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Ductile Iron VS Cast Iron - From A 20-year-old Foundry Engineer

16th July, 2025
Author: HULK Metal

Strictly speaking, ductile iron should be a type of cast iron, because the definition of cast iron is the general term for iron that can be used for casting. In order to make it easier for you to understand the difference between ductile iron and cast iron, we will summarize it at the beginning of the article:




Ductile Iron VS Cast Iron:


Comparison of Definitions:


Cast iron is a general term for alloys mainly composed of iron, carbon, and silicon. In addition to the above components, it may also contain other elements.

Ductile iron is the abbreviation of ductile iron. It is composed of iron, carbon, silicon, and other elements. It is named because the graphite in its internal structure is spherical.

 

From this point of view, ductile iron is a type of cast iron. The only difference is that the carbon content of ductile iron exists in the form of spherical graphite, while that of cast iron may exist in the form of flake graphite (gray cast iron), flocculent graphite (malleable cast iron), vermicular graphite (vermicular graphite cast iron), cementite (white cast iron) or a mixture of graphite and cementite (mock cast iron).


Comparison of Ingredients:

 

Cast iron: Industrial cast iron generally contains 2.5% to 4.0% carbon. Carbon exists in cast iron mostly in the form of graphite, and sometimes in the form of cementite. In addition to carbon, cast iron also contains 1% to 3% silicon, as well as elements such as manganese, phosphorus, and sulfur. Corrosion-resistant cast iron also contains nickel, chromium, molybdenum, aluminum, copper, boron, and vanadium. Carbon and silicon are the main elements that affect the microstructure and properties of cast iron.

 

Ductile iron: The chemical composition of ductile iron is C = 3.0% to 4.0%, Si = 2.0% to 3.2%, P ≤ 0.1%, S ≤ 0.07%. We can also add some other elements to it to change its performance.

 

From this point of view, the composition of ductile iron and cast iron is similar. The only difference is the elements added in actual use to change the performance.

 

Comparison of Classification:


There are many ways to classify cast iron:

There are mainly incision color, shape characteristics of carbon elements, chemical composition, production methods and organizational properties.

Classification of ductile iron: matrix characteristics, chemical composition, performance characteristics, etc.

 

Comparison of Internal Structure:


Because the internal structure of cast iron varies greatly depending on the type. It will not be described here.

The internal structure of ductile iron is mainly composed of spheroidal graphite and matrix, and the matrix is divided into ferrite, pearlite, ferrite and pearlite mixed, and bainite.

 

Comparison of Performance:


Because the performance of different types is too different, cast iron and ductile iron can be replaced in some environments. The specific material to be used needs to be considered comprehensively.

 

Application Comparison: 


Cast iron can be used in more scenarios because it has more categories, while ductile iron is relatively rare. In most scenarios, the performance of ductile iron can be compared with some steels.

 

Below, I will make a detailed comparison from the aspects of their definition, acquisition method, types, common international grades and their respective applications.

 

Ductile iron:

 

Ductile iron, also known as spheroidal graphite cast iron, is a cast iron material with unique mechanical properties. Through a specific treatment process, the graphite of ductile iron is distributed in a spherical shape, making it have higher strength and toughness than traditional cast iron.

 

Obtaining Ductile Iron:


The acquisition of ductile iron is mainly processed by adding magnesium, cerium or rare earth elements to molten iron to make the graphite form spherical during solidification. The specific steps are as follows:

 

Smelting: Melt pig iron, scrap steel and alloy elements in a high-temperature furnace.

 

Inoculation treatment: Add inoculants such as ferrosilicon or calcium-silicon alloys to molten iron to improve the morphology and distribution of graphite.

 

Spheroidizing treatment: Add spheroidizing agents such as magnesium, magnesium-iron alloys or rare earth metals to transform graphite from flakes to spheres.

Casting: Cast the treated molten iron into a mold, and after cooling and solidification, you will get a ductile iron casting.

 

Of course, the above four steps are just a general introduction. In this process, you also need to pay attention to temperature control, the time of each link and other details.

 

Chemical Composition, Structure, and Properties of Ductile Iron


The chemical composition of ductile iron is w C = 3.6% ~ 4.0%, w Si = 2.0% ~ 3.2%, w P ≤ 0.1%, w S ≤ 0.07%. Ductile iron has a high carbon and silicon content to reduce the tendency of white cast iron and ensure the spheroidizing effect. The sulfur and phosphorus content is low to reduce their harmful effects. Because S reacts with Mg in the spheroidizing agent to form MgS slag inclusions, it increases the consumption of the spheroidizing agent and reduces the spheroidizing effect; P is easy to form phosphorus eutectic, split the matrix, and significantly reduce the toughness of ductile iron, while also increasing the tendency to form looseness. The matrix structure of ductile iron includes ferrite, pearlite and "ferrite + pearlite".

 

Because the graphite of ductile iron is spherical, the degree of its cutting of the matrix is greatly reduced, and its mechanical properties are higher than other cast irons, and it can exert 70% to 90% of the performance of the matrix structure. The casting performance, shock absorption, friction reduction, and machinability of ductile iron are better than those of cast steel. However, ductile iron has a large tendency to overcool, which is easy to produce white spots; it is also prone to defects such as insufficient pouring, cold shut, shrinkage holes, and shrinkage. Therefore, the smelting and casting process requirements of ductile iron are higher than those of gray cast iron.

 

Heat Treatment of Ductile Iron


Annealing:


The purpose of annealing ductile iron is to eliminate internal stress, eliminate white spots, and decompose the cementite in pearlite to obtain a ferrite matrix with high plasticity and toughness. The annealing heating temperature is 720 to 760℃. When there is white spots in the cast structure, the annealing heating temperature is 900 to 950℃ to promote the decomposition of cementite. After heat preservation, it is cooled to 600℃ with the furnace and air-cooled out of the furnace.

 

Normalizing:


The normalizing of ductile iron can be divided into high-temperature normalizing and low-temperature normalizing. After high-temperature normalizing, ductile iron can obtain pearlite, with refined structure, high strength, hardness and wear resistance. The normalizing heating temperature is 880-920℃ and the furnace is completely austenitized and then air-cooled. After low-temperature normalizing, ductile iron can obtain "pearlite + ferrite matrix", with high plasticity and toughness, but poor strength compared to high-temperature normalizing. The normalizing heating temperature is 840-860℃ and the furnace is partially austenitized and then air-cooled. After normalizing, ductile iron should be subjected to stress relief annealing at a heating temperature of 550-600℃ to eliminate the stress caused by normalizing.

 

Quenching and Tempering:


After quenching and tempering, ductile iron obtains a tempered troostite matrix with good comprehensive mechanical properties. It is often used as important structural parts such as diesel engine crankshafts and connecting rods. The quenching heating temperature is 860-880℃, and the hardness after quenching in oil is 58-60HRC, and it is tempered in time. The tempering heating temperature is 550-600℃, and it is air-cooled after being taken out of the furnace.

 

Isothermal Quenching:


After isothermal quenching, ductile iron can obtain a lower bainite matrix with high hardness, wear resistance and certain toughness. Isothermal quenching is suitable for parts with smaller cross-sectional dimensions. The heating temperature is 840-950℃, and after heat preservation, it is isothermal in 250-350℃ nitrate salt, and after completing the lower bainite transformation, it is air-cooled after being taken out of the furnace.

 

Types of Ductile Iron

 

Due to its chemical composition, mechanical properties and use environment, ductile iron can be divided into many types. Common classification methods include chemical composition, mechanical properties and application fields.

 

Classification by chemical composition

 

Ordinary Ductile Iron: The main components are iron, carbon and silicon, with basic mechanical properties.

 

High-strength Ductile Iron: On the basis of ordinary ductile iron, a certain amount of alloy elements such as copper, nickel, chromium, etc. are added to improve strength and hardness.

 

Heat-resistant Ductile Iron: Heat-resistant elements such as chromium and molybdenum are added to improve the stability of the material in high temperature environments.

 

Comparison Table of Ductile Iron Grades:


In order to help readers understand the specific performance of different grades of ductile iron, I provide a comparison table of common ductile iron grades, covering international standards and main performance indicators.

 

standard

Grade

Tension Strength (MPa)

Yield Strength (MPa)

Extension  (%)

Hardness

(HBS)

Matrix Structure

Common Applications

GB

QT400-18

400

250

18

130-180

Ferrite

Automobile and tractor chassis parts, wheel hubs, motor housings, brake shoes, couplings, pumps, valve bodies, flanges, etc.

EN-GJS

EN-GJS-400-18

ASTM

60-40-18

GB

QT400-15

400

250

15

130-180

EN-GJS

EN-GJS-400-15

ASTM

60-40-15

GB

QT450-10

450

310

10

160-210

EN-GJS

EN-GJS-450-10

ASTM

65-45-12

GB

QT500-7

500

320

7

170-230

Ferrite + Pearlite

Motor frame, transmission shaft, spur gear, sprocket, housing, bracket, connecting rod, rocker arm, crank, clutch plate, etc.

EN-GJS

EN-GJS-500-7

ASTM

70-50-05

GB

QT600-3

600

370

3

190-270

EN-GJS

EN-GJS-600-3

ASTM

80-60-03

GB

QT700-2

700

420

2

225-305

Pearlite

Automobile, tractor transmission gears, crankshafts, camshafts, cylinder blocks, cylinder liners, steering knuckles, etc.

EN-GJS

EN-GJS-700-2

ASTM

100-70-03

GB

QT800-2

800

480

2

245-335

EN-GJS

EN-GJS-800-2

ASTM

-

GB

QT900-2

900

600

2

280-360

Bainite

High-strength gears (such as automotive rear axle spiral bevel gears, large reducer gears), internal combustion engine crankshafts, camshafts, etc.

EN-GJS

-

ASTM

120-90-02

 

 

The above is a detailed introduction to ductile iron, which can help you better choose a grade as a more suitable material for castings. In order to make it easier for you to compare ductile iron and cast iron, we will introduce the definition, acquisition method, classification, and performance of cast iron below.


Cast Iron:

 

Cast iron is a general term for an iron-carbon alloy with a carbon content generally between 2% and 4%.

 

Obtaining Cast Iron

 

The process of obtaining cast iron mainly includes the following steps:

 

Smelting: melting pig iron, scrap steel and alloying elements in a blast furnace or electric furnace to produce molten iron.

 

Tempering: adjusting the composition and properties of molten iron by adding silicon carbide, ferrosilicon or other alloying elements.

 

Degassing: removing gases and impurities from molten iron by blowing oxygen or adding deoxidizers during the smelting process.


Casting: casting the treated molten iron into a sand mold or metal mold, and forming a cast iron part after cooling and solidification.

 

Types of Cast Iron

 

Cast iron can be divided into many types according to its chemical composition and microstructure. The main types of cast iron include gray cast iron, white cast iron, ductile cast iron and malleable cast iron.

 

Classification

 

There are several main ways to classify cast iron:

 

Classification by Microstructure

 

Gray cast iron: The graphite in the microstructure is flaky and the fracture is gray.

 

White cast iron: The carbon in the microstructure exists in the form of cementite, the fracture is white, hard and brittle.

 

Nodular cast iron: The graphite is distributed in spherical shape and has high strength and toughness.

 

Mallet cast iron: The carbides in white cast iron are decomposed into flocculent graphite through heat treatment to improve its toughness.

 

Classification by Alloying Elements:


Ordinary cast iron: The main components are iron, carbon and silicon.

Alloy cast iron: A certain amount of alloying elements such as chromium, nickel, molybdenum, etc. are added to ordinary cast iron to improve its mechanical properties and corrosion resistance.

 

Classification by Performance:


Wear-resistant cast iron: It has high hardness and wear resistance and is suitable for working conditions with greater wear.

Heat-resistant cast iron: It can maintain stable mechanical properties under high temperature conditions.

Corrosion-resistant cast iron: It has strong resistance to chemical media such as acids and alkalis.

 

Common Classification

 

According to the above classification method, the following are several common types of cast iron and their characteristics:

 

Gray Cast Iron:


Features: Graphite is flaky and has a gray fracture. It has good casting and vibration reduction properties, but is brittle.

 

Application: It is widely used in the manufacture of engine blocks, machine tool beds, pipe fittings and valves.

 

White Cast Iron:

 

Features: White fracture, high hardness but high brittleness, difficult to process.

 

Application: Mainly used to manufacture wear-resistant parts, such as coal mill liners, rollers and crusher gear plates.

 

Ductile Iron:

 

Features: Graphite is spherical, and its strength and toughness are better than gray cast iron, close to steel.

 

Application: Used to manufacture high-strength and high-toughness mechanical parts, such as gears, crankshafts, pipe fittings and pressure vessels.

 

Malleable Cast Iron:


Features: Through annealing, the carbides in white cast iron are decomposed into flocculent graphite, which has high toughness and plasticity.

 

Application: Widely used in the manufacture of automotive parts, pipe joints and agricultural machinery.

 

Alloy Cast Iron

 

Features: By adding alloy elements such as chromium, nickel and molybdenum, the wear resistance, heat resistance and corrosion resistance of cast iron are significantly improved.

 

Application: Mainly used to manufacture special parts that are heat-resistant, wear-resistant and corrosion-resistant, such as high-temperature furnace tubes, wear-resistant liners and chemical equipment.

 

Summary:

Through this article, you can understand that ductile iron is a special cast iron. The carbon element exists in the form of spherical graphite and can be used to produce various parts in our lives through casting technology. Cast iron can be made into ductile iron using a special smelting process. When choosing the type of cast iron, we need to consider the casting properties comprehensively.

If you have not learned about ductile iron vs. cast iron through this article, you can contact our engineers. They will explain it to you in detail.


 



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