Characteristics and prevention measures of pore precipitation in gray cast iron

1. Characteristics of pore precipitation in gray cast iron

The precipitation porosity in gray cast iron parts is a common and specific casting defect. It is mainly caused by the sharp decrease in the solubility of gases (mainly hydrogen and nitrogen) dissolved in the molten iron during the cooling and solidification process, which cannot be completely released and precipitate in the form of bubbles and remain inside the casting. The main characteristics of precipitated pores are as follows:

a. Location characteristics: Mostly occurring in the hot spots, thick and large sections, or core areas of the final solidification of castings: These areas have a slow solidification rate, providing more sufficient time for gas evolution, accumulation, and growth. Often inside the casting (away from the surface): Although sometimes close to the surface, it is usually located in the inner or central area of the casting wall thickness, unlike subcutaneous pores that closely adhere to the skin. Usually stay away from the gating system and risers: because the gating riser area solidifies later and has lower pressure, gas is more likely to migrate and escape to these areas. The precipitation pores are more likely to form at isolated hot nodes far away from these "exhaust channels".

b. Shape and size characteristics: Shape: Small holes that are mostly circular, elliptical, or teardrop shaped. If multiple bubbles gather at the solidification front and grow along the dendrites, they may also form worm like, tadpole like, or irregular shapes distributed along grain boundaries. Size: Usually relatively small, with a diameter range of around 0.5mm to 3mm. But it may also be larger, especially at thick and large sections. Inner wall: Smooth, clean, and shiny (like a mirror), which is one of the most typical characteristics of precipitated pores. Because bubbles are formed inside the molten iron, their walls come into direct contact with the liquid metal without oxidation or contamination.

c. Distribution characteristics: Isolated or small clustered distribution: can appear individually, but more commonly, several or more stomata gather together to form local small clusters. They are usually not dispersed or evenly distributed (which is the case when the dissolved gas content is extremely high). Scattered but relatively concentrated in location: Within a thick and large cross-section or hot spot area, there may be multiple scattered gas pore points.

d. Distinctive features from other pores: Distinction from invasive pores: Invasive pores are usually larger and more irregular, with rough and oxidized inner walls, and may contain slag (because gas comes from external sources such as sand moisture, paint decomposition, etc., and gas invasion may carry slag). Invasive pores are often located on the upper surface of castings or near the surface of the mold cavity/sand core. Difference from subcutaneous pores: Subcutaneous pores are located below the surface of the casting (1-3mm) and are needle shaped or elongated, sometimes only discovered after processing or cleaning. The formation of subcutaneous pores is often related to chemical reactions on the surface of molten iron (such as FeO+C ->Fe+CO), and oxidation may also occur on the inner wall. Difference from reactive pores: Reactive pores (such as CO pores produced by carbon oxygen reactions) usually have an oxidized color (blue or dark) on the inner wall, with a more irregular shape, and are often accompanied by slag or inclusions.

e. Related characteristics of formation reasons: closely related to the original gas content of molten iron: molten iron with high hydrogen and nitrogen content is more likely to produce precipitation pores. Closely related to solidification speed: thicker and slower cooling areas have higher risks. Related to molten iron treatment: the use of damp, corroded, and oily furnace materials, damp inoculants/spheroidizers, excessive stirring, and high overheating temperatures of molten iron (increasing suction) can all increase the tendency for precipitation pores. Summary of key identification points: Location: casting thickness, large cross-section, hot spot, and core. Shape: mainly round/oval/teardrop shaped, or worm shaped. Inner wall: Smooth, clean, and shiny (the most important feature!). Size: Small to medium, usually less than 3mm. Distribution: Isolated or small clusters, concentrated in local areas. Identifying these features is crucial for accurately determining the type of porosity, tracing the root cause of defects (such as raw materials, melting processes, inoculation treatments, pouring temperatures, casting designs), and developing effective preventive measures. Measuring the gas content (especially hydrogen content) of molten iron is usually a key verification step when suspecting that it is a pore formation.

Where does the gas from the precipitating pores in gray cast iron come from? The gas in the pores of gray cast iron mainly comes from the gas dissolved in the molten iron during the melting and pouring process. These gases precipitate due to a sharp decrease in solubility during the cooling and solidification of the molten iron. Its generation and dissolution mechanism involves complex physical and chemical processes, with the core gases being hydrogen (H ₂) and nitrogen (N ₂), and a small amount possibly involving carbon monoxide (CO).

The main sources and dissolution processes of these gases are as follows:

a. Source and generation mechanism of core gas

a. 1. Hydrogen (H ₂) - the main source of evolved gases: moisture and oil in furnace materials: moist furnace materials (pig iron, scrap steel, recycled materials), rust (Fe ₂ O ∝· nH ₂ O), oil or organic matter (such as cutting oil, plastics) decompose at high temperatures: 2H ₂ O → 2H ₂+O ₂ C ₘ H ₙ (hydrocarbons) → mC+(n/2) H ₂ Water vapor in the melting environment: moisture in damp melting furnaces, undried ladles, tools or coverings. Furnace atmosphere: The atmosphere containing H ₂ O generated by fuel combustion (such as natural gas, coke oven gas). Moisture absorption of inoculants/additives: inoculants or alloys such as ferrosilicon and ferromanganese absorb moisture from the air. Dissolution mechanism: Iron can dissolve hydrogen gas when it is in a high-temperature liquid state. At high temperatures, the solubility is relatively high (up to 5-7 ppm at 1500 ℃), but during solidification, the solubility drops sharply to about 1/3~1/2 (almost insoluble in the solid state)

a. 2. Nitrogen (N ₂) - an important source, especially in high nitrogen furnace materials. Source: nitrogen-containing alloys/furnace materials: scrap steel (especially alloy steel), nitrogen-containing pig iron, nitrogen in carburizers. Nitrogen in furnace gas: About 78% of air is N ₂, which is inhaled when molten iron is exposed to air or stirred in electric arc furnaces or induction furnaces. Resin sand/coating decomposition: furan resin and amine curing agents decompose to produce nitrogen-containing gases (such as NH3) HCN）。 Dissolution mechanism: The solubility of nitrogen in molten iron also increases with temperature, but is affected by the composition of the molten iron (carbon and silicon reduce nitrogen solubility). The solubility significantly decreases during solidification (solid solubility is extremely low).

a. 3. Carbon monoxide (CO) - Secondary but possibly involved source: Carbon (C) in molten iron reacts with dissolved oxygen (O) or oxides (such as FeO): (Note: CO bubbles usually form reactive pores rather than atypical precipitation pores, but may coexist under specific conditions).

3. How to prevent and control the occurrence of gas pore defects: prevention strategy: cutting off the gas source+promoting escape

a. Strictly control the furnace material and melting environment: the furnace material is dry, rust free, and free of oil stains. Fully dry the ladle and tools (>800 ℃). Avoid excessive overheating (>1500 ℃) and prolonged insulation.

b. Optimize molten iron treatment: inoculant/alloy pre baked (200~300 ℃). Use low nitrogen resin sand or reinforced molding sand for exhaust.

c. Process design assisted exhaust: Install cold iron to accelerate solidification in thick and large areas. Reasonably design the riser and exhaust channel to facilitate gas migration towards the riser.

d. If necessary, perform degassing treatment: introduce inert gas (such as Ar) to drive hydrogen, or add degassing agent (such as rare earth alloy).

Summary: The gas that precipitates pores in gray cast iron is essentially H ₂ and N ₂ dissolved during the melting process of molten iron, originating from moist/nitrogen-containing furnace materials, furnace gas, and improper operation. During solidification, supersaturation precipitates due to a sudden decrease in solubility, and is eventually captured by dendrites to form smooth circular pores on the inner wall. Controlling the source gas dissolution and optimizing the solidification process are the key to cure the problem.