Intelligent Testing and Analysis of Dissimilar Steel Welds for Industrial Throttle Flowmeter

. Te throttling fowmeter used in utility boilers has been used as a measuring instrument for a long time. However, its safety performance, such as welding, lacks enough attention. Te manufacturing welds of the throttling fowmeter are welded with austenitic stainless steel and pearlitic heat-resistant steel. Cracks and other defects are generally found in the inspection and detection of the manufacturing welds of throttling fowmeters in service, which have serious potential accidents. To fnd out the relationship between weldingdissimilar steels and defects, the microhardness indentation method was used to measure the residual stress of welding. Combined with the self-developed calculation software of microscopic indentation residual stress distribution, we used the diference between the color of the indentation area and the surrounding area to quickly measure the indentation strain collected by the microscope using the computer image recognition algorithm, which greatly improved the accuracy and speed of the microscopic indentation residual stress test. Te results show that the residual stress at the weld of dissimilar steel is relatively large, and the mechanical property test is generally unqualifed. Te microindentation residual stress analysis method based on computer image recognition is used to quickly, intuitively, and accurately reveal the direct relationship between the combination of dissimilar steel welding materials and the generation of cracks and other defects.


Introduction
Te throttling fowmeter used in utility boilers is ubiquitous and is applied in high-temperature and high-pressure environments. Te welding seam is generally welded between austenitic stainless steel and pearlitic heat-resistant dissimilar steel. Te quality of the welding seam is very important, which is related to whether the throttling fowmeter can be used safely. Many scholars have carried out research on the welding problems of austenitic stainless steel and pearlitic heat-resistant dissimilar steel and the detection and analysis methods of welding defects.
Defects in the form of pores and slag inclusions usually appear in the welds of austenitic stainless steel and pearlitic heat-resistant dissimilar steel, thus causing local strain concentration. In austenitic steel and pearlitic steel, pores of 1 to 3 mm and slag inclusion will reduce the thermal fatigue life of welded joints by more than 10 times [1]. Scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy adopted by Gotalskii et al. [2] directly observed the weld interface of austenitic steel and pearlitic steel dissimilar alloy welds, and they found that there are two types of interfaces: austenite/martensite and martensite-like/ferrite. Pan and Zhang [3] proposed a method to reduce the width of such martensite interfaces. Schmidova et al. [4] performed hardness measurements in critical areas of pearlitic and austenitic steel welds and translated them into carbon concentration changes, which confrmed the presence of large concentrations and structural inhomogeneity at the fusion line, and such fusion zones were considered to be unstable [5]. Li et al. [6] and Nelson et al. [7] also analyzed the fusion zone and found it to be characterized by chemical inhomogeneity between the weld and the base metal. Danielewski et al. [8] conducted metallographic analysis of the weld and found that in stainless steel, the heat-afected zone was very narrow, and the growth of austenite grains was not observed, while the slender grains of ferrite formed a discontinuous network around the austenite grain. Another problem with dissimilar steel welds between austenitic steel and pearlitic steel is the presence of decarburization and carbide interlayers, which have an adverse efect on the mechanical properties of the welding seam [9]. Nikulina et al. [10] studied the fash butt welding of high-carbon pearlitic steel and chromium-nickel austenitic steel, and the results showed that layered pearlite colonies containing chromium and nickel and thin austenite interlayers were formed in the weld zone. Vishniakas [11] found that the failure of austenitic electrode welds was viscous, and individual parts were quasibrittle failures, while the failures of ferritic and pearlitic electrode welds were moderate. Elagin et al. [12] found that during long-term high-temperature heating, the formation of nitride particles and grain refnement help to improve the microstructure stability of the weld, inhibit the development of carbide reactions, and reduce the microstructure inhomogeneity of the weld. Terefore, the carbides are distributed more uniformly in the fusion zone with pearlitic steel, thereby reducing the hot brittleness of dissimilar steel welds. Li and Yan [13] used plasma arc welding to weld dissimilar steels of pearlitic steel and austenitic steel and also found that a platelike martensitic hardened layer was formed in the welding zone, carbon migrated on both sides of the welded joint, and there was a central area of lower hardness. Chu et al. [14] analyzed the failure causes of a certain weld and found that there were intergranular holes and cracks around the main crack, and the main failure mechanism was creep cracking. Dupont [15] summarized the welding of dissimilar steels, where premature failure is usually caused by the following factors: abrupt changes in the microstructure and mechanical properties at the fusion line; large diferences in coefcient of thermal expansion (CTE) between ferritic and austenitic alloys; the formation of interfacial carbides, which leads to the formation of creep voids; and the preferential oxidation of ferritic steels near the fusion line.
For the inspection of welds, some standards and manuals have been issued for reference [16,17]. In addition, except for some traditional inspection methods, new inspection methods are also emerging. Tuvander et al. [18] combined magnetic force microscopy (MFM) with scanning electron microscopy to successfully study austenitic and duplex stainless steel weld metals with diferent ferrite levels. Hempel [19] conducted an in-depth study on the residual stress state of dissimilar steel welds using X-ray and neutron difraction and confrmed that the residual stress on the surface of austenitic tubes during sample preparation was greatly afected by the machining technology. Barat et al. [9] experimentally confrmed that the acoustic emission method can be used to detect both typical welding defects in welded joints of diferent structural grades of steel as well as difusion interlayers. Lyubimova et al. [20] proposed that the combination of X-ray fuorescence analysis, X-ray difraction, and microhardness determination with traditional inspection methods (visual or ultrasonic inspection [21], etc.) can improve the operational reliability of dissimilar steel welds. Finite element analysis of welding deformation and residual stress was performed by Bouchard [22] and Rong et al. [23], which can be used to predict the transient behavior of welding deformation and residual stress. Woo et al. [24] and Eisazadeh et al. [25] used the neutron diffraction method to measure the residual stress of diferent welding layer thicknesses and diferent welds. Te transverse residual stress caused by welding was related to the existence of the martensite phase in dissimilar welds. Rathod et al. [26] concluded that radiography techniques could not detect small inclusions in welds. Terefore, the quality of detection should be checked by additional NDT detection such as ultrasound [27]. Ultrasonic testing requires materials that are acoustically isotropic, whereas austenitic welding materials are highly anisotropic due to the dendritic structure created by the cooling process during welding. Bulavinov [28] addressed this issue with an in-depth understanding of sound propagation in welded structures through elastodynamic simulations to support the assessment of the local structure of the weld. Juengert [29] developed two ultrasonic inspection techniques and validated them on planar specimens with artifcial and real defects; both reconstruction techniques gave quantitative inspection results and allowed the determination of defect sizes. Lugin [30] proposed a new method to detect hidden defects using lateral heat fow, which can fnd hidden defects/cracks that cannot be detected by traditional thermal detection methods. It is a common method to collect welding stress data using Internet of Tings technology [31,32].
Xia et al. [33] developed a method based on microindentation, which could predict the length of the crack as a function of the residual thermal stresses. Mulford et al. [34] described a procedure for extracting simple constitutive parameters from microindentation tests to construct the entire stress versus strain curve. Frutos et al. [35] addressed the determination of residual stresses in sandblasted austenitic steel by ultramicroindentation techniques using a sharp indenter. Te results showed good agreement with those obtained by synchrotron radiation on the same specimens. Yonezu et al. [36] proposed a method to evaluate the residual stress and plastic strain of an austenitic stainless steel using a microindentation test. A numerical experiment with the fnite element method (FEM) was carried out to simulate an indentation test for SUS316L with various plastic strains (prestrains) and residual stresses. Liu et al. [37] investigated the microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718, and residual stress evaluation in microstructure scale by Vickers microindentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample.
Although dissimilar steel welding of austenitic stainless steel and pearlitic heat-resistant steel is widely used in utility boiler pipes, and domestic and foreign research studies have also been carried out on the problems of dissimilar steel welding and the detection and analysis methods of welding defects, less research has been done on the welding of dissimilar steels on the throttling fowmeter. For a long time, the throttling fowmeter was only used as a measuring instrument, and its safety performance, such as welding, lacked attention. It was not until 2016 that the explosion of the fowmeter of the Dangyang Power Plant in Hubei caused serious casualties, and all parties in the society began to pay attention to its safety. In this paper, the standard nozzle fowmeter (hereinafter referred to as the nozzle fowmeter) in the throttling fowmeter is taken as the research object, the structure and process of the welding seam of the throttling fowmeter are simulated, and diferent combinations of dissimilar steel welding materials are selected to make the processing test pieces. Ten, the microhardness method combined with the microindentation residual stress distribution calculation software was used to test the residual stress and mechanical properties of the weld of the specimen. Te relationship between welding of dissimilar steels and defects is analyzed, and the reasons why throttling fowmeters are pervasive in defciencies are revealed, which provides a direction for the next step of the fowmeter's structural transformation and processing technology improvement.

Problems Existing in the Welding Seam of Throttling Flowmeter
Te throttling fowmeters used in utility boilers are similar in structure. Te example used in this case is the standard nozzle fowmeter, also known as the nozzle fowmeter, as shown in Figure 1. In order to ensure strength as a pressurebearing part in a high temperature and high pressure environment, the front and rear clamping rings are generally made of pearlitic heat-resistant steel. To ensure the geometric size of the throttling part and guarantee the measurement accuracy as a measuring element as well as maintain the cleanliness of the surface and prevent oxidation at high temperature, the throttling parts are generally made of austenitic stainless steel. Due to the inconsistent materials of the front and rear clamping rings and the throttling parts, the welding seam has a problem of dissimilar steel welding.
To be specifc, the welding of austenitic stainless steel and pearlitic heat-resistant steel, and the welding seam is Vshaped. Te author collected 9 out of 53 nozzle fowmeters from 8 thermal power companies in a targeted manner, involving 4 user units and 5 manufacturing units, also taking into account the diferent materials, operating parameters, different media, and operating times of the nozzle fowmeter. After taking advantage of the convenient conditions of the laboratory, by means of dissection, nondestructive testing, scanning electron microscope, energy spectrum analysis, mechanical performance tests, etc., it was found that none of the nozzle fowmeters survived. To be specifc, cracks and other associated defects were found in the manufacturing welds of the nozzle fowmeters that were sampled, as shown in . Te defect rate is 100%, and the high incidence of fowmeters is shocking.

The Basic Situation of Welding Specimens of Throttling Flowmeter
To verify the relationship between the welding of dissimilar steel for the throttling fowmeter of a utility boiler and the defects such as cracks commonly found on the weld, the structure and process of the welding seam of the throttling fowmeter were simulated, and diferent combinations of dissimilar steel welding consumables were selected to make processing test pieces. Ten, the residual stress and mechanical properties of the welds of the specimens were tested by appropriate test methods. Te structure and welding process of the throttling fowmeter weld were simulated, and diferent combinations of dissimilar steel welding consumables were selected, including process test pieces 1 to 4 (see Figures 6-9). Te hardness and welding residual stress of the specimens under diferent welding processes were analyzed, and the mechanical properties of the specimens were also tested; the results are shown in Table 1.

Selection of Residual Stress Test Method
Te traditional measurement techniques of residual stress of welded components can be roughly divided into two categories: the mechanical release measurement method with certain damage and the nondestructive physical measurement method. Tese measurement methods are basically only macroscopic measurement methods, which means the test process is difcult to repeat and the experimental data are widely distributed. In addition, mechanical methods, such as the blind hole method will cause greater damage to the measurement samples. In recent 10 years, due to the development of thin flm materials and nanotechnology, traditional measurement methods have been unable to meet the experimental requirements, and a new residual stress measurement technology has emerged: the use of microhardness indentation to measure residual stress. Te basic measurement principle of residual stress measured by the microhardness indentation method is that there is a linear relationship between residual stress (strain) and indentation area ratio. When there is tensile stress in the sample, depressions will occur around the indentation, and the area of the indentation will be relatively small. When there is compressive stress, there will be bulges around the indentation, and the area of the indentation will be relatively large. Tat is why the hardness method for measuring the residual stress of the material is determined by the ratio of the indentation area on the surface of the sample.
According to the theory of OliverWC and PharrGM, the residual stress is sensitive to the amount of metal accumulated around the indentation during the pressing process, so the primary condition for accurate residual stress measurement is to accurately measure the area change of the indentation. Te introduction of the parameter indentation area ratio C 2 is given below:

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A nom is the projected area of indentation under stress state; A is the area under stress-free state. Te residual stress is fnally obtained by calculating the residual strain due to the area change caused by the residual stress.
However, the microhardness indentation method also has some limitations. For example, in order to test the residual stress on the cross section of the weld, it is necessary to take a sample of the weld section, which destroys the integrity of the weld inevitably and only the residual stress in the two-dimensional direction of the cutting plane can be measured, which means the actual residual stress at the test point cannot be completely refected. Due to the irregularity of the indentation shape afected by residual stress, the accuracy and time-consuming nature of traditional dimensional measurement methods limit the application of this test method in practical engineering. In addition, the      Mobile Information Systems    Mobile Information Systems 5 residual stress is calculated through the relationship between the residual stress and the residual strain. Te residual stress between adjacent test points sometimes has a large diference, with peak values and some discontinuities.
To fnd out the distribution rules of welding residual stress on the welding seam cross section of the throttling fowmeter, the microhardness indentation method was used in this welding residual stress test. Te author used the selfdeveloped "microindentation residual stress distribution calculation software" and utilized the diference between the color of the indentation area and the surrounding area. Te fow of the algorithm is shown in Figure 10. Te computer image recognition algorithm was used to quickly measure the indentation strain collected by the microscope, which greatly improved the testing accuracy and speed of residual stress by microindentation. Te software functions included image recognition of indentation topography, automatic calculation of actual area of indentation, theoretical area of indentation, residual strain, and residual stress. It can perform one-click batch identifcation and calculation of indentation photos.
Te key to improving the accuracy and speed of microindentation measurement lies in the rapid and accurate determination of the indentation area of complex shapes that are actually deformed by residual stress. Te traditional microscope cannot measure the size and calculate the area of the actual indentation with concave-convex arc features. Using the light and shade diference between the indentation and the surrounding material in the metallographic photo, the computer image recognition method is used to directly extract the sum of the pixels of the dark indentation as the actual indentation area, and the diagonal pixel length between the four endpoints is extracted to obtain the theoretical Indentation area. Te ratio of the two can cancel the infuence of the pixel unit and the length unit and avoid the difculty of the actual indentation size measurement and area calculation.

Measurement of Welding Residual Stress on Welding Specimens of Throttling Flowmeter
Weld seam section samples were taken from samples 1#, 2#, 3#, and 4#, respectively, and the samples taken to avoid the arc starting point were recorded as 1#-1, 2#-1, 3#-1, and 4#-1. Tey were lightly corroded, respectively, and weld distribution can be seen. On the 1#-1 sample, a test point was taken every 2 mm, and a total of 27 columns and 13 rows were taken, totaling 351 points ( Figure 11). Te microhardness indentation method was used for stress testing, and the microindentation residual stress distribution calculation software was used too. Te schematic diagram of the stress distribution is shown in Figure 12. Te minimum stress is 97 MPa, and the maximum stress is 291 MPa. On the 2#-1 sample, a test point was taken every 2 mm, with a total of 23 columns and 13 rows, for a total of 299 points (Figure 13. Te schematic diagram of stress distribution is shown in Figure 12; the minimum stress is 106 MPa, and the maximum stress is 386 MPa. On the 3#-1 sample, a test point was taken every 2 mm, with a total of 23 columns and 13 rows, for a total of 299 points (Figure 14. Te stress distribution diagram is shown in Figure 12. Te minimum stress is 122 MPa, and the maximum stress is 398 MPa. On the 4#-1 sample, a test point was taken every 2 mm, with a total of 21    Figure 12. Te minimum stress is 102 MPa, and the maximum stress is 346 MPa.

Mechanical Properties Test of Welding Specimens of Throttling Flowmeter
According

Analysis of Test Results of Welding Specimens of Throttling Flowmeter
Welding residual stress and mechanical properties test situations are given below: (1) Te welding material of specimen 1 is heat-resistant steel as the base and heat-resistant steel welding rod cover. Except for the welding of dissimilar steel at the welding place between the bottom layer and the stainless steel throttling piece, other welding materials are of the same type. Te mechanical properties test refects that the tensile strength of the welded joint is 539 MPa. Te tensile test and impact test results of the welded joint meet the requirements of the specifcation. However, cracks appeared in some bending specimens of the bottom weld fusion line position of the bending test. Te welding residual stress of the specimen is larger than that of the base metal. Most of the residual stress of the weld is between 150 and 250 MPa, and a small part of the point stress is between 250 and 300 MPa. Te closer the weld is to the bottom, the greater the welding stress, which is close to 60% of the tensile strength, and there is a certain risk of cracking. (2) Te welding material of specimen 2 is heat-resistant steel and stainless steel transition welding wire as the base and the heat-resistant steel welding rod cover. Due to the transition of welding wire, the heatresistant steel welding rod will not be in contact with the stainless steel throttling parts. Te mechanical performance test refects that the tensile strength of the welded joint is 500 MPa, and the results of the tensile test, bending test, and the impact Mobile Information Systems test of the welded joint all meet the requirements of the specifcation. Te welding residual stress of the specimen is larger than that of the base metal. Since there was no heat treatment performed, most of the residual stress of the weld is between 250 and 350 MPa, and a small part of the point stress is between 350 and 400 MPa. Te closer the weld is to the bottom, the greater the welding stress, which is close to 80% of the tensile strength, and there is a greater risk of cracking. (3) Te welding material of specimen 3 is the stainless steel wire as the base and the heat-resistant steel welding rod cover. Tere is a problem of direct welding of dissimilar steel between the stainless steel wire and the heat-resistant steel welding rod. Te mechanical performance test refects that the tensile strength of the welded joint is 500 MPa, and the tensile test of the welded joint meets the requirements, while the bending test and impact test do not meet the specifcation requirements. Te welding residual stress of the specimen is larger than that of the base metal. Since no heat treatment is performed, most of the residual stress of the weld is between 250 and 350 MPa, and a small part of the point stress is between 350 and 400 MPa. Te closer the weld is to the bottom, the greater the welding stress, which is close to 80% of the tensile strength, and there is a great risk of cracking. (4) Te welding material of specimen 4 is the stainless steel wire as the base and the heat-resistant steel welding rod cover. Tere is a problem of direct welding of dissimilar steel between the stainless steel wire and the heat-resistant steel welding rod. Te mechanical performance test refects that the tensile strength of the welded joint is 500 MPa, and the tensile test of the welded joint meets the requirements, while the bending test and impact test do not meet the specifcation requirements, but the impact test result is higher than that of specimen 3. Te welding residual stress of the specimen is greater than that of the base metal, and the specimen has been heat treated. Most of the residual stress of the weld is between 150 and 250 MPa, and a small part of the point stress is between 250 and 300 MPa. Te closer the weld is to the bottom, the greater the welding stress, which is close to 60% of the tensile strength, and there is a certain risk of cracking.

Conclusion
Trough the testing of welding residual stress and mechanical properties of specimens with four diferent welding methods, the test results are comprehensively analyzed below: (1) Tere are diferences in residual stress between the weld passes of the fowmeter welding specimen. Te residual stress near the bottom of the weld is larger, and the residual stress near the end of the weld is smaller. Te maximum residual stress is located at the root of the weld, and the local strength is close to 60-80% of the tensile strength of the material, indicating that there is a risk. (2) If the stainless steel throttling parts are directly welded with heat-resistant steel welding materials or stainless steel welding material base and heatresistant steel welding material cover, the mechanical properties tests are generally unqualifed, and the residual stress at the joint of dissimilar steel is large, indicating that this welding process should be avoided. (3) Te stress of the welded seam of the specimen after heat treatment is obviously lower than that of the welded seam without heat treatment, and the test results of mechanical properties are also better than those of the specimen without heat treatment, indicating that the post-weld heat treatment process can efectively improve the performance of the fowmeter and can be popularized and applied.

Data Availability
Te data that support the fndings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.