Cold and Hot work
Heat treatment of steel
Iron is the basic element of all steels. If it is not combined with any other element, it is only named, iron. Whenever it is allied with carbon, it becomes steel. Whichever elements with which it is combined, steel takes completely different chemical and mechanical properties. Let us review some of the elements with which iron is often combined.
|Carbon||The higher there is carbon in steel, the higher will be its yield. Its roughness and its resistance will improve and steel will get a better response to heat treatments.|
|Manganese||The ductility increases and also a better response to the heat treatments.|
|Sulphurize||Improve the machinability.|
|Silicium||Acts like deoxydant and increases the resistance of steel. It affects also the speed of diffusion of zinc in steel at the time of galvanization.|
|Molybdene||It improves roughness and hardenability when it is added in proportions of 2 to 4% (as in stainless steel), it will improve the resistance of steel to the attacks of the chlorides.|
|Chromium||If it is used in small quantities, it improves the response to heat treatments and abrasion. When its percentage is larger (between 12 and 25%), it improves the resistance to oxidation and corrosion.|
|Nickel||Improves the hardenability and the resistance to corrosion. Moreover, it increases the stability of steel in highest temperature.|
|Lead||Increases the machinability of steel.|
|Phosphorus||Decreases the ductility and improves the machinability and the resistance to corrosion.|
|Sulphur||Decreases strongly the crossover. It affects the quality surface and the weldability when it is combined with manganese to obtain sulphides of manganese, it improves the machinability.|
|Aluminium||Deoxydant and refiner of grain, it affects also the speed of diffusion of zinc in steel.|
|Vanadium||Delays the growth of grain and the shape of the carbonitrides which increases the resistance of steels HSLA at the time of hot rolling.|
|Titanium||Stabilizes the structure and improves the efficacity of the hardeners in steel.|
By combining some of these elements, a wide variety of steels can be created and respond to more sophisticated applications.
Cold and Hot work
Steel, after being refined is put into pockets where the elements of alloy are added and also reducing agents. Steel in fusion then is put into ingots whose weight reaches sometimes until 18 tons. One can also directly give it its form according to the continuing cast.
One puts steel in ingots and one lets it be solidified. After we retrieve the hot ingots and place them into a hot furnace where temperature is maintained at 1200 celcius, during a period which can reach 1h30. From there, ingots pass to the rolling mill for the transformation into billets or flagstones.
When it is still hot, the piece is transformed into desired shapes: round, square, hexagon, or other. These laminated products belong to the hot rolled category. One can recognize the billets by the blue scales on the exterior shell.
In the cold rolled category, one has retrieved the scales in a bath of acid and one makes them pass by rollers or matrices to the forms and desired dimensions.
Heat treatment of steel
Moreover, in order to modify the mechanical and chemical properties of steel, large variety of heat treatments can be executed. Here is a brief explanation on the most current treatments.
|Annealing||Refers to the heating and the slowed down cooling of materials solid with an aim of withdrawing its stress, refining its structure or while changing its ductility, toughness, and others properties.|
|Air hardening||Process in which steel is heated to a hardening temperature and cooled in air. Only steels containing a great percentage of Carbon will demonstrate a great increase in their roughness.|
|Carburizing||Referres often to case hardening. This process consists in heating the ferrous base beyond the melting temperature in a material containing carbon. Carbon, absorbed by the surface of steel will obtain a really hard and wear resistant steel.|
|Normalisation||Consists in heating steel at a temperature around 100 F under the critical temperature (of deformations) and to cool in still air. It will result in a steel with a more uniform structure of grain and a better machinability.|
|Stress relieving||To heat steel under critical temperature to eliminate deformation and tension thus increasing also the machinability and weldability or cold working.|
|Hardening||To heat and quench steel to improve its roughness.|
|Drawing back||To re-heat after a hardening treatment of critical tempature to change the hardness of steel.|
|Soak and return||Heating of steel at an austenization temperature followed by a liquid hardening or by a pulverization, then reheat to an appropriate temperature and a cooling in a liquid or in the air.|
The majority of steels will indicate as follows their percentage of Carbon: A.I.S.I./S.A.E.
1010: 0.10 of a percent of Carbon
1020: 0.20 of a percent of Carbon
1045: 0.45 of a percent of Carbon
Several organizations and societies have established standards to classify steels, especially the following products:
|1) steel plates:||A.S.T.M. and A.S.M.E.|
|2) steels sheets :||A.I.S.I.|
|3) steel rods:||S.A.E. and A.I.S.I.|
|4) structural steel and profiles C.S.A. and A.S.T.M.||C.S.A. et A.S.T.M.|
Here are the complete names of the abbreviations:
|A.S.T.M.||American society for testing material www.astm.org||www.astm.org|
|A.S.M.E.||American society of mechanical engineers||www.asme.org|
|C.S.A.||Canadian standards association|
|A.C.N.O.R.||Canadian association of standardization|
|S.A.E.||Automotive society of engineers|
|A.M.S.||Aerospace material specification|
A.C.N.O.R. standards classify structural steels such as the strong plates, the profile, the tubes and bars as the following:
|Type G||Steel for general construction |
|Type W||Weldable steel|
|Type WT||Weldable steel for low temperature application|
|Type R||Structural steel resistant to atmospheric corrosion|
|Type A||Weldable structural steel resistant to atmospheric corrosion|
|Type AT||Structural steel resistant to corrosion ans suitable for low temperature applications|
Strong plates in low allied steel quenched and tempered
|Type QT||Strong plates in low allied steel, quenched and tempered with resilience properties improved at low temperature|