Austenitic Stainless Steels
Austenitic Stainless Steels
Stainless steel is the term used to describe an extremely versatile family of engineering materials, which are selected primarily for their corrosion and heat resistant properties. All the stainless steels contain principally iron and a minimum of 10.5 % chromium (Cr). At this level, chromium reacts with oxygen (O2) and moisture in the environment to form a protective, adherent, and coherent oxide film which envelops the entire surface of the material. This oxide film (known as the passive or boundary layer) is very thin (2 to 3 nano-metres).
The passive layer on stainless steels shows a truly remarkable property, i.e., when damaged (e.g. abraded), it repairs itself since chromium in the steel reacts rapidly with oxygen and moisture in the environment to reform the oxide layer. Increasing the chromium content beyond the minimum of 10.5 % confers still higher corrosion resistance. Corrosion resistance can be further improved, and a wide range of properties provided, by the addition of 8 % or more nickel (Ni). The addition of molybdenum (Mo) further increases corrosion resistance (in particular, resistance to pitting corrosion), while nitrogen (N2) increases mechanical strength and improves resistance to pitting.
Reference is frequently made to stainless steel in a singular sense as if it is one material. Actually, there are over 50 stainless steel alloys. Three general classifications are used to identify stainless steels. They are (i) metallurgical structure, (ii) the AISI (American Iron and Steel Institute) numbering system namely 200, 300, and 400 series numbers, and (iii) the unified numbering system (UNS).
The family of stainless steel has several branches, which can be differentiated in a variety of ways e.g., in terms of their areas of application, by the alloying elements used in their production, or, perhaps the most accurate way, by the metallurgical phases present in their microscopic structures. There are five classes of stainless steel namely (i) austenitic, (ii) ferritic, (iii) martensitic, (iv) duplex, and (v) precipitation hardening. These are named as per their micro-structure resembles a similar micro-structure in steel. The properties of these classes differ but are necessarily the same within the same class.
Within each of these groups, there are several ‘grades’ of stainless steel defined as per the ranges of their composition ranges. These ranges of composition are defined in national and international standards, and within the specified range, the grade of stainless-steel shows all of the desired properties (e.g. corrosion resistance and / or heat resistance and / or machineability).
The fundamental criterion in the selection of a stainless steel is normally that it can survive with virtually no corrosion in the environment in which it is to be used. Good engineering practice sometimes needs that the materials be selected for sufficient, but finite, service life. This is especially true for high-temperature service, for which creep and oxidation lead to limited life for all materials. The choice among the stainless steels which can be used in that environment is then based on the alloy from which the component can be produced at the lowest cost, including maintenance, over the intended service life.
The ferritic stainless steels are less expensive for the same corrosion resistance but sometimes are found lacking because of (i) lack of toughness, as is the case at sub-ambient temperatures or in thicknesses higher than around 1.5 mm, (ii) lack of high ductility, specifically if more than around 30 % elongation is needed, and (iii) susceptibility to high-temperature embrittling phases when moderately alloyed.
The less-expensive martensitic grades are used instead of austenitic when high strength and hardness are better achieved by heat treatment rather than by cold working, and mechanical properties are more important than corrosion resistance. This is also the case for the more expensive precipitation hardening grades, which can achieve corrosion resistance only matching the least corrosion resistant of the austenitic alloys.
Duplex grades match austenitic grades in corrosion resistance and have higher strength in the annealed condition but present the designer with challenges with regard to embrittling phases which can form with prolonged exposure to high temperatures and only moderate ductility like the ferritic alloys.
Austenitic stainless steels are mainly segregated into the following two series namely 200 series and 300 series.
200 series – Stainless steels with a low nickel and high nitrogen content are classified as 200 series. These are chromium-nickel-manganese (Cr-Ni-Mn) austenitic stainless steels. Grade 201 is hardenable through cold working while the grade 202 is a general-purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.
300 series – The most common austenitic stainless steels are iron-chromium-nickel steels and are widely known as the 300 series. In this series the most widely used austenitic stainless steel is the grade 304, also known as 18/8 stainless steel for its composition of 18 % chromium and 8 % nickel. The second most common austenitic stainless steel in this series is the grade 316, also called marine grade stainless steel, used primarily for its increased resistance to corrosion. A typical composition of 18 % chromium and 10 % nickel, commonly known as 18/10 stainless steel, is frequently used in cutlery and high-quality cookware.
Besides the above two series, there are super grades of austenitic stainless steel which show high resistance to chloride pitting and crevice corrosion because of high molybdenum content (higher than 6 %) and nitrogen additions. Higher nickel content ensures better resistance to stress-corrosion cracking (SCC) than the stainless steels of the 300 series. The higher alloy content of super austenitic steels makes them more expensive.
The straight grades of stainless steel contain a maximum of 0.08 % carbon (C). In these grades, there is no requirement of minimum carbon in the specification. The ‘L’ grades are used to provide extra corrosion resistance after welding. The letter ‘L’ after a grade of stainless steel indicates low carbon (as in 304L). The carbon is kept to 0.03 % or under to avoid carbide precipitation. Carbon in steel, when heated to temperatures in what is called the critical range (430 deg C to 870 deg C) precipitates out, combines with the chromium and gathers on the grain boundaries. This deprives the steel of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By controlling the quantity of carbon, this is minimized. For weldability, the ‘L’ grades are used. However, the ‘L’ grades are more expensive. In addition, carbon, at high temperatures imparts great physical strength.
The ‘H’ grades contain a minimum of 0.04 % carbon and a maximum of 0.10 % carbon and are designated by the letter ‘H’ after the steel grade. ‘H’ grades are primarily used at extreme temperatures as the higher carbon helps the material retain strength at extreme temperatures.
All austenitic stainless steels contain a small quantity of ferrite. Conventional austenitic grades of stainless steel can contain traces of delta ferrite, for improved weldability. Normally this quantity of ferrite is not enough to attract a normal magnet. However, if the balance of elements in the steel favours the ferritic end of the spectrum, it is possible for the quantity of ferrite to be sufficient to cause a significant magnetic response. Also, some types of stainless steels are deliberately balanced to have a considerable quantity of ferrite.
The austenitic grades are the most commonly used grades of stainless mainly since, in several cases, they provide very predictable levels of corrosion resistance with excellent mechanical properties. Using them wisely can save the design engineers considerably costs in their product. They are a user-friendly metal alloy with life-cycle cost of fully manufactured products lower than several other materials. The austenitic alloys can have compositions anywhere in the portion of the Schaeffler-Delong stainless steels constitution diagram labeled austenite shown in Fig 1. This diagram has designed to show which phases are present in alloys in the as-solidified condition, such as found in welds. Hence, it also applies to castings and continuously cast products.
Fig 1 Schaeffler-Delong stainless steels constitution diagram
As a practical matter of castability, the composition of most commercial alloys falls along the zone of several percent ferrite as cast. The salient feature of austenitic alloys is that as chromium and molybdenum are increased to increase specific properties, normally corrosion resistance, nickel or other austenite stabilizers are to be added if the austenitic structure is to be preserved.
Austenitic stainless steels are most easily recognized as non-magnetic. They are extremely formable and weldable, and they can be successfully used from cryogenic temperatures to the red-hot temperatures of furnaces and jet engines. Stainless steels play an important role in the modern world, even if its tonnage represents only around 2 % of the total steel production. If the cost of the nickel which helps stabilize the austenitic structure of these steels, these steels can be used even more widely.
Austenitic stainless steels are the most common and widely known types of stainless steels. They make up over 70 % of total production of stainless steels. These steels contain around 16 % to 25 % chromium and sufficient nickel and / or manganese to retain an austenitic micro-structure (Fig 2) at all the temperatures from cryogenic region to the melting point of the stainless steel. Austenitic stainless steels can also contain nitrogen in solution. Although nickel is the alloying element most commonly used to produce austenitic stainless steels, nitrogen can also be used to produce austenitic stainless steels. Both chromium and nitrogen contribute to the high corrosion resistance of these steels. The austenitic stainless steels are more easily recognized because of their non-magnetic properties. Austenitic steels are non-magnetic because of the non-magnetic characteristic of the face centered cubic (fcc) structure of austenite (Fig 2).
Fig 2 Structure of austenitic stainless steel
Austenitic stainless steel (ASS) was invented in the beginning of the 20th century. The continuing development this steel has resulted in complex steel compositions with substantial quantities of alloying elements. These alloying elements are of course introduced in the steel for one or more reasons but the final aim is mainly to achieve better mechanical properties (especially high creep strength and high creep-rupture ductility) and / or higher corrosion resistance (especially oxidation resistance in the case of high temperature application). As usual, the benefits of such additions invariably come attached to unavoidable disadvantages of which the most important are the potential microstructural instability and difficult processing of the material.
Properties and of stainless steels – Austenitic stainless steels are non-magnetic and are not heat treatable. They cannot be hardened by heat treatment. However, they can be cold worked to improve hardness, strength and stress resistance. A solution annealing (heating within the range 1,000 deg C to 1,200 deg C followed by quenching or rapid cooling) restores the stainless steels original condition, including removal of alloy segregation and re-establishment of ductility after cold working. Stainless steels can be subjected to solution annealing. Because of the solution annealing the carbides, which can have precipitated (or moved) at the grain boundaries, are put back into solution (dispersed) into the matrix of the metal by the annealing process. ‘L’ grades are used where annealing after welding is impractical.
Austenitic stainless steels can be made soft enough (i.e. with yield strength of around 200 newton per square millimeters (N/sq mm) to be easily formed by the same tools which work with carbon steel, but they can be made incredibly strong by cold work, up to yield strengths of over 2,000 N/sq mm. Their austenitic (fcc, face centered cubic) structure is very tough and ductile down to absolute temperature. They also do not lose their strength at high temperatures as rapidly as ferritic (bcc, body centered cubic) iron base alloys.
Austenitic grades of stainless steels are the most common used grades, mainly since they provide very predictable level of corrosion resistance with excellent mechanical properties. The least corrosion resistant versions can withstand the normal corrosive attack of the everyday environment which people experience, while the most corrosion resistant grades can even withstand boiling seawater. Austenitic stainless steels have good formability and weldability, as well as excellent toughness, particularly at low, or cryogenic, temperatures. Austenitic grades also have a low yield stress and relatively high tensile strength. They have excellent corrosion resistance and excellent high-temperature tensile and creep strength.
Austenitic stainless steels are not very strong materials. Typically, their 0.2 % proof stress is around 250 N/sq mm and the tensile strength between 500 N/sq mm and 600 N/sq mm, showing that these steels have substantial capacity for work hardening, which makes working more difficult than in the case of mild steel. However, austenitic stainless steels possess very good ductility with elongations of around 50 % in tensile tests.
Austenitic stainless steels are also highly resistant to high temperature oxidation because of the protective surface film, but the normal grades have low strengths at high temperatures. Those steels stabilized with titanium (Ti) and niobium, grades 321 and 347, can be heat treated to produce a fine dispersion of TiC or NbC which interacts with dislocations generated during creep. One of the most commonly used alloys is 25Cr20Ni with additions of titanium or niobium which possesses good creep strength at temperatures as high as 700 deg C.
Austenitic stainless steels are ductile over a wide temperature range, from cryogenic to creep temperatures. They do not display brittle fracture. Their tensile strength is high at low temperatures. They can be work hardened to high levels of strength by cold forming.
Austenitic stainless steels are less resistant to cyclic oxidation than are ferritic grades since their higher thermal expansion coefficient tends to cause the protective oxide coating to spall. They can experience stress corrosion cracking if used in an environment to which they have insufficient corrosion resistance. The fatigue endurance limit is only around 30 % of the tensile strength (against 50 % to 60 % for ferritic stainless steels). This, combined with their high thermal expansion co efficient, makes them especially susceptible to thermal fatigue. However, the risks of these limitations can be avoidable by taking special precautions.
The salient feature of austenitic stainless steels is that as chromium and molybdenum contents are increased to increase specific properties, normally corrosion resistance, nickel or other austenitic stabilizers are to be added if the austenitic structure is to be preserved. The tensile properties in the annealed state not surprisingly relate well to composition. The 0.2 % yield strength applies to the austenitic stainless steels.
Austenitic stainless steels have several advantages from a metallurgical point of view. Their properties include good to excellent corrosion resistance. They can be work hardened. They can be easily machined and fabricated to tight tolerances. They have smooth surface finish which can be easily cleaned and sterilized. They are temperature resistant from cryogenic to high heat temperatures.
Chemical compositions – Austenitic stainless steels constitute a very large steel class in terms of alloys and usage. In addition to iron, the main components are chromium to improve corrosion resistance and nickel to stabilize austenite. Chromium contents range from 15 % to 26 % and nickel contents from 5 % to 37 %. The 200 series has a lower nickel content than the 300 series. These steels have a high manganese content up to 15.5 % and also a high nitrogen content which partly replaces nickel as austenite stabilizer. In some steels one can find 2 % to 4 % of molybdenum. Molybdenum is primarily introduced for improving the resistance against pitting corrosion but it is also efficient in promoting solid solution hardening. More recently developed steels, known as super-austenitic stainless steels, can contain up to 6 % molybdenum.
The term super-austenitic relates to austenitic stainless steels containing large quantities of chromium, nickel, molybdenum and nitrogen, resulting in an iron (Fe) content close to or less than 50 %. One of the most well-known super-austenitic stainless steels is the UNS S32654 (also known as 654 SMO) with a composition of Fe-0.02C-3Mn-24Cr-7.3Mo-22Ni-0.5Cu-0.5N %.
In the majority of the austenitic stainless steels, the maximum silicon (Si) content is 1 %. However, higher silicon contents between 1 % and 3 % can improve oxidation or scaling resistance. Even higher silicon contents up to 5 % are used in certain cases for improving the corrosion resistance in nitric acid (HNO3). Other alloying elements such as copper, boron (B), or sulphur (S) are sometimes added to the austenitic stainless steels. Using low carbon content (such as AISI 304L, 316L and 317L) or / and titanium or niobium stabilized alloys (such as AISI 321 and 347), it is possible to minimize intergranular attack in austenitic stainless steels.
The family of austenitic stainless steels has a wide variety of grades precisely tailored for specific applications such as house-hold and community equipment, transport, food industry, industrial equipment, chemical and power engineering, cryogenics, and building industry. The optimum choice of the grades depends on the service needs and this needs a clear understanding of the metallurgical parameters, which control the micro-structure and hence the mechanical properties, formability, and corrosion resistance. The physical metallurgy of stainless steels explains the tendency of alloying elements to form different phases, the transformation of austenite to martensite during cooling or straining, hardening processes, and formation of inter-metallic phases.
The chemical composition, and temperature have influence on the different physical properties of austenitic stainless steel such as coefficient of expansion, thermal conductivity, and magnetic permeability. There is variation in mechanical properties, such as tensile, fatigue, and creep strengths of austenitic stainless steels with temperature, composition, and microstructure. The mechanisms to strengthen the austenitic stainless steels are appropriate thermo-mechanical treatments, and grain refinement etc. Austenitic stainless steels lend themselves remarkably to deep drawing and cold rolling, where their work-hardening characteristics enable high strength levels to be achieved. Weldability is excellent, and welds, which do not transform to martensite during air-cooling, have mechanical properties similar to the base metal.
Austenitic stainless steels have several advantages from a metallurgical point of view. They can be made soft enough (i.e., with a yield strength of around 200 MPa) to be easily formed by the same tools which work with carbon steel, but they can also be made incredibly strong by cold work, up to yield strengths of over 2,000 MPa. Their austenitic (fcc, face-centered cubic) structure is very tough and ductile down to absolute zero. They also do not lose their strength at high temperatures as rapidly as ferritic (bcc, body-centered cubic) iron base steels.
The least corrosion-resistant versions can withstand the normal corrosive attack of the everyday environment which people experience, while the most corrosion-resistant grades can even withstand boiling sea-water. If these steels are to have any relative weaknesses, these are given below.
The first is that the austenitic stainless steels are less resistant to cyclic oxidation than are ferritic grades since their higher thermal expansion coefficient tends to cause the protective oxide coating to spall. The second is that they can experience stress corrosion cracking if used in an environment to which they have insufficient corrosion resistance. The third is that the fatigue endurance limit is only around 30 % of the tensile strength (against around 50 % to 60 % for ferritic stainless steels). This, combined with their high thermal expansion coefficients, makes them especially susceptible to thermal fatigue. However, the risks of these limitations can be avoidable by taking proper precautions.
The traditional way of displaying the austenitic stainless steels is to present 302 as a base. Fig 3 shows one such diagram. Diagrams such as these treat alloys as an evolutionary family tree and subtly mislead. Several alloys have been pushed toward obsolescence because of advances in processing. For example, 321 has been developed as an alloy in which the detrimental effects of carbon have been negated by addition of titanium. The widespread adoption of the argon oxygen decarburization (AOD) in the 1970s made this alloy unnecessary, except for special circumstances, since carbon can be cheaply removed routinely. Likewise, 302 gave way to the lower-carbon 304, for which the even lower carbon 304L is normally substituted and dually certified to qualify as either grade. While low carbon prevents sensitization, stabilized grades can still be preferred for special applications such as type 321 in aerospace and type 347 in refinery service. Similar inertia keeps the higher-nickel 300 series as the de-facto standard when the more cost-efficient high-manganese 200 series is the logical basic grade. The relevant types of austenitic alloys can nonetheless be rationalized with this diagram.
Fig 3 Family of Austenitic stainless steels
As chromium is added, oxidation resistance and corrosion resistance increase. Since nickel equivalents (manganese, nitrogen, and carbon etc.) are also to be added in matching quantities, austenite stability is also increased. If molybdenum, a chromium equivalent, is added, corrosion resistance but not oxidation resistance is improved. And, if nitrogen is the austenite stabilizer added to balance increases of chromium or molybdenum, then corrosion resistance is also increased. With small exceptions, that is the rationale of austenitic grade design. Silicon is used as an alloying element to promote oxidation resistance and resistance to corrosion by oxidizing acids. Copper is used to promote resistance to sulphuric acid (H2SO4). Rare earths make a more stably oxidation-resisting scale. Niobium increases creep resistance. Sulphur (S) and selenium (Se) increase machinability.
Austenitic stainless steels are classified into three groups namely (i) lean alloys, (ii) chromium nickel alloys, and (iii) chromium, molybdenum, nickel, and nitrogen alloys.
Lean alloys – Stainless steels with less than 20 % chromium and 14 % nickel fall into this category. Examples of these alloys are grades 301, 304 and 201. These are the largest portion of all the stainless steels being produced. These stainless steels are normally used when high strength or high formability is the main objective since the lower, yet tailorable, austenite stability of these stainless steels gives a high range of work hardening rates and good ductility. Richer stainless steels (e.g. grade 305) with minimal work hardening are the high alloy steels. The general-purpose stainless steel (grade 304) is within this group. Stainless steels of this category have sufficient corrosion resistance to be used in any indoor or outdoor environment. These stainless steels are easily weldable and formable and can be given several attractive and useful surface finishes.
Chrome nickel alloys – These stainless steels are used when the objective is high temperature oxidation resistance. This can be improved by silicon and rare earths. If the application needs high temperature strength, carbon, nitrogen, niobium (Nb), and molybdenum can be added. Stainless steels of grades 302B, 309, 310, 347 and different proprietary alloys are in this group.
Chromium, molybdenum, nickel, and nitrogen stainless steels – These stainless steels are used when corrosion resistance is the main objective. Elements such as silicon (Si) and copper (Cu) are added for resistance to specific environments. This group of stainless steels includes 316L, 317L, and 304L, and several proprietary grades.
Wrought alloys normally have cast counter-parts which differ mainly in silicon content. Types which need improved machinability have a high content of controlled inclusions, sulphides, or oxy-sulphides, which improve machinability at the cost of corrosion resistance. Carbon is kept below 0.03 % and designated an ‘L’ grade when prolonged heating because of the multi-pass welding of heavy section (higher than around 2 mm) or when welds needing a post-weld stress relief are anticipated.
Lean austenitic alloys constitute the largest portion of all stainless steel produced. These are mainly 201, 301, and 304. 316L in this group is a corrosion-resisting alloy since it is so pervasively used as a service center sheet item. Alloys with less than 20 % chromium and 14 % nickel fall into this category. Since they are stainless, it is normally taken for granted that these alloys do not corrode, and these alloys have sufficient corrosion resistance to be used in any indoor or outdoor environment, excluding coastal. These grades are easily weldable and formable and can be given several attractive and useful surface finishes, so they are very much general-purpose alloys. The typical compositions vary with end use, raw material cost factors, and the preference of a given manufacturer. The compositions of standard alloys are frequently fine-tuned to the intended end use.
The main difference among the lean austenitic alloys lies in their work-hardening rate. The leaner is the alloy, the lower is the austenite stability. As unstable alloys are deformed, they transform from austenite to the much harder martensite. This increases the work-hardening rate and increases ductility since it delays the onset of necking since higher localized deformation is more than offset by higher localized strain hardening. These grades are best viewed as a continuum with a lower boundary at 16 % Cr-6 % Ni and an upper boundary at 19 % Cr-12 % Ni. This represents the range from minimum to maximum austenite stability. This is the main distinction within this grade family. The basis for this is described below.
Martensite and austenite stability – The formation of martensite at room temperature can be thermo-dynamically possible, but the driving force for its formation can be insufficient for it to form spontaneously. However, since martensite forms from unstable austenite by a diffusion less shear mechanism, it can occur if that shear is provided mechanically by external forces. This happens during deformation, and the degree to which it occurs varies with composition as per ‘Md30 (deg C) = 551 – 462(% C + % N) – 9.2(% Si) – 8.1(% Mn) – 13.7(% Cr) – 29(% Ni + Cu) – 18.5(% Mo) – 68(% Nb) – 1.42 (GS – 8)’ (equation 1). This is the temperature at which 50 % of the austenite transforms to martensite with 30 % true strain. It is to be noted that even elements which are chromium equivalents in promoting ferrite are austenite stabilizers in that they impede martensite formation. This temperature is the common index of austenite stability.
This regression analysis has been generated for homogeneous alloys. If alloys are inhomogeneous, such as occurs when they are sensitized or when solute segregation occurs, as from welding, then the equation applies on a microscopic scale. Sensitized zones (i.e., the regions near grain boundaries where chromium carbides have precipitated) have a much higher tendency to transform to martensite. Fig 4a and 4b show the changes in phase structure as a function of composition over ranges which encompass these alloys.
Fig 4 Iron-chromium and iron-nickel phase diagrams
Martensite can be present in two different forms. The alpha′ form is the bcc magnetic form, while epsilon is a non-magnetic, hcp (hexagonal close-packed) form. The formation of epsilon against alpha′ is related to the stacking fault energy of the alloy, which is given by ‘Y300 SF (mJ per square meter) = Y0 SF + 1.59Ni – 1.34Mn + 0.06(Mn)square – 1.75Cr + 0.01(Cr)square + 15.21Mo – 5.59Si –60.69(C + 1.2N)square root + 26.27(C + 1.2N)(Cr + Mn + Mo)square root + 0.61[Ni x (Cr + Mn)]square root (equation 2)’.
Epsilon martensite formation is favoured in alloys of lower stacking fault energy. The fcc structures deform by slip between (111) planes. Viewed from these planes, the structure is a series of ABCABC atom arrangements. Slip between planes can result in an ABCA/CAB structure. This so-called stacking fault generates an hcp structure. With lower stacking fault energies, these are more readily formed, and epsilon pre-dominates. The stacking fault can also be viewed as two partial dislocations with the material between them faulted. These partial dislocations, when generated in abundance, cannot readily slip past one another and hence pile up, increasing work-hardening rates.
As in carbon and alloy steels, the martensite transformation can take place simply by cooling, but in the lean austenitic alloys the temperatures are well below ambient. The more stable alloys do not transform even with cryogenic treatment. Fig 5a shows the variation of martensite formation with temperature and true strain for 304. Martensite formed in these alloys is quite stable and does not revert until heated well above the temperatures (Fig 5b) at which it has been formed. The carbon levels of austenitic stainless steels are always relatively low, so strain-induced martensite is self-tempering and not brittle.
Fig 5 Property variation with temperature
Martensite has been found to form in unstable austenite because of the electrochemically induced super-saturation by hydrogen. Under conditions of cathodic charging, superficial layers have been found to transform to epsilon under conditions of intense hydrostatic compression. During subsequent outgassing, alpha’ has been found to form because of the reversals in the stress state. Martensite hence formed is, of course, susceptible to hydrogen embrittlement.
Mechanical properties – The tensile properties in the annealed state not surprisingly relate well to composition. The 0.2 % yield strength and tensile strength, respectively, are reported to follow the equations ‘YS(MPa) = 15.4[4.4 + 23(% C) + 32(% N) + 0.24(% Cr) + 0.94(% Mo) + 1.3(% Si) + 1.2(% V) + 0.29(% W) + 2.6(% Nb) + 1.7(% Ti) + 0.82(% Al) + 0.16(% ferrite) + 0.46(d to the power -1.5)’ (equation 3), and ‘TS (MPa) = 15.4[29 + 35(% C) + 55(% N) + 2.4(% Si) +0.11(% Ni) + 1.2(% Mo) + 5(% Nb) + 3(% Ti) + 1.2(% Al) + 0.14(% ferrite) + 0.82 (d to the power -1/2)’ (equation 4). In each case, d is the grain diameter in millimeters.
Another study has given the relationships as =’ YS(MPa) = 120 + 210[root (N +0.02)] + 2Mn + 2Cr + 14Mo + 10Cu + (6.5 – 0.054delta)delta + [7 + 35(N + 0.2)]d to the power -1/2’ (equation 5), and ‘TS = 470 +600(N + 0.2) + 14 Mo + 1.58delta + 8d to the power -1/2’ (equation 6). Again, d is grain diameter in millimeters, and delta is percent ferrite. The claimed accuracy for the latter set of equations is 20 MPa and is said to apply to both austenitic and duplex stainless steels, but clearly the tensile strength relationship is to break down for leaner alloys, such as 301, in which tensile strength increases with decreasing alloy content because of the effect of increasing alloying causing less transformation to martensite, which inarguably produces higher tensile strengths in austenitic stainless steels. Equation 3 is also to be favoured over equation 5 in that it accounts for carbon explicitly.
One other hardening mechanism is possible in austenitic stainless steels, and that is precipitation hardening. Majority of the precipitation-hardening stainless steels are unstable austenite, which is transformed to martensite before the precipitation hardening takes place. One commercial alloy, A-286, is entirely austenitic and employs the precipitation within the austenite matrix of Ni3 (titanium, aluminum) for strengthening.
Austenitic stainless steels do not have a clear yield point but can begin to deform at as little as 40 % of the yield strength. As a rule of thumb, behaviour at less than half the yield strength is considered fully elastic and stresses below two-thirds of the yield strength produce negligible plastic deformation. This quasi-elastic behaviour is a consequence of the several active slip systems in the fcc structure. Even highly cold-worked material shows this phenomenon, although stress-relieving cold-worked material causes dislocations to ‘lock in place’ and form more stable dislocation arrays which break loose at a higher and distinct yield point.
The tensile properties of austenitic stainless steels with unstable austenite, that is, those with Md30 temperatures (equation 1) near room temperature, are very strain rate dependent. This is simply because of the influence of adiabatic heating during testing increasing the stability of the austenite. Tests run under constant temperature conditions, either by slow strain rates or use of heat sinks, produce lower tensile strengths. Hence, reported tensile strengths are not to be taken as an absolute value but a result which can be considerably changed by changes in testing procedure, even with accepted norms and standards.
Highly cold-worked austenitic stainless steels are frequently used for their robust mechanical properties. Few metallic materials can match the very high strengths they can achieve. Very lean 301 can be cold worked to yield strengths on the order of 2,000 MPa because of its unstable austenite transforming to martensite. When cold worked to lower degrees, it can provide very high strength while keeping impressive ductility.
Austenitic stainless steels have exceptional toughness. The ambient temperature impact strength of austenitic stainless steels is quite high. This is not surprising in view of their high tensile strengths and high elongations. What is most remarkable is the absence of a transition temperature, which characterizes ferritic and martensitic materials. Fig 5c shows impact strength of the different types of the stainless steel against temperature. This again is because of the multiplicity of slip systems in the fcc structure and the fact that they do not need thermal activation. This makes the austenitic stainless steels, especially the 200 series, the optimal cryogenic material, surpassing the 9 % nickel martensitic steels in cost, toughness, and, of course, corrosion resistance.
Precipitation of carbides and nitrides – Carbon is normally considered as an undesirable impurity in austenitic stainless steel. While it stabilizes the austenite structure, it has a high thermodynamic affinity for chromium. Because of this affinity, chromium carbides, M23C6, form whenever carbon reaches levels of super-saturation in austenite, and diffusion rates are sufficient for carbon and chromium to segregate into precipitates. The solubility of carbon in austenite is over 0.4 % at solidification but decreases greatly with decreasing temperature. The solubility is given log(C ppm) = 7771 – 6272/T Kelvin’ (equation 7).
The equilibrium diagram for carbon in a basic 18%Cr10%Ni alloy is shown in Fig 6a. At room temperature, very little carbon is soluble in austenite, even the 0.03 % of L grades is mostly in a super-saturated solution. The absence of carbides in austenitic stainless is because of the slow diffusion of carbon and the even slower diffusion of chromium in austenite. At a carbon level of 0.06 %, which is found in most 304, super-saturation is reached below around 850 deg C. Below this temperature, super-saturation increases exponentially, while diffusion decreases exponentially. This results in precipitation rates which vary with temperature and carbon level as shown in Fig 6b. At these temperatures, grain boundary diffusion is much more rapid than bulk diffusion, and grain boundaries provide excellent nucleation sites, so precipitation occurs along grain boundaries. And, since carbon diffuses several orders of magnitude more rapidly than chromium, carbon diffuses to and combines with chromium essentially in situ, depleting the grain boundaries of chromium in solution.
Fig 6 Carbon solubility and precipitation of Cr23C6
Fig 7a shows that the local chromium depletion is such that the chromium level can become low enough that it has not even enough to be stainless and certainly much lower corrosion resistance than the surrounding area. This zone, since it is lower in chromium, also has very unstable austenite and is quite prone to martensite formation. Fig 7b shows how the locus of precipitation changes with time and temperature. Carbon relatively far from grain boundaries in the interior of grains remains in super-saturation until much longer times and much greater super-saturation since bulk diffusion is needed for the nucleation and growth of these precipitates.
Fig 7 Depletion of chromium and variation of carbide precipitation
The key observation is that any solid-state precipitation of a chromium-rich precipitate necessarily causes local chromium depletion and a resulting loss of corrosion resistance. Much longer-term heat treatment is needed to eliminate these depleted zones by re-homogenization of slowly diffusing chromium than the short time needed to form them. This is very evident for carbides, but also true for oxides. Underneath chromium-rich oxide scales is a layer depleted in chromium and lower in corrosion resistance. This is why not only scale from welding is to be removed, but also the underlying chromium-depleted zone.
Other precipitation processes which give rise to chromium depletion are alpha and chi and the solid-state precipitation of oxides, nitrides, and sulphides. Chromium precipitates which form in the liquid alloy do not cause depletion of chromium locally since no chromium gradients are set up around them during precipitation as diffusion in the liquid is very rapid. Hence, primary carbides, oxides, and sulphides are not per se harmful to corrosion resistance. But, if the same compounds form and grow in the solid state, chromium depletion occurs.
Alloying elements can have a major influence on carbide precipitation by their influence on the solubility of carbon in austenite. Molybdenum and nickel accelerate the precipitation by diminishing the solubility of carbon. Chromium and nitrogen increase the solubility of carbon and hence retard and diminish precipitation. Nitrogen is especially useful in this regard (Fig 8a).
Fig 8 delay in carbide precipitation and variation in hardness
Increasing austenite grain size accelerates precipitation, as does cold work, especially in the interior of grains, where diffusion is improved by increased defect density. Nitrogen is much more soluble than carbon and does not give rise to sensitization phenomena as does carbon even though Cr2N can be a stable phase when the solubility limit is exceeded. The solubility is over 0.15 % in austenite, so its precipitation seldom has the possibility of occurring, but it does become an issue in ferritic stainless steels in this regard, for which solubility is much lower. Manganese and chromium increase the solubility of nitrogen in austenite.
Stabilization – Before carbon is easily lowered to harmless levels, it is found that adding more powerful carbide formers than chromium can preclude the precipitation of chromium carbides. Titanium and niobium are the most useful elements in this regard. They form carbides with solubility which follows the equation type ‘log[M][X] = + A – H/RT’ (equation 8). For titanium carbide and niobium carbide, the respective solubilities are ‘log[Ti][C] = 2.97 – 6780/T’ (equation 9) and log[Nb][C] = 4.55 – 9350/T’ (equation 10). )
Oxides and sulphides are more energetically favourable than are carbides and nitrides of these metals. Hence, any additions made to form carbides are to be sufficient to account for the prior formation of these compounds. Nitrogen also competes with carbon for available titanium or niobium. Hence, for successful getting of all the carbon, there is to be sufficient titanium or niobium available to combine stoichiometrically with all these species present. This needs in rough terms that titanium exceeds four times the carbon plus nitrogen, or that niobium exceeds eight times, after accounting for the oxygen and sulphur. It is going to be a mistake to ignore the titanium-consuming capacity of oxygen and sulphur unless they have been minimized by refining, which can be done quite readily.
Even if sufficient titanium or niobium are present to combine with all carbon, kinetic considerations can result in that not occurring. High temperatures, such as encountered in welding, dissociate carbides. If quenched from this state, carbon can be free to form Cr23C6 if it is reheated to temperatures above 500 deg C.
Carbon has always been considered totally undesirable from a corrosion point of view because of its tendency to form chromium carbides. Recently, however, new processes have been developed to super-saturate carbon in austenite below the temperatures at which it has sufficient mobility to form carbides. This so called colossally super-saturated austenite results in very high hardness (Fig 8b) and corrosion resistance over limited depths. From this, however, it can be seen that carbon, like nitrogen, is actually beneficial to corrosion resistance in solid solution, although this is not observed at the normal concentrations. It is possible to see that if it can be kept in solution it is to be appropriate to give it a factor of around 10 in the pitting resistance equivalent number (PREN) equation ‘PREN = % Cr + 3.3(% Mo) + 30(% N) + 10(% C)’ (equation 11). This is consistent with the similar thermodynamic interaction coefficients which carbon and nitrogen share with regard to chromium.
Usage of austenitic stainless steels – Nickel which stabilizes the austenitic structure of these steels restricts their widespread usage since nickel increases the costs of these stainless steels. Other steels can offer similar performance at lower cost and are preferred in certain applications, for example ASTM A387 is used in pressure vessels but is a low-alloy carbon steel with a chromium content of 0.5 % to 9 %. Low-carbon versions, for example 316L or 304L, are used to avoid corrosion problems caused by welding. Grade 316LVM is preferred where bio-compatibility is needed (such as body implants and piercings).
Austenitic grades of stainless steels are the most commonly used grades, mainly because they provide very predictable level of corrosion resistance with excellent mechanical properties. Using them wisely can save the designer of a product significant cost. These steels are user friendly metal alloy with life cycle cost of fully manufactured products lower than many other materials.
Austenitic stainless steels are those steels which are normally used for stainless application. Some of the applications for austenitic stainless steel include (i) kitchen sinks, (ii) architectural applications such as roofing and cladding, (iii) interior decoration, (iv) roofing and gutters, (v) doors and windows, (vi) kitchen-ware, cutlery and cook-ware, (vii) benches and food preparation areas, (viii) food processing equipment, (ix) heat exchangers, (x) ovens and furnace parts, and (xi) chemical tanks.
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