Machinery's Handbook, 31st Edition
1538 Powder Manufacturing Processes Sintering temperature is below melting point (generally within 70 to 90 percent of the melting point of the metal or alloy), and the metal remains unmelted. The temperature in the furnace is high enough to get metal into the recrystallization zone. In this temperature zone, the metal particles begin to recrystallize into each other. Sintering times average from 1 to 1.5 hours for small parts, such as bushings, to 3 hours for average-size ferrous parts. However, for tungsten parts, average sintering time may go to 8 hours. Sintering furnaces have three sections: first is a burn-off section for volatilizing the lubricants in the green compact in order to improve bond strength and prevent cracking; second is a high-temperature section for sintering; third is a cooling section. Hardening sintered parts involves applying a controlled cooling rate in a separate cool- ing section of the sintering furnace. In the cooling section of the sintering furnace the parts are cooled in a protective atmosphere in order not to be oxidized in contact with air. Dissociated ammonia or nitrogen-based atmospheres are commonly used. Vacuum atmospheres are used for certain metals, such as stainless steel, titanium, and tungsten. The cooling speed, especially in the temperature range of 1560 to 930°F (850 to 500°C) also affects the mechanical properties, due to phase transformations in the material. During the sintering, a moderate dimensional change takes place. Most materials shrink, but some alloying elements, such as copper, cause material growth. The design of the die must compensate for any anticipated dimensional change. In Fig. 15 is shown a typical heat-treatment cycle in a PM sintering operation.
Key:
Part Temperature Furnace Temperature
Preheat
Sinter
Cooling
0
Time
Fig. 15. Typical Heat-Treatment Cycle in Sintering Operation Secondary Operations: Although PM parts can be used as finished parts, they also may need some additional operations after sintering to improve their properties or to impart special characteristics to the parts. Repressing: A second pressing operation (wherein the part is squeezed in a closed die) serves to decrease porosity (i.e., improve density) for applications where density is very important to achieve the required mechanical or other physical properties. After repress ing, the parts are sintered a second time. Sizing: Sizing is forcing the part through a finish die to provide dimensional accuracy. Coining: Coining is a pressworking operation on a sintered part to improve the surface finish by further densification or to press details into its surface. Finishing Operations : Some powder metal parts require additional finishing operations that may include: machining to produce holes or threaded holes, and various other geo metric features by milling or other processes; grinding to improve dimensional accordance and surface finish; plating to improve resistance to wear and corrosion and sometimes to improve appearance; and heat treating for hardness and other mechanical properties.
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