Pipeline hard spots are localized areas on a pipe’s surface with a significantly higher hardness than the surrounding material. These spots result from a different microstructure that forms during a localized thermal cycle, which typically occurs during plate or pipe manufacturing. The specific thermal cycle determines the present microstructure; see Figure 1. The microstructure expected in hard spots with high levels of hardness is martensite, which has distinct mechanical and magnetic properties compared to the base steel.1,2
How do hard spots arise?
Hard spots usually occur due to accidental, rapid cooling during hot rolling. They can also form during welding operations in the pipe mill or during pipeline construction. Because these features originate in the manufacturing process, they often pass the initial hydrostatic tests and remain in the pipeline throughout their operational life.1,2
What are the types of pipeline hard spots?
Martensitic hard spots: Areas with very high hardness caused by rapid cooling.
Non-martensitic hard spots: Regions with a different microstructure than the surrounding steel, but with lower hardness levels.1
What are the risks of pipeline hard spots?
Although hard spots are generally stable, they can pose a latent threat to pipeline integrity. Under certain conditions, they can become initiation points for cracking, especially hydrogen-induced cracking. This risk increases when hard spots are combined with other stress factors, such as bending, strain, or corrosion. Cracking in hard spots is less likely if they are covered by an effective coating. However, due to several pipeline incidents in recent years, regulatory authorities have highlighted hard spots as a concern, making their detection and monitoring essential for pipeline safety.
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Source reference
- The Competence Club, ROSEN, 2025.
- The Encyclopedia of Pipeline Defects, ROSEN UK, Edition 3, 2017.