# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 4 Discussion
Differential analysis of the tensile data allows us to estimate the contribution by Nb precipitation strengthening. The result of this analysis is shown for the alloys with the highest Nb content for the two sub-critical temperatures in Figure 11a. The precipitation strengthening is generally larger at 800 \({}^{\circ}\)C than at 600 \({}^{\circ}\)C. Furthermore, the magnitude of the strengthening is with 80 to 100 MPa rather equal in all alloys. At 600 \({}^{\circ}\)C, the precipitation kinetics is slower and differences between the alloy systems become apparent. In alloys with an increased Mn content, the strength increase remains smaller and this effect is becoming even more pronounced when Mo is added. This can be due to the lower activity of carbon caused by Mn and Mo retarding the precipitation kinetics of Nb. On the other hand, when Si is reduced as in alloy V6, precipitation appears to be accelerated. It is assumed that the lower Si content allows for the better formation of nano-cemente particles [16], which serve as a precursor for the nucleation of NbC [17]. In the ICZ at a 900 \({}^{\circ}\)C peak temperature, niobium precipitation will occur even faster. Yet it is difficult to estimate the precipitation strengthening effect since simultaneously the entire matrix microstructure changes, also affecting the strength. The very pronounced grain refinement in the ICZ microstructure of Nb-alloyed variants certainly contributes to strength. This is caused for one part by the increased A\({}_{\mathrm{clp}}\) temperature of the Nb alloyed steel vs. the Nb-free steel, which is a solute drag effect of Nb. The delayed partial transformation from ferrite to austenite enhances the nucleation rate for austenite grains. Secondly, the precipitation of Nb occurring simultaneous obstructs the growth of recrystallized ferrite and newly formed austenite grains. Re-distribution of carbon is more efficient due to shorter diffusion distances between grains in the refined ICZ microstructure. Therefore, the carbon content in newly formed austenite grains is more balanced and on average lower. This makes the formation of MA phase less likely, which is confirmed by the disappearance of the dual phase character (low YTR) that is clearly observed in the Nb-free steel. Molybdenum increases the hardenability of carbon enriched austenite so that a higher share of the hard phase is present in Mo-alloyed steels. This reflects in lower ICZ toughness values for the Mo-alloyed steels. However, increasing Nb content offsets the Mo effect via the carbon balancing explained before. The respective carbon-reduced austenite grains have a higher martensite-start temperature and will experience severe self-tempering during the down-cooling cycle. Accordingly, toughness improves rapidly in Mo-alloyed steels with increasing Nb content. The reduced Si content in alloy V6-d results in a particularly low A\({}_{\mathrm{c3p}}\) temperature (875 \({}^{\circ}\)C) for a reason that is not yet known. As a consequence, the peak temperature of 900 \({}^{\circ}\)C in the ICZ heating cycle causes an almost complete austenite transformation, preventing carbon enrichment in individual grains. Therefore, the formation of hard phases despite Mo alloying is unlikely. Indeed, the toughness of steel V6-d is on the same good level as found for the best Mo-free alloys. When heating up to the CGZ peak temperature of 1300 \({}^{\circ}\)C, all NbC particles present in the as-rolled plate will dissolve. The time required for dissolving such particles having typically a size below 100 nm is estimated to be less than one second. The TiN particles, however, remain stable. Upon down-cooling niobium's solubility decreases and NbC should re-precipitate. Precipitation of NbC does not easily occur inside the austenite grain in the absence of externally applied plastic deformation. The lattice mismatch between the austenite matrix and the NbC phase, both having an fcc structure, is around 25% [18]. Better nucleation conditions for precipitation are given in the austenite grain boundaries.