8 Key Techniques to Boost Low-Temperature Impact Toughness of Ductile Iron

Studies have confirmed that in different temperature environments, variations in the matrix structure of ductile iron have a significant impact on its low-temperature impact toughness. Among them, ductile iron with higher ferrite content and better plasticity typically achieves more ideal low-temperature impact toughness indicators. The following details the core measures to improve the low-temperature impact toughness of ductile iron from multiple technical dimensions and verifies their technical accuracy.

I. Optimizing Chemical Composition Control

1. Reducing Harmful Element Content

Elements that promote or stabilize pearlite formation need to be strictly controlled, such as manganese (Mn), vanadium (V), zirconium (Zr), niobium (Nb), titanium (Ti), chromium (Cr), molybdenum (Mo), tungsten (W), copper (Cu), lead (Pb), antimony (Sb), etc. Two key elements require special attention:

• Manganese (Mn)：It has a particularly significant impact on the impact toughness and ductile-brittle transition temperature of ductile iron. Studies have shown that for every 0.1% increase in manganese content, the ductile-brittle transition temperature of ductile iron rises by 10°C to 12°C. Therefore, in production, low-manganese pig iron and scrap steel should be preferred as raw materials to reduce manganese input from the source.

• Copper (Cu)：Although it is a neutral element and has little effect on increasing pearlite content, as copper content increases, the ductile-brittle transition temperature of ductile iron gradually rises, and impact toughness decreases significantly, so its content must be strictly controlled.

2. Reasonably Regulating Ferrite-Forming Elements

The content of ferrite-forming elements such as carbon (C), silicon (Si), calcium (Ca), barium (Ba), aluminum (Al), and bismuth (Bi) should be appropriately increased, but the dosage balance must be maintained. Among them, the regulation of silicon (Si) is particularly critical:
Silicon is a strong graphitization-promoting element, which helps increase ferrite content, but excessive silicon will significantly reduce impact toughness. Data shows that for every 0.1% increase in silicon content, the ductile-brittle transition temperature rises by 5.5°C to 6°C. If the silicon content reaches around 4%, even if the ductile iron has a full ferrite matrix, it will be too brittle to withstand room-temperature impact loads. Therefore, for ductile iron requiring low-temperature impact performance, the silicon content is usually controlled between 1.6% and 2.0%.

II. Optimizing Casting Cooling Rate Control

For ductile iron with a specific composition, the cooling rate during the eutectic stage can significantly affect the matrix structure: the slower the cooling rate, the higher the ferrite content (the thicker the casting wall, the slower the cooling, and the higher the ferrite proportion). However, it is necessary to avoid coarse grains and graphite nodules caused by excessively slow cooling. Specific measures include:

• Using molding materials with low thermal conductivity (such as dry sand, resin sand) and appropriately increasing mold thickness (known as "increasing sand allowance");

• Reducing or avoiding the use of chills;

• For thin-walled parts, the cooling rate can be slowed down by appropriately increasing the pouring temperature, while extending the mold opening time. When conditions permit, castings can be placed centrally to reduce heat dissipation.

III. Optimizing Heat Treatment Processes

Experimental data (as shown in Figures 4 and 5) indicates that heat treatment can effectively increase ferrite content, significantly improving elongation and impact toughness. Annealing promotes element diffusion at high temperatures, refines matrix lattices and grains, and stabilizes ferrite content and performance. In addition, heat treatment can appropriately relax strict requirements on some elements in raw and auxiliary materials; for small and medium-sized castings with substandard performance, defects can be compensated through heat treatment.

IV. Refining Grains and Increasing Eutectic Cell Count

There is a significant negative correlation between material grain size and fracture stress: when the grain size exceeds a critical value, brittle fracture is likely to occur. Grain refinement can reduce the ductile-brittle transition temperature, thereby improving low-temperature impact toughness. Core measures include:

1. Adopting Synthetic Cast Iron Melting Process

Using scrap steel and returned ductile iron as main raw materials, molten iron is smelted by carbon increasing with graphite and silicon increasing with ferrosilicon or silicon carbide. Since the melting points of carbon and silicon are higher than that of molten iron, they mainly enter the molten iron through diffusion and dissolution, forming a large number of [C] microcrystals. These microcrystals can serve as high-quality exogenous nucleation substrates for proeutectoid or eutectic graphite, effectively refining grains.

2. Implementing Multiple Inoculation Processes

The core of inoculation is to deoxidize, desulfurize, and form exogenous grains, thereby increasing graphite nucleation capacity, refining grains, and improving the number of graphite nodules and ferrite content. Practice has shown that after three inoculations (especially instantaneous inoculation with 0.3–1mm barium-containing inoculants during pouring), although the inoculant dosage is small, the effect is significant.

V. Purifying Molten Iron to Reduce Inclusions

Material fractures are mostly transgranular or intergranular. Inclusions inside grains or at grain boundaries weaken material bonding force, becoming crack sources or propagation paths under impact loads, reducing low-temperature impact resistance. Purification measures include:

1. Molten Iron Pretreatment

• Deoxidation and Desulfurization：For manufacturers using cupola-electric furnace duplex melting, shaking ladle injection or pneumatic desulfurization can be adopted to reduce the sulfur content in the original molten iron to below 0.02%. Desulfurizers (such as CaO, CaC2) have weak deoxidation ability and can be supplemented with deoxidizing elements such as calcium, barium, and aluminum; deoxidation and desulfurization are also necessary for direct electric furnace melting.

• Overheating and Standing：Increasing the melting temperature (≥1500°C for scrap steel carburization processes) and extending the holding time promote the floating of inclusions; standing the nodularized molten iron for 1–3 minutes facilitates the floating of oxides and sulfides of magnesium, barium, aluminum, and iron.

2. Strengthening Slag Removal and Filtration

• "Multiple covering" can reduce contact between molten iron and air during smelting and pouring, lowering oxygen content; "frequent slag removal" can gather residual oxides and sulfides to achieve iron-slag separation;

• A slag collecting ladle with a filter is installed in the pouring system to prevent solid and liquid slag from entering the mold cavity, stabilize molten iron flow to reduce secondary oxidation slag, and promote the floating of slag particles.

VI. Controlling Harmful Element Segregation and Inclusion Formation

• Reducing Grain Boundary Segregation Elements：Strictly control the content of segregation-prone elements such as manganese, antimony, tin, arsenic, and titanium;

• Reducing Oxide and Sulfide-Forming Elements：Elements such as calcium, barium, aluminum, magnesium, and rare earths easily form oxides and sulfides, so their contents should be reasonably reduced.

VII. Selecting Special Nodulizers and Inoculants

Nodulizers and inoculants for producing low-temperature impact-resistant ductile iron must follow three principles:

• Stable Nodularization and Inoculation Effects：The deviation of nodulizer components (such as magnesium, rare earth, calcium, barium) must be ≤±0.3%. Meanwhile, ensure the stability of molten iron temperature, sulfur and oxygen content, and operating processes (such as tapping speed and position) to avoid slow tapping causing molten iron to directly impact the nodulizer.

  
  

• Strong Graphitization Ability：Magnesium and rare earth are main nodularizing elements but tend to form chill. It is necessary to use magnesium as the main component, supplemented by rare earth, and matched with strong graphitizing elements such as calcium, barium, and bismuth.

  
  

• Low Slag-Forming Ability：Reduce slag content in nodulizers and inoculants (such as magnesium oxide, rare earth oxides, etc.), and control calcium and barium contents (both have strong slag-forming ability).

VIII. Balancing the Contradiction in Nodularizing Element Dosage

There is a contradiction between the dosage of elements such as magnesium, rare earth, calcium, and barium in nodulizers and inoculants, and nodularization effect and low-temperature impact performance: excessive dosage will lead to high residual elements, increased oxide and sulfide slag, and reduced impact performance; insufficient dosage will affect nodularization effect and matrix structure. Therefore, it is necessary to accurately select special nodulizers, inoculants, and supporting processes according to molten iron quality, casting size, shape, wall thickness, and pouring time to achieve dosage balance.

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DAWN | Pig Iron & Castings Procurement Advisor
18 years in the foundry trenches give me an edge: I know how pig iron’s chemistry impacts casting quality and can troubleshoot defects like cracks and porosity. With a 1M MT/year pig iron and 60k MT/year casting output from our in-house factory, plus 200+ verified suppliers on our platform, we offer fast price comparisons. Expect a 24-hour inquiry response—my goal? Not just closing deals, but being your go-to partner in the foundry world.

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