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Dull finish and tensile variation of s.s wirerods due to improper annealing

2003

Hi everybody,

I have a problem in heat treatment of stainless steel coils. During annealing to 1050 deg. cent., the ss wire rods which are adhered to the castable doesn't get properly heated and gives dull finish after pickling. The top wirerods are found to be ok. The bottom wirerods also give variations in tensile strength(on higher side). We have a direct light diesel oil(LDO) fired furnaces. I would like to have suggestions on changing annealing standards, utilisisation of flue gases(to the maximum) produced while heating or any change in design of furnaces. Suggest some literature (Indian authors ) also.

Thanks friends, regards,

Vijayan R Nair
- Mumbai, Maharashtra, India

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2003

Hi,

To begin, I recommend as references MIL-H-6875H HEAT TREATMENT OF STEEL (downloadable from http://assist2.daps.dla.mil/quicksearch/) and The Making, Shaping and Treating of Steel (either 9th or 10th edn.), especially the chapters The Manufacture of Steel Wire and Steel Wire ProductsÏ and the section Heat Treating FurnacesÏ in the chapter Principles of Heat Treatment of Steel.Ï These give information about annealing temperatures, times and atmospheres for various stainless steel grades. The book gives information on wire manufacturing, furnace and gas details.

You have 2 basic problems: improper temperature distribution and trying to achieve a neutralÏ atmosphere utilizing combustion gases. As your furnace does not seem to have bottom heating, perhaps the simplest way to minimize the temperature distribution problem is to raise the product off the castable using ceramic bricks, and add some type of deflector (maybe SS sheet) to direct some of the combustion gases under the coil. This would also avoid contamination of your SS with the unspecified castable material.

Achieving a neutral atmosphere (neither oxidizing nor carburizing) utilizing combustion gases in an oil-fired furnace is probably not feasible. In the US, natural gas-fired furnaces are used under controlled combustion conditions, which gives an atmosphere consisting of nitrogen and controlled amounts of methane, carbon monoxide and carbon dioxide. The dewpoint of the gas is kept below ù40 F, thus preventing oxidation. The furnace construction must prevent loss of gas and air infiltration, and the exiting gas is burned off. Even if your furnace design is suitable, the oil fuel would cause problems (sooting) if the air supply were limited. Another problem with oil fuel is a relatively high sulfur content, which may contribute to the poor surface finish. A consulting firm below, Burns & McDonnell, should be able to tell whether your furnace oil burners can be replaced with natural gas type and what other changes may be necessary.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
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