Welding of Cast Iron: Explained
Welding cast iron, a common material in machinery and construction, presents unique challenges due to its high carbon content and brittleness. This article dives into various methods like shielded metal…
Welding cast iron presents unique challenges due to its brittle nature and susceptibility to defects. This article provides essential tips and techniques for successfully welding cast iron, covering both hot and cold welding methods, material selection, and common issues such as cracks and porosity. Readers will learn how to choose the appropriate welding rods, preheat procedures, and post-welding treatments to ensure strong, durable repairs. Whether for minor fixes or significant restorations, these insights will help achieve reliable and effective welding outcomes.
Many individuals have questions regarding the possibility of welding cast iron. They are uncertain if cast iron can indeed be welded, as well as the methods that can be utilized and precautions that should be taken.
Related reading: 7 Types of Iron Castings You Should Know
Let’s discuss the methods and precautions for cast iron welding and how to select welding rods.
Related reading: How to Choose the Right Welding Rod?
Cast iron refers to items produced by casting molten iron.
While cast iron has a lower strength and plasticity compared to steel, it boasts excellent abrasiveness, shock absorption, castability, and machinability.
Due to the simplicity of manufacturing equipment and low production costs, it is frequently used for producing large machine components such as boxes, shells, fuselages, and bases.
Some crucial parts with minimal impact, such as the crankshaft of small diesel engines, are manufactured from ductile iron.
However, cast iron’s poor weldability restricts its use in welded structures.
Presently, welding of cast iron mainly serves for repair and maintenance, with very few instances of using it for creating composite parts.
Iron castings are prone to defects such as air holes, pinholes, slag inclusions, cracks, and pits due to several factors.
The commonly used repair equipment includes argon arc welding machines, resistance welding machines, and cold welding machines.
For casting defects with low quality and appearance requirements, argon arc welding machines, and other welding machines that generate high heat and speed can be used for repair.
Related reading: 10 Different Types of Casting Process
Cast iron can be welded, but its weldability is poor.
Cast iron refers to articles that are cast using molten iron. Unfortunately, various factors can lead to defects such as air holes, pinholes, slag inclusions, cracks, and pits. These defects can affect the appearance quality, internal quality, and use quality of the cast iron.
Appearance quality refers to various factors that affect the surface roughness, surface defects, size deviation, shape deviation, and weight deviation of castings.
It mainly includes the casting’s metallographic structure, and the presence of holes, cracks, inclusions, and segregation in the casting.
Service quality refers to the cast iron’s durability under different working conditions, including wear resistance, corrosion resistance, cold and heat resistance, fatigue, shock absorption, and other properties, as well as machinability, weldability, and other process properties.
Due to different factors, iron castings may have defects such as air holes, pinholes, slag inclusions, cracks, and pits.
The commonly used repair equipment includes argon arc welding machines, resistance welding machines, and cold welding machines.
For repairing casting defects with low appearance and quality requirements, high heat and high-speed welding machines such as argon arc welding machines can be used.
The quality of cast iron has a great influence on the performance of mechanical products.
For instance, the wear resistance and dimensional stability of machine tool castings have a direct impact on the accuracy and maintenance lifespan of the machine tools.
Likewise, the size, profile accuracy, and surface roughness of impellers, shells, and internal cavities of hydraulic parts in various pumps significantly influence the working efficiency, energy consumption, and cavitation development of pumps and hydraulic systems.
Additionally, the strength and heat shock resistance of cast copper parts, such as cylinder blocks, cylinder heads, cylinder liners, piston rings, and exhaust pipes, of internal combustion engines have a direct impact on the lifespan of the engine’s operation.
Common cast iron welding is divided into hot welding and cold welding.
The hot welding method for welding cast iron involves preheating the casting to a temperature of 500 to 600 ℃. For this method, cast iron cored welding rod Z248 or steel cored cast iron welding rod Z208 is typically used, resulting in a weld that is similar in composition to the cast iron and therefore homogeneous.
However, preheating is not necessary for cold welding of cast iron, and pure nickel electrode Z308 or nickel iron electrode Z408 is generally used for this method. The resulting weld composition is different from that of cast iron and is therefore heterogeneous.
During welding, it is important to polish the welding surface to prevent impurities and ensure a firm surface after welding.
Welding iron castings is possible, however, the weldability is typically poor and poses difficulties.
Usually, repairing cast iron through welding is not recommended.
For minor appearance defects without any stress requirements, they can be rectified using Z308 welding rod and manual arc welding. However, it is necessary to use a hammer to continuously knock after welding to remove residual stress and prevent cracks.
In case of major defects, oxyacetylene gas welding can be employed. The welding material should be cast iron rod of the same material as the casting, and borax should be used as the flux. Preheating is essential before welding, with the temperature set between 850-900℃.
After welding, heat treatment is necessary, which can be quite cumbersome. However, repair welding is not permitted for key stressed components. The welding technique adopted depends on various factors such as material, size, thickness, complexity, defect type and size, cutting process, and technical requirements of the casting.
Related reading: Welding Basics You Should Know
In order to meet various welding requirements, it is important to prepare beforehand by removing oil stains and sand inclusions, beveling, or preheating.
The welding methods available include gas welding, brazing, manual arc welding, and manual electroslag welding.
Gas welding, in particular, is classified into three types: hot welding, heating stress relief zone method, and non-preheating gas welding.
Manual arc welding is divided into cold welding, semi hot welding, non preheating welding and hot welding.
Gray cast iron contains high levels of carbon, sulfur, phosphorus, and other impurities. It has low tensile strength, brittleness, and very little plastic deformation ability, which ultimately leads to poor welding performance.
The primary issue with gray cast iron welding is the susceptibility to the formation of white and hardened structures, along with cracks in the welded joint.
Welding gray cast iron can result in the formation of a white cast structure in both the weld metal and heat-affected zone. This outcome depends on the welding materials employed during the welding process and the cooling rate following welding.
Related reading: 8 Problems in Welding Dissimilar Materials
Preheating, heat preservation, and slow cooling are effective measures to reduce and avoid white spots during welding.
For homogeneous welding, preheating between 400-700 ℃ can generally prevent the formation of white spots in the weld and fusion zone.
When heterogeneous welding is used, it is typically performed at room temperature without preheating the weldment, which makes it difficult to completely avoid the formation of white spots in the fusion zone.
However, the generation of white spots can be reduced by using low-temperature preheating, post-welding heat preservation, and slow cooling.
Welding cast iron can be a challenging task due to its hardness. However, with proper welding methods and materials, success can be achieved. Additionally, a set of suitable welding process measures is needed to cooperate with the welding repair process.
Related reading: How to Choose Welding Materials for Stainless Steel?
There are several common methods for welding gray cast iron, including arc welding and gas welding, as well as brazing and manual electroslag welding.
Arc welding is the most widely used method, with arc welding with electrodes being the most common. Gas shielded welding, on the other hand, is less frequently used.
Welding cracks in cast iron are caused by the low strength and poor plasticity of the material, as well as welding stress.
Therefore, preventing welding cracks primarily involves reducing or eliminating welding stress.
In practice, there are several methods for arc welding repair, including cold welding, hot welding, semi-hot welding, and non-preheating welding. Gas welding methods include hot welding, heated stress relief zone, and non-preheating welding.
By using these welding methods effectively and reasonably, good results can be achieved.
It is easy to form a white spot structure, which is mainly caused by the fast cooling rate of the weld and a lack of graphitization elements.
Solution:
a. To achieve optimal results, it is recommended to adjust the chemical composition of the weld by increasing the blackening element and selecting an appropriate welding rod.
b. In order to prevent cracking and improve blackening conditions, it is suggested to reduce the cooling rate of the weld. This can be achieved by preheating before welding and using a slow cooling process after welding.
c. Brazing method is preferred for welding repairs because it prevents the cast iron from melting.
② Note: Cast iron has a high carbon content, low strength, poor plasticity, and is prone to shrinkage. As a result, it is susceptible to cracking and even peeling under the welding stress after cooling.
Solution:
a. Adopt thermal welding method;
b. Heating stress relief zone method;
c. Arc cold welding method;
d. Gas shielded welding (commonly used thin wire CO2 shielded welding);
e. Take special technological measures, such as silk planting.
③ It is easy for pores to form in cast iron welds, mainly due to the rapid cooling rate and the small semi-melting range. This often results in insufficient time for the gas present in the molten pool to escape, leading to pore formation.
Solution:
a. Clean carefully before welding;
b. Adopt thermal welding method;
c. Select welding rod reasonably.
④ Poor weld formation is mainly caused by the silicon oxide film that is difficult to melt when cast iron is welded, which will reduce the fluidity of molten metal and make it difficult to obtain beautiful welds.
Solution:
a. Select welding materials with good fusibility with cast iron;
b. Select reasonable welding parameters;
c. Gradually improve operation skills.
As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.
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