# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 5 Conclusions  
Increasing addition of niobium raises yield strength and improves toughness simultaneously in the as-rolled state of the considered extra-low carbon steels. The combined addition of molybdenum and niobium provides the highest strength fulfilling the requirements for grade X80.  
Regarding the inter-critical heat-affected zone, niobium-free steels have the lowest yield strength and toughness. These steels exhibit a low yield-to-tensile ratio, which is caused by a fraction of martensite islands dispersed in recrystallized ferrite. Adding niobium in increasing amounts raises the yield strength as well as the yield-to-tensile ratio. The recrystallized ferrite grain size is being severely refined and martensite formation is being suppressed. Consequently, ICZ toughness is improving with increasing niobium content.  
Molybdenum addition increases the amount of hard phase in the inter-critical heat-affected zone leading to a decrease of ICZ toughness. The hard phase appears predominantly as tempered martensite. At higher niobium additions this toughness-reducing effect of molybdenum is being diminished. Lowering the silicon content further reduces the formation of hard phases.  
Regarding the coarse-grained heat affected zone, titanium-free steels show low CGZ toughness due to excessive grain coarsening. A small addition of titanium, typically in near-stoichiometric ratio to residual nitrogen present in the steel, counteracts pronounced grain coarsening. Increasing niobium content can further improve toughness in combination with titanium. At the highest niobium addition, however, individual grains showed abnormal coarsening in several alloys. The mechanism for this abnormal grain growth must be investigated in more detail. Increasing niobium content leads also to more pronounced precipitation strengthening reducing CGZ toughness. Thus, the actual toughness performance of the coarse-grained heat affected zone is controlled by the counteracting effects of grain refinement and precipitation.  
Toughness in the coarse-grained heat affected zone after post-weld heat treatment is controlled by the degree of in-situ precipitation of niobium that was prior dissolved during the CGZ heat cycle. The potential of in-situ precipitation increases naturally with the niobium content in the steel. However, by anticipating or delaying in-situ precipitation, PWHT embrittlement can be reduced to an acceptable level. This requires careful consideration of the integral alloy concept, the selected PWHT temperature and an appropriate multi-pass welding schedule. Molybdenum, particularly, was found to improve hot ductility, which helps to avoid stress relaxation cracking.  
[PERSON] evaluated and analyzed the data and wrote the paper. All interpretations of the observed effects reflect [PERSON]'s personal opinion.  
This work was funded in part by the International Molybdenum Association (IMOA), London, UK and by Companhia Brasileira de Metalurgia e Mineraao (CBMM), Sao Paulo, Brazil.  
The author sincerely acknowledges the experimental work and data preparation by [PERSON], [PERSON] and [PERSON] of Voestalpine Grobblech GmbH.  
The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.