Here’s a chance for you, the reader, to comment on a topic that I consider of relevance to the industry.

A question came to me through a commercial heat treater operating batch integral-quench (IQ) furnaces produced by the same manufacturer. The furnaces have only one apparent difference between them, that being the radiant-tube material. Both furnaces are pulse-fired running natural gas. Furnace 1 has vertical 8-inch-diameter radiant (“U”) tubes, and Furnace 2 has vertical 6-inch-diameter silicon-carbide tubes. The temperature uniformity and carbon uniformity is reported to be well below the industry requirement.  

However, the heat treater reports a 20% increase in demand for additive gas on the silicon-carbide-heated furnace. The results of increased additive gas are reported to be across the board – irrespective of the recipes run in the furnaces.  

The additive-gas usage is determined by setting the furnaces at the same 930°C (1700°F) temperature – empty of product – and manually increasing the carbon potential to 1.20%C and recording the gas flow required to keep the chamber at setpoint. The endothermic carrier gas flows at a rate of 650 cfh (18 m3/hour) with a controlled dew point of +5°C (+9°F) and a target dew point of 2°C (+3.5°F). The enrichment gas is propane. The furnaces are controlled by oxygen probes.  

The heat-treater’s investigation (through other plants in its organization) claims that the reason for this added consumption is because the alloy tubes act as a catalyst to the additive gas where the silicon carbide tubes do not. So, here are the questions:

1. Is it reasonable to assume that the alloy tubes act as a catalyst?
2. Do the tests run prove this or is there a better test?
3. What are the true advantages and disadvantages of one type of tube over another with respect to gas consumption? With respect to other factors?
4. As more and more alloy (roller rails, chain guides, muffles, etc.) is being eliminated from furnaces, will this result in less-optimum carburizing gas consumption?
5. Where is the market headed and why?

I await your answers!