Tool steels

Two different tool steel compositions were laser clad and tested, WR10 (Fe-12.5Cr-1.1Mo-4.0V-2.3C in wt.%) and WR6 (Fe-5.3Cr-1.3Mo-11.5V-2.9C in wt.%), the latter being substantially better in abrasion wear resistance (950 vs. 113 mg). At the same time, it was the best monolithic laser coating studied here. According to XRD analysis WR10 powder consisted predominantly of austenite, whereas laser coating consisted of austenite,

Table 24. Influence of matrix material on abrasion wear resistance and microhardness.

Coating Volume fraction

Carbide size (μm)

Mass loss (mg)

Volume loss (mm³)

STDE V

COV (%)

HV₁ HV_0.3 Metco 16C +

CrC

(70/30)

(80/20)¹ 30-106 87 12.0 11.9²

15 17 840 870³ 1500⁴ Stellite 6 +

CrC

(65/35) (83/17)¹

30-106 116 14.8

14.2²

26 24 920 830³ 1590⁴ 1) volume ratio according to image analysis

2) volume loss calculated on the basis of average matrix/carbide volume ratio in coating measured with image analysis

3) matrix microhardness 4) carbide microhardness

Figure 121. Optical micrographs taken from the transverse cross-section (perpendicular to cladding direction) of Stellite 6 + 70C-NS (65/35 vol.%) coating.

Figure 122. SEM micrographs taken from the wear scar of Stellite 6 + 70C-NS (65/35 vol.%) coating. Rubber wheel rotated from left to right.

martensite/ferrite and only small amounts of VC (actually the peaks coincided with V4C3, V8C7 and VC0.88/V32C28). Austenite to martensite/ferrite ratio, as evaluated roughly from the integrated peak areas (γ(111) vs. α(110)), was 9 for powder and 1.3 for laser coating. These peaks were the highest ones and no preferred orientation was detected. Microhardness of this coating was approximately 560 HV1. According to XRD analysis WR6 powder consisted of austenite, VC and small amounts of martensite/ferrite. WR6 laser coating consisted of fine, round and star-like, VCs (~10 vol.%, 0.6–2.0 µm in diameter), network of FeCrVMo-carbides in a matrix of austenite and martensite/ferrite. Austenite to martensite/ferrite ratios were 3.2 and 0.6 for powder and laser coating, respectively. These ratios are overestimated since one of the VC peaks overlaps with γ(111). The best and most often used peaks to evaluate austenite to martensite ratios are α(200) and γ(220). Unfortunately α(200) peak was so weak that its integrated intensity could not be measured. Average microhardness of the WR6 laser coating was 848 HV1. Nital (4%) etching revealed reheated zones associated with overlapping, which were approximately 40 HV1 harder than non-reheated zones (870 vs. 830 HV1). This hardness increase originated probably from the transformation of austenite to martensite due to reheating. Despite high hardness, this alloy was the only laser coating among the second group of abrasion wear tested materials that survived the cladding without cracking (even without preheat). This resulted from the volume expansion associated with martensite formation during cooling, which acted against tensile stresses that formed during cooling.

Vickers pyramid indentations (3 and 10 N) did not initiate cracks indicating excellent fracture toughness. Representative micrographs of WR6 laser coating is shown in Figure 123.

Reheated and non-reheated zones together with hardness values are illustrated in Figure 124.

SEM micrographs taken from the wear scar are shown in Figure 125. In contrast to MMCs discussed above, deep and long scratches caused by angular silica particles are clearly seen.

VCs were definitely too small and mean free path between them was too high to prevent apparent wear mechanisms, micro-ploughing and –cutting, to occur.

Figure 123. SEM micrograph of WR 6 tool steel laser coating. Volume fraction of vanadium carbides is approximately 10% according to image analysis. Carbides are 0.6-2.0 µm in diameter.

Abrasion wear resistance of WR6 tool steel was further enhanced by adding externally 30 vol.% of coarse VC particulates (45–106 µm). Optical micrograph of this coating is shown in Figure 126. Mass and volume losses decreased from 113 to 89 mg and from 15.3 to 13 mm³. This improvement was, however, rather modest. VC addition increased bulk hardness of the

Figure 124. Optical micrograph of etched transverse (perpendicular to cladding direction) cross-section of WR6 tool steel laser coating. Reheated zones and increased microhardness values are clearly seen.

Figure 125. SEM micrographs taken from the wear scar of the WR6 laser coating. Rubber wheel rotation direction was from left to right. In contrast to MMCs deep and long scratches/grooves are clearly visible.

coating from 850 to 1030 HV1 thanks to high hardness of the externally added VCs (1940 HV0.3). Matrix hardness was 840 HV0.3. According to XRD, WR6 laser coating reinforced with VCs contained significantly lower amount of martensite/ferrite than WR6 laser coating alone. Austenite to martensite/ferrite ratio was 3.6 compared with 0.6 for WR6 laser coating alone. Due to peak overlapping discussed above, these values are overestimated particularly for WR6 reinforced with externally added VCs. If comparison is made on the basis of integrated peak areas of γ(200) vs. α(110), austenite to martensite/ferrite ratios for WR6 alone and WR6 reinforced with externally added VCs are 0.16 and 0.56, respectively. This comparison indicates lower martensite/ferrite content in WR6 reinforced with externally added VCs. Those reheated zones with increased hardness were not detected either. Similar to other MMCs, matrix wore more than carbides and they were tightly bonded to the matrix as revealed by the SEM images taken from the wear scar (Figure 127). Fracture toughness remained high since Vickers pyramid indentation at a load 10 N did not initiate any cracks in the matrix.

3.4.5 (Ti, Mo)C - MMC

(Ti, Mo)C laser coatings were prepared from the powders, which were manufactured with SHS technique. As a result of SHS and subsequent crushing, individual powder particles (45–

125 µm), angular in shape, contained spherical or cubic (Ti, Mo)C particulates less than approximately 3 µm in diameter/diagonal. Matrix materials were Stellite 6, nickel and Colmonoy 42-P2 (NiCrBSi). (Ti, Mo)C particulates were spherical in Stellite 6 (Figure 21)

Figure 126. Optical micrographs taken from the transverse cross-section (perpendicular to cladding direction) of WR6 + VC (70/30 vol.%) coating. Vickers pyramid indentation at a load 10 N did not initiate any cracks indicating excellent fracture toughness compared with many other MMCs produced in this study.

Figure 127. SEM micrographs taken from the wear scar of the WR6 + VC (70/30 vol. %) laser coating. Rubber wheel rotation direction was from left to right.

and cubic in nickel matrix (Figure 23). Particulates in Colmonoy 42-P2 powder resembled particulates in nickel matrix just the corners were rounded (Figure 25).

3.4.5.1 (Ti, Mo)C – Stellite 6

Two different mass ratios were used; (Ti, Mo)C – Stellite 6 (50/50 wt.%) (SHS 1378) and (Ti, Mo)C – Stellite 6 (20/80 wt.%) (SHS 1389). The latter one was obtained by mixing the desired amounts of SHS 1378 and Stellite 6 powders. Laser coating made of SHS 1378 powder exhibited the best resistance against abrasive wear in used conditions among all the laser coatings tested. Owing to high volume fraction of fine and hard (Ti, Mo)C particulates, approximately 64 vol.% according to image analysis, coating was characterized with the bulk hardness of 1100 HV1 and very short mean free path between particulates. The representative micrographs of SHS 1378 laser coating in low and high magnifications are illustrated in Figures 128 and 129. It was observed that during solidification some carbides tended to form carbide chains or strings, which are seen on the right hand image in Figure 129. In general, Vickers pyramid indentation at a load of 10 N did not initiate cracks in the matrix, but in the vicinity of these carbide chains, some cracks formed and they propagated along the chains.

Occasional large pores as well as vertical cracks were also detected. XRD patterns were identical for powder and coating which means that no additional phases formed during cladding. This was expected since possibly dissolved TiCs recrystallizes to dendritic TiC.

XRD patterns for SHS 1378 powder and laser coating are shown in Figure 130. In addition to (Ti, Mo)C and fcc-Co, hcp-Co was also detected.

Figure 128. Optical micrograph taken from the transverse cross-section (perpendicular to cladding direction) of as-laser-clad SHS 1378 coating. Large black dots are pores. Some large particles at the lower end of the image have experienced incomplete melting. Due to exceptionally high melt viscosity irregularities formed on the surface.

Figure 129. SEM and optical micrographs taken from the transverse cross-section (perpendicular to cladding direction) of SHS 1378 laser coating. Carbide chains/strings are seen in optical micrograph.

Despite short mean free path between (Ti, Mo)C particulates Stellite 6 matrix wore down at the front of carbides in relation to wear direction as displayed in Figure 131. Carbide surfaces were free from scratches. Some of them cracked but not as severely as dense-coated WCs (Amperit 522) and fused WC/W2Cs (Woka 9604) discussed earlier.

Figure 130. XRD patterns measured from SHS 1378 a) powder) and b) coating.

Figure 131. SEM micrographs taken from the wear scar of SHS 1378 laser coating. Arrow indicates the rubber wheel rotation direction.

3.4.5.2 (Ti, Mo)C – Ni-based matrices

With pure nickel as matrix two different mass ratios were used; (Ti, Mo)C – Ni (50/50 wt.%) (SHS 1377) and (Ti, Mo)C – Ni (35/65 wt.%) (SHS 1380). The latter one was obtained by mixing the desired amounts of SHS 1377 and Ni powders. Due to low matrix hardness (~200 HV), coatings made of these powders, were among the worst MMC laser coatings studied here. Carbide volume fractions varied largely in coating made of SHS 1380. This resulted from the incompletely melted or mixed single SHS 1377 particles found in the coating. One representative example is shown in Figure 132. Similar feature was observed in coating made of Co-based SHS 1389, which was also the blend. These incompletely melted or mixed powder particles had better wear resistance than other areas as revealed by the SEM image taken from the wear scar of SHS 1389 in Figure 133. Carbide chains or strings were observed again and they were susceptible to cracking as illustrated in Figure 132.

Another Ni-based (Ti, Mo)C coating was prepared from the powder; (Ti, Mo)C – Colmonoy 42-P2 (50/50 wt.%) (SHS 1436). Due to higher matrix hardness (36 HRC ~ 355 HV), this coating outperformed (Ti, Mo)C – Ni coatings clearly in wear resistance, but was still worse than even SHS1389 ((Ti, Mo)C – St 6 (20/80 wt.%)). This result emphasizes again the role of matrix hardness in given abrasive conditions.

Figure 132. SEM and optical micrographs of the transverse cross-section (perpendicular to cladding direction) of SHS 1380 coating. Carbide chain/string is seen in optical micrograph.

Vickers pyramid indentation at a load 10 N cracked it. Original powder was the blend of SHS 1377 and Ni. For the reason of incomplete melting of single SHS 1377 powder particles, there are regions with high (51 vol.%) and low carbide (37 vol.%) fractions.