The effects of the alloying elements in Stainless Steels
The alloying elements each have a specific effect on the properties of the steel. It is the combined effect of all the alloying elements and, to some extent, the impurities that determine the property profile of a certain steel grade. In order to understand why different grades have different compositions a brief overview of the alloying elements and their effects on the structure and properties may be helpful. It should also be noted that the effect of the alloying elements differs in some aspects between the hardenable and the non- hardenable stainless steels.
Alloying elements have the capability to block slip planes. In the case of chromium added to iron (Fe-Cr alloys), there is another advantage, which consists of a tremendous increase in the corrosion resistance of the new metallic alloy compared to pure iron. There are more than a hundred chromium-containing metallic alloys. Each one was developed for a particular need, and each has its own set of properties and characteristics.
The most important characteristics common to all chromium-containing alloys, among them stainless steels, is that they contain sufficient chromium to make them corrosion resistant, oxidation resistant and/ or heat resistant.
All of these alloys contain chromium (Cr), manganese (Mn), silicon (Si), carbon (C), nitrogen (Ni), sulphur (S), and phosphorus (P), and may contain: nickel (Ni), molybdenum (Mo), titanium (Ti), niobium (Nb), zirconium (Zr), copper (Cu), tungsten (W), vanadium (V), selenium (Se), and some other minor elements.
Chromium (Cr)
Chromium is a silver-grey transition metal with a relative atomic mass (12C=12) of 51.996, an atomic number of 24, and a melting point of 1,875°C and a density of 7.190 kg/dm3. It is in group VI of the periodic table. Chromium has a body-centered-cubic (b.c.c.) crystal structure.
This is the most important alloying element in stainless steels. It is this element that gives the stainless steels their basic corrosion resistance. The corrosion resistance increases with increasing chromium content. It also increases the resistance to oxidation at high temperatures. Chromium promotes a ferritic structure.
Nickel (Ni)
Nickel is a silver-white transition metal with a relative atomic mass (12C=12) of 58.69, an atomic number of 28, a melting point of 1,453°C and a density of 8.902 kg/dm3. It is in group VIII on the periodic table. It has a face-centered-cubic (f.c.c.) crystal structure.Nickel is ferromagnetic up to 353°C, its Curie point.
The main reason for the nickel addition is to promote an austenitic structure. Nickel generally increases ductility and toughness. It also reduces the corrosion rate and is thus advantageous in acid environments. In precipitation hardening steels nickel is also used to form the intermetallic compounds that are used to increase the strength.
Molybdenum (Mo)
Molybdenum is a silver-white transition metal with a relative atomic mass (12C=12) of 95.94, an atomic number of 42, a melting point of 2,610°C and a density of 10.22 kg / dm3. It is in group VI on the periodic table. Molybdenum has a body-centered-cubic (b.c.c.) crystal structure.
Molybdenum substantially increases the resistance to both general and localized corrosion. It increases the mechanical strength somewhat and strongly promotes a ferritic structure. Molybdenum also promotes the formation secondary phases in ferritic, ferritic-austenitic and austenitic steels. In martensitic steels it will increase the hardness at higher tempering temperatures due to its effect on the carbide precipitation. An increase in the chromium and molybdenum content mainly increases resistance to localized corrosion (pitting and crevice) and is particularly effective in the ferritic grades.
In austenitic and duplex alloys, nitrogen also has a beneficial influence on the pitting corrosion resistance. In order to quantify these composition effects, empirical indices have been derived to describe the resistance to pitting corrosion in the form of Pitting Resistance Equivalent (PRE). For ferritic steels, the formula employed is:
PRE = % Cr + 3.3 (% Mo), where the concentrations are in weight %,
While for austenitic and duplex grades it is: PRE (N) = % Cr + 3.3 (% Mo) + K*(% N)
where K = 16 for duplex stainless steels K = 30 for austenitic stainless steels.
Manganese (Mn)
Manganese is a grey-white metal with a relative mass (12C= 12) of 54.938, an atomic number of 25, a melting point of 1,245° C and a density of 7.43 kg / dm3. It is in group VII on the periodic table. Manganese has a complex cubic crystal structure.
Manganese is generally used in stainless steels in order to improve hot ductility. Its effect on the ferrite/austenite balance varies with temperature: at low temperature manganese is an austenite stabilizer but at high temperatures it will stabilize ferrite. Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in austenitic steels.
Promotes the stability of austenite, at or near room temperature and improves hot working properties. Addition of up to 2% manganese has no effect on strength, ductility and toughness. Manganese is important as a partial replacement of nickel in 200 series stainless grades.
Silicon (Si)
Silicon is a dark-grey metalloid (silicon is not considered as a metal) with a relative mass (12C = 12) of 28.085, an atomic number of 14, a melting point of 1,414° C, and a density of 2.33 kg / dm3. It is in group IV on the periodic table. Silicon has a diamond (dia) structure. Silicon increases the resistance to oxidation, both at high temperatures and in strongly oxidizing solutions at lower temperatures. It promotes a ferritic structure.
In small amounts, silicon confers mild hardenability on steels. Small amounts of silicon and copper are usually added to the austenitic stainless steels containing molybdenum to improve corrosion resistance in sulphuric acid. In austenitic stainless steels, high-silicon content not only improves resistance to oxidation but also prevents carburizing at elevated temperatures.
Increases scaling resistance by forming a tight initial scale, which will withstand cyclic temperature changes. It resists carburizing at high temperatures and slightly increases tensile strength and hardness. Small amounts of silicon are added to all grades of stainless for deoxidizing.
Carbon (C)
Carbon is a strong austenite former and strongly promotes an austenitic structure. It also substantially increases the mechanical strength. Carbon reduces the resistance to intergranular corrosion.
In austenitic, ferritic, and duplex stainless steels, it is kept to low levels (typically 0.005% C to 0.03% C in low carbon grades) to retain the desired properties and mechanical characteristics. In ferritic stainless steels carbon will strongly reduce both toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels carbon increases hardness and strength.
In the martensitic steels an increase in hardness and strength is generally accompanied by a decrease in toughness and in this way carbon reduces the toughness of these steels.
Nitrogen (N)
Nitrogen is as a relatively inert gas, with a relative atomic mass (12C = 12) of about 14, and an atomic number of 7. It constitutes about 78% of the Earth’s atmosphere by volume and 76% by mass. Each molecule of nitrogen is made up of two nitrogen atoms linked together extremely strongly. In a nitrogen molecule, the three lines joining the two atoms represent a triple bond.
Nitrogen is a very strong austenite former and strongly promotes an
austenitic structure. It also
substantially increases the mechanical strength. Nitrogen increases the
resistance to localized corrosion, especially in
combination with molybdenum.
In austenitic and duplex stainless
steels, nitrogen content
increases the resistance to localized corrosion
like pitting or intergranular.
This is due to the precipitation of Cr2N nitride instead of Cr23 C6 carbide. In ferritic stainless steels nitrogen will strongly reduce toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels nitrogen increases both hardness and strength but reduces the toughness.
Copper (Cu)
Copper enhances the corrosion resistance in certain acids and promotes an austenitic structure. In precipitation hardening steels copper is used to form the intermetallic compounds that are used to increase the strength.
Titanium (Ti)
Titanium is a hard silver metal with a relative atomic mass (12C= 12) of 47.867, an atomic number of 22, a melting point of 1,668° C and a density of 4.54 kg / dm3. It is in group IV on the periodic table. Titanium has a hexagonal close-packed (h.c.p.) structure.
Titanium is a strong ferrite former and a
strong carbide former, thus lowering the effective carbon content and promoting a ferritic structure in two ways.
In austenitic steels it is added to
increase the resistance to intergranular corrosion but it also increases the mechanical properties at high
temperatures. In ferritic stainless steels titanium is added to improve toughness and corrosion resistance
by lowering the amount of interstitials in solid solution. In martensitic steels titanium lowers the martensite
hardness and increases the tempering
resistance. In precipitation hardening steels titanium is used to form the intermetallic compounds that are used to increase
the strength.
Titanium is a highly reactive element which
forms stable TiN precipitate in the liquid phase,
in the presence of nitrogen (N). In the presence of both C and N, titanium
nitrides TiN (in the liquid phase)
and titanium carbides TiC (in the solid phase) form, the latter surrounding the former. The most commonly
used stabilizing element for stainless steel is titanium. The stoichiometric amount of Ti required for full
stabilization is described by the following equation:Ti ≥ 4 (%C) + 3.4 (%N)
However, greater levels of Ti are required
for full stabilization because Ti reacts with sulphur to form stable Ti sulphides,
Ti2S. In practice, the generally accepted level of Ti required to fully stabilize a stainless steel, must therefore satisfy the following
criteria:
Ti ≥ 0.15 + 4 (%C +%N)
Titanium also improves resistance to pitting
corrosion since stable Ti2S have been shown to
form in preference to manganese sulphides (MnS) which are known to act as pit initiation sites. In low alloy steels,
titanium has a strong desire to unite with carbon, nitrogen and oxygen. When dissolved in steel, titanium is
believed to increase hardenability;
however, the carbide-forming tendency of this element is so strong that it is frequently in the steel structure as
undissolved carbides and in this way decreases
hardenability.
Niobium (Nb)
Niobium is a shiny, white transition metal with a relative atomic mass (12C=12) of 92.906, an atomic number of 41, a melting point of 2,468° C and a density of 8.57 kg / dm3. It is in group V on the periodic table. Niobium has body centered cubic (b.c.c.) crystal structure.
Niobium is both a strong ferrite and carbide former. As titanium it
promotes a ferritic structure. In austenitic steels it is added to improve the resistance to intergranular corrosion but it also enhances mechanical
properties at high temperatures. In martensitic steels niobium lowers the hardness and increases the tempering
resistance. In U.S. it is also referred to as
Columbium (Cb).
In stainless steels, as
far as corrosion resistance is concerned, it is well known that stabilizing the grade by Nb additions
prevents the risk of intergranular corrosion in heat affected zones. To prevent this niobium is added in sufficient
amounts, depending on the carbon and nitrogen
(ferritic types) levels.
The theoretical
amount of niobium required for full stabilization based on
stoichiometric calculation is described by the equation:%Nb ≥ 0.2 + 5 (%C +
%N)
In ferritic stainless steels, the addition of niobium is one of the most effective methods for improving thermal fatigue resistance.
Titanium and Niobium as Stabilizing Elements: Dual Stabilization
Stabilizing elements are added to Fe-Cr-(Mo) alloys and Fe-Cr-Ni-(Mo) alloys to prevent sensitization to intergranular corrosion following a sojourn of the alloy within the temperature range in which precipitation of chromium carbide might occur. The function of these stabilizing elements is to combine preferentially with any carbon for Fe-Cr-Ni- (Mo) alloys and with any carbon and nitrogen for Fe-Cr-(Mo) alloys that might otherwise precipitate as chromium carbide and a chromium nitride (Fe-Cr-(Mo) alloys). This leaves the chromium in solid solution in the alloy where it belongs and the full corrosion resisting qualities of the alloy are therefore preserved.
Dual stabilization with titanium and niobium provides the best mechanical properties for weldments. This behavior might be related to a better grain size control and probably to a modification of the nature and morphology of precipitates. The optimal dual stabilisation content is given by the equation:% Ti + 4/7 (% Nb) ≥ 0.15 + 4 (C + N)
The advantages of this dual stabilisation can be summed up as follows:
- The TiN precipitates formed in the liquid phase act as nucleation sites for crystal growth, resulting in a fine grained equiaxed structure that improves the mechanical properties of weldments.
- Less Nb is required
for full stabilisation, thereby further reducing
the amount of low melting
phases that might form at the
grain boundaries.
Aluminium (Al)
Aluminium improves oxidation resistance, if added in substantial amounts. It is used in certain heat resistant alloys for this purpose. In precipitation hardening steels aluminium is used to form the intermetallic compounds that increase the strength in the aged condition.
Cobalt (Co)
Cobalt is a silvery-white transition metal with a relative atomic mass (12C=12) of 58.933, an atomic number of 27 and a melting point of 1,495° C and a density of 8.92 g/cm3. It is in group VIII on the periodic table. Cobalt has a hexagonal close packed (h.c.p.) crystal structure.
Cobalt only used as an alloying element in martensitic steels where it increases the hardness and tempering resistance, especially at higher temperatures.
Vanadium (V)
Vanadium increases the hardness of martensitic steels due to its effect on the type of carbide present. It also increases tempering resistance. Vanadium stabilizes ferrite and will, at high contents, promote ferrite in the structure. It is only used in hardenable stainless steels.
Sulphur (S)
Sulphur is added to certain stainless steels, the free-machining grades, in order to increase the machinability. At the levels present in these grades sulphur will substantially reduce corrosion resistance, ductility and fabrication properties, such as weldability and formability.
When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present the weak spots at the grain boundaries are greatly reduced during hot working.
Cerium (Ce)
Cerium is one of the rare earth metals (REM) and is added in small amounts to certain heat resistant temperature steels and alloys in order to increase the resistance to oxidation and high temperature corrosion.
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