**3. Results and Discussion**

### *3.1. Nondestructive Testing: Visual Testing Results*

During the visual tests of selected hardfaced composite wear-resistant plates produced by the automated FCAW and MMA methods, imperfections of cracks (100) perpendicular to the hardfacing direction and surface spatter (602) were found, as presented in Figure 7. *Materials* **2020**, *12*, x FOR PEER REVIEW 10 of 25

**Figure 7.** Overview and cross‐section of the hardfaced composite wear‐resistant plate produced by the automated flux‐cored arc welding self‐shielded (FCAW‐SS) alloy. **Figure 7.** Overview and cross-section of the hardfaced composite wear-resistant plate produced by the automated flux-cored arc welding self-shielded (FCAW-SS) alloy.

The hardfacing process is an important process associated with welding; therefore, it is necessary for the verification of the applied hardfacing technology according to accepted standards. For hardfaced composite wear‐resistant plates, the ISO 15614‐7 [40] standard is suitable. However, applying this standard precisely can be difficult. In acceptance tests, wear‐resistant hardfaced layers are often unable to conform to acceptable quality levels and cannot be accepted without remarks. Regular transverse cracks in hardfaced layers contribute to the reduction of stress levels in hardfaced elements and can act as a lubricant reservoir. In the context of product application, transverse cracks in the hardfaced layer can be deemed acceptable. The hardfacing process is an important process associated with welding; therefore, it is necessary for the verification of the applied hardfacing technology according to accepted standards. For hardfaced composite wear-resistant plates, the ISO 15614-7 [40] standard is suitable. However, applying this standard precisely can be difficult. In acceptance tests, wear-resistant hardfaced layers are often unable to conform to acceptable quality levels and cannot be accepted without remarks. Regular transverse cracks in hardfaced layers contribute to the reduction of stress levels in hardfaced elements and can act as a lubricant reservoir. In the context of product application, transverse cracks in the hardfaced layer can be deemed acceptable.

#### *3.2. Hardness Measurements Test Results 3.2. Hardness Measurements Test Results*

The results of the mean Rockwell hardness test on the working surface of wear‐resistant plates, arc hardfaced with alloys from groups T Fe15 and T Fe16 and subjected to examinations, were significantly higher compared to the Hardox 400 reference material. The results from the five measurement points for each specimen are presented in Figure 8. The hardness measurements placed the hardness of examined plates in the range of 60–70 HRC (from approximately 700 HV to over 900 HV), which is mostly consistent with additional material characteristics and published data in scientific articles [1,41–44]. Only the hardness of the Alphachrom 700 wear‐resistant plateʹs working surface was lower than that declared by the manufacturer, which could be caused by high dilution of the weld metal. The average value of the hardness test for the PHWP wear‐resistant plate was the lowest among the T Fe16 alloys. The reason for this phenomenon is the high plasticity of the metallic matrix. No major dispersion in the hardness results was observed. The results of the mean Rockwell hardness test on the working surface of wear-resistant plates, arc hardfaced with alloys from groups T Fe15 and T Fe16 and subjected to examinations, were significantly higher compared to the Hardox 400 reference material. The results from the five measurement points for each specimen are presented in Figure 8. The hardness measurements placed the hardness of examined plates in the range of 60–70 HRC (from approximately 700 HV to over 900 HV), which is mostly consistent with additional material characteristics and published data in scientific articles [1,41–44]. Only the hardness of the Alphachrom 700 wear-resistant plate's working surface was lower than that declared by the manufacturer, which could be caused by high dilution of the weld metal. The average value of the hardness test for the PHWP wear-resistant plate was the lowest among the T Fe16 alloys. The reason for this phenomenon is the high plasticity of the metallic matrix. No major dispersion in the hardness results was observed.

**Figure 8.** Comparison of the working surface hardness arc hardfaced with alloys from groups T Fe15

and T Fe16: wear‐resistant plates and the Hardox 400 reference material.

**Figure 7.** Overview and cross‐section of the hardfaced composite wear‐resistant plate produced by

The hardfacing process is an important process associated with welding; therefore, it is necessary for the verification of the applied hardfacing technology according to accepted standards. For hardfaced composite wear‐resistant plates, the ISO 15614‐7 [40] standard is suitable. However, applying this standard precisely can be difficult. In acceptance tests, wear‐resistant hardfaced layers are often unable to conform to acceptable quality levels and cannot be accepted without remarks. Regular transverse cracks in hardfaced layers contribute to the reduction of stress levels in hardfaced elements and can act as a lubricant reservoir. In the context of product application, transverse cracks

The results of the mean Rockwell hardness test on the working surface of wear‐resistant plates, arc hardfaced with alloys from groups T Fe15 and T Fe16 and subjected to examinations, were significantly higher compared to the Hardox 400 reference material. The results from the five measurement points for each specimen are presented in Figure 8. The hardness measurements placed the hardness of examined plates in the range of 60–70 HRC (from approximately 700 HV to over 900 HV), which is mostly consistent with additional material characteristics and published data in scientific articles [1,41–44]. Only the hardness of the Alphachrom 700 wear‐resistant plateʹs working surface was lower than that declared by the manufacturer, which could be caused by high dilution of the weld metal. The average value of the hardness test for the PHWP wear‐resistant plate was the

the automated flux‐cored arc welding self‐shielded (FCAW‐SS) alloy.

matrix. No major dispersion in the hardness results was observed.

in the hardfaced layer can be deemed acceptable.

*3.2. Hardness Measurements Test Results*

**Figure 8.** Comparison of the working surface hardness arc hardfaced with alloys from groups T Fe15 **Figure 8.** Comparison of the working surface hardness arc hardfaced with alloys from groups T Fe15 and T Fe16: wear-resistant plates and the Hardox 400 reference material.

and T Fe16: wear‐resistant plates and the Hardox 400 reference material.

#### *3.3. Abrasive Wear Test Results*

The results obtained in the metal-mineral wear resistance test of selected wear plates hardfaced with alloys from groups T Fe15 and T Fe16 (Table 3) were compared to the wear resistance of a Hardox 400 steel plate, resulting in a relative abrasive wear resistance number. The surface view of the samples after the metal-mineral abrasive wear resistance test performed according to ASTM G65-00: Procedure A is presented in Figure 9. Selected surface areas of the representative samples after the metal-mineral abrasive wear resistance test observed under a confocal microscope and scanning microscope are presented in Figures 10 and 11. Among the composite hardfaced wear-resistant plates examined, two produced by the automated FCAW-GS method, with trade names CastoDur Diamond Plate® 4695 and Vecalloy 752 Plate®, deserve special attention. The corresponding relative abrasive wear resistance was 15 and 19 times higher than the Hardox 400 reference material. The average results of mass loss in the ASTM 65 test of the CastoDur Diamond Plate® 4695 and Vecalloy 752 Plate® plates were presented by Górka et al. [45] and Gucwa et al. [46]. Under the conditions of the experiment performed, the lowest relative metal-mineral abrasive wear resistance was achieved by the hardfaced composite wear-resistant plate under the trade name Hardplate™ 100S (Figure 12). The average value of the metal-mineral wear resistance of the PHWP plate was higher than the result obtained for the CastoDur Diamond Plate® 4695 plate made by NanoAlloy using the FCAW-SS automated hardfacing process.

automated hardfacing process.

*3.3. Abrasive Wear Test Results*

The results obtained in the metal‐mineral wear resistance test of selected wear plates hardfaced with alloys from groups T Fe15 and T Fe16 (Table 3) were compared to the wear resistance of a Hardox 400 steel plate, resulting in a relative abrasive wear resistance number. The surface view of the samples after the metal‐mineral abrasive wear resistance test performed according to ASTM G65‐ 00: Procedure A is presented in Figure 9. Selected surface areas of the representative samples after the metal‐mineral abrasive wear resistance test observed under a confocal microscope and scanning microscope are presented in Figures 10 and 11. Among the composite hardfaced wear‐resistant plates examined, two produced by the automated FCAW‐GS method, with trade names CastoDur Diamond Plate® 4695 and Vecalloy 752 Plate®, deserve special attention. The corresponding relative abrasive wear resistance was 15 and 19 times higher than the Hardox 400 reference material. The average results of mass loss in the ASTM 65 test of the CastoDur Diamond Plate® 4695 and Vecalloy 752 Plate® plates were presented by Górka et al. [45] and Gucwa et al. [46]. Under the conditions of the experiment performed, the lowest relative metal‐mineral abrasive wear resistance was achieved by the hardfaced composite wear‐resistant plate under the trade name Hardplate™ 100S (Figure 12). The average value of the metal‐mineral wear resistance of the PHWP plate was higher than the result

**Figure 9.** Surface view of the representative sample after the metal‐mineral abrasive wear resistance results performed according to ASTM G65‐00: Procedure A. Samples from the: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000 wear plate; (**c**) Kalmetall W 143 wear plate; (**d**) CDP*®* 1001 wear plate; (**e**) Abradur 64 wear plate; (**f**) CDP*®* 4624 wear plate; (**g**) HCCr wear plate; (**h**) CDP*®* 4666 wear plate; (**i**) HCNb4B wear plate; (**j**) CDP*®* 4695 wear plate; (**k**) Vecalloy 752 Plate wear plate; (**l**) PHWP wear plate; (**m**) Hardox 400 abrasion‐resistant steel. **Figure 9.** Surface view of the representative sample after the metal-mineral abrasive wear resistance results performed according to ASTM G65-00: Procedure A. Samples from the: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000 wear plate; (**c**) Kalmetall W 143 wear plate; (**d**) CDP® 1001 wear plate; (**e**) Abradur 64 wear plate; (**f**) CDP® 4624 wear plate; (**g**) HCCr wear plate; (**h**) CDP® 4666 wear plate; (**i**) HCNb4B wear plate; (**j**) CDP® 4695 wear plate; (**k**) Vecalloy 752 Plate wear plate; (**l**) PHWP wear plate; (**m**) Hardox 400 abrasion-resistant steel.

(**a**)

wear plate; (**m**) Hardox 400 abrasion‐resistant steel.

(**k**) **(l)**

(**m**) **Figure 9.** Surface view of the representative sample after the metal‐mineral abrasive wear resistance results performed according to ASTM G65‐00: Procedure A. Samples from the: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000 wear plate; (**c**) Kalmetall W 143 wear plate; (**d**) CDP*®* 1001 wear

**Figure 10.** View of the surface of the representative sample after the metal‐mineral abrasive wear resistance test observed under a confocal microscope: (**a**) HCCr wear plate; (**b**) CDP*®* 4666 wear plate; (**c**) PHWP wear plate. **Figure 10.** View of the surface of the representative sample after the metal-mineral abrasive wear resistance test observed under a confocal microscope: (**a**) HCCr wear plate; (**b**) CDP® 4666 wear plate; (**c**) PHWP wear plate.

(**a**)

(**c**) PHWP wear plate.

(**b**)

(**c**)

**Figure 11.** View of the surface of the representative sample after the metal‐mineral abrasive wear resistance test observed under a scanning electron microscope: (**a**) HCCr wear plate; (**b**) CDP*®* 4666 wear plate; (**c**) PHWP wear plate. **Figure 11.** View of the surface of the representative sample after the metal-mineral abrasive wear resistance test observed under a scanning electron microscope: (**a**) HCCr wear plate; (**b**) CDP® 4666 wear plate; (**c**) PHWP wear plate. Hardox <sup>400</sup> p01 p02 116.2260 116.7526 114.7526 115.2773 1.4734 1.4753 1.4744 7.7620 189.9510 1.00 ‐ Remarks: <sup>1</sup> Mass loss in 30 min; <sup>2</sup> relative abrasive wear resistance to Hardox 400 steel; <sup>3</sup> surface layer manufactured in a three‐pass process; <sup>4</sup> surface layer manufactured in a one‐pass process.

**Weld Metal, %**

p02 160.5386 160.3787 0.1599 0.1777 7.4756 23.7707 7.99 22.8 CDP <sup>4666</sup> p01 p02 161.9005 164.5962 161.7996 164.4729 0.1009 0.1233 0.1121 7.4894 14.9678 12.69 20.7 **Figure 12.** Relative metal‐mineral abrasive wear resistance (ASTM 65‐00, procedure A) of wear plates hardfaced with alloys from groups T Fe15 and T Fe16 in relation to Hardox 400 abrasive wear resistance. **Figure 12.** Relative metal-mineral abrasive wear resistance (ASTM 65-00, procedure A) of wear plates hardfaced with alloys from groups T Fe15 and T Fe16 in relation to Hardox 400 abrasive wear resistance.

0.1200 0.1091 7.3559 14.8316 12.81 21.3

0.1092 0.0993 7.7208 12.8614 14.77 22.5

0.0682 0.0758 7.6816 9.8677 19.25 22.7

0.0982

0.0894

0.0834

HCNb4B p01

CDP <sup>4695</sup> p01

Vecalloy <sup>752</sup> p01

(EDS) (Figure 15).

p02

p02

p02

174.3838 176.8729

p02 163.5409 163.4401 0.1008

155.4632 155.8611

167.8436 168.2761 174.2856 176.7529

155.3738 155.7519

*3.4. Metallographic Test Results and Results of the X‐ray Diffraction Analysis*

167.7602 168.2079

(**a**) (**b**)

PHWP <sup>4</sup> p01 161.8431 161.7440 0.0991 0.0999 7.7112 12.9617 14.65 17.2

The microscopic metallographic examinations enable the determination of the microstructures of the plated layer of wear‐resistant plates hardfaced with alloys from groups T Fe15 and T Fe16 and the Hardox 400 steel reference material (Figure 13). Moreover, X‐ray diffraction analysis allowed

#### *3.4. Metallographic Test Results and Results of the X-ray Di*ff*raction Analysis* **Figure 12.** Relative metal‐mineral abrasive wear resistance (ASTM 65‐00, procedure A) of wear plates

p02 163.5409 163.4401 0.1008

114.7526 115.2773

116.2260 116.7526

Hardox <sup>400</sup> p01 p02

*Materials*

The microscopic metallographic examinations enable the determination of the microstructures of the plated layer of wear-resistant plates hardfaced with alloys from groups T Fe15 and T Fe16 and the Hardox 400 steel reference material (Figure 13). Moreover, X-ray diffraction analysis allowed determining the phase composition. The selected diffractograms are presented in Figure 14. The results of X-ray diffraction analysis were confirmed by means of energy-dispersive spectroscopy (EDS) (Figure 15). hardfaced with alloys from groups T Fe15 and T Fe16 in relation to Hardox 400 abrasive wear resistance. *3.4. Metallographic Test Results and Results of the X‐ray Diffraction Analysis* The microscopic metallographic examinations enable the determination of the microstructures of the plated layer of wear‐resistant plates hardfaced with alloys from groups T Fe15 and T Fe16 and the Hardox 400 steel reference material (Figure 13). Moreover, X‐ray diffraction analysis allowed determining the phase composition. The selected diffractograms are presented in Figure 14. The results of X‐ray diffraction analysis were confirmed by means of energy‐dispersive spectroscopy (EDS) (Figure 15).

*Materials* **2020**, *12*, x FOR PEER REVIEW 15 of 25

Remarks: <sup>1</sup> Mass loss in 30 min; <sup>2</sup> relative abrasive wear resistance to Hardox 400 steel; <sup>3</sup> surface layer manufactured in a three‐pass process; <sup>4</sup> surface layer manufactured in a one‐pass process.

1.4753 1.4744 7.7620 189.9510 1.00 ‐

1.4734

**Figure 13.** *Cont.*

*Materials* **2020**, *12*, x FOR PEER REVIEW 17 of 25

*Materials* **2020**, *12*, x FOR PEER REVIEW 17 of 25

T Fe15, T Fe16, and the reference material: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000 wear plate; (**c**) Kalmetall W 143 wear plate; (**d**) CDP*®* 1001 wear plate; (**e**) Abradur 64 wear plate; (**f**) CDP*®* 4624 wear plate; (**g**) HCCr wear plate; (**h**) CDP*®* 4666 wear plate; (**i**) HCNb4B wear plate; (**j**) CDP*®* 4695 wear plate; (**k**) Vecalloy 752 Plate wear plate; (**l**) wear plate PHWP; (**m**) Hardox 400 abrasion‐resistant steel. **Figure 13.** Microstructure of wear-resistant material of wear plates hardfaced with alloys from groups T Fe15, T Fe16, and the reference material: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000 wear plate; (**c**) Kalmetall W 143 wear plate; (**d**) CDP® 1001 wear plate; (**e**) Abradur 64 wear plate; (**f**) CDP® 4624 wear plate; (**g**) HCCr wear plate; (**h**) CDP® 4666 wear plate; (**i**) HCNb4B wear plate; (**j**) CDP® 4695 wear plate; (**k**) Vecalloy 752 Plate wear plate; (**l**) wear plate PHWP; (**m**) Hardox 400 abrasion-resistant steel. T Fe15, T Fe16, and the reference material: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000 wear plate; (**c**) Kalmetall W 143 wear plate; (**d**) CDP*®* 1001 wear plate; (**e**) Abradur 64 wear plate; (**f**) CDP*®* 4624 wear plate; (**g**) HCCr wear plate; (**h**) CDP*®* 4666 wear plate; (**i**) HCNb4B wear plate; (**j**) CDP*®* 4695 wear plate; (**k**) Vecalloy 752 Plate wear plate; (**l**) wear plate PHWP; (**m**) Hardox 400 abrasion‐resistant steel.

**Figure 13.** Microstructure of wear‐resistant material of wear plates hardfaced with alloys from groups

**Figure 13.** Microstructure of wear‐resistant material of wear plates hardfaced with alloys from groups

**Figure 14.** *Cont.*

*Materials* **2020**, *12*, x FOR PEER REVIEW 18 of 25

**Figure 14.** Selected diffractograms of the wear‐resistant material of wear plates hardfaced with alloys from groups T Fe15 and T Fe16: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000; (**c**) Kalmetall W 145 wear plate; (**d**) Abradur 64; (**e**) CDP® 4624 wear plate; (**f**) HCCr wear plate; (**g**) CDP® 4695 wear plate; (**h**) Vecalloy 752 Plate® wear plate; (**i**) PHWP wear plate. **Figure 14.** Selected diffractograms of the wear-resistant material of wear plates hardfaced with alloys from groups T Fe15 and T Fe16: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000; (**c**) Kalmetall W 145 wear plate; (**d**) Abradur 64; (**e**) CDP® 4624 wear plate; (**f**) HCCr wear plate; (**g**) CDP® 4695 wear plate; (**h**) Vecalloy 752 Plate® wear plate; (**i**) PHWP wear plate. from groups T Fe15 and T Fe16: (**a**) Hardplate™ 100S wear plate; (**b**) Alphachrom 7000; (**c**) Kalmetall W 145 wear plate; (**d**) Abradur 64; (**e**) CDP® 4624 wear plate; (**f**) HCCr wear plate; (**g**) CDP® 4695 wear plate; (**h**) Vecalloy 752 Plate® wear plate; (**i**) PHWP wear plate.

**Figure 14.** Selected diffractograms of the wear‐resistant material of wear plates hardfaced with alloys

(**a**) (**a**)

**Figure 15.** *Cont.*

*Materials* **2020**, *12*, x FOR PEER REVIEW 19 of 25

results of EDS (Energy Dispersive Spectroscopy) point microanalysis, mag., 15,000×, high tension, 20 kV (PHWP sample): (**a**) view of the carbide structure; (**b**) point chemical analysis of the study area (measurement points: 1, 3, 5). The metallographic analysis of the wear‐resistant plates hardfaced with an abrasion‐resistant layer of group T Fe 15 alloys revealed the existence of the martensitic Fe‐Cr‐C ternary system phase **Figure 15.** Sample BSE (Back Scattered Electrons) image of the surface layer microstructure with the results of EDS (Energy Dispersive Spectroscopy) point microanalysis, mag., 15,000×, high tension, 20 kV (PHWP sample): (**a**) view of the carbide structure; (**b**) point chemical analysis of the study area (measurement points: 1, 3, 5).

in all of the samples and an austenitic Fe‐Cr‐C ternary system phase in some of the samples. These

**Figure 15.** Sample BSE (Back Scattered Electrons) image of the surface layer microstructure with the

The metallographic analysis of the wear-resistant plates hardfaced with an abrasion-resistant layer of group T Fe 15 alloys revealed the existence of the martensitic Fe-Cr-C ternary system phase in all of the samples and an austenitic Fe-Cr-C ternary system phase in some of the samples. These two phases formed a metal matrix. For the hardfaced layers with a high chromium cast alloy composition, it is presumed that a primary austenitic phase was partially transformed into a martensitic phase during the hardfacing thermal cycle. The degree of transformation varied among the examined wear-resistant plates. Several metal carbides can be observed in the X-ray diffractogram patterns. The main identified metal carbide in all the layers produced by hardfacing with alloys from group T Fe15 is the primary Cr7C<sup>3</sup> carbide, and Cr23C<sup>6</sup> eutectic carbides can sometimes be observed. Moreover, in the abrasion-resistant layers of Kalmetall W 143, Abradur 64, CastoDur Diamond Plate® 4666, and HCNb4B plates, indices of NbC phases were found. The microstructures of abrasion-resistant layers hardfaced with alloys from group T Fe16 demonstrated a more complex composition characterized by the presence of borocarbides and molybdenum borides.

The hardfaced layer of the Hardplate™ 100S wear-resistant plate (Figure 13a) was characterized by a chromium cast iron structure with a fine Cr7C<sup>3</sup> carbide precipitation of approximately 20 µm and a microhardness in the range of 950–1450 HV, uniformly distributed in the austenitic matrix (Figure 14a). The weld metal dilution was around 22%.

A similar microstructure was observed in the Alphachrom 7000 wear-resistant plate (Figure 13b). The hardfaced layer was characterized by an austenitic structure with a precipitation of primary Cr7C<sup>3</sup> chromium carbides oriented perpendicularly to the surface (Figure 14b). The weld metal dilution was the highest and exceeded 25%.

In contrast, the hardfaced layer of the Kalmetall W 145 wear-resistant plate (Figure 13c) was identified as a chromium and carbon-rich iron alloy with a hard martensitic matrix structure, with 30 a vol % of primary Cr7C<sup>3</sup> chromium carbides, a microhardness around 2200 HV, and niobium carbides NbC with a microhardness around 2400 HV (Figure 14c). The carbides were oriented parallel to the working surface, which increased abrasive wear resistance. The calculated weld metal dilution was under 24%.

The microstructure of the hardfaced layer of the CastoDur Diamond Plate® 1001 wear-resistant plate (Figure 13d) was composed of primary Cr7C<sup>3</sup> chromium carbides with a microhardness in the range of 1500–2200 HV and M2B metal borides with a microhardness of 1800 HV, regularly and densely situated in a plastic metal matrix. The base metal content was low, which promotes uniform and high abrasive wear resistance on the whole cross-section [26,47]. The hardfaced layer was distinguished by the lowest weld metal dilution rate of just over 19%. Abrasive wear of the surface layer of the wear plate resulted from microcutting. In the area of the abrasion, material discontinuities were present, caused by shearing unevenness of the surface with abrasive. The hardfaced layer of the Abradur 64 wear-resistant plate was characterized by a hypereutectic microstructure with chromium and niobium carbide precipitations (Figure 13e). The metallographic examination revealed the existence of a buffer layer with a high chromium, nickel, and boron content, presumed to act as a barrier for crack development. The MMA hardfaced wear-resistant layer of the Abradaur 64 plate, similar to the CastoDur Diamond Plate® 1001 plate, was characterized by a uniform distribution of chromium and niobium carbides on the whole cross-section (Figure 14d). The weld metal dilution under 21%.

The hardfaced layer of the CastoDur Diamond Plate® 4624 wear-resistant plate (Figure 13f) was characterized by a high chromium cast alloy microstructure of approximately 30 vol % of primary Cr7C<sup>3</sup> chromium carbides (Figure 14e) 1800–2000 HV in hardness, uniformly distributed in the matrix. The carbide volume and distribution positively impacted the abrasive wear resistance of the plate. The weld metal dilution was slightly above 21%.

In contrast, the matrix of the hardfaced layer of the HCCr wear-resistant plate (Figure 13g) was austenitic in structure, with a high volume (over 50 vol %) of precipitations, mainly Cr7C<sup>3</sup> chromium carbides (Figure 14f). Due to the high chromium and carbon content in the weld, a metal hardfaced layer with high abrasion resistance was achieved. The weld metal dilution was under 23%.

The above-characterized wear-resistant layers were subjected to intensive abrasive wear. The volumetric material loss was in the range of 23–32 mm<sup>3</sup> . The wear mechanism was deeply grooved, with plastic deformation of the contact area and raising of both groove edges (Figure 9a,b) or microcutting (Figure 9c–g). The wear was accompanied by the formation of surface defects, e.g., scuff marks or microcraters.

The hardfaced layer of the CastoDur Diamond Plate® 4666 wear-resistant plate (Figure 13h) was characterized by a supereutectic ferrous alloy microstructure with a high chromium content. The microstructure was composed of a high fraction (over 50 vol %) of hard (2000–2100 HV) Cr7C<sup>3</sup> chromium carbides, niobium carbides NbC, and chromium borides Cr2B, as well as other metal compounds distributed in a hard–austenitic matrix. The calculated weld metal dilution was under 21%.

The hardfaced layer of the HCNb4B plate (Figure 13i) was characterized by an alloyed austenitic microstructure with optimally oriented Cr7C<sup>3</sup> chromium carbide precipitations and hard niobium carbides NbC and a hardness of approximately 1800 HV. The weld metal dilution was over 21%.

The structure of overlays produced on the CastoDur Diamond Plate® 4695 wear plate (single weld layer on a carbon steel substrate) is presented with eight micron-scale images in Figure 13j. The high volume fraction of the ultrahard complex borocarbides (M23(BC)6), metal carbides (MC), and metal borides (M2B) is finely distributed within a mesomorphous α–Fe alloy matrix (Figure 14g). The abrasion-resistant layer possessed very high abrasive wear resistance due to the in situ formation of a high fraction of the complex borocarbide phase (−70 vol %). The weld metal dilution was over 22%.

The FCAW-GS hardfaced layer of the Vecalloy 752 Plate® wear-resistant plate was unique in the microstructure of the fine martensitic ferrous alloy (Figure 13k). The microstructure was similar to composite layers reinforced with tungsten carbide (WC). The microstructure was composed of cubic molybdenum borides (33–50 vol %), with a hardness on the level of tungsten carbide, embedded in a hard martensitic matrix with a high fraction of primary and eutectic Cr7C<sup>3</sup> and Cr23C<sup>6</sup> chromium carbides, niobium carbides, and metal borides (Figure 14h). The size of the complex molybdenum borides was around 10 µm, with spacing lower than 50 µm. The weld metal dilution was around 23%. The slightly lower abrasion wear resistance of the wear plate CDP 4695 compared to the Vecalloy 752 wear plate can be explained by the lesser volume fraction of stable borocarbides, carbides, and borides in the deposit.

The hardfaced layer of the PHWP wear-resistant plate (Figure 13l) was characterized by a highly alloyed chromium carbide microstructure with a high concentration of complex carbides. The microstructure was composed of a high eutectic fraction of hard (1000–1600 HV) Cr23C<sup>6</sup> chromium carbides; niobium carbides NbC, with a hardness of approximately 1800 HV and molybdenum borides Mo2B; and other metal compounds distributed in a plastic austenitic–martensitic matrix (Figure 14i).

Advantageous properties under the conditions of abrasion by mineral grains and erosion were also revealed by padding welds made of iron-based materials in the Fe-Cr-C-Mo-B alloy. The addition of boron increases the flowability and wettability of the metal and lowers the melting point. The abrasive wear resistance of borides was much higher than that of carbides. According to Hejowski [36], an increase in the boron content with constant chromium content increases the volume fraction and hardness of the primary phases. The calculated weld metal dilution was the smallest and amounted to 17%.

The wear mechanism of the wear-resistant layers presented in Figure 9h–l consisted only of the microcutting of the contact surface by abrasive grains. A volumetric loss of 10 to 15 mm<sup>3</sup> was measured. As a result of the wear process, strengthening phases embedded in the ductile matrix were revealed.

#### *3.5. Linear Regression Model*

In the majority of cases, the increase in hardness coincides with the increase in wear resistance, especially in hardfaced wear plates. Research conducted by Marulanda-Arévalo et al. [1] and Ban et al. [4] demonstrated a strong linear relationship between the hardness and the wear resistance of the hardfaced wear plates. ASTM G 64 tests were performed on samples that had undergone a grinding treatment. The surface condition of the wear plate was taken into account for the purpose of determining wear resistance.

The obtained abrasive wear resistance test results of arc-hardfaced alloys from groups T Fe15 and T Fe16 were used to obtain a mathematical linear regression model describing the dependence between metal-mineral wear resistance and the hardness of hardfaced layers. A linear predictor function with an equation in the form y = a + bx, where x is the explanatory variable (surface hardness) and y is the dependent variable (abrasive wear resistance), was used (Figure 16).

and y is the dependent variable (abrasive wear resistance), was used (Figure 16).

The obtained abrasive wear resistance test results of arc‐hardfaced alloys from groups T Fe15 and T Fe16 were used to obtain a mathematical linear regression model describing the dependence between metal‐mineral wear resistance and the hardness of hardfaced layers. A linear predictor

**Figure 16.** Regression and correlation analysis between matrix hardness and abrasive wear resistance. **Figure 16.** Regression and correlation analysis between matrix hardness and abrasive wear resistance.

Pearson's r correlation coefficient was calculated for the proposed model and was equal to 0.8702, which indicates the high accuracy of the description of the linear model between the hardness of the layer and its resistance to abrasive wear. The calculated coefficient of determination R² = 0.7572 proves the accuracy of the model. The proposed mathematical linear regression model describes the variability of the dependent variable (abrasive wearresistance) and the independent variable (surface layer hardness) by nearly 80%. Pearson's r correlation coefficient was calculated for the proposed model and was equal to 0.8702, which indicates the high accuracy of the description of the linear model between the hardness of the layer and its resistance to abrasive wear. The calculated coefficient of determination R<sup>2</sup> = 0.7572 proves the accuracy of the model. The proposed mathematical linear regression model describes the variability of the dependent variable (abrasive wear resistance) and the independent variable (surface layer hardness) by nearly 80%.

#### **4. Conclusions 4. Conclusions**

The aim of this research was to compare abrasive wear resistance and determine the linear correlation between surface hardness and the metal‐mineral wear resistance of 11 commercially produced and industrially applied wear‐resistant composite plates and hardfaced layers produced by a patented covered tubular electrode with a special chemical composition of the metallic core. The comparative analysis allowed for the formulation of the following conclusions: The aim of this research was to compare abrasive wear resistance and determine the linear correlation between surface hardness and the metal-mineral wear resistance of 11 commercially produced and industrially applied wear-resistant composite plates and hardfaced layers produced by a patented covered tubular electrode with a special chemical composition of the metallic core. The comparative analysis allowed for the formulation of the following conclusions:


M2B metal borides, or nearly cubic complex Mo2B molybdenum borides, Cr7C<sup>3</sup> primary chromium carbides, Cr23C<sup>6</sup> eutectic chromium carbides, and Nb6C<sup>5</sup> niobium carbides densely distributed in a martensitic matrix.


In the future, the results of laser and plasma layers hardfaced with alloys from the Ni20 group and metal-mineral abrasive wear resistance will be published by the author.

**Funding:** The research was founded by the Silesian University of Technology Rector's proquality grant and the Faculty of Mechanical Engineering Silesian University of Technology Dean's proquality grant.

**Conflicts of Interest:** The author declares no conflict of interest.
