimize the mechanisms that cause these cracks to appear. Some measures for preventing hot shortness include:
- Select the right materials – Steels with low residual phosphorus or sulfur content can minimize the impurities that segregate at the grain boundaries.
- Control the heat during welding – Lower heat during welding can reduce the amount of time impurities have to gather at the grain boundaries.
- Minimize weld pool contamination – Low melting point contaminants such as sulfur and copper in the weld pool can make the welded area prone to cracking.
- Use appropriate filler material – Filler material with manganese can help reduce the occurrence of hot shortness. The manganese combines with sulfur impurities to form manganese sulfide. This compound has a significantly higher melting point than iron sulfide, and therefore is more resistant to cracking during solidification.