# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 4 Discussion
Furthermore, niobium, as well as carbon, segregates to the grain boundary resulting in a locally much higher [Nb]-[C] solubility product. Precipitation of Nb should occur more efficiently during or after the phase transformation from austenite to the bcc phase. However, due to slow kinetics, it is expected to be incomplete. The precipitation strengthening effect has been estimated as shown in Figure 10(b) by performing a differential analysis of the tensile data of the CGZ treated steels. The magnitude of precipitation strengthening ranges between 60 and 150 MPa. The highest strengthening is found in alloy V2-d containing lower Mn and no Mo. The otherwise identical Ti-free alloy (V1-d) shows clearly lower strengthening. Likewise, this is observed when comparing alloy V4-d and V5-d, where also the Ti-free alloy exhibits clearly lower strengthening. Possibly, the larger austenite grain size in the Ti-free alloys causes a more unequal distribution of Nb (due to boundary segregation) and increased diffusion distances. When applying post-veld heat treatment to the CGZ treated steels, two main metallurgical effects can be expected, being a tempering of the matrix and further precipitation of solute Nb. The grain size will not be altered. Based on the tensile data, the softening in the matrix due to recovery processes is for all alloy systems between 40 and 50 MPa. The forthcoming additional precipitation strengthening is displayed in Figure 10(b). It is largest in the Ti-free steels and generally larger in the alloys with the higher Mn addition and those with Mo alloying. This means that more Nb remained in solution after the CGZ treatment due to the slower precipitation kinetics in these particular alloys. Summing up the strengthening contributions after the CGZ and PWHT cycles results in rather similar values for all alloys with the exception of Si-reduced steel. Figure 11: Precipitation strengthening effect in HAZ sub-zones for alloy variants containing 0.10%Nb: (a) Sub-critical zone at peak temperatures of 600 and 800 \({}^{\circ}\)C; (b) coarse-grained zone (peak temperature 1300 \({}^{\circ}\)C), precipitation effect of PWHT (560 \({}^{\circ}\)C/30 min.) and sum of precipitation strengthening (CGZ + PWHT). The lower reduction of area observed in steels with increasing Nb content during the hot tensile deformation can be due to in-situ precipitation. Freshly formed precipitates may obstruct dislocations generated during the hot deformation and reduce the ductility of the matrix. It is feasible that this effect is stronger in the grain interior while the area near to the grain boundaries might be depleted from niobium as it has already precipitated after the CGZ cycle. This can lead to narrow areas in the vicinity of former austenite grain boundaries where plastic deformation will concentrate and cause rupture and a limited reduction of area. The observation that Mo alloying improves the hot ductility can be due to a retardation of in-situ precipitation and also to increased hot strength as Mo segregates to the grain boundary area. This should counteract the local concentration of plastic deformation. At the higher hot deformation temperature of 620 \({}^{\circ}\)C (Figure 9b), the decrease of reduction of area with increasing Nb content is more pronounced since the precipitation kinetics is quicker. Yet also at this temperature, the precipitation retarding effect of Mo is evident. The understanding that in-situ precipitation of NbC during hot deformation is the main cause for reduced ductility allows designing practical solutions to alleviate problems when post-weld heat treating steel with higher Nb addition. Firstly, the post-weld heat treatment can be done at the lower side of the typical temperature range of 500 to 600 \({}^{\circ}\)C, preferably below 550 \({}^{\circ}\)C, slowing down the kinetics of in-situ precipitation. Secondly, molybdenum should be co-alloyed to niobium for additionally retarding the precipitation kinetics and stabilizing the niobium-depleted areas in the vicinity of grain boundaries.