Volume 6 Preprint 42
Failure of Super Heater of Boiler Tubes
Shahid Tufail Sheikh
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Volume 6 Paper C100
FAILURE OF SUPER HEATER OF BOILER TUBES
DR. SHAHID TUFAIL SHEIKH PSO
MINERALS AND METALLURGY RESEARCH CENTRE PCSIR
LABORATORIES LAHORE-54600 PAKISTAN
Boiler tubes of super heater failed very early when put into service, three samples of
these along with the sample of tube working with out any problems as reference
standard were evaluated for their Microstructure, Chemical analysis, Elemental
distribution using Electron Probe Micro Analyzer and Corrosion resistance in a
Combined Cyclic Corrosion tester in order to establish the cause of early failure. The
oxygen content, film or scale forming properties, residual stresses built into a part
being the main causes of boiler tubes corrosion. The composition of both the failed
tubes and the one working contains chromium and molybdenum in the right amount
and are used in the same conditions, the early failure of the tubes, was attributed due
to the poor processing during manufacturing, the formation of blow holes and
segregation of the carbides at the boundary, also the localized heating increases the
residual stresses already incorporated in the tubes during metal working, these stresses
crosses the limit of deformation tolerable causing the formation of inter crystalline
cracks leading to the bursting of the tubes. ````````
Corrosion in hot water systems particularly in boiler tubes is mostly due to the
dissolved oxygen (1), it may be uniform corrosion, pitting, inter granular cracking.
Water being a uniform solvent can dissolve all the materials present in the earth crust,
in presence of carbon dioxide the solubility of the water increases (2). The poor
quality water, high pressure and elevated temperature may also causes premature
failures. It is thus important that steel used in boiler tubes construction must be creep
and oxidation resistant at high temperature. The primary cause of the inter granular
attack is the presence of an inhomogeneous condition at the grain boundary. This may
be due to the segregation mechanism or of the inter granular precipitation. Metal
working techniques involves plastic deformation includes rolling, swagging,
explosive forming, and others. Because metalworking involves residual stresses being
built into a part, altering the microstructure and there is a limit to the amount of
Plastic deformation tolerable. Residual stresses can also be built into a worked piece,
stresses particularly tensile are detrimental to the work pieces and these stresses
increases with the amount of deformation and also lack of uniform working through
the thickness of a part. Residual stresses are deleterious because they increase the
effect of load on a component by adding to the applied load. Stress corrosion also
prolongs the loading. Applied stress due to over heating can cause cracking, low
melting point inclusion can also cause failure at high temperature by melting. Applied
stresses may cause cracking either of inclusion or at the inclusion-matrix interface.
Gas holes and porosity formed during welding process also propagates cracking.
Four samples of the super heater boiler tubes, including one good quality provided
were analyzed to determine their chemical composition. The results are as follows:
tube No.1 tube No.2 tube No.3
The microscopic examination reveals that tube of Japanese origin have
pearlitic-ferritic structure with very little or no inclusions and segregation at the
boundaries. Tubes of local origin contains inclusions along with the porosity in them,
corrosion pits were also present and chromium carbide segregating at the boundaries.
ELECTRON PROBE MICROANALYSER STUDY:
Structural Mapping At High Magnification:
Mapping of the four tubes were carried out at 25.0 KV AND 1.08
Ampere current. It shows the presence of heavy elements such as chromium,
molybdenum at the grain boundaries in the case of tube 2 and 3. Also the presence of
sulphur is indicated along with the manganese around the pores. Distribution of heavy
elements is more or less uniform in the sample of original tube.
Spot analysis were carried out non the four samples reveals the presence of sulphur
and manganese around the pores particularly in the samples of the tubes N0. 2 and 3.
The sample of the original tube have lesser amount of the sulphur in it.
The scanning electron microscope (SEM) reveals that porosity is prominent in the
samples of the fractured tube, heavy elements carbides are segregating at the grain
boundaries and the presence of non metallic inclusions in the fractured tube sample.
X-ray measurements were carried out at the accelerating voltage of 25-30 Kv volts.
The studies also confirms the presence of sulphide inclusions, in the samples of the
fractured tube particularly at the place where fracture has occurred and pores are
CYCLIC CORROSION TESTING:
The corrosion accelerated salt spray test of four tubes was carried out for 120 hours at
35°C and 70% relative humidity, using 5% sodium chloride solutions acidified with
acetic acid to a pH of 5.5. The number of corrosion spots on the fractured tubes 1-3 is
significantly more than the number of spots on the surface of the original tube.
Corrosion in most cases is caused by ordinary natural water, the amount of dissolved
oxygen is the controlling factor. In closed water system the oxygen concentration is
limited and practically determines the corrosion rate. Also, the variation in the content
of dissolved oxygen is the most active cause of pitting on the surface of the corroding
metal. In the absence of oxygen, all kinds of corrosion are reduced to a negligible rate
in most natural water.
Hot water systems when kept full of water and protected from undue contact with the
air practically shows no corrosion, because the oxygen present originally is exhausted
after a short time. On the other hand, hot water system, using the same water as in the
closed water systems failed in few years. Raw water is heated and added to the system
as the heated water is drawn out, and as result a fresh supply of oxygen is continually
brought in. The rapid corrosion in such cases is indicated by the decrease in dissolved
oxygen as the water flows through the system. Corrosion in steam, power plant is
more active in the hot feed water line, or where the water happens to impinge on the
metal of the boiler. Corrosion in such cases is caused by the comparatively large
amounts of dissolved oxygen present in these parts of the system and elevated
temperature. Corrosion boilers is the resultant of two groups of contending forces.
The first is the tendency of the metal to go into solutions, the initial corrosion thus
initiated is thus maintained by the depolarization effect of free oxygen and evolution
of hydrogen determined by the pH. Corrosion in steam boilers and accessories occurs
mainly in the following forms by uniform attack, intergranular attack, pitting; and
erosion combined with corrosion or cavitation effects in feed water pumps and
heaters. The solubility of all the materials constituting the earth crust is increased in
the presence of carbon dioxide or alkalies in water. Consequently natural waters are
impure. Many of these are entirely unfit for boiler use without pretreatment. Use of
poor quality water in high pressure boiler leads to high corrosion rates, sometime
accompanied by embrittlement or cracking of the metal. Operating under such
conditions may result in tube failures (due to overheating or pitting)
Steam boiler in operation is fairly efficient degassifier and may liberate into steam
practically all the dissolved gases in the feed water, particularly when evaporation is
progressing at a normal rate and the fed water is introduced above the water line. The
feed water should of course, always be in the low neutral or alkaline zone. When the
boiler are banked or used intermittently, or at a low rate of evaporation, the oxygen
should be keep as low as practicable. Dissolved oxygen is probably the greatest
accelerator of corrosive action in boilers and accessories. Differences in oxygen
concentration due to irregular sludge deposits, etc. account for much of the pitting in
the water space of boiler system.
Carbon dioxide causes serious corrosion to turbines, condensers, pipes. This occurs
only where condensation of the steam takes place. Carbon dioxide dissolves in water
forming carbonic acid, thus lowering pH influences the corrosion in boiler, mainly by
its effects on carbonates scales. Higher alkalinity is required to inhibit corrosion with
the higher concentrations of dissolved oxygen or soluble salt. But boiler operating at
very high pressure, the accelerating affect of temperature is so great that usually of
high pH (10.0-11.0) and zero oxygen is desirable.
The concentration of water increases with the increase in the concentration of soluble
salts present if sufficient free oxygen is also available, conditions become more
favourable to localized corrosion. This variation in concentration may give use to
concentration cell and localized corrosion, thus to ensure uncontaminated steam, it is
desirable to maintain relatively low and fairly uniform concentration of soluble salts
in the boiler water. Sulphuric acid may be generated in the boiler where the water has
been previously treated with coagulants; especially alum unless sufficient alkali is
added to form stable sulphates.
Cracking in steam boiler steel generally includes one or the other of two types of
stress corrosion. All such fatigue failures result mainly in trans crystalline cracks.
Another type of stress corrosion cracking is caused by the combined action of stress
close tot he elastic limits in bleaching systems in which caustic boiler water has been
highly concentrated. The grain boundaries of steel are chemically weak and failure
due to high stress and concentrated caustics solution is predominantly inter
Inter crystalline cracking occurs due to the four simultaneous conditions:
The boiler water must contain substances particularly hydroxides capable of
producing intergranular damage when concentrated in contact with steel stress.
There must be a joint or seam into through which leakage of this boiler water
may occur slowly.
The boiler water must concentrated with the joint or seam.
The steam must be over stressed locally where it is exposed to chemical
concentration. Such conditions may develop in seams in riveted drums and in rolled
tube seats and result in embrittlement of the steel and ultimate failure by cracking.
Steel used for boiler construction must have greater resistance to creep and oxidation
at temperature above 240°C.The solution of boiler water corrosion problem lies in the
removal of the causes of corrosion by adequate feed water conditioning since low
alloy steels posses very little advantage over plain carbon steels of good grade in their
resistance to aqueous corrosion. Low alloy steel containing chromium, molybdenum
eliminate the graphitization tendency in steam piping at elevated temperature.
Cracking occurs because of over heating. Any low melting point constituents at
segregated areas that are resent may melt at the temperature used to preheat the metal
for working, causing a weakness known as hot shortness. This can happen in steel
with a high sulphur content. Another potential for cracking exists in those metals
where the working causes precipitation which can result in increased yield strength
and reduced ductility. High temperature also causes the growth of grains which result
in weakening of the metal.
Gas holes and porosity are caused by the evolution of solidifying metal and
entrapment of the gas in solidifying metal. This gas may result form various
resources, a decrease in solubility upon cooling form the liquid
state, reaction of metallic oxide with C to form Co and Co2 and the reaction of liquid
metal with H2O in green sand mole.
The increase service life of the original tube as compared to the locally purchased
tube is due to the following:
Segregation of the heavy elements carbides in the fractured tubes, as compared
to the original tube. Inclusions and porosity is more significant in the fractured tube 23 whereas it is considerably less than the original tube.
Rapid material removal is more pronounced in the fractured tubes 1,2 and the
early failure is due to the development of porosity and inclusion which weakens the
metallic bond, thus when they are subjected to high pressure to steam at elevated
temperature, cracks developed at these weak point eventually leading to their bursting.
Walker, W. H., Anna M. Cederholm, and L. N. Bent, The corrosion of
Iron and Steel, J. Am. Chem. Soc., 28, 1251-1264, 1907; 473-474, 1908.
Hatfield, W. H., Corrosion as affecting the metal used in the
arts, Engineer, 134, 639-643, 1922.