Propane torch gas cutting set up for cutting steel plate by hand and machine.

There are several options available when it comes to cutting steel, and in this article, I would like to address oxy-fuel gas cutting and more precisely a method where propane gas used as a fuel.  There are many benefits to using propane gas instead of acetylene however there are some shortcomings too.

Propane is much cheaper gas to buy, it is readily available and much safer to use, transport and store.  There are also some differences that can be used for the benefit of skilled operators. For example, propane burns at lower temperatures compared to acetylene,  and this can be used in gas welding or braising of very thin metals or meshes. As far as propane gas cutting of steel the biggest benefit of propane is safety factors and cost. It is a little harder to use propane due to lower burning temperature and especially it is harder to ignite and keep flame going outside in the wind.

In order to understand the gas cutting process, we need to be aware of one very particular phenomena.  In the atmosphere of pure oxygen, steel can burn at a temperature lower than its melting temperature. If you ever picked up a hot smoldered piece of wood out of the fire and blow at it, you see a red glow of wood as it gets burned without a flame. It is a kind of similar thing that happens to the steel in the environment of pure oxygen. The heating flame is there just to heat up the steel to that kindling temperature and then the jet of pure cutting oxygen burns away that very localized area of steel. The concentrated jet of pure cutting oxygen not only burns oxidizes the steel in the line of the cut, but it also removes all of the products of oxidation out of the cut gap preventing fusing to the freshly cut sides. This can be observed as a jet, fountain, or stream of molten metal under the material that we cut.

Stainless steel can also be cut by using gas cutting equipment but it requires constant use of special flux during the cutting process. This happens due to the physical properties of oxides and their high thermal resistivity. The oxides melt at the higher temperatures than parent materials, that is why one needs to use flux that can break down those oxides. Naturally, with technological progress, it is a very uncommon sight to see the oxy-fuel cutting of stainless steel in the production environment. This type of gas cutting  widely substituted by more effective processes like plasma cutting or abrasive cutting where possible.

Let’s talk about the structure of gas cutting flame, what we can learn from it, and how we can adjust the flame to suit the job we have.

Here is a section of the images that show different types of flame. The very first is a normal flame where

Normalising gas cutting flame

A – is a cold zone also known as restoring or normalizing flame. This part of the flame is not hot enough to melt steel but its presence ensures that no atmospheric oxygen can get to the molten steel and produce impurities in the form of oxides. This flame also slows down the rate of cooling of any molten steel and that aids any impurities to float to the surface and form slag instead of being trapped in the steel.

B –  is a core of the flame which also is the hot part of the flame. The most vigorous reactions of oxidation of the fuel happen in the core of the flame.

C – is a Hot zone, the area where the flame is the hottest. It is just the outside of the core of the flame as on the opposite side of the core almost at the edge of the nozzle is a cold area as two cold gasses are cool before they ignite.

Knowing this lets look into the carbonising flame, it happens when we have fuel rich situation where in the flame there is more fuel than can be combusted with supplied oxygen.

Carbonising Gas Cutting Flame

A – First of all we can notice that the area of the cold normalizing flame is really large as unburned fuel in the flame burns away when it mixes with atmospheric oxygen causing broader propagation of the flame and wide normalizing zone.

B – The core of the flame is not perfectly formed and has very smooth feathered edges.

C – Hot zone is not defined

D – We can observe traces of fuel that was not burned in the core of the flame due to the lack of supplied oxygen.

Here is another type of flame and it called oxidising as it excessive in the amount of oxygen present in the flame as it’s quantity is far greater then necessary to completely consume all available fuel.

Oxidising gas cutting flame.

A – Here we see that cold zone of normalising flame is drastically reduced.

B The core of the flame is very defined and is much smaller as the time of complete fuel bun is way shorter due to an increased amount of oxygen.

C – The edge of the flame core nearing hot zone is super sharp

 

There are certain preferences in gas welding and brazing that dictate various types of flames to use depending on the type of materials that one working with, but that is a subject for another article. Let’s look at what happens in the gas cutting process when we try to cut with oxidizing flame.  When you look at the image below the first of all you notice a new item on the list E – is a jet of pure oxygen that is supplied through an independent channel in the center of the nozzle.

Cutting gas jet inside of oxidising flame.

As you can see now the normalizing flame that we used to mark as  – A, now almost none existent. B and C remaining the same as described already.

If we look at the normal flame with cutting jet (Picture below) in it, we can clearly see that normalizing flame – A, is well established along the whole length of the cutting jet – E, and there is no excessive carbon in the flame as out flame core – B is defined and proportionate.

Perfect Gas Cutting flame with oxygen jet.

I hope that now you have a little better understanding of the different types of flames used in gas welding and also gas cutting of steel plates. Here is a video that will give you a little more information on the subject

https://youtu.be/fCPJNJ8WFxA

As you probably noticed in this video about gas cutting, i said that the slag that we have after gas cutting steel is oxidised matter, essentially iron oxide along with small quantities of various oxides of alloying metals that can be found in different steel grades. If you remember any chemistry from the school, you will also remember that common rust is also iron oxide. Comes up a question, how come that iron oxide that we get from gas cutting steel is black in colour and iron oxide that is rust is brown?

Iron occurs naturally in two different forms: Iron (II) and Iron (III).

•    Scientific Name of Oxide of Iron (II) is Ferrous Oxide (FeO).

•    Scientific Name of Oxide of Iron (III) is Ferric Oxide (Fe2O3).

Iron also plays a role in blood transport as it has a good oxygen-carrying capacity or it has a strong affinity with oxygen.

One of the most important functions of iron is the transportation of oxygen to the blood. Iron’s main purpose is to carry oxygen in the hemoglobin of red blood cells throughout your body so your cells can produce energy.

Also, iron improves oxygen storage through myoglobin. And hence iron also plays a

vital role in the human body by making ferrous oxide and then transferring the bound oxygen to the blood.

Rusting is a type of Chemical Change. It results in the formation of Iron Oxide which is

an entirely new substance. Chemical Reaction of Rusting is as follows:

Iron + Oxygen (from environment) + Water (Humidity) —> Iron Oxide (Rust) Fe + O2 + H2O —>FeO and Fe2O3

Amazing thing is that the colour of our blood comes from the red blood cells and they get their colour from tiny particles of rusted iron. It is an amazing and humbling realization that even the most advanced and technological thing that we do as building submarines or skyscrapers is simply a manipulation of ourselves as part of such skyscraper is within each one of us.

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