# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 2 Materials and Methods  
### Definition of the Test Materials  
Low-carbon (0.035%) steels with a systematic variation of alloying elements were designed as specified in Table 1. Such alloys are being used for pipe steels with high requirements regarding toughness and sour gas resistance. Several of these alloy compositions are in actual industrial use for pipe related strip and plate products. Thereby, lower-Mn and Mo-free variants cover X60-X65 grades whereas the higher alloyed variants are being used for X70 and X80 grades (X refers to minimum yield strength, number in ksi). All alloy variants (V1-V6) were produced as 300 kg laboratory melts in an induction furnace. Each sub-variant (a-d) was cast into a 75 kg ingot and the niobium level was increased in the remaining melt to the next level. The ingots were forged into square bars (\(75\times 75\) mm) at 1200 \({}^{\circ}\)C. These bars were subsequently cut into segments, which were subjected to the following rolling schedule:  
* Reheating temperature: 1150 \({}^{\circ}\)C +/\(-10\)\({}^{\circ}\)C.
* Rouging rolling in four passes to 35 mm. * Thermo-mechanical rolling to 13 mm in five passes. * Final rolling temperature: 850 \({}^{\circ}\)C +/\(-20\)\({}^{\circ}\)C
* Water cooling or in selected case air cooling. The produced steel plates were characterized by tensile testing and the resulting values are displayed in Figure 2a. The alloy variants cover a yield strength (R\({}_{\text{p0.2}}\)) range from 350 to 600 MPa. Thereby the increase of the Nb level in each variant clearly relates to a strength increase, amounting to 50-160 MPa when adding 0.1%Nb. The tensile strength correlates well with the yield strength indicating an average yield-to-tensile ratio (YTR) of 0.78. The toughness of each alloy variant was characterized by standard Charpy V-notch testing using a \(10\times 10\) mm sample size (acc. to DIN 10045). The upper-shelf toughness of these steel alloys scatters around 200 J. This level is relatively low for such low-carbon steel. An industrially produced reference material (X65) having nearly the composition of variant V2-c showed an upper-shelf toughness of 370 J in comparative tests. The ductile-to-brittle transition temperature, i.e., the temperature at which 50% of the upper-shelf energy is reached (DBTT T50%, Figure 1b) is also lower in the industrial material (\(-125\)\({}^{\circ}\)C) as compared to the laboratory alloy V2-c (\(-85\)\({}^{\circ}\)C). The differences in the toughness behavior are due to a lower cleanness of the laboratory \begin{table}
\begin{tabular}{c|c|c|c|c|c|c|c|c} \hline \hline
**Alloy** & **Symbol** & **C** & **Si** & **Mn** & **Cr** & **Ti** & **Mo** &
\begin{tabular}{c} **Nb** \\ **(varied in all alloys)** \\ \end{tabular} \\ \hline
**V1** & \(\bigcirc\) & & & & **0.00** & & \\ \hline
**v2** & \(\bigcirc\) & & & & & **0.00** & \\ \hline
**v3** & \(\bigtriangleup\) & **0.30** & & & & **0.015** & & **a: 0.00** \\ \hline
**v4** & \(\bigtriangleup\) & & & & **0.20** & & **b: 0.02** & \\ \hline
**v5** & \(\bigtriangleup\) & & & & **0.00** & **0.15** & & \\ \hline
**v6** & \(\blacksquare\) & & **0.10** & & & **0.015** & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variation of alloy compositions in mass % subjected to heat-affected zone (HAZ) simulations (symbols refer to all subsequent diagrams). melts, which were produced under non-vacuum conditions. However, the comparison between the various laboratory melts allows identifying the toughness influence of the alloying elements relative to each other. The DBTT results of all laboratory steels are shown in Figure 2b. It is apparent that the DBTT in each alloy system decreases with increasing Nb addition, irrespective of the strength level of the base alloy. This beneficial behavior is related to the pronounced microstructural refinement induced by Nb micro-alloying. The two data points marked as \"PF\" in Figure 2b have a mostly polygonal ferritic (PF) microstructure in contrast to the acicular ferritic microstructure of all other alloys.