Welding is a well-known skill that allows the fusion of different metals by using a filler which is heated to liquid and then hardens on cooling to create a blend. There is a common myth that cast iron cannot be welded successfully due to the high carbon content which makes it difficult to achieve a safe and proper joint. There are very few people left with the skillset needed to weld cast iron today.
Cast iron is known for its durability and strength, but it can also be notoriously difficult to work with. Here we will investigate the world of welding cast iron, its challenges, our techniques, and the dos and don’ts of this intricate process.
Indeed, cast iron cannot be welded like other metals due to its unique properties. Cast iron is a group of iron-carbon alloys known for its excellent castability and high wear resistance. However, due to its makeup, it is not easily malleable, it is also brittle and prone to cracking when subjected to excessive heat or stress. But, can you weld cast iron successfully? The short answer is yes, we have been doing so for years with our tried and tested methods and reliable flux.
Various types of cast irons are produced using diverse heat treatment and processing methods, including gray iron, white iron, malleable iron, ductile iron, and compacted graphite iron.
Gray iron is distinguished by the presence of flake-shaped graphite molecules within the metal. When the metal fractures, it breaks along these graphite flakes, resulting in the characteristic gray appearance on the fractured surface. The name “gray iron” is derived from this distinctive feature.
The size and matrix structure of the graphite flakes can be controlled during production by adjusting the cooling rate and composition. Gray iron is also easier to machine than other cast iron variants, and its wear resistance properties make it one of the most widely used cast iron products.
White iron, on the other hand, results from the right carbon content and a rapid cooling rate, causing carbon atoms to combine with iron to form iron carbide. Consequently, there are minimal to no free graphite molecules in the solidified material. When white iron is sheared, the fractured surface appears white due to the absence of graphite. The cementite microcrystalline structure of white iron is hard, brittle, and boasts high compressive strength and excellent wear resistance. In specific applications, having white iron on the surface of a product is desirable
One of the most well-known grades of white iron is Ni-Hard Iron, which gains exceptional properties for low-impact, sliding abrasion applications through the addition of chromium and nickel alloys. White irons and Ni-Hard irons fall under the ASTM A532 classification, known as the “Standard Specification for Abrasion-Resistant Cast Irons.
Malleable iron can be produced from white iron through a heat treatment process involving prolonged heating and cooling. This process breaks down iron carbide molecules, releasing free graphite molecules into the iron. Various cooling rates and alloy additions result in malleable iron with a microcrystalline structure.
Ductile iron, also known as nodular iron, acquires its unique properties by incorporating magnesium into the alloy. The presence of magnesium causes the graphite to form in a spheroidal shape instead of the flake structure found in gray iron. Different grades of ductile iron are created by manipulating the microcrystalline structure around the graphite spheroids through casting or heat treatment.
Ductile iron’s ability to deform upon impact, rather than shatter, makes it an ideal choice for products like our cast iron bollards, especially in areas with vehicular traffic.
Compacted graphite iron combines the characteristics of both gray and white iron. Its microcrystalline structure is formed around blunt graphite flakes that are interconnected. To suppress the formation of spheroidal graphite, alloys like titanium are used. Compacted graphite iron exhibits higher tensile strength and improved ductility compared to gray iron. The microcrystalline structure and properties of compacted graphite iron can be adjusted through heat treatment or the addition of other alloys.
Using alternative metals to weld cast iron doesn’t work as well
Using alternative metals for brazing, a process that joins two or more metal surfaces by allowing molten metal to flow into the joint, or for welding components in situations where heating and cooling occur (such as engine cylinder heads), poses challenges related to differential expansion and contraction of the inserted material, often nickel alloy. This can result in cracks forming at the interface and the introduction of heat stress during the process. Additionally, machining the implanted metal is difficult due to its hardness, making the precise restoration of cast iron components a challenging task.
What problems arise from using metal stitching?
Some of the same concerns mentioned in the previous section could also apply to metal stitching and mechanical jointing processes, especially where strength and warranty concerns may play a part in decision-making. For example, a stitching process is likely to involve the use of a dissimilar material. You may also have concerns over approval from classification societies where the component under repair is a pressure vessel like water cooler engine blocks/heads & heat exchangers.
Welding Techniques for Cast Iron
To successfully weld cast iron, you need to choose the right welding technique. At Cast Iron Welding Services, we specialise in the Gas Fusion Welding Process. Our expertise in gas fusion welding ensures the creation of a uniform, stress-free weld with improved metallurgical and machining properties compared to the original parent metal.
Using cast iron, to repair cast iron removes all the risks and concerns. Gas fusion welding is a re-casting or foundry process. It protects the integrity of the original component at a molecular level and makes for a consistent repair.
Successful gas fusion welding of cast iron (FusionCast™) relies on the use of fusion rods that precisely match the material of each specific component being restored. A small sample of the parent metal is analysed so that the correct fusion rod can be selected. The selected rod is made up of a similar grade of cast iron along with trace additives that assist with the fusion process. Both of equal importance.
A controlled environment, at high temperature, is required to make fusion possible. Starting at 600°C, and locally to something in the region of 1300°C the environment makes it possible for a skilled artisan to float in the new cast iron, so that it becomes as one with the original material.
The carefully tailored heating, and subsequent cooling, has the additional benefit of relieving stress built up, not only during the fusion process but, in the day-to-day use of the component before the repair takes place. The parent metal is thus returned to its original state.
How do I know which repair process a supplier is recommending?
Ask what material is being added during the process. The answer should simply be “cast iron”. No nickel cladding or brazing – just pure cast iron.
Large Bore Engine Cylinder Heads
The process is of particular benefit when it is used for the reconstruction of cylinder heads on Caterpillar/MaK, Rolls Royce/Bergen, MAN Diesel, Peilstick, Deutz, Mirrlees and Ruston engines from power plants and ships. The onerous operating conditions, particularly around the combustion face of the head, mean that dissimilar metals associated with other methods may not last.
Engine blocks and Manifolds
Where the items can be made available at a Cast Iron Welding Services facility, almost anything can be repaired. Cracks and frost damage can be restored using cast iron only. No dissimilar metals and therefore no risk of subsequent cracking at the interface.
Pressure vessels & Pump housings
Where there is pressure involved, or where the component is used in conjunction with critical process, a repair that restores the original material rather than introducing alternatives, is often all that the class or insurance authorities will accept.
Machine tools – presses etc.
Where there is force, torque or tension to the frame or body of machinery, a molecular repair with cast iron, provides a new full life cycle with the assurance that the tool will be back to full capacity.
Vintage and Heritage components
It’s not just about the viability of the repair process. In heritage and vintage settings, there is a desire to “keep it original” by only using cast iron to repair cast iron items. An engine block from a classic Edwardian vintage car will not be as aesthetically pleasing with dissimilar metal or stitching.
For listed structures, there is often a legal requirement to maintain the integrity of the restoration by avoiding “modern” methods and opting for a traditional and sympathetic repair process.
89.4% reduction of carbon emissions with cast iron gas fusion welding (FusionCast™)
Proper fusion cast iron repair, carried out in line with the detail above, reduces carbon emissions, when compared to replacement. There is an obvious benefit to repairing rather than replacing any item, but with cast iron, the benefit is dramatic and measurable.
In 2020, Cast Iron Welding Services began to take life cycle assessment seriously and initiated an independent and accredited study based around a typical remanufacturing requirement. Using the example of a large cylinder head from a Wärtsilä 46 diesel engine, the emissions related to its original manufacture (mining, production and logistics etc.) were compared to the class approved remanufacture process used at Cast Iron Welding Services.
There was an evidenced based 89.4% saving on carbon emissions when we remanufacture the cylinder head vs buying a new one. There is also substantial reduction in cost too. See all the Environmental Benefits of Cast Iron Repairs on Cylinder heads in our carbon reduction report commissioned, then independently verified, in accordance with ISO 14040 & 14044.
Common Mistakes to Avoid
Numerous common errors can result in unsuccessful cast iron welds, such as inadequate preheating, rapid cooling, the use of incorrect filler materials, and attempting welding without proper training. When considering the pros and cons of welding cast iron, the advantages include the restoration of cast iron components which is a highly cost-effective alternative to replacement, and the preservation of the original appearance and design, which is most beneficial in heritage and vintage work. However, it should be noted that this process demands skill and experience and carries the risk of cracking and further damage if not executed correctly by professionals who are used to working with cast iron.
Welding Cast Iron with Skill and Care
The fusion process of repairing cast iron was developed in 1946 by the current owner of Cast Iron Welding Services grandfather, Harold Palmer. Back in those days, the process was used predominantly to repair faulty castings from the many foundries in the local area.
The basic skills are now third generation, but the development continues.
Conventional welding is simply not applicable for use with cast iron, but that does not mean this durable and extremely common material cannot be repaired. Gas fusion welding (FusionCast™) procedures are difficult to master and very few artisans have the skills required. Seek out gas fusion welding operatives where the component in question is of value to you. Repairing cast iron, with cast iron; and no other material makes sense.