Delube and Preheat Atmospheres

Delubing involves the burning and removal of hydrocarbon lubricants used to compact the powdered metal. Removal occurs early in the sintering process in the low-temperature delube zone of the furnace. Small amounts of oxidizing gases in the atmosphere cause effective lubricant burning without oxidizing parts or decarburizing steel parts. The most effective oxidants are controlled amounts of water and carbon dioxide.

Without such oxidizing gases, lubricants tend to decompose thermally into hydrogen and carbon >540 °C (1000 °F). Hydrogen is given off as a gas, but carbon remains as a sooty deposit. If a suitable oxidant is present, however, a chemical reaction (oxidation) rather than thermal decomposition occurs. The carbon in the lubricant combines with the oxidant and is removed as a gaseous mixture of carbon monoxide and carbon dioxide. For example, assuming CHy is a waxlike lubricant and water is the oxidizing agent, the following reaction applies:

Delubing in a sintering atmosphere is performed most efficiently when a wet (high dew point) atmosphere is used. Atmosphere circulation should also be provided; atmosphere flow toward the furnace entrance is desirable. Ideally, the maximum temperature of the P/M compacts during delubing should range from 425 to 650 °C (800 to 1200 °F).

Sooting is primarily caused by improper delubing. The majority of sintering problems are caused by incomplete or improper delubrication. Sooting occurs when hydrocarbon vapors from the lubricants decompose thermally by the following reaction:

CH4(g)- >2H2(g) + C(s) Sooting can also be explained by the following reaction:

Sooting problems can be classified into four types to help take corrective action as described in the following paragraphs (Ref 3).

Adherent soot appears as a black stain and is not easily rubbed off. It can occur on the top, bottom, and the sides of the P/M part. The lubricant oozes to the surface and does not fully vaporize or oxidize before the part enters the hot zone. The causes can be any one or more of the following:

• Dew point too low in the preheat

• The preheat length too short

• Part density too high

• Incorrect preheat temperature

• Belt loading too high

• Incorrect temperature profile

The short term solution is to increase the preheat dew point using a humidified nitrogen system. Reducing belt loading and belt speed can also help. Ideally, the preheat should be lengthened or a rapid burn off unit installed to accelerate the delubing process.

Granular soot looks like black snow and appears mostly on the top surface of the part. The flaky black soot can be easily blown off. It is caused when lubricant vapors thermally decompose into black soot and rains down onto the part. Granular soot is caused when the following conditions occur:

• The dew point is too low in the preheat

• Lubricant vapors migrate into the hot zone

• The forward velocity of atmosphere is too low

• The atmosphere profile shifts due to plant drafts

• Improper design of exhaust stacks

Solutions for granular sooting include increasing total atmosphere flows, redirecting the flow, and redistribution of flows into different inlets. It is also helpful to control up and down drafts in the exhaust stacks.

Soot deposits on belts are indicative of granular sooting. Correcting sooting problems in general will help get rid of soot on the belts. Even small amounts of sooting tend to accumulate along the belt edges over extended periods of time. Soot on belts can lead to carburization and embrittlement of the belts.

Shiny soot appears as a black uniform and shiny coating on all exposed surfaces of the part. It is caused by the catalytic cracking of natural gas on the P/M part surface in the hot zone. The obvious solution is to decrease the amount of natural gas addition. Changing the location of the natural gas addition can also help solve this problem.

Blistering/rippling can be seen on the P/M part and sometimes the part crumbles into powder at sharp edges. Blistering is an extreme case of sooting, which occurs within the P/M part. It is commonly seen in nickel-containing grades and P/M parts with very high densities. Another cause is too rapid a heating rate in the preheat zone. Solutions include decreasing belt speed and belt loading rates. Increasing wet N2 flows and adding a pre-preheater can be helpful.

Preheating Zone and Oxide Removal. As the sintering process progresses, the furnace performs another function in addition to delubrication. At —650 °C (1200 °F), generally near the end of the delube zone and through the hot zone (■-1120 °C, or 2050 °F for iron and steels), the atmosphere begins to strip surface oxides on the metal particles that comprise the part.

In the preheat zone, the atmosphere must reduce surface oxides on the metal particles to ensure a clean, metallic surface on the particles comprising the P/M compact. This permits admixed graphite to diffuse into the iron particles, thus creating a pearlitic microstructure during cooling. Graphite diffusion into the particles is essentially completed when parts reach-1040 °C (1900 °F).

Clean particle surfaces further improve bonding of the particles at >1100 °C (2010 °F). Thus, the greater the oxide reducing effect of the atmosphere, the stronger the sintered bond. Furthermore, porosity between the particles becomes rounded, thus improving the structural integrity and toughness of the part.

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