Material Heating Times

Q: Are there any guidelines for determining the proper length of time for billets to attain the ready-to-forge temperature, after direct-charging into an operating slot furnace set at the proper temperature? In other words, what is the proper heating time for different section sizes of material charged directly into a furnace operating at the preset forging temperature?

Both the ASM Metals Handbook and ASM's Forming and Forging Handbook state that "for stock up to 3 in. in diameter, the heating time per inch of section thickness should be no more than 5 minutes for low-carbon and mediumcarbon steels, or more than 6 minutes for low-alloy steels. For stock 3 to 9 in. in diameter, the heating time should be no more than 15 minutes/inch of thickness." Does this statement apply to the length of time required once the surface temperature has attained proper temperature, or to the total heating time?

The surface temperature of billets is easily determined through the use of optical pyrometers, thermocouples and/ or heat-indicating crayons and paints, but our debate always centers around the core temperature of the billets.

(Note, Any response must assume that the BTU output of our slot furnaces are not the limiting factor, and that the spacing of our billets in the furnace chamber allows for the maximum exposure to the furnace atmosphere. For discussion's sake, our operating temperature is 2,300°F and the material grade is 1018 low carbon.)

A: From my experience in the industry, we generally used the "13 -16 minutes per inch" guide for forging temps for alloy steels in large rotary or pusher furnaces. This assumed that the stock centers never quite reached the furnace settings for forging.

For example, furnace settings of 2,350°F were generally OK for 7-in. square, alloy-steel billet to be heated to near this temp in 1.5-2 hurs, or to a metal temp of about 2,300°F. In pusher furnaces, the square billets often are in contact with each other, so heat transfer is slower.

We used optical pyrometers to check the billet temps sighting on a "black body" hole in test billets. I was the senior forge shop metallurgist and responsible for setting temps for a wide range of alloys. For austenitic stainless, we soaked a little longer, usually about 1-2 minute more per inch.

I should add that this was our technique for box and rotary furnaces. Pusher furnaces required the longer times. I do not have much experience with slot furnaces, but the general rule you cite for 9-in. billets probably is OK.

I am not sure about the quote of 5 min./inch for the smaller bar. That seems short, except for induction heating. However, experience is the best teacher with slot furnaces because they have much more aggressive gas circulation than box furnaces and are more likely to experience near-flame impingement on the bar surfaces. For this reason, be sure that the edges of the billet are not being overheated by the hot gas exposure.

There is nothing better than an experiment to verify any published data. I like to verify the actual temperatures for specific forgings using optical systems in the "black-hole" method, or with billets with thermocouples imbedded in the centers.

You later supplied the following additional information based on research and experimentation at your plant. You stated that once the surface temp approaches the furnace setpoint, the material is ready to go. Your charting thus far was indicated for 1018 material heated to 2,300°F at:
- 2 in. diameter @ 10 minutes, or 5 min./inch.
- 3 in. diameter @ 16 minutes, or 5 min./inch.
- 4 in. diameter @ 28 minutes, or 8 min./inch.
- 5 in. diameter @ 45 minutes, or 9 min./inch.

You also state that these times represent the cross-sectional area of the billet. Your additional data was excellent data for my file. My old data were based on raising the temperatures of the billet centers to within 50° of the surface temp. I did not have any actual data from bar/billet smaller than 5 in. square or round. Also, your data seems to back up my suspicion that the slot furnace has more aggressive heat transfer from the gas flame.

You have filled in some blanks for my reference.

H. James Henning answers forgers' technical and operational questions. For more than 40 years he held key technical positions in the forging industry, most recently as director of technology for the Forging Industry Asso-ciation. He is president of Henning Education Services, Columbus, OH, specializing in customized education and training in forging technologies. Send questions to ASK JIM, c/o FORGING, 1300 E. 9th St., Cleveland, OH 44114-1503. Or e-mail: [email protected] n Guidelines and recommendations offered here are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.

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