# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 3 Results
### Toughness Characteristics of HAZ Sub-Zones  
Principally, the transition from ductile to brittle fracture mode occurs in bcc-type steel when the yield strength approaches the cleavage stress level [10]. Lower temperature and higher strain rate increase the yield strength so that under impact loading conditions at a certain low temperature the yield strength exceeds the cleavage stress causing brittle failure as a consequence. Refining the  
Figure 6: Evolution of hardness and yield-to-tensile ratio (YTR) in the heat-affected zone of (**a**) alloys V4; (**b**) alloys V6. Measured transformation temperatures during up-heating are indicated. microstructure also increases the yield strength (Hall-Petch relationship), yet the refinement has an even larger effect in terms of increasing the cleavage stress. Hence, microstructural refinement effectively lowers the DBTT. The yield strength is also increased by solid solution, precipitation and dislocation strengthening. These mechanisms have an adverse effect on the DBTT. Accordingly, for evaluating the alloy influence on the heat affected zone toughness it is most interesting to analyze the transition temperature (DBTT T50%) as a function of the actual yield strength in the inter-critical and coarse-grained sub-zones. Figure 7a displays the measured DBTT values as a function of the yield strength for the ICZ. The most obvious observation is that the DBTT decreases with increasing Nb content, despite strongly increasing yield strength. This can be due to pronounced grain refinement by Nb in the ICZ (Figure 5b,c). Molybdenum alloyed variants tend to have a higher DBTT at a given yield strength level than Mo-free alloys. This discrepancy, however, diminishes with increasing Nb content. The Mo-alloyed variants at lower Nb contents contain an increased amount of carbon-rich phase, which probably is tempered martensite. The highest DBTT is observed for the steel without micro-alloy (no Ti, no Nb). The low-Si Mo-alloyed variant, however, shows a low DBTT in the micro-alloy-free state. In the CGZ the DBTT values are generally much higher than in the ICZ for all alloy variants. This can be due to the significantly coarser grain size and the absence of ductile polygonal ferrite in the microstructure. The absence of Ti in the alloy leads to clearly higher DBTT in comparison to the Ti-alloyed variant. A significant influence of molybdenum for otherwise identical alloys cannot be detected. In several of the alloy systems, the addition of Nb results in a slight decrease of the DBTT, followed by a marked increase for the highest Nb content of 0.1%. The difference in DBTT is most obvious between alloys V4 (MoNbTi) and V5 (MoNb). Comparing the microstructures of the CGZ indicates that the Ti-free alloy V5 has generally large prior austenite grain size (Figure 8), which decreases somewhat towards the highest Nb addition. The grain size distribution is also quite inhomogeneous. This is congruent with the high DBTT found for these alloys. On the contrary, alloy V4 has much smaller and rather homogeneous grain size. Yet, for alloy V4 containing 0.1%Nb individual large grains are observed, which are probably the result of abnormal grain growth. The occurrence of individual large grains appears to have a significant negative influence on the DBTT. Figure 7: Ductile-to-brittle transition temperature (DBTT T50%) determined by Charpy testing: (a) Simulated intercirtal HAZ (ICZ) with a peak temperature of 900 \({}^{\circ}\)C; (b) simulated coarse-grained HAZ (CGZ) with a peak temperature of 1300 \({}^{\circ}\)C.  
The comparison of alloy V2-c with the corresponding industrial reference X65 steel reveals that the DBTT T50% is approximately 30 K lower in the HAZ sub-zones. In the ICZ the industrial steel has a DBTT of \(-\)130 \({}^{\circ}\)C as compared to \(-\)103 \({}^{\circ}\)C in alloy V2-c.