The Secret Art of Welding

Welcome back to Metallurgy Monday!

Today we're going to talk a little bit about welding metallurgy. Most people, even professional welders, don't have a solid understanding of how electric welding affects the material. Since my specialty is iron and its alloys we’ll stick with that! If anyone has questions about other metals, I can either brush up on what I know or direct you to someone or some resource that knows more than I do (also known as ALL of them, haha).

Welding at its basis is defined as the joining of two metal elements, and generally when we talk about welding these days we talk about electric-arc methods which are CRAZY and varied. Other methods can include forge-welding (joining two or more elements without a filler in a fire with a hammer or press), torch welding (using an oxy-fuel torch setup and a filler material), all that way up to friction-welding, vacuum welding and WELDING USING FREAKING EXPLOSIONS! Seriously, look up explosive welding.

They use it to do such boring and pedestrian things as bond copper and aluminum to high-end stainless steel frying pans. It’s nuts, and the history of how it was discovered is also ridiculous. No matter the method, there are three important conditions that need to be met in order for a metal to weld, and almost every metal will weld if these conditions are met.

1. We need perfectly clean surfaces

2. We need perfect contact between surfaces

3. We need an inert atmosphere around the weld (no pesky oxygen)

   
   

Now, obviously, not all of these things are possible in every situation and especially not in pretty much anywhere other than a high-end laboratory setting. So how does welding work in industry? Regardless of whether it’s an oxy-fuel torch, a stick welder, MIG, or TIG, the weld is shielded by an inert atmosphere that is artificially created.

An oxy-fuel torch is run in the neutral range, meaning there isn’t too much fuel or oxygen in the flame. This prevents the build-up of oxides and allows the filler material to bond to the clean pieces being welded while the outer “envelope” of the flame shields the metal as the torch moves along the workpiece. A stick welder utilizes a rod coated in a material known as “flux”, which melts in the arc, absorbing oxygen and shielding the weld area, then depositing behind the arc further protecting the weld. MIG uses two main methods, hard wire and flux-core, but both utilize an automatic wire feed. Flux-core is the same as stick welding, but the flux is manufactured as a core inside the wire. Hard wire uses a shielding gas such as carbon dioxide or argon which floods the weld area and protects the weld from oxygen. TIG welding is similar, but uses an electrode made of tungsten to create the arc, a shielding gas, and a filler rod fed in by the operator’s other hand.

So that solves the inert atmosphere issue, how do we solve the other two?

The trick is that as the metal melts in the welding space a few things are happening: There is a partially molten base below the weld area, the weld itself being a liquid and crystallizing behind the weld, and the area between the electrode or torch being PUMPED full of plasma from the arc, gasses from the torch or shielding gases, and metal oxides and vapors coming off the electrode/filler material. This is a WILDLY dynamic area with CRAZY high heat and - we won’t get TOO into it - if this melting is controlled properly we end up with perfect contact between our surfaces and perfectly clean surfaces (between the action of flux/shielding and the prior mechanical cleaning of surfaces plus heat)

In this area, as the weld progresses, there forms a region we call the Heat Affected Zone, or HAZ. As with everything, it goes deeper than that too, but the HAZ is basically what makes the microstructure of the weld develop.

Oh, look! Here’s a snazzy photograph of the microstructure of a weld! Looking closely you can see a VERY sexy weld, and a good visualization of how it formed as it cooled from molten to solid. The HAZ is labeled, but otherwise pretty obvious as a different microstructure from the material that was welded. 

The lighter area is sometimes called the “Unmixed Zone”, and the transition between that and the edge of the HAZ proper is referred to now and then as the “Partially Melted Zone”, and together these are called the “Fusion Zone”. How relevant these are depends on how far you zoom in. Like I said, we’re going to just stay with the HAZ in general. 

The cool thing is that in the HAZ all the metallurgical action are what we call “solid-state” reactions. These are the result of the intense heat of the weld in a relatively small surface area. Depending on the alloy of steel this causes anything from minor grain growth and recrystallization up to crazy things like forming new phase changes, precipitates and stresses that can seriously mess up the weld as it cools unless cooling is slowed or the metal is preheated to a certain point.

Like everything in steels, the formation of the HAZ is a function of time-over-temperature. Depending on the rate of heating over the rate of cooling, and the alloy of the steel, lots of strange things can happen. We usually refer to Continuous Cooling Transformation (CCT) diagrams to see what those transformations might be, at what temperatures, at what times. We will cover those later down the road.

The reason this matters is that the rate of recrystallization has a pretty direct effect on the strength and ductility, and grain growth, and THAT matters when it comes to things that also matter, for example strength and flexibility.

One more cool thing before we wrap this up! The molten metal in the weld pool is formed by surface tension, sort of the same as water forming raindrops. The beautiful structure in the weld cross-section is formed by the weld cooling from the bottom up to the top, which is held in place by surface tension as it cools, and is just one more remarkable thing about steel that we rarely see and take for granted every day.

So the next time you see a weld on a railing, or a shelf bracket, or something you walk past or use every day, think about and be thankful for the beautiful structure inside that fillet of metal that keeps our whole crazy world together.