Lecture 40. Ultra high strength steel. Lecture 40. Ultra high strength steel

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1 Lecture 40 Ultra high strength steel Lecture 40 Ultra high strength steel 1

2 Introduction Steel has unique transformation characteristics that allow us to get a wide range of properties with very little alloy addition. In the last two classes we have seen how these concepts can be exploited to improve the strength of steel significantly with very little loss of ductility. The major emphasis has been on controlled thermo mechanical processing and controlled cooling so as to get extremely fine grains in the final product. Apart from controlled rolling we also looked at three different processing techniques that could give unusual strength and ductility. These are as follows: Inter critical heat treatment giving dual phase (DP) steel consisting of ferrite martensite structure having an excellent combination of strength and formability Patenting that uses isothermal transformation at the nose of the TTT diagram so that the structure can be easily cold drawn into wires having tensile strength approaching the theoretical strength of ideal solids Deformation processing (commonly known as ausforming) of rapidly cooled austenite in the bainitic bay followed by quenching to get fully martensitic structure having extremely high strength and reasonable ductility Although the processing routes we looked at so far can give attractive combination of high strength and ductility in steel there is a major limitation on the section size of the product in which this can actually be achieved. In this lecture we would look at how the problem can be overcome by alloy addition. Reasons for alloy addition: Apart from carbon all other alloy elements found in steel are more expensive than iron. The obvious question that comes up is why do we go for expensive alloy addition when such a wide range of strength & toughness can be achieved in carbon / micro alloyed steel? One of these we are already familiar with. This is the section size limitation. The optimum combination of strength and toughness can be obtained by controlled processing only if the product is thin. The strength and toughness are not the only considerations for the selection of steel. There are several other factors as well. The reason for alloy addition can be summarized as follows: 2 To overcome the section size limitation by increasing harden ability To improve strength to weight ratio To improve machinability To improve formability To improve Corrosion / oxidation resistance To improve magnetic properties (soft / hard) To improve Creep resistance

3 To improve tempering resistance & overcome embrittlement To improve weldability (By and large most alloy steels are more difficult to weld) We know that plain carbon steel is not necessarily a binary Fe C alloy. It would always have some amount of Mn & Si. These are added primarily to remove oxygen from molten iron. Besides these it would invariably some impurities like S, P, O, N, H etc. The list of intentionally added alloy elements includes Mn (beyond 1%), Si (beyond 0.3%), Cr, Ni, Mo, W, V, Ti, Nb, B, Al etc. There are certain grades of steel where S & P are also added intentionally to meet certain other specifications. Role of major alloy elements in steel: Ferrite, maternsite, austenite and carbide are the four phases that might be present in steel. Some of these may exist as separate constituents consisting of intimate mixtures of two phases like pearlite or bainite. It may be worthwhile to know about the role of individual alloying elements present in steel. 3 Carbon (C): Present mostly as carbide. Its solubility in austenite is more than that in ferrite. It increases the strength and harden ability of steel but decreases the ductility, formability and weld ability of steel. Manganese (Mn): It has good solubility in all the three phases present in steel (ferrite, carbide and austenite). It is an austenite stabilizer. It decreases A 1 & A 3 temperatures. If present in excess (beyond 12%) the structure may be totally austenitic (non magnetic). Hadfield Mn steel has 1%C and 12%Mn. It exhibits extremely high work hardening behavior. It is used as components where you need the material to become harder during service. Examples: helmets, railway crossings, shovels, jaw crushers etc. It is present in most steels as de oxidizer and as an element to fix S as MnS so as to overcome the hot shortness due to S. It increases the strength, toughness, shock resistance (by decreasing DBTT), harden ability and hot formability of steel. It may have an adverse effect on weldability. Silicon (Si): It is usually added as a deoxidizer because of its high affinity for oxygen. It is a ferrite stabilizer. It raises the critical temperatures of steel. Its solubility in ferrite is more than in austenite. Steel having higher silicon (>6%) cannot be heat treated. It gives solid solution strengthening. It decreases weldability and magnetic hysteresis loss but improves the strength, hardenability and oxidation resistance of steel. High silicon steel (4%Si) is used as transformer core because of its high magnetic permeability and low eddy current loss (because of high electrical resistivity). Aluminum (Al): It is primarily added as a deoxidizer because of its extremely high affinity for oxygen. It is mostly present as oxide inclusions (Al 2 O 3 ). It also has good affinity for

4 4 nitrogen. It forms aluminum nitride (AlN), because of this it is intentionally added to steel that are given nitriding treatment. Al killed steel are known as inherently fine grain steel. This is because of the presence of extremely fine oxides of Al at prior austenite grain boundaries. Therefore Al killed steel has low DBTT (higher impact toughness) and low hardenability. Phosphorous (P): P has a fairly good solubility in ferrite. It increases the strength and oxidation resistance of steel but decreases its impact toughness, formability and weldability. Therefore in most commercial steel it is kept within 0.035%. It is a ferrite stabilizer like Si it is a gamma loop forming element. Its solubility in ferrite is more than that in austenite. Steel having excess P (beyond 0.6%) may not respond to heat treatment. It has a tendency to form Fe 3 P. Steel having P may respond to precipitation hardening. Except for certain grades of electrical, weathering resistant or bake hardenable grades of steel it an undesirable alloy element in steel. Sulfur (S): The presence of excess S in steel makes it brittle because of the formation of low melting (~1000 C) Fe FeS eutectic around the grain boundaries of austenite. This is why such steels are difficult to form. Therefore Mn is added to fix S and MnS so that there is little S left in molten steel to form Fe FeS eutectic. The S in steel is always present as MnS inclusions. This gets elongated during hot rolling. The presence of such elongated inclusions makes steel anisotropic (it has higher elongation & strength along the rolling but poor through thickness strength & ductility). Therefore like P, S too is controlled within 0.035%. The presence of MnS inclusions helps the formation of chips during machining. Therefore machine able grades of steel popularly known as free cutting steel may have 0.3%S with adequate Mn to fix it as MnS. Oxygen (O): It has a tendency to form iron oxide at the grain boundaries. This would make the steel brittle. Therefore every attempt is made to control the amount of oxygen in molten steel during solidification by adding S & Al as de oxidizer. Rimming steel has higher amount of dissolved oxygen therefore it has higher DBTT than killed steel. Hydrogen (H): This being the smallest atom it can easily penetrate steel through its grain boundaries during various stages of processing or during service. The formation of hydrogen bubbles by the reaction H + H = H 2 creates additional local stress. In combination with the external stresses it may lead to cracking or brittle failure. This is known as hydrogen embrittlement. High strength steels are most susceptible to such failures. Nitrogen (N): Like C it also forms interstitial solid solution with Fe. Its solubility in ferrite is a little more that of C in ferrite. The presence of soluble interstitial is responsible for the yield point phenomenon in steel. It makes steel susceptible to strain ageing. It increases its strength at the cost of its ductility. In the presence of other elements like Cr

5 5 & Al it may form strong nitrides that could give higher strength. It is often added in precipitation hardenable grades of stainless steels. Chromium (Cr): It increases the corrosion resistance, hardenability, room as well as high temperature strength of steel. It has good solubility in both austenite () and ferrite () but its solubility in is more than that in. It is a ferrite stabilizer. If the amount of Cr present is beyond a limit the steel may not transform into austenite at all. Such steels do not respond to conventional hardening treatment. Cr promotes the tendency to form a protective oxide film on the steel. If %Cr > 12% the steel becomes stainless (it does not rust or corrode). It has strong affinity for both C & N present it steel. It forms strong carbides and nitrides that act as obstacle to dislocation motion and thereby improve the elevated temperature strength of steel. Nickel (Ni): It is an austenite stabilizer. Its solubility in austenite is higher than that in ferrite. If the amount of Ni in steel exceeds a specified limit, the FCC form of iron (austenite, usually stable at higher temperature) may become stable even at room temperature. Such steels would not respond to the conventional hardening heat treatment. Austenitic steel may have at least 8% Ni. Ni is used in several grades of hardenable steel. It increases hardenability and strength with little in ductility. It increases toughness and decreases DBTT significantly. Several grades of steel used for cryogenic application have significant amounts of Ni. Molybdenum (Mo): It increases hardenability and high temperature strength of steel. Almost all grades of steel for elevated temperature applications have Mo. It is a ferrite stabilizer but has a very strong affinity for C. It can form very stable carbides having very high melting point. This is responsible for the red hardness of tool steel having high amount of Mo. It is often present in high speed tool steel (HSS). Mo decreases the risk of temper embrittlement. This is why 0.5% Mo is present in several hardenable grades of steel. Mo addition alters the shape of the CCT diagram. It consists of two C curves one for peartic transformation and the other for bainitic transformation. Tungsten (W): It increases hardenability, wear resistance, and high temperature strength of steel. It is a ferrite stabilizer and strong carbide former. It is widely used in high speed tool steel (HSS). Vanadium (V): It is a strong carbide / nitride former. It is present in small amounts in several grades of micro alloyed steel. It is also an important alloying element present in HSS. Titanium (Ti): It is a strong carbide / nitride former. It is one of the common alloy additions in micro alloyed steel. Austenitic stainless steel having small amounts of Ti is not susceptible to weld decay or sensitization. It is also used in HSS steel.

6 Niobium (Nb): Like Ti it is a strong carbide / nitride former. It is one of the common alloy additions in micro alloyed steel. Austenitic stainless steel having small amounts of Nb is not susceptible to weld decay or sensitization. Copper (Cu): The presence of small amounts of Cu ( %) in steel gives good resistance to atmospheric corrosion. It has very little solubility in steel. The presence of Cu may cause hot shortness because of its low melting point. %Cu is rarely exceeds 0.35%. This problem may be overcome in presence of Ni. Cu can contribute to precipitation hardening. There are several weld able grades of Cu containing HSLA steel. Lead (Pb): It is insoluble in steel. It is added to certain grades machine able steel. The presence of fine globules of Pb favors chip formation during machining. Boron (B): It increases harden ability of steel significantly even though the amount B is in ppm range. Like Mo, B too splits CCT diagram of steel into two separate pearlitic and bainitic C curves. It a good absorber of neutrons. It is present in several grades of steel used in nuclear reactors. If present in excess it may form iron borides. Ultra high strength steel (UHSS) for aerospace applications: Most of the commercial UHSS are used in quenched and tempered conditions. The microstructure of such steel consists of tempered martensite. Therefore hadenability is a major consideration for the design and selection of appropriate steel. The performance of UHSS steel in actual service depends a lot on the shape and size of precipitates and inclusions in particular. The most harmful are the inclusions that deform easily during thermo mechanical processing. Sulfides and silicates come under this category. These are often present as elongated stringers in hot rolled steel. This may adversely affect the short transverse strength, toughness and ductility of the component. This is why to exploit the full potential of UHSS steel there has considerable efforts to produce clean steel having very little inclusions. Often adoption of techniques like vacuum arc re melting, electro slag refining and ladle treatment to control the shape and size of inclusions may be necessary. This apart from the additional heat treatment that is necessary adds up to the cost of such steel. Therefore these are used only for applications that demand high strength to weight ratio and durable products such as aerospace component (landing gears), rocket motor casing or heavy duty bearings. A few of the most common UHSS and their compositions are given below: En24 / AISI 4340: 0.4C 1.8Cr 0.8Ni 0.25Mo: OQ+T AISI 4335V: 0.35C 1.8Cr 0.8Ni 0.25Mo 0.2V: OQ+T H11: 0.35C 5.0Cr 1.5Mo 0.4V: AC + T Maraging steel: 0.02C 18Ni 3 9Co/Mo 0.6Ti 0.1Al: AC+ CW + Aged 6 The first three come under the category of hardened and tempered steel. All of these have significant amounts of alloy element apart from C. AISI 4340 or 4335V needs oil quenching whereas H11 having still higher alloy addition hardens even on air cooling. Under oil

7 quenched (OQ) or air cooled (AC) condition the structure of such steel may consist of martensite and retained austenite. This is the stage when the hardness or the strength of such steel is possibly the highest. Tempering restores the toughness of the steel although its strength may drop a little. It also gives structural and dimensional stability through the transformation of retained austenite to a more stable structure. Both AISI 4340 and 4335V can give YS as high as 1800MPa with 5% elongation under hardened and tempered condition. Fig 1 gives an idea about the CCT diagram of AISI 4340 steel. It has a bainite bay. The existence of such a region where super cooled austenite is fairly stable makes this steel amenable to ausforming T C 300 Ferrite precipitation A + F A+P+F A 8.3 C/S A + B 0.3 C/S C/S 0.02 C/S M +A M+F+B+P M M + B F + P Time, sec M+B+F Fig 1: A schematic CCT diagram of AISI 4340 steel. The values of time & temperature are given only to have rough idea about the cooling rates. The carbon equivalent (CE) is lower than that of H11. It behaves as hypoeutectoid steel. It does not need very high austenitizing temperature. It usually needs oil quenching. The C curves for B & P are different because of Mo. Note that there is a bay where super cooled austenite (A) can exist over a very long duration. The steel is amenable to ausforming. If cooling rate is higher than 8.3C/s you get 100%M. This gives the highest hardness. Extremely slow cooling gives ferrite pearlite structure. Tempered martensite gives the best combination of strength and toughness. Fig 2 gives the CCT diagram of H11 steel. This has very high amount of Cr. Its carbon equivalent is higher than 0.8%. Therefore its transformation characteristic is expected to be similar to that of hypereutectoid steel. In ausformed condition H11 has significantly YS higher strength than that of AISI 4340 steel. UHSS steels are mostly used under hardened and tempered condition. 7

8 1000 Carbide precipitation 890 A + C 825 A+P+C T C A + C A+B+C 300 A+M+C Time, sec Fig 2: A schematic CCT diagram of H11 steel. The values of time & temperature are given only to have rough idea about the cooling rates. Although %C in this steel is around 0.35 its carbon equivalent (CE) is much higher. It behaves as hyper eutectoid steel. A very high temperature is required to dissolve all carbides in austenite. It is an air hardening type of steel. If section size is large oil quenching may be needed. Note that there is a bay where super cooled austenite (A) can exist over a very long duration. The steel is amenable to ausforming. Martensitic in a deformed austenite is likely to have finer lath size. The presence of carbides restricts the grain size of austenite. Therefore tempered H11 steel has the optimum combination of strength and toughness. Maraging steel: The amount of carbon in this steel is negligible. The martensite that you get in this steel on quenching is not hard. It BCC structure.it can be cold worked. On ageing its strength increases significantly. This is due the precipitation of extremely fine coherent precipitates such as Ni 3 (TiAl) and Ni 3 Mo. Slide 1 gives a plot that describes the effect of ageing temperature and cold work on the kinetics of precipitation. The peak hardness is achieved at around 500 C. It has very high strength and high fracture toughness as well. It can be easily welded. 8

9 Maraging steel Low C makes martensite soft (BCC). Can be cold worked. On aging Ni 3 (TiAl), Ni 3 Mo precipitates form. HV T 500 CW+AC+ aged AC+ aged Low C : excellent weldability YS ~ 1800 MPa K IC ~ 120 MPa m Slide 1 Stain induced pptn gives higher strength. Expensive: rocket casing Stainless steel Alloy: Cr (corrosion resistance), Ni (corrosion austenite stabilizer), Mo (Pitting resistance) T Austenite stabilizer: Ni, Co, Mn, Cu, C,N Ferrite stabilizer: Cr, Al, V, Si, Nb, Mo, Ti, W Binary alloy L 12.7 %Cr Cr >12.7 : no Ferritic steel: (Cr-17C) > 12.7 Martensitic: (Cr-17C) < 12.7 F: AISI 430: 16Cr 0.12C M: AISI 410: 12Cr 0.15C M: cutlery, turbine blade Slide 2 9 Stainless steel: All grades of stainless steel must have at least 12%Cr. If the amount of Cr exceeds this threshold a continuous impervious (coherent) film of Cr 2 O 3 forms on the steel. This prevents corrosive environment to come in contact with its surface. This is how it protects steel against corrosion. Apart from Cr, Ni and Mo too help improve its resistance to corrosion. Slide 2 gives a list of various alloying elements that might be present in stainless steel. On the basis of their effect on the stability of gamma or alpha iron these are classified either as an austenite or

10 a ferrite stabilizer. The former includes Ni, Co, Mn, Cu, C and N whereas the latter includes Cr, Al, V, Si, Nb, Mo, Ti, and W. Slide 2 also includes a part of the binary Fe Cr phase diagram. Cr is a gamma loop forming element. If % Cr exceeds 12.7% it remains as alpha (ferrite) until it melts. Therefore it cannot undergo austenite to martensite transformation. The condition that favors martensitic transformation in iron chromium binary alloy is %Cr < In the presence of additional alloying elements the phase diagram gets modified. Therefore the expression for the maximum Cr that could be there in a harden able grade of stainless steel gets modified. For example in the presence of C the modified expression is (Cr 17C) < Note that C is an austenite stabilizer. This means that if C is present in the steel you could afford to have a little more Cr and yet the steel would respond to hardening treatment. All grades of stainless steel have very high concentration of alloying elements. Depending on their concentration and whether these are austenite or ferrite stabilizer stainless steel can be classified into 3 major groups. These are as follows: Ferritic stainless steel: effective concentration of ferrite stabilizer is more than 12.7%. Slide 1 gives the composition of one of the most commonly used ferritic stainless steel AISI 430. It has 16%Cr and 0.12%C. It does not undergo ferrite to austenite transformation on heating. Therefore it cannot be hardened by heat treatment. Its strength can be increased only by solid solution strengthening and work hardening. Martensitic stainless steel: effective concentration of ferrite stabilizer less than a specified limit. Slide 1 gives the composition of AISI 410 steel the most common martensitic grade of stainless steel. It is used for cutlery and turbine blades. There are several other martensitic grades of stainless steels having higher Cr. But higher amount of Cr may make it unsuitable for conversion to maternsite. This needs to be balanced by additional amounts of austenite stabilizer like Ni. Example 16Cr5Ni or 13Cr4Ni. Austenitic stainless steel: effective concentration of austenite stabilizer is more than a specific limit. This is the most popular and widely used stainless steel. The most common grade of austenitic steel has 18%Cr8%Ni. Like ferritic steel it can be only be hardened by solid solution strengthening or work hardening. 10

11 Schaeffler diagram 30 A A A+F A: Austenite M: Martensite F: Ferrite M+A+F M+F F F+M 0 %Cr 40 %Ni Slide 2 (Cr) eq = Cr+2Si+1.5Mo+5V+5.5Al+1.75Nb+1.5Ti+0.75W (Ni) eq = Ni+Co+0.5Mn+0.3Cu+30C+25N Schaeffler diagram (see slide 2) is a useful tool to have a rough idea of the microstructure of stainless steel as a function of Cr and Ni, the two major alloying elements present in these steels. Cr is a ferrite stabilizer. There are several other alloying elements that stabilize ferrite may also be present in such steels. On the basis of the ability of such alloying elements to stabilize ferrite in comparison to that of Cr an expression for Cr equivalent has been empirically evaluated. Such an expression is given in slide 2. Similarly the combined effect of all austenite stabilizers can be represented in the form of Ni equivalent (Ni) eq. Slide 2 also gives as expression for the same. Slide 2 also gives a schematic representation of Schaeffler diagram for Cr Ni steel. This was originally developed to represent the effect of chemical composition on the structure of rapidly cooled weld metal. Nevertheless it gives an approximate idea on the various possible structures that can be obtained in Cr Ni steel. This suggests that apart from ferrite, martensite or austenite you can also have steel having more than one phase. You can have a stainless steel consisting of austenite and ferrite. This is popularly known as duplex stainless steel. The toughness of duplex steel is better than that of ferritic steel. It also has a better resistance to stress corrosion cracking. However such a diagram is a function of the temperature from which the steel is rapidly cooled. Most often the temperature is around 1050 C. It also assumes that carbon remains in solution. The predictions may not match if carbon is present as carbides. 11

12 Austenitic steel AISI: 200 series Ni substituted grade: cheaper AISI 300: Cr Ni steel: more expensive AISI 304: 0.08C 18Cr 8Ni Popular, sensitization AISI 316: 0.08C 18Cr 10Ni 2 Mo: Good acid / pitting resistance AISI 321: 0.08C 18Cr 10Ni 0.4Ti: stabilized AISI: C 18Cr 10Ni 0.8Nb: stabilized Slide 3 Excellent strength & ductility. YS can increased to ~ 1000MPa by CW 12 Austenitic steel: Slide 3 gives the composition of a few common grades of austenitic stainless steel. AISI 300 series is the most popular and widely used stainless steel. It is nonmagnetic unlike the other two (Ferritic or martensitic). It does not get attracted by magnet. This is the easiest way to check whether a given piece of stainless steel is indeed austenitic. AISI 304 is the post popular grade of austenitic steel. This is often referred to as 18/8 stainless steel. Increased Ni and addition of Mo gives higher corrosion and pitting resistance in acidic resistance medium (example: AISI 316). There is a direct connection between the resistance to pitting and the absence of chromium carbide precipitates in this steel. You normally expect the structure to be 100% austenite (a single phase structure). Long thermal exposure in the range of C or during slow cooling though this range of temperature carbides rich in Cr may form. The most common carbide that forms in austenitic steel is Cr 23 C 6. This has around 70%Cr. As a result the surrounding region gets depleted of Cr. We know that if %Cr in the matrix falls below 12% the protective film of Cr 2 O 3 does not form. Therefore the regions surrounding these carbides act as anodes in a corrosive medium. These are the locations where pits develop. The grain boundaries are the favored locations for the formation of precipitates. If the numbers of such precipitates along the grain boundary are too many it may provide easy path for the growth of inter granular crack. The phenomenon is known as sensitization. The formation of such a zone is a very common problem encountered during welding of austenitic steel. The heat affected zones may have a network of very closely spaced Cr 23 C 6 precipitates. The inter granular attack during service in such steel is commonly known as weld decay. Slide 4 explains the phenomenon with the help of a set of diagrams. The lower sketch shows how the concentration profile near a precipitate gets altered due the formation of Cr 23 C 6. Each of the grain boundary

13 precipitate is surrounded by a region having low Cr. This is illustrated with the help of a sketch in slide4. The presence of an interconnected zone having lower concentration of Cr along the grain boundary makes the steel susceptible to failure. Sensitization: weld decay Cr + C = Cr 23 C 6 : 70% Cr occurs at C 18% Cr depleted region A A A Cr 23 C 6 70% A A 12% %Cr profile Cr 23 C 6 GB area get depleted of Cr & loose its stainless characteristic. How to overcome this? Quench from above 800C Reduce % C in steel Add strong carbide former Slide 4 How do we avoid sensitization? There are three possible ways to overcome the problem. These are listed in slide Avoid prolong thermal exposure to the temperature regime where precipitation of Cr 23 C 6 may take place. Even slow cooling through this regime should be avoided. Precipitation is a nucleation and growth process where diffusion plays a major role. Therefore the only way to suppress such precipitation is fast cooling or quenching. In order to take all chromium carbide into solution you need to heat the steel beyond 800 C. The preferred temperature is around C. Once all the carbides have dissolved you need to quench the steel to suppress the precipitation of carbides. Often you may need to anneal austenitic steel. Annealing temperature is around 900C. After soaking at this temperature the steel must be quenched. The second option is to reduce %C in austenitic steel. Carbon is an austenite stabilizer. If you reduce %C you may have to increase the amount of Ni or add any other austenite stabilizer so that the steel remains austenitic. The third option is the addition of very strong carbide formers in the steel. The most common alloy elements added to fix carbon in stainless steel are Ti and Nb. Slide 4 gives the approximate compositions of a Ti stabilized (AISI 316) and a Nb stabilized (AISI 347)

14 stainless steel. Note that the amount of Ni in these grades is higher than that of AISI 304. This is because both Ti and Nb are ferrite stabilizers. In addition the concentration of carbon in austenite gets reduced as it gets fixed as Ti/Nb carbides. Ti and Nb carbides form at relatively higher temperatures ( C). Their affinity for carbon is so high that little carbon is left in austenite to react with Cr to form Cr 23 C 6. Therefore steels having Ti/Nb are not susceptible to sensitization. Sigma phase embrittlement: Austenitic steels are often used as high temperature components because of its resistance to oxidation and creep resistance. Prolong high temperature exposure may also lead to the formation of a brittle inter metallic compound known as sigma phase having 50%Cr and 50%Fe (CrFe). It is a hard, brittle, nonmagnetic intermediate phase having tetragonal crystal structure. Usually steels having not sufficient austenite stabilizer are more susceptible to sigma phase embrittlement. AISI 304 stainless is commonly used in fluid catalyst cracking units in petro chemical plants. It may be subjected to a temperature of C. Brittle sigma phase may form along its grain boundaries. This makes the steel brittle. Therefore old high temperature components must be handled with care. It should not be subjected to any impact load or excessive stress during start up, shut down or maintenance work. Mn containing stainless steel: The presence of high amount of Ni makes austenitic stainless very expensive. Ni is primarily added to stabilize austenite at room temperature. Mn too is an austenite stabilizer. It can be used to replace Ni in austenitic steel. Apart from this Cu and N are the two other austenite stabilizer. Mn containing steel has very high work hardening index because it decreases stacking fault energy significantly making cross slip (the main mechanism for strain softening) difficult. AISI 200 series (18 18Cr, 7 9Mn, 2 5Ni, < 0.25N) represent several grades of Mn substituted austenitic steel. These are relatively cheaper than the 300 series. The mechanical properties are similar but corrosion resistance is slightly inferior. 14 Creep resistant steel: Steel is a major material for the construction of power generating units where it may have to withstand a temperature up to 600 C for a period of 20 to 25 years. Any material when it is subjected to load at a temperature beyond 0.4T m (T m denotes melting point in degree C) undergoes a time dependent deformation called creep. As a result its load bearing capacity would decrease with time. When it falls below a specified limit it must be replaced otherwise it would fail or rupture. Therefore a high temperature material has a finite life. The load bearing capacity of such a material is expressed in terms of its rupture strength for a specified period at a given temperature. For example if 100,000hr (approximately 11years) rupture strength of steel at 600 C is known to be 40MPa means it can withstand this stress for 11 years. Clearly you need very long hours of testing to evaluate the ability of steel (or any other material) to

15 withstand high temperature. There are standard methods of accelerated testing procedures where tests are done at a little higher temperature. For example to get an estimate of long term rupture strength at 600 C you may need tests at 650 C. We now do have hundred thousand hour rupture strength of a number of steel that are suitable for high temperature service. These are often referred to as creep resistant steel. The obvious questions that come up are: What type of steel is likely to have high creep resistance? What would you look for while selecting steel for high temperature service? Would you only look for high rupture strength? A few of the important factors for the selection of such steel are as follows: Creep resistance (Mo increases creep resistance of steel) Oxidation resistance (Cr improves oxidation resistance) Structural stability (Stable precipitate that does not coarsen improves creep strength) Formability (Billets must be shaped into seamless tubes) Weldability (Lower C equivalent means better weldability) Creep resistant steel must withstand oxidizing environment. For example the surface of the tube that comes in contact with super heated steam at high pressure is likely to get oxidized. This results in a loss of section size or its load bearing capacity. Cr is known to improve the oxidation resistance of steel. This is why it is present in almost all grades of creep resistant steel. Prolong thermal exposure may favor a number of phase transformations in steel during use. This includes graphitization, coarsening of carbides and formation of new precipitates. One of the earliest creep resistant steel is one having 0.5%Mo. The structure of such steel consists of ferrite and carbide. Ferrite is no doubt stable but carbide (Fe 3 C) is a meta stable phase. It decomposes into ferrite and graphite (Fe 3 C = 3Fe + C). Such transformations lead to a loss of its load bearing capacity. In order to avoid this, the subsequent improved grades of steel have higher amounts of carbide forming elements. One of the most popular grades of creep resistant steel has 2.25%Cr 1%Mo. The presence of Cr in carbide increases its stability. 15 Creep resistant steel has higher high temperature strength. Its flow stress is likely to be higher. However ferritic steels are hot rolled when it is in austenitic state. Most of the carbides would go into solution. Therefore its flow stress may not be very high. Nevertheless forming of creep resistant steel is always more expensive. Welding is the most common technique to join creep resistant steel. Weldability depends on carbon equivalent. However creep resistant steel may have significant amounts of Cr, Mo and

16 other alloy elements. As a result its carbon equivalent may be high even though %C might be around 0.15%. It may need special care to weld these. Preheating and post weld heat treatment may be required. Which type of steel has higher creep resistance? (Austenite or Ferrite): You can have both ferritic and austenitic steel. Which of these is likely to have higher creep resistance? The answer lies in the magnitude of self diffusion coefficients of Fe in the two. Ferrite is BCC it has lower packing density whereas austenite is FCC is close packed structure. The diffusivity in austenite is much lower than that in ferrite. Equations 1 2 give the self diffusion coefficients of Fe in and iron / : At 900 C / (1) / : At 900 C / (2) Consider a temperature of 900 C where both & iron is stable. Note that the diffusivity in austenite is significantly lower than that in ferrite. Therefore creep rate of austenite is lower than that of ferrite. Besides low creep rate and high resistance to oxidation often there are a few other requirements to ensure long life of critical components at high temperatures. Frequent start up and shutdown of high temperature units made need good low cycle fatigue resistance (stain controlled fatigue). A lowelastic modulus gives better low cycle fatigue resistance. In addition thermal stress that develops during start up and shut down depends on the coefficient of thermal expansion and thermal conductivity. The magnitude of thermal stress is given by. Clearly a combination of low thermal conductivity (this ensures low T), low elastic modulus (E) and low coefficient of thermal expansion () means high thermal stress. Table 1 gives a comparison of the two steel with respect to each of these requirements. Apart from better phase stability and higher creep resistance on all other counts ferritic steels appears to be more attractive for high temperature application at least up to 600 C. Therefore for the most part of the last century there have been considerable efforts to raise the temperature capability of ferritic steel. Table 1: A comparison of the properties of ferritic and austenitic steel for high temperature application 16 Criteria for selection Ferritic steel Austenitic steel Preference Creep rate High Low Lower creep rate gives longer life Elastic modulus Low High Lower modulus gives higher fatigue resistance Thermal expansion Low High Lower expansion gives lower thermal stress Phase stability Low High High: higher temperature capability Thermal conductivity High Low High: lower thermal stress

17 Cost Low High Low cost Here is a list of common creep resistant high temperature materials used in power generation units. It also includes the maximum permissible metal wall temperature for prolonged use. 0.5C steel: maximum service temperature 450 C 1Cr0.5Mo: maximum service temperature 500 C 2.25Cr 1Mo: maximum service temperature 550 C 9Cr1MoVNb: maximum service temperature 600 C Austenitic steel: maximum service temperature 700 C Ni base super alloy: maximum service temperature 900 C The first four in the list are ferritic steel. These are used in air cooled and tempered condition. The tempering temperature is at least 100 C higher than the expected service temperature. This gives the steel sufficient structural stability. The microstructure consists of a dispersion of carbides in ferrite matrix. The initial improvements in the temperature capability were achieved empirically. Mo was found to be responsible for higher creep resistance whereas Cr improved its temperature capability by raising its resistance to oxidation. 2.25Cr1Mo was found to be the optimum choice. It continued to be the most commonly used steel for super heater, re heater and steam pipes of thermal power plants. 9Cr1Mo steels were popular in cases where higher oxidation resistance was needed. Its rupture strength was inferior to that of 2.25Cr1Mo steel. The lower creep resistance was later found to be due to the precipitation Mo 6 C. This can be delayed by the addition of strong carbide formers like Nb/V to 9Cr1Mo steel. Adoption of this strategy led to the development of several new grades of creep resistant ferritic steel. The evolution of modified grades of 9Cr1Mo steel was due to the growing knowledge and our understanding of the basic mechanisms of strengthening. Currently 9cr1Mo steel having a small amount of V/Nb (the total amount may not exceed 0.5% depending on its carbon concentration) is the most common material of construction for the high temperature components of super critical thermal power plants. 17 Creep resistant microstructure: Creep occurs due to bulk diffusion, grain boundary diffusion or dislocation glide. Therefore in order to improve the creep resistance one has to make each of these more difficult. For example diffusion at a given temperature depends on the concentration gradient. If the grain size is large the distance a vacancy needs to travel is longer. Therefore the concentration gradient is lower. This is why coarse grain structure exhibits better creep resistance. Coarse grain also means fewer paths for faster diffusion along the grain boundaries. This too indicates that coarse grain structures would favor lower creep. Here is a list of a few choices that can improve the creep resistance of engineering materials:

18 High melting point: alloy element that increases melting improves creep resistance. Coarse grain: Si killed steel has better creep resistance than Al killed steel. Low stacking fault energy: this makes cross slip difficult thus inhibits strain (creep) softening Precipitates: inhibits dislocation glide therefore resists creep deformation Glide plane Climb Glide Glide Fig 3: Schematic representations of creep deformation due a combination of climb and glide motions of dislocations in crystalline solid. Climb depends on the rate at which vacancies can diffuse in the lattice. This being a thermally activated process is the slowest and therefore it is the rate controlling step. Clearly shorter inter particle spacing and higher height to climb would result in slower creep rate. This follows directly from the Orowan equation (see text). Figure 3 would help you understand how by controlling the microstructure the creep resistance of steel can be improved by making dislocation glide more difficult. The average distance () a dislocation can glide is determined by the array of precipitates. It depends on the volume fraction and the size of the precipitates. The strain rate due to dislocation glide is given by Orowan equation: where denotes dislocation density and b is the Burgers vector. The average dislocation velocity (v) may be given by /.The time (t) has two components. One denotes the time to glide (t glide ) and the other denotes the time required to climb (t climb ) over the obstacle or the precipitate. The dislocations in a glide plane cannot move until the one at the tip of the pileup at the precipitate could climb over the precipitate. Climb is controlled by diffusion. Therefore:. On the basis of this it comes out that: 18 (3) Equation 3 suggests that creep (or strain) rate can be lowered by decreasing inter particle spacing () and increasing time to climb. How can one increase t climb? A possible way is to

19 increase the height a dislocation needs to climb. This can be achieved by making the precipitate large. The both can be satisfied if volume fraction of precipitate can be increased. There are super alloys having very high volume fraction of precipitates (~70%). These are known for very high creep resistance. However this cannot be applicable to steel where the volume fraction of precipitate is very less. In this case you need to have an optimum combination of precipitate size and inter particle spacing. However there is another way to improve the creep strength of steel. This is by decreasing the density of mobile dislocation density. Unlike 2.25Cr1Mo steel modified 9Cr1Mo steel has a very high dislocation density. A part of it is mobile and a part is immobile. During the primary stage of creep mobile dislocation density in this steel decreases significantly. In short the creep strength is controlled by an optimum combination of stable dislocation network, precipitate size and inter particle spacing. Summary: In this lecture we looked at how alloy addition helps in improving the strength and toughness of steel. Most of these are used in hardened and tempered condition. You can get extremely high strength and toughness even in components that have sizeable thickness. This is the main advantage of alloying. Besides overcoming the section size limitation it also helps in overcoming several other limitations of plain carbon steel. The role of various alloy elements on the properties and performance of steel has also been covered. Besides a few hardened and tempered grades of ultra high strength steel we also have learnt about age harden able variety of steel called maraging steel, steel that can withstand corrosive environment like those having very high amounts of Cr known as stainless steel and also those that can survive 20 to 25 years of high temperature exposure known as creep resistant steel. In short we looked at Effect of alloying elements on steel Hardened & tempered ultra high strength steel Maraging steel Stainless steel Creep resistant steel Exercise: Compare the crystal structures of two alloy steels having nominal carbon content (a) 18%Cr (b) 18%Cr8%Ni. Can these be hardened by conventional quenching from a high temperature? 2. High speed cutting tools are made of 18W4Cr1V0.6C and it is used in hardened and tempered condition. Unlike normal hardened and tempered plain carbon tool steel it does not lose its cutting ability even when it is red hot. Explain why it is so.

20 3. How is the mechanism of strengthening in maraging steel different from that in conventional ultra high strength steel? 4. Explain why commercial steels / alloys having lower stacking fault energy exhibit higher creep resistance. 5. Does all steel suitable for ausforming exhibit transformation induced plasticity? Answers: 1. (a) BCC: ferrite (b) FCC: austenite. 18%Cr steel cannot be transformed to austenite which is the main criterion for conventional hardening process by heating & quenching. This is because amount of ferrite stabilizer present is beyond loop. Therefore it cannot be hardened by this method. 18Cr8Ni on the other hand is austenitic at room temperature. Steels that could be hardened by conventional process develop martensitic structure on quenching to room temperature. Therefore this too cannot be hardened as above. However through cold work and subzero quenching some martensite may form in 18Cr8Ni steel. 2. High speed tool steel retains its hardness even when it is red hot because it has significant amount of strong carbide former which precipitates during the 4 th stage of tempering (gives secondary hardening) before these are put to use. The precipitate is coherent with the matrix. This gives it excellent micro structural stability to resist particle coarsening and retain its hardness. 3. Martensite in normal steel is hard primarily due to carbon. Presence of excess carbon is responsible for tetragonal distortion. This is why martensite in normal steel has BCT structure. Maraging steel has very little carbon. Therefore martensite that forms here is BCC. In virtual absence of carbon it is soft. It can be cold worked to give desired shape and increase its strength. On aging at around 500⁰C inter metallic compounds precipitate. This results in further increase in strength. Therefore apart from solid solution strengthening unlike conventional steel martensite in maraging steel derives its strength from cold working and precipitation hardening All creep resisting alloys have precipitates which obstruct movement of dislocation that is responsible for deformation. Usually precipitates are strong. Stress required to shear these is high. Therefore dislocations held up at obstacles can move either by climb (restricted to edge dislocation) or cross slip (restricted to screw dislocation). There are also restrictions imposed by crystal structure on slip planes and burgers vectors of mobile dislocations. If stacking fault energy is high the partials join to form a perfect dislocation that could either cross slip or climb depending on its character. If SFE is low

21 the distance between the two partial is large. It would require higher stress to join these. Therefore lower SFE gives higher creep resistance. 5. No not all. Only steels whose M s temperature is little lower than room temperature and M d temperature is higher than room temperature would respond to transformation induced plasticity if it is heavily deformed above M d temperature. Such steels are in metastable state during service. If a crack develops in the steel martensite would form at crack tip. Such a transformation is accompanied by volume expansion resulting in local deformation. The region surrounding the crack tip would resist such deformation. Therefore more work has to be done so that the crack would grow. This gives the steel high yield strength and toughness. Such steels are known as TRIP steel. Appendix Types of steel (A broad classification) 21 Structural steel: (AISI 1010, 1020 etc) available as sections, sheet, plate, deep drawing quality has l and low carbon & fine grain Heat treatable steel for high strength & toughness: carbon steel (thin sections) & low alloy steel having less than 5% alloy addition Carburizing steels: low carbon steel subjected to case hardening: gears Nitriding steel: low carbon steel with nitride forming elements such as Cr and Al Free cutting steel: excellent machinability due to the presence of MnS inclusions (S is added intentionally) Spring steel: medium carbon ( C) used in hardened & tempered condition, it should have good harden ability Bolting steel: should have good formability to allow thread rolling Creep resistant steel: super heater, re heater, steam pipes, headers of power boiler Cryogenic steel: should have very low DBTT, though austenitic steels are preferred there are much cheaper ferritic steels having very low DBTT Stainless steel: high Cr steel available as austenitic, ferritic, martensitic and precipitation harden able grades Tool / die steel: hot work, cold work or high speed tool steel (HSS) Ball bearing steel: used in hardened & tempered condition Electrical steel: soft magnetic grades have low carbon and high Si (CRGO / CRNO), hard magnets need high carbon and high hardness High strength low alloy steel (HSLA): fine grain steel having micro alloy addition Nb/Ti/V

22 Dual phase steel: ferrite martensite or ferrite austenite steel Maraging steel: 18% Ni low carbon martensitic steel that responds to age hardening having ultra high stregth 22