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.

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.

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