Ferritic Structures
Materials with predominately ferritic structures are but seldom subject to grinding, being usually machined instead with geometrically defined cutting edges. In the case of grinding, low strength and hardness as well as the high deformability of ferritic structures leads to frequent clogging of the grinding wheel. For this reason, the use of open-pored grinding wheels and dressing conditions that create a high initial effective peak-to-valley height in the grinding wheel topography are advantageous. Clogging tendencies can also be countered with an increase in cutting speed, by means of which smaller depths of cut are produced. However, it must be considered that the higher thermal strain results from increased cutting speeds, which increases the material’s tendency to clog. Thus, conflicting effects are to be expected here.
Due to the low hardness of ferrite, no particular requirements are placed on the grain material with respect to hardness and wear resistance.
Pearlitic Structures
Due to the higher strength and hardness of pearlitic structures as opposed to ferrite, the abrasive tool is subject to a stronger abrasive wear effect during machining. Furthermore, the higher strength leads to higher grinding forces and thus to higher strains on the grain cutting edges, resulting in higher machining temperatures. Favoured by the lower deformability of pearlitic structures and the smaller clogging tendency related to this, grinding wheel topographies with a low effective peak-to-valley height can be employed. In this way, lower roughness values can be realised.
Austenitic Structures
Austenitic materials are characterised by ductile material behaviour. In grinding, the tool tends to clog, for which reason grinding wheels with higher effective peak-to-valley heights are to be recommended. In contrast to ferrite however, high grinding forces also cause more stress on the grinding wheel because of the higher strength of austenite.
Martensitic Structures / Heat-treated Structures
Martensitic and heat-treated structures exhibit high hardness and strength levels. This results in heavier stress on the grinding wheel and in high grinding forces. To achieve a low amount of grinding wheel wear, high workpiece hardness requires high grain hardness. For this reason, the use of cBN is well suited to high-speed grinding if high surface qualities, high chip removal rates and high formal and dimensional precision are required. By increasing the cutting speed, the depths of cut and thus the mechanical stress on the grain cutting edges can be reduced. Precious corundum and sol-gel corundum are also common grain materials in the machining of hardened steel materials. However, more wear is to be expected than with cBN.
Especially in the case of martensitic or heat-treated structures, excessive thermal load on the material, leading to tempered zones and/or brittle hardening zones (known as “grinding burn”), must be avoided. This can be counteracted with customised tool specifications and machining parameters as well as with an effective cooling lubricant.