Probably all of us are familiar with the terms welding and know its uses. Welding is used to join to metals by fusing them together. Most of us also know how welding works.
You take to metal pieces, heat them on the edges where you want to join them, let it cool down, and done.
The pieces are magically fused. Well, scientifically would be a better term to use! Either way, all we know is that the metal melts on heating and than solidifies on cooldown. When both the pieces are touching it other, they fuse and join when cooled down.
However, in this simple seeming process, there’s quite a bit of process taking place which we don’t bother thinking about. Why should we? Our work is getting done after all. But some people do like to know about it and that’s great.
Curiosity is definitely great. There is one question that appears quite frequently when talking about metal welding. Is welding of metal a physical or chemical change?
Surely enough, if you’ve tried reading about welding, you’ve probably seen this question somewhere, or pondered upon it yourself. Let’s dive a bit into the process.
Is welding of metal a physical or chemical change?
Now, it’s obvious that physical change is happening when you do welding. Two pieces are being joined together. Their physical form is changed, at least slightly. There is no doubt in it whatsoever.
But, since metals and heating is involved, it is possible that the question arises. Can there be a chemical change happening? Maybe, maybe not. Since we heat the metals, it is possible that a chemical process is happening.
Whether it can be considered a chemical change or not, we can only know by taking a look at the whole process. Before that, let’s run through quick definitions and examples of physical and chemical change.
- A physical change is just the change of physical form. No new materials are formed and only the form changes. The composition of the material doesn’t change. For example, grinding, cutting, shattering, etc. are all physical changes. Phase changes are also physical changes and not chemical. For example, boiling water or freezing it is a physical change. The molecular structure (H2O) doesn’t change in ice or water vapour. Boiling would be a chemical change only if the molecular structure of the water decomposes to H2 and O2.
- Chemical changes can also be called chemical reaction. Here, the molecular structure of the ingredients (reactants) changes and forms new products. For the simplest example, burning of wood is a chemical change. When you burn wood, it turns into ash, carbon dioxide, and water.
The process of welding
The whole welding process happens on heating and cooling. Two metal pieces are placed together and concentrated heat is applied via flame, electric current, or friction.
The heat causes the metal to melt and since their edges are together and heat is being applied on the edges, a puddle of liquid metal is formed. This liquid metals is the melted form of both the edges, and when it cools down, it solidifies and forms a joint.
One solid piece of metal is formed using two. Usually some kind of wire or extra metal is used to fill any gaps and strengthen the seam. So, is welding of metal a physical or chemical change? Is there even any chemistry involved? Let’s take a look.
Is there any chemistry involved in welding?
At the very basic level, yes, chemistry is involved. However, it only causes phase change when melting and freezing. It is a bit more complicated than just saying that welding is melting the edges of two metal pieces and letting them join on cooling down.
That is, of course, if we go in depth into it. Without going much into technicalities, let’s see some key points that help us decide what kind of change happens. Molten metal is quite reactive when exposed to oxygen.
It might cause metal oxides to form and that would be considered a chemical change. But if that happens, the weld seam can weaken. That is why protection is used while welding to keep that problem at bay.
To deal with this problem, different techniques are used in different methods of welding. If we talk about TIG (Tungsten Inert Gas) welding, an inert gas is used to shield the welding arc and molten metal from air. That means, the molten metal does not react with oxygen.
There is also some metallurgy in welding. If we heat metal for extended periods to a high temperature, or melt/solidify it, it undergoes some changes in crystalline structure. If we talk about steel, it has different stable and metastable phases.
This is because iron and carbon can behave differently with each other at different temperatures. Depending on the temperature, the carbon in steel can dissolve or precipitate within the iron.
This might happen in different ratios according to temperature. There’s a lot more to it but in short, there can be changes in the crystalline structure of the metal.
The are that melts during welding and solidifies later can have a different crystalline structure after the process. Even the other nearby areas affected by the heat can undergo changes in the crystalline structure.
This is a chemical transformation and can affect the mechanical properties of the metal, sometimes to an extent that you might have to re-temper that part to get it back in shape again. There are also certain measures taken for this in most welding techniques.
As we tried to explain briefly and in a simplified manner, there are definitely chances of chemical transformation happening like the change in crystalline structure.
But usually there is just change of phase from solid to molten, and from molten to solid again. Is welding of metal a physical or chemical change? Yes, you guessed it right! It’s a physical change.
As we saw in the definitions, change of phase in considered physical and not s chemical change. However, as mentioned above, there are chances of chemical reaction happening if molten metal is exposed to oxygen.