Welding cast iron can be really difficult and almost impossible especially if you have little or no experience. You would require the appropriate knowledge in knowing the exact steps to take when you are welding a cast iron. Any wrong step can result in ruining the whole iron, especially when you are dealing with critical parts of the iron. In fact, it might be a wise thing to request for the help of a professional in the field for success.
However, if you know just the right things to do welding cast iron can be really valuable and it will save you some money and time. And more importantly, you would have added an important one to your skillset. There are some important things to note if you are new and just getting yourself started in welding a cast iron successfully.
If you will be doing your cast iron welding in-house, There are four things to note before you even start the process:
- You need to be able to recognize the type of alloy you want to work on
- Your casting should also be cleaned thoroughly
- The preheat temperature must have determined for your casting
- Deciding on the correct welding method to use on the cast irons.
Understanding Your Alloy Type:
The cast iron falls under the iron-carbon alloy. Cast irons have a high content of carbon or around two to four percent. And this is what makes cast irons very hard. Their hard nature affects the ductility of cast irons making the malleability of these cast irons almost impossible unlike the wrought iron or even steel. There is either expansion or some compression during the heating or cooling of cast iron. When cast iron expands from too much heat it cannot stretch as other irons instead, it cracks from the tensile stress and as a result, it becomes even very difficult or even impossible to weld it unless perhaps if you combine it with other alloys.
Also, and more importantly, you have to understand that there are different alloys for cast irons and each alloy type has their different level of difficulty in welding them. They are: grey irons and the white irons
The very common cast iron around us is the grey cast irons. During the manufacturing process, the carbon pours out into flakes of graphite turning to a ferrite or pearlite crystalline microstructure. Although the process is also difficult, especially for the newbies because those graphite flakes can cause weld metal embrittlement when they enter the weld pool, however, as for gray iron casts, they are more malleable. And as a result, their ductility allows for easier welding than the other types of cast irons.
As for white iron cast, their carbon content can be easily retained as iron carbide without having to pour our as flakes of graphite during welding. However white iron cast are impossible to weld because of their very hard nature. Also they have a cementite crystalline microstructure which makes them brittle during welding. All these make white iron cast impossible to weld.
You have to be an expert in recognizing the alloy type of your cast iron. If you are an expert, you should be able to tell the alloy type when you take a look at the fresh cracks made on the cast irons. In fact, the safest way for you to be able to identify which type of cast iron can be welded it to check the manual of that product. And you can even get in touch with the manufacturer if there are no clear indications about the alloy type in the product package details.
Cleaning the Weld Surfaces of you Cast Iron
Putting the alloy type aside, there should be adequate preparation time for the cleaning of your casting before starting the welding process. During the preparation time of your cast iron, it is very important that you get rid of all the surface materials on your cast irons if you actually want to totally clean the weld area of your casting. You can start by ridding any gummed label, scratching the paint away, removing any oil of grease and any other materials attached to the surface of your weld in order to expose the bare cast iron of your product. You can apply some heat on the weld area but for a short duration if you want to remove any stuck gas on the weld area of your cast iron.
You have to understand that the degree of how clean your iron is, determines how better your welds you will get. And there are likely stuck fu And you can test the readiness of how ready the surface of your cast iron is for cleaning. How do you go by this? Well, you can just pour a weld pass on the weld area of your metal. If there is still an impure element on the weld area there will be some porosity. Then you can grind off the weld pass and do some cleaning again. And you can repeat the whole testing process again and again until the porosity disappears totally.
There are two important steps during your cast iron weld: Pre-heating and Cool Welding
When your cast iron gets stressed as a result of much heat, cracking is very likely to occur. Therefore you need to constantly check the heat produced from time to time. You must be able to control the heat production to avoid any crack. The main reason for controlling heat during your welding is to have the thermal expansion of the cast iron in proper check. Since your metal would normally expand whenever it gets very warm.
There might be no stress when the metal expands and gets warm at the same speed. However, when you concentrate the heat on the small area you want to weld, tensile stress starts building upon that area because the metal expansion is restricted to that area, and the only cooling that happens come from the areas surrounding the weld area. So the stress level will depend on the rate at which other surrounding areas get warm. Unlike the cast iron, other metals would normally contract after the stretching and expansion, but for cast iron when the weld area expands from much heat, it gets stressed and when it gets stressed it does not contract like others, instead, it cracks. This is as a result of the poor ductile nature of the cast iron.
However, pre-heating the whole cast iron will reduce the tensile stress of the weld area by balancing the thermal degree of the surrounding areas as against the weld area in order to avoid too much stress that comes from the welding. The welding technique you choose will determine the level at which you will preheat your casting iron. If your welding technique will produce higher heat, then the temperature of your pre-heating must also be higher. If however, you are not able to achieve the desired temperature for your pre-heating then the best method is to go for the welding method that has low melting point wires with low-temperature output during the welding process.
Another important factor to consider is the rate at which your weld cools as relative to the surrounding areas. If you have got a metal that cools very fast then some stress may occur in the weld area causing some brittleness and some crack. However, if your metal cools slowly, both the contraction and hardening process is slow and tensile stress is reduced. So the faster the rate at which your iron cools the higher the rate at which your weld area contracts and gets stressed. Trying to cool your welded piece faster, is never helpful and will only cause further damage. What you can try is balancing the temperature by reducing the cooling process. You can try to reduce the cooling process of your iron by putting your metal into a warm oven or sand.
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Choosing your welding technique should be determined by the type of cast iron alloy that you want to weld. However, the general techniques that are usually adopted by professionals are oxy-acetylene, stick welding, and brazing.
Stick welding or shielded metal arc comes with a flux covered electrode that is consumable. Your choice of the electrode will be determined by the application, the matching color that is required, and how much machining you will be doing after the welding. You have to choose the right filler wire and electrodes during your stick welding and there are only three types of filler wire or electrodes that work well for your stick welding:
- Nickel alloy
- Copper alloy
- Cast iron
As for the nickel alloy electrodes, they are definitely the most common for your welding when it comes to casting iron welding. This is because nickel-iron is much stronger and it does not expand much when it gets heated. With this, the tensile stress is much reduced and the possibility of cracking is eliminated.
It is the electric arc between the weld area and the electrode that melts the cast iron and join them together. Instead of the base metal, the arc is pointed at the weld pool to reduce dilution. And to reduce the stress caused by heat, you can check for the standard current settings as indicated in the manual. Since nickel alloy has a slow heat output they do not need any pre-heating, but as for copper and cast alloys, you would require a pre-heat temperature of about 260 degrees Fahrenheit.
The oxy-acetylene torch is used to supply energy for welding instead of the electric arc. There are also electrodes for oxy-acetylene welding and you can use the copper alloy or the cast alloy electrodes for welding your cast iron. And to avoid loss of silicon and formation of white iron in your weld, you must pay every attention and care to not oxidize the cast iron during the welding process. Also, the melting of the welding rod should be done inside the molten weld pool instead of the flame in order to balance the temperature of your weld area as against the surrounding areas. In all, oxy-acetylene welding is really a good welding technique even for newbies because you can reduce the cooling or heating welding processes and you can easily regulate the temperature of the components with it.
The brazing welding technique is also a good technique for melting the cast iron together because of the low impact on the base of the metal. The welding rod supplies the filler that goes in line with the cast iron surface. The melting point of filler has a lower melting point in comparison to the weld cast iron and this is what ensures the adherence of the filler to the surface instead of diluting with the cast iron.
However, it is very important to clean the surface of the weld area well, because the melting depends on the porosity of the base metal surface. In other words, if the base metal is dirty it won’t stick. Also, the use of flux has proven very effective in preventing the oxides from forming. And it increases the wetness of the filler to allow for the proper joining of the metal parts. Also, fluxes are really crucial for welding because they help clean metal surfaces.
Cast iron welding is not an easy task and the quick safer way would be to seek the help of a professional to do the welding for you. However, you can also get the process right and successfully weld your cast iron if you carefully follow the processes of recognizing your alloy, cleaning and preparing your material well before welding, and selecting the correct welding method for your cast iron. And if perhaps some cracks still manifest during your welding, you may just adopt the peening method of gently hammering your metal as it cools off. However, you need to understand that the peening should be done while your iron is still softer and malleable. Cast iron welding can be difficult but in the end, they are great and long-lasting if you get a successful weld.