# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 3 Results
### Post-Weld Heat Treatment  
During post-weld heat treatment, the strength of the material is lowered, thus, residual stresses introduced to the weld zone can relax by plastic deformation. The reduction of area during a hot-tensile test indicates how much deformation the steel can withstand without cracking. Hot-tensile testing was performed at 560 and 620 \({}^{\circ}\)C with steel that was prior subjected to the CGZ simulation heat cycle. Figure 9 shows the measured reduction of area as a function of the measured hot tensile strength.  
Taking the original tensile strength in the CGZ as the reference, heating to 560 \({}^{\circ}\)C reduces the tensile strength by 150-200 MPa. When heating to 620 \({}^{\circ}\)C the strength reduction is in the range of 230-330 MPa. At both temperatures, the steels alloyed with molybdenum reveal a higher strength than those without it. Increasing the niobium content systematically raises the strength. The reduction of area decreases with increasing tensile strength for all alloys at both temperatures. This decreasing effect is stronger at higher temperatures. At the same hot-tensile strength level, molybdenum alloyed steels allow a significantly higher reduction of area. The comparison of the reduction of area at 560 \({}^{\circ}\)C between alloy V2-c (RA = 32%) and the industrial X65 steel (RA = 50%) reveals again the better performance of the latter.  
Finally, the toughness behavior after the post-weld heat treatment at 560 \({}^{\circ}\)C for 30 min is analyzed in Figure 10a. One finds a strong correlation of the DBTT T50% with the measured yield strength. The majority of the data can be fitted very well by a linear relationship indicated by the dashed line. Only the data of alloy V1 are deviating from this line. The yield strength of the higher Nb-alloyed variants (0.05%Nb and 0.10%Nb) significantly increases after the heat treatment by values in the range of 50 to 175 MPa. The Nb-free alloys show little change in yield strength. For the alloys containing 0.02%Nb, the strength increase is in the range of 20 to 30 MPa. In the higher Nb-alloyed variants the DBTT increases severely by 50 to 100 K. In the Nb-free variants, the DBTT drops by 10 to 30 K. In alloys containing 0.02%Nb the DBTT increases in the range of 10 to 30 K. For comparing the data sets before and after heat treatment, Figure 7b is re-plotted in Figure 9b on the same scale as Figure 9a. It is apparent that Nb precipitation moves the data points to higher strength and DBTT. For the Nb-free alloys, tempering appears to lower the DBTT while the yield strength remains nearly unaffected. Alloying of molybdenum does not seem to have a particular influence on the toughness behavior after  
Figure 9: Reduction of area vs. hot-tensile strength of coarse-grained zone (CGZ) treated material at a test temperature of (**a**) 560 \({}^{\circ}\)C and (**b**) 620 \({}^{\circ}\)C.  
this heat treatment. Performing the same post-weld heat treatment on the CGZ of the industrial X65 steel results in a DBTT of -55 \({}^{\circ}\)C whereas in the comparable laboratory steel V2-c it is -21\({}^{\circ}\)C.