**4. Conclusions**

Grade 91 steel is usually applied in situations where the temperature and pressure are very demanding. The weldability of these steels presents well-known challenges, which are usually overcome using complex thermal cycles before and after the welding process. This work intended to test other thermal cycles with a view to shorten the treatment time, making the process more productive and more environmentally friendly. Thus, five different thermal cycles were drawn and tested: (a) welding without any pretreatment, post-treatment, or PWHT; (b) using the thermal cycle recommended by the construction codes and steel manufacturers; (c) using a pretreatment and the PWHT, using as well a six-month waiting period between welding and PWHT without phases' transformation time, in order to study the possible development of cold cracking; (d) using the previous strategy but including the post-welding treatment and; finally, (e) using the same strategy as that in (b) but excluding the phases' transformation time.

The hardness and microstructure assessed in P\_APP\_T00 indicate that this strategy cannot be used, because if the mechanical strength is even higher than that in the P\_APP\_T01 case, the risks of further cracking are higher, making this option not safe. The non-tempered martensite in the MZ and HAZ induces a clear hardness increase, which can be capable of inducing further cracks in service. Thus, this option was discarded due to these facts.

The last strategy (P\_APP\_T08) for thermal cycles around the welding process produced closer results in terms of mechanical properties at both room and elevated temperatures, comparing with the reference: P\_APP\_T01. A slight reduction in yield strength (2% at room temperature and 3.9% at elevated temperature) and ultimate strength is reported (2% at room temperature and 9% at elevated temperature), as well as a reduction of about 39% in the elongation at room temperature. However, at elevated temperature, the elongation is even a little bit higher than in P\_APP\_T01 conditions. Since this steel is usually used in high-temperature applications, this strategy seems to be the best among the other strategies tested in this work.

Thus, the main contribution brought by this work can be stated as the development of a new heat-treatment strategy to be applied in welding of grade 91 steels that is less time consuming and more

environmentally friendly, providing very close results compared to the solution currently recommended by construction codes and steel manufacturers.

**Author Contributions:** Conceptualization, A.P.P. and F.J.G.S.; methodology, A.P.P. and F.J.G.S.; validation, F.J.G.S., O.C.P. and A.B.P.; formal analysis, A.P.P.; investigation, A.P.P.; resources, A.P.P.; data curation, A.P.P. and F.J.G.S.; writing—original draft preparation, F.J.G.S.; writing—review and editing, A.P.P., O.C.P. and A.B.P.; supervision, F.J.G.S., O.C.P. and A.B.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thanks to ARSOPI company, which provided the best conditions to carry out this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
