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Engineering   Fabrication   Maintenance

Understanding the process of welding steel

By nothingbutnet, May 23 2016 10:55AM

If you understand welding you know that there's no chemical reaction taking place. What's happening is concentrated heat is being applied via friction, electric current, or flame. The heat melts down the edges of 2 separate pieces of metal and creates a liquid metal puddle shared between the 2 pieces. Once that puddle cools down it solidifies and forms into one solid piece of metal where once there was two.


Most generally there is a bit of extra metal added in to fill in the gaps to help form a stronger seam. However, that is not always necessary. Welding is sort of like throwing ice cubes into a cup and adding water, then freezing the whole cup. What you wind up with a one solid cup of ice. There is no chemical reaction taking place other than the normal melting and freezing changes.


Naturally, as with anything concerning civil engineering, things are a bit more complicated than these examples. To begin with, molten metal is normally very reactive with oxygen during metal fabrication. You must have some type of protection or else metal oxide can form and weaken your weld seam. There are different welding methods that deal with this kind of problem in different ways. Stick welding utilizes flux (found on the sticks) that boils off around the arc into a protective gas. This arc reacts with the impurities keeping them out of your puddle. TIG welding is a technique that uses an inert gas blanket (argon) to help shield off the arc and the puddle from the air.



Metallurgy


Whenever you melt or solidify or just heat a metal to an intense temperature for long periods of time, it will undergo changes to the crystalline structure of that metal. You have lots of stable and meta-stable solidifying phases when you use steel because of the way carbon and iron behave differently together when heated to different temperatures.


The carbon in the steel can be dissolved within the iron in varying ratios. It depends on the temperatures as well as the cooling rates you expose your steel to. Wikipedia explains metallurgy like this -


You may have an instance where the melted area and its 'heat affected zone' that is close-by might have a whole different crystalline structure than all the rest of your metal. It could contain more carbon grains or have different sized metal grains. The reason this is important is because this kind of chemical transformation, with different crystalline structures, could have significantly different mechanical properties.


Welding could ruin those crystal structures when metal working, so much that you'll need to re-temper one part so you can get it back to the proper structure. A good welding technique will include heat control to help minimize the overall size of your heat-affected zone. There are times when you can even weld using a different alloy than the pieces to be fused together that you get from a filler metal. That will cause the re-solidified puddle to be even stronger than your base metals as well as offset any heat damage.



The Chemistry


That's right! The chemistry can get to be very interesting once you begin to study it. However, it is nearly always 'phase change chemistry' (meaning from liquid to solid) as well as between various solid phases and crystalline structures. There really shouldn't be too much in regard to chemical reactions as they are usually defined.



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