SELECTION OF MATERIALS FOR HIGH TEMPERATURE DUTIES IN BOILERS

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 undergo structural changes.

2.     PROBLEMS:

          The crucial most high temperature problems encountered in the design of steam boilers are associated with �Super Heaters� and �Re-heaters�.  The necessity of high superheat of steam mostly creates two major problems.  The first of these consists in prevention of fouling of Super Heater external surface by slag whereas the second problem is faced in selection of appropriate �Super Heater� tube materials since high temperature reduces the material strength and high pressure increases the stress.

3.     OUTCOME OF USING IN-APPROPRIATE MATERIAL:

          In case suitable material is not used, it fails prematurely at high temperature on account of one or more of the following reasons: -

I.            COLD WORKING:

The cold worked material must not be used at temperatures where re-crystallization may occur; because the �Creep Resistance� of Metal reduces during �Re-Crystallisation� process.

II.         graphitization:

Graphitization that is, the breakdown of �Carbides� into Free Iron & Graphite, of some steels, may occur after a considerable length of time at temperatures ranging between 427⁰C to 650⁰C. Graphitization may cause serious losses of strength and toughness in zones affected by Heating in this temperature range.

III.       CORROSION and OXIDATION:

Because of the increased rate of chemical reaction at high temperatures, corrosion and oxidation may give rise to serious problems. Corrosion may occur uniformly over the surface or be limited to small areas.  In such cases it is called �Pitting Corrosion�. The rate of uniform corrosion may be expressed as milligrams loss per sq. centimetres per day. The resistance of steel to high temperature oxidation increases commendably by additional elements, such as Chromium or Silicon.

4.     MATERIAL SELECTION:

          From the material selection stand point, Super Heater Tubes are most critical element in Steam Boilers. The Metal Temperature tends to be 50⁰C above than the Steam Outlet Temperature for Super Heater Coils. In selecting materials for Boilers it is normal practise to use the steel, which is most appropriate to the local temperature conditions. Several types of steels are used in such conditions. Commencing with, Carbon Steel for Main Bank & Furnace Tubes to Carbon-Molly or Chromium-Molly Alloys to various grades of Stainless Steels in the High Temperature Zone.

The following list is typical of material selection in modern Steam Boilers.

Sr.	Materials	Material Specification	Max. Metal Temperature
			(0F)	(0C)
(a)	Carbon Steel	BS 3059 St.320/ 360/ 440	800	425
(b)	Carbon � Mo.	SA 209	900	480
(c)	1 to 1� Cr.,  � Mo.	SA 213 T-11	1000	538
(d)	2� Cr., 1 Mo.	SA 213 T-22	1050	565
(e)	18 Cr., 8 Ni. + Ti.	SA 213 T-321	Over 1050	Over 565

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