White cast iron is a great material to make the blade of shot blasting machines and parts of mud pumps. What are the features of white cast iron and what heat treatments are commonly applied to it?
What is White Cast Iron?
The section of white cast iron is bright white and exists in iron in the form of cementite. White cast iron is hard and brittle, not easy to cut, and rarely used directly for casting parts. Generally, the carbon content of white cast iron is about 2.5%, of which the silicon content is below 1%. No graphite precipitation, except a small amount of carbon dissolved in ferrite, most of it exists in cast iron in the form of cementite.
White Cast Iron Composition
| Element Name | Chemical Symbol | Typical Content Range (wt%) | Main Role and Effect |
|---|---|---|---|
| Carbon | C | 2.0% – 3.6% | Forms carbides; the key factor determining hardness and wear resistance. Higher content increases carbide, but may lower toughness. |
| Silicon | Si | 0.5% – 1.9% | Strongly promotes graphitization. White cast iron keeps Si low to inhibit graphite and obtain a white structure. |
| Manganese | Mn | 0.25% – 0.8% | Stabilizes carbides and neutralizes sulfur, often forming MnS. |
| Phosphorus | P | ≤ 0.2% | Generally an impurity; higher amounts increase brittleness. |
| Sulfur | S | ≤ 0.2% | Generally an impurity; increases casting defects and brittleness, so strict control is needed. |
| Chromium | Cr | Low alloy: 1.0–3.0% Medium alloy: 5–10% High alloy: 10–30% |
The most important alloying element; strongly promotes carbide formation. Forms (Fe,Cr)₃C and (Fe,Cr)₇C₃, etc. At >9%, can form M₇C₃ and M₂₃C₆ types of carbides, giving excellent hardness and wear resistance. |
| Nickel | Ni | 3.0% – 5.0% | Improves hardenability and refines structure; commonly used together with Cr. |
| Molybdenum | Mo | 0.3% – 3.0% | Increases hardenability, refines matrix, and improves toughness. |
| Boron | B | 0.15% – 1.0% | Strong carbide former (e.g., Fe₂B, Cr₂B); improves wear resistance. |
| Copper | Cu | 2.0% – 3.0% | Improves toughness and corrosion resistance; sometimes used as a partial substitute for nickel. |
White Cast Iron Properties
The properties of white cast iron derive from its unique microstructure, where almost all carbon exists as hard cementite (Fe₃C) rather than graphite. The table below details its various performance indicators.
| Property Category | Key Indicators/Features | Detailed Description & Typical Values |
|---|---|---|
| Physical Properties | ||
| Density | ~7300 kg/m³ (similar to steel) | |
| Melting Point | ~1200°C | |
| Thermal Conductivity | Poor | Large amounts of cementite reduce thermal conductivity, causing parts to heat up and cool down quickly, which can induce thermal stress and cracking. |
| Castability | Moderate | Good fluidity (especially in hypoeutectic composition), but large shrinkage and thermal cracking tendency require strict casting process control. |
| Mechanical Properties | ||
| Hardness | Very high | Mainly composed of cementite, typically white cast iron hardness > HB 500. High-chromium alloy white cast iron can reach HRC 58–65 (about HV 1300–1800). Special carbides such as (Fe,Cr)₇C₃ can achieve macro hardness of 1300–1800 HV or above. |
| Compressive Strength | High | Able to withstand large compressive loads; compressive strength is high. For example, low-alloy white cast iron can reach 150–180 MPa, alloy white cast iron can reach 320–360 MPa or higher. |
| Toughness & Plasticity | Very poor | Extremely brittle, almost no plasticity. Not suitable for parts bearing impact loads, but can tolerate certain static loads (like liners). |
| Wear Resistance | Excellent | Outstanding wear resistance under dry abrasive conditions, thanks to large amounts of hard cementite or alloy carbides. |
| Elastic Modulus | Comparable to gray cast iron | For example, standard gray cast iron HT300 is about 130 GPa. |
| Machinability | Very poor | Due to extreme hardness, cannot be machined by conventional cutting; only grinding or special processing (like EDM) is feasible. |
| Chemical Properties | ||
| Main Constituent Phase | Cementite (Fe₃C) | Alloying elements may form special alloy carbides, such as (Fe,Cr)₇C₃ or (Fe,Cr)₂₃C₆, further enhancing wear resistance. |
| Corrosion Resistance | Ordinary white cast iron is poor; high-chromium white cast iron is good | High-chromium white cast iron (>12% Cr) resists oxidation and corrosion; suitable for harsh, corrosive, and abrasive environments such as mining and cement industries. |
| Oxidation Resistance | High-chromium white cast iron is good | Special carbides such as (Fe,Cr)₇C₃ provide good oxidation resistance even at high temperatures, suitable for parts like kiln grates. |
| Thermal Properties | ||
| Thermal Stability | High-chromium white cast iron is good | High-chromium white cast iron maintains hardness and wear resistance at high temperatures, especially when alloyed for heat-resistant castings. |
| Thermal Fatigue Resistance | Poor | Due to brittleness and low plasticity, thermal fatigue resistance is poor, and parts subject to temperature cycling are prone to cracking. |
White Cast Iron Applications
White cast iron holds significant importance in industry due to its high hardness and wear resistance. The table below details its main types and uses.
| Type | Main Application Fields | Core Features & Application Notes |
|---|---|---|
| Ordinary White Cast Iron | Ploughshares, grinding discs, guide plates, etc. | High carbon content (approx. 2.5%–3.6%), low silicon (usually <1.0%) to ensure carbon exists as cementite (Fe₃C), giving high hardness. Hardness and wear resistance increase with carbon content, but also brittleness. Mainly used for wear-resistant parts under low impact loads. |
| Low Alloy White Cast Iron | Applications with moderate wear and toughness requirements | Small amounts of Cr, Mo, Cu, B, etc. are added to increase carbide microhardness and strengthen the matrix, thus improving wear resistance. Carbides are mostly continuous and network-like, so brittleness remains high. |
| Medium Alloy White Cast Iron (mainly Cr-based) | Wear-resistant parts with some impact load | Typically contains about 9% Cr, forming (Cr,Fe)₇C₃ carbides in the structure. These carbides have extremely high hardness (HV 1300–1800) and are distributed as isolated rods or plates, improving toughness and strength over ordinary and low alloy white cast iron. |
| High Alloy White Cast Iron (mainly high-chromium white cast iron) | Core wear-resistant parts in mining, cement, power, road construction, refractory materials, such as slurry pump wetted parts, crusher hammers, ball mill liners, etc. | Cr content is typically 12%–20% or higher, making it the most widely used and best-performing white cast iron. The main carbide is (Cr,Fe)₇C₃, which is hard and isolated. Usually requires heat treatment (quenching + tempering) to transform the matrix to martensite, achieving both high hardness and good toughness—especially suitable for severe wear conditions. As-cast hardness of high-chromium white iron is usually above HRC45. |
Why Heat Treat White Cast Iron?
Heat treatment of white cast iron is primarily carried out to overcome its inherent drawback of being “hard but brittle.” By employing various heating and cooling techniques, the internal structure of the iron can be adjusted to achieve properties suitable for actual applications.
White Cast Iron Heat Treat Benefits:
1. Improving Machinability:
The main reason for the high hardness of white cast iron is the presence of large amounts of cementite in its structure. In some alloyed white cast irons, a significant amount of martensite may also exist in the as-cast state, further increasing the hardness. In such cases, soft annealing (also known as graphitizing annealing) is used: the casting is heated to a high temperature (such as 880–900°C or above) and held for a period to allow partial decomposition of cementite and transformation of martensite into softer pearlite. This reduces the overall hardness to a level that can be machined.
2. Relieving Residual Stresses from Casting:
During the cooling process, different parts of a casting cool at different rates, which can create considerable residual stress inside the material. For white cast iron, especially the highly alloyed types, this is a critical issue because the material is already very brittle. If these stresses are not removed, the castings are highly prone to cracking under vibration or environmental changes. Stress relieving annealing (usually heating to 500–560°C, or 800–900°C for high-alloy white iron, followed by slow cooling) can effectively remove these stresses and ensure the stability of the casting.
3. Fundamental Transformation: Producing Malleable Iron:
One of the most important uses of white cast iron is as a raw material for the production of malleable cast iron. Through prolonged annealing (sometimes tens of hours), the networked cementite in white cast iron is decomposed into clusters of graphite. After this treatment, the material transforms from the almost non-ductile white cast iron into malleable iron, which has much higher toughness and ductility. This makes it suitable for producing complex-shaped parts that require some toughness, such as various pipe fittings. Depending on the process and the final structure, blackheart and whiteheart malleable iron can be produced.
4. Optimizing Final Service Properties:
For some white cast iron parts used directly as wear-resistant components (like high-chromium white cast iron), heat treatment processes such as quenching and tempering are used to optimize the matrix structure. Quenching produces a highly hard martensitic matrix, and subsequent tempering relieves stress and stabilizes the structure. This ensures that, while maintaining high wear resistance, the component also achieves better overall mechanical properties.
White Cast Iron Heat Treatment
The hardness of white cast iron is high, it is very brittle, so it is rarely used to make machine parts directly. Generally speaking, it is very important to select a reasonable heat treatment process according to the chemical composition and working conditions of cast iron. The commonly used heat treatment processes are stress relief annealing, softening annealing or normalizing, quenching, and tempering. In addition, most white cast iron parts are malleable iron billets, which need to be graphitized to decompose most or all of the cementite to form flocculent or spherical graphite, converting white cast iron into malleable cast iron.
White Cast Iron Heat Treatment Steps:
1) Stress relief annealing
It is mainly used for high silicon and high chromium alloy white cast iron. Because of the large internal stress in casting, parts will crack during vibration or environmental change if the residual internal stress is not removed in time. Generally, the annealing heating of iron castings is relatively low, while for high alloy white iron castings, the structure of white iron is relatively stable, so the annealing temperature is relatively high, and the process specification is 800~900 ℃ for 1 to 4 hours, after the heat preservation is completed, the furnace shall be cooled to 100~150 ℃ and discharged for air cooling.
2) Soft annealing
For some alloy white iron castings with higher hardenability, there is a large amount of martensite in the cast iron, but there is less cementite, and the hardness is very high. Parts made of such materials need to be machined, so the softening annealing process must be used to transform the martensite into pearlite so that the hardness of the cast iron can be reduced to the extent that it can be machined.
3) Quenching and tempering
It is mainly used for low silicon, low phosphorus, and sulfur wear-resistant white iron castings with low carbon content. Because of its high alloy element content, carbides are difficult to dissolve in austenite. Quenching treatment (heating temperature of 950-1000 ℃) can dissolve more carbides in austenite, and obtain martensite structure with higher hardness after cooling, thus ensuring good wear resistance of iron castings. In order to reduce internal stress and avoid cracking, it can also be quenched in stages with nitrate, and finally tempered at 180-200 ℃ for 1-3h.
4) Isothermal quenching
The purpose of quenching is to obtain a certain amount of bainite to ensure good comprehensive mechanical properties. The carbon content of white cast iron is generally 2.2%~2.5%, and the sulfur and phosphorus content should be less than 0.1%. After the heat preservation is completed at 890~910 ℃ austenite temperature, the required mechanical properties can be obtained by holding the temperature for 90 min in 280~300 ℃ nitrate solution and air cooling out of the furnace.
What Material is Produced by Heat Treating White Cast Iron?
By subjecting white cast iron to specialized heat treatment processes, its internal structure can be altered, transforming it from a hard and brittle material into one with different mechanical properties. The most typical and important material result from heat treating white cast iron is malleable cast iron.
White Cast Iron Heat Treatment Product:
-
Blackheart Malleable Cast Iron:
This is the most common type. The white cast iron casting is heated to about 900–1000°C and held to allow the cementite to decompose. Then, it is further held at 720–760°C for a long time, causing the cementite to break down into clusters of graphite, known as “temper carbon.” The final structure consists of ferrite with embedded clusters of graphite, giving the material good ductility and toughness. -
Pearlitic Malleable Cast Iron:
This is obtained by adjusting the cooling rate or through alloying so that the metallic matrix contains more pearlite rather than ferrite, resulting in higher strength and hardness while still retaining some ductility. -
Whiteheart Malleable Cast Iron:
Produced by a decarburization heat treatment in an oxidizing environment, resulting in a ferrite matrix with less graphite. This type is less common and mainly used in specific applications.
High Chromium White Cast Iron Heat Treatment
High chromium white cast iron is renowned for its outstanding wear resistance, not only because of its special chemical composition but also due to precise heat treatment procedures that optimize its microstructure. These heat treatments achieve the best combination of hardness and toughness.
Main Heat Treatment Processes and Their Purposes
- Softening Annealing:
Used to reduce casting stress, improve machinability, and relieve residual stress.
Typical example: Heating to 920–1000°C, holding, then cooling slowly to 600°C. - Quenching & Tempering (Harden + Temper):
This process increases the hardness and wear resistance of the material while ensuring adequate toughness.
Typical example: Quenching at 950–1020°C in air or oil, then tempering at 200–300°C. - Stress Relief Annealing:
Used to reduce internal stresses caused by casting or machining, improving the stability of the parts.
Typical example: Heating to 200–300°C and holding for 8 hours. - Solution Treatment:
Applied to some special high chromium cast irons to dissolve alloy elements into the matrix and improve comprehensive properties.
Typical example: Heating to 1140–1180°C and holding for 16 hours before air cooling.
Key Technical Points and Principles
- Microstructural Transformation:
Heat treatment changes the cast iron’s internal metallic structure. For instance, the common quench + temper process transforms the matrix into high-hardness martensite, while alloy carbides remain, resulting in a combination of wear resistance and toughness. - Balance of Properties:
The main goal is to balance hardness (for wear resistance) and toughness (for impact resistance). By selecting appropriate heat treatment procedures, you can tailor the material’s properties for specific applications, such as mining or cement equipment. - Critical Parameters:
- Quenching Temperature: Should not be too low (carbide dissolution incomplete, less hardening) or too high (causing excessive grain growth and cracks).
- Cooling Rate: Must be rapid enough to avoid soft structures but controlled to prevent deformation and cracking.
- Tempering After Quenching: Essential to relieve stress, stabilize the structure, and avoid cracking—typically performed for 4–6 hours after quenching.
Modern heat treatment technology continues to evolve, with some advanced technologies allowing for even more precise control of the cast iron’s microstructure and properties. For example, rapid heating, precise temperature control, and surface strengthening techniques are becoming more common in industrial applications.