Knowledge

 

Metallic inclusions in steel, as a key defect in the metallurgical process, not only disrupt the continuity of the metal matrix but also significantly reduce material performance through stress concentration effects, crack initiation mechanisms, and other pathways. According to their morphological characteristics and formation mechanisms, metallic inclusions can be classified into three major categories: foreign metal fragments, welding process inclusions, and casting specific structures. Their physical and chemical properties and distribution patterns directly determine the mechanical properties, processing performance, and corrosion resistance of steel.

I. Classification System of Metal Inclusions

(1) Foreign Metal Fragments

This type of inclusion mainly originates from the residual iron alloy that is not completely melted during the steelmaking process. The typical morphology is irregular angular particles, with the main components being high-melting-point metals such as tungsten and molybdenum. When the size of alloy additive exceeds 30mm, the risk of incomplete melting significantly increases. For example, during the vacuum induction melting of high-speed steel, if the size of ferrovanadium is not controlled below 20mm, the residual vanadium particles will cause the cutting tool edge to break. The hazards are as follows:

1. Abnormal microhardness: The Vickers hardness of tungsten inclusions can reach 800-1200HV, which is 3-4 times that of the base material (200-300HV).

2. Fatigue crack source: Under alternating loads, angular particles cause stress concentration coefficients of 3-5 times, accelerating crack propagation.

3. Deterioration of processing performance: During rolling, it leads to surface cracks, reducing the yield rate by 15-30%.

 

 

(2) Welding Process Inclusions

1. Tungsten Inclusion Defect: In tungsten inert gas welding (TIG), when the current is below 120A, the tungsten electrode tip droplets cannot fully enter the molten pool, forming white bright spherical particles with diameters ranging from 0.2 to 1.5mm. A welding experiment on an aero-engine turbine disk showed that tungsten inclusion defects reduced fatigue life by 62%.

2. Spherical Inclusion Phenomenon: Spherical inclusions formed by the molten pool encapsulating metal spatter, mainly composed of iron oxide. In pressure vessel welding, spherical inclusions cause a 40% reduction in weld impact toughness and need to be eliminated through multiple grinding passes.

(3) Casting Special Structures

1. Cold Slag Defects: Surface-oxidized metal lumps (0.5-3mm in size) are entrapped in the molten steel during the casting process, resulting in a surface-to-center composition gradient. In the casting of automotive crankshafts, cold slag causes surface hardness fluctuations of up to 50HRC, leading to premature wear.

2. Inclusions: Abnormal eutectic structures formed by local composition segregation, such as (Cr,Mo)6C type carbides in Cr-Mo steel. In the casting of gas turbine blades, inclusions reduce high-temperature creep strength by 28%.

II. Multi-dimensional Impact of Metal Inclusions on Steel Properties

(1) Mechanism of Mechanical Properties Degradation

1. Strength Fluctuation:

   - Microscopic inclusions (<1μm) may increase the yield strength by 5-10% through grain refinement.

   - Macroscopic inclusions (>100μm) cause a 15-20% reduction in tensile strength. For instance, 150μm-sized Al₂O₃ inclusions in a certain bearing steel shortened the contact fatigue life by 73%.

2. Toughness attenuation:

  - Sulfide inclusions (MnS) cause hydrogen-induced cracking, reducing the impact energy from 120J to 35J at -20℃.

  - The sharp corner effect of alumina inclusions reduces the fracture toughness KIC value by 40%. The brittle fracture accident rate of a certain pressure vessel steel increased threefold due to inclusions.

3. Plasticity loss:

  • The elongation decreases exponentially with the increase of inclusion content. When the area fraction of inclusions exceeds 0.5%, the elongation drops by 50%.

  • The reduction of area is sensitive to the size of inclusions. Inclusions of 10 μm grade reduce the reduction of area by 25%, while those of 100 μm grade cause a 45% decrease.

(2) Constraints on Machining Performance

1. Deterioration of cutting performance:

   - Hard inclusions (such as TiN) increase tool wear rate by 3 to 5 times. In a precision gear processing, due to inclusions, the tool life dropped from 800 pieces per edge to 150 pieces per edge.

   - Plastic inclusions (MnS) cause built-up edge, worsening the surface roughness Ra value from 0.8 μm to 3.2 μm.

2. Limited formability:

   • In deep drawing steel plates, 0.5 μm-level inclusions reduce the cupping value by 20%, causing the earing rate to increase by 15%.

   • During forging, inclusions trigger cracks. The forging scrap rate of a certain shaft part decreased from 8% to 1.2% due to inclusion control.

(3) Corrosion Resistance Degradation Pathways

1. Acceleration of Galvanic Corrosion:

   - Inclusions of dissimilar metals (such as Cu particles) form microcells with the base metal, increasing pitting corrosion rates by 2-3 orders of magnitude.

   - A steel used in a certain offshore platform, due to 0.1% Cu inclusions, saw its annual corrosion rate increase from 0.02 mm to 0.15 mm.

2. Stress corrosion sensitivity:

   • In chloride environments, stress concentration at the tips of inclusions reduces the stress corrosion critical stress σth by 60%.

   • In nuclear power equipment steels, the proportion of stress corrosion cracking accidents caused by inclusions accounts for 42%.

III. Analysis of Typical Cases and Control Strategies

(1) Aviation Engine Blade Fracture Incident

The turbine blade of a certain type of aviation engine fractured after 1,200 hours of service. The failure analysis revealed the following:

1. Inclusion characteristics: 0.8mm-sized TiN particles with a hardness of 2,200HV.

2. Fracture mechanism: Stress concentration at the inclusion tip initiated fatigue cracks, which rapidly propagated at 820°C.

3. Control measures:

   - Implement a dual-process of vacuum induction melting followed by electroslag remelting.

   - Pre-treatment of raw materials: Crush Ti alloy to a particle size of less than 15mm.

   - Melting control: Maintain a superheat of 120°C to ensure complete dissolution of TiN.

(2) Improvement of Fatigue Failure of Automotive Gears

The gear of a heavy-duty truck transmission experienced surface spalling after 300,000 kilometers. Root cause analysis:

1. Inclusion influence: 0.3mm-sized Al₂O₃ inclusions led to a 55% reduction in contact fatigue strength.

2. Improvement plan:

   - Optimize deoxidation process: Use Al-Ca composite deoxidizer.

   - Refining treatment: RH vacuum circulation degassing to reduce [O] from 30ppm to 15ppm.

   - Continuous casting protection: Control the basicity of the tundish cover agent between 3.0 and 3.5.

3. Effect verification: The fatigue life of the gear was increased to 800,000 kilometers, reaching 2.6 times the design requirement.

IV. Frontier Detection Technology and Control Direction

(1) Multi-modal Detection Technology

1. Ultrasonic Detection: The detection rate for metal inclusions ≥ 2mm is 95%, with the characteristic signal being high-amplitude bottom wave attenuation.

2. Backscattered Electron Imaging: Identifies tungsten inclusions (W content > 95%) through compositional differences, with a spatial resolution of 0.1 μm.

3. Fully Automated Electron Microscopy Analysis: The ParticleX system enables automatic statistics of 300 inclusions per hour, increasing efficiency by 20 times compared to manual methods.

(II) Process Control Innovation

1. Electromagnetic purification technology: By applying an alternating magnetic field to the molten steel, inclusions aggregate into larger particles and float up. After a certain special steel plant adopted this technology, the total amount of inclusions decreased by 42%.

2. Optimization of the mold flow field: By using a turbulence suppressor + dam structure, the inclusion floatation rate increased from 68% to 89%.

3. Continuous casting protective pouring: The long nozzle + submerged entry nozzle sealing system keeps the secondary oxidation oxygen increase within 0.5 ppm.

V. Conclusion and Outlook

The influence of metallic inclusions on the properties of steel shows a significant size effect and composition dependence:

1. When the size of inclusions is greater than 50 μm, the fatigue life reduction rate is linearly related to the size (R² = 0.92).

2. The damage to toughness caused by brittle inclusions (Al₂O₃) is 2.3 times that of plastic inclusions (MnS).

3. By adopting the combined deoxidation + electromagnetic purification + tundish flow control technology, the cleanliness can reach ASTM E45 standard grade A (total inclusion area < 0.05%).

The future development directions should focus on:

1. Developing an intelligent inclusion monitoring system to achieve real-time control of the steelmaking process.

2. Researching the regulation technology of nano-scale inclusions and exploring their strengthening mechanisms.

3. Establishing a machine learning-based inclusion-performance prediction model to guide precise smelting.

By controlling the morphology, size and distribution of metallic inclusions through a system, the reliability of steel can be significantly enhanced, providing material support for the manufacturing of high-end equipment.

 

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