heating and re-heating...........?

Date 05-22-2007 10:03 Edited 05-22-2007 10:07

Things "change " every time you heat ANY metal......you are steadily transforming it every time to you heat it....however ASTM36 is very adaptable to this...after all its made to be welded in many fashions ( i.e. in the middle of a beam etc.) Therefore it keeps its properties for the most part....a soft steel like this is very forgiving of heat. I would be more worried about stresses from flexing it too much in a heated state...forming stress cracks on the surface which might lead to trouble later...

After you are done forming it....buff or scotchbrite it real good and look for stress lines......if you dont have any big fissures you should be ok but it will probably be a little weaker/softer then original.

yes you gurus this is a shade tree answer ...but it is accliable to the question.

By cryogenicshaun (**)

Date 05-22-2007 12:10

It probably wouldn't be a bad idea to take a few hardness readings in the heat affected zone once you've completed the heating and re-heating process.

By js55

Date 05-22-2007 14:08

As Tommy said you will probably not do much damage to an A36 as long as you can ensure you have remained below transformation (about 1340). Over time carbides will precipitate and soften the material but this will take time. Hardnesses are probably a good idea for this reason. A couple of cycles or three should be OK.
In fact, when you have to add additional PWHT time under Nuke quals to accomodate the poissibility of weld repairs and additional PWHT, cook times of 8 to 12 hours are not unusual. This is well beyond the code min.
This has been done to a certain extent as well in the old hot slab bending of carbon steel pipe. They didn't always hit the correct angle the first time.
The heat regime response of most materials is not linear. Most of the change takes place in a relatively short period of time. Except of course for those alloys that rely on specific precipitations with their own time frame. Carbon isn't one of those.

By Joseph P. Kane (****)

Date 05-22-2007 14:06

Under ideal circumstances this re-heating could go on indefinitely, because 1200 Deg. F. is below the lower theoretical transformation temperature. However your heating method can introduce carbon, Oxygen, Nitrogen and Hydrogen into the steel, and that can affect it. But this could also happen on the very first heat!

If part of the metal is bent when the temperature is in the 670 to 900 degree F. range, this can contribute to hardenning of the metal through variuous mechanical mechanisms. If there is a rolling defect such as a seam, especially one with spurious inclusions on the boundry, the effect of the chemistry of the inclusion or even the mechanical barrier caused by the inclusion can have an effect.

Mainly depending on the Carbon content, the transformation range is above 1360 degrees for ASTM A36 steel. Reheating up to 1200 degres F. should not be a factor by itself.

By 803056

Date 05-24-2007 03:59

You are dealing with a low carbon base metal that essentially will not respond to a quenching treatment, i.e., it will not harden if heated above the transition temperature and quenched. There isn't sufficient carbon to produce enough martensite to be of any consequence. That being said, I'm not advocating quenching the A36 from high temperatures, i.e., above 1400 degrees.

Heating the A36 above the transformation temperature will reduce the tensile strength, relieve any residual stress present, and restore ductility. Remember the blacksmith? The blacksmith performs most of the metal forming at temperatures on the order of 1700 to 2100 degrees with out any ill effects. Spring back is zero at the higher temperatures. Forging operation are performed in the same temperature range. Deep drawing operations which result in extensive plastic deformation require repeated annealing or normalizing operations to prevent cracking and tearing. These heat treatments restore the ductile properties of the metal before further drawing operations.

At the high temperatures, plastic deformation takes place with ease. If after working the material, it is cooled in still air, it is essentially normalized and stress free. If the material is held at these temprature for an extended time, grain coarsening can occur, but it is unlikely you will be holding it at these temperatures for the time necessary to experience grain coarsening.

The danger, as I see it, heating the base metal to a temperature that barely approaches dull cherry puts the metal in a temperature range that is in transition, i.e., transformation from BCC to FCC has started, but the temperature is too low to push the allotropic transformation to completion. There is a high probability that the metal can cool to a temperature less than 700 degrees while performing the bending operation. The concern is that the temperature is close to the blue brittle range where the steel is brittle and cracks easily.

To see what I mean, try heating a piece of the A36 to a temperature between 300 and 600 degrees and try to bend it. Don't be surprised if it fractures.

Best regards - Al