How to Weld Cast Iron?

Many people have some doubts about whether cast iron can be welded.

They do not know whether cast iron can be welded, what methods can be used if cast iron is welded, and what measures can be taken.

Related reading: 7 Types of Iron Castings You Should Know

How to Weld Cast Iron? 1

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?

What is cast iron?

Cast iron is a general term for articles cast by molten iron.

Although cast iron has lower strength and poorer plasticity than steel, it has advantages of good abrasiveness, shock absorption, castability and machinability.

Moreover, due to simple manufacturing equipment and low production cost, it is often used to manufacture large machine parts such as box, shell, fuselage and base.

Some important parts with little impact, such as crankshaft of small diesel engine, are made of ductile iron.

However, the poor weldability of cast iron limits its application in welded structures.

At present, the main application of welding in cast iron is the welding repair and repair of cast iron, and there are few occasions for producing composite parts.

How to Weld Cast Iron? 2

Due to various factors, iron castings often have defects such as air holes, pinholes, slag inclusions, cracks and pits.

The commonly used repair equipment is argon arc welding machine, resistance welding machine, cold welding machine, etc.

For casting defects with low quality and appearance requirements, argon arc welding machines and other welding machines with high heat generation and high speed can be used to repair them.

Related reading: 10 Different Types of Casting Process

How to Weld Cast Iron? 3

Can iron castings be welded?

Cast iron can be welded, but its weldability is poor.

Articles cast with molten iron are collectively called cast iron.

Due to various factors, defects such as air holes, pinholes, slag inclusions, cracks and pits often occur.

Including appearance quality, internal quality and use quality.

How to Weld Cast Iron? 4

Appearance quality refers to the surface roughness, surface defects, size deviation, shape deviation and weight deviation of castings;

It mainly refers to the casting, metallographic structure and holes, cracks, inclusions, segregation, etc. existing in the casting;

Service quality refers to the working durability of cast iron under different 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 various factors, iron castings often have defects such as air holes, pinholes, slag inclusions, cracks and pits.

The commonly used repair equipment is argon arc welding machine, resistance welding machine, cold welding machine, etc.

For casting defects with low quality and appearance requirements, argon arc welding machines and other welding machines with high heat generation and high speed can be used to repair them.

How to Weld Cast Iron? 5

The quality of cast iron has a great influence on the performance of mechanical products.

For example, the wear resistance and dimensional stability of machine tool castings directly affect the accuracy and maintenance life of machine tools;

The size, profile accuracy and surface roughness of impellers, shells and internal cavities of hydraulic parts of various pumps directly affect the working efficiency, energy consumption and cavitation development of pumps and hydraulic systems;

The strength and heat shock resistance of cast copper parts such as cylinder block, cylinder head, cylinder liner, piston ring and exhaust pipe of internal combustion engine directly affect the working life of the engine.

Cast iron welding type

Common cast iron welding is divided into hot welding and cold welding.

How to Weld Cast Iron? 6

The hot welding method for welding cast iron is to preheat the casting.

The Preheating temperature to 500 or 600 ℃.

Generally, cast iron cored cast iron welding rod Z248 or steel cored cast iron welding rod Z208 is used.

The weld after welding is similar to the cast iron composition and belongs to homogeneous weld.

Preheating is not required for cold welding of cast iron.

Generally, pure nickel electrode Z308 or nickel iron electrode Z408 is used.

The weld composition is different from that of cast iron and belongs to heterogeneous weld.

The welding surface shall be polished during welding to prevent impurities and make the welding surface more firm after welding

How to weld cast iron?

Iron castings can be welded, but the weldability is poor and it is difficult.

Generally, repair welding is not allowed for cast iron.

How to Weld Cast Iron? 7

If it is only a small defect in appearance and has no stress requirements, it can be completed by Z308 welding rod and manual arc welding.

However, after each welding, a hammer should be used to knock continuously to remove residual stress and prevent cracks.

If the defects are large, oxyacetylene gas welding can be used.

The welding material is cast iron rod of the same material as the casting, and borax is used as the flux. However, preheating is required before welding.

The preheating temperature is high, generally 850-900℃.

Heat treatment is required after welding, which is cumbersome.

Repair welding is not allowed for key stressed parts.

And the welding method depends on the material, size, thickness, complexity, defect type and size, cutting process and technical requirements of the casting.

Related reading: Welding Basics You Should Know

Prepare before welding according to different welding requirements, such as removing oil stain and sand inclusion, beveling or preheating.

Welding methods include gas welding, brazing, manual arc welding and manual electroslag welding.

Among them, gas welding is divided into hot welding, heating stress relief zone method and non preheating gas welding;

How to Weld Cast Iron? 8

Manual arc welding is divided into cold welding, semi hot welding, non preheating welding and hot welding.

1. Welding of grey cast iron

Gray cast iron has high content of carbon, sulfur, phosphorus and other impurities, low tensile strength and brittleness, and almost no plastic deformation ability, which determines its poor welding performance.

The main problem is that the welded joint is prone to white and hardened structures and cracks.

How to Weld Cast Iron? 9

Welding gray cast iron may produce white cast structure on both weld metal and heat affected zone, which depends on the welding materials used during welding and the cooling rate after welding.

Related reading: 8 Problems in Welding Dissimilar Materials

Preheating before welding, heat preservation during welding and slow cooling after welding are effective measures to reduce and avoid white spots.

For homogeneous weld, preheating to 400-700 ℃ can generally avoid white spot in weld and fusion zone.

When heterogeneous welds are used, it is usually cold welding (i.e. fusion welding process at room temperature without preheating the weldment).

It is difficult to completely avoid white spots in the fusion zone.

If low-temperature preheating and post welding heat preservation and slow cooling can be used, the generation of white spots can also be reduced.

2. Welding process of grey cast iron

How to Weld Cast Iron? 10

Cast iron is a hard to weld metal material.

Practice shows that in addition to the selection of welding methods and welding materials, it also needs a set of suitable welding process measures to cooperate with welding repair to achieve success.

Related reading: How to Choose Welding Materials for Stainless Steel?

The common methods for welding gray cast iron are arc welding and gas welding, in addition to brazing and manual electroslag welding.

In arc welding, arc welding with electrodes is the most widely used, while gas shielded welding is less.

Cast iron welding cracks are caused by low strength and poor plasticity of cast iron and welding stress.

Therefore, the prevention of welding cracks mainly starts from reducing or eliminating welding stress.

Cold welding, hot welding, semi hot welding and non preheating welding are summarized in the practice of arc welding repair: gas welding includes hot welding, heated stress relief zone and non preheating welding.

Reasonable use of these welding methods can achieve good results.

Causes of specific difficulties in welding repair of cast iron and corresponding solutions

① It is easy to form white spot structure, which is mainly caused by fast cooling rate of weld and insufficient graphitization elements.

Solution:

a. Adjust the chemical composition of the weld, increase the blackening element, and select the appropriate welding rod;

b. Reduce the cooling rate of the weld and improve the blackening conditions, that is, preheating before welding and slow cooling after welding;

c. The brazing method is used for welding repair, because the brazed cast iron does not melt.

② It is easy to produce cracks because of the characteristics of cast iron such as high carbon content, low strength, poor plasticity and easy shrinkage.

 After cooling, it is easy to produce cracks and even peel off under the action of welding stress.

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 to form pores, which is mainly caused by the fast cooling rate of cast iron welds and the small semi melting range, which often makes it too late for the gas in the molten pool to escape and form pores.

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.

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