Knowledge

 

High-performance cast iron (including high-strength gray cast iron and ductile iron) melted in medium-frequency furnaces, due to its pure molten iron, high overheating temperature and large tendency of undercooling during crystallization, poses higher requirements for inoculation treatment. Traditional 75% silicon iron inoculant is prone to deterioration and has limited effect, thus various high-efficiency inoculants have become an inevitable choice.

 

I. Types and Characteristics of Commonly Used High-Efficiency Nodulizers

High-efficiency nodulizers are usually composite nodulizers formed by adding one or more strong graphitizing elements to ferrosilicon.

Silicon-barium inoculant (FeSi-Ba)

Characteristics: Barium (Ba) can significantly delay inoculation decline and extend the effective inoculation time. It is highly suitable for production scenarios where molten iron needs to be kept warm or transported for a relatively long time. It has excellent graphite promotion ability and strong deoxidation capacity.

Application scenarios: Large and medium-sized castings, automated production lines (such as brake disc and brake drum production lines), and situations requiring prolonged pouring.

 

Silicon-strontium inoculant (FeSi-Sr)

Characteristics: Strontium (Sr) is one of the most effective elements for eliminating white cast iron and reducing the tendency of carbide formation. Its key advantage lies in the fact that it hardly desulfurizes and can effectively protect the original MnS nucleation cores in the molten iron.

Application scenarios: Primary low-sulfur molten iron (S < 0.06%), thin-walled castings, and castings with extremely high requirements for hardness uniformity and machinability (such as engine blocks, cylinder heads, and hydraulic valve blocks).

 

Silicon-calcium inoculant (FeSi-Ca)

Characteristics: Calcium (Ca) has strong deoxidizing and desulfurizing capabilities, effectively purifying molten iron and providing nucleation cores. However, it is necessary to be cautious as its residue (CaO) may cause "calcium rust spots" defects.

Application scenarios: When the sulfur content in molten iron is relatively high (S > 0.08%), it is used to adjust the sulfur content by its mild desulfurization effect. It is mostly used as a pretreatment agent or as a component of composite inoculants, and its use as a sole inoculant in the later stage has become less common.

 

Compound inoculants (such as Si-Ba-Ca, Si-Sr-Ba, etc.)

Characteristics: They combine the advantages of multiple elements, complementing each other. For instance, Si-Ba-Ca has both anti-decay and iron water purification capabilities; Si-Sr-Ba, on the other hand, not only has strong anti-white cast iron properties but also enhanced anti-decay performance.

Application scenarios: The most widely used type. Different ratios of compound inoculants can be selected based on the different grades of castings and the conditions of the molten iron, making them the most versatile.

 

Rare earth inoculant (such as FeSi-RE)

Characteristics: Rare earth (RE) elements have extremely strong deoxidizing and desulfurizing capabilities and can neutralize the hazards of certain interfering elements. They can effectively improve the morphology of graphite and increase the content of pearlite.

Application scenarios: It is commonly used for inoculation in ductile iron; in gray cast iron, it is used to treat furnace materials containing trace amounts of harmful elements (such as Ti, Pb, Bi), or to produce ultra-high strength gray cast iron.

 

II. Principles for Selecting Efficient Inoculants

The core of the selection principle is "Adapt to local conditions and match requirements", which requires comprehensive consideration of the following four dimensions:

Principle One: Based on the original sulfur content of the molten iron (the most core principle)

This is a decisive factor, directly corresponding to different inoculation philosophies.

High sulfur molten iron (S > 0.08%): Select inoculants with mild desulfurization capabilities, such as silicon-calcium series (FeSi-Ca) or calcium-containing composite inoculants (such as Si-Ba-Ca). Utilize their desulfurization capabilities to adjust the sulfur content to the ideal range and provide nucleation cores.

Low sulfur molten iron (S < 0.06%): Must select "sulfur-protecting" type inoculants, with the first choice being silicon-strontium series (FeSi-Sr) or strontium-containing composite inoculants. Protect the precious MnS cores from being destroyed and fully leverage their strong ability to eliminate white cast iron.

Principle Two: Based on the characteristics and performance requirements of the castings

For thin-walled and complex castings: They are extremely sensitive to white iron tendency. The silicon-strontium inoculant is the first choice due to its strongest ability to eliminate carbides, ensuring uniform hardness and machinability.

For thick and heavy castings: The focus should be on anti-decay performance. To avoid the deterioration of graphite morphology caused by inoculant decay, silicon-barium or barium-containing composite inoculants can be selected.

For castings with high surface quality requirements/export castings: To avoid the "calcium rust spot" defect, low-calcium or calcium-free strontium-based or barium-based inoculants should be preferred.

Principle Three: Based on the Production Process Flow

Long pouring cycle: If the time interval from inoculation completion to the pouring of the last box of castings is long, a silicon-barium-based inoculant with strong anti-decay ability must be selected.

On-the-flow inoculation / in-mold inoculation: Generally, special inoculants with uniform particle size, fast dissolution speed, and high efficiency are used, such as strontium-based or special composite inoculants.

Principle Four: Based on the Comprehensive Balance of Cost and Benefit

Although the unit price of high-efficiency inoculant is higher than that of 75% silicon iron, its addition amount can usually be reduced by 30% to 50%, and its effect is far superior to the latter.

The comprehensive cost needs to be calculated: the cost savings brought by the improvement of the yield rate, the enhancement of performance stability, and the reduction of scrap rate and rework rate due to the high-efficiency inoculant are often much higher than the price difference of the inoculant itself.

 

Summary and Practical Suggestions

Data First: It is essential to be equipped with a front-of-furnace thermal analyzer to accurately measure the carbon-silicon equivalent (CE) and sulfur (S) content of the original molten iron. This is the sole basis for scientifically selecting inoculants.

Small-scale Testing: When changing the type or grade of inoculant, conduct a front-of-furnace triangular test block test first to observe the size of the white cast iron and the morphology of graphite, quickly verify the effect, and determine the optimal addition amount.

Simplification and Standardization: After achieving stable production, the furnace charge formula and one dominant high-efficiency inoculant should be fixed as much as possible to reduce production variables, which is beneficial for process control and quality stability.

In conclusion, for the medium-frequency furnace melting of high-performance cast iron, discarding the single 75 silicon iron and scientifically selecting efficient inoculants based on the principle of "determining the type by sulfur content, determining the characteristics by part thickness, and determining the variety by process" is the key technical means to achieve a leap in product quality.

 

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