Alloy steel is steel alloyed with other elements in amounts of between 1 and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The differentiation between the two is somewhat arbitrary; Smith and Hashemi define the difference at 4%, while Degarmo, et al., define it at 8%. However, most commonly alloy steel refers to low alloy steel.
These steels have greater strength, hardness, hot hardness, wear resistance, hardenability, or toughness compared to carbon steel. However, they may require heat treatment to achieve such properties. Common alloying elements are molybdenum, manganese, nickel, chromium, vanadium, silicon and boron.
Types of stainless steel
There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For greater hardness and strength, more carbon is added. When subjected to adequate heat treatment, these steels are used as razor blades, cutlery, tools, etc.
Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.
Stainless steel chair in use in Rio de Janeiro, Brazil. The austenitic AISI 304 is proper for use in sea regions due to its high oxidation-resistance
Stainless steels are also classified by their crystalline structure:
* Austenitic, or 300 series, stainless steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless, is often used in flatware. Similarly, 18/0 and 18/8 are also available. Superaustenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus the 300 series. The higher alloy content of superaustenitic steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications.
The low carbon version of the Austenitic Stainless Steel, for example 316L or 304L, are used to avoid corrosion problem caused by welding. The "L" means that the carbon content of the Stainless Steel is below 0.03%, this will reduce the sensitization effect, precipitation of Chromium Carbides at grain boundaries, due to the high temperature produced by welding operation.
* Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, due to the lower Chromium and nickel content. They are also usually less expensive. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. These alloys can be degraded by the presence of σ chromium, an intermetallic phase which can precipitate upon welding.
* Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (0-<2%), and carbon (about 0.1-1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic.
* Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel. There is a rising trend in defense budgets to opt for an ultra-high-strength stainless steel when possible in new projects, as it is estimated that 2% of the US GDP is spent dealing with corrosion. The Lockheed-Martin Joint Strike Fighter is the first aircraft to use a precipitation-hardenable stainless steel—Carpenter Custom 465—in its airframe.
* Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50/50 mix, although in commercial alloys, the mix may be 40/60 respectively. Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19–2