Bulk and microhardness analysis

In Figure 110 an example of a VHN hardness profile is shown, which is superimposed onto an optical image (OM) image of a conventional radial sample (P0-cast). The three main zone can be distinguished based on the hardness profile and macro-etching effect (dark and white areas on the surface): 1) hard zone; 2) hardness reduction; 3) low stabilized.

Figure 110. Example of VHN hardness measurements along the conventional radial sample (P0-cast)

In Figure 111, VHN profiles along the conventional HSS shells (P0-samples, high carbon group, Table 2) after different HTs are shown. The lines represent an averaged data: 5 actual values as one point on a curve. As it can be seen, bulk hardness measurements and microstructural observations (discussed in Section 4.2) of the conventional material indicate almost no effect of any sort of conventional HTs (yellow, purple and green lines). The only exception is the hardness measured after preliminary HT, showing considerable reduction, or softening (red line, Figure 111). These were the original measurements made, stimulating the current work in developing a more efficient HT, which would homogenize the hardness through the shell.

Figure 111. Averaged VHN profiles of conventional material (P0 samples, Table 2) vs. HT

More complete hardness data of samples from the Table 2, corresponding to overall bulk HRC profiles, are shown in Figure 112. In general, the hardness drop at the fusion line between

bonding region depends on the centrifugal casting (mixing) and the width of the intermix zone, which is controlled by the casting rate and time intervals between solidification and pouring. An exceptional example is the CPC sample, which shows an immediate drop, because it has no transient region due to its different casting method (continuous pouring for cladding). Most significantly, the red line (P9DT) located above most of the profiles shows uniform, stable hardness from the surface to the bonding. It was formed by the non-conventional HT applied to the refined cast (dash blue line below).

Figure 112. Overall bulk HRC profiles of HSS radial samples

Just the data comparing the conventional (original) and non-conventional (improved) approaches is given in Figure 113. The hardness of the new cast and new HT (Type I) in the figure stays uniform at 60 HRC right up until the core is encountered. The material starts as the as-cast (Type II in Figure 113), which is also uniform until the core is encountered, but at a lower hardness (HRC 55). If the original HT is applied to the new cast (Type III in the Figure 113), surface

hardness is improved (HRC 60), but it decreases to < HRC 50 toward the core. It is still an improvement over the conventional cast and HT (Type IV in Figure 113), which has an HRC hardness that is also non-uniform, not exceeding HRC 55 and decreasing drastically from about 20mm depth to the core at 60 mm. Thus, the new material produced by the new cast and HT (Type I) has superior hardness characteristics when compared to the original material (Type IV), and thus is expected to have superior service life.

Figure 113. Selected bulk HRC profiles of conventional and improved materials

In order to validate the initial assumption concerning the matrix enrichment with alloying elements after a higher austenitization HT, micro-VHN hardness measurements were performed.

Figure 114 shows an example of micro-indents in the matrix and boundaries.

Figure 114. Micro-VHN of the matrix and carbides (Olympus and Keyence OM)

A series of indentations were made at the surface, middle and next to the bond locations, considering material condition and HT type, Figure 115.

Figure 115. Micro-VHN of the matrix

From Figure 115 it is seen that the matrix strength is increased by as much as 7%, 18%, and 28% at the surface, middle and bonding region, respectively, and becomes more uniform after the new HT (red curve). Clearly, the shell is now heated with sufficient time and unifromity;

therefore, the matrix is well homogenized, which can be concluded by comparing the as-cast condition (decrease in hardness closer to the intermix zone) and after the non-conventional HT

(no reduction). Additionally, looking at the conventionally hardened matrix profile and refined as-cast, it is evident that there is almost no difference. This result was also shown with the bulk hardness measurements mentioned in Figure 111, where initial as-cast values almost remained at the same levels to those of the conventionally heat treated material. When examining Figures 111 and 115, it can be suggested that the effect of preliminary HT is to soften the matrix of the bulk material. It was shown that this was achieved through matrix spheroidization, i.e. formation of incoherent spherical alloy carbides (see Section 4.2.2). The matrix softening of the as-cast can cause a reduction in hardness that is 2.5-2.7 times lower. The subsequent hardening then increases it up to 3 times. Thus, the bulk hardness can be increased by 1.2-2 times after the hardening HT.

After triple tempering the hardness remained uniform along the shell (P9TT, Figure 112), and a reduction of only 2 HRC points was measured, bringing the total value down to 58 HRC.

Thus, with non-conventional HT, higher and more uniform attainable hardness is achieved along the HSS shell in the radial direction. The matrix shows enhanced response to the high temperature hardening as a result of the secondary precipitation of fine alloy carbides and increased matrix hardness.

Additionally, some relevant observations of the area of indentation itself were made, using SEM. Local micro-deformations in the vicinity of HRC indenter (RT) were located and given in Figure 116. The tangential stresses are schematically shown with the arrows on the sample surface.

The improved matrix of P9DT was deformed plastically without any sign of cracks; although eutectic Mo-, V-carbides contain multiple parallel cracks that were oriented perpendicular to the tangential force direction (Figure 116). By default, the carbides are 3-4 times harder than the matrix and, evidently, cannot be plastically deformed. But due to their separate and uniform

dispersion, the cracks do not connect and propagate through the matrix; they are basically entrapped within these fine carbides.

It is worthwhile to mention the role of carbide morphology and shape- that more round, idiomorphic carbides, e.g. MC (dark phase) might not have any crack-like features at all, presumably, due to their “by-passing” the stress field. In contrast, any boundary changes or sharpness plays a significant role as a stress concentrator within the area of the carbide; hence, brittle cracks can easily propagate from these spots.

Figure 116. HRC indenter and local deformation of the matrix (P9DT)

Moreover, the refined as-cast structure, with finer prior austenite grain boundaries and fine sub-grain structure promotes higher toughness when compared to the conventional cast. These

types of simple findings help one to visualize and surmise the carbide-matrix response to in-service loading and how to select the proper carbide configuration, which can be controlled by the solidification rate and HT.

In conclusion, the hardness measurements of the bulk material and the matrix were performed, validating the development of the improved high temperature hardening HT. The bulk HRC measurements of all considered alloys were compared and analyzed, to find where the improved HSS material demonstrates the highest and uniform attainable hardness in the shell. The micro-hardness measurements (VHN) of the enriched matrix showed increased values along the shell in contrast with the conventional samples. Additionally, some assumptions and observations of local deformations after HRC hardness testing in the vicinity of indentation areas were made, confirming proper fracture toughness behavior of the matrix.