Welding cast iron might be one of the most tricky subjects out there when it comes to welding. For you to claim that you can weld cast iron properly it will take a lot of practice and failures along the way. The first thing you need to take care of though is to gain all the knowledge about it before you try it out. you need to have your basics clear as it will give you further confidence to deal with any issues which arise along the way. Let get into it, shall we?
Welding cast iron can be tricky and you have be extra careful while dealing with it because of its brittleness and low ductility. Keep in mind to use the right electrode for the job and to pre-heat the metal when needed. Temperature management is the most important part of dealing with cast iron and if done properly your cast iron can be welded without any cracks or stress.
UNDERSTANDING CAST IRON
Cast iron consists of iron (obviously), some amounts of silicon, and carbon. The presence of carbon can vary between 1.7 and 4.5 percent. Cast iron in general can be divided into further subcategories depending upon the chemical composition of the alloy. Depending upon the type of cast iron you are working with, the difficulty of welding can also vary.
The first and the most popular type of cast iron is grey cast iron, it is generally considered to be more weldable when compared to white cast iron because of its higher ductility. Grey cast iron consists of graphite flakes which are formed when carbon precipitates into some sort of pearlite or ferrite crystalline microstructure. One drawback of using this type of cast iron is that during welding, some graphite can get mixed with the weld pool and can cause embrittlement.
The second type is the white cast iron, this is considered to be unweldable by a lot of people because of its extremely low ductility. White cast iron consists of iron carbide and an extremely brittle cementite crystalline microstructure.
Some of the other less used types consist of ductile, nodular, and malleable irons which are considered to be easier to weld.
It is very important to check which type of cast iron you will be working with, try to check the specification from the manufacturer. One way to check whether you have grey or white cast iron is to look at the color of the fracture point.
WHY IS IT CONSIDERED A DIFFICULT METAL TO WORK WITH?
The culprit behind the low weldability of cast iron goes to its carbon content. When heat is applied to the metal during welding, the carbon migrates towards the area where the heat is being applied, and this further increases the already brittle structure leading to the formation of cracks on the metal. A lot of people consider welding cast iron to be a 50% success rate job which is pretty alarming if you think about it.
CAN YOU WELD CAST IRON WITH STICK WELDER?
Whenever you speak about cast iron among a group of welders you can be sure that you will hear a lot of horror welding stories along the way. Although you can use gas welding, stick welding is actually preferred by a lot of people. You just need to be careful about picking the right electrode and the heating requirements.
ROD OPTIONS TO WELD CAST IRON
Selecting the correct electrode is the most important step while welding anything as a matter of fact, especially cast iron.
Electrodes that are suitable for welding cast iron consists of a graphite rich flux which counters the problem of carbon migration due to the heat applied during welding. This presence of graphite chemically ties up the carbon present in the cast iron.
These electrodes can be further divided into two types, pure nickel, and ferro-nickel electrodes. These two types are good for specific tasks. Ferro- nickel is composed of about 47% nickel and 53% steel, they are also way cheaper than pure nickel electrodes. Ferro nickel rods are considered more suitable when you have to weld cast iron to steel.
On the other hand, if you are looking for a softer weld deposit that is more malleable I would suggest going for a pure nickel variant. Although most of the time you should be fine with ferro nickel unless the specifications especially call for a pure nickel.
I have read in some blogs about people using copper electrodes as well, namely, copper-tin alloy and copper aluminium. I am not too sure about the efficacy of using these so make sure to do your research before you try it out.
Some people when they are in a pinch and need to perform a quick repair job also go for a mild steel electrode which can get the job done okay-ishly. Although I haven’t tried this before and I would suggest getting the proper electrodes required for the job.
HOW TO WELD CAST IRON (THE PROCEDURE)
The welding can be divided into three phases, cleaning, pre heating and the actual welding. Let’s take a look at each one of them further.
It is very important for you to make sure that the workpiece is free from any contaminants like grease, oil, paint. I know this process is a common procedure when taking on any welding project but that is because of its importance. You can even try to heat the metal just a little, this can release some of the trapped gas inside the metal.
Let me explain to you why this step is important while welding cast iron. The only way to weld on cast iron and not making it crack under stress is through proper heat control. Cast iron is brittle and will crack under stress, but there is no stress when the entire metal piece expands or contracts at the same time. So, if the entire metal piece is heated there will be no stress on the metal.
On the other hand, when a small area on the workpiece receives heat and the rest of the surrounding area is cold there will be a lot of stress formation on the metal. This localized heating is also known as HZ (Heat-affected Zone).
So what does the preheating accomplish? It reduces the thermal gradient when performing the weld. In other metals that are comparatively more ductile, this localized heating can be offset because of the stretching of the metals to some extent but this quality is missing from cast iron. Pre-heating greatly reduces this stress between the HZ and the rest of the body. Make sure to heat the metal evenly and slowly to further reduce any stress. Never heat cast iron above 1400F as it can reach the critical stage.
If for some reason you can’t undertake the pre-heating process, I would suggest going for a welding process that requires or produces less heat or go for a low melting point heating rod.
You also have to keep in the cooling rate, similar to heating, rapid cooling can lead to the formation of stress on the metal and can cause cracks.
It is a good idea to place the workpiece in the sand after welding for 24 hours to let the welded area cool off very slowly.
Make sure the arc is directed towards the weld pool and not the base metal. I would also recommend trying using the lowest current settings possible according to the specifications. This can go a long way to lower the stress. It is also suggested to try not to lay long beads, you should go for small, one inch segments to minimize over localized heating.
This step is also called staggering which involves you welding another small area while the previous area gets to cool down a little.
If you are not preheating the metal before welding you should look into peening after completing the weld to prevent cracks on the surface. Welding in a flat position will make the weld easier and try to maintain a medium arc length.
Peening means opposite compressive stress to counteract the stress on the cast iron, it can be done by moderately striking the workpiece.
After the weld is completed comes another important step, which would be the cooling process. Try to slow down the rate of cooling of the workpiece as much as you can.
POLARITY AND SETTINGS FOR WELDING CAST IRON
You should keep the welder settings at the lowest amperage possible to conduct the weld. As far as I know, you can use either AC or DC+ polarity to complete the weld, of course, the final polarity settings depend on the electrode being used.
CAN YOU WELD CAST IRON TO STEEL
In short, yes, it is possible to weld cast iron with steel. But you need to keep a few things in mind before you do it.
The first step you should take is to check which type of cast iron or alloy you are working with. If you have this information wrong the preheating temperature and electrode selection can be wrong.
Also, keep I mind the difference in melting temperatures of both the metals. Cast iron generally has a lower melting temperature. Because of this, you have to be extra careful when dealing with thinner cast iron pieces. You will have to adjust the settings on the welder to offset the difference in melting points.
CAN I WELD CAST IRON ENGINE BLOCK/EXHAUST MANIFOLD.
You can weld both exhaust manifolds and engine blocks and each one of them have a different requirement. Exhaust manifolds, for example, are pretty common in terms of repair work. The good news is that a lot of times they break like a ceramic plate and you can piece together the parts to get it in its original shape. A lot of exhaust manifolds are constructed from basic weldable cast iron and the material itself does not present much of a challenge.
Though it is important to keep in mind that exhaust manifolds are made from thin pieces of cast iron and can get worn out over time, this can create some challenges when you are going for the weld.
Engine blocks are usually thicker compared to manifolds and therefore are a bit easier to weld on. But one thing you should keep in mind is the more critical nature of welding an engine block and you should always get the final piece checked by someone for cracks.
FREQUENTLY ASKED QUESTIONS
It can take some people a lot of time to be able to weld a piece of cast iron successfully. I will try to address some of the common questions that people have, I hope it helps you.
Although welding techniques differ with each person you ask, a lot of people actually prefer gas welding on cast iron over stick welding. The general opinion in the welding world however, is that stick welding should be the go-to option for anyone.
The reality is that the process is only difficult if you don’t know the metallurgical properties of the metal. If you are aware of the brittle nature of cast iron and what to do about it, then it shouldn’t be much of an issue after some practice.
The only reason why this is done is to avoid any localized heating which can lead to the formation of cracks on the surface.
To sum it all up, cast iron welding can be very tricky if you don’t take the necessary steps like preheating and controlling the cooling rate. You also have to make sure to select the right electrode for the job and try to identify the correct type of cast iron you will be working on. Make sure to implement other techniques as well when needed such as peening, staggering, etc.
One thing I can tell you, it will take a lot of practice before you can master welding on cast iron.