yvonne@nydcasting.com 
1. Metallographic structure of martensitic wear-resistant ductile iron
Martensite
Martensite is the main matrix structure, generally high-carbon hidden needle martensite. Under the metallographic microscope, it is in the form of needles or laths, with fine and interlaced layers. Due to the supersaturated solid solution of carbon in α-Fe, it has high hardness and high strength, and is the key phase to provide wear resistance.
Spheroidal graphite
Graphite is evenly distributed in the matrix in a spherical shape, which is a typical feature of ductile iron. Spheroidal graphite can effectively reduce stress concentration, improve the toughness and strength of the material, and also has a certain auxiliary effect on the wear resistance of the material, making it less likely to crack and peel off when subjected to friction and impact.
Residual austenite
After heat treatment, a small amount of residual austenite will exist, usually distributed between martensitic laths or needle-shaped structures. It can improve the toughness of the material to a certain extent. When subjected to external force, the residual austenite can undergo phase transformation and absorb energy, which helps to improve the wear resistance and impact resistance of the material.
Alloy carbides
When alloying elements such as chromium, molybdenum, and tungsten are added, alloy carbides such as M₇C₃, M₂₃C₆, etc. (M represents metal elements) are formed. They are generally distributed in the matrix in fine particles or blocks, with high hardness, which can significantly improve the wear resistance and corrosion resistance of the material and enhance the overall performance of the material.
Possible presence of bainite
In some cases, there may also be a small amount of upper bainite or lower bainite. Upper bainite is generally feather-shaped, and lower bainite is needle-shaped or bamboo-leaf-shaped. The presence of bainite can improve the strength and toughness of the material to a certain extent, and also contribute to improving the wear resistance.
2. Chemical composition control
（1）. Carbon (C)
Range: 3.4%~3.9%
Function: Stabilize austenite and promote graphite spheroidization, but it is necessary to avoid excessive carbon content, which will lead to excessive carbon content in martensite after quenching.
（2）. Silicon (Si)
Range: 1.1-1.4% in raw iron, 2.1-2.5% in final silicon (conventional); Function: Strengthen the matrix, but excessive amount will reduce toughness, and needs to be adjusted in combination with the wall thickness of the casting.
（3）. Manganese (Mn)
Range: ≤1.2% (conventional); Function: Improve hardenability, but positive segregation will reduce toughness and needs to be strictly limited.
（4）. Phosphorus (P), Sulfur (S)
P: ≤0.15% (conventional); Dual-phase structure can be relaxed to ≤0.1%
S: ≤0.03% (conventional); needs to be controlled by desulfurization treatment (such as magnesium spraying).
（5）. Alloy strengthening elements
Nickel (Ni): 0.5%-1.0%
Molybdenum (Mo): 0.1%~0.3% (improve high temperature strength, but need to be controlled to avoid grain coarsening). Chromium (Cr): 0.2%~0.5% is added to some grades to improve wear resistance, but the precipitation of carbides at grain boundaries must be avoided.
（6）. Spheroidizing elements
Magnesium (Mg): 0.035%~0.055% (residual amount), to ensure graphite spheroidization.
Rare earth (Re): 0.01%~0.03% (residual amount), desulfurization and stabilization of spheroidization effect.
Reference to a casting production composition case
Conventional martensitic ductile iron: C3.4~3.9%, Si2.2~2.5%, Mn≤1.2%, P≤0.15%, S≤0.03%, Mg0.03~0.05%, Re0.02~0.03%.
Key control principles
1. Carbon-silicon balance: Carbon equivalent (CE) is usually controlled at 4.2%~4.8%, taking into account both casting performance and hardenability.
2. Impurity elements: Sulfur needs to be deeply desulfurized (≤0.01%), and phosphorus is controlled at ≤0.06% to reduce grain boundary brittleness.
3. Heat treatment adaptation: Martensitic ductile iron needs to be quenched (such as oil cooling) to obtain high hardness (59~61HRC), but deformation caused by excessive cooling should be avoided.
Through the above-mentioned composition control, martensitic ductile iron can take into account high strength (tensile strength ≥1000MPa) and certain toughness, which is suitable for wear-resistant parts (such as grinding balls) or high-load scenarios.
3. Melting and pouring temperature control of martensitic wear-resistant ductile iron
1. Melting temperature control
The temperature of the molten iron needs to be controlled at 1450~1550℃ to ensure that the alloy elements are fully melted and the burning loss is reduced.
The temperature of the molten iron before spheroidizing is recommended to be 1500~1530℃ to ensure that the spheroidizing agent reacts fully and the magnesium absorption rate is high.
2. Pouring temperature control
（1）. Pouring temperature range
Conventional pouring: 1360~1420℃, depending on the wall thickness and structural complexity of the casting.
Thick and large parts: 1370~1390℃ is recommended to avoid graphite floating or shrinkage due to low temperature.
Thin-walled parts: It can be appropriately increased to about 1400-1450℃, but the cooling rate needs to be controlled to prevent deformation.
（2）. Pouring process optimization
Pouring speed: Rapid filling is required to reduce oxide inclusions. Bottom pouring or step pouring is recommended for thick-walled parts.
Pouring time: From the end of spheroidization to the completion of pouring, it needs to be controlled within 10~15 minutes to avoid spheroidization decay.
Pouring temperature fluctuation: It needs to be stable within ±15℃ to prevent uneven structure due to temperature fluctuation.
4. Heat treatment of martensitic wear-resistant ductile iron
Quenching
Heating temperature: Generally between 850-950℃. In this temperature range, the pearlite, ferrite and other structures in the matrix can be fully transformed into austenite, laying the foundation for the subsequent acquisition of martensitic structure. For example, for martensitic wear-resistant ductile iron with high carbon content and moderate alloying element content, the heating temperature can be selected to be around 900℃.
Heating speed: It needs to be reasonably controlled according to factors such as the size, shape and complexity of the casting. For castings with simple shapes and small sizes, the heating speed can be appropriately increased; while for castings with complex shapes and large sizes, a slower heating speed should be used to prevent deformation or cracking caused by excessive thermal stress.
Holding time: usually 1-3 hours. The purpose is to ensure that austenitization is fully carried out and that the alloying elements are fully dissolved and homogenized. For example, for castings with a thickness of 20-30mm, the holding time at 900℃ is about 1.5-2 hours.
Cooling medium: commonly used are oil, water, brine, etc. Oil quenching has a relatively slow cooling speed, which can reduce quenching stress and is suitable for castings with complex shapes and high deformation requirements; water quenching has a fast cooling speed and can obtain higher hardness and strength, but the quenching stress is large, which is suitable for castings with simple shapes and high hardness and strength requirements; salt water cools faster than water and can be used for castings requiring higher hardness and wear resistance, but it may increase the risk of cracking of castings.
Tempering
Tempering temperature: generally between 150-350℃. Low temperature tempering (150-250℃) can eliminate part of the quenching stress, stabilize the structure, improve toughness, and maintain high hardness and strength; medium temperature tempering (250-350℃) can further eliminate stress and improve toughness, which is suitable for occasions with high toughness requirements.
Tempering time: usually 2-4 hours. If the tempering time is too short, the stress is not fully eliminated and the structure stability is poor; if the tempering time is too long, the hardness may drop too much and affect the wear resistance.
Tempering times: generally 1-3 times. For some large, complex or extremely high performance castings, 2-3 tempering may be required to ensure the stability of structure and performance.
In actual production, it is also necessary to combine the specific casting composition, size, shape, performance requirements and production equipment and other factors to optimize the heat treatment process parameters through experiments and practice to obtain the best martensitic wear-resistant ductile iron performance.