Table 3: Schematically representation of the three zones.
|Dewaxing Zone||Process Us||Temperature (℃)|
|4||Diffusion of carbone||900-1050|
|5||De carburation (atmosphere)||1000-1300|
• First step: Dewaxing to eliminate the lubricant in the green part between 300 °C et 800 °C.
o The delubrication or dewaxing is the major challenge with sintering process. In order to obtain the sufficient delubrication the furnace should be supplied with an appropriate atmosphere correct distributed between the furnace zones. The dewaxing step can take place under a dry reducing atmosphere with the formation of microcraks at the surface of the specimens due to a poor lubricant removal or under a wet oxidizing atmosphere with the presence of oxygen and water vapor that is capability to remove the lubricant. The oxidizing reaction can lead to the formation of a superficial oxide on the powder particles that must be reduced in the high sintering zone.
• Second step: After the first step with dewaxing, the sintering process is the next step. The goals of this step are the following:
o Reduction of metal (metal oxide Metal + oxygen)
o Formation of sintering bridges between the individual powder particles
o Avoid a decarburization of the specimen
o Sintering time (20 to 35 minutes) and sintering temperature between 1120 and 1250 ℃ the choice of the protective atmosphere under production conditions are the important parameters to obtain the best mechanical properties that depend strongly on the type powder (bonded powder, diffusion alloyed powder.) and on the chemical composition (amount of carbon, nickel, molybdenum, copper, chromium).
• Third step: After the sintering step, the third step is the cooling zone with or without rapid cooling system. For the rapid cooling, the furnace atmosphere is used after a cooled at temperature below 100 ℃ and blows onto the parts. The cooling rate is about 2-4 ℃/s. The goal of the rapid cooling is to obtain a full martensitic microstructure or a bainitic-martensitic microstructure. For this reason the PM steels materials have to be alloyed with elements like Chromium, Molybdenum and copper that give the desired microstructure at cooling rate between 2-4 ℃. A carbon content of 0.4 to 0.6 wt.-% is recommended for a rapid cooling effect. In conclusion to the remarks, the PM steel FeCuC do not lead to the formation of martensite because a high cooling rate > 10 ℃/s is necessary.
• The technique offers good manufacturing economy by providing a one step process without a secondary heat treatment like carburizing.