Manu said:
The points of the gentlemen in the video:
- "blood red" isn't a reliable way of controlling the temp.
- The "blood red"/1600F range is too high for a stress relief. 1600F is high enough to anneal the material / reducing its mechanical strenght.
- why buying such a good material (4130) to reduce its mechanical strength by a heat treatment? a structure is as strong as the weakest point/part. it would be cheaper to buy a lower grade material.
- unless you have a good understanding of what your are doing and control what you are doing, it is better not to do anything after welding.
Well MAYBE, just kinda depends on some more factors and what you are trying to achieve.
Annealing, Normalizing, and Hardening all refer to the same physical phenomenon in an alloy, the formation of different percentages of micro structures. There are a few different structures that in form alloy steel but whats important for this conversation is the formation of martensite, which increases hardness of the material. The amount of martensite that is developed is directly related to the thermal history of the metal, or how fast the material is cooled from a given temperature.
If the steel is cooled very quickly you end up with large amounts martensite, making the material hard, strong, but brittle and unreliable. On the other hand if the material is heated up and let to cool slowly the material will have more ductility and higher fracture toughness. Anytime some says they heat treated, annealed, normalized, tempered, or hardened a material all they are saying is that they changed the micro-structure of the material to vary the properties by some means. Now that we know that steel alloys are affected by temperature, time to cool, and transformation of the micro-structure we can look at with a bit more understanding whats going on at the weld site.
So they where talking about TIG welding and we will exam it from that vantage point first then look at gas welding. First 4130 when purchased is not in a hardened state, if it was you could not cut it saw, file, or machine it, chances are it is in a "normalized state". Which means they heated it up evenly to some point and let it cool "slowly" in air or in a kiln to promote a uniform structure with uniform properties. So now lets weld our tube with our tig torch, the metal is heated very quickly by the arc of the torch, much higher then "blood red". Filler rod is added to the weld puddle we go about our welding, till we finish the weld and cut the torch off. All the time we are welding away we are heating the metal surrounding the weld site, forming what is referred to as the heat affected zone. The metal in the heat affected zone is raised to various temperature depending on the distance from the weld puddle and the intensity of torch, the further away from the puddle the cooler the temperature. Once the torch is off the weld begins to cool and the metal being an excellent conductor it will draw the heat away from the weld
quickly. So what happens if steel cools very quickly, as you can see from the diagram it develops martensite making it hard and possibly brittle. Tig torches are very good at putting heat right where you need it with out putting to much heat into the rest of the metal, this has lots advantages for distortion but yields welds that tend to harden some what due to the small amount of metal that is heated. The degree to which the weld hardens depends on lots of factors from the thickness of the weld, the size of the heat affected zone, ambient temperate, the type of material your working on, plus some other factors. Yeah so super simple stuff going on in the metal at the weld right. The take away, is that metal is heated way above 1600f to weld but the weld does not become "annealed" because the surrounding material quenches the weld making it cool quickly and some what hardens it. In fact chances are the tig weld will be harder then the original tube.
On the other hand if you gas weld your 4130, the heat affected zone is much larger then a tig. So in essence you have heated up more metal than the tig torch did so the weld will cool slower therefore making the joint softer then the tig torch weld. The more metal you heat up the longer it takes to cool, resulting in a different micro-structure then the quicker cooling tig weld.
So in regards to heating something till its blood red is bad for the material, well MAYBE. It depends once again on how fast the material cools down from blood red, if the material cools quickly it will harden, to slowly it will anneal. Most aircraft tube are thin and cylindrical, which means they have lots of surface area for the given volume of steel. The high surface area to volume means the steel can cool quickly and a quick cooling steel becomes hard steel as stated before. Most aircraft that are gas welded will have to have the weld clusters re heated to tweak the frame for alignment, when you reheat the cluster it goes "blood red" so you can manipulate the frame. The weld clusters have so many tubes attached to them that they draw the heat away quickly leaving the metal in its original state of hardness.
Manu said:
You mention "normalize". isn't normalizing something different than annealing? isn't normalize a process that relief the residual stress created by welding or other production process? something like letting a part resting for 6 months, or shot peening, or heating to a "low" temperature and have a very slow (controlled) cooling.
IN reference to stress and stress relief, the micro structure has a certain amount of give to it and can bend and deform so much before breaking. As you use up the give in a material it becomes harder, this is often called work hardening, once you use all the give the part breaks. How ever you can reset the amount of deformation possible through heat treating and making the metal once again soft, hence why its called stress relief or annealing. Shot peening is the direct opposite your putting stress into the outside of the part to make a layer that is harder to increase the strength of the part.