# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 4 Discussion  
The present study has demonstrated that the influence of various alloying elements on the HAZ properties requires a careful analysis of individual metallurgical effects such as grain size, hardenability and precipitation status. In the investigated alloy variants, Nb and Ti are the elements with potential for precipitation, Mo and Mn reduce the carbon activity and increase the hardenability, and Si increases the carbon activity in austenite [11; 12]. Both, Mo and Nb are large atoms having the tendency of segregating to grain boundaries and exerting solute drag [13; 14]. Titanium added up to the stoichiometric ratio with respect to nitrogen contained in the steel forms very temperature-stable TiN particles, which exert grain boundary pinning in hot austenite. Niobium carbide precipitates provide also grain boundary pinning. However, NbC precipitates form at a lower temperature than TiN and typically niobium's precipitation is incomplete. Thus, in the interpretation of the present results, one always has to reflect on the status of niobium, which in thermodynamic equilibrium is described by established solubility equations. The solubility of niobium carbide, where [Nb] and [C] are the alloy concentrations in mass percent, is described as a function of the absolute temperature, T, as [15]:  
\[\log\ [\text{Nb}][\text{C}]=3.14-7753/\text{T}. \tag{1}\]  
Accordingly, niobium additions of 0.02, 0.05 and 0.10% to the present 0.035% carbon steel dissolve at temperatures of 960, 1040 and 1120 \({}^{\circ}\)C, respectively. The solubility of titanium nitride carbide is similarly described as [15]:  
\[\log\ [\text{Ti}][\text{N}]=5.40-15790/\text{T}. \tag{2}\]  
In the present alloys, the nitrogen content [N] is 0.0070% so that according to Equation (2) TiN is stable beyond the upper-temperature limit of the austenite phase field for these alloys being around 1450 \({}^{\circ}\)C. Consequently, at a peak temperature of 900 \({}^{\circ}\)C simulating the inter-critical sub-zone both, NbC and  
Figure 10: Influence of post-weld heat treatment (PWHT) on strength and toughness: (**a**) DBTT (T50%) vs. yield strength after a relaxation treatment at 560 \({}^{\circ}\)C for 30 min; (**b**) DBTT (T50%) vs. yield strength before relaxation treatment (same data as Figure 7b, same trendline as Figure 9a). TiN particles are both stable. At a peak temperature of 1300 \({}^{\circ}\)C simulating the coarse-grained sub-zone, however, NbC particles should be dissolved whereas TiN particles are still stable. Therefore, TiN particles are available to control the austenite grain size in the CGZ simulation at 1300 \({}^{\circ}\)C. This is indeed obvious when comparing the CGZ microstructures of Ti-free alloys (coarse-grained) with those of Ti-added alloys (fine-grained). Niobium carbide particles will fully re-dissolve during the up-heat cycle to 1300 \({}^{\circ}\)C. Solute Nb acts in the hot stage only by solute drag on austenite grain boundaries, which can contribute to finer grain size especially in combination with TiN particle pinning. This combined effect reflects an improved toughness. It is, however, also seen that for high Nb addition (0.10%) individual grains can grow abnormally large, leading to a significant decrease in toughness. In the as-rolled plates, a significant amount of niobium is expected to be in solution, out of equilibrium. This is due to the strong cooling after rolling and the low cool-stop temperature, preventing substantial precipitation of niobium as its precipitation kinetics is rather slow. Hence, any heat treatment of such an Nb super-saturated steel above temperatures of 500 \({}^{\circ}\)C and below the equilibrium dissolution temperature given by Equation (1) will effectively lead to Nb precipitation. The higher the temperature is within that range, the more complete will be the precipitation. Accordingly, Nb precipitation can be expected for the HAZ simulations at sub-critical and inter-critical peak temperatures.