*2.2. Materials*

The examinations were performed on a segment of 16Mo3 pressure vessel steel pipe 180 mm in length, 51 mm in outer diameter and the wall thickness of 5 mm surfaced with a singular layer of nickel-based superalloy Inconel 625 through robotized plasma transfer arc and laser surfacing. In both surfacing methods, additional material in the form of metallic powder 50–150 µm in diameter was used. The surfaced layer was not subjected to post surfacing heat treatment. Chemical composition of base and additional material, acc to the manufacturer and obtained by spectrometric analysis, are presented in Tables 1–4. Mechanical properties of nickel-based superalloy weld metal are presented in Table 5.

**Table 1.** Chemical composition of 16Mo3 steel according to manufacturer data (Margo Ltd., Stalowa Wola, Poland).


Notes: 1) carbon equivalent calculated according to International Institute of Surfacing (IIW) guidelines.

**Table 2.** Chemical composition of 16Mo3 steel according to spectrometric analysis.


Notes: In the table mean values of 5 measurements were presented.

**Table 3.** Chemical composition of Inconel 625 superalloy; Böhler L625 (EN NiCr22Mo9Nb) powder according to manufacturer data (voestalpine Böhler Surfacing Germany GmbH, Hamm, Germany).


**Table 4.** Chemical composition of Inconel 625 superalloy, Böhler L625 (EN NiCr22Mo9Nb) powder deposit weld according to spectrometric analysis.


Notes: In the table mean values of 5 measurements were presented, 1) no Tantalum was identified.



Notes: 1) metal in as welded condition (without PWHT), test temperature 20 ◦C, unless otherwise specified.

Prior to surfacing, according to the manufacturer recommendations, the powder was subjected to drying, by baking at a temperature of 200 ◦C for 10 min and mixed in a laboratory planetary stirrer. After preparation, the powder was placed in feeding hopper of a used powder surfacing station. The base material surface was prepared by cleaning the outer pipe surface from rust, mill scale, and grease. The base metal surface preparation consisted of abrasive blasting in cabinet sandblaster followed by mechanized brushing to remove any remaining electro corundum from surface and chemical degreasing. The used abrasive material was electro corundum normal brown with particle diameter 850–1000 µm (F22 acc. Federation of European Producers of Abrasives). The chemical agent used for degreasing was Tetrachloroethylene. Surface roughness parameters measured after preparation were Ra = 12 µm, Rz = 85 µm. The prepared pipes were mounted on an automated surfacing station. The station consisted of a horizontal pipe rotator (with self-centering chucks for pipe mounting) and pipe cooling equipment (with liquid coolant contact on the inner pipe surface). Additional station equipment changed based on surfacing technology applied.

### *2.3. Plasma Processing*

PPTA was carried out with surfacing machine Durweld 300T PTA with machine powder plasma surfacing torch PT 300AUT (Durum Verschleiss-Schutz GmbH, Willich, Germany), mounted on industrial robot Fanuc R-2000iB (FANUC Ltd., Oshino-mura, Japan) arm.

Based on a preliminary surfacing trial, nine parameter sets were used for the fabrication of samples. The initial evaluation of samples has shown the fabrication of surfaced layers of acceptable quality. Optimal parameters of automated PPTA enabling the formation of nickel-based superalloy Inconel 625 surfaced layers on Surface of pressure vessel steel 16Mo3 pipe of sufficient quality are presented in Table 6.


**Table 6.** Processing parameters of robotic plasma powder transferred arc surfacing of Inconel 625 superalloy layer on the outer surface of the 16Mo3 steel pipe.

Notes: 1) defined as the resultant velocity of rotational pipe movement and linear industrial robot manipulator movement parallel to pipe rotation axis, 2) Argon 5.0 (99.999%) acc. ISO 14175—I1: 2009 [43] was used as plasma, shielding and transport gas, 3) calculated acc. to the formula: E\_u <sup>=</sup> <sup>k</sup>·(U <sup>×</sup> I)/v The thermal efficiency coefficient for plasma transferred arc k = 0.6 was used.

### *2.4. Laser Processing*

The powder laser surfacing process was carried out on the robotized station, equipped with a modern laser system for surfacing mounted on a six-axis robot system, ABB IRB 2600 (Asea Brown Boveri, Zurich, Switzerland). A 2 kW 808 ± 5 nm wave length high power direct diode laser Rofin DL020 (ROFIN-SINAR Technologies Inc., Hamburg, Germany) with a rectangular beam with the top-hat intensity distribution in the slow-axis direction and a near Gaussian in the fast-axis direction was used in this study. The laser spot size in the focal plane, measured by the Prometec Laserscope UFF100, was approximately 1.8 mm × 6.8 mm. The fast-axis of the beam spot was set parallel to the traverse direction and the focal plane of the beam was positioned at the surface of the substrate material. The powder was injected directly into the molten pool by an off-axis powder injection system. To ensure a uniform powder distribution on the surface of the molten pool, the geometry of the powder injection nozzle has been fitted to the laser beam spot. Details of the used powder injection system are available elsewhere [12,16]. Argon was used as a shielding gas. In an effort to establish the range of optimal surfacing parameters, a series of single-pass clads have been made at laser powers of 1000, 1200, 1400, and 1600 W with traverse speeds ranging from 2.6 to 4.7 mm/s. The powder feed rate was in the range of 5 to 15 g/min. The optimal surfacing parameters (Table 7) were determined as the parameters providing the single-pass overlapping clad with a uniform and low fusion penetration into the base material.

The schematic of outer 16Mo3 steel pipe surface surfacing with nickel-based superalloy Inconel 625 by powder plasma transferred arc surfacing and powder laser beam surfacing is presented in Figure 1.

Figure 1.

the outer surface of the 16Mo3 steel pipe.


**Table 7.** Processing parameters of robotic laser powder surfacing of Inconel 625 superalloy layer on the outer surface of the 16Mo3 steel pipe. Laser power, (W) 1200 1400 1600 1600 1000 1000 1000 1000 1200 Surfacing velocity 1), (mm/s) 3.9 3.9 3.9 4.7 3.9 3.3 3.3 2.6 2.6

**L1 L2 L3 L4 L5 L6 L7 L8 L9** 

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**Table 7.** Processing parameters of robotic laser powder surfacing of Inconel 625 superalloy layer on

Notes: 1) defined as the resultant velocity of rotational pipe movement and linear industrial robot manipulator movement parallel to pipe rotation axis, 2) Argon 5.0 (99.999%) acc. ISO 14175—I1: 2009 [43] was used as shielding and transport gas, 3) defined as the laser power divided by the traverse speed. The schematic of outer 16Mo3 steel pipe surface surfacing with nickel‐based superalloy Inconel 625 by powder plasma transferred arc surfacing and powder laser beam surfacing is presented in

**Figure 1.** Schematic of outer 16Mo3 steel pipe surface surfacing with nickel‐based superalloy Inconel 625: (**a**) movement of plasma/laser head and pipe rotation, (**b**) arrangement of surfacing passes on the pipe surface. **Figure 1.** Schematic of outer 16Mo3 steel pipe surface surfacing with nickel-based superalloy Inconel 625: (**a**) movement of plasma/laser head and pipe rotation, (**b**) arrangement of surfacing passes on the pipe surface.

#### *2.5. Methodology of Research 2.5. Methodology of Research*

To establish quality and reveal surfacing defects such as cracks, discontinuities, gas pores, non‐ uniform geometry or lack of surfaced layer adhesion visual (VT) and penetrant‐dye (PT) testing were carried out on each sample. Surfaced layers properties evaluation was based on macro and microscopic metallographic examinations, surfaced layer thickness measurements, HAZ measurements, determination of base material content in surfaced layer, chemical composition analysis, and X‐ray diffraction of the most outer area of surfaced layer, microhardness testing and surfaced layer scratch resistance testing. To establish quality and reveal surfacing defects such as cracks, discontinuities, gas pores, non-uniform geometry or lack of surfaced layer adhesion visual (VT) and penetrant-dye (PT) testing were carried out on each sample. Surfaced layers properties evaluation was based on macro and microscopic metallographic examinations, surfaced layer thickness measurements, HAZ measurements, determination of base material content in surfaced layer, chemical composition analysis, and X-ray diffraction of the most outer area of surfaced layer, microhardness testing and surfaced layer scratch resistance testing.

#### 2.5.1. Non‐Destructive Testing 2.5.1. Non-Destructive Testing

Visual and penetrant-dye testing was carried out in accordance with procedures, material, and equipment from normative ISO 17637 [44] and ISO 3452–2 [45]. Visual testing was based on a direct inspection with the naked eye of the surfaced layer surface procedure. Before the testing surface

subjected to observation were cleaned and dried. Penetrant-dye inspection was carried out with the use of color contrast penetrant System Designation Type II, Sensitivity 2 (PT ISO 3452-2 II Cd-2). 2.5.2. Metallographic Examination

use of color contrast penetrant System Designation Type II, Sensitivity 2 (PT ISO 3452‐2 II Cd‐2).

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Visual and penetrant‐dye testing was carried out in accordance with procedures, material, and equipment from normative ISO 17637 [44] and ISO 3452–2 [45]. Visual testing was based on a direct

#### 2.5.2. Metallographic Examination Microscopic examinations were carried out on a cross‐section subjected to typical metallographic specimen preparation. The samples were etched in two stages: steel pipe base

Microscopic examinations were carried out on a cross-section subjected to typical metallographic specimen preparation. The samples were etched in two stages: steel pipe base material structure was revealed by etching in FeCl3Et (Mi19Fe) solution, nickel-based superalloy structure was revealed by electrochemical etching in reagent: 20 cm<sup>3</sup> HCl, 10g FeCl3, 30 cm<sup>3</sup> distilled water. Etching parameters were selected by trial. Observation and recording of macro- and microstructure was carried out on Olympus SZX7 stereoscopic microscope (Olympus Corporation, Tokyo, Japan) and Olympus GX 71 inverted metallographic microscope (Olympus Corporation, Tokyo, Japan). Obtained macroscopic images enabled the determination of surfaced layer thickness, HAZ depth and base material content in the surfaced layer. Chemical composition examinations, including iron content determination, on the surface area of the nickel-based superalloy surfaced layer were done on XRF X-MET8000 Expert mobile spectrometer (Hitachi High-Technologies Corporation, Tokyo, Japan). Precise determination (surface and volumetric) of surfaced layer chemical composition were carried out on ZEISS SUPRA 25 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) by means of Energy Dispersive Spectroscopy (EDS) method. High tension of 15 kV and probe current of 5 nA were used. X-ray diffraction testing, enabling phase content determination in surfaced layer, were carried out on X'Pert Pro PANalytical diffractometer (Malvern Panalytical Ltd., Malvern, UK) Cu (λ = 1.54056) lamp. X-ray diffraction was done in Bragg-Brentano geometry. material structure was revealed by etching in FeCl3Et (Mi19Fe) solution, nickel‐based superalloy structure was revealed by electrochemical etching in reagent: 20 cm3 HCl, 10g FeCl3, 30 cm3 distilled water. Etching parameters were selected by trial. Observation and recording of macro‐ and microstructure was carried out on Olympus SZX7 stereoscopic microscope (Olympus Corporation, Tokyo, Japan) and Olympus GX 71 inverted metallographic microscope (Olympus Corporation, Tokyo, Japan). Obtained macroscopic images enabled the determination of surfaced layer thickness, HAZ depth and base material content in the surfaced layer. Chemical composition examinations, including iron content determination, on the surface area of the nickel‐based superalloy surfaced layer were done on XRF X‐MET8000 Expert mobile spectrometer (Hitachi High‐Technologies Corporation, Tokyo, Japan). Precise determination (surface and volumetric) of surfaced layer chemical composition were carried out on ZEISS SUPRA 25 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) by means of Energy Dispersive Spectroscopy (EDS) method. High tension of 15 kV and probe current of 5 nA were used. X‐ray diffraction testing, enabling phase content determination in surfaced layer, were carried out on X'Pert Pro PANalytical diffractometer (Malvern Panalytical Ltd., Malvern, UK) Cu (λ = 1.54056) lamp. X‐ray diffraction was done in Bragg‐ Brentano geometry.

#### 2.5.3. Hardness Measurements 2.5.3. Hardness Measurements Surfaced layer hardness testing was done using Vickers method on Nexus 423D stationary

Surfaced layer hardness testing was done using Vickers method on Nexus 423D stationary hardness tester (Innovatest Europe BV, Maastricht, Nederland). Hardness testing was carried out acc. to ISO 6507 [46]. The test load used was 300 gf (2.942 N). Hardness tests were performed in 9 test points on polished metallographic transversal crosssection of chosen surfaced layers manufactured by powder plasma surfacing and powder laser surfacing. hardness tester (Innovatest Europe BV, Maastricht, Nederland). Hardness testing was carried out acc. to ISO 6507 [46]. The test load used was 300 gf (2.942 N). Hardness tests were performed in 9 test points on polished metallographic transversal crosssection of chosen surfaced layers manufactured by powder plasma surfacing and powder laser surfacing.

#### 2.5.4. Scratch Test 2.5.4. Scratch Test

To establish scratch resistance and kinetic friction coefficient of surfaced layers, macroscale scratch tests—Revetest (RST) on Revetest Xpress Scratch Tester machine (Anton Paar Instruments, Graz, Austria) were performed. Scratch tests were carried out with the use of Rockwell diamond indenter of radius R = 200 µm. Constant Load Scratch Test (CLST) mode of scratch testing was used. The indenter travel direction was parallel to the pipe axis and velocity was v = 0.3 mm/min on test length l = 5 mm with constant load P = 100 N. Schematic diagram of the scratch test is presented in Figure 2. To establish scratch resistance and kinetic friction coefficient of surfaced layers, macroscale scratch tests—Revetest (RST) on Revetest Xpress Scratch Tester machine (Anton Paar Instruments, Graz, Austria) were performed. Scratch tests were carried out with the use of Rockwell diamond indenter of radius R = 200 µm. Constant Load Scratch Test (CLST) mode of scratch testing was used. The indenter travel direction was parallel to the pipe axis and velocity was v = 0.3 mm/min on test length l = 5 mm with constant load P = 100 N. Schematic diagram of the scratch test is presented in Figure 2.

**Figure 2. Figure 2.** Schematic diagram of Schematic diagram of Inconel 625 nickel-base superalloy surfaced layer scratch test. Inconel 625 nickel‐base superalloy surfaced layer scratch test*.* 

#### **3. Results and Discussion 3. Results and Discussion 3. Results and Discussion**

#### *3.1. Non-Destructive Testing Results 3.1. Non‐Destructive Testing Results 3.1. Non‐Destructive Testing Results*

During surfaced layers manufactured by powder plasma surfacing and powder laser surfacing no surfacing defects of type: cracks (100), surface pores (2017), excessive weld metal (502), incorrect weld toe (505), spatter (602) and other were found. Manufactured surfaced layers had high surface smoothness and symmetry of surfaced seam overlaps (Figure 3). Both powder plasma surfacing and powder laser surfacing enabled the formation of surfaced layers with quality level B. According to ISO 5817 [47] norm level B corresponds to the highest quality of manufactured layers. During surfaced layers manufactured by powder plasma surfacing and powder laser surfacing no surfacing defects of type: cracks (100), surface pores (2017), excessive weld metal (502), incorrect weld toe (505), spatter (602) and other were found. Manufactured surfaced layers had high surface smoothness and symmetry of surfaced seam overlaps (Figure 3). Both powder plasma surfacing and powder laser surfacing enabled the formation of surfaced layers with quality level B. According to ISO 5817 [47] norm level B corresponds to the highest quality of manufactured layers. During surfaced layers manufactured by powder plasma surfacing and powder laser surfacing no surfacing defects of type: cracks (100), surface pores (2017), excessive weld metal (502), incorrect weld toe (505), spatter (602) and other were found. Manufactured surfaced layers had high surface smoothness and symmetry of surfaced seam overlaps (Figure 3). Both powder plasma surfacing and powder laser surfacing enabled the formation of surfaced layers with quality level B. According to ISO 5817 [47] norm level B corresponds to the highest quality of manufactured layers.

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**Figure 3.** Photograph of Inconel 625 nickel‐based superalloy surfaced layer on 16Mo3 steel pipe: (**a**) after surfacing, (**b**) after dye‐penetrant application, removal and developer application (visible pipe length *L* = 180 mm, diameter *D* = 51 mm and thickness *t* = 5 mm). **Figure 3.** Photograph of Inconel 625 nickel-based superalloy surfaced layer on 16Mo3 steel pipe: (**a**)after surfacing, (**b**) after dye-penetrant application, removal and developer application (visible pipe length *L* = 180 mm, diameter *D* = 51 mm and thickness *t* = 5 mm). Photograph Inconel 625 superalloy surfaced on 16Mo3 after surfacing, **b**) penetrant developer (visible length *L* = 180 mm, diameter *D* = 51 mm and thickness *t* = 5 mm).

#### *3.2. Metallographic Test Results 3.2. Metallographic Test Results 3.2. Metallographic Test Results*

Carried out macroscopic examinations of Inconel 625 surfaced layer and 16Mo3 steel pipe base material did not reveal any defect such as: cracks, lack of penetration, gas pores, and other types of discontinuities in fusion line area both powder plasma surfacing and powder laser surfacing. Lack of defects presence in the fusion line area indicates correct parameter selection and sufficient preparation of the pipe surface. Sample macrostructures of the fusion line between nickel‐based superalloy surfaced layer and steel pipe base material are shown in Figure 4. The measured thickness of nickel‐based superalloy layer manufactured by powder plasma surfacing was 1.2–1.7 mm (Table 8). The measured thickness of nickel‐based superalloy layer manufactured by powder plasma surfacing was 0.6–1.3 mm (Table 9). The measured values are lower than recommended 2.5 mm [8,9]; however, taking into consideration the high price of nickel alloy additional material and the difference in heat transfer coefficient between nickel‐ and iron‐based alloys surfaced layers can be considered to be conforming to criteria for exploitation and optimal [35,36]. Surfaced layers were manufactured as a single layer surfaced without post‐weld heat treatment. Carried out macroscopic examinations of Inconel 625 surfaced layer and 16Mo3 steel pipe base material did not reveal any defect such as: cracks, lack of penetration, gas pores, and other types of discontinuities in fusion line area both powder plasma surfacing and powder laser surfacing. Lack of defects presence in the fusion line area indicates correct parameter selection and sufficient preparation of the pipe surface. Sample macrostructures of the fusion line between nickel-based superalloy surfaced layer and steel pipe base material are shown in Figure 4. The measured thickness of nickel-based superalloy layer manufactured by powder plasma surfacing was 1.2–1.7 mm (Table 8). The measured thickness of nickel-based superalloy layer manufactured by powder plasma surfacing was 0.6–1.3 mm (Table 9). The measured values are lower than recommended 2.5 mm [8,9]; however, taking into consideration the high price of nickel alloy additional material and the difference in heat transfer coefficient between nickel- and iron-based alloys surfaced layers can be considered to be conforming to criteria for exploitation and optimal [35,36]. Surfaced layers were manufactured as a single layer surfaced without post-weld heat treatment. out Inconel surfaced and base material did not reveal any defect such as: cracks, lack of penetration, gas pores, and other types of and of defects presence in the fusion line area indicates correct parameter selection and sufficient surface. line nickel superalloy surfaced layer and steel pipe base material are shown in Figure 4. The measured thickness manufactured by 8). The measured thickness of nickel‐based superalloy layer manufactured by powder plasma surfacing 0.6–1.3 recommended mm [8,9]; however, taking into consideration the high price of nickel alloy additional material and the difference in transfer ‐ ‐surfaced be considered to be conforming to criteria for exploitation and optimal [35,36]. Surfaced layers were as a layer surfaced without post

(**a**) (**a**) **Figure 4.** *Cont.*

(**b**)

**Figure 4.** Sample macrostructures of the fusion line between nickel‐based superalloy surfaced layer and steel pipe base material (**a**) powder plasma surfacing (sample P2), (**b**) powder laser surfacing (sample L4). **Figure 4.** Sample macrostructures of the fusion line between nickel-based superalloy surfaced layer and steel pipe base material (**a**) powder plasma surfacing (sample P2), (**b**) powder laser surfacing (sample L4).

**Table 8.** The average thickness of the surfaced layers, iron content in the coating layers and HAZ width for samples obtained in the process of plasma powder transferred arc surfacing (PPTA), Table **Table 8.** The average thickness of the surfaced layers, iron content in the coating layers and HAZ width for samples obtained in the process of plasma powder transferred arc surfacing (PPTA), Table 6.


Surfaced layer base metal content 2), *U* (%) 10.7 5.0 3.2 4.2 3.7 5.1 4.1 4,0 3.8 Iron content in surfaced layer 3), *Fe* (wt. %) 11.8 5.2 4.0 4.8 4.2 5.5 4.5 4.3 4.3 Notes: 1) Sample was not considered consecutive examinations as iron content was to high, 2) base metal content in surfaced layer was calculated by formula U = P/(S + P) where P—fused material area on cross-section S—added material area on cross-section P, 3) mean value of five measurements.

Notes: 1) Sample was not considered consecutive examinations as iron content was to high, 2) base

metal content in surfaced layer was calculated by formula U = P/(S + P) where P—fused material area on cross‐section S—added material area on cross‐section P, 3) mean value of five measurements. **Table 9.** The average thickness of the coating layers, iron content in the coating layers and HAZ width for samples obtained in the process of laser surfacing, Table 7.


Average visible HAZ depth *s*, (µm) 1030 1128 1330 841 1070 1180 1120 1350 1340 Surfaced layer base metal content 2), *U* (%) 11.5 13.6 14.0 3.9 4.8 6.9 12.4 20.6 29.8 Notes: 1) Sample was not considered consecutive examinations as iron content was to high, 2) base metal content in surfaced layer was calculated by formula U = P/(S + P) where P—fused material area on cross-section S—added material area on cross-section P, 3) mean value of five measurements.

Iron content in surfaced layer 3), *Fe* (wt. %) 10.4 14.9 15.1 5.1 6.8 7.5 8.9 19.1 28.7

Notes: 1) Sample was not considered consecutive examinations as iron content was to high, 2) base metal content in surfaced layer was calculated by formula U = P/(S + P) where P—fused material area on cross‐section S—added material area on cross‐section P, 3) mean value of five measurements. Microscopic metallographic examinations revealed the ferritic‐perlitic microstructure of 16Mo3 steel base material (Figures 5d and 6d). In the case of both surfacing technologies in HAZ diversified microstructure was observed from martensitic, through martensitic‐bainitic to ferritic‐bainitic. Moreover, high heating and cooling rates of the base material in the heat affected zone, due to higher achievable surfacing velocity, have lowered grain growth rates observed in the case of powder plasma surfacing (Figures 5c and 6c) [37–39]. In both powder plasma surfacing and powder laser surfacing depth of HAZ was low, a probable cause was liquid cooling of pipe inner surface during the process. Visible HAZ depth evolved with the change of heat input into the material. With the Microscopic metallographic examinations revealed the ferritic-perlitic microstructure of 16Mo3 steel base material (Figures 5d and 6d). In the case of both surfacing technologies in HAZ diversified microstructure was observed from martensitic, through martensitic-bainitic to ferritic-bainitic. Moreover, high heating and cooling rates of the base material in the heat affected zone, due to higher achievable surfacing velocity, have lowered grain growth rates observed in the case of powder plasma surfacing (Figures 5c and 6c) [37–39]. In both powder plasma surfacing and powder laser surfacing depth of HAZ was low, a probable cause was liquid cooling of pipe inner surface during the process. Visible HAZ depth evolved with the change of heat input into the material. With the increase of surfacing linear energy depth of the base material layer subdued to microstructural changes. In the range of examined parameters slightly higher depth of HAZ occurred as a result of powder laser surfacing, but did not exceed 1350 µm (Table 9).

increase of surfacing linear energy depth of the base material layer subdued to microstructural changes. In the range of examined parameters slightly higher depth of HAZ occurred as a result of

powder laser surfacing, but did not exceed 1350 µm (Table 9).

J/mm.

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**Figure 5.** View of the microstructure of Inconel 625 superalloy coating layer obtained in the process of plasma powder transferred arc surfacing on the outer surface of 16Mo3 steel pipe (sample P2): (**a**) coating layer area, mag. 100×, (**b**) general view HAZ area mag. 100×, (**c**) HAZ area at the fusion line, mag. 500×, (**d**) base material, mag. 200×. **Figure 5.** View of the microstructure of Inconel 625 superalloy coating layer obtained in the process of plasma powder transferred arc surfacing on the outer surface of 16Mo3 steel pipe (sample P2): (**a**) coating layer area, mag. 100×, (**b**) general view HAZ area mag. 100×, (**c**) HAZ area at the fusion *Materials*  line, mag. 500 **2020**, *12*, x FOR PEER REVIEW ×, (**d**) base material, mag. 200×. 11 of 17

**Figure 6.** View of the microstructure of Inconel 625 superalloy coating layer obtained in the process of powder laser surfacing on the outer surface of 16Mo3 steel pipe (sample L4): (**a**) coating layer area, mag. 100×, (**b**) general view HAZ area, mag. 100×, (**c**) HAZ area at the fusion line, mag. 500×, (**d**) base material, mag. 200×. **Figure 6.** View of the microstructure of Inconel 625 superalloy coating layer obtained in the process of powder laser surfacing on the outer surface of 16Mo3 steel pipe (sample L4): (**a**) coating layer area, mag. 100×, (**b**) general view HAZ area, mag. 100×, (**c**) HAZ area at the fusion line, mag. 500×, (**d**) base material, mag. 200×.

One of the main factors determining iron content in the surfaced layer is surfacing linear heat input. According to [7,11,15] the linear heat input should no exceed 300 J/mm. In the case of PPTA

other technology parameters unchanged, which resulted in the sufficiently low iron content was 340

is notable.

*3.3. Results of the XRD Analysis*

Surfaced layers in each case were composed of organized, primary, fine-grained microstructure of dendrites and precipitations in interdendritic space—which is typical for nickel-based superalloy Inconel 625. Packets were orthogonal, dendritic, and oriented in the heat dissipation direction (Figures 5a and 6a). The structure was highly uniform and possessed no gas pores or cracks on the microscopic level. Slight grain refinement on the boundary layer between surfaced material and base material was observed. (Figures 5b and 6b). For each layer, slight fusion into the base material (enabling monolithic bonding of the surfaced layer with the pipe outer surface) was observed. One of the base criteria for the outer surface layer of heat exchangers pipe exploitation in biomass and waste fueled furnaces is the assessment of iron content in the surface layer. The guidelines of 7% maximal wg. content of Iron in case of automatic surfacing and 10% maximal wg. content of Iron in case of manual surfacing should not be exceeded [2,10]. Exciding the maximal iron content in the surfaced surface layer cand lead to iron oxide (Fe2O3) formation, which due to discontinuous and lamellar structure are prone to flaking during exploitation [2,10]. Measured iron content in the case of powder plasma surfaced layers, with the exclusion of sample P1, was in the range of 4.0–5.5 wg.% (Table 8), whereas in the case of powder laser surfaced layer only samples L4, L5, L6 had around 7 wg.% of iron (Table 9). EDS chemical composition microanalysis (Figure 7) revealed that the interdendritic area is rich in Niobium content. Iron and Chromium are equally present intra- and interdendritically. Iron presence in the surface layer is due to the mixing of base and additional metal. Chemical composition of interdendritic precipitations suggests the presence of γ/NbC eutectics, probably derivative of Ni2Nb Laves phase. *Materials* **2020**, *12*, x FOR PEER REVIEW 12 of 17

(**a**)

**Figure 7.** Sample BSE image of surfaced layer microstructure with results of EDS point microanalysis, mag. 3000 x, high tension 15 kV (sample L4): (**a**) a view of dendritic structure, (**b**) chemical analysis of intradendritic area (measurement point 1), (**c**) chemical analysis of interdendritic area (measurement point 2), higher content of Nb compared to Inconel 625 powder chemical composition **Figure 7.**Sample BSE image of surfaced layer microstructure with results of EDS point microanalysis, mag. 3000×, high tension 15 kV (sample L4): (**a**) a view of dendritic structure, (**b**) chemical analysis of intradendritic area (measurement point 1), (**c**) chemical analysis of interdendritic area (measurement point 2), higher content of Nb compared to Inconel 625 powder chemical composition is notable.

powder laser surfaced layer and powder plasm surfaced layers examinations main counts peaks from nickel are observed on diffractogram with angles: 2θ = 43.66°, 50.85°, 75.34°, 90.83°, and 96.74°. However, main count peaks for pure nickel crystalline structure from JCPDS‐ICDD database angles are: 2θ = 44.51°, 51.85°, 76.37°, 92.94°, and 98.45°. In the case of analyzed sample counts peaks from crystallographic planes γ (111), γ (200), γ (220), and γ (311) were observed with slightly higher angle 2θ. This difference can be caused by the change of lattice parameters due to the solid solution of Inconel 625 alloying elements in Ni lattice and strengthening phases precipitations. In the surfaced layer Ni‐Si phase with lattice Miller indices (101), (111), (120), (121), (301), and (310) was found. Moreover, in the surfaced layers manufactured by powder plasma surfacing or powder laser

surfacing no other phases with parameters comparable to pure Ni were found.

One of the main factors determining iron content in the surfaced layer is surfacing linear heat input. According to [7,11,15] the linear heat input should no exceed 300 J/mm. In the case of PPTA with intensive liquid cooling of the inner pipe surface, modified heat dissipation mode enabled obtaining of sufficiently low iron content in the surfaced layer when linear surfacing energy was kept under 500 J/mm. Maximum linear heat input into the material in case of powder laser surfacing, with other technology parameters unchanged, which resulted in the sufficiently low iron content was 340 J/mm.

#### *3.3. Results of the XRD Analysis*

X-ray diffractogram for singular surfaced layer of nickel-based superalloy Inconel 625 manufactured by powder plasma surfacing (sample P2) was presented in Figure 8. Similarly in powder laser surfaced layer and powder plasm surfaced layers examinations main counts peaks from nickel are observed on diffractogram with angles: 2θ = 43.66◦ , 50.85◦ , 75.34◦ , 90.83◦ , and 96.74◦ . However, main count peaks for pure nickel crystalline structure from JCPDS-ICDD database angles are: 2θ = 44.51◦ , 51.85◦ , 76.37◦ , 92.94◦ , and 98.45◦ . In the case of analyzed sample counts peaks from crystallographic planes γ (111), γ (200), γ (220), and γ (311) were observed with slightly higher angle 2θ. This difference can be caused by the change of lattice parameters due to the solid solution of Inconel 625 alloying elements in Ni lattice and strengthening phases precipitations. In the surfaced layer Ni-Si phase with lattice Miller indices (101), (111), (120), (121), (301), and (310) was found. Moreover, in the surfaced layers manufactured by powder plasma surfacing or powder laser surfacing no other phases with parameters comparable to pure Ni were found. *Materials* **2020**, *12*, x FOR PEER REVIEW 13 of 17

**Figure 8.** Examples of X‐ray spectrum of overlay weld in the surface of Inconel 625 superalloy layer obtained in the process of plasma powder transferred arc surfacing (sample P2). **Figure 8.** Examples of X-ray spectrum of overlay weld in the surface of Inconel 625 superalloy layer obtained in the process of plasma powder transferred arc surfacing (sample P2).

#### *3.4. Hardness Measurements Test Results 3.4. Hardness Measurements Test Results*

Measured in the course of macroscopic metallographic examinations of powder plasma surfaced pipes and powder laser surfaces pipes, the depth of HAZ (Figure 5) was confirmed in microhardness tests. Mean microhardness HV0.3 results tested on cross‐section on 16Mo3 steel plates surfaced with nickel‐base superalloy Inconel 625 manufactured by surfacing were presented in Tables 10 and 11. The presented hardness results are mean from five measurements done at 0.1 mm intervals in the base material, HAZ, and surfaced layer. The base material was characterized by hardness in range 160–168 HV0.3. In the range of used surfacing parameters, the surfacing thermal cycle increased the hardness of HAZ to maximally 210 HV0.3 in case of plasma surfacing and 260 HV0.3 in case of laser Measured in the course of macroscopic metallographic examinations of powder plasma surfaced pipes and powder laser surfaces pipes, the depth of HAZ (Figure 5) was confirmed in microhardness tests. Mean microhardness HV0.3 results tested on cross-section on 16Mo3 steel plates surfaced with nickel-base superalloy Inconel 625 manufactured by surfacing were presented in Tables 10 and 11. The presented hardness results are mean from five measurements done at 0.1 mm intervals in the base material, HAZ, and surfaced layer. The base material was characterized by hardness in range 160–168 HV0.3. In the range of used surfacing parameters, the surfacing thermal cycle increased the hardness of HAZ to maximally 210 HV0.3 in case of plasma surfacing and 260 HV0.3 in case

The surfaced layer manufactured by powder plasma surfacing was characterized by hardness of over 240 HV0.3, while the hardness of the powder laser surfaced layer decreased sharply, from 261 HV0.3

**Table 10.** Average microhardness HV0.3 measured on the cross‐section of 16Mo3 steel pipes plasma

Base material (16Mo3) 161.5 165.3 163.8 162.7 162.9 163.1 162.9 161.4 160.7 Heat affected zone 209.8 193.4 196.4 199.2 190.5 186.1 182.2 180.3 178.8

(Inconel 625) 242.8 245.4 241.0 246.2 248.7 249.6 242.2 245.1 242.9

**Sample Designation P1 P2 P3 P4 P5 P6 P7 P8 P9 Mean Microhardness HV 0.3** 

to 187 HV0.3, with the increase of base material content in the surfaced layer (Table 11).

clad with Inconel 625 superalloy powder (Table 6).

**Microhardness Test Area** 

Surfaced layer

surfacing. The HAZ microstructure evolved with the increasing distance from the fusion line. Near the fusion line bainitic‐ferritic microstructure with a hardness of around 200–210 HV0.3, as a result of laser surfacing. The HAZ microstructure evolved with the increasing distance from the fusion line. Near the fusion line bainitic-ferritic microstructure with a hardness of around 200–210 HV0.3, as a result of normalization, was present. In the distance of around 1.5 mm from the fusion line, microstructure changed to ferritic-pelitic-bainitic with hardness in the range 170–180 HV0.3. In the distance of 2.0 mm and more from the fusion line, the perlitic-ferritic microstructure of base material was present. The surfaced layer manufactured by powder plasma surfacing was characterized by hardness of over 240 HV0.3, while the hardness of the powder laser surfaced layer decreased sharply, from 261 HV0.3 to 187 HV0.3, with the increase of base material content in the surfaced layer (Table 11).

**Table 10.** Average microhardness HV0.3 measured on the cross-section of 16Mo3 steel pipes plasma clad with Inconel 625 superalloy powder (Table 6).


**Table 11.** Average microhardness HV0.3 measured on the cross-section of 16Mo3 steel pipes laser clad with Inconel 625 superalloy powder (Table 7).

