Steel

Steel is an alloy of iron. It is a material with high strengh that is widely used on Earth for construction. It is technologically well known.

Steel consists mainly of iron and is distinguished by its carbon content, and can be produced on Mars from local resources. The carbon part can be from 0,02% to 2,06%. Other additives include nickel, aluminium, calcium, silicon, manganese, chrome, vanadium, molybdenum and tungsten. The percentage of the parts decide upon the properties of the steel. The tensile strength varies from 250 MPa to more than 2000 MPa. It can be recycled easily by melting and forging for new parts.

The low quantity of oxygen in the martian atmosphere and the absence of liquid water means that exposed steel may be less prone to rust.

Terminology

Since ancient times, hard ferrous metals suitable for weapons were known (in the terms appropriate for the language) as steel whereas iron was used for softer ferrous metals. The meanings were changed in the beginning of the 20th century, so that steel was used only for ferrous metals within a certain band of carbon content. Below that, it is known as iron and above it as cast iron. The element also retains the name iron.

However, usage in industry is not always technically correct. Steel producers may use terms such as ultra-low carbon steel for low-carbon iron alloys. The term soft iron also refers to low-carbon iron.

Wrought iron originally referred to a low-carbon iron containing bands of slag impurities. However, this material is no longer commonly produced and the term is often (if inaccurately) used to refer to any hand-wrought (as opposed to machine-wrought) metal.

Use cases

Steel can be made with a deliberate set of properties, making it usable for

a great variety of beams and plate for building construction
containers and machine casings, including single-piece pressure vessels
tools with low abrasion properties
steel cable with high tensile strength
Rocket thrust chambers may be made from steel. This is especially true of rockets which burn for short durations, where a simple thick-walled steel chamber is superior to complex cooling systems.^[1]

Types of steel

The two main types of steel are carbon steel and alloy steel.

Carbon steels are alloys of iron and carbon with other elements not exceeding specified maximum fractions (which vary by element). Mild steel is on the low end of carbon content and is often used for structural purposes. Medium- and high-carbon steels are increasingly harder but more brittle. High-carbon steel can be used for hard tools such as knives or screwdrivers and is more elastic, making them suitable for springs. However, alloy steels are often used for these applications.

The most significant type of alloy steel is stainless steel, which is an alloy of (typically in this order) iron, chromium, usually but not always nickel or vanadium and, lastly, carbon. When exposed to air, a layer of chromium oxide forms on the stainless steel and helps to prevent rust. This comes at the cost of significantly weakening the steel, which can be partially alleviated by the addition of either nickel or vanadium. Stainless steel has an improved resistance not just to oxygen, but to chemical corrosion in general.

Open issues

How can steel be made from local Martian resources? Probably, this is not done in a conventional furnace with coke, and hence the steel plant does not need to remove the carbon excess from the raw iron. However, smelting iron from iron ore without carbon is very difficult. Meteoric iron may be a good raw material for steel or wrought iron. It can be used directly: it does not require smelting.