Properties of Carbon Steels
Properties of Carbon Steels
Steels are mainly composed of iron and carbon and special properties in steels are achieved by introducing additional alloying elements. The term steel is normally considered to mean an iron-based alloy containing carbon in quantities less than around 2 %. Carbon steels (sometimes also termed plain carbon steels, ordinary steels, or straight carbon steels) can be defined as steels which contain only residual quantities of elements other than carbon, except those (such as silicon and aluminum) added for deoxidation and those (such as manganese and cerium) added to counter-act certain harmful effects of residual sulphur. However, silicon and manganese can be added in quantities higher than those needed strictly to meet these criteria so that arbitrary upper limits for these elements have to be set which normally 0.6 % for silicon and 1.65 % for manganese are accepted as the limits for carbon steel. Carbon steel is one of the most widely used materials.
The primary alloying element in steel is carbon. Since carbon is such a powerful alloying element in steel, there are considerable differences in the strength, hardness, and ductility achievable with relatively small variations in the levels of carbon in the composition. However, other important factors, such as material fabrication, heat treatment, component fabrication, and fabrication processes, can result in considerable changes to the properties of the carbon steels.
Carbon steel is available in virtually all product forms, including both the forms needed for pressure-containing applications and the shapes needed for structural applications. Carbon steel is used in boilers, pressure vessels, heat exchangers, piping, and other moderate-temperature service systems in which good strength and ductility are desired. Carbon steels are used around the world because of its easy availability, cost, properties, ease of fabrication, availability in several shapes and forms, weldability, and so on.
When selecting a steel material for a particular application, user has to be confident that it is suitable for the loading conditions and environmental challenges it is going to be subjected to while in service. Understanding and control of the properties of the steel material is hence necessary. The main properties of steel are (i) physical properties, (ii) metallurgical properties, (iii) mechanical properties, and (iv) corrosion properties. Properties of steel depends on (i) its chemical composition, (ii) its heat treatment, and (iii) its mechanical working.
The feasibility of using a carbon steel is determined by assessing whether or not it is metallurgically suitable for the application. Evaluations can include tensile and fatigue strengths, impact resistance, size of the part compared with the need for through hardenability, fabricability, ductility and / or machinability, potential of heat treating, service temperature of the part, and corrosion resistance. When the desired characteristics can be achieved with a plain carbon grade, majority of the users select this less costly steel. If critical strength requirements or other specified needs are beyond the inherent capabilities of carbon steel, then alloy steel is the obvious choice.
One consideration worth noting is that the majority of the carbon steels are not through hardening on heat treatment, except in relatively thin sections. The selection of a specific grade of steel is a more complex decision. Grade selection is to be undertaken in consultation with the metallurgists, and broad practical experience, as well as the experience of other users who have put carbon or alloy steel for the same or similar use.
Physical properties of steels
There are several physical properties which are of importance in case of steels. The physical properties of steel are related to the physics of the material, such as density, thermal conductivity, elastic modulus, Poison’s ratio etc. These properties are described below.
Mean coefficient of linear thermal expansion – It is the ratio of the change in length to the original length at a reference temperature, ‘T0’, per degree of temperature change, where ‘T0’ is normally room temperature. If ‘L0’ is the length at ‘T0’ and alpha (a) is the mean coefficient of linear thermal expansion, the length at temperature ‘T1’ (Lt), is given by the equation Lt = L0[1 + a(T1-T0)].
Instantaneous coefficient of linear thermal expansion – it is the rate of the change in length at a specific temperature.
Modulus of elasticity (E) – It is the measure of rigidity or stiffness of a material, the ratio of stress below the proportional limit to the corresponding strain or it is the slope of a stress-strain curve in the range of linear proportionality of stress to strain. It is also known as Young’s modulus.
Thermal conductivity – It is the quantity of heat transmitted ‘k’ because of the unit temperature gradient, in unit time under steady conditions in a direction normal to a surface of unit area and when the heat transfer is solely dependent on the temperature gradient.
Thermal diffusivity – It is the constant in the heat conduction equation describing the rate at which heat is conducted through a material. It is linked to thermal conductivity ‘k’, specific heat ‘Cp’, and density ‘d’, through the equation ‘Thermal diffusivity = k/d x Cp’.
Electrical resistivity – It is a measure of how strongly a material opposes the flow of electric current and is represented by Greek letter rho (r). Electrical resistivity (r) = RA/L, where ‘R’ is the resistance, ‘A’ is the cross-sectional area, and ‘L’ is the length of the sample.
Specific heat – It is the quantity of heat, ‘Cp’, measured in calories, needed to raise the temperature of one gram of a substance by one degree Celsius.
Density – It is the mass per unit volume of a solid material.
Specific gravity – It is the ratio of the density of a substance to the density of water.
Shear modulus (G) – It is the ratio of shear stress to the corresponding shear strain for shear stresses below the proportional limit of the material. Values of shear modulus are normally determined by torsion testing. Shear modulus is also known as the ‘modulus of rigidity’.
Melting point – It is the temperature at which a metal changes from solid to liquid, the temperature at which the liquid and the solid are at equilibrium.
Poisson’s ratio – It is the absolute value of the ratio of transverse (lateral) strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material.
Typical physical properties of steels are given in Tab 1.
|Tab1 Physical properties of steels|
|Melting point||deg C||1,371-1,540|
|Thermal conductivity||Watts per metre – kelvin||24.3-65.2|
|Electrical resistivity at 70 deg C||micro-ohm-meter||0.213|
|Shear modulus at room temperature||GPa||75-80|
Chemical properties of steels
Iron is the basic component of steel. When carbon (C), a non-metal, is added to iron (Fe) in quantities up to 2.0 %, the result is an alloy known as steel. Composition of steel mainly consists of iron and other elements such as carbon, manganese, silicon, phosphorus, sulphur, and alloying elements. A large number of elements in wide ranging percentages are used for the purpose of alloying of steels. Variations in chemical composition of steels are responsible for a great variety of steel grades and steel properties.
The chemical compositions of the materials are also established by the material specifications for each type or grade of material in standards. The elements which are not identified are not to be present in more than trace quantities, except iron, which is of course the primary constituent of carbon steels.
Each element which is added to the basic steel composition has some effect on the properties of the steel and how that steel reacts to the processes of working and fabrication of steels. The chemical composition of steel also determines the behaviour of steel in different environments. Steel standards define the limits for composition, quality, and performance parameters for different steel grades.
The heat analysis is given unless otherwise noted. Although this is the analysis taken from the molten heat and given on the certified material test report, the actual composition of the end product can vary in excess of the heat analysis because of the fluctuations which occur during solidification and processing. The limits on the product analysis are hence somewhat less restrictive than those of the heat analysis. For determination of chemical composition either instrumental analysis by the use of optical emission spectrometer (OES) or wet analysis methods are used.
The metallurgical structure and the carbon content are major contributors to the overall properties of the different carbon steels. Steels classified as carbon steel can also contain small quantities of other elements, such as chromium, nickel, molybdenum, copper, vanadium, niobium, phosphorous, and sulphur. Each element which is added to the basic constituent of iron has some effect on the end properties of the steel and how that material reacts to the fabrication processes. The alloying additions are responsible for several of the differences between the different types or grades of carbon steels. The elements normally added to iron and their effects on the steel are described below.
Carbon – Carbon is the most important alloying element in steel and can be present up to 2 % (although most welded steels have less than 0.5 %). The carbon can exist either dissolved in the iron or in a combined form, such as iron carbide (Fe3C). Increased quantities of carbon increase hardness and tensile strength as well as response to heat treatment (hardenability). On the other hand, increased quantities of carbon reduce weldability.
Manganese – Steels normally contain at least 0.3 % manganese, which acts in a three-fold manner. It (i) assists in deoxidation of the steel, (ii) prevents the formation of iron sulphide inclusions, and (iii) promotes higher strength by increasing the hardenability of the steel. Quantities up to 1.5 % are normally found in carbon steels.
Silicon – Only small quantities (0.2 %, for example) are normally present in rolled steel when silicon is used as a deoxidizer. However, in steel castings, 0.35 % to 1 % is normal. Silicon dissolves in iron and tends to strengthen it. Weld metal normally contains around 0.5 % silicon as a deoxidizer. Some filler metals can contain up to 1 % silicon to provide improved cleaning and deoxidation for welding on contaminated surfaces. When these filler metals are used for welding of clean surfaces, the resulting weld metal strength is markedly increased. The resulting decrease in ductility can present cracking problems in some situations.
Sulphur – This is an undesirable impurity in steel rather than an alloying element. Special effort is made to eliminate or minimize sulphur during steelmaking. In quantities exceeding 0.05 %, it tends to cause brittleness and reduce weldability. Additions of sulphur in quantities from 0.1 % to 0.3 % tend to improve the machinability of steel but impair weldability. These types of steels are referred to as free machining steels.
Phosphorus – Phosphorus is also considered to be an undesirable impurity in steels. It is normally found in quantities up to 0.04 % in the majority of the carbon steels. In hardened steels, it tends to cause embrittlement. In low-alloy, high-strength steels, phosphorus can be added in quantities up to 0.1 % to improve both strength and corrosion resistance, although it is not normally added for this reason in carbon steels.
Chromium – Chromium is a powerful alloying element in steel. It is added for two principal reasons namely (i) it greatly increases the hardenability of steel, and (ii) it markedly improves the corrosion resistance of steel in oxidizing types of media. Its presence in some steels can cause excessive hardness and cracking in and adjacent to the weld. Stainless steels contain chromium in quantities exceeding 12 %.
Molybdenum – Molybdenum is a strong carbide former and is normally present in alloy steels in quantities less than 1 %. It is added to increase hardenability and to increase the high temperature strength.
Nickel – Nickel is added to steels to increase the hardenability. It performs well in this function since it frequently improves the toughness and ductility of the steel, even with the increased strength and hardness. Nickel is frequently used to improve steel toughness at low temperatures.
Vanadium – The addition of vanadium result in an increase in the hardenability of steel. It is very effective in this role, hence it is normally added in very small quantities. In quantities higher than 0.05 %, there can be a tendency for the steel to become embrittled during thermal stress relief treatments.
Niobium – Niobium, like vanadium, is normally considered to increase the hardenability of steel. However, because of its strong affinity for carbon, it can combine with carbon in the steel to result in an overall decrease in hardenability.
Other alloying elements – Some carbon steel specifications allow additions of certain other elements, but they are not deliberately added. The specifications in steel standards can list these elements as a specified addition to the steel, but the addition is minor in carbon steels.
Even when other alloying elements are not present, high carbon content can result in high local hardness. However, other alloying elements also contribute to the overall hardenability of the steel. This effect can be normally quantified by the determination of the carbon equivalence (CE) of the steel. CE is defined by several formulas, and it is important that close attention be paid to the formula being used. It is important that any CE determination be calculated using the actual chemical analysis rather than the maximums specified in materials specifications. If this is not done, the calculation results in an unrealistically high CE. CE is a useful parameter in determining the behaviour of steel during the process of welding of steel. The two popular formulas used for CE are (i) CE = %C + %Mn/6 + (%Cr + %Mo + %V)/5 + (%Cu + %Ni)/15, and (ii) CE = %C + (%Mn + %Si)/6 + (%Cr + %Mo + %V)/5 + (%Cu + %Ni)/15.
Microstructural properties of steels
Steels take the form of a crystalline structure in the solid state. The crystalline structure and the alloying elements added to pure iron give carbon steel the ability to have a wide range of properties, which make it one of the most useful materials in the industry today. The crystalline structure of carbon steel can include body-centered cubic (ferrite), face-centered cubic (austenite), or body-centered tetragonal (martensite) forms. The crystalline structure forms in several directions during solidification from the liquid state of the steel.
Solidification starts from initiation points and continues until the crystalline structure which is formed runs into another island which started from a different point. Each of these islands of a single orientation is a grain which exists as a singular structure. The size of these grains also contributes to the properties of the steel and also affects the ability of the steel to form certain micro-structures. As the steel cools, carbon steel crystalline structures are forced to change from one structure to another and these are called phase transformations. The different structures have different limits of solubility of the alloying elements, primarily carbon in carbon steels.
The micro-structure can also contain other compounds, such as metallic carbides, interspersed with the crystalline form. The complex micro-structure of carbon steel includes the crystalline structure, the grain size, and the size and frequency of the interspersed metallic compounds. Carbon steels can exist in different micro-structures or combinations of micro-structures. The micro-structures of carbon steels include not only the crystalline structure but also different metallic carbides or compounds in different arrangements. Pearlite, upper bainite, and lower bainite are examples of the arrangements which can exist. Fig 1 gives micro-structures of carbon steels.
Fig 1 Micro-structures of carbon steels
Pearlite is an arrangement of thin alternating and roughly parallel lamellar platelets of ferritic (body-centered cubic) structures with iron carbides (Fe3C) called cementite. The lamellar platelets can be coarse or fine, but they are frequently recognizable with optical microscope. Bainite is an arrangement of aggregates of ferrite with distributions of precipitated carbide particles. However, the arrangement can take different forms, hence the terms upper bainite and lower bainite. Upper bainite consists of small ferrite grains which form in plate-shaped sheaths. These grains are interspersed with the cementite which forms at relatively high temperatures. Lower bainite consists of needle-like ferrite plates containing a dispersion of very small carbide particles. Fig 2 shows an example of the growth of upper and lower bainite.
Fig 2 Growth of upper and lower bainite
The different micro-structures or crystalline structures of carbon steels have considerably different properties which are determined by alloy content (again, primarily by carbon) and the different thermal cycles which can exist during fabrication and heat treatment.
Transformation behaviour – The crystalline structure of pure iron is ferrite at room temperature. The room temperature form of ferrite is called alpha ferrite. At higher temperatures, the ferritic structure is unstable and transforms into a face-centered cubic structure called gamma austenite. At even higher temperatures, the austenitic structure can again transform into a higher temperature form of ferrite, which is called delta ferrite. Iron-carbon phase diagram (Fig 3) represent the crystalline structures, or phases, of the carbon steels in an equilibrium state which are determined by very slow cooling from liquid steel.
Fig 3 Iron carbon phase diagram
This is not a realistic view of the micro-structural phases which exist during normal fabrication processes since the heating and cooling rates affect considerably the temperatures at which the suggested phase transformations occur. This effect can be seen in the temperature difference between A1 temperature, the equilibrium lower transformation temperature, and Ar1 temperature, the lower transformation temperature upon cooling. There is also a lower transformation temperature upon heating, Ac1 temperature, which is somewhat higher than A1 temperature. The Ac1 temperatures depict the start point of the transformation between the alpha ferrite and the gamma austenite upon heating. The phase diagram in Fig 3 also shows an equilibrium upper transformation temperature, A3 temperature. Similar to the variations noted for A1 temperature, there are also upper transformation temperatures upon heating and cooling (Ac3 and Ar3 temperaures, respectively).
The transformation temperatures indicate the points at which the structure becomes an unstable form and begins to undergo a transformation to a different crystalline structure. It can be seen that carbon steels, with a typical maximum carbon content of less than 0.35 %, have a transformation temperature range which varies with the carbon content and the rate of heating or cooling. The ferritic structure at room temperature has a relatively low ability (probably less than 0.008 %) to contain carbon atoms in the space between the iron atoms (interstitially). The face-centered cubic structure has a much higher affinity for carbon and can contain as much as around 2.1 %.
Carbon which cannot be contained interstitially can exist in other forms, such as iron carbides or carbides of other metal elements. In a carbon steel micro-structure, iron carbides can appear as platelets or particles of cementite (Fe3C). A micro-structure which has alternating platelets of ferrite and cementite is called pearlite. With certain rates of cooling, the carbon steel micro-structure can also be bainite. Bainitic structures represent a variety of ferrite aggregates with a distribution of small iron carbide precipitates.
Upon heating the carbon steel micro-structure through the transformation range, the ferrite transforms into an austenitic structure. Since the austenitic structure has a much higher solubility of carbon, the iron carbides dissolve, and the carbon enters into solution with the austenitic iron micro-structure. This is a time-dependent and temperature-dependent mechanism which takes longer if the cementite particles or platelets are large. An increased rate of heating also has the effect of needing a higher temperature to complete the dissolution. Fig 4 gives an example of the transformations which are expected when low-carbon steel is heated rapidly.
Fig 4 Schematic representation of plain carbon steel (0.2 % carbon) when heated rapidly to the temperature shown
Shortly after full austenization has been completed and upon the temperature reaching a point slightly above the upper transformation temperature Ac3, the grain size is quite small. Upon subsequent cooling, this fine grain structure is essentially maintained. However, if the steel is heated to a higher temperature before cooling, the grain size becomes larger, and the result is a coarser grain structure in the room temperature structure. The temperature reached during thermal cycles upon heating above the transformation temperatures (such as during a welding process) has a considerable effect on the end properties of the steel.
Transformations of even higher significance occur during cooling from the austenitic structure of carbon steel. The austenitic structure can contain a much higher level of carbon than the ferritic structure, which can contain a maximum of only around 0.008 % carbon. When austenitized carbon steel is cooled very slowly (when it is equilibrium cooled, essentially), ferrite grains begin to form just below the Ar3 temperature (the upper transformation temperature upon cooling). These ferrite grains cannot contain the typical carbon content levels of carbon steel, and hence as a result, the content increases in the austenite grains, the reverse of what happens when the ferritic grains are heated through the transformation temperatures. As the steel is cooled further toward the Ar1 temperature (the lower transformation temperature upon cooling), more ferrite is formed at the grain boundaries of the austenite, and the austenite continues to gain carbon content. This can continue until the Ar1 temperature is reached, at which point the austenite can contain as much as around 0.77 % carbon (the eutectoid composition). This can be seen in Fig 5 in the illustrations from point ‘c’ down to point ‘d’ just above the Ar1 temperature (marked as 723 deg C).
Fig 5 Transformation of carbon steel with slow cooling
When the structure cools further to just below the Ar1 temperature (as represented by point d just below Ar1 temperature in Fig 5), the high-carbon austenite transforms to ferrite and cementite since the ferrite is not able to accommodate the high carbon content. This results in the pearlitic micro-structure in which the ferrite and the cementite are arranged in alternating lamellar platelets. Considerable differences in the transformation mechanism are realized when the carbon steel is cooled more rapidly than the slow (essentially equilibrium) cooling described. The formation of ferrite and pearlite from austenite is a nucleation and growth mechanism. With slow cooling, there is adequate time for this mechanism to occur. As the cooling rate increases, the austenite can be undercooled to a temperature below the Ar1 lower transformation temperature. When this happens, changes occur in the micro-structure of the steel.
The different micro-structures which can result from this more rapid cooling are described here. Equilibrium cooling of typical carbon steel results in a ferritic structure with grains of pearlite. In this case, the carbon in the austenite has the time to diffuse into the cementite platelets and to allow the ferrite platelets to form. The result is a coarse pearlite with ferrite grains which is formed at the grain boundaries. If the austenite is undercooled slightly before transformation can occur, the result is a finer pearlitic structure since the time for the carbon to diffuse into the cementite platelets is shortened. Also, the nodules of pearlite and the grains of ferrite tend to be smaller. Strength and hardness are increased as a result.
The existence of bainitic structures is possible in carbon steels. Bainitic structures occur when the undercooling of the austenite is such that pearlite can no longer form and the formation of martensite has not yet started (i.e., the martensite start temperature [Ms] has not been reached). Bainite can take different morphologies (patterns) as either upper bainite, or lower bainite, depending on the temperature at which it forms. Upper bainite is somewhat harder and tougher than the pearlite if it forms. Lower bainite is not as hard as martensite but can be much tougher.
If the cooling rate is too rapid to allow nucleation and growth mechanisms (this condition is called the critical cooling rate), the result is that the trapped carbon is forced into the crystalline lattice. Instead of forming ferrite structures, the austenite lattice shears and results in a body-centered tetragonal structure called martensite. This martensitic transformation occurs without diffusion of the carbon and hence occurs very rapidly. In addition, once the austenitic structure is undercooled to the point at which the carbon cannot diffuse and additional ferrite cannot form, the only remaining transformation which can occur upon further cooling is to martensite. The temperature at which martensite begins to form from austenite is the ‘Ms’ temperature. Since ferrite cannot form, martensite continues to form as the temperature decreases from any existing austenite until all of the austenite is transformed, which occurs at the martensite finish temperature, or ‘Mf’ temperature. This carbon steel martensitic structure is known to be both hard and strong but lacks ductility and toughness in the untempered state. The resulting maximum hardness is closely related to the carbon content of the steel and the percentage of martensite which is formed. Fig 6 shows the relationship between carbon content and maximum achievable hardness in steels.
Fig 6 Carbon content and maximum achievable hardness in steels
Heat treatment – The transformation of carbon steel from one micro-structure or crystalline structure to another also makes the steel heat treatable, or in other words, it allows for changes in the properties of the steel just by going through different heating and cooling cycles, without a change in the overall chemical composition of the steel. This characteristic can also result in property changes occurring during fabrication processes such as hot bending / forming, welding, and brazing. The material specifications in steel standards provide the heat treatments needed to achieve the properties necessary for the specific steel.
Heat treatment is highly dependent on the manufacturing methods used for the product, and the requirements can range from not required heat treatment to subcritical heat treatments (such as precipitation heat treatment, tempering, or stress relief) to high-temperature (austenitizing) heat treatments (such as quench hardening, annealing, or normalizing) which can be followed by a tempering heat treatment. If no heat treatment is needed, the properties of the steel are dependent on the steelmaking practice, the chemistry, and the fabrication processes used. Descriptions of common heat treatments are given below.
Annealing is a very broad term used to describe a variety of heat treatments, but it is a process customarily applied to remove stresses or work hardening. For the purpose of the heat treatment used on carbon steels in the material specifications, the more specific term full annealing better describes the process. Full annealing is defined as ‘annealing a steel object by austenitizing it and then cooling it slowly through the transformation range’. The result is that the maximum transformation to ferrite and to coarse pearlite is achieved, which corresponds to the lowest hardness and strength. Full annealing of carbon steels needs likely the steel to be heated to 845 deg C to 900 deg C for 1 hour 30 minutes for each additional 25 mm above 25 mm thickness.
Normalizing is a specific term defined as ‘heating a steel object to a suitable temperature above the transformation range and then cooling it in air to a temperature substantially below the transformation range’. For several carbon steels, the cooling rate in air is not rapid enough to prevent considerable transformation from austenite into ferrite and a pearlitic micro-structure. Higher alloy, air-hardenable steels can be considerably hardened by normalizing. The normalizing temperature is typically 55 deg C above the upper critical temperature.
Hardening of steels is done by quench hardening. Quench hardening is frequently used prior to a tempering heat treatment. It is defined as ‘hardening a steel object by austenitizing it and then cooling it rapidly enough so that some or all of the austenite transforms to martensite’. Quench hardening is normally the first step in a heat treatment which then include a tempering heat treatment. The martensitic steel is excessively hard and strong with characteristic low toughness, so the tempering treatment is used to recover some of the more desirable properties. The carbon steel is typically heated to 815 deg C to 870 deg C and then quenched in a medium selected to cause the desired cooling rate.
Tempering is defined as ‘reheating a quench hardened or normalized steel object to a temperature below Ac1 temperature and then cooling it at any desired rate’. Tempering allows some of the carbon atoms in the strained martensitic structure to diffuse and form iron carbides or cementite. This reduces the hardness, tensile strength, yield strength, and stress level but increases the ductility and toughness. Tempering temperatures and times are interdependent, but tempering is normally done at temperatures between 175 deg C and 705 deg C and for times ranging from 30 minutes to 4 hours.
Stress relieving is frequently associated with tempering and can occur simultaneously with tempering. It is defined as ‘heating a steel object to a suitable temperature, holding it long enough to reduce residual stresses, and then cooling it slowly enough to minimize the development of new residual stresses’. Locked-in (residual) stresses in a component cannot exist at a higher level than the yield strength of the material. An increase in the temperature of steel lowers the yield strength and hence relieves some of the stresses. Further reduction in the residual stress can occur because of a creep mechanism at high stress relief temperatures. Stress relieving has a time-temperature relationship similar to tempering. Although some stress relief occurs very quickly as a result of the lower yield strength at temperature, additional stress relief occurs by the primary creep mechanism. Stress relief temperatures are typically 600 deg C to 675 deg C for carbon steels.
Precipitation heat treatment is less common in carbon steels since the precipitates desired are normally carbides of alloying elements other than iron. However, some of the carbon steels include a small quantity of those elements, such as chromium, molybdenum, niobium, or vanadium. Precipitation heat treatment is defined as ‘artificial aging in which a constituent-precipitates from a super-saturated solid solution’. Since precipitation hardening is not normally used to increase the strength of carbon steels, this does not apply.
Mechanical properties of steel
The behaviour of steels under external load defines their mechanical properties. Deformations are normally described in terms of stress of force per unit area and strain or displacement per unit distance. Using the stress-strain relation, a person can distinguish elastic and plastic regimes. At small stress, the displacement and applied force obey the Hooke’s law and the sample returns to its original shape upon unloading. Exceeding the so-called elastic limit, upon strain release the steel is left with a permanent shape.
Within the elastic regime, the elastic constants play the primary role in describing the stress-strain relation, whereas in the plastic regime the mechanical hardness expresses the resistance of material to permanent deformations. Plastic deformations are facilitated by dislocation motion and can occur at stress levels far below those needed for dislocation-free crystals.
Mechanical hardness can be related to the yield stress separating the elastic and plastic regions, above which a substantial dislocation activity develops. In an ideal crystal, dislocations can move easily since they only experience the weak periodic lattice potential. In real crystal, however, the movement of dislocation is impeded by obstacles, leading to an increase of the yield strength. In particular, in solid solutions the yield stress is decomposed into the Peierls stress (it is the minimum shear stress required to move a single dislocation of unit length in a perfect crystal) needed to move a dislocation in the crystal potential and the solid-solution strengthening contribution because of the dislocation pinning by the randomly distributed solute atoms. The Peierls stress of pure metals is found to be approximately proportional to the shear modulus. Dislocation pinning by random obstacles is controlled by the size and elastic misfit parameters. The misfit parameters, in turn, can be derived from the composition dependent elastic properties of bulk solids.
Mechanical properties of steels are defined as the reaction of the steels to certain types of external forces. The material specifications in the steel standards establish the needed minimum mechanical properties for each type or grade and for each class of material covered. In cases where a range is identified, the property has both a minimum and maximum value. These property values can change with a different edition of the steel standards. Some mechanical properties are not needed to be determined and are not listed in the standards. These standards also specify the methods for preparation of test samples and the procedure for testing.
The design tensile and yield strengths of carbon steel typically decrease with an increase in temperature. However, this is not in fact the actual behaviour of the carbon steel since the actual tensile strength can decrease slightly and then increase because of strain aging. The design values are modified so that the design tensile strength is not allowed to increase with temperature. The following are the mechanical properties which are applicable to carbon steels.
Tensile strength (TS) is the maximum force which the steel material withstands before fracturing. It is also called ultimate tensile strength (UTS). It is normally reported in terms of force per unit of area (newtons per square millimeter or Mega-pascal, MPa).
Yield strength (YS) is the force which the steel material can withstand before permanent deformation occurs. It is also reported as force per unit of area (newtons per square millimeter or MPa).
Proof load testing is frequently used inter-changeably with yield strength. It refers to the tension-applied load which a test sample is required to support without evidence of deformation.
Ductility is the ability of the steel material to deform without fracturing. It is normally reported in percentage as elongation and reduction of area in a cross section which has been purposely fractured.
Tensile strength, yield strength, proof load testing, and ductility of carbon steel are determined by performing a tension test in which a standard sample of the material is subjected to a pulling force which increases gradually until the material deforms, stretches, and fractures.
Hardness is the resistance of the steel material to penetration. It is different from hardenability which is a measure of the ability of the steel to respond to heat treatment. Hardness is measured by applying a standard force to the surface of the steel through a small, hardened ball point, and then measuring the diameter of the resulting impression. Hardness is normally reported as a value on one of two industry standard scales, Brinell or Rockwell. Brinell hardness is normally more accurate for measuring hardness of plate products. Fig 7 shows effect of carbon on properties of steel.
Fig 7 Effect of carbon on properties of steel
Impact strength is the ability of the steel material to withstand a high velocity impact. Impact strength is measured by subjecting a standard notched sample to a swinging weight. As it is frequently important to know how the steel performs in colder environments, this test is frequently done at sub-zero temperatures. Known as a notch test or Charpy test, the test needs three standard samples of a defined grain orientation. The results are reported in joules, showing the average of the three samples and the lowest value of the three, at the testing temperature.
Torque is the angular force needed to turn something. It is the tendency of a force to rotate a steel object around an axis. Torque testing is accomplished by inserting a torque transducer between the tool applying the force and the item against which the force is being applied. There are two different approaches to torque measurement namely reaction and in-line. In-line torque testing measures the torque needed to turn the rotating part while reaction measures the quantity needed to prevent the part from rotating.
Fatigue testing is performed on steel parts to simulate the progressive and localized structural damage which occurs when steel material is subjected to cyclic loading. Fatigue is the weakening of the steel material caused by repeatedly applied loads. It is the progressive and localized structural damage which occurs when the steel material is subjected to cyclic loading. The nominal maximum stress values which cause such damage can be much less than the strength of the material typically quoted as the UTS limit or YS limit.
Bend test determines the ductility or the strength of the steel material by bending the material over a given radius.
Shear testing measures the shear strength. Shear strength is the measure of the steel material’s response to shear loading, a force which tends to produce a sliding failure on the steel material along a plane which is parallel to the direction of the force.
Creep properties – Creep can be defined simply as time-dependent strain occurring under constant stress. There are basically three stages of creep identified namely primary, secondary, and tertiary. Primary creep is the initial instantaneous elastic strain from the applied load, followed by a region of increasing inelastic strain at a decreasing strain rate. Secondary creep occurs when the creep rate is nominally constant at a minimum rate. Tertiary creep is characterized by a drastically increased strain rate with rapid extension to fracture.
The allowable stresses permitted by the different construction codes are based in part on time-dependent creep properties. For carbon steels, these time-dependent properties dominate the allowable stress above around 400 deg C, although creep begins to occur in carbon steels at around 370 deg C. Since the creep rupture strength is heavily influenced by temperature, the allowable stress drops off rapidly above that temperature.
In addition, graphitization is also a time-dependent mechanism in carbon steels above 425 deg C, although this mechanism is not included in the development of allowable stresses since it is mostly unrelated to the stress level. Hence, other materials are frequently used in power plant applications for which continuous operation is expected at or near a design temperature above 425 deg C. Some codes do not give allowable stresses above 425 deg C, but this is out of concern for graphitization rather than creep.
Creep failure can be avoided through appropriate control over the temperature and the imposed stress. Unfortunately, with power plants, other factors enter into play. These factors are frequently related to the operation of the plant and can include the desire to operate at higher temperatures to increase efficiency, the build-up of corrosion products within the pipe or tube (this can expose the material to a higher localized temperature than that intended in a boiler), obstructions in the pipe or tube, and local flaws which can cause local stress concentrations.
The Larson–Miller relation, also widely known as the Larson–Miller parameter and frequently abbreviated LMP, is a parametric relation used to extrapolate experimental data on creep and rupture life of engineering materials. The LMP can be used to determine the expected life of a component. The LMP describes the equivalence of time at temperature for a steel under the thermally activated creep process of stress rupture. It permits the calculation of the equivalent times necessary for stress rupture to occur at different temperatures.
The Larson–Miller parameter is a means of predicting the lifetime of material versus time and temperature using a correlative approach based on the Arrhenius rate equation. The value of the parameter is normally expressed as LMP = T(C + log t), where ‘C’ is a material specific constant, frequently approximated as 20, ‘t’ is the time in hours, and ‘T’ is the temperature in kelvins. Creep-stress rupture data for high-temperature creep-resistant alloys are frequently plotted as log stress to rupture versus a combination of log time to rupture and temperature. Graphs for different material groups are available.
Initially, dislocations occur in the grain structure during the primary stage of creep. During the secondary stage, voids begin to form in the structure, starting at the grain boundaries. When these voids form an orientation and begin to link, the tertiary stage of creep starts, signaling impending failure under the same operating conditions.
Creep failures can occur in carbon steel materials when subjected to long-term overheating while under stress. Although carbon steel materials are normally not used under conditions where creep is expected, a number of factors can cause the material to see higher temperatures than expected, particularly within a boiler with a heat source external to the material. The build-up of an internal oxide scale or tube blockage can cause this overheated condition and, if left unresolved, can contribute to long-term failure.
An oxide build-up of just 0.025 mm can allow a tube temperature within a boiler to increase by around 1 deg C, an oxide build-up of 0.508 mm, hence, can result in an increase in the metal temperature by as much as 16 deg C. This increase can result in a considerable increase in cumulative damage by creep in the steel. The effects of this increase can readily be seen in the rapid loss of rupture strength in the carbon steel material with increases in the temperature.
Graphitization – Several major failures have occurred in carbon steels and carbon-molybdenum steels as a result of long-term service at elevated temperatures. The mechanism of these failures has been graphitization, a micro-structural change which occurs primarily in steels which have been deoxidized using aluminum. The pearlitic micro-structure is a mixture of ferrite and iron carbide (cementite). However, the cementite is unstable at higher temperatures and breaks down into essential pure iron and randomly dispersed carbon.
The breakdown occurs over a considerable period related to the temperature. This can result in a very localized failure of the weak pure iron associated with the brittle carbon. Frequently, the primary location for this failure is in the heat-affected zone of a weld at the point where the material is briefly heated above the lower transformation temperature (in the inter-critical zone). This occurs slightly away from the fusion line of the weld and can extend around the entire circumference of the pipe at a girth weld. The failure can be similar to a brittle failure and can hence be catastrophic. Some failures have resulted in complete separation of a pipe at a girth weld (a double-ended pipe break).
This mechanism was first recognized in the early 1940s, but significant failures have occurred much more recently, e.g., a graphitization failure occurred in August 1977 which resulted in several fatalities. Failures have occurred even though codes recognized the mechanism and took steps to limit the use of the carbon steels and carbon-molybdenum steels to temperatures at which this mechanism is not expected.
The issue is that several plants have already been designed and built using these materials at higher temperatures. The temperature above which graphitization is expected to occur is around 427 deg C for carbon steels. Although the materials used today have much better resistance to graphitization because of the use of silicon instead of aluminum as a deoxidizer, they are still susceptible. Long-term operation of carbon steels at temperatures above 427 deg C is to hence to be avoided. Modern power plant design does not allow carbon steels to be used for long-term operation at the high temperature at which graphitization can occur.
However, the failure mechanism is still a concern because of the material identification or design errors. The recommended method to determine if graphitization is present, is to examine a sample metallurgically. A bend test of the material helps to determine the degree of graphitization which has occurred. Bend test results which show failure with a bend angle from around 30-degree down to around 10-degree or less indicate extensive to severe graphitization. Results which show failure with a bend angle from around 90-degree to around 30-degree indicate moderate to heavy graphitization. Results with a bend angle higher than 90-degree and up to 180-degree indicate mild to no graphitization.
Mild to moderate graphitization can be rehabilitated by heating to around 955 deg C for about 2 hours, followed by slow cooling and a final heat treatment of around 675 deg C for about 4 hours. This method is not recommended for more severe graphitization since the graphite particles cannot fully dissolve back in the ferritic matrix and can also leave voids in the material. The more frequent approach to repair of a graphitized weld joint is to remove the weld and heat-affected zone beyond the point of graphitization and to reweld and perform PWHT (post weld heat treatment). This does not prevent graphitization from reoccurring, but it is intended to delay any further problem for several years. Complete resolution of the problem likely needs replacement with material which is not susceptible to graphitization.
Fatigue properties – The fatigue properties of steels can be affected by mechanical discontinuities, metallurgical discontinuities, microstructures, and environmental / service conditions. The fatigue life is typically expressed with a fatigue design (S-N) curve, such as that shown in Fig 8 for medium strength steel. This curve shows the characteristic of ferrous materials that have a fairly well-defined fatigue limit or endurance limit (the stress level at which a failure is not likely to occur, regardless of the number of cycles). The fatigue limit for the medium carbon steel in Fig 8 is slightly less than 50 % of the fracture strength load under which fatigue failure is not likely to occur, even if the number of cycles exceeds about 10,000,000 cycles.
Fig 8 Typical S-N curve for medium strength carbon steel
Mechanical discontinuities which have a considerable effect on fatigue include the planar flaws perpendicular to the direction of the stress, such as under-cut, sharp entrance angles at the weld toe, cracks, non-fusion flaws, incomplete penetration, and mis-match. Metallurgical discontinuities are those for which the micro-structure is crack-sensitive, such as those with high hardness, low toughness, or high residual stresses. These discontinuities can frequently occur within the heat-affected zone of a weld or in the weld itself. The environment and the service also affect the fatigue strength of a component since corrosion and creep also contribute to an acceleration of fatigue.
Effect of grain growth on properties – The grain size of carbon steel materials can vary depending on the practices used during the initial steelmaking, alloying, heat treatment, or recrystallization. Initial steelmaking practices can include using aluminum as a deoxidizer which also has the effect of reducing the grain size, or adding other grain-refining elements, such as niobium, vanadium, or titanium. Heat treatment can also result in grain growth or refinement by austenitizing at different temperatures. An aging heat treatment on material which has been recrystallized after cold working can increase the grain size. Fine-grained micro-structures tend to have better toughness, and steels which have been specifically treated to have a fine-grained structure are used for low-temperature applications. A fine grain size is ASTM 5 or higher (higher numbers are finer), 7 is typical. The opposite effect is true for creep rupture properties, creep rupture strength is higher for coarse-grained microstructures than it is for fine-grained microstructures. A coarse grain size is typically in the range of ASTM 1 to ASTM 5.
Corrosion properties of carbon steels – As per ISO 8044:2010, ‘corrosion is the physico-chemical interaction between a metal and its environment, which results in changes in the metal’s properties and which may lead to significant functional impairment of the metal, the environment, or the technical system of which they form a part’. Corrosion is seen when there is a change in the metal’s or system’s properties which can lead to an undesirable outcome. This can range simply from visual impairment to complete failure of technical systems which cause big economic damage and even present a hazard to the people.
Corrosion can be defined broadly as the destruction or deterioration of metal by direct chemical and electro-chemical reaction with its environment. Most simply stated, metallic corrosion is the reverse of electro-plating. The metal being corroded forms the anode while the cathode is that being electro-plated. Metallic corrosion occurs since in several environments, the majority metals are not inherently stable and tend to revert to some more stable combination of which the metallic ores as found in nature are familiar examples.
Carbon steel is the most widely used engineering material. It has relatively limited corrosion resistance. The cost of corrosion of carbon steel to the total economy is very high. Since the carbon steels represent the largest single class of alloys in use, both in terms of tonnage and total cost, the corrosion of carbon steels is a problem of enormous practical importance. In carbon steel, the typical corrosion process can be regarded as the thermodynamically favoured reverse reaction of the metal-winning (extraction) process as shown in Fig 9.
Fig 9 Chemical reaction of iron during corrosion and metal extraction reaction
Carbon steel (which include mild steels) is by its nature has limited alloy content, normally less than 2 % by weight for the total of all additions. Unfortunately, these levels of addition do not normally produce any remarkable changes in general corrosion behaviour. One possible exception to this statement is the weathering steels, in which small additions of copper, chromium, nickel, and / or phosphorus produce significant reductions in corrosion rate in certain environments. At the levels of various elements in which they are present in the carbon steel, the elements have no significant effect on corrosion rate in the atmosphere, neutral waters, or soils. Only in the case of acid attack, an effect observed. In this case, the presence of phosphorus and sulphur markedly increase the rate of attack. Indeed, in acid systems, the pure irons appear to show the best resistance to attack.
Corrosion reactions take place when conditions are thermodynamically in favour of the chemical reactions. When this happens, then potential other factors drive the speed of the reaction (kinetics of the reaction). The rate of corrosion is highly dependent on the environment, in which the carbon steels are used. In solving a particular corrosion problem, a dramatic change in attack rate can frequently be attained by altering the corrosive environment. Since corrosion is such a multi-faceted phenomenon, it is normally useful to attempt to categorize the different types based upon the environmental basis, such as atmospheric corrosion, aqueous corrosion, corrosion in soils, concrete, and boilers etc.