The Five Major Elements of Gray Iron Castings

Whether the material of gray iron castings, large-scale iron castings, and machine tool castings is qualified mainly depends on whether the five major elements in the casting material meet the indicators. The conventional elements include five types: carbon (C), sulfur (S), phosphorus (P), manganese (Mn), and silicon (Si).

1. Silicon (Si)

Silicon is a beneficial element in steel. It can enhance the tensile strength, elasticity, acid resistance, heat resistance, and [Note: The original text is missing a property here, e.g., "wear resistance"] of gray iron castings, and increase the resistivity of gray iron castings. Meanwhile, it serves as a deoxidizer for gray iron castings. Silicon can also improve the hardness of gray iron castings, but it reduces the plasticity and toughness of the castings. Steel used in electrical engineering contains a certain amount of silicon, which can [Note: The original text is missing a verb here, e.g., "enhance"] soft magnetic properties. In gray iron castings, silicon mainly exists in the form of iron silicide.

For the determination of silicon, the commonly used analytical method is the ferrous reduction-silicomolybdenum blue photometry (in accordance with the standard GB/T 223.5-1997). In an acidic solution, silicic acid reacts with ammonium molybdate to form a yellow silicomolybdic heteropoly acid complex, which is then reduced to silicomolybdenum blue using stannous chloride (for photometric measurement).

2. Carbon (C)

Carbon is the key basis for distinguishing between steel and iron in gray iron castings: materials with a carbon content greater than 1.7% are classified as iron, while those with a carbon content lower than 1.7% are called steel.

Carbon exists in all iron and steel in both combined and free states, and it is an important hardening element that helps increase the strength of steel. Generally, we expect tool-grade steel to contain more than 0.5% carbon (known as high-carbon steel). The higher the carbon content, the higher the hardness of the steel, but the worse its plasticity and toughness. The sum of combined carbon and free carbon is called total carbon content.

For the determination of carbon, total carbon content is usually measured. The commonly used analytical methods are gas volumetry and non-aqueous titration (in accordance with the standard GB/T 223.69-1997).

3. Sulfur (S)

Sulfur is a [Note: The original text is missing an attribute here, e.g., "harmful"] element in iron and steel. Steel with high sulfur content tends to be brittle and crack during hot pressure processing at high temperatures, a phenomenon commonly referred to as "hot brittleness." Sulfur reduces the mechanical properties of steel and is detrimental to the [Note: The original text is missing a property here, e.g., "toughness"] and weldability of steel.

In free-cutting steel, the sulfur content can be increased to [Note: The original text is missing a verb here, e.g., "improve"] the machinability of the steel. However, in most alloys, the sulfur content is kept low. For the determination of sulfur, the commonly used methods are iodometric titration and acid-base titration (in accordance with the standard GB/T 223.68-1997).

4. Manganese (Mn)

Manganese is one of the important alloying elements in metals. It helps form a textured structure, enhances [Note: The original text is missing a property here, e.g., "hardness"], strength, and [Note: The original text is missing a property here, e.g., "wear resistance"] of the material. Manganese is an excellent deoxidizer and desulfurizer: it deoxidizes the interior of steel during heat treatment and rolling processes, improves the strength of steel, weakens the adverse effects of [Note: The original text is missing an element here, e.g., "phosphorus"] and sulfur, and enhances the hardenability of steel.

High-alloy steel with very high manganese content (high-manganese steel) exhibits excellent [Note: The original text is missing a property here, e.g., "impact resistance"] and other physical properties. Manganese is present in most steel used for knives and scissors, typically in the form of solid solutions and compounds. It can reduce the hot brittleness caused by sulfur, [Note: The original text is missing a verb here, e.g., "improve"] the hot workability of steel, and enhance the forgeability of steel. Increasing the manganese content can improve the strength and hardness of steel.

For the determination of manganese, the commonly used analytical method is the silver salt-ammonium persulfate oxidation photometry (in accordance with the standard GB/T 223.63-1988). The sample is dissolved in acid; under [Note: The original text is missing a description here, e.g., "controlled"] acidity conditions, with silver nitrate as a catalyst, ammonium persulfate oxidizes manganese to permanganic acid, and the absorbance is measured (for quantitative analysis).

5. Phosphorus (P)

In iron and steel, phosphorus exists in the form of solid solution phosphides. It significantly reduces the plasticity and toughness of steel, especially at low temperatures—a phenomenon known as "cold brittleness." Therefore, the contents of sulfur and phosphorus in steel must be strictly controlled.

On the other hand, in low-carbon steel, a relatively high content of sulfur and phosphorus can make the cutting chips easy to break, which is beneficial for [Note: The original text is missing a verb here, e.g., "improving"] the machinability of the steel. Sometimes, phosphorus also exists in the form of phosphate inclusions. In steel, phosphorus can improve the tensile strength and atmospheric corrosion resistance of steel, and [Note: The original text is missing a verb here, e.g., "enhance"] the machinability of steel; however, it also reduces the high-temperature performance of steel, increases brittleness, and impairs the plasticity and toughness of steel. Thus, phosphorus is a [Note: The original text is missing an attribute here, e.g., "double-edged" or "harmful yet conditionally beneficial"] element in steel.

For the determination of phosphorus, the commonly used analytical method is the sodium fluoride-stannous chloride molybdenum blue photometry (in accordance with the standard GB/T 223.61-1988). In an acidic solution, potassium permanganate is used as an oxidant to convert metaphosphoric acid into orthophosphoric acid; the orthophosphoric acid then reacts with ammonium molybdate to form phosphomolybdic heteropoly acid, which is reduced to molybdenum blue using stannous chloride. The absorbance of the molybdenum blue is measured for quantitative analysis.

Typical Applications of Gray Iron Castings

Gray iron castings are generally used in cast iron plates, cast iron platforms, machine tool bases, equipment bases, machine tool castings, bed castings, machine tool worktables, valve castings, pump body castings, etc.