Microstructures of Iron and Steels
Microstructures of Iron and Steels
Iron is a chemical element with symbol Fe (from Latin word Ferrum). Its atomic number is 26 and atomic mass is 55.85. It has a melting point of 1,538 deg C and boiling point of 2,862 deg C. It is a metal in the first transition series. It is by mass the most common element on the earth, forming much of earth’s outer and inner core. It is the fourth most common element and the second most common metal in the earth crust. Steels contain over 95 % Fe.
Pure iron is polymorphic. Two allotropic phases exist for pure iron in solid state depending on the temperature. High purity iron is very weak. The ability of iron to accommodate heavy interstitial elements, namely carbon and nitrogen, is mostly responsible for the strength and the hardening effects.
Pure iron is a common metal but it is mostly confused with other materials such as steel and wrought iron. All these materials vary in composition. The carbon content of pure iron makes it unique and different from the other metals and ferrous alloys. The carbon content in pure iron is always less than 0.008 %. Wrought iron has a higher carbon content of up to 0.5 %. This shows how less the impurities are in the pure iron.
Pure iron has a body centred cubic (bcc) structure at ordinary temperatures and is ferro-magnetic with a curie temperature of 768 deg C. The magnetic and non-magnetic forms are normally referred to as alpha-iron and beta-iron respectively. Iron changes to the face-centered cubic (fcc) form (gamma-iron) on heating to 910 deg C and then back to the bcc structure form (delta-iron) at 1,394 deg C.
Cast iron is an iron-carbon cast alloy with other elements and is made by remelting pig iron, scrap, and other additions. For differentiation from steel and cast steel, cast iron is defined as a cast alloy with a carbon content (minimum 2.03 %) which ensures the solidification of the final phase with a eutectic transformation. Depending on chemical specifications, cast irons can be non-alloyed or alloyed. The range of alloyed irons is much wider, and they contain either higher quantities of common components, such as silicon and manganese, or special additions, such as nickel, chromium, aluminum, molybdenum, tungsten, copper, vanadium, titanium, plus others. Free graphite is a characteristic constituent of non-alloyed and low-alloyed cast irons.
Precipitation of graphite directly from the liquid occurs when solidification takes place in the range between the temperatures of stable transformation (Tst) and meta-stable transformation (Tmst), which are, respectively, 1,153 deg C and 1,147 deg C as per the iron-carbon phase diagram. In this case, the permissible under-cooling degree is dT max = Tst – Tmst. In case of a higher under-cooling degree, that is, in the temperatures below Tmst, primary solidification and eutectic solidification can both take place completely or partially in the meta-stable system, with precipitation of primary cementite or ledeburite.
Graphitization can also take place in the range of critical temperatures during solid-state transformations. The equilibrium of phases Fe (gamma) = Fe (alpha) + Fe3C occurs only at the temperature of 723 +/- 2 deg C, while equilibrium of phases Fe (gamma) = Fe (alpha) + C (graphite) occurs at the temperature of 738 +/- 3 deg C. Hence, in the range of temperatures 738 deg C to 723 deg C, the austenite can decompose only into a mixture of ferrite with graphite instead of with cementite
Steel is basically an alloy of iron and carbon to which sometimes other elements are added to achieve certain properties for specific performance of the steel. In steel, majority of the carbon is present as meta‐stable iron carbide called cementite. The upper limit of carbon content is 2 %. Iron-carbon phase diagram helps in guessing the structure of the steels and their properties.
The structure of steel depends on its composition. Steel normally has structure which develops under equilibrium rate of cooling. The steel on solidification is expected to have fully austenitic structure. It can be assumed to be homogeneous since the rate of cooling is considered to be slow. Depending on its composition, the steel has three types of structures. The three compositions of steel with regards to its structure are (i) % carbon – less than 0.02 (ii) % carbon – between 0.02 to 0.8. and (iii) % carbon – between 0.8 to 2.
The micro-structure of iron-based alloys is very complicated and diverse, being influenced by their composition, homogeneity, processing and section size. Micro-structures of castings look different than those of wrought products, even of the same composition and if given the same heat treatment. In general, it is easiest to identify heat-treated micro-structures after transformation and before tempering. For example, if a mixed micro-structure of bainite and martensite is formed during quenching, these constituents become more difficult to be identified reliably since the tempering temperature given to the product increases towards the lower critical temperature.
The micro-structure of ferrous alloys is a basic tenet of physical metallurgy. The composition and processing establish the micro-structure and the micro-structure influences several properties and service behaviour of steel. For maintaining control of the quality of steel products and to diagnose problems in processing, testing, or service, the micro-structure is to be identified first and, in some cases, quantified. This can only be accomplished when the person carrying out the metallography can properly distinguish the phases or constituents present. This also depends upon proper sample preparation and etching.
Terminology – There is a need to discuss the terminology describing the constituents in ferrous alloys, as there is confusion regarding certain terms and mis-usage. Certain terms, such as sorbite and troostite, were dropped from the metallographic lexicon in 1937 since they referred to micro-structural constituents inaccurately. However, such terms are still occasionally used.
The term ‘phase’ is frequently used incorrectly in reference to mixtures of two phases, such as pearlite or bainite. A phase is a homogeneous, physically distinct substance. Martensite is a phase when formed by quenching but becomes a constituent after tempering as it decomposes from body centered tetragonal (bct) martensite to bcc ferrite and carbides. In the process of describing and showing different phases and constituents in ferrous alloys, definitions are given.
Etchants – For ferrous metallography, nital and picral are the most widely used etchants for carbon and alloy steels. Nital consists of 2 milli-litre HNO3 (nitric acid) and 98 milli-litre ethanol or methanol (95 % or absolute, also amyl alcohol). Picral consists of 4 grams picric acid, 100 milli-litre ethanol or methanol (95 % or absolute, use of absolute alcohol only when acid contains 10 % or more moisture), and 4 drops or 5 drops 17 % zephiran chloride (wetting agent). Nital is normally used in concentrations of 1 % to 3 % HNO3 in ethanol or methanol. In solutions containing more than 4 % to 5% HNO3, only methanol is used, since ethanol becomes unstable as the concentration of HNO3 increases.
However, nital is not always the best reagent to use to properly reveal all micro-structures. It is unfortunate that some organizations prohibit use of picral since picric acid can be made to detonate under certain conditions. Picral is an excellent etchant for revealing certain micro-structural constituents in steel and accidents have been less common than for nital. Vilella’s reagent [5 milli-litres HCl, 1-gram picric acid, and 100 milli-litre ethanol or methanol (95 % or absolute)] is also exceptionally valuable for certain compositions and micro-structures. 10 % sodium meta-bi-sulphite (Na2S2O5) in water (10 % SMB) is a very good general-purpose reagent for steels, and safer to use than nital or picral, with a combination of the capabilities of both nital and picral.
If steels are to be examined for inclusions or nitrides, then the samples are not etched. For seeing the other micro-structural constituents, etching is needed. 2 % Nital is most commonly used. It is good for revealing the structure of martensite. Nital is also very good for revealing ferrite in a martensite matrix and for revealing ferrite grain boundaries in low-carbon steels. Picral is better for revealing the cementite in ferritic alloys and the structure of ferrite-cementite constituents, pearlite and bainite.
Nital and picral both dissolve ferrite but the dissolution rate of nital is a function of crystal orientation while the rate of picral is uniform. Aqueous sodium meta-bi-sulphite reveals ferrite grain boundaries, colour some of the ferrite grains (some stay white), reveals pearlite and bainite much like picral but also etches martensite nicely, as-quenched or tempered. Other reagents have their uses, especially when dealing with higher alloy steel grades, such as tool steels and stainless steels, or when trying to selectively reveal certain constituents or prior-austenite grain boundaries.
There are also ‘tint’ etchants which can be used to colour specific constituents in steels. These can be quite useful for identifying constituents, for studying grain size, and for detecting segregation and residual deformation. There are etchants which colour either ferrite or austenite. Unlike standard etchants which reveal only a portion of the grain boundaries, a colour etchant colours all grains. If the grains have a random crystallographic orientation, then a wide range of colours, randomly dispersed, is obtained. If texture is present, then a narrow range of colours is observed. Since colour etchants are selective, they are very useful for image analysis work where the contrast between what is needed to be measured and what is not needed to be measured is to be maximized. Klemm’s reagent [50 milli-litre saturated (in H2O) Na2S2O3 solution and 1-gram K2S2O5 (potassium meta-bi-sulphite)] is a popular ‘tint’ etchant.
Ferrite grain boundaries can be etched using nital in majority of the carbon steels, although Marshall’s reagent (100 milli-litre H2O, 8 grams oxalic acid, and 5 milli-litre H2SO4) appears to be the better etchant for low-carbon and decarburized steels. Ferrite grain boundaries in low-carbon, 1 % silicon, titanium bearing steels are difficult to delineate. In this case, Beraha’s tint etchant (3 grams K2S2O5, 10 grams Na2S2O3, and 100 milli-litre H2O) differentiates the grains by colour. Tint etchants such as Beraha’s or sodium meta-bi-sulphite (Na2S2O5) is not to be agitated. The sample is to be pre-etched 2 seconds to 3 seconds in picral or nital. A sample etched in a tint etchant cannot be re-immersed if the structure is too light, but repolishing for 15 seconds using 0.3 micrometre Al2O3 removes the tint etchant.
Pearlite, cementite (Fe3C), and carbides in all carbon steels can be etched using 4 % picral with zephiran chloride. Alloyed steels when etched using 4 % picral, can need a few drops HCl (hydro-chloric acid) for each 100 milli-litres of solution to improve contrast among the phases. Pearlite does not always appear lamellar, since the spacing between the cementite and ferrite constituting the phase cannot always be resolved using an optical (light) microscope. Special cases need other etchants to reveal pearlite, cementite, and other carbides. Bainite can be etched using picral or a picral-nital procedure in much the same way as pearlite.
Martensite, low-carbon lath-type, can be etched using 2 % nital or an 8 % aqueous solution of Na2S2O5. High-carbon plate-type martensite can also be etched in nital or Na2S2O5, although nital is preferred. Multi-phase micro-structures with martensite combined with bainite, pearlite, or ferrite can be etched using a 10 % aqueous solution of Na2S2O5 without over-etching any single phase. Internal oxidation is etched using 4 % picral. Non-metallic inclusions, if etched for 2 second to 4 second in aged 4 % picral, are more sharply contrasted.
A wide range of constituents is encountered in carbon and alloy steels. Single-phase constituents include austenite, ferrite, delta-ferrite, cementite, different alloy carbides, and martensite, as well as different inter-metallic phases, nitrides, and non-metallic inclusions. Two-phase constituents include tempered martensite, pearlite, and bainite. Non-metallic inclusions consisting of two or more phases can be present in steels.
Alpha Iron and ferrite – Fully ferritic steels are obtained only when the carbon content is low. The most obvious micro-structural features are the ferrite grain boundaries. Ferrite is a soft low-strength phase. If the ferrite grain size is fine, good ductility and formability are achieved. Since ferrite has a bcc crystal structure, ferritic steels show a transition from ductile to brittle behaviour as temperature decreases or as strain rate increases.
Alpha iron, strictly speaking, refers only to the bcc form of pure iron which is stable below 910 deg C, while ferrite is a solid solution of one or more elements in bcc iron. Frequently these terms are used as synonyms, which is not correct. Ferrite can precipitate from austenite in acicular form under certain cooling conditions. Strictly speaking, acicular means the shape of needle-like in three dimensions. However, this is not the actual shape of acicular ferrite in three dimensions. Fig 1 shows the appearance of ferrite grains in a low-carbon steel.
Fig 1 Ferrite grain structure in low carbon steel.
Nital is normally used to reveal the grain boundaries but it is orientation sensitive and does not bring out all of the ferrite grain boundaries. However, if a tint etchant which colours ferrite is used then all of the grains can be clearly observed.
There are also ferritic stainless steels with high chromium contents and very little carbon. Ferrite grain structures can be quite difficult to reveal in ferritic stainless steels using standard immersion or swabbing reagents. Ferrite is a very soft, ductile phase, although it loses its toughness below some critical temperature.
Gamma iron and austenite – Gamma iron, as with alpha iron, pertains to only the fcc form of pure iron which is stable between 910 deg C and 1394 deg C. Austenite is a solid solution of one or more elements in fcc iron. Again, these terms are frequently used interchangeably, but that is not correct. For heat treatable steels, austenite is the parent phase for all transformation products which make ferrous alloys so versatile and useful commercially.
Austenite is not stable at room temperature in ordinary steels. Chromium-nickel steels, known as austenitic stainless steels, is a family of very important grades where austenite is stable at room temperature. Fig 2 shows an example of the micro-structure of type 316 austenitic stainless steel. In Fig 3a, a 316 sample was swab etched with Kalling’s No. 2 reagent (ethyl alcohol – 85 % to 95 %, copper (II) chloride – 0.5 % to 5 %, hydrogen chloride – 2 %n to 7 %, methyl alcohol – 1 % to 5 %, and isopropyl alcohol – 1 % to 5 %) (‘waterless’ Kalling’s) and not all of the boundaries are revealed. Using a tint etch which colours austenite in such grades, all the grain structure can be brought out. There are other austenitic iron-based alloys, such as Hadfield manganese steel and iron-nickel magnetic grades. Fig 2 shows austenitic grains with annealing twins revealed with different procedures.
Fig 2 Austenitic grains with annealing twins revealed with different procedures
Austenite is a soft, ductile phase which can be work hardened to high strength levels. For case-hardened carburized steels and high-carbon, high alloy steels, such as tool steels, use of an excessively high austenitizing temperature dissolves excessive quantities of carbide which depresses the temperatures where martensite begins and completes its transformation to such an extent that austenite is present (but not necessarily stable) at room temperature (called retained austenite).
Fig 3 shows an example of excessive retained austenite (and coarse plate martensite) in a D3 (2.1 % carbon, 12 % chromium, 0.5 % nickel, and 0.35 % manganese) tool steel sample which has been austenitized at 1,120 deg C, well above the recommended temperature, which dissolved an excessive quantity of carbide leading to partial hardening. The micro-structure shows the coarse plate martensite and the massive Cr7C3 carbide which has not been dissolved. Excessive retained austenite in tool steels is normally detrimental to die life, since it can transform to fresh martensite and cause cracking, or reduce wear resistance.
Fig 3 Over-austenitized steel with excessive quantities of retained austenite
Retained austenite in a carburized gear tooth is not normally detrimental as the teeth are not normally shock loaded, so that the retained austenite transforms to martensite, and the toughness of the austenite, when stabilized, can be beneficial. There are grades of stainless steel where the composition is balanced to produce around equal quantities of ferrite and austenite at room temperature. Fig 4a shows the microstructure of 2205 duplex stainless steel.
Fig 4 Micro-structures showing austenite and delta ferrite
Obtaining fully austenitic steels needs careful balancing of chemical composition, that is, large quantities of the austenite-stabilizing elements (carbon, nitrogen, nickel, and manganese) are to be present compared with those elements which stabilize ferrite. Examples of fully austenitic ferrous alloys are austenitic stainless steels and austenitic manganese steel. The most visible micro-structural features of these single-phase alloy steels are the austenite grain boundaries. These alloy steels also contain annealing twins in the wrought, solution annealed conditions. Austenite is also a soft low-strength phase. However, cold working produces substantial strengthening and, if extensive, can produce strain-induced martensite. Because of their fcc crystal structures, austenitic alloy steels remain ductile irrespective of temperature or strain rate unless phase changes occur.
Delta iron and delta ferrite – Delta iron is the bcc form of pure iron which is stable above 1,394 deg C to the melting point of 1,538 deg C, while delta ferrite is the stable high temperature solid solution of one or more elements in bcc iron. Delta ferrite can be observed in as-cast austenitic stainless steels (it is normally to be put into solution after hot working and solution annealing), in some precipitation hardened stainless steels, when the composition is not balanced to avoid it, in some martensitic stainless steels, and in some tool steels.
Delta ferrite is normally considered detrimental to transverse toughness when it is present in a hardened structure. Delta ferrite is not always detrimental. When austenitic stainless steels are welded, the composition of weld metal is adjusted to produce a certain level of delta ferrite in the as-cast structure to minimize the risk of hot cracking. Fig 4b shows the structure of type 312 weld metal used to weld 316 stainless-steel. The delta ferrite forms in the last regions to solidify and has an inter-connected dendritic appearance.
Cementite and graphite – Carbon in iron exists as either as cementite or graphite. Cementite is a compound of iron and carbon with the approximate formula Fe3C and an ortho-rhombic crystal structure. Some substitution of other carbide forming elements, such as manganese and chromium is possible. Hence, it is more normal to refer to the formula as M3C, where M stands for metal. But, only small quantities of the different carbide forming elements can be substituted before alloy carbides of other crystal structures and formulae are formed.
Cementite or iron carbide, contains 6.67 % carbon, corresponding to the formula Fe3C. It is normally the terminus for the Fe-C phase diagram. Cementite is hard. Pure Fe3C has a hardness of around 800 HV (Vickers hardness). Substitution of other elements for some of the iron in cementite increases the hardness appreciably. The hardness of cementite is up to around 1,400 HV for highly alloyed M3C. Cementite is brittle. Only limited quantities of cementite are present in steels because of its brittleness. Fig 5 shows cementite in white cast iron. Picral is a good etch for revealing carbides as it outlines the massive cementite and also reveals the cementite in the pearlite. Several reagents can preferentially colour cementite.
Fig 5 Cementite and pearlite in white cast iron and graphite in hyper-eutectic gray iron
Graphite is the stable form of carbon in iron (mainly observed in cast iron) while cementite is meta-stable and can transform to graphite under long-term, high-temperature exposure. Graphite in cast iron can take several forms, such as flakes (Fig 5) of different sizes and distributions in gray cast iron and nodules in ductile cast iron. But there are other shapes such as temper nodules in malleable cast iron and short, stubby flakes in compacted cast iron. Graphite is occasionally observed in steels, where it is either deliberately created (as in graphitic tool steels or accidentally created by long-term, high-temperature exposure.
Pearlite – Pearlite is a mixture of ferrite and cementite in which the two phases are formed from austenite in an alternating lamellar pattern. Formation of pearlite needs relatively slow cooling from the austenite region and depends on the steel composition. Pearlite forms at temperatures below the lower critical temperature of the steel in question and can be formed isothermally or by continuous cooling. As the hardenability of the steel decreases, the cooling rate can be increased without the formation of other constituents. As isothermal reaction temperature decreases or the cooling rate increases, the inter-lamellar spacing decreases. The strength and toughness of pearlitic steels increase as the inter-lamellar spacing decreases.
Since the maximum solubility of carbon in ferrite is nearly zero at room temperature and a fully pearlitic micro-structure is obtained when a steel containing 0.76 % carbon is slowly cooled from the austenite region, the volume fractions of ferrite and pearlite can be estimated. In low-carbon steels, ferrite forms before the eutectoid reaction, which produces pearlite, and is termed pro-eutectoid ferrite. Below around 0.4 % carbon, the pro-eutectoid ferrite forms as equi-axed patches and is the continuous phase. Above around 0.4 % carbon, the pro-eutectoid ferrite normally exists as isolated equi-axed patches or as a grain-boundary layer, depending on thermal history.
Carbon steels are referred to as hypo-eutectoid, eutectoid, or hyper-eutectoid when their carbon contents are below 0.76 %, around 0.76 %, or above 0.76 % respectively. In the case of hyper-eutectoid steels, excess cementite above the quantity needed to form pearlite precipitates in the austenite grain boundaries before the eutectoid reaction. This excess cementite is referred to as pro-eutectoid cementite. A grain-boundary cementite network embrittles such steels.
Pearlite increases the strength of carbon steels. Refining the inter-lamellar spacing also increases the strength and the toughness, as well. In a slowly cooled steel, the quantity of pearlite increases to 100 % as the carbon content increases to the eutectoidal carbon content, 0.76 %. Pure ferrite (no carbon) has a hardness of around 70 HV. The hardness of a fully pearlitic eutectoidal steel varies with the inter-lamellar spacing from around 250 HV to 400 HV as the spacing decreases. Fine pearlite is the most desirable structure for wire drawing, where extremely high strengths can be achieved. Pearlite can be cold drawn to exceptionally high tensile strengths, as in piano wire, which also shows considerable ductility.
Carbon steels are widely used in the hot-rolled condition. The austenite grain size of the steel as it enters the final rolling pass establishes the relative sizes of the ferrite and pearlite produced during subsequent air cooling, but the cooling rate influences the fineness of the pearlite, the morphology of the pro-eutectoid ferrite, and the quantities of the different constituents.
Pearlite is a meta-stable lamellar aggregate of ferrite and cementite which forms at temperatures below the lower critical temperature (the temperature where austenite starts forming from ferrite and cementite upon heating). With time and temperature, the cementite in the pearlite becomes spheroidized, that is, it changes from a lamellar to a spheroidal shape. Spheroidizing the cementite reduces the strength and hardness of the sample while increasing its ductility. The degree of change is a function of the carbon content of the steel.
Pearlite is formed by the eutectoid reaction. A eutectoid transformation is an isothermal, reversible reaction in which a solid solution (austenite) is converted into two intimately mixed solid phases, ferrite and cementite. All eutectoidal products are lamellar, even in non-ferrous systems. For steels with carbon contents below the eutectoidal value, ferrite precipitates before the eutectoidal transformation and is called pro-eutectoid ferrite. Fig 6 shows pro-eutectoid ferrite and lamellar pearlite in a sample of steel.
Fig 6 Micro-structure of pro-eutectoid ferrite and pearlite
In Fig 6, the ferrite is white while the pearlite is dark as the lamellae are much too finely spaced to be resolved at this magnification. Fig 6 also shows coarse pearlite in a fully annealed sample of 4140 alloy steel where the lamellae can be resolved. The cementite lamellae appear dark while the ferrite remains white. In steels with carbon contents above the eutectoidal composition, cementite precipitates in the grain boundaries before the eutectoid reaction occurs and is called pro-eutectoid cementite.
Bainite – If the cooling rate is faster than air, or if alloying elements are added to the steel to increase hardenability, a different two-phase constituent can be observed, called bainite. Bainite is a lath-like meta-stable aggregate of ferrite and cementite which forms from austenite at temperatures below where pearlite forms and above the temperature where martensite starts to form. Bainite is normally classified as upper bainite or lower bainite. Upper bainite forms isothermally or during continuous cooling at temperatures just below those which produce bainite. Lower bainite forms at still lower temperatures, down to the Ms temperature or slightly below in certain cases.
The appearance of bainite changes with the transformation temperature being called ‘feathery’ in appearance at high temperatures and ‘acicular’ at low transformation temperatures. The feathery appearance of upper bainite is also influenced by carbon content and is most appropriate for grades with high carbon contents. The acicular description is not a prefect description of the shape of lower bainite.
Formation of upper bainite begins by growth of long ferrite laths devoid of carbon. Since the carbon content of the ferrite laths is low, the austenite at the lath boundaries is enriched in carbon. The shape of the cementite formed at the lath boundaries varies with carbon content. In low-carbon steels, the cementite precipitates as discontinuous stringers and isolated particles, but at higher carbon contents the stringers are more continuous. In some cases, carbide is not precipitated, but is retained as austenite or transforms to plate martensite.
Lower bainite has a more platelike appearance than upper bainite. The ferrite plates are broader than those in upper bainite and are more similar in appearance to plate martensite. As with upper bainite, the appearance of lower bainite varies with carbon content. Lower bainite is characterized by formation of rod like cementite within the ferrite plates. Fig 7 shows micro-structures of upper bainite and lower bainite in partially transformed 5160 alloy steel samples.
Fig 7 Micro-structures of upper bainite and lower bainite
Martensite – When carbon or alloy steels are hot worked, they are in the austenitic condition. Subsequent cooling results in the transformation of austenite to other phases or constituents. If a carbon or low alloy steel is air cooled after hot rolling, a diffusion-controlled transformation occurs, when ferrite precipitates first, followed by pearlite. Normalizing is a heat treatment process used to refine the grain structure of carbon and low-alloy steels. The steel is austenitized at a somewhat higher temperature than used for quench hardening, followed by air cooling to produce fine ferrite and pearlite.
Martensite is formed if the cooling rate from the austenitizing temperature is rapid enough (a function of section size, hardenability, and quench medium). Martensite is a generic term for the bct phase which is formed by the diffusion less transformation. and the parent and product phases have the same composition and a specific crystallographic relationship. The degree of tetragonality increases with carbon content. Tempering decreases the strength of martensite, but increases its toughness. However, tempering of alloy steels within certain temperature ranges can reduce toughness because of embrittlement (temper martensite embrittlement or temper embrittlement). However, tempering, along with composition selections, can permit achievement of a wide range of useful strengths and toughness.
Martensite can be formed in alloy steels where the solute atoms occupy interstitial sites, as for carbon in iron, producing substantial hardening and a highly stained, brittle condition. However, in carbon-free alloy steels with high nickel contents, such as maraging steels, the solute atoms (nickel) can occupy substitutional sites, producing martensites which are soft and ductile. In carbon-containing steels, the appearance of the martensite changes with carbon in the interstitial sites.
Martensite is not an equilibrium phase in steels. Formation of martensite depends on chemical composition and cooling rate from the high-temperature austenite region. Unlike other austenite transformation products, martensite normally forms instantaneously once the sample is cooled below a specific temperature, the martensite start (Ms) temperature, which is a function of the carbon and alloy content of the parent austenite phase. The transformation is completed when the sample reaches a lower temperature, the martensite finish (Mf) temperature. The hardness of martensite is governed primarily by carbon content, but is also influenced by the alloy content. The ability to form martensite in a steel as a function of section size and quench rate depends on the hardenability of the steel. Hardenability is increased by increasing carbon and alloy contents and by enlargement of austenite grain size. Grain size is rarely coarsened to improve hardenability in wrought steels, since majority of the mechanical properties are reduced.
Low carbon steels produce ‘lath’ martensites while high carbon steels produce ‘plate’ martensite, frequently incorrectly called ‘acicular’ martensite, when all of the carbon is dissolved into the austenite. Fig 8 shows lath martensite. When quenched from the proper temperature, so that the correct quantity of cementite is dissolved and the grain size is quite fine, martensite appears virtually feature-less by light microscopy, as shown in Fig 8 for 52100 bearing steel. Fig 8 also shows the structure of martensite in nearly carbon-free 18Ni250 maraging steel.
Fig 8 Martensite micro-structure in steels
Carbon content markedly influences the nature of martensite. Basically, two types of martensite can be formed in steels. At low carbon contents, lath martensite is formed. The laths are present in a packet arrangement in which the individual laths within the packet have essentially the same orientation. At high carbon contents, plate martensite is formed. The plates form as individual lenticular crystals in a wide range of sizes. At intermediate carbon contents, mixtures of lath and plate martensite are obtained.
The strength and hardness of martensite varies linearly with carbon in austenite up to around 0.5 % carbon. As the carbon in the austenite increases beyond 0.5 %, this curve starts to flatten and then goes downward because of the inability to convert the austenite fully to martensite (retained austenite becomes increasing present). Hence, when high carbon steels are heat treated, the austenitizing temperature is selected to dissolve no more than around 0.6 % carbon into the austenite.
Because of the important influence of grain size on the properties of martensitic steels, much effort has been made on reducing the grain size of such steels. However, unlike ferritic and austenitic alloy steels, the critical grain size for martensitic steels is that of the parent austenite phase, that is, the prior austenite grain size. Delineation of the prior austenite grain boundaries in martensitic steels using selective etchants is difficult, but frequently can be achieved. In general, the low-carbon martensitic steels are more difficult to etch in this manner than medium-carbon and high-carbon steels. In the case of lath martensite, the packet size is also an important micro-structure measurement. The hardness and strength of martensite increase with increasing carbon content. However, it also becomes more brittle.
Other constituents – There are other minor constituents in steels, such as non-metallic inclusions, nitrides, carbo-nitrides, and inter-metallic phases, such as sigma and chi. Non-metallic inclusions are of two types namely (i) indigenous inclusion, and (ii) exogenous inclusion. Indigenous inclusions form as a natural result of the decrease in solubility of oxygen or sulphur which occurs as the metal freezes. Exogenous inclusions are introduced from external sources, for example, slag or refractories, which enter the steel and become trapped during solidification. In majority of the cases, these included phases are undesirable.
Several terms are used in reference to inclusions. Nitrides and carbo-nitrides result when certain nitride forming elements are present in adequate quantities, e.g., aluminum, titanium, niobium, and zirconium. A certain quantity of nitrogen is always present in the melt and this varies with the melting procedure used. Electric furnace steels normally have around 100 ppm (parts per million) nitrogen while basic-oxygen furnace steels have around 60 ppm nitrogen. Aluminum nitride is extremely fine and can be seen only after careful extraction replica work with the TEM (transmission electron microscopy). The other nitrides are frequently visible in the light microscope, although sub-microscopic size nitrides can also be present. Sigma and chi can be produced in certain stainless steels after high temperature exposure.