Analysis of residual stresses resulting from the surface preparation for X-ray diffraction measurement
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There is no consensus in the literature on the need to remove preprocessing layers from the material prior to the measurement of residual stresses by X-ray diffractometer. Thus, the purpose of this work was to evaluate the residual stresses induced by material preprocessing and its evolution during the preparation of the surface by electrolytic removal. Sample surfaces were pre-processed by grinding and sandblasting and the resulting residual stresses were measured by X-ray diffractometry. At each removal stage, the evolution of residual stresses, hardness and microstructure of the surface were verified. It was concluded that different preprocessing methods can induce surface residual stresses of either tension or compression, reaching different depths. Removal by electrolytic method of the modified layer has shown itself capable of reducing significantly the magnitude of the residual stresses induced by preprocessing. On the other hand, the depth of deformed grains or surface hardness proved to be incapable of predicting the depth of induced residual stresses. Finally, it was discussed whether or not the layers removed by this method reveal the subsurface stresses and if the removal should take place before or after a second processing.
KeywordsResidual stress X-ray diffraction Preprocessing Surface stresses Electrolytic removal
Lachmann et al.  ranked measurement speed, portable equipment and safety as reasons for residual stress (RS) measurement through X-ray diffractometry (XRD) to be one of the most used techniques, both in research and in industrial applications. Still according to Lachmann et al., the main limitation of RS measurements by XRD is the limited penetration of the X-rays. Owing to this low penetration, the measurement result represents stresses state only at the surface. These authors, however, point out to the fact that the measurement results can be disguised by material preprocessing. As reported by Robinson et al. , Hatori et al.  and De Giorgi , RS due to preprocessing are concentrated on the workpiece surface and can have both tensile and compressive nature.
Robinson et al.  measured RS along the thickness of a cold-forged aluminum plate. On the plate surface, the measurement reached − 300 MPa, with a tendency of turning into tensile stresses at deeper layers. Hattori et al.  studied RS in cold rolled aluminum plates. The RS measured was of compressive nature on the surface and of tensile nature at deeper positions. De Giorgi  quantified RS on cold rolled plates of AISI 301. The difference between the RS values taken from the surface and from a 0.6-mm-deep layer reached up to − 150 MPa. Che-Haron and Jawaid  found a greater than 0.010-mm-thick plastically deformed surface after titanium alloy shaft turning. Nasr et al.  observed a deformed surface thickness of 0.140 mm after turning AISI 316 steel. Although in the last two cited papers RS were not measured, it can be deduced that the plastic deformation implies the presence of RS.
Prior to the welding operation, for instance, the plate region near the groove must be cleaned by grinding, machining or shot blasting. As a result, manufacturing related preprocessing superficial modifications can be eliminated in the region under cleaning, but a new surface change is imposed. AISI 316 steel plates blasted with Al2O3 or ZrO showed compressive stresses of 400 MPa in surface layers up to 0.080 mm deep, according to Multinger et al. . Youssef et al.  indicated that grinding can induce tensile RS with a value close to the rupture stress of AISI 316L steel. Thus, it is important to know the RS induced by preprocessing and to find out whether it is necessary or not to eliminate them before XRD measurements.
When thermal stresses due to welding are generated on the material, the pre-existing stress field on the surface will certainly assume another value. This fact itself does not characterize a problem for RS measurement, because this is the real situation. However, as XRD measurement occurs on a thin surface, the resulting stress field may not accurately represent what it is desired to measure (which are the stresses that actually will influence on the mechanical behavior of the component). Nevertheless, when one wants to study the parameters that govern RS generation due to welding, unquestionably the pre-existing stress field on the surface will disguise the results of a XRD measurement. In these cases, to guarantee the quality of RS measurement by XRD, the pre-deformed surface is usually eliminated by electrolytic processes, which do not induce plastic deformations.
However, some authors remove 0.100 mm of material before XRD appraisal, others apply a deeper removal. For example, Gou et al.  and Hilson et al.  made an electrolytic removal of 0.100 mm thick before measuring RS by means of XRD in arc welded tubes (although, none of the researching groups verified if the deformed surface was completely eliminated or not). Rai et al. , on the other hand, consider the removal of a 0.020 mm layer as sufficient to eliminate the oxide film effect on measurement results of RS in arc welded tubes. In addition, Brown et al.  and Tsuji et al.  used electrolytic polishing to prepare the surface before RS measurements. However, the thickness of the removed layer was not specified. Harati et al.  did not make any material removal and measured the RS in pieces as received. Furthermore, in all the cited papers, the authors did not take into account the type of manufacturing related preprocessing applied to the plate surface.
On the other hand, there are explicit recommendations upon the surface preparation prior to measuring given in standards, such as one from ASTM International , in which a 0.250 mm removal before the measurement of RS by XRD is recommended in section 8.1. Another standard, from British Standards Institution  also recommends removal of work-hardened surface, but in this case the thickness is not specified. Conversely, based on the results of literature review, the thickness of the surface deformed by several manufacturing processes varies greatly from one process to another and sometimes a recommended removal of 0.250 mm may be excessive (by removing this thickness, the RS of interest, as RS induced by welding process, may be altered). A relevant issue, also not usually observed by those who investigate or measure RS, is when the altered surface cleanliness must be done. It could be done before or after the stress-generating operation, for example, welding. All the cited papers did it afterward. The questionable would be: if the surface layer removal is done after applying a new RS generation processing (such as welding), would not the new stresses be removed together with the preprocessing stresses? Even when there is surface preparation before measurements, for example, as seen in Gou’s et al.  and Hilson’s et al.  papers, how can it be ensured that the entire surface changed by a given pre-processing was fully eliminated?
Finally, another phenomenon that apparently affects measurement results is the relaxation of RS over time. The relaxation of RS induced by welding process was recently reported by Estefen et al. , when they observed the continuous relaxation of RS during the first 14 days after restrictions removal.
As has been seen, there are antagonistic procedures and recommendations applied to surface preparation before the measurement of RS by XRD. It can be noticed a lack of information upon the alteration that a given type of surface processing can imply in measurement results. Therefore, this study’s aim was to evaluate the RS induced by preprocessing and the evolution of their intensities during the surface preparation by electrolytic removal, considering the potential temporal relaxation over a maturation period.
2 Methodology, equipment and materials
Three batches of plates (AISI 1020 carbon steel, 50 × 50 × 6.35 mm) were prepared. One of them was used for tests with a surface preparation by grinding, a second one for replication with the same preparation and a third one in which the surface preparation was sandblasted. To relieve pre-existing stresses and normalize the microstructure, the three batches of test plates were prior to grinding/sandblasting, individually heated at a temperature of 650 °C, at a rate of 865 °C/h, kept for 2 h and cooled at an average rate of 224 °C/h to room temperature. The average residual stresses measured on the first, second and third batches after the heat treatment were − 70 ± 32, − 8.7 ± 21 and 122 ± 3 MPa, respectively. The search for reasons for the plates to have not reached the same residual status after the heat treatment was not the focus of this work, because the heat treatment objective was to stress relieve and normalize the microstructure, as successfully reached. One could say that these variations could be due to different removal positions of them in relation to the “mother plate”, different removal positions inside the furnace (gradients of temperature), etc.
The plastically deformed surfaces were removed by electrolytic etching with a solution of 19% H3PO4, 9% H2SO4 and 72% H2O. The removal procedure and electrolyte removal parameters were previously published elsewhere (Mishchenko et al. ). Briefly, the electrolyte process adjustments were to obtain approximately 0.020, 0.040, 0.060, 0.080, 0.100, 0.120 and 0.140 mm removals. For each target thickness of removal, two test plates were used, for repeatability control. One of the plates was used only for hardness and deformed thickness measurement. RS and removed thickness measurements were performed on both plates. After measuring the RS, the second plate was saved as an evidence.
This study has extended to evaluate the possible stresses relaxation over time, as suggested by Estefen et al. . It is important to mention that the existence of this phenomenon in experimental conditions used in this study will potentially impact the analyzes described above. For this purpose, the value of RS was measured on a test plate ground on both sides, that is, to say on the top surface (face) and the bottom surface (back side). Even assuming that the present paper results would not have to match Estefen’s et al. (distinct RS source, material, experiment conditions, etc.), it was decided to monitor the RS temporal evolution in the ground plate. RS measurements were made for 18 days at an interval of 6 days and no surface removal was applied.
2.1 RS value correction as a function of the removed layer thickness
It is important to point out that all RS values thereafter presented here are corrected using Eq. (1), except the ones (Sect. 3.1) on test plates without layer removal [by principle, Eq. (1) cannot be applied over results from a plate without material removal].
3 Results and discussion
3.1 Residual stresses temporal evolution and the experiment repeatability
3.2 Evolution of residual stresses and surface microstructure in relation to the layer removed thickness
3.3 Residual stresses in test plates with surface treated by sandblasting
The fact that grinding has presented on its surface RS tensile values above the material yield stress and close to or higher than the material rupture limit (in agreement with literature) suggests that the depth of RS induction by grinding is close to 0.140 mm for a non-alloy carbon steel (other values can be reached for alloyed steels, with higher ultimate stress limits). Sandblasting, because it exerts compressive deformation, may even reach greater depth, but the high values of RS reached indicate the saturation point proximity. It is determined that the recommendation in section 8.1 of the ASTM standard , that is to say, a 0.250 mm removal before RS measurement by XRD, is well consistent, but depending on the application may be exaggerated, considering the relaxation that can be introduced due to excessive material removal. However, a temporal quantification of RS values along the removal would be the correct way to know if the RS due to preprocessing would have already been removed, at the expense of long time and increasing costs.
Still by the present results, it is suggested that the surfaces layers removal by electrolytic methods would be a way of measuring the RS field in the material thickness, overcoming a XRD technique limitation. Nevertheless, this paper’s authors believe that more studies must be done on this topic. For instance, it is not clear whether the reduction of reached RS as the removed depth is raised (regardless of being tensile or compressive), mean the existent stresses or just the reflex of restriction removal, which was avoiding the stress mechanical relaxation.
On the contrary, this study is able to support a reasoning about either material removal prior to RS measurements should be happen before or after another material processing. For instance, in welding case, the process will generate a particular stress field close to and at the weld bead. If the electrolytic removal is done after welding and before measured by XRD, it may happen that the own stresses generated by the welding process are being removed (therefore, not measured). It may be better to remove the pre-existing surface stresses (in the case of measurements close to the weld bead, not on the bead surface) prior to welding, so that they do not disguise the stresses generated by the weld. In contrast, the argument could be that the correct thing would be to never remove a surface layer before applying XRD, because the final stress is what matters (and the preexisting is part of it). It seems that the removal before welding, not after, is the most correct method to study the welding parameter effect upon the RS generation, since in this case it is necessary to eliminate sources of noise. However, if quantification of RS of an in-service component after a processing is the target, no surface removal should be applied besides the normal cleaning employed before the processing (groove grinding in welding, for instance).
Different preprocessing methods can induce residual stresses in the metal surfaces RS with distinct modes (tensile or compressive) and depth;
Surface treatment with layer removal by electrolytic method has shown itself capable to eliminate the RS induced by preprocessing and a removal layer of about 0.200 mm would be safe to suppress the RS induced by grinding of unalloyed carbon steels;
Deformed grains depth visualized by optical microscopy or surface hardness proved to be incapable of predicting the induced RS depth.
A second purpose of this study, which was to evaluate the potential temporal relaxation during a maturation period of the material prior to the RS measurements, led to the deduction that relaxations does not happen if the material is left resting long time before XRD measurement, at least to reasonably small and non-restrained materials.
Finally, if the intention is to assess the effect of welding parameters on RS generation in a weld joint, surface removal should be applied before welding, not after it. However, if the aim is to quantify the residual stress of a component under service, surface removal dedicated to XRD measurements should not be employed.
Therefore, it is believed to have produced subsidies for the RS measurement procedures in metal plates that undergo pre or post processing prior to RS measurement by XRD. Nevertheless, for future studies, it remains that the task of classifying whether the removed layers through this method reveal or not the subsurface stresses and to demonstrate that removal should take place before or after a second processing.
The authors would like to thank the Center for Research and Development of Welding Processes (Laprosolda) of Federal University of Uberlandia and the Brazilian Nanotechnology National Laboratory—LNNano, for the laboratorial infrastructures. The authors also would like to acknowledge the financial support provided by the Brazilian National Council for Scientific and Technological Development (CNPq), through Grants number 302863/2016-8 and 149308/2014-0, and from the Minas Gerais State Agency for Research and Development (FAPEMIG), through Project number TEC—APQ-01992-15.
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