Typical metals

Metal structure

All metal are recognised as having a crystalline structure. The crystals are geometrically regular in shape. The molecules are attracted to each other by “binding forces” which are non-direc — tional and encourage these molecules to take up a regular shape. Whilst all solids have some tendency to become crystal­line, metals are likely to form the most regular and packed arrangement.

Where impurities are present, the crystals like to form around them. The metallurgist tries to improve the strength of the mate­rial, by controlling the order of the metal crystals and introduc­ing other elements necessary to improve some particular prop­erty desired for the alloy.

Carbon steels

Small percentages of carbon are introduced into steel to im­prove its strength. At the same time this may reduce its ductibility and weldability. Approximate physical properties are as shown in Figure 7.2.

Typical metals

Ultimate tensile strength N/mm2 Figure 7.2 Typical strength of steel with varying carbon contents Typical properties of such steels are shown in Table 7.1

Type

Low carbon steel

Structural

Steel

Steel

Casings

Machined part steel

% Carbon

0.1

0.2

0.3

0.4

% Manganese

0.35

1.4

0.75

Yield stress N/mm2

220

350

270

480

Ultimate tensile stress N/mm2

320

515

490

680

Table 7.1 Carbon content versus strength of steels

Low-alloy and alloy steels

Low-alloy steels have small amounts of chromium, magne­sium, molybdenum and nickel to increase certain physical properties. Alloy steels have an even larger percentage of these elements, together with silicon, vanadium and others to give increased strength and hardness.

Cast irons

These are iron and carbon alloys which have somewhat more than 2% carbon. They may be subdivided into grey and white varieties.

Grey cast iron

These types have a grey appearance with a structure of ferrite, pearlite and graphite. The latter exists as either flakes or spheres. Nodular or spheroidal graphite cast iron is obtained by adding magnesium which helps the graphite to form spheres. This material is widely used for the hubs of centrifugal fan im­pellers.

White cast iron

This material is hard and brittle due to its structure of cementite and pearlite. It is difficult to machine and is therefore used for wear resisting components. In the past it has been used for cast scroll segments of mill exhausters.

Malleable cast iron

These are forms of cast iron which are heat treated to improve their ductility whilst retaining their high tensile strength. Three types are usually recognised:

Whiteheart — which is heated with an iron compound to give a ferrite outer skin and a ferrite/pearlite core

Blackheart — which is soaked at high temperature to break down the cementite and then slowly cooled to produce ferrite and graphite.

Pearlite — very much the same as Blackheart, but cooled faster to produce a higher strength

Stainless steels

This term describes a group of steel alloys containing over 11 % chromium. There are four main categories, which in turn may be subdivided into many different proprietorial and generic grades.

Austenitic — which contain 17 to 25% chromium combined with 8 to 20% nickel and/or magnesium and other trace alloying elements. They are easily weldable due to the low carbon con­tent and in their raw state are non-magnetic. Magnetism can however, be induced by heavy working. Good strength is com­bined with high corrosion resistance.

Ferritic — again have a high chromium content greater than 17% together with medium carbon content and small quantities of molybdenum and silicon. Good corrosion resistance rather than high strength and generally non-hardenable. Magnetic.

Martensitic — have a high carbon content up to 2% and a low chromium content generally around 112%. Difficult to weld. Magnetic.

Duplex — grades contain both austenitic and ferritic phases. High tensile strength at normal temperatures is combined with good corrosion resistance due to the addition of trace ele­ments. Weldable, but becomes brittle above 300 °C.

Non-ferrous metal and alloys

This term is used for all those metals or alloys which do not con­tain iron as the base element. Apart from copper they are rarely used in a pure form and hence the term alloy is often more ap­propriate. Some typical properties of these alloys are given in Table 7.2.

Main constituent

Ultimate tensile strength N/mm2

Typical alloys

Aluminium

100 to 500

Duralumin, silumin

Copper

200 to 1100

Brasses, cupronickels, aluminium & tin bronzes, gunmetal

Magnesium

150 to 340

Nickel

400 to 1200

Monel®, Inconel®, Hastelloy®, Nimonic®

Titanium

400 to 1500

TiCu, TiAl, TiSn

Zinc

260 to 360

A, B, ZA12

Table 7.2 Properties of non-ferrous alloys 122 FANS & VENTILATION

Aluminium alloys

These are widely used in the fan industry where lightness com­bined with strength is desired. Whilst pure aluminium is rela­tively weak, the addition of small quantities of other elements can increase its strength and hardness enormously. Mechani­cal properties can also be improved by work hardening.

There are now a very large number of aluminium alloy grades available in both casting grades and sheet form.

Axial flow fan blades and hubs are frequently cast in grades such as LM6 and LM31. Readers are referred to relevant Brit­ish, European and International standards for further information.

Centrifugal fans can have impellers and casings fabricated from relevant sheet grades, many of which are weldable. Again reference to standards is recommended.

The use of silumin, a grade containing about 12% silicon has especial properties for fans in explosive atmospheres. When subject to a grinding action, the material tends to fracture, be­fore frictional deformation and heat can result.

Copper alloys

Whilst copper in its pure form may be used for electrical compo­nents, its alloys are of particular interest to the fan engineer. Thus brasses may be used as anti-spark features at the bound­aries between close running, stationary and rotating parts (see Chapter 8). In this case admiralty brass, which has a small lead content, is particularly good. It has been widely used in fans for coal mines and offshore oil rigs. Some authorities, however, re­strict the use of alloys containing lead and its acceptance should be verified.

The fans used for the ventilation of oil tanker holds have to be of intrinsically non-sparking design. In such cases the complete impeller may be made of aluminium bronze together with po­tential rubbing parts.

Magnesium alloys

Not used to any extent in the fan industry, due to their flammability. There may however be a use for them in certain special applications.

Nickel alloys

Nickel is commonly alloyed with copper, chromium and iron to produce a range of materials with high temperature and corro­sion resistance. The Nimonics® and Hastelloy® have been ex­tensively used for high temperature fans (in excess of 500 °C) whilst Monel® has been used for fan shafts, due to its ability to withstand shock loads (when dampers have to close in mi­cro-seconds or large “lumps” pass through the fan).

Titanium alloys

Titanium may be alloyed with many other elements to produce a range of materials which are extremely light, strong and resis­tant to many corrosive gases and vapours. In consequence they may be used to produce a lightweight impeller which can rotate at high speed to produce high pressures. Anything is possible, so long as you can afford it!

Zinc alloys

Particularly useful for the production of small die cast parts, due to the ease of casting. Provided that stresses and shock loads are not high, then a zinc alloy may be acceptable.

Posted in Fans Ventilation A Practical Guide


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