In terms of inspection, what is a critical factor to investigate in regards to a hot spot on carbon steel vessels?

When investigating a hot spot on carbon steel vessels, the temperature history of the spot is crucial. Understanding the temperature variations that the area has undergone provides insights into the thermal stresses and potential degradation mechanisms that could affect the integrity of the vessel. Repeated or prolonged exposure to elevated temperatures can lead to material changes such as hardening, softening, or even localized thinning of the steel, which are significant for assessing the vessel's overall condition and safety. Additionally, temperature history can help identify potential correlations with operational conditions or maintenance issues that need to be addressed.

While the presence of moisture, external appearance, and the location of welds can be relevant factors affecting vessel integrity, they do not provide the same direct assessment of the thermal effects on the material. Moisture presence can be an indicator of corrosion but does not specifically relate to the thermal phenomena at the hot spot. The external appearance may offer some clues about surface conditions but not about the internal temperature impacts. Similarly, weld locations may influence stress distributions but don't directly inform the thermal history of the specific hot spot being analyzed. Therefore, focusing on the temperature history is essential for understanding the implications of the hot spot and making informed decisions regarding the vessel's continued use or the need for repairs.