spheroidization

With many steels in which a completely spheroidal structure is difficult to obtain with a regular procedure, preheating is helpful. This is mainly applicable to hypoeutectoid steels, but it has also been useful with some low alloy hypereutectoid steels. Preheating consists of heating the steel at a temperature about 25 to 100 degrees F below the critical before the steel is austenitized. The preheating may be followed by cooling to room temperature before the austenitizing heating, but obviously it is more economical to hold the steel at the preheating temperature and then to raise it immediately to the austenitizing temperature. The purpose of preheating is to agglomerate the carbides in the steel so that they will be more resistant to solution in the austenite during the subsequent heating. The presence of undissolved carbides, or carbon concentration gradients, in the austenite is conducive to the formation of a spheroidal, rather than a lamellar, structure when the austenite transforms. This treatment has been found effective in a number of carbon and low alloy steels with 0.40 to

0.80% C. The temperature for 1018 would be from 1575 to 1650 F and furnace cooling at 50 degrees per hour to 1300 degrees.

Dixon

Ok for steel with 40 to 80 points of carbon. But 1018 is 18 points. How is the spheroidization occuring with such low percentages of carbon?

-- Ed Huntress

Without going into a lot of reasearch, I would have to say you're right. The term spheroidization, at a certain carbon point probably "morphs" with the term "normalization". I would suspect the original question had to do with improved machining properties, which I would personally handle with machining techniques.

Dixon

Yeah, it could be. I'm not presuming to know the answer, I was just curious.

The problem with learning technical subjects like metallurgy at the

*technician's* level, as I have, rather than at the engineering level, is that you can be victimized by the simplifications they use to teach you. I was taught that speroidization requires a lot of dissolved carbon, which is precipitated out of the cooling metal as spheres or other rounded, chunky shapes, rather than as flakes or laminations. It's mostly of interest in relation to cast irons.

But there could well be something else going on here that I'm unaware of, and it would be interesting to know if you can get spheroidization in low-carbon steel.

-- Ed Huntress

Ed, I talked to a metallurgist friend and he said that the spheroidization process at 18 points carbon would be so insignificant that you would never notice a machining change. However, there would be very slight spheroidization of the small % carbon, but it would only be detectable by microscope. Something about pearlite strings. I personally never heard of anyone bothering with it in steel, only cast iron.

Dixon

Thanks, Dixon. It's good to hear it from someone who's really into it.

-- Ed Huntress

thanks all for the replies.

my aim of spheroidizing the steel is to reduce hardness and increase machinability of the steel..

how to determine the cooling rates?1.cooling rate while cooling to room temperature 2cooling rate frm above critical temp to below critical temperature where we hold the sample

if possible please tell me the standard treatment you follow in industry or tell the resources where i can find out information about treamments being followed in industry to achieve spheroidization?? regards vikram