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Heat & quench method

Heat & quench method

Heat & quench method

I am currently in Panama City, Panama Building a museum By Frank Gery.
 The col.s for one of the roofs are covered with a concrete pour, so in order to set the embeds which are welded to the col.s with 1" plate approx. 3'x 1' Makeing a "T" connection The embeds them selves are 2'x3'x1 1/2" the embeds are only 3mm-8mm in offset so to move them with out cutting or other destrugtive methods we began to heat and quench areas to move the embeds which worked just fine but the inspetors said that is not acceptible because we are changeing the tinsel strength of the steel and must submit an R.F.I to the engineer. My question is at what temp does mild steel begin to change? I know it does but not at the temp I am subjecting it to.   

RE: Heat & quench method

Most lower grade structural steel is simply hot rolled low carbon steel, per ASTM A36, for example.  If this is what you have, then heating it to almost any temperature has very little affect, if any, on the tensile properties.  It sounds like you are heating and water quenching to get the metal to move.  Again, with low carbon steels, there is little you can do, except for severe cold working, to affect the tensiles, especially on the downside.  Cold working actually raises the tensiles.  Water quenching from very high temperatures (i.e. 1500F deg or higher) may increase tensiles, but not much, due to the low carbon content.  Even HSLA (high strength, low alloy) structural steels like ASTM A709 or A572 are not typically affected by heating and cooling that you describe.   But if you have very high strength steels (e.g. A514, like T1), then you can't heat it very high (sometimes as low as 800F) without possibly lowering the tensiles. That depends on the tempering temperature.  Mostly these temps are higher .. in the 1000F and up.  So it sounds like you are safe in doing what you want.  Dull red heat gets you in the tempering range.

RE: Heat & quench method

Using heat to move structural steel is fine provided it is controlled. However, I do not agree with quenching immediately after heat application. Quenching at excessive heat can induce surface cracks in stress risers - like threads and result in reduced ductility of the steel.

Heat is used for two reasons - one is to reduce the yield strength of the material to allow it to deform easier along with using thermal stresses to help move the material and induce residual stresses during cooling. I would favor spot heating followed by forced air cooling.

Your inspector is correct and based on what you described the Engineer should be involved to evaluate your technique.

RE: Heat & quench method

I agree too with MetEngr's advice ... if you have a geometric configuration such that the possibility of inducing cracks exists, then I would be more cautious.  I could not picture the geometry from your description. There are easy ways to inspect for cracks in most applications (e.g. MT, PT, VT, UT).  But if you are simply heat shrinking to move flat surfaces on low carbon steel, then I maintain that it sounds like a safe procedure.  Heat cambering and straightening is somewhat common practice for instance, in structural beams, for bridge construction.  I think the issue was one of effect on tensile strength, not physical damage.  I believe my initial post addresses that issue.

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