Optimum sintering conditions for bushings of automobile engine valve guides and the effect of some factors on their physicomechanical properties and structure

Abstract

1. The optimum conditions for sintering 8.0–8.5-kg boat loads of valve guide bushings may be taken to be a temperature of 1160°C and a boat pushing period of 20 min. 2. Carbon segregation along the bushing height leads to instability of strength properties. 3. The material of bushings from PZh2M2 powders surpasses in physicomechanical properties that of bushings from PZh2M3 powders. 4. Nonuniform compaction, resulting in increased porosity in the central part, has an adverse effect on the strength properties of bushings, even when the latter's over-all porosity is of the order of 20% (which meets the requirements of TU 02-69 specification for valve guide bushings). When one-fifth or two-fifths of a bushing at either end has a higher porosity, the strength of the bushing remains unchanged. 5. In an investigation of the temperature distribution in an OB-357 furnace during the sintering of bushings, it was established that, when the operation is performed with a 20-min period at 1160°C, sintering temperature is maintained over three boat lengths, which ensures an optimum sintering time, 1 h, and imparts satisfactory strength to bushings. 6. A study was made of the dimensional changes of bushings from PZh2M2 and PZh2M3 powders under various sintering conditions. A knowledge of these changes is indispensable in die calculations if parts of the required degree of dimensional accuracy are to be produced.