CHAPTER7
Materials of Construction
7.1.INTRODUCTION
This chapter covers the lection of materials of construction for process equipment and piping.
Many factors have to be considered when lecting engineering materials,but for chemical process plant the overriding consideration is usually the ability to resist corrosion. The process designer will be responsible for recommending materials that will be suitable for the process conditions.He must also consider the requirements of the mechanical design engineer;the material lected must have sufficient strength and be easily worked. The most economical material that satisfies both process and mechanical requirements should be lected;this will be the material that gives the lowest cost over the working life of the plant,allowing for maintenance and replacement.Other factors,such as product contamination and process safety,must also be considered.The mechanical properties that are important in the lection of materials are discusd briefly in this chapter.Several books have been pu
blished on the properties of materials,and the metal-working process ud in equipment fabrication,and a lection suitable for further study is given in the list of references at the end of this chapter.The mechanical design of process equipment is discusd in Chapter13.
A detailed discussion of the theoretical aspects of corrosion is not given in this chapter, as this subject is covered comprehensively in veral books:Revie(2002),Fontana(1986), Dillon(1986)and Schweitzer(1989).
Corrosion and corrosion prevention are also the subject of one of the design guides published by the Design Council,Ross(1977).
7.2.MATERIAL PROPERTIES
The most important characteristics to be considered when lecting a material of construction are:
1.Mechanical properties
(a)Strength tensile strength
(b)Stiffness elastic modulus(Young’s modulus)
(c)Toughness fracture resistance
(d)Hardness wear resistance
(e)Fatigue resistance
(f)Creep resistance
2.The effect of high and low temperatures on the mechanical properties
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3.Corrosion resistance
4.Any special properties required;such as,thermal conductivity,electrical resistance,magnetic properties
5.Ea of fabrication forming,welding,casting (e Table 7.1)
6.Availability in standard sizes plates,ctions,tubes
7.Cost
Table 7.1.A guide to the fabrication properties of common metals and alloys
M a c h i n i n g
C o l d w o r k i n g
H o t w o r k i n g
C a s t i n g
W e l d i n g
A n n e a l i n g t e m p .°C
Mild steel
S S S D S 750Low alloy steel S D S D S 750Cast iron
baptiste giabiconiS U U S D/U Stainless steel (18Cr,8Ni)S S S D S 1050Nickel S S S S S 1150Monel S S S S S 1100Copper
(deoxidid)D S S S D 800Brass
S D S S S 700Aluminium S S S D S 550Dural S S S S 350
eligible
Lead S S Titanium S
S
U
U
D
S Satisfactory,D Difficult,special techniques needed.
U
Unsatisfactory.
7.3.MECHANICAL PROPERTIES
Typical values of the mechanical properties of the more common materials ud in the construction of chemical process equipment are given in Table 7.2.
7.3.1.Tensile strength
The tensile strength (tensile stress)is a measure of the basic strength of a material.It is the maximum stress that the material will withstand,measured by a standard tensile test.The older name for this property,which is more descriptive of the property,was Ultimate Tensile Strength (UTS).
The design stress for a material,the value ud in any design calculations,is bad on the tensile strength,or on the yield or proof stress (e Chapter 13).
Proof stress is the stress to cau a specified permanent extension,usually 0.1per cent.
7.3.2.Stiffness
Stiffness is the ability to resist bending and buckling.It is a function of the elastic modulus of the material and the shape of the cross-ction of the member (the cond moment of area).
286CHEMICAL ENGINEERING
Table7.2.Mechanical properties of common metals and alloys(typical values at room temperature)
Tensile0.1per cent Modulus of
strength proof stress elasticity Hardness Specific
(N/mm2)(N/mm2)(kN/mm2)Brinell gravity Mild steel4302202101002007.9 Low alloy steel4206602304602101302007.9 Cast iron1401701401502507.2 Stainless steel
(18Cr,8Ni)>5402002101608.0 Nickel
(>99per cent Ni)500130*********.9 Monel6501701701202508.8 Copper
(deoxidid)20060110301008.9 Brass
(Admiralty)4006001301151002008.6 Aluminium
(>99per cent)801507030 2.7 Dural40015070100 2.7 Lead3015511.3 Titanium500350110150 4.5 7.3.3.Toughness
上海留学Toughness is associated with tensile strength,and is a measure of the material’s resistance to crack propagation.The crystal structure of ductile materials,such as steel,aluminium and copper,is such that they stop the propagation of a crack by local yielding at the crack tip.In other materials,such as the cast irons and glass,the structure is such that local yielding does not occur and the materials are brittle.Brittle materials are weak in tension but strong in compression.Under compression any incipient cracks prent are clod up. Various techniques have been developed to allow the u of brittle materials in situations where tensile stress would normally occur.For example,the u of prestresd concrete, and glass-fibre-reinforced plastics in pressure vesls construction.
A detailed discussion of the factors that determine the fracture toughness of materials can be found in the books by Institute of Metallurgists(1960)and Boyd(1970).Gordon (1976)gives an elementary,but very readable,account of the strength of materials in terms of their macroscopic and microscopic structure.
7.3.4.Hardness
The surface hardness,as measured in a standard test,is an indication of a material’s ability to resist wear.This will be an important property if the equipment is being designed to handle abrasive solids,or liquids containing suspended solids which are likely to cau erosion.
7.3.5.Fatigue
Fatigue failure is likely to occur in equipment subject to cyclic loading;for example, rotating equipment,such as pumps and compressors,and equipment subjected to pressure cycling.A comprehensive treatment of this subject is given by Harris(1976).
MATERIALS OF CONSTRUCTION287 7.3.6.Creep
Creep is the gradual extension of a material under a steady tensile stress,over a prolonged period of time.It is usually only important at high temperatures;for instance,with steam and gas turbine blades.For a few materials,notably lead,the rate of creep is significant at moderate temperatures.Lead will creep under its own weight at room temperature and lead linings must be supported at frequent intervals.
The creep strength of a material is usually reported as the stress to cau rupture in 100,000hours,at the test temperature.
7.3.7.Effect of temperature on the mechanical properties
The tensile strength and elastic modulus of metals decrea with increasing temperature. For examp
le,the tensile strength of mild steel(low carbon steel,C<0.25per cent) is450N/mm2at25ŽC falling to210at500ŽC,and the value of Young’s modulus 200,000N/mm2at25ŽC falling to150,000N/mm2at500ŽC.If equipment is being designed to operate at high temperatures,materials that retain their strength must be lected.The stainless steels are superior in this respect to plain carbon steels.
Creep resistance will be important if the material is subjected to high stress at elevated temperatures.Special alloys,such as Inconel(International Nickel Co.),are ud for high temperature equipment such as furnace tubes.
The lection of materials for high-temperature applications is discusd by Day(1979). At low temperatures,less than10ŽC,metals that are normally ductile can fail in a brittle manner.Serious disasters have occurred through the failure of welded carbon steel vesls at low temperatures.The phenomenon of brittle failure is associated with the crystalline structure of metals.Metals with a body-centred-cubic(bcc)lattice are more liable to brittle failure than tho with a face-centred-cubic(fcc)or hexagonal lattice.For low-temperature equipment,such as cryogenic plant and liquefied-gas storages,austenitic stainless steel(fcc)or aluminium alloys(hex)should be specified;e Wigley(1978). V-notch impact tests,such as the Charpy test,are ud to test the susceptibility of mat
erials to brittle failure:e Wells(1968)and BS131.
The brittle fracture of welded structures is a complex phenomenon and is dependent on plate thickness and the residual stress prent after fabrication;as well as the operating temperature.A comprehensive discussion of brittle fracture in steel structures is given by Boyd(1970).
7.4.CORROSION RESISTANCE
The conditions that cau corrosion can ari in a variety of ways.For this brief discussion on the lection of materials it is convenient to classify corrosion into the following categories:
1.General wastage of material uniform corrosion.
2.Galvanic corrosion dissimilar metals in contact.
3.Pitting localid attack.
4.Intergranular corrosion.
288CHEMICAL ENGINEERING
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5.Stress corrosion.
6.Erosion corrosion.
7.Corrosion fatigue.
8.High temperature oxidation.
9.Hydrogen embrittlement.
Metallic corrosion is esntially an electrochemical process.Four components are necessary to t up an electrochemical cell:
1.Anode the corroding electrode.
2.Cathode the passive,non-corroding electrode.埃及艳后传奇
3.The conducting medium the electrolyte the corrodingfluid.
4.Completion of the electrical circuit through the material.
Cathodic areas can ari in many ways:
(i)Dissimilar metals.
(ii)Corrosion products.
(iii)Inclusions in the metal,such as slag.
(iv)Less well-aerated areas.
(v)Areas of differential concentration.
(vi)Differentially strained areas.
pricked7.4.1.Uniform corrosion
夹子英语
This term describes the more or less uniform wastage of material by corrosion,with no pitting or other forms of local attack.If the corrosion of a material can be considered to be uniform the life of the material in rvice can be predicted from experimentally determined corrosion rates.
Corrosion rates are usually expresd as a penetration rate in inches per year,or mills per year(mpy)(where a mill D10 3inches).They are also expresd as a weight loss in milligrams per square decim
etre per day(mdd).In corrosion testing,the corrosion rate is measured by the reduction in weight of a specimen of known area over afixed period
of time.
ipy D 12w
tA
7.1
where w D mass loss in time t,lb,
t D time,years,lool
A D surface area,ft2,
D density of material,lb/ft3,
as most of the published data on corrosion rates are in imperial units.
In SI units1ipy D25mm per year.
When judging corrosion rates expresd in mdd it must be remembered that the penetration rate depends on the density of the material.For ferrous metals100mdd D0.02ipy.
What can be considered as an acceptable rate of attack will depend on the cost of the material;the duty,particularly as regards to safety;and the economic life of the plant.For
MATERIALS OF CONSTRUCTION289 the more commonly ud inexpensive materials,such as the carbon and low alloy steels, a guide to what is considered acceptable is given in Table7.3.For the more expensive alloys,such as the high alloy steels,the brass and aluminium,thefigures given in Table7.3should be divided by2.
Table7.3.Acceptable corrosion rates
Corrosion rate
ctm是什么ipy mm/y
Completely satisfactory<0.010.25
U with caution<0.030.75
U only for short exposures<0.06 1.5
Completely unsatisfactory>0.06 1.5
The corrosion rate will be dependent on the temperature and concentration of the corrosivefluid.An increa in temperature usually results in an incread rate of corrosion; though not always.The rate will depend on other factors that are affected by temperature, such as oxygen solubility.
The effect of concentration can also be complex.For example,the corrosion of mild steel in sulphuric acid,where the rate is unacceptably high in dilute acid and at concen-trations above70per cent,but is acceptable at intermediate concentrations.
7.4.2.Galvanic corrosion
If dissimilar metals are placed in contact,in an electrolyte,the corrosion rate of the anodic metal will be incread,as the metal lower in the electrochemical ries will readily act as a cathode.The galvanic ries in a water for some of the more commonly ud metals is shown in Table7.4.Some metals under certain conditions form a natural protectivefilm; for example,stainless steel in oxidising environments.This state is denoted by“passive”in the ries shown in Table7.4;active indicates the abnce of the protectivefilm.Minor
Table7.4.Galvanic ries in a water
Noble end
(protected end)18/8stainless steel(passive)高辛烷值
Monel
Inconel(passive)
Nickel(passive)
Copper
Aluminium bronze(Cu92per cent,Al8per cent)
Admiralty brass(Cu71per cent,Zn28per cent,Sn1per cent)
Nickel(active)
Inconel(active)
Lead
18/8stainless steel(active)
Cast iron
Mild steel
Aluminium
Galvanid steel
Zinc
Magnesium
