General Information

EFFECTS OF ALLOY ELEMENTS ON MICROSTRUCTURE AND HARDENABLENESS OF STEEL

As a common rule, when it is required more strength, ductility and toughness than obtained from simple carbon steel, in an important section, the steel that will be added one or more alloy elements during the production, is called alloyed steel. Alloy elements have usage increasing year by year. They usually are produced as ferro alloys and sometimes produced pure elements as nickel and oxide compounds as nickel and molybdenum. In these forms,they are added to molten steel in amounts that  determined for structural steel.
Alloy elements are added to steel for purposes that least two items have an importance, as mentioned below:

  • Improving the nechanical properties during the control of factors that affects hardenableness and allowing high tempering temperature while high strength and well ducyility conditions are going on,
  • Improving mechanical properties in high and low temperatures,
  • Increasing the resistant against chemical effects and high temperature oxidation,
  • Effecting the different properties as magnetic permeability and neutron absorbtion.

Carbon
It is the most important alloy element. It is necessary for constitution of cementite (and the other carbides), perlite, spheroite, beynite, and ferro-carbon martensite. The hardness of ferro-carbon martensite will be increased with incresing of carbon content of steel and it reaches maximum at approximately %0,6 C. Toughness, ductility and welding ability of perlitic steels will be decreased with increasing of carbon content.

Manganese
It is present at all commercial steels normally. It is very important in production of steel, because it makes molten steel deoxide and makes the hot machining of steel easy with decreasing of sensitivity to be ripped hot. Also it combines with sulphur for constituting manganese- sulphur connectives that will increase machinability of steel. Manganese increases strength of ferrite a bit and also increases the hardenableness of steel too much. Adding mangenese for increasing effect of normalisation that is a heat treatment process which it is required for strengthening the steel and forming perlite well, is result of these two effects. Manganese decreases the martensite forming temperatures during hardening, thus it increases the possibility of protection of ostenite in hardened steels.

 Silicon
It is one of the main deoxidant used in the production of steel. In a steel, amount of this element that is taken into no consideration in spesification, depends on deoxidation application that determined for production. Completely extinguished steels usually contain %0,15- 0,30 silicon for deoxidation. It can be reduced the amount of silicon in steel, if the other deoxidant are used. Silicon increases the strength of ferrite a bit without causing to decrease in ductility. In much amounts, it increases the resistant of steel against to be poured as a sequin. But with these much amounts, the process is usually difficult.

Chromium
It is used for increasing corrosion and oxidation resistant, high temperature strength, hardenableness and abrasion resistant in low alloyed steel, in much carbon combination. Chromium – carbides need high osteniting temperature for dissolving. Simple chromium steels can be rather brittle, also it has sensitivity to temper brittleness.

Nickel
It is used in low alloyed steels for improving low temperature toughness and increasing the hardenableness. It is considered that nickel decreases sensitivity of steel against changing in heat treatment and distortion and breakage during hardening. It strengthens the ferrite so the steel. Nickel is effectual in giving form to the alloyed steel that has high strength, toughness and hardenableness when it is used with chromium and molybdenum.

Molybdenum
It increases the hardenableness of steel and between certain limits it is especially effective in continuation of hardenableness. In amounts of especially %0,15-0,30, this element minimizes the sensitivity of steel against tempering brittleness. Hardened steels that contain molybdenum, must be tempered at high temperature. Molybdenum is an element that is present alone in amount of increasing high temperature rolling and crawling strength. It transforms the ostenite to the beynite with delaying transforming it to the perlite, thus it can be obtained beynite with continual cooling the steel that contains molybdenum.

Copper
It is added to steel for improving the resistant to the atmospheric corrosion. The amount that will be added to steel for this purpose, changes from %0,2 to %0,5. Copper is harmful to surface quality and hot machining treatment. Because it settles down in grain boundaries during hot machining treatment.

Vanadium
It will be added to steel for hindering the enlarging of grain during heat treatment. In grain size control, it improves both strength and toughness of hardened and tempered steels. Adding vanadium up to %0,05 increases hardenableness of steel. But adding much amounts decreases hardenableness. Because it probably constitutes carbides that dissolve hardly in ostenite.

Nyobium
It decreases crossing temperature from one phase to other phase and increases the strength of low carbon alloyed steels. It gives well grain size, delays tempering and increases high temperature strength of steel. Because it constitutes very stable carbides, during heat treatment it decreases the hardenableness of steel with decreasing carbon amount which dissolves in ostenite.      

Titanium
It can be added to boroned steel, because it combines quickly with oxygen and nitrogen in steel. Thus effect of boron increases in increasing of hardenableness of steel. Otherwise contribution of titanium to hardenableness usually can be careless, because it can be dissolved a bit in presence of much carbon.
Titanium is used as first deoxidant in steel and effectual inhibitor the grain size, but as alloying element it has very much carbide constitution tendency. When titanium will be added %1,5–2,0, carbide constitution tendency is much hard in % 0,5 carboned steels.

Zirconium and Cerium
Zirconium and Cerium that are used mostly in high strengthly, low alloyed (HSLA) steels, are added for checking form of inclusions. Thus hardness of steel increases.

Boron
It is usually added maount of %0,0005–0,003. It increases hardenableness of steel. It is effective in especially low carbon contents. Because boron doesn’t act on strength of ferrite, it is used for formableness, machinableness and hardenableness of steel without getting lost ductility.     

Lead
Lead is added to steel for improving machinability. It doesn’t dissolve in steel but it keeps the form of microscopic spheres. It causes liquid metal brittleness at temperatures near melting temperature. 

Aluminium
It is used for machining hot and checking grain size of steel. Also it deoxidizes steel. The extinguished steels with aluminium have perfect toughness, because they usually have well grain size. One of special usage is in steels that are prodeced for nitration.  

Calcium
Sometimes it is used for deoxidizing for steels. It helps to control the form of nonferrous inclusions in HSLA steels. Thus toughness will be improved. The steels that are deoxidized by calcium usually have beter machinability than steels that are deoxidized by silicon and aluminium. 

Cobalt
Cobalt dissolves in all rate of gamma ferrous and it has %75 of solubility in alpha ferrous. When cobalt dissolved in ferrite, it hardens or strengthen ferrite and thus it hinders the softening in high temperatures. Carbide formation tendency is near or more a bit than carbide formation tendency of ferrous. Cobalt has negative effect for %0,40 carbon steels and it quickens formation to soft parts instead of delaying the formation and affecting the hardenableness as the other elements. However it is seen that cobalt increases the hardenableness in chromium steels with low carbon.  

Phosphorus
It increases the strength and hardenableness of steel but decreases ductility and toughness hard. It increases temper brittleness sensitivity of alloyed steel with medium carbon and especially steels with simple chromium. It can be added purposely to steel for improving the machinability and resistant of corrosion.  

Sulphur
It is harmful to widthwise strength and stroke resistant of steel. But it affects a bit to lengthwise properties. Also it affects surface quality and weldability negative. Sulphur seems manganese-sulphur binding. One of the functions of manganese is forming compound with sulphur and hindering to form of ferrous-ferroussulphur otectoide which has low melting temperature. These sulphur hinderings increase machining of steel. Sulphur is added purposely to some steels for improving the last machining.           

Nitrogen
It increases strength and hardenableness of steel but decreases ductility and toughness. Nitrogen forms nitrure particles that controls the grain size of steel, in aluminium content extinguished steels. Thus both toughness and strength increase. Nitrogen decrases the effect of boron in hardenableness of steel.

Oxygen
Oxygen that is probably contained in Rimmed Steels mostly, increases strength of steel but decreases the toughness much.   

Hydrogen
Hydrogen that dissolved in steel during production, makes steel brittle very much. This effect is not same with brittleness at the end of electro-coating and pickling. The brittleness which is occured by hydrogen dissolves during production, causes to be poured as a sequin during cooling from rolling temperatures. Dissolved hydrogen rarely affects final products.   

Tin
Tin can make steel sensitive against to temper brittleness and ripping hot.