Surface quality in the flat surface grinding of porous materials

Conclusions

• 1. The presence of pores in a sintered material being ground affects the process of microrelief formation in the surface layer and its physicomechanical properties and intensifies the influence of processing factors on the quality of sintered articles.

• 2. With increase in porosity, the supporting surface area decreases. Values of R_a, and R_z provide very little indication of differences in the shape of irregularities.

• 3. The grinding of a porous material not only changes its surface microhardness (by up to 20–30%), but also reduces the porosity of a thin surface layer of the material, i.e., densifies its porous skeleton. This phenomenon is an inevitable manifestation of the processing heredity of grinding. By changing grinding conditions, it is possible only slightly to increase or decrease the degree of densification of the porous skeleton of the surface layer compared with some nominal level, which is determined by the starting porosity and the mechanical properties of the material being ground.

• 4. When quenched ZhCh25Kh3 material of 10–30% porosity is ground, tensile stresses are set up in its surface layer. The magnitude of these stresses on the surface, being as high as 50–60 kg/mm², exceeds the ultimate strength of the material. This is attributable to intense strengthening of the surface layer (work-hardening and densification of the porous skeleton), which is not allowed for in calculations.

• 5. In the grinding of quenched porous materials consisting of iron and cast iron powders, use should be made of strengthening and densifying process parameters ensuring a Class 8 or 9 surface finish and resulting in the formation of a surface layer having good wear resistance, namely, a grinding depth of 0.03–0.05 mm, an axial feed of up to 12–15 m/min, and a transverse feed of up to 0.5–0.9 mm/stroke. For most purposes, medium-structure ceramic-bonded wheels from white electrocorundum of No. 25–40 grain size are recommended. Where it is desirable to minimize the densification of the porous skeleton of the surface layer, for example, in the case of parts intended for operation involving friction under restricted lubrication conditions, use should be made of wheels from Elbor of No. 10–16 grain size bonded with KB material of 100% concentration (grinding depth 0.01–0.015 mm, transverse feed 0.3–0.5 mm/stroke, axial feed 3–5 m/min).

• 6. Grinding is an effective means of controlling the service properties of the surfaces of porous sintered machine components during production.