Subcritical vs. Intercritical Annealing for cold heading 2

[Drewman030]

I am working on reducing cycle time and temperatures for a variety of low and medium carbon steel coils for subsequent cold heading (bolt manufacturing).

I always had the belief that inter-critical annealing is better because although it uses higher temperatures it allows for shorter cycle times (controlling process bottlenecking) but the attached paper seems to propose the opposite. (It's only the abstract/first page).

What annealing philosophies are used to produce a more cost effective/less time extensive spherodized structure? Thanks in advance for your help!!

http://resources.metapress.com/pdf-preview.axd?code=n67r5p3155385341&size=large

 

[CoryPad]

That is a great technical article by a giant (Hosford) in the field of metal forming. I recommend following his work. You can perform some simple trials yourself to learn more about the tradeoff between cost (time and temp) and performance (flow stress and ductility) for your specific application.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[redpicker]

No argument with CoryPad's advice.

In general, however, I think that for plain carbon steels, the inter-critical route would lead to shorter times. With low-alloy steels, however, sub-critical can be faster because long times are needed once you have dropped back below the A1. The alloying elements slow the austenite transformation so long hold times are needed to prevent lower temperature transformation products from forming.

rp

 

[TVP]

The short answer is it depends on the product form (hot-rolled wire rod vs. cold drawn wire) and the thermomechanical history at the hot rolling mill. Intercritical annealing is really only needed for breaking up the poor microstructure (large pearlite size, presence of bainite, etc.) that occurs in some higher carbon steels after hot-rolling. Most of the modern steel mills with Stelmor cooling beds are capable of producing relatively fine ferrite + pearlite structures that respond well to subcritical annealing. Exceptions are larger diameter rods that cannot be properly Stelmor cooled, garbage grades like SAE 1541, and steel producers that don't like their wire drawing mill customers. Newer techniques like thermomechanical rolling (hot rolling at low temperatures in the austenite region) produce even more refined structures, especially in highly alloyed grades that are prone to formation of bainite & martensite, which also favor subcritical annealing over intercritical.

If you are talking about annealing cold-drawn wire, then there is essentially no contest: the strain induced during the cold-drawing process accelerates the breakdown of pearlite during subcritical annealing. The research by O'Brien & Hosford (also published in Industrial Heating and Met Trans) was very good. Some other good references on this subject are as follows:

Robert L. Draker and Krish Naidu, “Control of Surface Carbon During Intercritical And Subcritical Annealing”, Wire Journal International, January 2000, p 96-103.

Krish Naidu, et al, "Quality annealing economically", Wire Journal International, May 1983, p 63-73.

D. Hernández-Silva, et al, "The Spheroidization of Cementite in a Medium Carbon Steel by Means of Subcritical and Intercritical Annealing", ISIJ International vol 32, no. 12 (1992)

http://www.journalarchive.jst.go.jp...tional1989&cdvol=32&noissue=12&startpage=1297

K. Aihara & S. Kanbara, "Influence of Prior Structure upon Spheroidization Rate and Cold Forgeability of Annealed Wire", The Sumitomo Search, No. 42, April 1990, p 1-8.

R. Kienreich, et al, "Improved Cold Formability by Thermo-Mechanical Rod Rolling", Steel Research International, Vol 74 (No. 5), 2003, p 304–310.

H. Hata, et al, "Development of High Quality Wire Rod through Thermomechanical Control Processes", Kobelco Technology Review, No. 25, April 2002, p 25–29.

T. Ochi, et al, "Special Steel Bars and Wire Rods Contribute to Eliminate Manufacturing Processes for Mechanical Parts", Nippon Steel Technical Report, No. 80, July 1999, p 9–15.

Ebner Industrieofenbau GmbH

http://www.ebnerus.com/

LOI Thermprocess

http://www.loi.de/en/loi/menuprodukte/inner.php3?currentNumber=2.2&currentIsExpanded=1

 

[Drewman030]

Thanks for all your help!! Great sources for plenty of trials in the near future.

 

[stanislasdz]

Great answer TVP thanks