# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 3 Results
### Mechanical Characteristics of HAZ Sub-Zones  
Figure 5 indicates the change in mechanical characteristics for alloy V3 after exposure to peak temperatures in the range of 300 to 1300 \({}^{\circ}\)C. The hardness measurements indicate significant softening in the ICZ, i.e., after heating to 900 \({}^{\circ}\)C peak temperature. The softening is most pronounced for the Nb-free steel losing 72 HV10 compared to the base material. For the alloy with the highest Nb addition, the hardness drop is only 36 HV10. In the CGZ, the behavior is similar yet less strong. Here the Nb-free alloy is softening by 52 HV10 while the highest Nb-alloyed variant shows no hardness loss. Already the smallest Nb addition of 0.02% reveals a significant softening resistance in both, ICZ and CGZ. The measured HV10 hardness relates quite well to the measured tensile strength by a factor of 3.2. Instead of reporting yield and tensile strength data as a function of the peak temperature, it is more interesting looking at the evolution of the yield-to-tensile ratio (YTR). The latter is more conclusive with regard to microstructural changes. In Figure 5 the YTR evolution is plotted as an interpolation of the data obtained at 300, 600, 800, 900, 1100 and 1300 \({}^{\circ}\)C for the Nb-free and 0.1%Nb containing alloy V3. Generally, the YTR of the 0.1%Nb steel is consistently higher than that of the Nb-free steel. The YTR increases form its base material when the peak temperature is raised to sub-critical values (<A\({}_{\text{clp}}\)). A maximum of the YTR appears between 600 and 800 \({}^{\circ}\)C, which is more pronounced and appears at a somewhat higher temperature in the 0.1%Nb alloy as compared to the Nb-free alloy. In the latter, the YTR shows a sharp drop to a pronounced minimum value of 0.54 in the ICZ. The YTR of the 0.1%Nb alloy, on the contrary, drops only to 0.75 in the ICZ. In the CGZ, the YTR increases from its minimum value in the ICZ to a level between 0.7 and 0.8. The increase of the YTR in the sub-critical temperature range of 600-800 \({}^{\circ}\)C is typically related to tempering and precipitation phenomena. Tempering of hard carbon-enriched phases in the original material reduces primarily the tensile strength while precipitation of solute micro-alloys primarily increases the yield strength. Particularly the precipitation behavior of Nb will be discussed in paragraph 4 in more detail. The very low YTR of the Nb-free steel in the ICZ is a clear indicator for a microstructure with dual-phase character. The dispersion of the martensitic phase in a soft matrix provides a low yield and high tensile strength. Such dual-phase steels are deliberately produced by annealing in the inter-critical temperature range followed by fast cooling. Metallographic analysis using Le Pera etchant reveals the presence of martensite in the ICZ material. In the optical microscope, these phases appear with a white or brownish color. Figure 5b shows the presence of a small fraction of martensite islands dispersed in a fully recrystallized polygonal ferrite matrix of the Nb-free steel. The original acicular ferritic microstructure of the base material has completely disappeared. In the 0.1%Nb-alloyed steel (Figure 5c) the ICZ microstructure also appears fully recrystallized. However, the polygonal ferrite grains are much finer than those in the Nb-free alloy. The amount of martensite islands in the microstructure is almost negligible. Grain refinement has a stronger effect (Hall-Petch relationship) on the yield strength than on the tensile strength. Thus, with decreasing grain size the YTR increases. The strength of the refined polygonal ferrite is however still lower compared to the acicular ferritic base material as the sub-structure within the grain is lost due to recrystallization. Adding molybdenum in alloy V4 shows qualitatively the same mechanical behavior as in alloy V3 (Figure 6a) and similar effects of niobium. However, the hardness scattering range between Nb-free and 0.1%Nb containing alloys in the ICZ and CGZ is less wide.