# Technological Processes for Steel Applications in Nuclear Fusion
## 4 Fusion Welding
### Effect of PWHT on Tensile Properties  
The tensile properties of all investigated welds without PWHT are shown in Figure 19, in comparison to the base material. Only the 12 mm TIG and EB welds delivered enough material for tests at temperatures up to 600 \({}^{\circ}\)C. The other welds have been tested only at room temperature. The effect of a single PWHT at 700 \({}^{\circ}\)C for 2 h was studied by tensile tests at room temperature, 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C. For the full PWHT (i.e., the standard heat treatment), only room temperature tests could be performed. All tensile specimens from the TIG welds fractured clearly out of the fusion zone, that is, inside or just at the base material. The determination of the fracture location for TIG welded specimens is difficult without a microstructural examination of the corresponding cross sections. Since most tensile specimens from the 12 mm TIG welds fractured nearly in the middle of the gauge length, it may be assumed that only here the fractures are mostly located in the fine-grained, soft HAZs. Figure 18: Lowest temperature in the upper shelf (LTUS) for 5 mm, 10 mm, and 12 mm welds after different post-welding heat treatments. The notches of the Charpy specimens were fabricated either in the weld center or in the HAZ (as indicated). For comparison, DBTT of the base material is plotted as a horizontal line in the graphs. Transverse tensile (the definition is given in [32]) tests characterize the strength of the whole joint, that is, they reveal the weakest part of the joint. If the fractures appear in the fusion zone, this means that the weld is the weakest part. Possible reasons might be defects, unsuitable chemical compositions of filler wires, oxidized surfaces before welding. A fracture in the HAZ usually indicates that the joint is good but that microstructural changes or diffusion processes during welding have led to a weak area between the fusion zone and the base material. Such soft regimes can only be removed by PWHTs (in the case of EUROFER97 by an austenitization step). Finally, fractures in the area of the base material indicate that the according joint has no weak part, i.e., the weldment is acceptable, at least with respect to tensile loading or straining. Within certain scattering (10 MPa for tensile, 50 MPa for yield strength, 15% for total and 4% for uniform elongation) the tensile test results of all welds agree well with those of the base material. That is, the tensile behavior is not dominated by the weld zone but by the base material. In other words, the welds show a higher strength, which has been expected. For a specific determination of the tensile behavior of the welds, the procedure according to [33] (longitudinal tensile test) has to be applied. Since this requires a larger amount of weld material, such a characterization could not be performed. However, from the instrumented Charpy tests, as described in the previous section, dynamic yield strength values were computed, which are closely correlated to the weld zone. This provided much higher values compared to the tensile results. Nevertheless, different heat treatments have been applied to selected welds and tensile tests have been performed at room temperature. The results are also plotted in Figure 19. For comparison, specimens have been prepared without PWHT, with a PWHT of 700 \({}^{\circ}\)C/2 h, and with the standard heat treatment (980 \({}^{\circ}\)C/30 min + air quenching + 760 \({}^{\circ}\)C/2 h). Similar to before, differences in strength  
Figure 19: Tensile properties of the EUROFER base material in the condition as delivered (squares) and of the different welds without and with two different PWHTs. The different welds are not discerned. They include 5-, 10-, and 12-mm EB, laser, and TIG welds, as described in the previous sections. (tensile as well as yield strength) are not recognizable.