David N. French, Sc.D
President of David N. French, Inc., Metallurgists,
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (6 printed pages)
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.
Table 1 -- Most Frequently Used Steels
|1 1/4 Chromium-
|2 1/4 Chromium-
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
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
At about 2760oF, the steel begins to solidify. The first solid that
forms is a body-centered cubic "delta ferrite." At a temperature of about 2700o
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
As an aside, all hot forming and shaping employed to make boiler tubes and
piping is done in the austenite temperature range of 1650oF to 2000oF.
With the continued cooling, the face-cantered-cubic austenite begins to
transform to body-centered-cubic ferrite at a temperature of around 1600oF,
and again, the exact temperature depends on the composition. Continuous cooling
to 1340oF changes the relative amounts of ferrite and austenite
until at 1340o
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.
The transformation from austenite to ferrite and iron carbide requires an un-mixing of the carbon. The carbon completely dissolves in austenite, and virtually none dissolves in ferrite. When the cooling is slow enough, this separation of dissolved carbon in austenite to a separate constituent, iron carbide, occurs in an orderly way, and pearlite forms. Pearlite is a sandwich of alternating layers of ferrite and iron carbide. When cooling rates are too
rapid, there is no time for the formation of iron carbide and pearlite. The carbon is trapped in the austenite, which is unstable at low temperatures.
Rapidly cooled austenite does change its atomic arrangement to martensite, a hard, brittle material with a lattice that is a distorted cube, called
"body-cantered tetragonal." This transformation can be an important concern
during welding and can lead to underbead cracking.
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
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
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.
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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