Cryogenic Temperature And Pressure Process

Reduction of powder and die temperature in warm compacting technique favors lowering energy consumption of special heating equipments and prolonging die working life. Moreover, it improves powder flowability and die-filling ability. Particularly, if it is feasible to fabricate high-density iron P/M materials by pressing with cold powder, there is no need for press to be equipped with expansive powder heating equipment even and die heating system, well solving the recent severe problem that hinders the widespread industrializing application of warm compaction. In the present paper, two kinds of lubricant system suitable for lower temperature application e.g. less than 100℃ and fit for ambient temperature application were, respectively, designed for lower temperature warm compaction and pressing with cold powder. In warm compaction at lower temperature, partially pre-alloyed powders were used for compaction, and influences of process parameters such as lubricating mode including die wall lubrication, powder temperature, die temperature, lubricant content as well as pressure on green density were investigated. After sintering and subsequent heat treatment, mechanical properties of specimens with different densities were tested, respectively. In the research of pressing with cold powder to fabricate high-density iron P/M materials, however, pre-mixed powders with iron annealing and the ones without iron annealing were used for compacting, effects of dietemperature, lubricant content and compacting pressure on green density were investigated. In addition, in this paper, a new powder compacting densification model described as C(P)=(Vo-V)/Vo=aP/(l+bP)+cexp(-d/P) was developed based on Cooper-Eaton modified model and Kawakita theory, and warm compacting densification mechanisms were analyzed according to Cooper-Eaton modified model and the newly developed model, respectively.In warm pressing at lower temperature without die wall lubrication, the results show that the optimum powder and die temperatures are 100°C and 120°C, respectively. The optimum lubricant content in powders is 0.65%. For Fe-1.5Cu-0.5C and Fe-1.5Ni-0.5Mo-0.5Cu-0.5C powders, the densities of 7.42g/cm3 and 7.41g/cm3, respectively, can be achieved under the pressure of 686MPa. As the pressure increases to 735MPa, the two green densities get to the same value of 7.44 g/cm3. The low temperature warm compacting effectiveness gets to the density value by conventional warm compaction. After sintering and subsequent heat treatment, the ultimate tensile strengths of Fe-Ni-Mo and Fe-Cu-C materials with the density of about 7.45g/cm3, respectively, reach 1200-1400MPa and 1000-llOOMPa. In lower temperature warm compaction with die wall lubrication, the fairly ideal lubricant content in powders is reduced to 0.2-0.3%. For Fe-1.5Cu-0.5C and Fe-1.5Ni-0.5Mo-0.5Cu-0.5C powders, the densities of 7.48 g/cm and 7.39 g/cm , respectively, can be achievedunder the pressure of 686MPa. Warm compaction with die wall lubrication favors much higher density especially for powders with better compressibility. Density variation reaches 0.04g/cm3 due to different die wall lubricants and their concentrations. The ejecting pressure of die wall lubricated pressing is 35-45% lower than that of merely interior lubricated compaction. As for densification mechanisms, the contribution ratio of particle rearrangement to the whole powder densification is only 50% by the application of Cooper-Eaton modified mathematical model, however, the value comes to 65-76% with the analysis of the newly developed densification equation. The new model better fits the true experimental results.In pressing with cold powder, pre-mixed powders with iron annealing treatment and the ones without iron annealing treatment were both used for conventional cold press and merely heating die compacting mode. The results show that green density is not sensitive to lubricant content in powders, fairly ideal value is 0.1-0.3%. For powders without iron annealing treatment under the pressure of 676-763MPa, densities of 7.20-7.30g/cm3 and 7.35-7.45g/cm3, respectively, can be achieved by conventional cold press and only heating die pressing mode. As pressure increases to 833 MPa, the densities compacted by the two processes mentioned above are 7.36 g/cm3 and 7.48 g/cm3, respectively. However, for powders by iron annealing treatment with the pressure of676-763MPa, higher densities of 7.25-7.35g/cm3and 7.40-7.50g/cm3 can, respectively, be achieved by conventional cold pressing and pressing with only die heating. As pressure increases to 833 MPa, the densities pressed by the two processes above are 7.40 g/cm3 and 7.52 g/cm3, respectively. The results concerning ejecting pressure measurement show that the ejecting pressure in pressing with cold powder is obviously lower than that in warm compaction at lower temperature, even as low as that in die wall lubricated warm compaction. For densification mechanisms analyzed by the new model, particle rearrangement plays a more important role in the whole powder densification process as compared to plastic deformation.