selection of materials for high temperature duties in boiler
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8/12/2019 Selection of Materials for High Temperature Duties in Boiler
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217362139.doc
1. EFFECT OF TEMPERATURE:
With the increase in temperature of metal, its strength and resistance to
corrosion in such environment decreases, the metal/s may even undergostructural changes.
2. PROBLEMS:
The crucial most high temperature problems encountered in the design ofsteam boilers are associated with Super Heaters and e!heaters. Thenecessity of high superheat of steam mostly creates two ma"or problems. Thefirst of these consists in prevention of fouling of Super Heater e#ternal surfaceby slag whereas the second problem is faced in selection of appropriateSuper Heater tube materials since high temperature reduces the material
strength and high pressure increases the stress.
3. OUTCOME OF USING IN-APPROPRIATE MATERIAL:
$n case suitable material is not used, it fails prematurely at high temperatureon account of one or more of the following reasons% !
I. COLD WORKING:The cold wor&ed material must not be used at temperatures where re!crystalli'ation may occur( because the )reep esistance of *etalreduces during e!)rystallisation process.
II. GRAPHITIZATION :+raphiti'ation that is, the brea&down of )arbides- into ree $ron +raphite, of some steels, may occur after a considerable length of timeat temperatures ranging between 012 3) to 453 3). +raphiti'ation maycause serious losses of strength and toughness in 'ones affected byHeating in this temperature range.
III. CORROSION AND OXIDATION:6ecause of the increased rate of chemical reaction at hightemperatures, corrosion and o#idation may give rise to seriousproblems. )orrosion may occur uniformly over the surface or be limitedto small areas. $n such cases it is called 7itting )orrosion. The rate ofuniform corrosion may be e#pressed as milligrams loss per s8.centimetres per day. The resistance of steel to high temperatureo#idation increases commendably by additional elements, such as)hromium or Silicon.
ALLIED ALLOY
PRODUCTS9!5:, $ndustrial ;rea, 533:, Haryana, $ndia
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