Summary of Powder Metallurgy (ASM Vol.7)

Sintesis = (penyatuan unsur-unsur atau bagian-bagian ke dalamsuatu bentuk yang menyeluruh) ----------------------------------------------------------------------------------------------------------------------------------------1. Compaction pressures ranging from 550 to 830 MPa (40 to 60 tsi) are common in commercial practice for stainless steel because : "stainless steel powders require higher compacting pressures and have lower green strength, compared with low-alloy ferrous powders." 2. The green strength of stainless steel compacts, is influenced by compaction pressure and the type of lubricant. 3. Lubricants that provide high green strength, such as stearic acid generally cause lower compactibility. 4. lubricant selection is an important factor in determining successful application and fabrication of P/M stainless steels. 5. Stainless steel powders with lower carbon and nitrogen contents also improve compressibility and compactibility (green strength). 6. Powder annealing also improves green strength. 7. Sintering is the most important step in processing stainless steel P/M parts. Because carbon lowers corrosion resistance, the lubricant must be removed to prevent carbon diffusion into the part. 8. medium-density stainless steels are sintered typically at 1120 to 1150 °C (2050 to 2100 °F), except when improved mechanical properties and corrosion resistance are required. 9. Higher sintering temperatures up to 1315 °C (2400 °F) are used for improved mechanical properties and corrosion resistance. 10. Sintering is generally performed at temperatures around 2/3 to 4/5 of the absolute melting point or solidus of the material for a single-component system. 11. Multi-component powder mixtures are generally sintered near the melting point of the constituent with the lowest melt temperature. 12. Sintering times are typically 20 to 60 min under a protective atmosphere. Widely used furnace atmospheres include endothermic gas, exothermic gas, dissociated ammonia (DA), hydrogen, hydrogen-nitrogen mixtures, and vacuum. The main function of the

atmosphere is to protect a part from oxidation or nitridation, as might occur when heating in air. 13. Inadequate sinter generally is indicated by low strength, low hardness, and improper dimensions. Causes of inadequate sintering are often related to the atmosphere, but several factors may be involved, such as: · Sintering temperature too low · Insufficient reducing agent · Dew point too high in the hot zone · High O2 content in hot zone · Incorrect green density · Incorrect belt speed or time at temperature Corrective action includes: · Measure and control dew point and O2 content in the hot zone · Measure and increase H2 content · Check and correct belt speed and/or powder compositions 14. Sintering temperatures depend on the material and desired properties. Alloy steels typically require higher sintering. temperatures (Table 3) and times to promote homogenization. 15. Sintering atmospheres primarily control chemical reactions between the materials being processed and the furnace surroundings. 16. Sintering atmospheres may also prevent the loss of alloying elements existing in the material being processed. 17. In common operating practice, the most important functions of select atmospheres are to aid in the reduction of oxides on the surfaces of the metal particles in the compact and to control carburization and decarburization of iron and iron-base compacts caused by oxygen, water vapor, and carbon dioxide when present in improper proportions with respect to the hydrogen and carbon monoxide contents of the sintering atmosphere. Iron oxides are reduced by hydrogen carbon monoxide, and carbon. 18. Carburization is caused by carbon monoxide and by hydrocarbons such as methane. 19. The most frequently used atmospheres in commercial sintering of P/M iron and steel materials are endothermic, exothermic, dissociated ammonia, pure hydrogen, and

nitrogen-base atmospheres. Of these, endothermic gas is the most widely used, followed by dissociated ammonia. 20. Although other hydrocarbon gases may be used, methane and propane are the most commonly used bases for the production of endothermic gas and exothermic gas atmospheres. 21. These furnace atmospheres consist of mixtures of nitrogen, water vapor, carbon monoxide, hydrogen, carbon dioxide, and methane. 22. The degree of combustion that occurs is controlled by the amount of air admitted to the process 23. methane is strongly reducing to iron oxide 24. Dew point is a measure of the dryness of the atmosphere gas 25. Generally, low dew point gases increase the reduction potential of the atmosphere 26. Exothermic atmospheres are not strongly reducing to iron and are decarburizing at normal sintering temperatures. Their use in sintering iron-base materials is limited to applications that do not require a residual carbon content. Removal of water and carbon dioxide can improve the properties of this protective atmosphere. 27. Time of sintering also affects the amount of combined carbon formed 28. The higher the sintering temperature, the longer the sintering time. 29. The smaller the particle size of the powder from which the compacts are pressed, and the lower the green density of the compacts, the greater the shrinkage. 30. Compacts from powders of body centered cubic metals of a given particle size pressed to a given density and sintered a given length of time at a given homologous temperature show higher shrinkage than compacts from face-centered cubic metals sintered under corresponding conditions. 31. The higher the temperature, the greater the shrinkage. Shrinkage also increases with increasing sintering time 32. The rate of shrinkage is initially quite high, but then decreases with increasing sintering time 33. The higher the sintering temperature, the more rapid the decrease in shrinkage rate 34. Accordingly, high sintered densities can be obtained more readily by increasing sintering temperature than by increasing sintering time.

35. higher the compacting pressure, the smaller the sintering shrinkage, or the change from green to sintered density.

36. However, the sintered density is much improved by the higher green density. 37. In comparison to plain iron powder, stainless steel powders require substantially higher compacting pressures because of their high alloy content, which increases their hardness and work-hardening rates.

(a) TiC-based hard metals (cermets) up to 1600 °C. (b) 3000 °C, when direct sintering is used. (c) Highest temperature under pressured N2 atmosphere or in powder bed. (d) Low temperatures for liquid phase sintering. (e) Low temperatures for highly active powders.

(a) Not all stages apply and some stages may overlap. (b) Reduction of metal oxides other than iron oxide may require higher temperatures depending on the element and furnace atmosphere.

(a) Temperatures represent the range of typical or possible sintering temperatures.

Halaman : 1095

Outline for Duplex Stainless Steel Ferritic-Martensitic P/M using Mechanical Alloying Material Feritic/Martensitic Duplex Stainless Steel Powder Fe : ? Cr : gas atomized -325 mesh spherical, mean size 4 µm, tap density…? ZrO2 : ? Binder ? Mixing ? Molding ? Debinding ? Sintering ? Post-Sintering Quench in oil medium for obtain Final Density and properties ?

melting point of pure Fe = 1539 0C  so, the proper sintering temperature for feritic-martensitic stainless steel (Fe-Cr-ZrO2) that we going to sinter is near melting down one of the lowest constituent element that mixed (for multi-composition system), in this case, Fe has the lowest melting down other than Cr and ZrO2. melting point of pure chromium = 1857 0C melting point of zirconium = 1852 0C melting point of zirconia (ZrO2) = 2715 °C