White Cast Iron – Types, Microstructure and Applications

The most common abrasion-resistant ferrous materials having carbon greater than 2wt% in the form of cementite is called white cast iron. White faceted fracture due to the presence of cementite is of main characteristics of white cast iron. There are many cast irons used in the industry each having specific characteristics and applications, Follow Cast iron types for insight. 

White Cast Iron Microstructure Development

For a general microstructure development in high carbon ferrous material and to understand the effect of equilibrium cooling and fast cooling, Follow Microstructure development in Iron. Cooling or directional solidification has a huge impact on microstructure of White cast iron. If fast cooling is only allowed on the surface than chill cast iron is developed which contains grey structure (graphite flakes) within the center region. To grasp the importance of solidification, Follow cast-iron types. By reading Cast iron types article, you will have an idea about how simple the solidification process can create various cast-iron types that can be used for multiple applications.

Here, we are going to discuss the microstructure of white iron in detail…

One important term which will be of main concern is Carbon equivalent (CE). Basically, along with Carbon, Silicon increases the graphitization process and increases the probability of graphite nucleation. That’s_ why silicon is also considered along with carbon to determine the final Cast Iron type. Following formula of CE is being used for microstructure development;

Phase diagram of solidification is given below;

First, consider the S1 line, and pass the solidus line. Below the solidus line, dendrites of austenite start appearing in a molten pool of iron and carbon. Around these dendrites, a region of molten pool is enriched in carbon. Due to high carbon concentration and various oxide phases, carbon precipitates in form of flakes rather than cementite which is formed by the eutectic reaction. This nucleation of graphite flakes increases as temperature drops resultantly giving grey cast iron. To reduce carbon activity and prevent graphite flakes nucleation, following few steps are normally taken;

• Increase Mn, Cr, and Mo/Si ratio: Increase in alloying elements prevents carbon activity in the liquid pool and also restricts precipitation of flakes. This promotes carbide formation.
• Fast cooling: This results in a supercool of the liquid pool. With fast cooling, the temperature drops suddenly and lowers the activity of carbon. Carbon does not get enough time to nucleate out of the molten pool. So, after passing from the eutectic line, lediburite (eutectic mixture of austenite and cementite) forms directly from a liquid pool of iron and carbon. 
• Pressure: Pressure increase is always accompanied by carbon flakes nucleation. Application of pressure control may create a chilling effect in thick castings to prevent grey cast iron formation.

Structure of White Cast Iron

Final microstructure of white cast iron looks like below;

This microstructure depicts the Pearlitic matrix with a continuous thick cementite matrix formed in result of the eutectic reaction. The presence of a thick network is the main reason for the brittleness of respective microstructure.

White Iron Types

Now, we have mentioned here that various mechanisms are present which can produce white cast iron microstructure like alloying, extreme fast cooling, and pressure. These casting proceedings have certain effects on the microstructure of white cast iron. 

Due to these reasons, white cast iron is further divided into two groups; 

1. Low Alloy iron: Cast iron with alloy content less than 4%
2. High alloy iron: Cast Iron with alloy content greater than 4%

• Nickel iron
• Chromium iron

Low Alloy Iron

This is also called pearlitic white cast iron. We mentioned earlier, with relatively faster cooling, it is possible to prevent the nucleation of carbon flakes.

The microstructure of Pearlitic white cast iron is shown above.

The pearlitic structure can, also, be possible with a high concentration of alloy. With a variation of solidification processes and composition, microhardness and microstructure of Pearlitic white cast iron may vary. 

Hardness achieved in conventional sand mold and low alloy cast iron is 350 HV. HV is a Vicker microhardness test, detail of microhardness test and how to read hardness number can be found in Vicker Hardness Test. If metal mold is used instead of sand, the chilling effect increases and Pearlitic strips appear finer increasing hardness up to 500 HV. 

Pearlitic white cast iron is used Bucker elevators as shown in the picture below. It is also used in the agriculture industry due to low cost and high hardness.

Chill Cast Iron

Chill casting is produced by inserting metal plate inside the sand mold for extracting fast heat from a composition containing a high amount of silicon to cause graphitization in the rest of iron. This results in a white shell and grey core containing high hardness and toughness.

The most common application of Chill casting is a hammer used in Coal crushing and cement crushing. This hammer is used in milling and crushing plants with a hammer consisting of white cast iron structure and mounting-end consists of grey iron.

High Alloy Iron

High Alloy cast iron is a term commonly used for white cast irons having alloy content greater than 4%. The casting of such ferrous material is carried out for high abrasion resistant materials like parts needed in machinery for cutting and grinding.

Microstructure of High Alloy iron – Effect of Alloying Elements

Needle-like matrix depicts martensite. The region next to needle-like martensite is retained austenite. Continuous network showing above matrix is of metal carbides. These carbides can be of chromium, Vanadium or Iron. 

Alloy content has a variety of purposes. Just for insight, Chromium is added to for corrosion resistant and secondary carbides which enhances hardness. The metallic matrix can be adjusted from soft too hard to optimize the microstructure between hard and tough cast iron. 

Details of Alloying elements effect is given below;

Carbon: With the increase in carbon percentage, the percentage of carbide formation increases resultantly giving more hardness.

Nickel: Nickel promotes martensitic and bainitic transformation in a matrix of white iron. With the Pearlitic matrix, white cast iron appears to be soft with better toughness and impact absorption characteristics. With the addition of Nickel, the graphite field increases resultantly suppressing Pearlitic formation giving a high percentage of Martensite. If the Amount of nickel is high, more retained austenite will be formed which results in lower hardness. Optimum Nickel content is essential for optimum abrasion resistance and hardness of white cast iron.

Chromium: Alloy cast iron where high abrasion and wear resistance are essential especially in applications like crushing and grinding, Chromium is essential alloying addition. With Nickel’s addition, the graphitization process also speeds up generating high carbon flakes. Chromium addition suppresses the graphitization process initiated due to the addition of Nickel and carbon. It mainly exists in carbide phases generating more carbides and resultantly giving more hardness. Chromium to Nickel ratio is usually kept at 1:2 or 1:2.5. 

Silicone: Silicone is one of the very important foundry additions which increases melt fluidity and also removes absorbed oxygen in the melt. It acts as an oxidizer and removes all absorbed oxygen thereby removing important casting defects, but it is also a strong graphitized. With Silicone addition, chances of graphite flakes formation increase and, thereby, reducing abrasion resistance of white cast iron. So, silicon should be added to a minimal level to let silicon perform its casting duties and prevent it to be flakes promoter. 

Manganese: Manganese addition improves deoxidation and also the hardenability of white cast iron. Improvement in hardenability is not on par with Nickel but still, it delivers the result. It should be added up to a minimal level of 0.5%.

Molybdenum: It is added to overall improve martensitic formation in the center of casting along with Nickel. It exists mostly with carbide phases and helps in hardening the structure in the center of the casting.

Copper: Hardenability is also improved by copper addition, but improvement is only half as good as Nickel. It may also embrittle white iron due to the formation of needle-like precipitates after reacting with oxygen.

This Alloying addition divides high alloy white cast iron into two groups;

Sulfur and Phosphorus: They also reduce the abrasion resistance and should be kept to a minimal level.

Martensitic nickel white iron

ASTM A532-I is a class of martensitic nickel white iron. In low-alloy white iron, a matrix is made of pearlite. Pearlite is relatively soft and has low wear resistance. To increase the wear resistance of white cast iron, the Pearlitic matrix is shifted to martensite by introducing Nickel and Chromium in it. Nickel is added in between 5-8wt%. Nickel does not take part in carbide formation; it just delays the formation of pearlite and extends the austenitic field and reducing the chances of Pearlitic formation. The final microstructure contains, carbide phases embedded in martensitic and retained austenite matrix.

These types of white cast iron are also called Nickel Hard. 

This type of microstructure has a Vicker hardness of 550HV 30. By tempering at a temperature of 275 ͦC, retained austenite breaks into lower bainite increasing the hardness of microstructure further by 100HV30. With using metal instead of the sand mold, the finer martensitic microstructure is produced with 50HV30 Vicker hardness higher than conventional ones. 

Composition of Nickel hard iron is as follow;

Nickel hard white cast iron physical and mechanical properties are given below;

Martensitic chromium White iron

Applications where high abrasion resistance is required, like grinding mills, brick molds, shot blasting mold and equipment, and mining equipment, use of high chromium white cast iron is employed. In these applications, high abrasion resistance and little toughness is needed to resist impact loading. High chromium white cast iron is the best combination of abrasion resistance and toughness.

Chromium content variation and heat treatment can be used to adjust the mechanical properties as there is a trade-off between wear resistance and toughness of cast iron.

ASTM A 532 standard has two set standards for compositions and hardness. ASTM A 532 class – II covers chromium-molybdenum irons.

Added chromium content increases carbide concentrations thereby increases hardness. They have the highest hardness among all the white cast iron.

Chromium white cast iron composition;

Molybednum added improves hardenability of matrix thereby improving abrasion resistance. With lower chromium content, corrosion resistance is slightly more than below mentioned chromium alloy iron.

Properties of chromium alloy iron is as follow;

The most common applications of this class of iron are milling machine liners, shot blasting grits and slurry pumps. A Roller crusher is, also, a very common application used in the cement industry.

ASTM A class – III standard represents this class of white iron. Due to very high chromium content, corrosion-resistant of this type of white iron is very high.  With high chromium content, tough matrix and very high abrasion resistant white iron is possible.

Composition of high chrome white iron is as follow;

Properties of high chrome white iron are as follows;

Comparison of properties of White iron and rest of cast iron can be studied in cast-iron types. You can follow Cast Iron on Wikipedia for mode detail information.

F.A.Q about White Cast Iron

Is white cast iron weldable?

White cast iron contains a strong continuous network of carbides that are difficult to weld. Welding is normally not recommended for white cast iron. The structure of white cast iron also contains martensite. This combination of carbides and various hard matrix phases is prone to heat cracking. Welding and immediate cooling result in cracks in cast iron, that’s why white iron is not recommended for welding. 
Following are common repair methods which can be adopted for white cast iron;
— Addition of welding inserts
— Addition of Heli coil inserts
— Bolting
— Epoxy

Why White cast iron is hard and Brittle?

White iron is extremely hard and brittle. The answer lies in its microstructure. The microstructure consists of a thick continuous network of carbides embedded in the Pearlitic or martensitic matrix. This carbide network is extremely hard and resists any plastic deformation. This is the main reason for white cast iron to be hard and brittle. When crack generates within carbide network, crack flows immediately and no other micro cracks are generated. That’s why the surface of fractured white cast iron appears white.

How is white cast iron produced?

Most commonly cast iron is produced by casting technology. Other manufacturing methods involve mechanical working. Since cast iron is extremely hard and brittle that’s why it can not be molded in solid form. The only possible way to shape white cast iron is to cast material in a certain shape.

Do White iron sink stains?

Since white cast iron is non-porous due to the casting manufacturing method. Polished enamel of white iron makes the surface extremely shiny. Due to the flat surface and no surface porosity, stains can be easy to remove as they don’t stick inside the material. The surface can be cleaned pretty well.

Applications of White Cast Iron 

Common applications are;

• Dredge pumping
• Oil sand Applications
• Mining crusher parts
• Ball mill liners
• Roller crusher
• Crusher liners
• Grit blasting grates
• Lifting bars