Effect of Pr6O11 on the Microstructures and Mechanical Properties of High-Strength Steel Weld Metal

The effect of Pr6O11 on the microstructure and mechanical properties of high-strength steel weld metal was investigated by optical microscopy, scanning electron microscopy and mechanical testing. Three different contents of Pr6O11 were added to the flux-cored wires. The results demonstrate that the addition of 1% Pr6O11 can promote the refinement and spheroidization of inclusions, refine the grains, form acicular ferrites in the weld metal, and significantly improve the toughness. The addition of Pr6O11 promoted the formation of rare earth composite inclusions and acicular ferrites in the weld metal, refined the lath microstructure, inhibited the formation of martensite and bainite. The crack formation mode changed from the boundary cracking of the bainite clusters caused by the surface shear stress to the surface shear stress-induced decohesion of inclusion. Excessive addition of Pr6O11 will reduce the number of inclusion nucleation and deteriorate the mechanical properties. The wire No.2 with 1% Pr6O11 had the good comprehensive mechanical properties.


Introduction
The attainment of both strength and toughness is a vital requirement for most structural materials; unfortunately these properties are mutually exclusive [1]. Improving the toughness of high-strength steel weld metal is a hot spot in welding field. Alloying elements are one of the main factors that control the weld metal microstructure and mechanical properties. Therefore, with the development of high-strength steels, the research and development of micro-alloying welding consumables, and the study of the influence of microalloying elements on the microstructure and properties of welds and heat-affected zones are the hot spots in the field of welding. The volume fractions of acicular ferrites (AF) were decreased as boron contents increased from 32 to 103 ppm in high-strength steel welds. The upper bainite was formed in the weld metal with 103 ppm boron wire, while was not formed in the weld metal with the wire of 32 ppm and 60 ppm boron contents [2]. Effect of Zr on the chemical component and size distribution of Ti-bearing inclusions, favored the grain refinement of the welding induced, coarse-grained heat affected zone with enhanced impact toughness in Ti-killed steels [3]. In order to enhance the toughness of weld metal, the optimum Ti content of the welded metal in the HSLA steel should be around Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0064.v1 of 150 ppm. However, this amount also depends on the content of other alloying elements because the AF formation is influenced simultaneously by the proper combination of alloying elements [4]. the amount of AF in Ti-B-N system is controlled by the balance between BN as an energetically favourable site for AF nucleation and soluble boron which acts as a hardenability element suppressing grain boundary ferrite formation [5].
Many research has been done on the microstructure transformation of weld metals and mainly focused on the micro-alloys, such as Ti and Zr [6][7][8]. However, there have been few, if any, studies on the effect of rare earth elements on the strength and toughness of high-strength steel weld metal.
Therefore, rare earth oxide is the research object in this paper, and we conduct a systematic study on the effect of Pr6O11 on the microstructures and mechanical properties of high-strength steel weld metal.

Experimental procedure
The flux-cored wire has the advantage of strong adaptability. It is very convenient and easy to adjust the proportion and composition of powder in the flux, and can provide the chemical composition required by the weld. Therefore, the flux-cored wire is used in this experiment.  [9]. The pass arrangement and welding parameters are shown in Figure 1a and Table 1, respectively. The base metal plates were Q235 steel. The reason for selecting Q235 steel is that it is readily available and does not have much alloying elements to add to those in the weld metal. Furthermore, the groove faces and the contacting face of the backing steel of each weld were first buttered no less than two layers using the wire to be used for subsequent welding. Then, the buttered faces were cut down to 3 mm thick at an angle of 22.5°. The preheat and interpass temperature was measured by a digital non-contact laser infrared thermometer gun at the location shown in Figure 1a. After each welding pass, the assembly was cooled in still air at room temperature to 150-160°C without special control of cooling or post-weld heat treating.
The specimens were prepared according to AWS standards A5.29/A5.29M [9] and B4.0 [10]. The groove weld test assembly for mechanical properties is schematically shown in Figure 1a. The round tensile specimens and Charpy V-notch impact specimens are shown in Figures 1b and 1c. After preparing the standard specimens, the mechanical properties of the weld metal were tested with a WAW-6000 tensile test machine, and the yield strength, tensile strength and elongation were recorded. The Charpy V-notch impact test was performed by an JB30B impact testing machine after the impact specimens were cooled to -40°C. The chemical compositions of weld metals were determined with a Q4 optical emission spectrometer. Metallographic specimens were prepared and etched with 4% natal. The microstructure of the weld metal was observed using optical microscopy, and the proportion of the microstructure was calculated. The Vickers hardness of the weld metal was measured using a HXD-1000TMSC/LCD Vickers hardness tester. The fracture surfaces were examined by scanning electron microscopy (SEM) with a Tescan Vega3 microscope, and the inclusions in the dimples were analyzed by energy dispersive spectrometry (EDS).

Microstructure and mechanical properties of weld metals
The chemical compositions and mechanical properties of the weld metals are shown in Table 2 and With the gradual increase in Pr6O11, the low-temperature impact toughness of the weld metals obviously increased, and the low-temperature impact toughness of the weld metal increased first and then decreased.
The impact value of wire No.2 with 1% Pr6O11 was the highest, which was 70 J. The low-temperature impact toughness of the weld metal with 3% Pr6O11 added is basically the same as that of the weld metal improve the low-temperature impact toughness of the weld metal.      The addition of Pr6O11 can promote the formation of AF in the weld metal. The rare-earth elements can both spheroidize and refine inclusions because they usually enrich at inclusions in weld metal deposits [11]. For liquid metals, the new inclusion phases in spherical forms normally have the maximum surface tension under the premise of the same volume. This is mainly ascribed to the fact that spherical inclusions have the minimum thermodynamic driving force during its growing process in weld pool. On the other hand, the amount of inclusions can be enhanced by the rare-earth elements, which accordingly provides more nucleation positions for AF [11,12]. AF is considered an excellent microstructure component to improve the toughness through effective grain refinement, and AF grains can divide large austenite grains into fine individual regions to form a mixed microstructure of fine particles [13,14,15].
The AF microstructure has a large-angle grain boundary, which can effectively increase the crack extension path during fracture. The increase in energy required for crack extension helps improve the low-temperature impact toughness of the weld metal, which can explain the higher impact energy of wire can cause a decrease in strength and an increase in toughness. As the content of Pr6O11 excesses 1%, the inhibition behavior of the nucleation of GB and DUB was revitalized. The difference in the electronegativity between Pr and Fe is very large, which is not conducive to the formation of solid solution in the weld metal. The Pr will preferentially segregate in inclusions [17]. Excessive addition of Pr6O11 can increase the lath structure of the weld metal and lead to the coarsened grain. This is mainly ascribed to the fact that excessive addition of Pr6O11 leads to grain boundary segregation of Pr6O11, causing grain boundary contamination. The segregation of Pr in the grain boundaries and nearby areas reduces the system energy to a metastable state, which is thermodynamically beneficial [17].

Fracture morphology of weld metals
The fractures of different Charpy V specimens at -40°C are shown in Figure 5. With the addition of Pr6O11, the fracture mode changed from quasi-cleavage fracture to ductile fracture with dimples, the tear ridges were scattered between the cleavage fracture and the dimple, the width of the dimples decreased, the depth of the dimples increased, and the distribution of the dimples was even. Dimples could improve the low-temperature impact toughness of the weld metals, which is consistent with the low-temperature impact curve in Figure 2. Without Pr6O11, the dimples had the largest width and smallest depth, and the distribution of dimples was extremely uneven and concentrated in a few areas, which shows that the energy absorbed was low and the toughness was poor, as shown in Figure 5a. With the increase in Pr6O11, the dimple width in the fracture of the specimen decreased, the depth increased, and the distribution was more uniform, as shown in Figure 5b. the most inclusions, and the distribution of inclusions was the most uniform, as shown in Figure 5b. The area of cleavage fracture where the crack source was located in wire No.1 was the largest, and the river pattern was smooth and continuous. The cracks started from between the B clusters and propagated along the adjacent B clusters, and the arrow direction was the crack propagation direction, as shown in Figure   5e, which is consistent with the micrograph in Figures 4a and e. The crack source generated by the decohesion of an inclusion, and the pit after the inclusion fell off appeared at the crack source, as shown in Figure 5f. When the Pr6O11 content increased from 0% to 3%, the average area of the cleavage fracture where the crack source was located gradually decreased first and increased, the river pattern gradually  When Pr6O11 was not added, the cause of the cracks in the weld metal was the boundary cracking of the B clusters caused by the surface shear stress. The small B cluster size determines the high cleavage fracture stress, which delays the occurrence of cleavage fracture and makes the material have higher impact toughness [20]. Figure 5e and h shows the cleavage fracture caused by the boundary cracking of B clusters, which extended to the critical microfracture surface and through the entire specimen. The control condition for crack growth is: σy ≥ σf [21,22] (σy is the normal stress at the front of the gap, and σf is determined by the size of the critical fracture surface). The crack initiated in the coarsest grain zone, the critical event is the propagation of cracks at the coarsest grains. The cleavage fracture mechanism was different from the ductile fracture mechanism; it did not depend on the density of the second phase particles but on the size of the critical microfracture surface controlled by the phase composition of the microstructure. The coarsest B cluster was the weakest area in the microscopic composition, and the critical event of cleavage fracture was confirmed as crack propagation through the boundary of the B cluster. The lath structure was refined with high cleavage fracture stress, which delayed the occurrence of cleavage fracture and improved the impact toughness of the material. Figure 5f show the cleavage fracture caused by the decohesion of an inclusion, and the crack initiation was surface shear stressinduced decohesion of the inclusion. The cracks caused by the decohesion of an inclusion of three uninterrupted stages [21]: (1) the dislocations accumulated at the inclusion particles during material deformation, cracks were generated at the interface between the inclusion and the matrix at the end of the dislocation accumulation, (2) the cracks extend through the grain boundary; (3) the grain cracks pass through the grains The boundary expands into adjacent grains. then, the cracks extended through the entire specimen. The addition of an appropriate amount of Pr6O11 in the weld metal promoted the nucleation of inclusions and the formation of AF, which leaded to a change in the fracture mode of the weld metal and improved the impact toughness.