David N. French, Sc.D.
President of David N. French, Inc., Metallurgists, Northborough, MA
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)
TABLE 1. OXIDATION LIMITS
|Carbon steel (SA210, SA106)
|Carbon-1/2 Mo (SA209)
|1-1/4 Cr-1/2 Mo (T-11, P-11)
|2- 1 /4 Cr-1 Mo (T-22, P-22)
|18 Cr-8 Ni (304, 321, 347)
The service conditions of a fossil-fired boiler are among the most severe of any large, engineered structure. Flame temperatures may reach more than 3000oF during combustion of the fuel. The products of combustion may be corrosive to the materials of construction. Ideally, only pure water and steam would be used, but, in practice, deviations from the ideal are expected. Life expectancies of these steam generators are 25 to 30 years or more. Tube-metal temperatures within the furnace will be 750oF and up; in superheaters and reheaters, temperatures of 1150oF are common; and in the support materials, temperatures of 1400oF or 1500o F are possible. This article will present some of the changes expected in ferritic boiler tubes in service at elevated temperatures.
It will also approach the subject from the viewpoint of changes in the microstructure, changes that are normal and expected for ferritic steels and occur without necessarily leading to failure. These microstructural changes occur at temperatures high enough to allow some atomic mobility. For carbon steels (SA210, SA192, SA178) this temperature is about 800oF. Thus, below about 800oF, no changes in the microstructure would be expected. Structural steel, SA36, has a similar composition and metallurgical characteristics as SA210 but is used, for the most part, at service temperatures not much above 100oF (perhaps 125oF on a hot day in Arizona). No changes in the appearance of the ferrite and pearlite will ever occur. SA192, on the other hand, may be used in a low-temperature or primary superheater and may experience metal temperatures above 800o F and suffer changes to the pearlitic structure of unused tubes.
The pressure parts of utility boilers are made from carbon steel, low- alloy chromium-molybdenum ferritic steels, and 18-8 chromium-nickel austenitic stainless steels. In a few unusual cases, 5-9% chromium- molybdenum steels have been used in superheater and reheater applications. The oxidation limits, the temperatures at which wastage is excessive for safe service for 25-30 years, for these alloys are listed in Table I.
As part of the original design, changes in material are made as estimated metal temperatures reach the oxidation limits given above. Thus waterwalls of sub-critical boilers are carbon steel, occasionally SA209. T-11 and T-22 may be used for super-critical units where temperatures may be 900o F. Superheaters and reheater alloys are T-11, T-22 and the 18-8 stainless steels, depending on the expected temperature. Economizers are usually carbon steel, SA210 or SA178. Downcomers are carbon steel, and the main stream piping is usually P-22, 2-1/4 Cr-1 Mo.
While it is clear why oxidation limits are set for these alloys in elevated-temperature service, there are less obvious, but important, microstructural changes that occur unseen by casual observation. Oxidation or corrosion of the fireside is easily observed. However, even at temperatures below the specific oxidation limit, long-term microstructure degradation may affect the strength and fitness for continued service. For new materials of plain-carbon and low-alloy chromium and molybdenum ferritic steels, the expected microstructure is ferrite (nearly pure iron) and pearlite, a lamellar structure of alternating layers of ferrite and iron carbide. This structure is made by slowly cooling the finished product from about 1650o and is called the normalized structure. In this condition the steel will meet all specification requirements and is as strong as it will ever be. All microstructural changes that occur during service will decrease the strength from this pearlitic structure.
As a model for these microstructural changes, a plain-carbon steel similar to SA210, SA192, and SA178 for boiler tubes, or SA106 for piping and headers, will be used. The Rockwell B hardness is somewhat variable but usually around 75. The pearlite is made up of alternating layers of iron carbide and ferrite, referred to as a "lamellar structure." The amount of pearlite depends on the carbon content. The individual particle of iron carbide or cementite in the pearlite is shaped like a playing card, long in two dimensions and short in the thickness direction. Each cluster is referred to as a "pearlite colony." The apparent spacing or thickness of the iron carbide and ferrite layers will depend on how the pearlite colony is sliced. When sliced perpendicular to the blade, the spacing is close, perhaps too fine to be resolved; when cut at an angle, the apparent thickness is greater.
At temperatures above about 800o F, the plate- or blade-like shape is unstable, and will change to sphere-like shape. The internal energy of the carbide is reduced by changing its shape to a sphere. The excess surface energy is the driving force for the change. The process that leads to the new shape is known as "spheroidization." Another factor besides temperature that will promote spheroidization is high stress. For plain- carbon (SA178, SA192, SA210) and carbon-1/2 molybdenum (SA209) steels, the iron carbide itself is unstable and will transform to graphite and ferrite. Thus, a further step in the microstructural changes is the formation of graphite particles within the steel, a process known as "graphitization."
Graphitization is a well-known phenomenon; and, in fact, the Boiler and Pressure Vessel Code, Section I has notes in Table PG23.1 that warn of the change in iron-carbide to graphite for the plain-carbon steels and the carbon -1/2 molybdenum steels.
The changes to the shape of iron-carbide, spheriodization, and its decomposition into graphite, graphitization, are competing processes. At temperatures above about 1000oF, graphite will appear after spheroidization. At temperatures below about 1000o F, graphitization occurs before the steel is fully spheroidized; that is, the pearlite colonies are still sharply defined, but graphite particles are clearly visible.
The addition of about 1/2% chromium as an alloying element to steels will prevent the change from iron carbide to graphite. Thus, T-2, T-11, and T- 22 materials with 1/2%, 1-1/4%, and 2-1/4% chromium will not graphitize. However, the microstructures will change from ferrite and pearlite to spheroidized carbides and ferrite.
This change in microstructure occurs at elevated temperatures. As temperature increases, the time for transformation decreases. For example, in SA178 or SA210 steels, spheroidization will occur in a few hours at 1300oF, but will take several years at 800-825oF. For T-22, spheroidization takes many years at 950oF, but less than a year at 1100o F. Thus there is an equivalency between time and temperature; equal changes as viewed in the microstructure occur at different times that depend on the service temperature.
Concomitant with these microstructural changes is a decrease in mechanical properties, tensile strength, hardness, creep strength, etc. All are reduced.
The purpose of a detailed metallurgical analysis of a boiler tube is to identify the root cause of the failure or establish the overall material condition and predict the nearness of failure. The microstructure at the rupture, and the tube in general, play an important role in the re-creation of the thermal history. The microstructures just discussed will provide useful information in the study of high temperature-failures.
Editor's note: 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 of the ASME Boiler and Pressure Vessel Code for current requirements.