Typical samples that are successfully investigated by neutron diffraction to determine residual stress states are crankshafts, pistons, turbine blades, wheels, impellers, shot peened components, welds and composite materials.
Real life specimens are often of a complicated shape or material, making the measurements challenging. For example, aluminium components often have large grains, which necessitates either the use of a large gauge volume or specimen rocking during measurement in order to mitigate the effect of the grain size on the measurements. Thus the spatial or angular resolution can be significantly reduced. Titanium is also a difficult material to measure due to its large incoherent scattering, which increases the background.
The determination of the strain free reference value is often the most challenging part of an investigation. A number of approaches to this challenge are described in Section 3.6.4.
6.1. MEASUREMENTS OF STRESS/STRAIN STATE INDUCED BY A WELD DEPOSITED PASS
Using the strain diffractometer installed at the reactor LVR-15 in Řež, Czech Republic, the macrostrain distribution was measured in the vicinity of two different welds from Inconel 52 deposited on steel plates from 15Ch2MFA. This steel is used for the construction of reactor vessels. The thickness of the plates was 7 mm and the welds were deposited in form of a single bead or six beads in two layers. The single and the six bead welds had a height of 1 mm and 3.5 mm and width of 5 mm and 12 mm, respectively (see Fig. 31).
FIG. 31. Diagram displaying the sequence of welding.
The extent of the fusion zone in the strain scanning direction was approximately 0.3 mm. The gauge volume of 2 × 2 × 3 mm3 was located in the middle of the plate, i.e. at a depth of 3.5 mm, and the scanning was carried out along a line going orthogonally across the weld. Measurements were performed using the α-Fe (110) reflection plane. Figure 32 displays a photograph of one of the plates with the single bead (the welding direction is indicated by an arrow) and the results of the macrostrain scanning for both samples. Axes x, y and z are parallel to the longest edge of the plate, the medium wide edge and the shortest edge, respectively. For calculation of the macrostrain values, the strain free Bragg angle position was measured at the corner of the plate.
FIG. 32. Photo of the plate of 15Ch2MFA with a single bead weld and the measured strain components vs. the distance from the weld passes for both samples.
6.2. RESIDUAL STRESSES IN A BIMETALLIC STAINLESS STEEL ZIRCONIUM ADAPTER
A bimetallic stainless steel zirconium adapter [104] is used in some structures of RBMK reactor channels. This adapter is a complex cross-section cylinder (Fig. 33) with a steel outer layer and a zirconium alloy inner layer. It is manufactured by vacuum sintering at a temperature of 900°C. The thermal expansion of steel is three times larger than that of zirconium. The steel shell compresses the zirconium part when the adapter is being cooled down, which introduces residual stresses inside the bimetallic joint. These stresses could be a reason for crack nucleation or adapter failure. The aim of this work is to investigate the residual stress state of the stainless steel near the steel–zirconium alloy splice. The investigated regions are shown in Fig. 33 as cross-section A-B, cross-section 2 and cross-section 3. Experience shows that cross-section A-B is the most critical in view of fatigue failure where the so-called first zirconium screw tooth exists.
FIG. 33. An adapter in an RBMK reactor.
The study was performed using neutron diffraction on an FSD at the IBR-2 pulsed reactor in Dubna, Russian Federation, and on a high resolution neutron diffractometer at the LVR-15 reactor in Řež, Czech Republic. The measured results are shown in Fig. 34, from which it can be seen that residual stresses in all three cross-sections are compressive and therefore should have a beneficial influence on the fatigue performance of the adapter, which works under tensile load conditions. The important factor is the presence of a stress concentrator in cross-section A-B, close to the junction of the steel and zirconium parts of the adapter. In Fig. 34 this location is indicated by a thin circle.
FIG. 34. The measured residual stresses in the stainless steel part in the various cross-sections of the adapter. The wall of the adapter is shown in the region of the critical cross-sections. The thin circle indicates the location of the stress concentrator.
6.3. MEASUREMENT OF RESIDUAL STRESSES IN A BIMETALLIC NUCLEAR PIPING WELD
Numerous light water reactors for nuclear power production in existence in the world operate with a primary piping system made from stainless steel components. Such stainless steel piping is welded to pressure vessel nozzles that are made of low alloy ferritic steels in virtually all cases. In such dissimilar metal — or bimetallic — welds, the residual stresses do not only originate from the welding process, but they are also generated through the thermomechanical mismatch of the different materials involved.
On the HB4 Large Component Neutron Diffraction Facility at the High Flux Reactor in Petten, the Joint Research Centre has performed a series of measurements in a reduced size mock-up of such a bimetallic piping weld. In this particular experiment, the specimen was made of a ferritic steel grade A508 welded to a stainless austenitic steel grade 304L by applying a buttering layer and subsequently a multipass V groove circumferential weld. The specimen was 168 mm in diameter, ~400 mm in length and had a wall thickness of 25 mm.
Figure 35 shows the specimen on the diffractometer during measurements in the piping circumferential direction, i.e. the hoop direction. The interface between the low alloy steel and the stainless steel buttering layer can be easily recognized.
A hole was cut into the specimen for these measurements in order to reduce the neutron path length through the material. In the picture the incident neutron beam duct is positioned inside this hole so that the incident beam aperture is close to the measurement position.
Many of the measurement locations for this specimen were actually within the weld material. Welded regions often exhibit inhomogeneities and variations of the free-of-stress parameter in diffraction experiments. For this reason, dedicated reference specimens have been used here in order to account for such variations as much as possible. One of the reference specimens can be seen in Fig. 35 at the bottom, in front of the piping specimen.
The main aim of such measurements, in particular in relation to nuclear applications, is to provide for the validation of numerical predictions of the residual stresses. Figure 36 shows the measured residual stresses, along a line 3 mm from the outer surface of the pipe parallel to its axis, in comparison to the corresponding residual stress data derived from two different approaches for prediction. The blue line has been derived from a simplified approach where the welding process itself has not been modelled at all. In this case only a simple cooling from an elevated temperature of about 500–600°C has been modelled, whereby the residual stresses were generated by the mismatch of the materials during cooling [105]. The green line stems from a detailed bead by bead two dimensional simulation process [106].
FIG. 35. Bimetallic piping weld specimen during hoop direction measurements at the HB4 Large Component Neutron Diffraction Facility at the HFR Petten. The incident neutron beam duct is on the left and the diffraction neutron beam duct is on the right. At the front end of the diffracted beam duct, the cadmium mask providing for the beam aperture can be seen.
FIG. 36. Hoop (left) and axial (right) residual stresses near the outer wall of a 25 mm thick bimetallic piping weld. The data points correspond to the neutron diffraction measurements (horizontal error bars indicate gauge volume dimension); the blue and green lines correspond to the simplified and detailed numerical predictions, respectively.
It can be seen that, in this case, the results produced by the more detailed simulation compare much better to the measurement results than those from the simplified analysis do. The simplified analysis has also produced a significant underprediction of the residual stresses, which means that this approach should definitely not be used for component integrity analyses.