I believe the concern is that when you exceed 1330 degrees F you are above the lower transformation temperature. You can begin to initiate allotropic transformation and some of the microstructure can be transformed to austenite. The austenitic microstructure can decompose and result in the formation of martensite if the cooling isn't controlled.

This is a factor when fabricators use high heat to induce camber and sweep. I've seen the fabricators heat the steel to a good red heat and then quench it with water to speed the cooling process.

How hot did the steel get? I don't know! I didn't monitor it!
How much of the microstructure was transformed to austenite? I don't know! I'm not a metallurgist!
How much of the austenite decomposed to form hard martensite? I don't know! How the heck am I supposed to know that, I'm a welder, not a metallurgist!
How much carbon was present in the base metal? I don't know, I didn't ask for certified mill test reports when I ordered the material!
What is the base metal specification you are heating? I don't know, I cut off the end with the identifying marks!

Do you see where the engineer might have a concern? Fortunately, most steels used for structural framing contains sufficiently low carbon content where the hardenability is not that severe. Still, all involved should be wary of the consequences of heating above the temperature where allotropic transformation can begin and uncontrolled cooling rates.

A simple demonstration might help. Take a piece of steel plate and drill a 1/4 inch diameter hole in it using a electric hand drill. Then take the same plate, heat a spot to a bright red and quickly quench it in water. Drill another 1/4 inch hole in the area that was heated and then cooled. Was there any difference in the effort required to drill the second hole? Unless the carbon content is very low, the second hole should be more difficult to drill. The higher the carbon and other alloying content, the more difficult it will be to drill the second hole.

This isn't exactly what you described, but the cold steel surrounding the area you heated can act as a heat sink and begin to approximate the affects of quenching. This is very pronounced when making a tack weld on cold steel.

One thing to keep in mind when welding, is that multiple pass welding tempers the previous weld beads and HAZ, thus causing any martensite that may have been formed to be tempered. Tempered martensite is a microstructure that is strong and tough with good ductility. Another reason to follow a WPS that was developed to control the thermal cycles and resultant mechanical properties.

Best regards - Al

Hello JA, I believe there is a term referred to as "time at temperature" that can explain the differences of heats generated from welding and those that are imposed by external thermal heating. Various welding processes, by the nature of their physical differences, can have varying amounts of heat input over varying amounts of time. Metallurgically speaking, as 803056 pointed out, the resulting changes can cause detrimental results in some materials. A36 steel is very resistent to many heating issues due to it's metallurgical makeup, since it is low in carbon and other alloying elements it will tolerate many abuses without causing it to display cracking and similar thermal related issues. On the other hand, many of the alloyed and higher carbon content steels will basically self-destruct if a fairly strict welding and heating regimen isn't followed. That doesn't necessarily mean that preheats, postheats and other types of heat application can't be used, it simply means that for a given material there is likely to be a fairly tightly regimented sequence of application, along with time and temperature restrictions. I hope this has given you a different perspective to consider here. Regards, aevald