The Metallurgy of Power Boilers

David N. French, Sc.D
President of David N. French, Inc., Metallurgists, Northborough, MA

October 1990  

Category: Design/Fabrication 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (6 printed pages)

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Steels are alloys of iron and carbon, usually with one or more alloying elements added to improve some properties of the material (strength, high-temperature strength, oxidation or corrosion resistance, for example). By definition, steels contain at least 50% iron. For welded construction, the ASME Boiler and Pressure Vessel Code limits the carbon content to less than 0.35%. Thus, virtually all of the materials used in the construction and repair of pressure parts of boilers fall into this classification. Some high-temperature, corrosion-resistant alloys of nickel and chromium with less than 50% iron are not, strictly speaking, steels, but are still occasionally used. Further, steels are divided into two subcategories: ferritic steels and austenitic steels, depending on the arrangement of atoms within the solid.

Steels are used in boiler construction because they are inexpensive, readily available, easily formed and welded to the desired shape and, within the broad limits, are oxidation- and corrosion-resistant enough to provide satisfactory service for many years. Table 1 lists the most frequently used steels, some common tubing specifications and the maximum recommended service temperatures.

These five alloys cover probably 85% to 90% of the steels used of the many acceptable grades listed in the Code. There are others that may find specific applications, for example 1/2 Chromium-1/2 Molybdenum alloy SA213 T-2, 9 Chromium-1 Molybdenum alloy SA213 T-9, and corrosion-resistant, high-temperature alloys of nickel and chromium, SB-407.

The maximum useful temperature is determined either by corrosion or oxidation concerns that limit the useful life before premature failure or changes within the microstructure occur that weaken the steel too much for elevated-temperature service.

In order to understand the behavior of steels in boiler environments, a working knowledge of the fundamentals of the metallurgy of these materials is needed. For our model we will take the alloy of iron and carbon, historically the first steel. All matter is made up of atoms, and iron and steels are no exception. The way these atoms arrange themselves to form a solid is referred to as a "lattice." For convenience, assume the atoms of the metal are solid spheres in contact, similar to the stacking of billiard balls. For steels there are two arrangements that are important. Both are cubes, but the arrangement of the atoms within the cube differ. In one, referred to as "body-centered cubic" (BCC), one atom is at each of the eight corners of the cube, and one atom in the center, as shown in Figure la. The other arrangement, the "face-cantered cubic" (FCC), has one atom at the eight corners of the cube, and one atom in the center of each of the six faces of the cube, as shown in Figure lb. The body-cantered cubic arrangement is referred to as "ferrite," and the face-cantered cubic arrangement is called "austenite." The addition of the element carbon does not alter this arrangement. Carbon is a small atom and some will fit within the holes between the spheres of iron.

The amount of carbon that can fit within these FCC and BCC lattice arrangements differs. For the body-cantered cubic arrangement of ferrite, the amount of carbon that will dissolve, (that is, fit into the holes), is virtually nil, about 0.02%. For the face centered cubic austenite arrangement, about 2% carbon will dissolve in the lattice holes.

To start with, in our steel model, only iron and carbon is cooled from the molten condition as, for example during the fabrication of a steel casting or the solidification of a weld. The following changes occur during the slow cooling:

At about 2760^oF, the steel begins to solidify. The first solid that forms is a body-centered cubic "delta ferrite." At a temperature of about 2700^o F (the precise temperature depends on the exact composition), the steel is completely solid. On further cooling, at a temperature of about 2500 degrees F, the delta ferrite (body-centered cubic) transforms to austenite (face-centered cubic).

As an aside, all hot forming and shaping employed to make boiler tubes and piping is done in the austenite temperature range of 1650^oF to 2000^oF. With the continued cooling, the face-cantered-cubic austenite begins to transform to body-centered-cubic ferrite at a temperature of around 1600^oF, and again, the exact temperature depends on the composition. Continuous cooling to 1340^oF changes the relative amounts of ferrite and austenite until at 1340^o F, the remaining austenite transforms to pearlite. The pearlite is a mixture of ferrite and a carbon-rich constituent called "iron carbide" or "cementite." The ferrite is nearly pure iron, dissolving less than 0.02% carbon. The iron carbide has a lattice arrangement that is referred to as "hexagonal" and is more complex than the simple cubic arrangements shown in Figures 1a and 1b. The relative amounts of ferrite and iron carbide will differ depending on the amount of carbon within the alloy; higher carbon grades will have more pearlite than lower carbon grades.

Ferritic steels are "ferrite" and iron carbide (pearlite) at room temperature. Other than carbon, the principal alloying elements are chromium and/or molybdenum; T-l, T-ll, and T-22 are the common examples. When sufficient chromium and nickel (18% Cr and 8% Ni) are added, the FCC "austenite" lattice remains stable to room temperature; hence this class of steels is called "austenitic." Since these 18-8 chromium-nickel alloys have excellent corrosion resistance and do not show rust-colored corrosion products, they are referred to as "stainless" steels. The nickel-chromium alloys with less than 50% iron (for example, SB 407) are also austenitic, as their lattice arrangement is FCC as well.

Atoms of iron are quite small, about 100,000,000 would fit in an inch. Thus, useful sizes of material contain a huge number of individual atoms.

The next step in the building block of making useful shapes is a crystal or grain. All of the atoms arranged within a given lattice in the same orientation defines a crystal. The grain size is variable, but within steels is fairly small; about 1,000 to the inch or about 1 mil (0.001") in diameter, for example. Thus there are about 100,000 atoms of iron across and perhaps 1015 atoms (10 followed by 15 zeros) in an individual crystal.

Since crystals are small, another large number is needed to make a useful shape. Neighboring crystals or grains do not have the same orientation of the lattice. Where two grains come together and meet, they form a crystal or grain boundary. The lattice arrangement in these two crystals is the same, but the orientation is different. A grain may be characterized by long-range order in the atomic arrangement. At the grain boundary, individual atoms are not arranged regularly and therefore short-range disorder characterizes the grain boundaries.

The observation and interpretation of grain structure is called metallography; and, as shall be evident later, the appearance under the microscope can tell a great deal about the past history of a piece of steel. Grain boundaries will play important roles in the interpretation of some failures, and these features will be more fully covered then. The region of short-range atomic disorder is more easily corroded because of the imperfect bonding of neighboring atoms within the confusion of the grain boundary. A more rapid corrosion of these grain boundaries allows the crystal structure to be examined. A controlled corrosion, called "etching", of a smoothly polished surface attacks the disorganized grain boundaries more rapidly than it does the well-organized crystals themselves. When examined at a high magnification in a microscope, the light reflects off the crystals like a mirror, but is trapped within the groove of a grain boundary; and thus the grain boundary shows up as a black line within the field of view.

The addition of other metals to iron improves the strength. There are two kinds of alloying elements, substitutional and interstitial. When the metallic atoms are similar in size to iron, for example chromium, nickel, molybdenum, manganese, and silicon, the atoms can substitute for iron at individual lattice points, and are called "substitutional solid solutions." When small atoms are used, for example carbon, nitrogen, or boron, the small atoms (relative to the size of the iron atom), fill the holes within the lattice and are called "interstitial solid solutions." Carbon is by far the most common alloying element and has importance all out of proportion to its content. For example, 0.2% carbon will increase the strength of pure iron from about 40,000 psi to about 60,000 psi. To get the same 50% improvement in strength takes more than 2 1/ 2% chromium and 1% molybdenum, as a comparison of the strength of SA192 and SA213 T-22 will indicate.

This article has introduced the concepts of atomic arrangement or lattice, crystals or grains, and grain boundaries and metallography.

Metallurgical Failures in Fossil Fired Boilers , John Wiley & Sons, Inc., N.Y., N.Y. 1983.

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Editor's note: The previous article is reprinted from the October 1990 National Board BULLETIN. Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.

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