Cooling Times of Odd alloyed Steel 1

[Trackhead]

I would like to heat a piece of track work rail steel of the following profile to the following temperature distribution, all shown in the picture at the link.

This track work can be of that cross-section in the link above, but extruded up to 80 feet long and is made of a low-alloy steel with a pearlitic microstructure. I want to keep the pearlite microstructure, but I just want to heat about 3-4 feet in length of the rail extrusion to the temperature distribution shown in the above link. The chemical composition of the steel is shown in the below link.

If I were to heat the base of the rail (where it shows the temperature range to be 1000 to 1600 degrees Fahrenheit) to 1200 degrees Fahrenheit, and the head of the rail (where it shows the temperature range to be 650 to 850 degrees Fahrenheit) to 750 degrees Fahrenheit, how quickly could I bring the steel down to room temperature throughout its entire mass (72 degrees Fahrenheit) in the following quench scenarios while avoiding the formation of martensite in the steel?

1) Completely submerged in 72 degree Fahrenheit, still water.
2) Sitting on concrete at 60 degrees Fahrenheit in still air (observed to take 6-10 hours).
3) Cooled through application of water misting.
4) Sitting in front of a 48" diameter fan which produces 19500 CFM with the airflow running transverse to the length of the rail extrusion.
5) Cooled by direct application of liquid nitrogen (observed to take 12 minutes but unknown if the microstructure was changed).

Any advice in this realm would be much appreciated because I am very lost on this and I don't have any Jominy charts or anything to look at cooling rates or evaluate anything analytically for this specific type of steel.

Please and Thank You

 

[kingnero]

There is a website somewhere where you can find all kinds of TTT/CCT diagrams.
I probably still have them for european rail steel, but not on my server - will have to look through some of the old HDDs.
They will tell you the cooling rate and the final structure. However, you will certainly not end up with the same quality of steel as it comes from the mill!

Are you trying to mitigate the thermal deformation caused by thermite head repair by any chance?

 

[Trackhead]

@kingnero no I am trying to heat the rail to make it more malleable for bending, but at the same time I need a quick way to cool it down after it has been heated and bent, without significantly changing the hardness or the microstructure.

 

[bobjustbob]

At 1200°F you should not austenatize that steel. You are still in the temper range. You will soften the steel that is heated above it's prior temper temperature. Since you are not getting up to the austenite range, you cannot create martensite, no matter how quickly you cool it.

Bob

 

[Trackhead]

@bobjustbob Thank You I suspected as much. However, do you know if the reduction in hardness can be minimized by cooling the steel quicker after bringing it up to 1200 degrees F?

For example, if I had two pieces of rail steel with exactly the same composition which were both heated to 1200 degrees Fahrenheit, but one was cooled to 72 degrees Fahrenheit by submerging it in a dunk tank (which took 2 hours to cool it) and the other was cooled to 72 degrees Fahrenheit with liquid nitrogen (which took 12 minutes to cool); would the 2nd sample which was cooled more quickly retain more of the original hardness than the first sample which was cooled more slowly?

I suppose this is where I would need to take a look at hardness and cooling rate charts but I don't have any for this rail steel neither do I have any rules of thumb for the amount of mass in each section of the rail being cooled.

 

Dull to medium red is a safe temperature range for a short holding time.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[bobjustbob]

The tempering is dependent on the temperature that the steel reaches. The cooling rate from temper is not going to change your hardness. I would cool it in agitated water.

Bob

 

[Trackhead]

@bobjustbob Thank you that sounds ideal! I still don't have any charts or anything to work with to be able to tell ahead of time how quickly that would cool the steel from ~1,200 degrees Fahrenheit to 72 degrees Fahrenheit in agitated water. Would you happen to know a ballpark figure or a rule of thumb as far as the cooling time using that method on a 4 foot long section of the rail?

 

[bobjustbob]

Oh, I don't know. That really depends on the amount of water in the quench tank and how quickly the water is moving past the steel. 5 minutes maybe if you have enough water. Are you worried about throughput through your quench tank and into another operation?

Bob

 

[Trackhead]

@bobjustbob exactly. Doing it with high CFM fans may be the best thing ergonomically for this as well, so my main interest is the cooling time as well as the method. I can simulate these things with CFD but I was just curious if there was a metallurgic chart or rule of thumb or something that can give me a good educated estimate as far as the best setup to accomplish this.

 

[bobjustbob]

Where I have worked, we never had a chart or rule of thumb because we don't really care. There are materials where you need to ensure that it gets down to ambient, but this steel is not one of them. Your cooling rate does not matter. There is no metallurgical reason to quickly cool this down to ambient temperature. If you don't need to handle it immediately let it air cool. If you have a fan, add a fan. If you have a quench tank then go ahead and quench it. Are you going to be doing this a lot?

Bob

 

[Trackhead]

@bobjustbob and I agree sometimes ensuring the metallurgical aspects of the heat treatment are near perfect may be move valuable than cooling time, but here's my situation where I work:

I heat these rail in a furnace and let these rail air cool but it takes 6-10 hours, which inhibits throughput. I would like to speed things up as far as the heating and cooling times go while still accomplishing what needs to be done (making it malleable for lateral bending) and without significantly altering the properties of the rail.

I can induction heat every one of these and quickly cool it down with application of liquid nitrogen. This would be very fast and allow production of these rail to continue at an amazing pace.

I can heat them in an oven and sit them in a quench tank. This may not be as fast and I would have to re-route fluid flow lines to a tank and make room for a tank and a material flow plan for the rail to be moved into the tank, and setup the tank to have a set flow rate or to just be agitated, decide what quenchant is best, etc.

I can also heat them in a furnace and let them cool under fans. This will probably only be a little faster than letting them sit in open air and cool but a bit easier to maneuver in a material handling aspect.

There are a myriad of different approaches I can take. The heating time and time the rail is held at a specific temperature are not as important to me as the cooling time. I am mostly concerned with a way or a chart or rule of thumb; something that I can use to determine cooling times using different cooling times using different methods without ruining the rail. That way, I can better weigh my options.

 

[EdStainless]

What matters is the cooling rate for the first few hundred degrees, you don't want to cause distortion by uneven cooling. Fans should cool in this temp range quickly and uniformly. Then once you are down under 1000F spray (or mist) them with water to cool faster.
Agitated water is a faster quench than Liquid Nitrogen. The nitrogen boils on the surface and creates a gas layer that limits the cooling rate.
I do presume that you will be heating from the flange, and 1200F (maybe even 1250F) will be plenty safe from transforming the structure. Since the head will bend much easier it will only need to 600F or so to make it a little easier to form.

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P.E. Metallurgy