Alismatales

The steel is a Alliage containing Fer added with a small percentage of Carbone (of 0,008 with approximately 2,14 % in mass).

Outline of the composition, the advantages and the disadvantages

The percentage of carbon has an influence considerable (and rather complex) on the properties of steel: in on this side 0,008 %, the alloy is rather malleable and one speaks about “iron”; beyond 2,14 %, inclusions of carbon in form Graphite weaken the microstructure and one speaks about cast iron. Between these two values, the increase in the percentage of carbon tends to improve the mechanical resistance and the hardness of alloy; one speaks about steels “soft, semi-soft, half-hard, hard or hardened” (traditional classification).

One also modifies the properties of steels by adding other elements, mainly metal, and one speaks about allied steels . Moreover, one can still largely improve their characteristics by thermal treatments (in particular hardenings) fascinating on the surface or heart of the matter; one then speaks about treated steels .

In addition to these various potentialities, and compared to other metal alloys, the main interest of steels lies on the one hand in the office plurality of values raised in the fundamental mechanical properties:

load-carrying capacity: Modulus of elasticity, yield stress, mechanical resistance;
hardness;
impact resistance (Impact strength).

In addition, their cost of development remains relatively moderate, because the Minerai of iron is abundant on ground (approximately 5 % of the bark) and its rather simple Reduction (by addition of carbon at high temperature). Finally steels are practically entirely which can be recycled thanks to the die reinforces.

One can nevertheless recognize some disadvantages to them, in particular their bad resistance to the Corrosion, but which one can cure, either by various surface treatments (Peinture, Brunissage, Zingage, hot Galvanisation, etc), or by the addition of elements carrying out of the nuances known as “stainless”. In addition, steels not easily castable, therefore are recommended little for the bulky parts of complex forms (built machines, for example). Pig iron and cast iron then is preferred to them. Lastly, when their density is compromising (in air transport air for example), one turns to lighter materials (alloys containing Aluminum, Composites, etc), but sometimes much more expensive.

So steels remain privileged in almost all the technique scopes of application: public equipment (rails, indication), building (reinforcements, frames, ironwork, hardware), means of transport (body, transmission), component mechanics (screws and bolts, springs, cables, bearings, gears), tools of striking (hammers, gravers, matrices) and of cut (strawberries, drills, carry-plate). Steels are also very present in intended products at the general public (movable, kitchen utensils): this list is far from being exhaustive.

History of steel

The Hittites are regarded as the inventors of steel. Indeed, it was the first people to use iron to replace copper or bronzes to manufacture weapons (swords, shields). The history tells that they heated their iron weapons with white for pourfendre their adversaries with the combat, and that they had to end up realizing that to long, their weapons became increasingly resistant to the shocks (indeed, the human body is composed of carbon, component of steel), and that they thereafter sought to improve the system.

Since the Age of iron, one used the low hearth X to produce massiots made up of iron and steel, which was to then be worked with the hand by the Forgeron S.

One often regards Réaumur as the founder of the modern scientific iron and steel industry. He carries out very many experiments in order to improve manufacture of steel and publishes the result of his observations in 1712.

Steel appeared by the evolution of the Métallurgie, towards 1786. This year, three French scientists, Berthollet, Gaspard Monge and Vandermonde, three types of products obtained starting from the casting of the blast furnaces characterized: the Iron, the cast iron and steel. Steel was then obtained starting from the iron, itself produced by refining of the cast iron from the blast furnace. Steel was harder than iron and less fragile than the cast iron.

At the 19th century methods of direct manufacture of conversion of the cast iron appeared, with the Bessemer converter in 1856 (Henry Bessemer) the proceeded Thomas-Gilchrist in 1877 (Sidney Gilchrist Thomas and Percy Carlyle Gilchrist of dephosphorization of the cast iron and Siemens-Martin. These discoveries, allowing the mass production of a steel of “quality” (for the time), take part in the Industrial revolution. Lastly, towards second half of the 19th century, Dmitry Chernov discovers the polymorphic transformations of steel and establishes the binary diagram iron/carbon, making pass the metallurgy of the state of craft industry to that of science.

Manufacture of steel

See also: Manufacture of steel, On the manufacture of steel

Composition of steels

One distinguishes several types of steels according to the percentage of carbon which they contain:

steels hypoeutectoïdes (of 0,008 with 0,77 % of carbon) which are softest;
steels eutectoids (0,77 % of carbon);
steels hypereutectoïdes (of 0,77 with 2,11 % of carbon) which are hardest;

The crystalline structure of steels to thermodynamic balance depends on their concentration (primarily in Carbone but also of the other alloy elements), and on the temperature. One can also have structures except balance (for example in the case of a Trempe).

The structure of pure iron depends on the temperature:

in lower part of 721°C and above: 1400°C iron (iron α) has a crystalline structure cubic with body centered (crystalline structure with room temperature);
between 721°C and 950°C until: 1400°C iron (iron γ) has a crystalline structure Cubique with centered faces.

Nonallied steels (with carbon) can contain until 2,11 % in carbon mass. Certain steel alloys can contain more carbon by the addition of elements known as “gammagenes”.

Carbon comes from the process of Réduction of the ore, which is done with coke in a Haut-fourneau. According to the wished properties, one adds or one removes alloy elements:

the Bore reinforces the cohesion of the grain boundary , one adds some sometimes in low content (a few hundreds of ppm in mass);
the Soufre weakens steel, by precipitation of Sulfure S with the grain boundary , one thus removes it during the development;
the Nickel and the chromium protect from the Corrosion while coming to form a passive layer, they are thus present in steels known as “stainless”;
but also the Magnesium, the Aluminum, the Silicon, the Titanium, the Manganese, the Cobalt, the Zinc, the Yttrium…

Various “families” of steels

There exist steels slightly allied, with low percentage of carbon, and contrary to steels containing many alloy elements (for example, a Stainless steel typical contains 8 % of nickel and 18 % of chromium in mass).

Various classifications

Old French standards

In France, the alloys were initially classified according to their Ductilité: ,

Then, we sold them under the denomination “has R_max ” (for example, steel “has 33” had a breaking strength of 33 kg/mm ²).

Then, one classified them according to their elastic Limite R_e , under the denomination “E R_max ” (for example, steel “E 24” had a yield stress of 24 kg/mm ²). One can establish following equivalences between the two standards:

At the time, main concern was thus mechanical. One created other standards according to the fields. For example, for the tubes, one spoke about steel “You 37 has” (“You” for tube, “37” are the module with the rupture in kg/mm ², “has” indicates the purity).

Progressively, the composition of steel, the alloy, became increasingly important. One thus gave up the mechanical properties to indicate the content of various elements. For nonallied steels, one distinguished series DC from series XC; the latter had a more important control on the composition, and in particular a content of Soufre and Phosphore (weakening elements) lower. One indicated the percentage of mass expressed as a percentage carbon multiplied by 100:

series DC:

DC 10: average content carbon of 0,10%;
DC 20: average content carbon of 0,20%;
DC 35: average content carbon of 0,35%;

series XC

XC 10: average content carbon of 0,09%;
XC 12: average content carbon of 0,13%;
XC 18: average content carbon of 0,19%;
XC 25 : average content carbon of 0,26%;
XC 32 : average content carbon of 0,32%;
XC 38 : average content carbon of 0,38%.

For low alloy steels, one indicated the percentage of carbon like below, then the list of the alloy elements by order of decreasing content, followed by a coefficient of content for the element more concentrated, the content being obtained by dividing the coefficient by a factor of 4 or 10 according to the elements.

For example, steel 35 NCD 16 is a steel having approximately 0,35% of C, containing approximately Ni 4%, as well as Cr and Mo in pus low content. In fact, the standard indicates:

C: 0,30 - 0,37%;
Ni: 3,70 - 4,20%;
Cr: 1,60 - 2%;
Mo: 0,3 - 0,5%.

Strongly allied steels started with “Z”, follow-up of the percentage of carbon (like above), and of the list of the elements with their content - without multiplicative factor. for example, steel Z 6 CN 18-09 contains approximately 0,06% of C, approximately Cr 18% and Ni 9%.

Old standards of the United States

Example of denominations:

ASTM A53 and has 106: Rank has - Rank B - Rank C;
ASTM HAS 333: Grade 1 - Rank 6;
API 5 a: H 40 - J 55 - K 55 - NR 80;
5 L: Rank has - Rank B;
5 LX: X 42 - X 46 - X 52 - X 56 - X 60 - X 65 - X 70;
5 AX: P 105 - P 110 - S 135.

Old German standards

Example of denominations:

DIN 1629: St 35 - St 45 - St 52;
DIN 17-175: St 35-8 - St 45-8;
DIN 17-172: USt 34-7 - RSt 34-7 - USt 38-7 - RRSt 38-7.

Nonallied steels

Special steels (type C)

Their composition is more precise and purer and corresponds to definite uses in advance.

Their current applications are the drills (Perceuse S), Ressort S, driveshafts, matrices (moulds),…

Stainless steels

The Stainless steel is one of the three big families of steels which has a great corrosion resistance, to hot oxidation and the Fluage (unrecoverable deformation). It is a steel alloy with nickel and chromium. Its applications are multiple: domestic chemistry, nuclear power, but also knives and equipment. These steels contain at least 10,5 % of chromium and less than carbon 1,2%.

Other steel alloys

Steels slightly allied

No element of addition not exceeding 5 % in mass, they are used for applications requiring a high strength.

An example of standardized designation: 35 NiCrMo16. The first figure (35) represents the percentage of carbon multiplied by 100, the letters which follow are the elements of addition (Nor, Cr and Mo) and their respective percentages multiplied by one coefficent dependant of its nature defined by the table below.

Strongly allied steels

At least an element of addition exceeds the 5 % in mass, intended for quite specific uses, one finds refractory, tool steels there, Maraging (very high strength, used in aeronautics), Hadfields (very great wear resistance), with bearings.

An example of standardized designation: X2CrNi18-9 where X is the letter representing the high alloy steels, the first figure (2) represents the percentage of carbon multiplied by 100, the letters which follow are the elements of addition (Cr and Nor) and their respective percentages thus here one has a high alloy steel with 0,02 % of carbon combined with chromium with height of 18 % and of nickel to height of 9 %.

The high speed steels (HS) belong to this family and are described by letters HS followed by the content of the following alloy elements: W, Mo, V, Co

Multiphase steels

These steels are designed according to the principle of the composite : by heat treatments and mechanical, one manages to locally enrich the matter some in elements by Alliage. One then obtains a mixture of Phase S Dur are and of Phase S Ductile S, whose combination allows obtaining better mechanical characteristics. One will quote, for example:

the damask steels where ductile white layers low in Carbone absorb the shocks, and black, the richer in carbon, guarantees a sharp good * steels Dual Phase which are the modern variation of damask steel, but where the distinction between Phase Dur E (the Martensite) and Phase Ductile (the ferrite), are done more finely, on the level of the Grain.
steels TRIP (Transformation Induced Plasticity), where the ferrite is transformed partially into Martensite after a mechanical request. One thus begins with a steel Ductile, to lead to a steel of the type Dual Phase.

Properties of steels

They have a Modulus Young of approximately 210 GPa, independently of their composition. The other properties vary enormously according to their composition, of the thermomechanical treatment and the surface treatments to which they were subjected.

The thermomechanical treatment is association:

of a Heat treatment, in the form of a cycle heating-cooling (Hardening, returned…) ;
of a mechanical treatment, a deformation causing of the work hardening (Rolling, Forging, Wiredrawing…).

The surface treatment consists in modifying the chemical composition or the structure of an outside layer of steel. That can be:

a reaction in liquid phase (Chromatization, Carburation, Nitriding in salt bath, Galvanization…) ;
a reaction in gas phase (nitriding in liquid phase);
a projection of Ion S (ionic Establishment);
a covering (Painting, Zinking).

See also the detailed article treatments anti-wear.

Symbolic system

steel is the 7th level in the progression of the sporting Sarbacane.

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