Alloy cast iron - what are its types, properties and applications?

Cast iron, as we know from the text What is the difference between cast steel and cast iron?, is the most numerous group of high-carbon, iron alloys (Fe). More specifically, cast iron is iron-carbon alloy with a carbon content above 2%, with a maximum content ranging from 3.8% to 6.7%.

Carbon can occur in cast iron in the form of graphite, or be bound to iron in the form of iron carbide (Fe3C), i.e. cementite, the hardness of which oscillates between the hardness of corundum and diamond. The presence of a specific carbon phase depends on the cooling rate and the chemical composition of the alloy.

Today we will discuss alloy cast iron, its types, properties, composition and applications.

In this article you will learn:

• what is alloy cast iron and how is it made,
• what is the chemical composition and popular additives for alloy cast irons,
• the types and properties of different alloy cast irons and their particular applications,
• where alloy cast iron is used.

Alloy cast iron

Alloy cast iron is cast iron enriched with alloying additives, thanks to which the cast iron's resistance to weather conditions, chemicals, high temperatures, abrasion, etc. increases.

Additions in the form of silicon, nickel, chromium, molybdenum, aluminum and other elements allow the modification of certain chemical and physical properties of specific alloys, and thus adapting them to the specific requirements of individual castings.

The number of grades of alloy cast iron is very large and can be classified in various ways, due to:

• the content of alloying additives (low-alloy < 3% and high-alloy > 3%),
• the type of dominant element (e.g. silicon, nickel, chromium, manganese),
• properties (e.g. wear-resistant, corrosion-resistant, heat-resistant).

Alloy cast iron - manufacturing

Alloying elements are introduced into alloy cast irons, aside from alloying additives. The elements mentioned above are added to improve the performance properties of cast irons, such as:

• mechanical properties,
• physical properties,
• abrasion resistance,
• resistance to electrochemical and gas corrosion,
• resistance to weather conditions.

Their chemical composition is selected in a way that allows them to maintain the appropriate structure and properties. For this reason, additives with a graphitizing and whitening effect should be selected with particular attention.

Chemical composition of alloy cast iron - additives

A specific alloy cast iron, depending on the composition and working conditions, can obtain desired properties due to individual alloying additives.

Due to the large number of alloy cast irons, including standardized ones, it is impossible to discuss them comprehensively, hence we will give some examples using selected, most popular alloying elements.

Silicon

• increases corrosion resistance,
• improves alloys properties in contact with high temperatures (Si in the amount of 4.5 ➗ 8%), because it reduces the tendency to peeling of the surface layer,
• increases acid resistance (Si in the amount of 14 ➗ 17%, especially with 5% Cr and 1% Mo), especially to acids such as nitric, phosphoric, acetic and, in particular, sulfuric (caution! it is easily corroded in hydrochloric acid and alkalis).

Chrome

• improves resistance to crushing and cracking,
• increases corrosion resistance,
• increases resistance to temperature,
• castings made with its admixture are heat-resistant and creep-resistant.

Nickel

• provides optimal heat resistance,
• increases corrosion resistance.

Molybdenum

• increases corrosion resistance,
• increases the creep resistance.

Manganese

• binds to sulfur, removing its adverse effects in cast iron.

Aluminum

• castings with its addition are resistant to the effects of oxygen, carbon and sulfur gas environments,
• increases resistance to high temperature.

Types of alloy cast irons

As already mentioned, the most common classification of alloy cast irons is the one whose criterion is determined by the properties of the casting. According to PN-88/H-83144, they are divided into corrosion-resistant cast iron, abrasion-resistant cast iron and heat-resistant cast iron.

Corrosion-resistant 

This is a group of cast irons resistant to the decline of materials under exposure to external factors. This type of cast iron is dominated by high chromium and chromium-nickel cast irons. Within it, we also distinguish cast irons with the addition of nickel and molybdenum, which stabilize corrosion-resistant austenite by adding silicon, chromium or aluminum, creating a corrosion-resistant layer on the surface of the casting.

Abrasion-resistant 

This type of alloy cast iron is resistant to loss of mass, volume or thickness due to abrasion of the surface layer of the elements that work together. For example, nickel, high-chromium and high-manganese cast irons have such properties.

Heat-resistant 

Unalloyed gray cast iron is characterized by low heat resistance, which is improved by an alloy additive, for instance in the form of chromium or silicon.

In this category it is worth to mention:

• Silal: a material intended for castings of elements to work at temperatures up to 600-800 °C, with increased content of manganese and silicon. Its disadvantage is increased fragility.
• Nicrosilal: a material similar to silal, with the addition of 16 to 20% nickel. It has better strength parameters than silal.
• Niresist: it is a highly heat-resistant and corrosion-resistant materia. It contains about 2.5% of silicon, up to 2% of manganese, up to 16% of nickel, up to 4% of chromium and up to 8% of copper.

Alloy cast iron - application

Alloy cast irons are used in many industries, depending on the properties of a particular alloy.

This alloy castings can be found, for example:

• in lids of the manholes,
• pipes,
• ovens
• chemical industry,
• pharmaceutical industry,
• automotive industry,
• in castings used in conditions susceptible to contact with acids, rust or high temperature.