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AEB-L is a conventionally ingot-cast martensitic stainless steel designed and manufactured by Uddeholm Tooling AB (Sweden). Its nominal chemical composition (in weight percent) is as follows:

C = 0.65 Cr = 12.8 Si = 0.4 Mn = 0.65

Figure 1 shows the phase diagram of Uddeholm AEB-L stainless steel (in deg. Celsius) calculated with Thermo-Calc, coupled with TCFE3 thermodynamic database.

Figure 1. Phase diagram of Uddeholm AEB-L stainless steel (in deg. Celsius) calculated with Thermo-Calc, coupled with TCFE3

thermodynamic database. Silicon and manganese were excluded from thermodynamic calculations.

The equilibrium values for solidus and liquidus temperatures were calculated to be 1461 °C (2661 °F) and 1379 °C (2515 °F), respectively.

In the temperature range of 1144-1379 °C (2091-2515 °F) the

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microstructure of Uddeholm AEB-L stainless steel consists of just one single phase: austenite. Thus, if AEB-L steel is hardened from an austenitization temperature that is higher than 1144 °C (2091°F) the resulting martensitic microstructure will contain no primary carbides.

Below the temperature of 1144 °C (2091 °F) the chromium-rich M7C3 primary carbides start to precipitate from the austenitic matrix. At the austenitization temperature of, say, 1052 °C (1925 °F) the equilibrium amount of chromium-rich M7C3 primary carbides is 3.3 molar percent (2.6 volume percent). The equilibrium amount of carbon and chromium in the austenitic matrix at 1052 °C (1925 °F) is 0.44 wt. % and 11.4 wt. %, respectively. (The amount of carbon and chromium in the matrix is a good indicator of the steel's hardenability and corrosion resistance, respectively.)

The equilibrium value for A1 temperature (eutectoid temperature) was calculated to be 814 °C (1497 °F). Under equilibrium conditions the austenite in Uddeholm AEB-L stainless steel transforms into ferrite at this temperature.

Finally, please see additional information about the Fe-Cr-C ternary phase diagrams.

Free Downloads ? Phase diagram (in deg. Celsius) of Uddeholm AEB-L stainless steel ? Phase diagram (in deg. Fahrenheit) of Uddeholm AEB-L stainless steel

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Martensitic stainless steel such as 154CM contains about 4 wt. percent molybdenum (in addition to 1.05 wt. % C and 14.0 wt. % Cr). To determine the effect of 4 wt. % Mo on the Fe-Cr-C ternary system, consider Figures 5 and 6, which show the isothermal sections of Fe-4Mo-Cr-C quaternary phase diagram at 1000°C (1832°F) and 1100°C (2012°F), respectively.

Figure 5. Isothermal section of Fe-4Mo-Cr-C quaternary phase diagram at 1000°C (1832°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

According to Thermo-Calc calculations, the austenitic matrix of

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Fe-4Mo-14Cr-1.05C alloy at 1000°C (1832°F) has the following chemical composition (in weight percent):

Cr = 8.6 C = 0.33 Mo = 2.6

The amount of chromium-rich M23C6 primary carbides in Fe-4Mo-14Cr-1.05C alloy at 1000°C (1832°F) is calculated to be 16.8 mol. percent. It is worth noting that the addition of 4 wt. % Mo to the Fe-Cr-C system expands significantly the presence of M23C6 phase at the expense of the M7C3 phase (compare Figure 1 — Isothermal Section of Fe-Cr-C Ternary Phase Diagram at 1000°C — and Figure 3 — Isothermal Section of Fe-0.8Mo-Cr-C Quaternary Phase Diagram at 1000°C — with Figure 5).

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Figure 6. Isothermal section of Fe-4Mo-Cr-C quaternary phase diagram at 1100°C (2012°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

According to Thermo-Calc calculations, the austenitic matrix of Fe-4Mo-14Cr-1.05C alloy at 1100°C (2012°F) has the following chemical composition (in weight percent):

Cr = 10.6 C = 0.58 Mo = 3.4

The amount of chromium-rich M23C6 primary carbides in Fe-4Mo-14Cr-1.05C alloy at 1100°C (2012°F) is calculated to be 11.6 mol. percent.

The amount of chromium and molybdenum in the matrix is also an

indicator of the secondary-hardening response — in general, the higher the amount of chromium and molybdenum in the matrix, the stronger the secondary-hardening response during tempering (especially at higher tempering temperatures.)

Part 1: Fe-Cr-C Ternary Phase Diagrams

Part 2: Fe-0.8Mo-Cr-C Quaternary Phase Diagrams

Consulting Services To cover the costs of running this site, we accept consulting assignments to perform customer tailored Thermo-Calc and DICTRA calculations. If we cannot solve your problem, we will help you find at least one

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A high hardness level, a fine array of uniformly distributed primary alloy carbides, and an adequate matrix chromium content are the three most desired properties required to produce a knife with optimum properties. Ideally, a martensitic stainless steel grade for knife applications should, therefore, satisfy the following two fundamental requirements:

(1) The carbon content of the austenitic matrix has to be around 0.6 wt. pct. or higher in order to achieve the hardness of 63-64 HRC.

(2) The chromium content of the austenitic matrix has to be at least 12 wt. pct. in order to ensure corrosion resistance. (It should be said, however, that a part of matrix chromium can be replaced with molybdenum with little or no negative consequences for corrosion resistance.)

Isothermal sections of the Fe-Cr-C ternary phase diagram are a good starting point when it comes to understanding the various trade-offs between the austenitization temperature selected for heat treatment and the resulting chemical composition of the austenitic matrix.

The composition plane for the Fe-Cr-C ternary phase diagram at 1000°C (1832°F) is shown on Figure 1. The carbon content is plotted along the horizontal axis and the chromium content along the vertical axis of the composition plane.

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Figure 1. Isothermal section of Fe-Cr-C ternary phase diagram at 1000°C (1832°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

The isothermal section in Figure 1 shows the content of chromium and carbon in the various phases of Fe-Cr-C alloys that can exist at 1000°C (1832°F). The area labeled γ (the Greek letter gamma) represents austenite. If the composition of an Fe-Cr-C alloy is plotted on Figure 1 and it falls inside the γ area, the microstructure of that alloy will consist of austenite only, i.e., no carbides will be present at 1000°C (1832°F).

Consider now one of the most basic martensitic stainless steel grades such as AISI 440C (approximately 17 wt. pct. chromium and 1.075 wt. pct. carbon), plotted on the composition plane of Figure 1. Its chemical

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composition falls inside of the region labeled γ + M7C3. This means that if AISI 440C is heated to 1000°C (1832°F) its microstructure will consist of austenite and chromium-rich M7C3 primary carbides. Upon quenching the martensite formed from the austenite will contain chromium-rich M7C3 primary carbides dispersed within it.

It is worth noting that on Figure 1 the right-hand boundary of the austenite region is labeled \

important as it tells us the maximum amount of carbon that austenite can dissolve within itself — addition of more carbon would precipitate carbides.

The further the alloy composition lies to the right of the saturation line, the larger the volume fraction of chromium-rich M7C3 primary carbides it will contain. The presence of chromium-rich M7C3 primary carbides

renders the austenite depleted in both chromium and carbon relative to the overall chemical composition of the alloy.

Figure 1 can be used to determine the chemical composition of the austenite in an alloy such as AISI 440C. The austenite composition for AISI 440C at 1000°C (1832°F) is found at the point where the tie line drawn through AISI 440C intersects the carbon saturation line. It is worth noting that even though AISI 440C alloy contains 1.075 percent of carbon and 17 percent of chromium overall, the austenite that forms at 1000°C (1832°F) contains only around 0.3 percent of carbon and 11.7 percent of chromium (see Figure 1). The martensite that forms upon quenching has the same chemical composition as the austenite. The carbon and chromium contents of the martensite have, in turn, the effect on its hardness and corrosion resistance, respectively. Thus, AISI 440C martensitic stainless steel, when hardened from 1000°C (1832°F), does not satisfy the two requirements stated above (the carbon and chromium content of the matrix of at least 0.6 and 12 percent, respectively).

To demonstrate the effect of increasing the austenitization temperature on the volume fraction of primary carbides and the chemical composition of the austenite, consider the change in the position of the carbon saturation line in Figure 2.

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Figure 2. Isothermal section of Fe-Cr-C ternary phase diagram at 1100°C (2012°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

When the austenitization temperature is increased from 1000°C (1832°F) to 1100°C (2012°F), the content of carbon and chromium in the

austenitic matrix is increased from 0.3 % C / 11.7 % Cr to 0.5 % C / 13.2 % Cr. The volume fraction of chromium-rich M7C3 primary carbides is smaller at 1100°C (2012°F) than at 1000°C (1832°F), as graphically demonstrated by the length of the tie line in Figures 1 and 2.

The Second Edition of Heat Treater's Guide — Practice and Procedures for Irons and Steels (published by ASM International in 1995) recommends that AISI 440C be austenitized at 1010°C-1065°C (1850°F-1949°F). For

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maximum corrosion resistance and strength, the Guide recommends the upper end of the austenitization range. Such a recommendation is not surprising. The above given phase diagrams demonstrate that the higher austenitization temperatures will increase both the carbon and chromium contents of the austenitic matrix.

It is worth noting that the matrix of AISI 440C martensitic stainless steel does not simultaneously satisfy the 0.6 % C and 12 % Cr requirements, even at 1100°C (2012°F). Not surprisingly, AISI 440C is typically hardened to just 58-60 HRC.

Finally, the Fe-Cr-C ternary phase diagrams explain — at least in part — why some commercial martensitic stainless steel grades, such as Sandvik 12C27 (Fe-0.6C-13.5Cr) and Uddeholm AEB-L

(Fe-0.65C-12.8Cr), are in general considered to be well optimized for stailess knives.

Part 2: Fe-0.8Mo-Cr-C Quaternary Phase Diagrams Part 3: Fe-4.0Mo-Cr-C Quaternary Phase Diagrams

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What is a ternary phase diagram? A ternary phase diagram has three components. The three components are

usually compositions of elements, but may include temperature or pressure also. This type of diagram is three-dimensional but is illustrated in two-dimensions for ease of drawing and reading. Instead of being a

rectangular plot, it is a triangle. Ternary phase diagrams exist for many metallic alloys, but are also widely used in ceramics. Stainless steel (Fe-Ni-Cr) is a perfect example of a metal alloy that is represented by a ternary phase diagram. Stainless steel is a very common metal alloy. Almost everyone knows of an everyday object that is made with stainles steel. In the figure below stainless steel has been used to construct a portable hoist. You can find out more about other products made from stainless steel via the link provided in the Fig. 1 caption.

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Figure 1: Stainless steel hoist constructed by Halliday Products

Ternary phase diagrams are needed so that three components can be compared at once. For example, stainless steel has iron, nickel, and chromium compositions. To view all three compositions at the same time, a triangular plot is set up with an element at each of the vertexes with the temperature and pressure stated. In ceramic systems, sometimes compounds are located at the vertexes instead of elements. The derivation of the ternary plot is too complicated to go into, but the analytical derivation of a binary system is available along with the experimental method of determining the phase diagram

How do you read a ternary phase diagram? 12

Figure 2: Stainless steel phase diagram at 900 degrees Celsius (ASM 1-27)

Reading the compositions of iron, chromium and nickel at any point on the

stainless steel ternary phase diagram in Fig. 2 is simple. Instead of drawing one tie-line, as in a binary phase diagram , three lines are drawn, each parallel to a side of the triangle and going through the point in question. Extend the lines so they pass through an axes. To find the iron composition, the line drawn parallel to the axis opposite the Fe vertex is the one needed. The percent iron is then read off the axis.

For example, to determine the compositions of the 18-8 circle point near the lower left corner of Fig. 2, draw these lines: 1. draw the first line to be parallel with the axis opposite the Fe vertex, we find that the composition of iron is 74% , 2. next draw a line parallel with axis opposite the Ni vertex and read the composition of nickel to be 8% , 3. and finally draw a line parallel to the axis opposite the Cr vertex and we see that there is 18% of chromium at that point.

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The point described is then called 18-8 stainless steel, naming only the percentages of the chromium and the nickel; the iron content being dependent on the other two. This 'recipe' for stainless steel is the most common.

Figure 3: Stainless steel solidus projections over a range of temperatures (ASM 3-44)

Figure 3 represents the entire ternary phase diagram solidus projections of stainless steel over a range of temperatures under constant pressure. This picture illustrates how temperature affects solubility.

What about the pressure? Before now, the pressure was kept constant (1 atm), which is acceptable for most applications but in extreme pressure environments, a scientist must consider the pressure dependence of a system. The variance of pressure

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brings about a whole new dimension -- the fourth dimension. The pressure is usually not taken into consideration unless a gas phase is liable to be present. For information on the materials class that this subject is studied in, visit the MSE 3424 syllabus, Phase Equilibria and Crystal Chemistry.

Jamie Yeakle 4/28/96

http://www.eng.vt.edu/eng/materials/classes/MSE2094_NoteBook/ 96ClassProj/experimental/ternary2.html

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The ternary diagram of Ni-Cr-Fe is one of the most used ternary diagrams in history. The two types of alloys covered here from the Ni-Cr-Fe diagram are Stainless Steel and Inconel (tm).

Stainless Steel There are a vast number of types of Stainless steel. A metal alloy with a Chromium content greater than 11.5 % and an Iron content greater than 50 % is called a stainless steel. The stainless steels are broken into three major classes:

1. Chromium (11.5-17%) -iron alloys with carefully controlled carbon content. Can be heat treated to a magnetic martensite structure and are therefore known as martensitic stainless steels.

2. Chromium (17-27%) -iron alloys with low carbon content. Nonhardenable by heat treatment. Their crystal structure is magnetic ferrite and therefore are known as ferritic stainliess steels.

3. Chromium (16-26%) Nickel (6-22%) -iron alloys with low carbon content. Nonhardenable by heat treatment. Crystal structure of nonmagnetic austenite so are therefore called austenitic stainless steels.

The following table compares the composition and some basic properties of various types of stainless steel. A Comparison of Stainless Steels Young's Type % % % % % % Corrosion Melting Modulus of Nickel Chromiium Carbon Manganese Silicon Nitrogen mpy (1) Range °F x Steel 10^6,psi 201 301 302 304 309 3.5 - 0.15 1.0 16.0 - 18.0 5.5 - 7.5 5.5 max max 6.0 - 0.15 1.0 16.0 - 18.0 2.0 max 8.0 max max 8.0 - 0.15 1.0 17.0 - 19.0 2.0 max 10.0 max max 8.0 - 0.08 1.0 18.0 - 20.0 2.0 max 10.5 max max 19.0 - 0.2 1.0 24.0 - 26.0 2.0 max 22.0 max max 0.25 max 0 0 0 0 20 12 10 - 18 6 - 12 5 - 9 2550 - 2650 2550 - 2590 2550 - 2590 2550 - 2650 2550 - 2650 28.0 28.0 28.0 28.0 29.0 16

(1) Corrosion rate is in mils per year. The test was performed in 65% Nitric acid at 245 °F.

(2) Table references

Types of Stainless steel and specific uses of each.

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Stainless type 201

Stainless type 301 Stainless type 302 Stainless type 304 Stainless type 309

Inconel (tm) Inconel (tm) is a specialty alloy that uses higher percentages of Nickel and Chrome than Stainless steel, as well as many other elements in small quantities. It is actually a trademark name of Inco Alloys International and is in a group of metals known as the Nickel based super alloys. These small additions of other elements is solid-solution hardening. It is quite expensive and therefore usually reserved for applications when some type of stainless steel won't suffice. The following tables compare Inconels (tm) properties. A Structural Comparison of Inconel (tm) Alloys Type % % % % % % Ni Cr C Mn Si Fe % S % % % % % % % % Cu Al Ti P Co Nb B Mo 14.0 6.0 72.0 0.15 1.0 0.5 .015 0.5 600 - - 0 min max max max max max 17.0 10.0 0 0 0 0 0 0 58.0 21.0 1.0 0.1 1.0 0.5 .015 1.0 601 - - bal - 0 max max max max max 63.0 25.0 1.7 0 0 0 0 0 20.0 3.15 8.0 58.0 0.1 0.5 0.5 5.0 0.015 0.4 0.4 .015 1.0 625 - 0 - 0 - min max max max max max max max max max 23.0 4.15 10.0 718 50.0 17.0 0.08 0.35 0.35 .015 0.3 0.2 0.65 1.0 4.75 .006 2.8 bal .015 - - max max max max max - - max - max - 17

55.0 21.0 0.1 0.8 .008 800 32.5 21.0 46.0 0 max max max 0.8 1.15 0.4 0.4 0.4 0 5.5 0 0 0 3.3 0 (1) Table references

Corrosion of Inconel (tm) One outstanding characteristic of high-nickel alloys, like Inconel (tm), is their good resistance to a wide variety of corrosives. With few exceptions, high-nickel alloys do significantly better than martensitic, ferritic, and austenitic stainless steels in corrosive environments.

A Comparison of Inconel (tm) Properties Yield Strength Melting Rupture Strength; 100h at ____°F, Type 70°F Range °F ksi ksi 41.3 49.0 71 172 36.3 1600, 5.3 1600, 7.0 1600, 10.5 1200, 100 1800, 21 600 601 625 718 800 2471 - 2579 2471 - 2579 2300 - 2435 2300 - 2435 2471 - 2525 (1) Table references

The uses of Inconel (tm) are specific and quite costly:

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Piping on Nuclear Reactors Piping on Steam Generators

A Heat Exchanger on an unmanned ariel vehicle

Well Thermometers in high temperatures and pressures

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Since my example problem is supposed to be an introduction to the phase diagram, its calculations, and how it is perceived, most of my text is taken from Callister.

figure 1

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This figure contains the copper-nickel phase diagram. Its system is termed as being isomorphus.

A good interpretation of a binary phase diagram that is easy to understand and interpret is the Cu-Ni system. This diagram has three different phase regions,

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the alpha region, the liquid region, and the alpha + liquid region, which are defined by specific compositions and temperatures as illustrated in figure 1. Both points A and B are located in the alpha and the alpha + liquid regions respectively. The phase boundaries are separated by two lines. The line separating the liquid and the alpha + liquid regions is the liquidous line. The line separating the alpha and the alpha + liquid regions is the solidous line. The intersection of these two lines signify the melting temperatures of the two constituents individually. The Cu-Ni system is especially noted for its complete liquid and solid solubility of its constituents, and is thusly identified as an isomorphous system.

figure 2

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A section of the copper-nickel phase diagram is contained here. Both the compositions and the phase amounts of each constituent will be determined at point B.

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The determination of phase compositions in a single phase, is just how much of each phase is present at a given temperature. In figure 1 at point A, the alpha region is the only phase present; therefore, the composition at 1100 degrees Centigrade is a weight percent of sixty for Ni and forty for Cu. For a double phase region, the procedure intensifies. One has to first draw a horizontal line at the given temperature from the first phase boundary to the second phase boundary, which is depicted at point B in figure 2. This line is defined as the tie line. Next, at the intersection of each phase boundary with the tie line, vertical lines are drawn straight down until they intersect the x-axis where the composition of each constituent is identified. The composition of the liquid phase, CL, is 32 wt% Ni - 68 wt% Cu. Thusly, the alpha phase, CALPHA, has a composition of 43 wt% Ni - 57 wt% Cu.

Through the use of the lever rule, one can also determine the amount of each phase present. The lever rule is an expression which allows one to compute the phase amounts in a two phase alloy equilibrium situation. In a one phase region, the alloy is composed of 100% of that phase, such as the alpha region in figure 2 at point A. For a two phase region, one has to rely on the combination of the tie line and the lever rule expression.

As stated previously with the determination of phase compositions, the tie line is drawn at the specified temperature between the two-phase region and the composition of the alloy is identified (figure 2). The percentage of one phase is calculated by dividing the length of the tie line from the overall composition of

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the alloy to the phase boundary for the second phase by the length of the total tie line, which is denoted by subtracting the compositions of the constituents, and multiplying by one hundred. The percentage of phase two is calculated by using the same procedure.

Let's take a look at computing both the alpha and the liquid phases (figure 2). The tie line is drawn at point B, which is specified at twelve hundred fifty degrees Centigrade. The overall composition of the alloy is 35 wt% Ni, the composition of the alpha phase is 43 wt% Ni, and the composition of the liquid phase is 32 wt% Ni. (It is important to note that the composition for a binary alloy needs to be specified in terms of one of the constituents.) The mass fraction for the liquid may be computed by:

You may prefer the use of one equation over another; however, both render the same solution. The mass fraction for the alpha phase maybe computed similarly.

Again, both equations render the same solution. Conclusively, at equilibrium when both the temperature and the composition are known in a two-phase region for a binary alloy, then the lever rule may be utilized to assist in the calculations of the relative amounts or fractions of phases.

Now that an interpretation of the Cu-Ni System has been given, we need to take a deeper glance into the development of microstructure that occurs for isomorphus alloys during solidification.

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figure 3

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This figure represents the development of microstructure during the equilibrium solidification of a 35 wt% Ni - 65 wt% Cu alloy. Movie * Note: You will need and MPEG viewer to play this movie!

At thirteen hundred degrees Centigrade with an alloy composition of 35 wt% Ni - 65 wt% Cu, we will observe the cooling process slowly (figure 3). Equilibrium will be continuous in the given phase as long as cooling occurs very slowly. At point a, the alloy is completely liquid. This microstructure, which is represented by the circle inset in the figure, can be viewed by clicking on point a in figure 3. As the cooling process takes place, we reach the liquidous line. Here we reach a change in both the microstructure and the composition of the alloy. Point b is located roughly around twelve hundred seventy degrees Centigrade with its

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composition defined by the tie line. By clicking on point b in figure 3, one is able to see that the first solid (alpha) begins to form. As cooling continues from this point further, both compositions and relative amounts of each of the phases will change. With continued cooling, the fraction alpha phase will increase, as the fraction of the liquid will decrease. Its microstructure can be viewed by clicking on points c, d, and e in figure 3. It is obvious that the compositions and relative amounts of each phase will change; however, the overall alloy composition does maintain consistent at 35 wt% Ni - 65 wt% Cu. Point c signifies that the cooling process is half complete. The microstructure displays an approximate equal amount of alpha and liquid. At point d, there is a definite increase in alpha. Very little liquid can be viewed. Finally, point e is located after crossing the solidous line. Here the remaining liquid solidifies. The outcome has a uniform 35 wt% Ni - 65 wt% Cu composition which is then a polycrystalline alpha-solid solution (point e, figure 3). If the alloy is cooled beyond point e, there will be no microstructural or compositional changes. The solidification process is very extensive for equilibrium conditions must be maintained at all temperatures. Cooling has to take place slowly in order for the readjustments to occur in the compositions of the two phases, which will yield in the one phase diagram previously viewed. The diffusion process is the driving mechanism for the composition readjustments. Just as diffusion depends on time in order to maintain equilibrium during cooling, the composition readjustments need sufficient time.

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Example: Problem

For a 35 wt% Ni - 65 wt% Cu alloy at twelve hundred fifty degrees Centigrade, what phases(s) is (are) present? What is (are) the composition(s) of the phase(s)? Calculate the relative amount of each phase present in terms of mass fraction.

Solution

(a.) Locate this temperature-composition point on the phase diagram (point c in figure 3). It is located within the alpha plus liquid region; therefore, both the alpha and the liquid phases will coexist.

(b) Since two phases are present, it becomes necessary to construct a tie line across the alpha plus liquid region at twelve hundred fifty degrees Centigrade, as indicated in figure 3. The composition of the alpha phase corresponds to the tie line intersection with the alpha per alpha plus liquid solvus boundary about 43 wt% Ni - 57 wt% Cu. Similarly for the liquid phase, which will have a composition of approximately 3 wt% Ni - 70 wt% Cu.

(c) Since the alloy consists of two phases, it is necessary to employ the lever rule.

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There are various applications of the isomorphus Cu-Ni phase diagram that may be applied to every day experiences contained within this section.

Additional Information:

Various mechanical properties, which are explored in depth in MSE 3305 - Structure Property Relationships, of solid isomorphus alloys are affected by composition as other structural variables (e.g., grain size) are held constant. Below the melting temperature of the lowest-melting component, for all temperatures and compositions, only a single phase will exist. As a result, an increase in strength and hardness, or solid solution hardening will be experienced by each component through additions of the other component.

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The Schaeffler and Delong diagrams for predicting ferrite levels in austenitic stainless steel welds

Introduction Ferrite is important in avoiding hot cracking in during cooling from welding of austenitic stainless steels. 'Constitution diagrams' are used to predict ferrite levels from the composition by comparing the effects of austenite and ferrite stabilising elements. The Schaeffler and Delong diagrams are the original methods of predicting the phase balances in austenitic stainless steel welds.

Nickel and chromium equivalents A 'nickel equivalent' is calculated for the austenite stabilising elements and a 'chromium equivalent' ferrite stabilising elements. These are used as the axes for the diagrams, which show the compositional equivalent areas where the phases austenite, ferrite, martensite (and mixtures of these) should be present. Although intended to show the phase balance of weld fillers, these diagrams can also be used to illustrate the phase balance of the 'parent' material. There are different diagrams for different alloy systems.

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Schaffler Diagram The nickel and chromium equivalents use the formulae.

Ni (eq) = Ni + (30 x C) + (0.5 x Mn) Cr (eq) = Cr + Mo + (1.5 x Si) + (0.5 x Nb)

This gives a diagram that is useful for the austenitic steels, except those with nitrogen additions. The values for typical 304(1.4301) and 316(1.4401) compositions are shown below. . 304(1.4301) 316(1.4401) Ni (equiv) 10.15 13.15 Cr (equiv) 18.92 19.83 The diagram, identifying the phase boundaries is shown below.

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Duplex Stainless Steel

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless

steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.

Duplex stainless steels have a two-phase microstructure of austenite and ferrite grains.

The duplex structure gives this family of stainless steels a combination of attractive properties:

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Strength: Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.

Toughness and ductility: Duplex stainless steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades.

Corrosion resistance: As with all stainless steels, corrosion resistance depends mostly on the composition of the stainless steel. For chloride pitting and crevice corrosion resistance, their chromium, molybdenum and nitrogen content are most important. Duplex stainless steel grades have a range of corrosion resistance, similar to the range for austenitic stainless steels, i.e from Type 304 or 316 (e.g. LDX 2101?) to 6% molybdenum (e.g. SAF 2507?) stainless steels.

Stress corrosion cracking resistance: Duplex stainless steels show very good stress corrosion cracking (SCC) resistance, a property they have “inherited” from the ferritic side. SCC can be a problem under certain circumstances (chlorides, humidity, elevated temperature) for standard austenitics such as Types 304 and 316.

Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts of similar corrosion resistance. Due to the lower alloying content, duplex stainless steels can be lower in cost, especially in times of high alloy surcharges. Additionally, it may often be possible to reduce the section thickness of duplex stainless steel, due to its increased

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yield strength compared to austenitic stainless steel. The combination can lead to significant cost and weight savings compared to a solution in austenitic stainless steels.

Please download the updated brochure \Fabrication of Duplex Stainless Steels\

Practical Guidelines for the Fabrication of Duplex Stainless Steels, 2nd Edition (2.8 MB)

Chinese version: Practical Guidelines for the Fabrication of Duplex Stainless Steels, 2nd edition (3.7MB)

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Delong Diagram This refines the Schaffler diagram by taking account of the strong austenite stabilising tendency of nitrogen. The chromium equivalent is unaffected but the nickel equivalent is modified to

Ni (eq) = Ni + (30 x C) + (0.5 x Mn) + (30 x N)

The diagram, identifying the phase boundaries is shown below. This shows the ferrite levels in bands, both as percentages, based on metallographic determinations and as a ferrite number 'FN', based on magnetic determination methods.

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