Structural Steels

All the NPPs of the boiler, mainly the supporting structure and the plate work (flues, ducts, casing, hoppers, pent house, and bunkers), are made from structural steels. These steels are specified in the building codes. In a way there are no stringent requirements to be met as in the case of PP steels, except that as the structures become large, the sections become excessively heavy. Limited suppliers, protracted deliveries, and also much higher specific cost make fabricated sections from plates competitive at times when the welding and handling costs are lower. Structural steels required in a boiler are presented in Table 5.12.

The basic requirements of structural steel are good weldability and strength. Members of the cold structure are required to have adequate notch toughness if installed in very cold climates. Components for hot structures, such as buckstays and sling rods, are many times designed for temperatures <400°C and call for no particular high-temperature prop­erties as for PPs.

Table 5.13 gives the structural material codes to American standards and their nearest BS and DIN equivalents. In all the codes, the basic material grades and their properties are covered in specifications, whereas the dimensions and other related mechanical details of each rolled section are covered in individual standards.

Structural Quality Tubes and Pipes

AH tubes are thin tubes of structural quality in plain CS for main sections or weathering steel for cold sections.

TABLE 5.12

Structural Steels Required in Boilers

Items

Consisting

Material

Main Areas of Application

Rolled

Beams, channels,

CS

Boiler-, bunker-, and deaerator-supporting

Sections

Angles, flats

Structures, stiffeners for ducts, and buckstays

Plates

CS, AS

Column base and top plates, gussets, splices, top suspension girders, AH ends, and PP attachments

Steel

Sheets >4 mm

CS, AS, ss

Flues, ducts, casing, hopper, bunkers, and bunker

Sheets

Liners

Coils/thin

Sheets <2 mm,

GI, ss

Cladding and expansion joints

Sheets

Colored

Strips

CS, ss

Membrane bars, header and drum attachments, ladders, and platforms

Rounds

Black and bright bars

CS

Foundation bolts, suspension hangers, pipe supports, earthquake restraints, and drum slings

Fasteners

CS, AS

Nuts for foundation bolts, bolts, and nuts for ducts

Pipes

CS

Safety valve, silencer and IBD tank escape pipes, round ducts for air and gas, and hand rails for galleries

Tubes

<76.2 mm

CS, COR-TEN

Airheater main and stay tubes

Note: CS, carbon steel; AS, alloy steel; ss, stainless steel; GI, galvanized iron; COR-TEN™ weathering steel.

TABLE 5.13

Structural Steels to American Specifications and Their Equivalent BS and DIN Standards

American Specificati

Ons

BS Specification

DIN Specification

Designation

Items

C

(%)

Tensile

(kpsi)

Yield

(kpsi)

Tensile

(MPa)

Yield

(MPa)

A 36

Sections

0.26

58

36

400

250

4360-40- B, C

17000 R St 37.2

A 572 Gr 42

Sections

0.21

60

42

415

290

-43-A, B, C

St 44.2

A 572 Gr 50

Sections

0.23

65

50

450

345

-50-A, B, C

A 36/572Gr50

Sections

0.22

65

50

450

345

-50-A, B, C

A 53 Gr A

Pipes

0.25

48

30

330

207

3601-S 360

1629 St 37.0

A 53 Gr B

Pipes

0.30

60

35

415

240

430

44.0

A 501

Hollows

0.30

58

36

400

248

4360-40 B 43 D

17120 R St 37.2

Sections

St 44.3

Note: Indicated tensile and yield strengths are the minimum figures.

The other structural quality steel tubes used in boilers are the pipes for hand railing. Salient points of the AH tubes are as follows:

• AH tubes usually measure 50.8, 63.5, 70 mm, and occasionally even 76.2 mm OD (2, 2 1/2, 2 3/4, and 3 in.), with thickness ranging from 11 to 14 SWG (2-3 mm).

• These tubes are usually in ERW execution and can be ordered in exact lengths with ends annealed if tubes are to be expanded.

• COR-TEN is a trade mark of U. S. Steel Corporation for low-alloy weathering struc­tural steels containing small amounts of Cr, Ni, Si, Ti, Zr, and V, which offers four to eight times corrosion protection over plain CS against a mildly corrosive AH environment.

• COR-TEN is produced in grades A and B. COR-TEN A is preferred for its better resistance to weathering.

High-Temperature Castings

• Although pressure castings are rarely used in boilers except for valve bodies, they are used at several places as NPP high-temperature castings.

• In the form of SH and RH lugs, air nozzles, stoker links, grate castings, or soot blower hangers, high-temperature castings constitute hundreds of pieces of small sizes weighing a few grams to a few kilograms.

• As ash nozzles, ash doors, peep hole doors, or smelt spouts, high-temperature cast­ings may be few and of medium size of —50 kg or so.

• Big castings are used only in pulverizing mills for rings and balls or wheels.

High-temperature castings can experience temperatures above 550°C. Many of them are proprietary developments not conforming to the material codes entirely. High-temperature castings are of two types: (1) iron and (2) steel.

Heat-Resistant Iron Castings

Iron castings are used where the shapes are intricate and there is no welding requirement. Also the castings are not subjected to tensile or alternating forces. Stoker parts, ash doors, peep hole doors, etc., fall in this category. The majority of the iron castings are chrome cast­ings with the compositions shown in Table 5.14.

TABLE 5.14

Typical Composition of Heat-Resistant Chrome and Cr-Ni Iron Castings

Composition (%)

Temperature

C

Si

Mn

S

P

Cr

Ni

Typical Parts

<600°C

3.2-3.4

1.5-2.5

0.75-0.85

<0.12

<0.25

0.30-0.70

Ash/furnace doors,

600-700°C

3.2-3.4

1.5-2.5

0.75-0.85

<0.12

<0.25

0.70-1.10

Ash spouts

600-700°C

3.0-3.5

1.7-2.4

0.60-1.00

<0.12

<0.4

0.85-1.50

Grate bars— heat-resistant

600-700°C

<3.4

1.6-2.0

0.60-1.00

<0.1

<0.2

0.40-0.60

1.00-1.50

Grate parts—heat — and growth — resistant

600-700°C

3.0-3.5

1.7-2.4

0.60-1.00

<0.12

<0.4

0.60-1.00

0.80-1.50

Grate parts—wear — resistant

Heat-Resistant Steel Castings

These castings are of three types with increasing temperature-withstanding capabilities. Besides castings, the heat-resistant materials are also available as bars and sheets.

1. Ferritic steels are suitable up to 900°C. The principal alloying element is Cr.

2. Ferritic-austenitic steels have improved heat resistance and toughness over ferritic steels due to the addition of Ni along with Cr.

3. Austenitic steels. Addition of Ni in excess of 9% makes steel of austenitic struc­ture free from transformation. Owing to higher toughness, the steels are relatively insensitive to shock stresses but machining the castings is very difficult.

SH and RH lugs and burner impellers are some of the most highly heated parts in a boiler with temperatures approaching 1500°C where alloys with the following compositions give fairly satisfactory service.

• 25% Cr-12% Ni to A351 CH 20

• 25% Cr-20% Ni to A351 CK 20

Using fuel oils with high V or S shortens life due to corrosion at temperatures above

650°C because the compounds containing Na, V, and S melt at a temperature as low

As 565°C. Oil ash in molten conditions is excessively corrosive, demanding even higher alloys such as

• 50% Cr-50% Ni

• 60% Cr-40% Ni

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