# Effects of High Toughness and Welding Residual Stress for Unstable Fracture Prevention
## 3 Brittle Crack Arrest Test Results and Discussions  
### Fracture Toughness Effect in Brittle Crack Propagation  
The fracture toughness and welding residual stress were considered as factors affecting the brittle crack arrest. It was confirmed that a large toughness welding consumable with excellent fracture toughness could hinder brittle crack propagation. Therefore, high-toughness commercially available welding consumables were selected and brittle crack arrest tests were performed using different welding consumables, with varying toughness values in each weld layer. Table 7 summarizes the experimental results. In the case of test specimens 1, 2, and 3, the brittle crack propagated straight along the weld line and then completely fractured. However, in the case of specimen-4, it was arrested in the weld metal. Figure 6 shows the experimental results of specimens 1, 2, and 3. The weld consumables, which were applied to specimen-1, were given an impact toughness of 27 J at \(-60\)\({}^{\circ}\)C. In addition, the C weld consumables contained 61% (or more) Ni, making them suitable for cryogenic temperatures. However, a limit existed to the brittle crack arrest. Although specimens 2 and 3 were almost similar to specimen-1--and had a much lower toughness guarantee temperature (\(-196\)\({}^{\circ}\)C)--they failed to arrest the brittle crack in the weld metal. Figure 7 presents the test results of specimen-4; the brittle crack was arrested in the weld metal for this specimen, despite having the same test conditions as specimens 1, 2, and 3. Meanwhile, two types of large toughness welding consumable were applied in specimen-4, whose yield and tensile strengths were almost equal, i.e., higher than or equal to the strength of the base material, and the impact energy guarantee temperatures were \(-60\)\({}^{\circ}\)C and \(-196\)\({}^{\circ}\)C for 27 J and 94 J, respectively. The brittle crack was arrested in the weld metal after approximately 50 mm of straight propagation. In the macrostructure of Figure 7b, two types of welding consumables were used to weld each layer to obtain a different toughness. It was confirmed that the brittle crack was finally arrested due to the mismatch in toughness between the welding consumables--which have a large toughness--while the crack propagation ability decreased. It is believed that this is because the brittle crack cannot propagate in a straight line due to the difference in toughness and because the crack propagation is insensitive in the weld metal region with good toughness. In addition, in specimens 1, 2, and 3, it was observed that the welding amount of the large toughness-weld metal was insufficient to exert the effect of a toughness mismatch with the three types of welding materials, during which the amount of large toughness weld metal was only approximately 60% of the total weld metal area of specimen-4. Therefore, it was necessary to quantify the amount of large toughness weld to arrest the brittle crack propagation. Using the rainbow type welding bead layer method to arrest the brittle crack caused by the toughness mismatch revealed that apart from the fracture toughness of each welding consumable, the quantity of welding was also an important factor. Generally, one welding consumable was used to join the structure; however, two welding consumables with different toughness levels were used in this study to improve the brittle crack-arrestablity in the metal weld zone. This welding method is referred to as 'rainbow welding' in this study. Figure 6: Brittle crack arrest test results of specimens 1, 2, and 3. Figure 8 shows the Charpy impact toughness evaluation results for weld consumables B and G, which were applied to specimen-4 to arrest the brittle crack. The Charpy impact test was conducted at \(-40\)\({}^{\circ}\)C with weld metal (WM), fusion line (FL), and FL + 1 mm notch Charpy impact test specimens.