The importance of the abrasive cannot be overemphasized. The enormous differences in typical hardness values of abrasive grains are illustrated in Table 1.1 [after De Beers]. A value for a typical M2 tool steel is given for comparison. The values given are approximate since variations can arise due to the particular form, composition, and directionality of the abrasive.
In grinding, it is essential that the abrasive grain is harder than the workpiece at the point of interaction. This means that the grain must be harder than the workpiece at the temperature of the interaction. Since these temperatures of short duration can be very high, the abrasive grains must retain hot hardness. This is true in all abrasive processes, without exception, since if the workpiece is harder than the grain, it is the grain that suffers most wear.
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TABLE 1.1 Typical Hardness of Abrasive Grain Materials at Ambient Temperatures Units (GPa)
Source: From De Beers, 1983. With permission. |
The hardness of the abrasive is substantially reduced at typical contact temperatures between a grain and a workpiece. At 1,000°C, the hardness of most abrasives is approximately halved. CBN retains its hardness better than most abrasives, which makes it a wear-resistant material. Fortunately, the hardness of the workpiece is also reduced. As can be seen from Table 1.1, the abrasive grains are at least one order of magnitude harder than hardened steel.
The behavior of an abrasive depends not only on hardness but on wear mode. Depending on if wear progresses by attritious wear, microfracture, or macrofracture determines if the process remains stable or if problems will progressively develop through wheel blunting or wheel breakdown. This range of alternatives means that productivity is improved when grinding wheels are best suited for the particular grinding purpose.