Knowledge

 

I. Concept

In the field of metallurgy, instantaneous modification technology usually refers to a process method in which specific modifiers are added within an extremely short period of time (typically a few seconds or even less) during the solidification of metals or alloys, causing rapid and significant changes in their microstructure and properties. Different from the traditional modification treatment completed in the furnace, the instantaneous modification technology emphasizes the treatment at the moment of pouring or solidification to ensure that the modifiers can be evenly dispersed and take effect in a timely manner.

In the smelting of high-chromium white cast iron, the instantaneous modification technology achieves the transformation of the eutectic carbide morphology from continuous network to isolated spherical by precisely controlling the nucleation and growth kinetics during the solidification of the melt. By optimizing the microstructure of the cast iron, its comprehensive mechanical properties, especially wear resistance and toughness, are significantly improved.

 

II. Principles of Metamorphic Treatment

The core of metamorphic agent treatment lies in:

Rapid action: The modifier is rapidly introduced into the molten metal, usually at the moment of pouring or solidification, to ensure that it is evenly dispersed and takes effect promptly, avoiding oxidation, burning loss or precipitation due to prolonged exposure.

 

 

Alteration of nucleation and crystal growth: The primary function of the modifier is to change the nucleation conditions and crystal growth patterns during metal solidification. For instance, in cast iron production, adding a modifier can refine graphite, transforming it from flake to spherical or vermicular forms, thereby significantly enhancing the strength, toughness and ductility of the cast iron.

 

 

Improvement of macro and microstructure: By controlling the solidification process, the instantaneous modification technique can eliminate or reduce the formation of harmful phases, refine grains, and improve the grain boundary condition, thereby optimizing the comprehensive mechanical properties of the material.

 

III. Types and Addition Methods of Degradation Agents

The common types of modifiers include the following:

Titanium (Ti) based modifiers: Titanium combines with nitrogen in molten iron to form TiN, which serves as a heterogeneous nucleation core for austenite and eutectic carbides, thereby refining the grain structure.

Rare earth (RE) type modifiers: Rare earth elements are surface-active elements that can adsorb on the interface of carbides, reducing the interfacial energy and inhibiting the growth of carbides, thereby refining them. At the same time, rare earths can react with harmful elements such as sulfur and oxygen, playing a purifying role.

Niobium (Nb) and vanadium (V): These elements can act as carbide-forming elements, by forming refractory carbides (such as NbC, VC) to serve as heterogeneous nucleation cores, playing a role in grain and carbide refinement in high-chromium cast iron.

Boron (B): Research indicates that the modification treatment with boron can granularize the carbides in high-chromium white cast iron, thereby enhancing its toughness. Boron can react with carbon in the melt to form specific phases, which affects the growth of carbides.

Composite modifiers: To achieve better modification effects, composite modifiers containing multiple elements are usually used, such as Ti-RE, Nb-RE, V-RE, etc. These composite modifiers work in synergy, which can comprehensively improve the microstructure and properties of high-chromium white cast iron.

 

There are usually several ways to add modifiers:

In-stream modification: During the pouring process, modifiers in powder, block or wire form are added to the molten iron flow, allowing them to melt quickly and disperse evenly.

In-bag modification: Place the modifier at the bottom of the ladle. When the molten iron is poured into the ladle, the modifier melts under the impact of the molten iron and takes effect.

Online modification: Through dedicated equipment, real-time and precise modification treatment is carried out on molten iron on the casting pouring line.

 

IV. Regulation Mechanism of Growth Kinetics by Metamorphic Agents

1. Interface adsorption inhibits growth:

The atomic radius of rare earth elements (such as Ce) is relatively large (about 1.83A), which makes them prone to accumulate at the carbide-melt interface. By reducing the interface energy and hindering the diffusion of elements like C and Cr, the growth rate of carbides can be decreased by 40% to 60%. This anisotropic growth inhibition effect prompts the transformation of carbides from elongated rod-like shapes to equiaxed ones.

2. Composition undercooling induces spheroidization:

The addition of modifiers (such as Al) can locally alter the melt composition, creating a composition undercooling zone at the carbide front. This undercooled environment forces carbides to adopt a more stable spherical growth morphology to reduce surface energy. Research shows that when the Al content exceeds 0.3wt%, the aspect ratio of eutectic carbides can be reduced from 8:1 to less than 2:1.

3. Synergistic effect of melt flow:

Instantaneous Modification Treatment are often combined with melt treatment methods such as pulsed current and ultrasonic vibration. For instance, 45Hz pulsed current treatment can generate periodic electromagnetic forces in the melt, causing primary carbide particles to rotate and collide, disrupting their preferred growth direction and ultimately forming regular hexagonal block structures. This synergy between physical fields and chemical modification can further refine carbide size to 50-100μm.

 

V. Thermodynamic Optimization Mechanism of Phase Transition of Metastabilizer

1. Carbon activity regulation:

The affinity of modifiers such as Ti for carbon is higher than that of Cr, which leads to the preferential formation of TiC, consuming free carbon in the melt and reducing the driving force for the formation of M₇C₃. When the Ti content exceeds 0.51 wt%, the carbon activity in the melt decreases by 20% to 30%, inhibiting the formation of coarse primary carbides.

2. Compression of eutectic reaction range:

Modifiers such as rare earth ferrosilicon can reduce the eutectic reaction temperature range from 80-120°C to 30-50°C, shortening the time window for carbide growth. At the same time, rare earth elements neutralize harmful impurities in the melt (such as Pb and Sn), eliminating their promoting effect on carbide growth and further inhibiting the formation of continuous networks.

 

VI. Implementation Path of Typical Metamorphic Treatment Processes

 

Taking Ti-rare earth composite modification as an example, its operation process usually includes:

Melt pretreatment: Add 0.2-0.5 wt% Fe-Ti alloy to the melt at 1550-1600℃. The purpose is to generate TiC nucleation particles, providing a basis for the heterogeneous nucleation of eutectic carbides in the subsequent process.

Instantaneous Modification treatment: Add 0.1-0.3 wt% of rare earth ferrosilicon alloy 30-60 seconds before pouring, and achieve uniform dispersion by using melt turbulence.

Dynamic solidification control: Promoting the fragmentation and spheroidization of carbide particles through electromagnetic stirring (frequency 10-20 Hz) or pulse current (voltage 300-500 V).

Subsequent heat treatment: Combined with de-stabilizing annealing at 850-950℃, the residual austenite is decomposed into martensite, forming a harder matrix, thereby further optimizing the matrix's support for carbides.

This technological approach can enhance the impact toughness of eutectic carbides by 120% to 150%, while maintaining a high hardness of 60-65 HRC. It is particularly suitable for components with strict requirements for wear resistance and impact resistance, such as the impellers of slurry pumps. Future research directions include the development of nanoscale composite modifiers (such as TiC-B₄C core-shell structures) and machine learning-based process parameter optimization models to achieve precise control of carbide morphology.