I still don't know for sure if its tempered or not. But here is some info about welding to a tempered piece of steel.
I could write a pretty good sized book about steel and its hardening processes, but basically a tempered piece of steel has been heated to its transition point, and allowed to cool slowly.
At that point, it is is annealed, and is as soft and malleable as it can get. This is when all the bending, drilling, machining and other things are done to it.
Then, it is heated to the transition point again, and then cooled rapidly. Either by quenching it in plain water, salt water, or oil, depending on how likely it is to crack.
At this point, it is hardened, and is about as hard, rigid and brittle as it can get.
It is then slowly heated to a precise temperature, either directly, or in a series of steps, held at that temperature for a certain length of time, and then allowed to cool at a prescribed rate.
At this point it is tempered, and its strength, rigidity, and toughness is determined by that tempering process.
By that process, I could take a piece of steel, and make it where you could bend it with your hands, need hundreds of pounds of force to bend it, or have it snap in two instead of bending.
There is a ton of science that goes into that process. When you weld to it, you change its properties.
In a way you cannot predict. It may be harder, softer, more or less brittle, you just never know.
What is worse, you usually create a narrow transition zone between two sets of properties that creates a stress riser.
When you weld to a thick piece of steel, the part touching the weld is heated to well past the annealing point, and then, when the thick piece of steel next to it sucks the heat away, the rapid cooling causes that area to be much harder than the rest.
So, when its stressed, the original part bends slightly, but that part does not, and it usually cracks at the seam between the two.
Most of you have seen that without realizing it. Its when you weld two parts together, and it breaks right next to the weld in the nonwelded metal.
I have seen parts that are one wide weld. They were repaired, and broke right next to the weld. Then welded there, and broke a little farther along. Rinse, lather, repeat.
If you are going to have to weld to a tempered frame, I would suggest using a torch to heat the weld to a very dull red, not too hot, for a few minutes after welding to at least spread the change over a wider area, and then pounding it with a hammer as it cools to do some stress relief.
This might help.
But, DO NOT heat a wide area of the frame, you will be just making it worse if its tempered, by drawing the temper of more of the metal.
But, far better, is to drill and use bolts instead of welding. If you have a weld-on hitch, weld some pieces to it, and then drill those pieces and use bolts to mount them to the frame.
Its far better than taking a chance of a cracked frame or mount.
And, just because its tempered, does not mean it cannot be drilled. A good TiN drill, with lots of oil and a slow speed will drill some pretty hard stuff. You may have to start small to increase the pounds per square inch loading.
