Research on microstructure and properties of 3D plasma spray welding repair layer of continuous casting segmented rollers

In this paper, the Fe-Cr-Ni-Si powder was used in a 3D plasma spray welding machine to perform spray welding repair tests on continuous casting segmented rollers for service life improvement. The phase composition, hardness distribution, abrasion resistance and thermal stability of the spray welding layer were analyzed. Results indicated that the spray welding layer produced by 160 A welding current was more uniform and was with finer organization. The main phase zone was a single γ-Fe and the fusion zone was metallurgically combined with the base metal in a planar grain manner. The hardness of the spray welding layer was the maximum when the welding current was 160 A. The abrasion resistance for each position of the spray welding layer was uniform. The γ-Fe phase zone decreased and the α-Fe phase zone increased with reduction of average hardness after the thermal stability test. In industrial tests, the optimum spray welding parameters were verified and the service life of the continuous casting segmented rollers with spray welding was 2 longer than rollers using surfacing welding process.


Introduction
In continuous casting, the mould bending moon surface of the continuous casting to the final billet complete solidification, the segmented rollers at positions from meniscus of the mould to the final solidification are subjected to various stresses such as the drum belly force, the alternating thermal stress, the high-temperature billet contact heating and the frictional force [1][2][3].The harsh working environment can easily lead to the failure of the segmented rollers and stainless steel welding electrode overlay process was usually used for segmented rollers repairing.The repaired segmented rollers usually generate small meshes of thermal fatigue cracks [4].When the thermal crack exceeds the critical dimensions, the outer skin of the overlay layer may fall off directly, affecting product quality and efficiency [5,6].Therefore, the development of a new repairing process for continuous casting segmented rollers with low cost is important as it can reduce the repairing time and can improve economic benefits of steel manufactures.
Plasma spray welding can use variety alloy powders according to specific needs of repairing the workpiece [7][8][9].The metallurgical melting between the powder and the base material happened during the spray welding process.The bonding strength is higher and the residual stress after welding is smaller.At the same time, precise control of the shape for the spray welding layer can be achieved using the modelling software.
Lu et al [10] sprayed welded iron-based reinforcement layers with different concentration gradients in inner walls of automotive cylinders by using different compositions of Fe-Cr-M-C powders.The abrasion resistance was 18 times better than that of specimens without concentration gradients.Wang et al [11] prepared a nickelbased abrasion-resistant spray welding layer on the surface of pure titanium using plasma spray welding technology.The main alloying elements, the microstructures and the microhardness along the layer depth were distributed continuously and gradually in the spray welding layer.Comparative tests were carried out with the base material TA1 and results showed that the friction coefficient was reduced and the abrasion resistance was significantly improved.The wear surface of the spray welding layer also exhibited slight abrasive wear characteristics.Xu et al [12] chose WC, Co, and Fe-based alloy composite powders and enhanced coatings were prepared on the surface of 42CrMo steel using the plasma spray welding method.The hardness of the coating increased with the the mass fraction of Fe-based amorphous alloy.The abrasion resistance of the spray welding layer was much higher than that of carbon steels and the abrasion resistance of Fe-20 was better than that of other materials.
In this paper, the continuous casting segmented rollers were repaired using 3D plasma spray welding equipment with homemade low-cost iron-based powder.Different welding parameters were analyzed.The distribution of elements and the distribution of hardness from heat affected zone to the near-surface of the spray welding layer were analyzed and studied.The study is to obtain the process parameters for a significant increase in the service life of the continuous casting segmented rollers.

Materials
The 3D spray welding substrate is a segmented roller used by a slab caster in a steel continuous casting process.The material of the segmented roller is 42CrMo and the composition is shown in table 1.
The self-fusing Fe-Cr-Ni-Si alloy powder used for the 3D spray welding experiment was produced by an atomization process, which was modified from the composition of the electrodes used in the surfacing repair process.A small amount of Ni was added to maintain the austenitic phase zone.

Equipment
The equipment used in the experiment is the PDA-400D1-ST plasma spray welding machine.It consisted of a powder feeding system, plasma spray welding gun, intelligent robot arm and other components.During the plasma spray welding operation, 3D control of the spray welding layer can be implemented through the computer connected to the robot arm.The powder feeding system of the equipment has two independent powder feeding bins, which are respectively connected with two powder feeding holes at the welding torch.It can be used for the functions of mixing different types of powder and feeding powder in a specific proportion.

Methods
The 42CrMo substrate was cut into small pieces with dimensions of 200 × 200 × 10 mm 3 as test samples.After the surface clean, test samples were preheated to about 200 °C and then spray welded.Preheating can slow down the cooling rate after welding and reduce the welding stress to avoid cracks.The transfer arc current in the plasma spray welding process was the most critical parameter controlling the amount of heat being put into the weld and was also a key factor in the quality of the spray welding layer [13].Experiments were conducted with 200, 180, 160 and 140 A transfer arc welding currents to prepare different samples of the spray welding layer.
The spray welding layer sample was polished smoothly with water-abrasive sandpaper.Ultrasonically cleaned in acetone solution was carried out for 10 min and the sample was dried with an air duct for x-ray diffraction analysis.The spray welding layer was made into a sample with a size of 10 × 10 × 5 mm 3 .The microstructure was analyzed using optical microscopy.The microstructure with finer structures was observed using a field emission scanning electron microscope, and the elemental composition of the physical phase was analyzed using Energy Dispersive Spectroscopy (EDS).
The hardness distribution of the spray weld layer was analyzed using the HVS-1000 micro Vickers hardness tester.Test points were taken at an interval of 0.2 mm along the surface, extending to the middle and the base material.The loading force applied during the tests was 100 N with a loading time of 10 s.
The abrasive wear test was carried out by using the ML-100 wear test equipment.The remaining height of the specimen was measured for four sides at 10 min intervals and the average value was taken to characterize the abrasion resistance.The thermal stability test was carried out by heating the specimens to 800 °C (according to the working temperature in the actual production) in a resistance furnace.The sample was held for 2 min and was cooled in water for 30 s.The Specimens was drying with an air blower and was heated to 800 °C again.After repeating this step 100 times, the microscopic morphology was observed.

Results and discussion
3.1.Spray welding layer and heat affected zone structure Figures 1(a), (b), (c) and (d) show microstructure images of weld layer samples obtained using welding currents of 200, 180, 160 and 140 A, respectively.The bright white region on the left side represents the spray weld layer and the black area on the right side corresponds to the 42CrMo base metal.Figure 1 shows that the weld is well bonded to the base metal with a more pronounced fusion zone (depicted by the deep black area).The spray weld layer shows a typical cast structure, i.e. being predominantly comprised of numerous columnar grains and a certain number of equiaxed grains.Figure 1(a) shows the microstructure of the sample obtained using a welding current of 200 A. Due to the high heat input, the image reveals an abundance of dendritic grains perpendicular to the weld direction, along with columnar grains.Additionally, the image demonstrates a larger fusion depth which indicates a higher dilution rate.
In figure 1(b), the columnar grain structure within the spray weld layer is reduced and the anisotropy is observed.The directionality is less pronounced compared to figure 1(a) and a region of parallel columnar grains adjacent to the weld can be observed.At the junction of the spray weld layer and the base material, there is a bright white band (indicated by the arrow in figure 1(b)).This band is caused by the growth of planar grains at the interface, indicating metallurgical fusion bonding between the spray weld layer and the base material.The welding current used to prepare the sample is lower than that used in figure 1(a), resulting in a decrease of temperature gradient and changes in the degree of supercooling.Because as the current decreases, the heat input also decreases, the temperature gradient naturally decreases.There is a change in the degree of subcooling, but it is not large, and such conditions are favorable for the growth of equiaxed grains, so the number of equiaxed grains increases.
Figure 1(c) exhibits a further increase in the number of equiaxed grains and the large-scale columnar grain structure is nearly disappearing.The grain organization appears to be fine and uniform.This phenomenon is attributed to a further reduction in welding current, leading to a decreased temperature gradient approaching the critical point of solidification.At this point, solute redistribution occurs, causing the solidification temperature to rise, but the degree of subcooling does not change, and the degree of subcooling now far exceeds the degree of subcooling needed to form nuclei.So there will be a large number of new nuclei grow out, these nuclei will inhibit the growth of columnar crystals, so the large-scale columnar crystal organization disappears.These new nuclei will also inhibit each other, and eventually form a fine grain and uniform isometric crystal region [14].
Figure 1(d) represents the microstructure of the sample obtained using a welding current of 140 A. Due to the low welding current and heat input, the fusion depth is small.The fusion zone and heat-affected zone in the base material are barely noticeable.Although a shallow planar grainline region is observed at the interface, the edge appears excessively smooth, indicating poor bonding between the spray weld layer and the base material.Additionally, the reduced temperature gradient prevents the composition from achieving the necessary supercooling degree for nucleation, causing the reappearance of columnar grains.
Based on the analysis of the spray weld layer and heat-affected zone structures discussed above, it can be concluded that a welding current of 160 A can give a superior spray weld layer and heat-affected zone structure.

The phase composition of the spray welding layer
Due to the finer and uniform microstructures, only specimens prepared with a welding current of 160 A were chosen for microscopic and EDS analysis.Figure 2 shows the EDS line scan analysis result of the spray-welded layer.The scan was from the top of the weld layer to the heat-affected zone, moving from left to right.It can be observed that the elements on the left do not exhibit prominent peaks, indicating the absence of severe segregation of elements in the spray-welded layer.
The content of a variety of alloying elements in the heat-affected zone on the right side decreases sharply, and the content of iron elements rises, which is in line with the compositional content of the elements in the base material.This indicates that the weld layer and the base material are in a state of metallurgical bonding, but no significant dilution has occurred, and the composition of the weld layer is stable.
Figure 3 illustrates SEM image of the spray-welded layer produced with a welding current of 160 A. The result shows that the spray-welded layer primarily consists of three phases.The gray phase represents equiaxial and columnar grain structures, while the striped white phase is distributed around the columnar grains.The dark gray eutectic phase is dispersed within the white phase in the form of stripes and webs.By examining the EDS spectra in figure 4 and the XRD diffraction results in figure 5, it is determined that the main phases present in the spray-welded layer is a solid solution comprising α-Fe and γ-Fe, with elements such as Mo, Ni, Cr and Si, as well as M 23 C 6 -type carbides [15,16].During the spray welding process, the solidification begins with the equiaxed grain formation of the γ-Fe phase in the molten droplets.Subsequently, the white phase, representing the sub-eutectic organization, precipitates along the grain boundaries of γ-Fe as the eutectic temperature.This is followed by the appearance of striped eutectic microstructure.As the solidification progresses, the eutectic structure condenses into a network, exhibiting typical eutectic characteristics in the weldment layer [17].Through the EDS analysis, the elemental contents in different positions can be determined.The details are presented in table 2. It can be observed that the white phase in figure 4 with spectrogram position 1 and the mesh structure in figure 4 with spectrogram position 2 have chromium (Cr) element content with approximately 24%.In figure 4 with spectrogram position 3, the gray phase area contains about 12% Cr.Since the maximum solid solubility of Cr in γ-Fe is 12.8%, it can be concluded that the gray area with spectrogram position 3 corresponds to γ-Fe.As Cr is infinitely soluble in α-Fe, the white phase with position 1 and the mesh structure with position 2 contain a certain amount of α-Fe phase.
Due to the low carbon content in the powder, the formation of M 7 C 3 -type carbides is not possible.Therefore, Cr precipitates from the austenite phase region and to dissolve into α-Fe forming a Fe-Cr solid solution.It also combines with Fe and C in the white phase region to form (Fe, Cr) 23 C 6 carbides, which are distributed within α-Fe.These carbides have higher hardness, giving an enhancement in hardness and wear resistance of the spray-welded layer.
Molybdenum (Mo) was not detected in figure 4 spectrogram position 3, while the Mo content reached 3.01% and 1.14% with positions 1 and 2, respectively.This can be attributed to the fact that Mo is a ferritestabilizing element and tends to be enriched in the α-phase.Mo is known for forming various carbides such as Mo 2 C, MoC, Mo 7 C 3 and Mo 23 C 6 in steel.However, due to the low carbon content in the powder, the formation of multiple carbides is limited.Combined with the XRD diffraction results in figure 5, it can be inferred that Mo exists only in the form of M 23 C 6 carbides in the spray-welded layer, forming ternary carbides (Fe, Cr, Mo) 23 C 7 .These ternary carbides possess high hardness, contributing to the improved hardness, abrasion resistance and overall strength of the spray-welded layer [18].Although the content of Ni in the powder is up to 3.5%, it is difficult to form stable carbides between Ni and C. Combined with the XRD and EDS results in figure 5, it infers that the Ni element is mainly distributed in γ-Fe matrix to form a solid solution with Fe and Cr, which plays a role in stabilizing the austenite phase [19].At the same time, Si can form a thin film of SiO 2 at the steel surface when the steel is in a high-temperature, which can improve the oxidation resistance of the spray welding layer in the high-temperature service environment [20,21].

The microhardness of spray welding layer
Figure 6 presents microhardness distributions of plasma-sprayed layers at different currents.After calculation, the average hardness of the plasma-sprayed layers for currents 200 A, 180 A, 160 A and 140 A is determined as 568.7 HV, 570.26 HV, 580.9 HV and 552.4 HV, respectively.The hardness of the sprayed layer was maximum at 160 A, while the hardness of the sprayed layer was minimum at 200 A.
The welding current of 160 A is considered the most suitable in terms of heat input.It gives the formation of small and uniformly distributed austenite grains and an increased grain boundary area.Enables more uniform distribution of M 23 C 6 carbides between crystals.The presence of M 23 C 6 carbides significantly contributes to the enhancement of hardness and abrasion resistance, leading to an overall increase in hardness [22].

The abrasion resistance of spray welding layer
Figure 7 shows results of the abrasive wear test conducted on the spray-welded layer using a welding current of 160 A. The hardness of the spray weld layer is highest at this point, bringing good wear resistance.There is no significant difference in wear with various time intervals, indicating that the wear resistance remains consistently the same not excellent.The wear resistance is not deteriorated as the thickness of the weld layer decreases.This can be attributed to the appropriate welding current, which provides an optimal amount of heat input.As a result, the grain structure within the spray-welded layer and the heat-affected zone exhibit small and uniform grain size.The distribution of M 23 C 6 carbides between the grains is enhanced, improving the wear resistance.Consequently, the spray-welded layer maintains a stable level of abrasion resistance.This phenomenon is attributed to the temperature range in which α-Fe transforms into γ-Fe when heated to 800 °C.During this transition, the carbon solubility in α-Fe is limited.As γ-Fe transforms into α-Fe, excess carbon is precipitated, forming carbides through the combination of carbon and iron.With the continued rapid cooling and heating cycles, the precipitation of carbides steadily increases, resulting in the formation of the black spots observed in the Figure 9 displays the microhardness comparison of the spray-welded layer before and after the heat stabilization test.The average hardness of the sprayed layer before the heat stabilization test was calculated to be 580.90HV and it decreased to 535.08 HV after the test.This reduction in hardness can be attributed to the precipitation of a significant amount of carbon during the transition from γ-Fe to α-Fe, leading to the formation of carburization.These carburizers destroy the original organizational structure and make the hardness lower.During the course of the experiment, the M 23 C 6 -type carbides also decompose to form carburites, resulting in a  reduction of the M 23 C 6 -type carbides.The decrease in hardness is associated with a corresponding decrease in the content of M 23 C 6 -type carbides, which are known to enhance the hardness.Thus, the decrease in hardness can be attributed to the changes in the carbide content caused by the precipitation of carbon during the heat stabilization test.At the same time, it can be found that the hardness of the base metal exists a substantial reduction after the experiment, which is because a large amount of carburite is also generated in the base metal, which destroys the original organizational structure.However, there is no hard phase in the base metal like M 23 C 6 -type carbides in the weld layer, so the hardness decreases sharply.
3.6.Practical performance test after spray welding repair of continuous casting segmented rollers Industrial trials were carried out to compare the service life and performance of the continuous casting roller repaired by the same continuous casting machine overlay welding process.Before spray welding, the original surfacing layer of failed continuous casting segmented rollers was processed.The original surfacing layer was first rough turned by a lathe and then finish turned into the base material.Then the 3D plasma spray welding of about 8 mm was performed (as shown in figure 10(a)) and the spray welding layer was processed with a lathe after completion.The final thickness of the spray welding layer obtained is approximately 5 mm.The repaired continuous-casting segmented rollers were annealed with furnace cooling to eliminate residual stress.The spray welding layer after heat treatment is shown in figure 10(b).The average hardness of the spray welding layer reaches 52.4 HRC, which meets the requirements for use in steel mills.When the continuous casting machine was stopped for maintenance, it was found that the skin of the segmented rollers using the surfacing repair process was seriously broken and the total amount of steel product was only 400,000 tons.For the rollers repaired by the spray welding process, the total amount of steel product has reached 600,000 tons and the reticulated thermal fatigue cracks on the surface of the rollers are significantly decreased figure 11.

Conclusions
The conclusions drawn from the analysis and research of the Fe-Cr-Ni-Si spray-welded layer prepared using the 3D plasma spray welding process on a 42CrMo continuous casting sectional roll substrate are as follows: (1) The microstructure of the spray-welded layer, under a welding current of 160 A, exhibits a relatively uniform distribution of equiaxed grains and small-sized columnar grains.Metallurgical bonding is observed between the spray-welded layer and the base material in the form of planar grains within the fusion zone.The main phase is consisted of single-phase γ-Fe.Additionally, (Fe, Cr, Mo)23C6 carbides are  distributed in the intergranular region of the austenitic α-Fe, forming a framework that supports the austenitic matrix.
(2) The hardness of the sprayed layer is highest at a welding current of 160 A. The hardness distribution of the spray-welded layer, as analyzed through line scanning, correlates positively with the distribution of metal carbides.The results of the abrasive wear test indicate that there is no significant difference in wear amounts among the different time intervals.The variation in wear resistance across different locations within the spray-welded layer is minimal.
(3) The thermal stability test reveals that the spray-welded layer changes due to the shrinkage of the γ-Fe phase region and an increase in the α-Fe phase region.This leads to the formation of carburized compounds, resulting in a reduction in hardness.The average hardness decreases by 45.82 HV.
(4) Industrial testing demonstrates that the spray-welding method significantly improves service life of the roller.While the conventional repair process fails after processing over 400,000 tons of steel, the spraywelded repair process extends the service life by at least 50% and allows for processing over 600,000 tons of steel.Moreover, the spray-welded roller surface exhibits fewer net thermal fatigue cracks.

Figure 2 .
Figure 2. The results of EDS line scan analysis of the spray welding layer section at 160 A welding current.

Figure 3 .
Figure 3.The SEM image of the spray welding layer at 160 A welding current.

Figure 4 .
Figure 4.The quantitative analysis of EDS elements in the spray welding layer at 160 A welding current.

Figure 5 .
Figure 5.The XRD analysis result of the spray welding layer at 160 A welding current.

3. 5 .
The thermal stability of spray welding layer Figure8displays the microstructure image for the middle section of the sprayed layer with a spray welding current of 160 A. The sample was subjected to a thermal stability test with 100 cycles of water cooling.The image reveals the presence of numerous black spots at the grain boundaries, indicating the occurrence of carbide precipitation.

Figure 6 .
Figure 6.The HV of the spray welding layers at different currents.

Figure 7 .
Figure 7.The abrasive wear test results of the spray welding layer.

Figure 8 .
Figure 8.The microstructure of the spray welding layer after the thermal stability test.

Figure 9 .
Figure 9.The HV of the spray welding layer after the thermal stability test.

Figure 10 .
Figure 10.(a) The continuous casting segmented rollers after spray welding, (b) The continuous casting segmented rollers after heat treatment.

Figure 11 .
Figure 11.The total amount of steel of the continuous casting segmented rollers repaired by spray welding and surfacing welding of 600,000 tons and 400,000 tons.

Table 2 .
The quantitative analysis table of EDS elements in spray welding layer at 160 A welding current.