Ferrous metals

Introduction and standards for wrought steels The development of alloys of iron (which include wrought
iron, ste'el and cast irm) has been essential to a technological society. Iron ore is widespread, cheap and easily mined, and from it can be produced alloys with the widest range of properties of any material. Steels may be manufactured with properties which vary continuously from soft and ductile to strengths and hardnesses which, until the very recent development of ithe sialons, exceeded those of any relatively inexpensive material. There are a very large number of wrought steels and numbering systems. German specifications alone include 1400 grades. Because of the rapid evolution of ferrous metallurgy none of these classifications and numbers are ever fully up to date. In (engineering, standard steels are commonly referred to by their AlSIiSAE number" based on composition (Table 7.1). Th~e last two figures indicate the carbon content in the case of rion-stainless steels: the corresponding general British
specification" is BS 970: 1983 and earlier. In it steels are identified by a three-digit number denoting the type of steel
followed (in the case of non-stainless steels) by a letter denoting the type of specification - A for analysis, H for
hardenability or M for mechanical properties, followed by two numbers denoting carbon content. For stainless steels the first three figures are identical with the AISI figures; the letter is S, and the final two numbers are coded. Wherever possible, in the account of classes of steel which follows, the AISI/SAE and BS ranges will b'e given. There are also a number of specifications depending on product or application which do not always follow the BS 970 or AISI/SAE numbering system.

In addition to the standard steels, there are well-known steels which are recognized by designations used by their originators and are in no way inferior to standard steels. Many steels of  this kintj are recognized and used worldwide in aircraft specifications and are made by many different steelmakers, although they have not yet been recognized by the several national bodies that govern steel specifications. Cast irons and cast steels have separate specification numbering systems.

Types of wrought steel

There are at least eleven separate classes of wrought steel most of which are further sub-divided.
Carbon steel (AISI 1006-1572-BS 970 000-119) is the basic type which far exceeds all other metals in tonnage produced.
Low-carbon steels are sub-divided into hot-rolled (see Manufacturing Procedures below) and cold-rolled steel.
Hot-rolled steel has low strength (although the highercarbon versions can be heat treated to high hardness in small
sections) and low toughness but is readily available at low costand is easily formed, welded and machined. Cold-rolled steel
is harder. has good surface finish and dimensional tolerances. high strength at low cost and good machinability.
High-strength low-alloy steels are proprietary steels with low carbon made to SAE 950. They have significantly higher
strength and are easily formed and welded. Hardened and tempered steels (AISI/SAE 31-98 BS 970
50G599. including higher-carbon steels) are steels containing sufficient carbon and alloy to enable them to be heat treated to
the desired strength and toughness at the design thickness. They may have high toughness and high strength at elevated
temperature but they are more expensive than carbon steels and the higher-alloy steels have poor weldability and machinability.
Case-hardening steels are of relatively low carbon content (final BS specification Nos 12 to 25). They may be surface
hardened by carburizing, carbonitriding or nitriding when heat treatment will produce a very hard surface and a softer (but.
where necessary, strong) ductile core. They are used when wear resistance must be combined with core toughness.
Stainless steels (AISI 20G499, BS 970 300S499S) contain chromium in amounts above 12% so that the magnetite layer
formed on the surface of iron becomes, at lower chromium levels, a spinel and at higher levels chromic oxide. The
introduction of chromium into the oxide layer greatly increases its stability and integrity and provides muchincreased
resistance to corrosion and oxidation, but also influences significantly the structure and properties of the
underlying metal. The addition of further alloying elements results in five separate classes of stainless steel. These are,
respectively:

Ferritic stainless steels are alloys of iron with up to 18% chromium and relatively small amounts of other alloy.
They are ductile, the high-chromium versions have good corrosion resistance. and they are, relative to other stainless
steels, inexpensive. They have a tendency to grain growth and are therefore difficult to weld. The recently
developed ‘Low Interstitial’ ferritic stainless steels have chromium contents between 17% and 30% and very low
carbon contents. These are claimed to have outstanding  corrosion resistance but may be difficult to obtain.
Martensitic stainless steels are limited in chromium content to about 17% so that they may be hardened by quenching
to give high hardness and strength.

Austenitic stainless steels avoid the problems which result from the addition of chromium to the ferrite matrix by the
addition of nickel and other elements which change the structure to the high-temperature gamma form. The resultant
alloys may have very high corrosion resistance, good ductility and/or high hot strength. Their very highly alloyed
versions merge at iron contents below 50% into  nickel alloys (see Section 7.4.5) and the very high creep
strength versions are described as ‘superalloys of iron’. Duplex stainless steels have compositions which produce a
mixed ferrite/austenite structure. They have excellent mechanical strength and corrosion resistance but may be
difficult to obtain.

Precipitation-hardening stainless steels combine very high mechanical strength with excellent corrosion resistance. They
require complex treatments.

Intermetallic strengthened (maraged) steels may be formed or machined in the soft condition and then aged. The maraged
steels are very strong and very tough but very expensive. The steel classes described so far have been based on
composition and structure. Other classifications based mainly on application (which, to some extent, cut across the classification
already described) include: Electrical steels: very low-carbon steels containing about 3% silicon supplied as strip. hot or cold rolled. with the surfaces insulated;

Spring steels: in which a very high hardness can be produced by working, quenching or precipitation hardening. They
may be carbon, alloy or stainless; Tool steels: used for forming or cutting materials. Their essential properties are high hardness, resistance to wear and abrasion, reasonable toughness and, in the case of high-speed steels. high hot hardness;
Creep-resisting steels: with high creep and creep rupture strengths at high temperatures. They may range from
bainitic through martensitic to austenitic steels and superalloys.

The higher-temperature steels are also oxidation resistant because of chromium additions; Valve sreels: with high-temperature tensile and creep strengths and good high-temperature oxidation and corrosion resistance. In the UK the title is restricted to certain martensitic and austenitic steels but in other countries it includes all steels which may be used for IC engine poppet
valves.