The basis of the Contiblu furnace is that there is a sufficiently high concentration of oxygen available to oxidize the iron to obtain the desired Fe304 layer at the part surface and in the pores of the sintered part at the reaction temperature, as given by the equation: 

3Fe + 4H20 -> Fe304 + 4H2 

The critical parameter to achieve this transformation is the partial oxygen pressure, which not only includes the free oxygen in the furnace atmosphere but also all of the bound oxygen in other compounds, such as H20 and C02 The quality of Fe oxidation, the oxide layer thickness and penetration depth which directly relate to the hardness of the treated parts, is mainly determined by the ratio, k = H2/H20. 

When natural gas or propane (generally designated as CnHm) is completely burned, carbon dioxide and steam are generated: CnHm + (n+m/4)02 -> nC02 + 0,6mH20. Blueing in the Contiblu furnace also occurs in the presence of steam. The steam oxidizes the Fe sintered parts at temperatures from 500 to 600°C, generating hydrogen. The gas mixture (waste gas) is extracted from the reaction chamber together with the evaporating oil and emulsion residues and burnt. 

The heat released in this process is used for heating the furnace. Sarnes says this method results in the surprisingly low energy consumption of 1.5 m3 /hour of propane (or 4.5 m3 /hour of natural gas) for a furnace capacity of 100 kg/hour.

Cost savings

While the technical advantages of the Contiblu process allow furnaces to be constructed at a lower price that other continuous steam treatment systems, Sarnes says the design also offers cheaper operating costs. 

He says in operation a typical system only requires $ 1/hour for propane (or natural gas), while the energy consumption of an electrically heated steam furnace would be around 60 kW (30 kW for the furnace and 30 kW for the steam generator) or $ 6/hour. The difference of $ 5/hour results in an annual energy savings of $ 30 000, assuming 6000 operating hours per year and specific costs of electricity 9 cent per kWh and 4 cent per kWh of natural gas). 

Sarnes says customers using the Contiblu furnace have calculated savings of up to $ 0.25 per part, compared with a conventional electrically heated steam furnace, depending on the complexity, size and weight of the sintered parts. Increasingly, PM parts makers are requiring a detailed history of their sintered parts. 

To facilitate this, the all of the important process variables in the Contiblu furnace system are controlled and logged by personal computers (PCS). Once production information is generated it can be directly fed into other PCS connected to the network.