3.1. Microstructure and Chemical Composition of Remelted Layers
By TIG arc remelting of the samples with the Pulnierpulver nitriding powder on their surfaces, a remelted layers with a significantly different microstructures were formed on the surfaces of the samples of the tested steels compared to the samples that were not remelted. The differences in the microstructure of the remelted layers on particular steel grades (Figure 3a-c) are distinct. They can be attributed to the influence of the respective alloying elements.
The remelted layer on the 25CrMo4 sample (Figure 3a, detailed in Figure 4a) has a dendritic matrix formed by fine martensite and residual austenite, in which large martensite needles are deposited. The microstructures of the remelted layers were etched selectively to avoid over-etching of the interdendritic regions. Such resulting microstructure has been created by the primary formation of a dendritic structure formed by austenite, which subsequently transformed into martensite and residual austenite in the subsolid region by further cooling. The presence of martensite in dendrites has been described in [38-41]. The largest hardening was achieved on the surface of this sample. Needle-shaped martensitic formations are present to a depth of approximately 1 mm from the sample surface. It can be attributed to the positive influence of Cr and Mo on hardenability. EDX analysis (Figure 5) showed the presence of Cr and Mo in the remelted layer. The remelted area was also enriched with N, with the highest N content of all used steels.
The microstructure of the remelted region of the 30CrMoV9 sample has a dendritic character (Figure 3b, detail in Figure 4b), with the martensite needles concentrated locally only in the upper part of the remelted layer. The homogeneity of the remelted area is disturbed by cracks formed during solidification after remelting, resulting likely from a local toughness decrease of the remelted layer in the regions with martensite formed. The cracks in the 30CrMoV9 sample are mainly due to the higher Cr content in the remelted layer (it follows from the EDX analysis in Figure 6) . The content is almost twice the values of Cr content in the other two samples examined. The higher Cr content, with the content of V (not present in other used steels) and N, could cause a reduction of the remelted layer resistance to crack generation. Such a significant toughness reduction leading to crack generation was only local (in two positions shown in Figure 3b). The results of the EDX analysis confirm the presence of Cr and N in the remelted layer, even directly in the martensite (Figure 6), but the presence of V and Mo in the subsurface part of the remelted layer was not confirmed. It is the local absence of V and Mo at the measurement site (area of large needle-like formations). The presence of V and Mo was not detected in all measurements in the subsurface area of the remelted layer with needle-like formations and inside the dendrites (segregation of these elements). V and Mo were demonstrated only in the interdendritic regions).
The matrix of the remelted layer of the 41CrAlMo7 sample is also dendritic, formed by fine martensite and residual austenite, with relatively massive martensite needles segregated continuously to a depth of approximately 0.1 mm below the surface (Figure 3c). The results of the EDX analysis (Figure 8) showed the presence of Cr and N in the remelted region. The presence of Al and Mo in the remelted layer was also confirmed, but only in trace amounts without the potentiality of influencing its final properties.
The differences in the N contents in the particular remelted layers are not marked. The effect of Cr on the solubility of N in Fe is significant at higher Cr contents (above 10 %) . At low Cr contents, being our case, the effect of Cr on N solubility may not be significant. The positive effect of alloying nitride-forming elements (Cr, Al, V) on increasing the N solubility in Fe can also be reduced by the C content, which has an adverse effect on the solubility of N in Fe [44-45]. Disproportions in terms of comparing the Cr and N content in the remelted layers can also be caused by kinetic factors in the time of remelted layer cooling and the non-equilibrium conditions of the surface remelting process.
3.2. Hardness in Remelted Layers
The average hardness values HV 10 found at the measuring points xi in Figure 1 for test specimens without remelting, with remelting in argon 4.6 and with remelting of Pulnierpulver nitriding powder are shown in Figure. 9. The results show that the base hardness (non-remelted steels) was approximately the same for all the steels examined (the difference in values does not exceed 8 %). The measured values of the hardness of particular steels in the non-treated state correspond to the content of C and alloying elements in them.
The remelting in the shielding atmosphere of argon 4.6 alone caused an increase in the average hardness of all the steels examined. The most significant increase in hardness value was found for the 25CrMo4 sample by 246 HV 10 (i.e., by 155.9 %). Samples 30CrMoV9 and 41CrAlMo7 reached almost the same hardness values even after remelting in a shielding atmosphere of argon 4.6 (the difference was only 4 HV 10, i.e., 2.3 %), with the hardness growth by about 120 %. The most significant increase in hardness of 25CrMo4 steel was likely due to the higher N content in the remelted layer, which resulted in a higher level of strengthening (oversaturation) of the solid solution. The influence of Cr and other alloying elements did not significantly affect the hardness value. After remelting the investigated steels in the presence of nitriding powder, a significant increase in average hardness was achieved compared to steels remelted in a shielding atmosphere of argon 4.6. The highest increase was recorded for the 25CrMo4 sample by 502 HV 10 (i.e., by 124.1 %). This increase can be attributed mainly to the enrichment the remelted layer with nitrogen. The nitrogen in the remelted layer contributed to a significant strengthening of martensite in particular, as evidenced by the significantly higher hardness values obtained after remelting the nitriding powder compared to the values after remelting without its presence (Figure 9). Compared to the state without remelting, the sample 25CrMo4 reached the highest increase in average hardness after remelting in the presence of nitriding powder by 748 HV 10 (i.e., by 473.5 %). Such significant increases in hardness of the investigated steels are caused by an increase in hardenability caused by enriching the matrix with nitrogen. Figure 10 shows the dependence of the average values of the hardness HV10 on the average values of the nitrogen content in the remelted layers. There were five measurements performed in each remelted layer. The dependence of hardness on the nitrogen content can be expressed by the equation with the coefficient of determination R2 = 0.995:
The austenite formed by the cooling of the remelted layer contained nitrogen at interstitial positions. Upon transformation of nitrogen-enriched austenite to martensite, nitrogen entered the martensite structure. Since nitrogen was added to the system (from nitriding powder), it can be considered an alloying element. Nitrogen causes the solid solution to strengthen approximately twice as much as carbon .
A graphical representation of the course of hardness as a function of the distance from the surface of the sample obtained from the measurements at the measuring points yi of Figure 1 is shown in Figure 11. The highest hardness values were found on a 25CrMo4 sample, with the highest value reaching 936 HV 10. The increased hardness (above 800 HV 10) was measured to a depth of approximately 1.5 mm, the thickness of the layer with hardness above 900 HV10 corresponding to the layer in which they are present needle-like formations (Figure 3a and 4a, respectively). In the heat-affected zone, there was a relatively sharp decrease in hardness to the level of 326 HV 10 at a depth of 2 mm, while the decrease in hardness to the level of hardness of the base material (165 HV 10) was achieved at a depth of 2.5 mm.
Sample 30CrMoV9 showed a similar hardness profile as sample 25CrMo4, which was characterised by a relatively stable level of hardness to a depth of 1 mm (with a maximum value of 770 HV 10), a slight decrease to a depth of 1.5 mm to 605 HV 10 and a subsequent significant decrease in the heat affected area to the hardness level of the base material (169 HV 10).
The 41CrAlMo7 sample retained increased hardness (exceeding 650 HV10) to a depth of 1.5 mm. The hardness profiles in Figure 11 also show that the reinforcing effect of Mo (in interaction with the higher N content) alone was significantly higher than in the case of its combination with nitride-forming elements Al, resp. V. A more significant strengthening effect of V (in interaction with the higher Cr content) than Al is also visible. The influence of Cr alone on strengthening showed to be less significant.
The lower hardness values of the 30CrMoV9 and 41CrAlMo7 samples (Figure 11) in the remelted layers can be attributed to the fact that no needle-like shapes were present in the measured parts of the remelted layers, in contrast to the 25CrMo4 sample. Only a nitrogen-strengthened dendritic structure was found in these areas. In all cases, we noticed a significant decrease in hardness in the heat-affected zone because the strengthening effect of nitrogen was no longer applied here.
3.3. Abrasive Wear Resistance of Remelted Layers
All examined samples were tested on an abrasive cloth. The degree of abrasion resistance was the average weight loss of the samples achieved by a defined abrasive wear process. The values of the average weight losses of the examined samples are shown in Figure 12. The highest values of average weight losses were achieved in samples of investigated steels without remelting. These values can be taken as a basis for comparing the wear resistance obtained by TIG remelting. The lowest weight loss value was measured in the sample 30CrMoV9 (0.2037 g), which represents a difference of 7.9 % compared to the sample with the highest weight loss (25CrMo4). While the hardness values of the investigated steels without remelting were almost the same (Figure 9) regardless of their chemical composition, the values of the average weight losses show in particular the influence of the Cr content on the abrasion wear resistance. By remelting in a protective atmosphere of argon 4.6, the average weight losses (wear intensity) of all investigated steels decreased. In this case, the lowest weight loss of the sample was 25CrMo4 (0.1669 g), while the weight loss was reduced by 24.1% compared to the no remelting state. The difference between the highest and lowest value of the average weight losses of the samples remelted in the protective atmosphere argon 4.6 was 10.4 %. Significantly the lowest values of average weight losses were achieved by samples of all investigated steels remelted in the presence of nitriding powder. Best resistance to abrasive wear, i. e. the lowest weight loss in the state after remelting in the presence of nitriding powder, was found in the sample 25CrMo4 (0.1131 g), with a decrease in wear intensity of 48.5 % compared to the state without remelting (which is also the most significant percentage decrease in wear intensity of all steels examined) and 32.2 % compared to the state after remelting in a protective atmosphere argon 4.6.
The influence of the content of individual alloying elements is qualitatively similar to that of remelting in a protective atmosphere of argon 4.6. However, this effect is significantly amplified by doping the microstructure with nitrogen. The obtained results show that the changes in the microstructure of steel after remelting nitriding powder (Chapter 3.1) significantly increase the resistance to abrasive wear. The performed microstructural analyses indicate that the nitrogen-saturated matrix influences the abrasive wear resistance increase. This statement is confirmed by the fact that hardness and wear resistance values were significantly lower when remelted without the nitriding powder (only in the protective atmosphere of argon 4.6). EDX demonstrated the presence of N in the layers remelted in the presence of nitriding powder, from which we conclude that the use of the nitriding powder enriched the structure of the remelted layer by N (in all three investigated cases). The hardness values obtained indicate that the microstructure of the remelted layers is formed by martensite.
3.4. Mechanism of Abrasive Wear
The wear mechanisms of the remelted layers were analysed by SEM photographs of the worn surfaces of the examined samples. The worn surface of the 25CrMo4 sample showed a relatively high uniformity of wear and a regular (rectilinear) shape of the abrasive grooves (Figure 13a), which were mainly caused by the micro milling process. The micro-scoring mechanism was used only to a limited extent. Microplastic deformation is not present at all. It was probably due to the presence of nitrogen hardened high hardness martensite in the remelted layer of the 25CrMo4 sample. Samples of 30CrMoV9 and 41CrAlMo7 showed a very similar character to worn surfaces (Figure 13b, c), on which areas with a predominant creasing mechanism were visible, with the presence of unusual formations on both worn surfaces. In both cases, it was probably an abrasive particle trapped in a worn surface with local plastic deformation of its surroundings.
3.5. Evaluation of Objectives Achievement
The main result of the presented research is the verification of the process of layers with increased hardness and abrasive wear resistance forming by steel surface TIG remelting with powders of suitable chemical composition. This work builds on previous research with the same procedure used to create remelted layers containing carboborides. The microstructure of the remelted layers containing carboborides gained a significant hardness and wear resistance increase.
Based on that, we assumed the formation of hard nitrogen-based structures during remelting with the nitriding powder. The process resulted in remelted layers with a matrix significantly enriched in nitrogen. This enrichment was uniform throughout the remelted layer volume. The presence of nitrogen in the matrices of all remelted layers caused a significant hardness increase (five times on average) and abrasion resistance increase (almost twice) compared to not remelted steels. The effect of nitrogen on the increase in hardness and abrasive wear resistance appears to be significant, as evidenced by significantly higher values of the tested properties compared to the values obtained when remelting samples in a protective argon atmosphere.
However, the conditions for forming hard nitrogen-based phases could not be met. It was probably due to the kinetic parameters (high cooling rate not allowing precipitation processes to occur) and the nonequilibrium conditions of the remelting process. In the future, we would like to focus on the kinetic parameters control of the remelted layers cooling to achieve the formation of hard nitrogen-based phases.
The contribution of the paper lies in the demonstration of the possibility of using TIG remelting as a simple, cheap, and technologically accessible method for layers with improved hardness and abrasive wear resistance production. Based on the experiments performed, it is possible to improve this method in the future and achieve an industrial application.