S355NL steel is widely used in the design of offshore wind monopiles due to its excellent properties. This hot-rolled, normalized weldable fine-grain structural steel offers a minimum yield strength of 355 MPa. It is highly weldable for thicknesses ≤ 30 mm without requiring preheating. It is suitable for various welding techniques. It performs well in low-temperature environments with good toughness down to −50 °C (as per BS EN 10025-3).

However, BS EN 15614-1 sets strict requirements for hardness values across welded joints in S355NL. For non-heat-treated conditions, the Vickers hardness (HV10) must not exceed 380, and for heat-treated conditions, it must remain ≤ 320. Why is this critical? High hardness value can appear even at a single point on the surface. They can also be in the subsurface. These values can compromise the integrity of the whole weld. This is especially true in fatigue-sensitive applications. Because, it is often an indication of formation of unacceptable phase in the location.

Picture A shows a test piece (20 mm thick) that underwent a butt-weld procedure qualification in accordance with BS EN ISO 15609-1 and BS EN 15614-1, without post-weld heat treatment. Hardness testing, conducted per DNVGL-OS-C401 at a ISO/IEC 17025/UKAS-accredited laboratory, revealed that some hardness values exceeded the permissible limit, indicating non-compliance with BS EN 15614-1. This raises a critical question: What caused the hardness deviation?

To identify the root cause and prevent recurrence in the present condition, we need to approach this issue from both welding and metallurgical perspectives. Firstly, locations of the microhardness values need to be mapped. Picture B illustrates such hardness mapping, highlighting five locations (in red) where the hardness values exceeded the compliance limits.

Secondly, to determine the root cause with certainty, the following data must be collected and analysed:

1. Preheat temperature
2. Interpass temperature
3. Travel speed
4. Energy input (calculated using current, voltage values and travel speed)

Additionally, the numbering of sequence of bead deposition during welding (24 beads in total to fill the joint) would be beneficial.  The further analysis of these factors, along with insights into the hardness deviation, will be shared in Part 2. Stay tuned!