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Section 1 Materials of Chemical Equipments
Section 2 Design of Chemical Vessels
Section 3 Mechanical Design of Typical Equipments
Reference:Structural Analysis and
Design of Process Equipment
Basic of Chemical Basic of Chemical Machinery & EquipmentsMachinery & Equipments
Section 1 Materials Section 1 Materials of Chemical Equipmentsof Chemical Equipments
1.1 Summarization1.1 Summarization
Chapter 1 Materials Chapter 1 Materials and Selectionand Selection
Mechanical Properties
Physical Properties
Chemical Properties
Technical(processing) Properties
1.2 Properties of Materials1.2 Properties of Materials
I. Definition:
The capability of materials to resist
external forces, but does not deformation
beyond allowance or wreck.
1.Mechanical Properties:
II. Main Performance Index:
Five Index: Elasticity, Plasticity,
Strength, Hardness,
Toughness
Elastic State(curve ob)
i.proportional limit:
o
pp F
P
o
ee F
P
σ
ε
abc
d
.
p
es
b
o
Tensile curve of Low Carbon steel
Elasticity
ii.elastic limit:
Strength:
lodo
p
p
PAo
Stress= (MPa
)
shrinkage
4Ao= πdo2 Strain= l
lo
ultimate tensile stress σb
yielding point σs, creep limit σn
creep rupture strength σD fatigue limit (strength) σ-1
i. Yielding State
(near point c)
o
ss F
P
ii. Intensification State (curve cd)
T. S. (Tensile Strength)
o
bb F
P
oF
P 2.02.0
Conditional Yielding
When it is stretched to a certain degree,
there will be shrinkage ,and then break.
iii. Shrink Neck State (after d)
As Figure-1 showing:
For the metal materials which have no apparent
yielding phenomena,it’s stipulated that:
σ0.2 = stress in 0.2% of residue elongation
1) Yielding Point σs (MPa)
it’s the minimum value in yielding state or the point in which the apparent plastic deformation appears.
*Conditional yielding point σ0.2
*The stress of any point in the pressure vessel caused by pressure from medium should be below the elastic limit and cannot happen the plastic deformation.
抗拉强度 The maximum value of stress from the beginning of being stressed to the end of fracture.
2)Ultimate Tensile Stress σb (MPa)
σb
σs
Elastic state Plastic state
σ
ξ
Yielding point
i. Normal or low temperature:
considering: yield [yield/tensile] ratio: σs / σb
Generally speaking, σs <σb
σs/ σb ↓ , Plasticity ↑, Deformation ↑
σs/ σb ↑ , Plasticity↓, Deformation ↓
Strength Usage ↑
ii. Elevated temperature:
Creep Rate
mm/mm*h(P con.)
Temperature(0C)--1Cr18Ni9Ti
425 475 520 550
10-6 176 91 33 6
considering: σn and σD as well as the previous
3) Creep Limit σn
#The temperature in which metals creep
#Creep phenomena: 蠕变 When the materials is in high temperature and in certain stress, the stress increases as the time is going.
Carbon steel > 420 0CAlloy steel > 450 0CLight metal and alloy > 50-150 0C Pt, Sn Normal Temperature
τ
ε
o
ab
cd
# Creep Curve
#Creep limit σn (MPa):Definition: The ability of materials to resist the slowly plastic deformation under high temperature.Under certain temperature, the creepspeed does’t excess the stress stipulated.Stipulated creep speed: 10-7 mm / mm . H 10-6 mm / mm . H1% straining within 105 hours1% straining within 104 hours
Definition: 持久强度 under certain temperature, the material cracks in a stress after a period of stipulated time. This stress is called creep rupture strength.
4) Creep Rupture Strength σD (MPa)
Stipulated time: 105 hours
Because the designed life time of chemical equipments is commonly 105 hours, the stress under which material cracks is said to be rupture strength.
Creep rupture strength is the ability to resist
cracking under certain temperature and
load. The stronger the ability is, the longer it
will endure under the same conditions.
5) 疲劳极限 (Strength) σ-1(MPa)
Fatigue strength: the maximum stress, under which the materials do not happen fatigue destruction or failure after infinite times of alternate load action.
Fatigue phenomenon: the constructional
elements destruct under the alternate
load action.
Times of Fatigue Test: 106 ~ 108
塑性
延伸率 After the unit of structure is cracked by tensile force, the ratio of the total stretched length and the origin length is called Percentage Elongation, described by δ%
1)Definition: the ability of plastic deformation but not destructing under external force.2)Commonly used Index: Percentage Elongation Shrinkage of Sectional Area Cold Bending Property
%100%100 00
0
l
l
l
ll kk
lk — the gauge length after cracking, mm
l0— the origin gauge length, mm
△lk—the absolute length after cracking, mm
Figure as following:
The meaning of Percentage Elongation:
i) The value of reflects the degree of the
plastic deformation before the material cracks.
ii) The larger , the better the plasticity of material.
iii) Plastic material > 5%; Low carbon steel = 20~30%
iv) Hard brittle material < 5%; Cast iron = 1%
断面收缩率() After the unit of structure is cracked, the
ratio of the reduced area of the cross-section and the original (cross) sectional area is called Shrinkage of Sectional Area which is described by ψ%.
Fk—the minimum As after cracking , mm2
F0—original sectional area As , mm2
%100
F
FF k
Cold Bending Property
Welding joint
R
The larger the , the better the plasticity of the material. The of Low Carbon Steel is about 60%.
With R increasing, the plasticity of materials will be better and better.
i) Forming handling(process) and welding ease, such as bending and rolling 、 forging
press 、 cold impacting 、 welding and etc.
ii) Make the unit of structure to avoid cracking for deformation after bearing load.
iii) The Pressure Vessels and their spare parts
should have the characteristic.
The real meaning of the Plastic Index:
硬度I. Definition: when something which is
harder than material itself is pressed on the surface of it, it will resist the pressure by deformation or be damaged, such abilities are called Hardness.
II. The Hardness Index:
Brinell Hardness (HB)
Rochwell Hardness (HR)
维氏 Hardness (HV)
III. The test of HB:
d
D
P
)( )(
222 aMPdDDD
p
F
pHB
p ——Pressure, N D——The diameter of the rigid ball, mm d ——The diameter of the indent, mm F ——The area of the Indent, mm2
IV. The relationship of Hardness and Strength: Generally, good Hardness leads to good Strength and good resistance to wear and tear. Experimental Value (MPa):
Low Carbon Steel b ≈ 36 HB
High Carbon Steel b ≈ 34 HB
Gray Cast Iron b ≈ 10 HB
V. Application of Hardness in Engineering
冲击韧性 ak
1)Definition: The ability of materials to resist the impact load, i.e., the ability of materials that will make plastic deformation immediately and rapidly when suddenly attacked by dynamic loading.
)( )(
221
cmJ
F
HHG
F
Aa kk
Ak—— Impact Work, J
F —— The sectional area of the
notch in the unit, cm2
2) Impact Toughness
The larger is a k , the better is the ability of materials to resist the impact load.
For Mediate and Low Pressure Vessels,
a k≥30 ~ 35J/cm2 , commonly a k > 60 J/cm2.
The relationship between Toughness and
Plasticity:
Generally, stronger toughness makes stronger plasticity; but strong plasticity may not make strong toughness .
Hard Brittle Materials’ a Hard Brittle Materials’ a kk << << Plastic Materials’ a Plastic Materials’ a kk
a. Modulus of elasticity (E) (M Pa)
E
Nature of E: 1) It’s the index of materials’ ability to resist elastic deformation. E↑ , ability to resist deformation↑. E of steel is about 2 10╳ 5 ( M Pa ) . 2) For the same material, T ↑ , E↓ .
2.Physical Properties:
b. Poisson’s Ratio
'
(For steel: ≈ 0.3)
′—— transverse stress —— longitudinal stress
c. Thermal Expansion Coefficient ()
•Physical Meaning of :
When T increases by 1 , ℃ the increasing
length per unit length is called Thermal
Expansion Coefficient.
•Application of in Engineering.
)C/1(
tl
ltll
3.Chemical Properties:
Definition: It’s the chemical stability of
materials in medium, i.e. , it’s the nature that
whether the materials react with medium
chemically or electro-chemically leading to
corrosion.
Two index:a. Corrosion Resistance b. Resistance to Oxidation
a. 耐腐蚀性 —— the ability of metal materials to resist
the corrosion caused by the medium (such as atmosphere, water vapor, electrolyte).
b. 抗氧化性 1 ) Resist to high temperature oxidation;
2 ) Resist to oxide etch by other gaseous medium, such as water vapor, CO2 , SO2 , etc.
4.Technical (Processing) Properties:
A. Definition: All performances in physical, chemical and mechanical properties when materials are in processing, they make the Technical or Processing Properties of the materials.
B.Contents: as following
Casting Property ——Fluidity, Congealing Shrinkage Rate Forging Property ——Resistance to Thermal Fragment, Resistance to Oxidation, Thermo-plasticity. Welding Property ——Fluidity of parent material and welding flux in the melting state, Congealing, Shrinkage Rate, Thermo-plasticity. Machining Property——Hardness, Brittleness. Heat Treatment Property ——Heat Treatment Feasibility. Cold Bending Property——Plasticity, Toughness.
Review of 1.2:Properties of Materials: Four kinds (1)Mechanical Properties
(2)Physical Properties
(3)Chemical Properties
(4)Technical (Processing) Properties
What are their indexes respectively and what is the meaning of them all?
How to calculate them all?
1.3.Carbon Steel
and Cast Iron
The Classification of Carbon Steel and Cast IronA. According to their Chemical
Components :
Iron Carbon Alloy:
(>95%)Fe +(0.05% ~ 4%)C
+(~1%)(impure steel and cast iron)
B. According to the Carbon Content:
Steel C%=0.02~2%
Cast Iron C%>2%
Engineering Pure Iron C%<0.02%
Pure Iron Steel Cast Iron0.02 2 4 C%0
1.3.1 The structure of iron-carbon alloy steel 1. The structure of metal The micro-structure of metalچ (Structure in brief)
GrainGrain Boundary
Different structure cause different performance of materials.
Thick plate-like Graphite
Low strength
Thin plate-like Graphite
Mediate strength
Spherical Graphite
High strength
2. Isomeric Transformation
of Pure Iron同素异构
is the phenomenon that the crystal
configuration changes with the temperature
in the state of solid.
Classification:
t < 910 Cubic Lattice in Bulk Center, ℃
called “x-Fe”
t > 910 Cubic Lattice in Face Center, ℃
called “y-Fe”
The transformation accomplishes in 910 ℃
without T changing.
-Fe -Fe910℃
The transformation is as following:
The basic types of C existing in iron-carbon alloy:
3.Carbon and its existing form in steel
DissolutionChemical CombinationBlending
A. Dissolution C dissolute in the lattice of Fe to form
Solid Solution
—— Fe-C Solid Solution.
Solvent—— the element without changing
in lattice , Fe is the solvent
Solute —— the element dissolving in
solvent , C is the solute
Two kinds of common-used Solid Solution
Solubility of C
At room temperature 0.006 %
723 ℃ 0.02 %(maximum)
铁素体 (F):
The solid solution formed by C dissolving in
-Fe is called Ferrite.
•Characteristics:
Because the gap between atoms is small, the capacity to dissolve C is weak.
• Properties:
Low strength
Low hardness
Good plasticity
Good toughness
asab MPMP 170~90,280~200
aMPHB 0.8~5.5
%40~30
2/5.2~8.1 mMJak
奥氏体 (A):
The solid solution formed by C dissolving in
-Fe is called Austenite, it is denser than Ferrite.
The lattice of C keeps in that of -Fe, i.e.
Cubic Lattice in Face Center.• Characteristics:
Because the gap between atoms is large, the capacity to dissolve C is strong.
Solubility of C
723 ℃ 0.8 %
1147 ℃ 2.06 %(maximum)
• Properties:
High strength
High hardness
Good plasticity
Good toughness
No ironic magnetism
The transformation between F and A:
723 ~ 910℃Ferrite (F) Austenite (A)
Both F and A have good plasticity and they
are the structural basis of steels’ characteristic
of excellent plasticity.
The irons that dissolve C will take the
transformation between -Fe and -Fe in
different temperature.
B. Chemical Combination
C and Fe form the metallic compound
——Iron Carbide (Fe3C) whose crystal
structure is called 渗碳体 indicated
by “C”. C + 3Fe Fe3C
•Characteristics:
a)The carbon content of Cementite is high,
the mass proportion is 6.67%.
b)Hard and brittle (HB=78.4MPa)
c) Almost no plasticity and toughness
d) Low break-down strength (b≈35 MPa )
e) The Cementite is semi-stable compound, it will decompose into Fe and C at certain conditions, the extricated C exists in the form of graphite.
Fe3C C + 3Fe
C. Mechanical Blending (Mixture)
The alloy whose components are blending together in the state of liquid cam solidify into two types of mechanical mixtures:
a) Mixture formed by two solid solutions;
b) Mixture formed by a solid solution and metallic compound.
For example: 珠光体 (P) 、 Ladeburite (L) is a kind of
Mechanical Mixture.
Pearlite (P) = Ferrite (F) + Cementite (C)
Ladeburite (L) = Austenite (A) + Cementite (C)
Conclusion and review of 1.3.1:
The three basic structural types are
showed as following:
Ferrite (F)
Austenite (A)
Cementite (C)
1.3.2 The impure elements in the Carbon Steels and their effects
The main impure elements are:
Mn Si S P O N H
Mn is useful element.
Si is useful element.
S is harmful element.
P is harmful element.
O is harmful element.
N is harmful element.
H is harmful element.
A. Mn < 0.8%
—— the common existing impure element
Coming from the deoxidizing and desulfurizing agent in the process of smelting.
Function: eliminating S and O2.
They won’t effect the properties of steels if the content of both are little.
1.Manganese (Mn):
B. Mn > 0.8%
—— the alloy element intentionally Function: Mn can disolve in the
ferrite to form the solid solution strengthening the effect of ferrite.
Si < 0.5% —common existing impure element Coming from the deoxidizing agent and ore. Function:
Ability of deoxidation is stronger than Mn.
2FeO + Si 2 Fe + SiO2
Si can dissolve in the Ferrite and improve
the strength and hardness of steels.
2.Silicon (Si):
The existing form in the steels:
Forming solid solution with Ferrite.
or Remaining in the steels in the form of
deoxidation product——SiO2
3.Sulphur (S): Originating in the fuels in ore or which
are used in the process of smelting
——called Coke. The existing form:
FeS (S doesn’t dissolve in Fe) Function:
The low-melting-pointed compound
(985ºC) formed by FeS and Fe makes the steel unit crack in the process of hot-working, this phenomenon is called “Hot Brittle”.
Controlling of the content of S:
Common Steel
S < 0.055 ~ 0.07%
High Grade Steel
S < 0.03 ~ 0.045%
Super High Grade Steel
S < 0.02 ~ 0.03%
4.Phosphorus (P): Originating in the ore. Function:
P in steels can dissolves in -Fe and improves the strength of steels in normal atmospheric temperature & brittleness, but dramatically reduces their plasticity and toughness, this phenomenon is called “Cold Brittle”.
When the content of P in the steel is P=0.3%, the impact toughness ak = 0.
Controlling of the content of P: P < 0.06%
5.Oxygen (O): Originating in the air. Existing form:
O2 always exists in the steels in the form of non-metallic inclusion, such as FeO, SiO2 , MnO, MgO, Al2O3, etc.
Function: These oxidations is in the steels as
solid grains which are hard but brittle and damage the continuity of basic structure of steels sharply reducing the mechanical property of steels.
Eliminating the O2 in the process of smelting.
6.Nitrogen (N): Originating in the air. Function:
Low Carbon Steels with high-content of N2
are particularly lack of resistance to
corrosion.
Easy to form the air bubble to be loose.
Cause the phenomenon of “Age-hardening”.
Methods:
Adding Al and Ti to form AlN and TiN as if making the N fix in the steels (called N-fixed Treatment), this will eliminate the age-hardening.
7.Hydrogen (H): Originating in moist feed in steel-melting
stove, pouring system and the moist air, etc. Function:
Making the steels to be brittle
(H-Brittle)
Making the steels to be seriously defective
(Fish-eye)
Methods:
Improve the environment of smelting.
Clear up the moisture content in the feed.
Purify the steel liquid.
Review of 1.3.2:What are the impure elements? Mn, Si, S, P, O, N, H
What are their respective function
in steels (which are harmful and
which are useful) ?
What are “Age-hardening”, “Fish-eye”, etc.? What is the factor to form them?
1.According to the content of carbon (C%): Low Carbon Steel
Medium Carbon Steel
High Carbon Steel
1.3.3 Classification anddesignation of the equipments
i. 低碳钢 (C<0.25%)
Low strength and good plasticity,
used in chemical vessels in welding and
mechanical units with low loads.
ii. Medium Carbon Steel (C=0.25%~0.6%)
Medium strength and plasticity, used
as the important units of shaft, gear, top
cap of high pressure equipments and so on.
iii. High Carbon Steel (C>0.6%)
High strength and hardness, poor
plasticity, used as string, wire line and so on.
2.According to the smelting methods:
Full Killed Steel Boiling Steel Semi-killed Steel
i. 镇静钢 — completely deoxidized steel
a. Deoxidation by the strong deoxidizer Si
to reduce the content of oxygen to be
less than 0.01%, commonly it will be
0.002~0.003%.
b. Tough structure, uniform texture and solid.
c. The Pressure Vessels’ steels should apply the full killed steels.
d. Using ‘Z’ to indicate the designation or none.
ii. 沸腾钢 — incompletely deoxidized steel
a. Deoxidation by the strong deoxidizer Mn
to reduce the content of oxygen to be
0.03~0.07%.
b. Loose structure, inferior texture.
c. Commonly used in Support, Frame and the
like unimportant units.
d. Designation is ‘F’.
iii. Semi-killed 半镇静钢 — between the previous two Designation is ‘b’.
3.According to the quality:
Common Steel
High Grade Steel
Super High Grade Steel
i. 普通碳素钢
a. The content of the harmful elements
S & P can be a little more to be
(S≤0.055%, P≤0.045%)
b. Three kinds of the old designation of
Common Steels (GB700-79) : A——merely assuring the mechanical
properties (A1, A2, A3, ……A7)
B——merely assuring the chemical
components (B1, B2, B3, ……B7)
C——assuring both of the previous
points (C1, C2, C3, ……C5)
c. The new designation of Common Steels
(GB700-88):
For example: Q 235 — A. F
The first letter of the Chinese spell of the word “ 屈”The first letter of the Chinese spell of the word “ 屈”
The value of the steels’ yielding point with the unit of “MPa”
The value of the steels’ yielding point with the unit of “MPa”
The quality grade of steels
The quality grade of steels
The deoxidized method
The deoxidized method
Designation
Z Z
BD C B B
b.Z
Q275
F.b.Z
A
Q255
F.b.Z
A
Q235
F.b.Z
A
Q215
F.b.Z
Q195
Deoxidized Method
Grade
d. The applying range of the Common Steels in the chemical equipments:
ΘFull Killed Carbon Steel Plate: *Q235-A——suitable under the condition of P≤1.0MPa, t=0~350ºC, S≤16mm. The media shouldn’t be used in the Pressure Vessels with media ultra-hazardously toxic or high-hazardously toxic or with media as liquified petroleum gas.
*Q235-B——suitable under the condition of P≤1.6MPa, t=0~350ºC, S≤20mm. The media shouldn’t be used in the Pressure Vessels with media ultra-hazardously toxic or high-hazardously toxic.*Q235-C——suitable under the condition of P≤2.5MPa, t=0~400ºC, S≤30mm.
ΘBoiling Carbon Steel Plate:
*Q235-AF——suitable under the condition
of P≤0.6MPa, t=0~250ºC, S≤12mm.
The media shouldn’t be used in the
Pressure Vessels with media media,
ultra-hazardously toxic or
high-hazardously toxic or
combustible.
ii. 优质碳素钢a. Seriously control the content of S & P to
be (S & P≤0.04%)
b. Uniform texture, good surface quality, superior properties than Common Steels.
c. The number in designation indicates the percentage of the average content of C:
08 : C=0.08% 20: C=0.2%
d. Designation: Steels that commonly contain Mn (without indicating Mn)
08 10 15 20 22 25 30 35 45 50…… Steels that contain more Mn (Mn=0.7~1.2%) (indicating Mn) 30Mn 40Mn ……
iii. 超级优质碳素钢
(1)S & P≤0.03%
(2)Both the texture and properties of this
kind of steels’ are superior to that of
High Grade Steel.
(3) Designation: adding the letter ‘A’ after
the designation, such as 20A, 25A …...
(4) Indication of steels with different usage
by the letter of Chinese spelling: Boiling Steel: g such as 20g Vessel Steel: R such as 20R
Review of 1.3.3:Classification and designation of the
equipments according to:
(1)Content of carbon
(2)Smelting methods
(3)Quality
What are the concrete steels under each
of the previous titles?
How to explain the designation?
1.Bring forward the problem: Find out the method and path of altering
the properties of steels
2. 热处理的目的 Eliminating some shortages of steels
Improving some properties of steels
1.3.4 Heat Treating of Steels
3. 热处理的优点 Intensifying the metallic materials,
fully developing the potential of materials,
lightening the mass of equipments and
guaranteeing the security and expected life
of equipments.
4. 热处理的定义 Heat treatment is the technical process or treatments to steels in solid state according to the scheduled requirements like heating, keeping warm and cooling, their aims are to vary the internal structure and gain the desired properties.
5.Basic Theories of heat treatment:
When the basic components of steels (Fe) is heated to a certain degree, its lattice structure of steel will vary from one form to another as the temperature.
Ferrite (F) and Austenite (A) are both the solid solution of Fe, so they have the lattice structure of iron.
6.Processing steps of heat treatment:
Heating Keeping warm CoolingT
emp
erat
ure
/ºC
Time
Keeping warm
CoolingHeating
Cooling media and way of cooling:
Cooling in furnaceCooling in still air Cooling in oilCooling in waterCooling in brine
Cooling Capacity Cooling Speed
7.Heat Treating Process of steels:
Annealing
Normalizing
Quenching
Tempering
i. 退火 正火 (1)The function of annealing and normalizing
*Lowering hardness, improving plasticity,
making steels apt to the cold-work.
*Homogenizing the steel structure, refining
the grain, developing the mechanical
properties.
*Clearing up the internal stress, resisting
the deformation of workpieces.
(2)The selection of annealing and normalizing
*Aimed mainly at improving the
machinability, normalizing is better for
Medium or Low Carbon Steels; while
annealing for High Carbon Steels.
*Aimed mainly at developing the
mechanical properties and never need
any other heat treatments, normalizing
is better.
*From the aspect of economy, normalizing
is better than annealing.
ii. 淬火 (1)Process
Heating the steel pieces to the
quenching temperature, cool them
quickly in the quenching agents after
the warm-keeping treatment, then the
Austenite changes into the Matensite.
(2)Quenching Temperature
*Hypo-eutectoid Steel (C<0.8%)
heating above the A3 line 30~50ºC
*Hyper-eutectoid Steel (C>0.8%)
heating above the A1 line 30~50ºC
(3)Quenching Agent
*Mineral Oil, Water, and Brine.
*Generally speaking:
Carbon Steel, cooling in water and brine.
Alloy Steel, cooling in oil.
(4)Quenching Function
——developing the hardness, strength
and wear (abrasion) resistance.
*The emergency cooling in quenching is apt
to make flaw in the steel pieces, so the
tempering is commonly needed to clear
up the stress after quenching.
*Quenching and Tempering are always
combined to the technical process.
iii. 回火 (1)Process Heat the steel pieces which are already quenched to the certain temperature
(T<Tcritical), cool them quickly in still air after the warm-keeping treatment. (2)Purpose Reduce or clear up the internal stress of workpieces after quenching, stabilize the internal structure and gain the different mechanical properties.
(3)Types of Tempering *Tempering at low temperature ——after quenching, tempering between 150~250ºC. Function——reduces the internal stress and brittleness of quenching steels, and at the same time keeps the high hardness and high wear resistance. Usage ——in spares of various tools and ball bearing after carburation.
*Tempering at medium temperature
——after quenching, tempering
between 300~450ºC.
Function——reduce the internal stress,
reach the limit of high
strength and high elasticity.
Usage——in the treatment of various
spring.
*Tempering at high temperature ——after quenching, tempering between 500~680ºC. Function——gain the certain strength, have higher plasticity and impact toughness, i.e. excellent overall mechanical properties. Quenching + Tempering ——
Thermal Refining Usage——important spares, such as gear, rod, crank shaft, etc.
Review of 1.3.4:Types of Heat Treatments: (1)Normalizing (2)Annealing (3)Quenching (4)Tempering
What are their definition?What are their functions in the improvement of mechanical properties of materials?
1.The chemical components of commonly used cast iron: 95% Fe + (2.5% ~ 4%) C + ( ~1%) Purities
2.Structure: Pealite + Cementite + Ladeburite + Graphite
1.3.5 Cast Iron
3.Properties and Characteristics: Excellent casting property Good machinability Good wear resistance Excellent property to reduce vibration Low plasticity and brittleness Low tensile strength and high (ultimate) compression strength
4.Properties and Designation of commonly used cast iron:
Gray cast iron (HT)
Spherical graphite cast iron (QT)
High-silicon cast iron (G)
i. 灰口铁 (HT)
(1)Properties and characteristics
*C exists in the form of plate-like graphite
*Gray fracture
*Low mechanical properties
*Excellent corrosion resistance in H2SO4
and NaOH
(2)Designation
HT 150 — 330 HT 200 — 400
Tensile Strength b (MPa)
Tensile Strength b (MPa)
Bending Strength bb (MPa)
Bending Strength bb (MPa)
ii. 球墨铸铁 (QT)(1)Properties and characteristics *C exists in the form of spherical graphite *Have better strength and a certain plasticity and toughness, its overall mechanical properties are close to that of steels. *Better corrosion resistance than that of
HT except when it is in the acid solution.
(2)Designation
QT 400 — 15 QT 450 — 10 QT 450 — 10
Tensile Strength b (MPa)
Tensile Strength b (MPa) Elongation Percentage
%
Elongation Percentage%
iii. High-silicon cast iron (G)
(1)Properties and characteristics
*Adding amount of Si (14.5~18%) to
improve the corrosion resistance of
the cast iron.
Highly corrosion resistant media:
nitric acid, sulfuric acid,
phosphorus acid, acetic acid
Medium corrosion resistant media:
hydrochloric acid, 草酸 ( 甲酸 ), 蚁酸
Corrosive media:
caustic soda, hydrofluoric acid
*Low tensile strength, good hardness, brittle and easy cracking. *Impact non-resistant, hard to cut and cast only.
(2)Designation ST Si 15 R
Content of Si is 15%Content of Si is 15%
Mixing lanthanide Mixing lanthanide
Review of 1.3.5:Types of commonly used cast iron: (1)Gray cast iron (HT)
(2)Spherical graphite cast iron (QT)
(3)High-silicon cast iron (G)
What are their properties and
characteristics?
How to explain their designations?
1.4.
Common Low Carbon Steel
and Special Steel
used in Chemical Equipments
1.Shortages of Carbon Steel: i. Low strength and yield ratio (s/ b)
σs
σb
σ
ε
σs/σb of Carbon Steel is small
σσb
σs
εσs/σb of Alloy Steel is large
1.4.1 Problems
The strength comparison
between Carbon Steel and Alloy Steel
Material σ s
(MPa)
σ b
(Mpa)σ s / σ b
Carbon
Steel
Q235-AR 240 400 0.6
20 250 400 0.62
Alloy
Steel
16MnR 360 520 0.69
15MnV 400 540 0.74
18MnMoNbg 520 650 0.8
ii. Low strength at high temperature
The strength comparison (at high T)
between Carbon Steel and Alloy Steel
σS (Mpa) Material
S(mm)
20℃400℃450℃500℃
Q235-A 20~40 230 125 — —CS 20g 26~36 125 115 —
16Mn 27~36 310 190 170 —AS 18MnMoNbR 16~38 520 420 390 350
230
iii. Poor hardenability
iv. Inferior special physical and chemical
properties
2.Necessity to the development of modern industry and
Science & Technology
1.Alloy elements: i. Definition
The elements that are added on
purpose to develop the structure and
characteristics of steels.
ii. Main alloy elements
Cr Ni Mn Si Al Mo
V Ti Cu B Nb W Re
1.4.2 Effect of Alloy Elements to the properties of steels
2.Alloy Steel
Definition 合金钢 Alloy steels are those steels that
contain the alloy elements which develop the properties of steels.
3.Characteristics of the main alloy elements: i. Cr
(1)Cr>13%, corrosion resistance dramatically
(2)Strength, hardness, wear resistance,
oxidation resistance and hardenability all
(3)Plasticity and toughness
(4)Adds strength at high temperature
ii. Ni
(1)Enlarge the range of corrosion resistance
of stainless steel, especially improve the
resistance to base.
(2)Broad the -phase region as to be the
element that form the austenite.
(3)Develop the strength as well as keep
excellent properties of plasticity and
toughness.
(4)Improves strength at high T( 热强性 ?)
iii. Mn
(1)Develop the strength and impact
toughness at low temperature.
(2)Broad the -phase region.
(3) Counteracts sulfur brittleness.
(4)Increases hardenability.
iv. Si (1)Develops strength and fatigue durability at high temperature. (2)Improve heat resistance (3)Resistant to the corrosion of such media
as H2S and so on. (4)If amount of Si is too much, plasticity and impact toughness both (5)Strengthens steel (6)Increases hardenability
v. Mo (1)Develop the resistance of stainless steels to the chloride anion Cl-. (2)Enhances H corrosion resistance. (3)Improve the heat resistance. (4)Raises grain-coarsening temperature. (5)Mo<0.6%, plasticity . (6)Counteracts tendency toward temper brittleness.( 要否 ?)
vi. Al
(1)Restricts grain growth.
(2)Develops the impact toughness.
(3)Resistant to the corrosion caused by H2S.
(4)Improves the oxidation and heat resistance.
(5)Cheap, common substitute for Cr among
heat-resistant steels.
vii. Ti
(1)Restricts grain growth.
(2)Develops strength and toughness.
(3)Improves the oxidation and heat resistance.
(4)Stablizes C to prevent the
“inter-crystalline corrosion”.
(5)Prevents formation of austenite in high
chromium steels; prevents localized
depletion of chromium in stainless steel
during long heating.( 英文书上的 , 要否 ?)
viii. V (1)Developes high-temperature strength. (2)Increases hardenability. (3)Restricts grain growth. (4)Keeps the strength and improve the plasticity. (5)Resists tempering ( 英文书上的 , 要否 ?)
A
E S P H
WR
IT CR OR HR FD GR H
Cr
Ni
Mn
Si H2S
Mo Mo<0.6% HCl
Al H2S
Ti in-c
V
Re
Interpretation: AE——alloy element S ——strength P ——plasticity H/WR ——hardness and wear resistance IT ——impact toughness CR ——corrosion resistance OR ——oxidation resistance HR ——heat resistance FD ——fatigue durability GR ——grain refining H ——hardability in-c ——inter-crystalline
Review of 1.4.2:The main alloy elements: Cr Ni Mn Si Al Mo V Ti Cu B Nb W Re
What are their characteristics?
What are their functions in the improvements of mechanical properties of steels?
1.Definition: 普通低合金钢 They are the steels that are
formed by adding a few alloy elements at the basis of
Common Low Carbon Steel.
1.4.3 Common Low Alloy Steel
2.Composition: (1)C<0.2%
(2)Alloy elements
*Mn 1~1.5%
*Si Cr Ti V Nb Ni Al… 0.015 ~ 0.6%
3.Structure: Ferrite + Pearlite
4.Properties and characteristics: i. High strength and large yield ratio
ii. Excellent welding property
iii. Good resistance to the corrosion of
atmosphere
iv. Perfect properties at low temperature
5.Designation (GB1591-88) (New Designation GB/T1591-94)
16Mn 16MnR 16Mng 15MnV 15MnVR15MnVg 09Mn2V 18MnMoNbRThe number ahead is the percentage of the C
content, such as 16Mn (C = 0.16%).
Indicate the main alloy elements, the number thereafter is the percentage of that element. If it is less than 1.5%, it can be omitted.
Content of alloy elements:
1.5 ~ 2.49% Sign as “2” 2.5 ~ 3.49% Sign as “3” 3.5 ~ 4.49% Sign as “4”
1.Steels specially used in the
manufacture of boilers and vessels.
2.There are some special requirements
for boiler steel and vessel steel.
1.4.4 Boiler Steel & Vessel Steel
3.Commonly-used Designation: i. Boiler Steel
20g 22g 12Mng 16Mng 15MnVg
14MnMoVg 18MnMoNbg
ii. Vessel Steel
Q235-AR 20R 16MnR 15MnVR
09MnVR 18MnMoNbR
不锈钢 Stainless Steels are the kind of alloy
steels which are resistant to the
corrosion caused by atmosphere,
water or other soft caustic media.
1.4.5 Stainless Steel and
Corrosion (Acid) Resistant Steel
耐酸钢 Acid Resistant steels are the kind of alloy steels which are resistant to the corrosion
caused by acid or strong caustic media. As a rule, we called them both “Stainless Steel”. Examples:
*Chromium Stainless Steel
*Chromium-nickel Stainless Steel
1.Chromium Stainless Steel:
i. Component
< 0.2% C + (13 ~ 28%) Cr + Fe
ii. Construction
Ferrite or Martensite
(no Austenite even at high temperature)
iii. Theories of corrosion resistance (1)In the oxidizing medium, a oxide skin
Cr2O3 which is stable and tight will be formed, it has an effect on passivation, i.e. there is a passivation layer on the surface of the steels. (2)The degree of corrosion resistance depends on the content of C and Cr. The more Cr, the better the resistance The less C, the better the resistance
iv. Commonly-used Chromium Stainless Steel
1Cr13 2Cr13 0Cr13 0Cr17 0Cr17Ti
v. Designation
(1)The first number:
Average C content 平均含 C 量的千分数 0 : C < 0.1% 1: C≤0.15% 2: C≈0.2%
(2)The second number:
percentage of the average content of Cr
2.Chromium-nickel Stainless Steel: i. Component
≤ 0.14% C + ( 17~19% ) Cr + ( 8 ~11%) Ni + Fe
Briefly called “18 — 8” Steel
Typical Designation: 1Cr18Ni9Ti
ii. Construction
Single austenite structure at normal temperature
iii. Characteristics (1)High strength and good plasticity & toughness (2)Large range of suitable temperature -196 ~ 800 ℃ ℃ (3)Excellent technical properties (4)Good corrosion resistance
ΘNon-corrosive media:
cold phosphorus acid, nitric acid, acetic acid,
hydrogen sulfide, sulfate, nitride, base liquid,
petroleum chemicals, etc.
ΘCorrosive media:
hydrochloric acid, dilute sulfuric acid (<10%),
hot phosphorus acid, oxalic acid ( 草酸 ),
melting caustic potassium, melting caustic
alkali, Cl-, bromine (Br), iodine (I), etc.
(5)Inter-crystalline corrosion easily occurs
between 400~800 ℃ Θ晶间腐蚀 Definition of inter-crystalline corrosion:
It is the phenomenon that
the corrosion occurs between two
crystalline surfaces and causes the
grain boundary continuously damaged.
ΘNature:
It’s a kind of local and selective
corrosive damage.
ΘOccurring in:
Austenitic stainless steels
ΘReason:
Lack of Cr element in the grain boundary
ΘAustenitic stainless steels (C<0.14%):
*At high temperature (1050ºC)
C distributes completely in whole alloy.
*Between 400~800℃ C + Cr + Fe (Cr . Fe)23C6
Anode
Cr < 12.5%
Inter-crystallineCorrosion occurs
Separate out along the grain boundaryCr%
Cr lacking
Corroding minicell
Cathode—Grain
—Cr lacking region
Grain
(Cr . Fe)23C6
Cr lacking region
Grainboundary
ΘDamage: To be brittle, even softly beating can makes it break into dust. Have very low strength.ΘPreventive measures: *Solution heat treatment —— quenching again (1100~1150ºC) to dissolve C and Cr into the austenite. *Reduce the content of C —— preventing C to combine with Cr, then less Cr will be separated out. For example: 0Cr18Ni9 (C ≤ 0.08%) 00Cr18Ni9 (C < 0.03%)
*C stabilization treatment —— adding Ti or Nb to form TiC or NbC to stabilize C. For example: 1Cr18Ni9Ti 1Cr19Ni11Nb *Add microelement —— adding B can vary the nature of grain
boundary to prevent (Cr . Fe)23C6 to be separated out.
(6) 氢蚀 Pitting corrosion occurs in the media containing [Cl-] ΘMechanism: [Cl-] intrudes into the flaw of passivation
film (Cr2 O3) and reacts with metallic ion to form strong acidic salts ([M+] + [Cl-] → MCl) which can dissolve the passivation film —— the locally corroded film becomes a “passive- active” minicell —— with corrosion taking place.
ΘDamage:
Fast corrosion speed easily perforates the
thin (only several mini-meter thick) stainless
steel by corrosion.
ΘPreventive measures:
*Adding some alloy elements
The most effective elements to improve the
pitting corrosion resistance: Cr, Mo
Secondarily effective elements: Ni, Si, N, Re
*Cr≥25%, pitting corrosion won’t occur. 2%Mo improve pitting corrosion resistance dramatically, Mo and [Cl-]
form the protective film (MoOCl2) which can prevent the passivation film being perforated.*Materials resistant to the corrosion of [Cl-]: high Cr-Ni stainless steel containing Mo such as: 1Cr18Ni12Mo2Ti 00Cr20Ni30Mo2Nb 000Cr30Mo2
1.Heat-resisting Steel: i. Characteristics
(1)Excellent high-temperature oxidation
resistance (excellent high-T chemical
stability)
(2)Good high-T mechanical properties
(strength at high T 热强性? )
1.4.6 Heat-resisting Steel
and Low-temperature Steel
ii. Elements added
Cr Mo V Ti W Si Ni Al
iii. Commonly-used heat-resisting steel
(1)Oxidation resistant steel——
*mainly resistant to oxidation, but has
low strength.
*used in the parts that are heated directly
(800~1000℃) but small loaded.
such as: heating tube support, nozzle, etc.
*commonly used steels’ designation:
Cr13SiAl Cr25Ti Cr17Ti Cr25Ni12
(2) 热强钢—— *mainly resistant to creep but also resistant
to oxidation.
*used in the parts that are loaded at high T.
such as: heating tube, reactor, etc.
*commonly used steels’ designation:
12CrMo Cr5Mo 1Cr18Ni9Ti Cr25Ni20
2.Low-temperature Steel: i. Working temperature
< -20 Low temperature℃
-20 ~-40 Non-cryogenic temperature ℃ < -40 Cryogenic temperature℃ ii. Characteristics
(1)Excellent low-temperature toughness
(2)Excellent processing workability and
weldability
iii. Requirements of structure
(1)Low content of C (0.08~0.18%) ——
form homogeneous ferritic structure.
(2)Homogeneous austenitic structure is
desirable at cryogenic temperature.
iv. Elements added
Mn Al Ti Nb Cu V N
1.Plates (Sheet Materials):
1.4.7 Varieties andSpecifications of Steels
2.Tubes(Tubular Products):
4. CCS (Cast Carbon Steel) & FS (Forged Steel)
3. Shapes (Section Materials): i. Flat Steel (bar)
ii. I-Steel (beam or section)
iii. L-Iron (Angle Steel)
1.5.
Corrosion & Protection of
Chemical Equipments
1.5.1 Harm of corrosion
1.5.2 Evaluation Methods of the Corrosion of Metal
htF
pp 210 /mg K
ht
m2F
p1
p0
K
g
g
g/cm2·h
Time of corrosion action—
Contact Area of corrosive media and test piece—
WT after corrosion—
WT before corrosion—
Corrosion Rate—
1.According to the weight changes:
2.According to the corrosion degree:
yearF
KFt
yearF
p
year
hKa
KK
76.81000
36524
mm/year
Ka—Thickness variation per year mm/year
—Metallic density g/cm3
hFP
VandhFV __
F
ph
3.Three Grades’ Standard of Metallic Resistance to Corrosion:
Grade I: Ka < 0.1 mm/year (corrosion resistant)
Grade II: Ka = 0.1 ~ 1.0 mm/year (available)
Grade III: Ka > 1.0 mm/year (unavailable)
1.Definition: 化学腐蚀 The corrosion caused by
chemical reactions between metals and drying gas or non-electrolyte solution ( 非电解质溶
液 ?) is called Chemical Corrosion.
1.5.3 Chemical Corrosion
2.Characteristics: i. Corrosion products are on the metallic surface
ii. No electric current in the cause of corrosion
iii. The two natures of the products from
chemical reactions:
(1)Stability —— Passivation
(2)Unstability —— Activation
3.Examples of Chemical Corrosion: i. Metallic high temperature oxidation
(1)Oxidation resistance:
oxidized rapidly at high T
forming oxidation film
stopping oxidation
(2)High temperature oxidation of carbon steel
and cast iron:
Stable
Unstable
Stablelayer I: Fe2O3
layer II: Fe3O4
layer III: FeO
T > 570 oxidation layer forms ℃
inner layer Fe3O4
outer layer Fe2O3
T < 570 oxidation layer forms℃
T > 300 oxidation surface appears℃
T < 570 ℃ T > 570℃
Fe2O3
Fe3O4
FeO
Fe
Composition of ironic oxidation layer
(3)Solutions:
Adding some Cr Si Al to form stable
oxidation film of Cr2O3 SiO2 Al2O3 which
can prohibit the oxidation reaction from
proceeding.
ii. High temperature decarburization
(1) T > 700 ℃ oxidation and decarburization both exist
Fe3C + O2 3Fe + CO2
Fe3C + CO2 3Fe + 2CO
Fe3C + H2O 3Fe + CO + H2
(2)Result
*Cementite Ferrite
with Strength, hardness and Fatigue
Strength all decreasing.
*Forming the air bubble which is the crack
initiation point.
(3)Prevention
Adding Al or W
iii. 氢腐蚀 (hydrogen brittleness)At relevant low temperature and pressure
(T≤200 , ℃ P ≤5MPa), H2 won’t
corrode the carbon and alloy steels
apparently.At high T and P, the corrosion actions of
H2 to steels are obvious.
Mechanism of hydrogen corrosion:
Stage I —— “Hydrogen brittleness stage”
H disperses inward and dissolves.
Stage II —— “Hydrogen attack stage”
Chemical reaction vary the
structure of steels:
Fe3C + 2H2 3Fe + CH4
1.Definition: 电化学腐蚀 The corrosion caused by electrochemical reactions between metals and electrolytes is called Chemical Corrosion.
1.5.4 Electrochemical Corrosion
2.Mechanism: Anode reaction —— Me Me+ + e
Electron movement —— eanode ecathode
Cathode reaction —— D + ecathode [D e]
3.Conditions of electrochemical corrosion: i. There is potential difference on the parts of metallic surface or between different metals. ii. The parts which have potential difference are connected with each other or the anode is connected with cathode. iii. The metal with potential difference is in the electrolyte or the electrolyte where the anode and cathode are connected with each other.
4.Inter-crystalline corrosion i. Definition 晶间腐蚀 It is the phenomenon that the corrosion occurs between two crystalline surfaces and causes the grain boundary continuously damaged. ii. Nature It’s a kind of local and selective corrosive damage.
iii. Occurring in
Austenitic stainless steels
iv. Reason
Lack of Cr element in the grain boundary
v. Austenitic stainless steels (C<0.14%)
*At high temperature (1050ºC)
C distributes completely in whole alloy.
*Between 400~850℃ C + Cr + Fe (Cr . Fe)23C6
Anode
Cr < 12.5%
Inter-crystallineCorrosion occurs
Separate out along the grain boundaryCr%
Cr lacking
Corroding minicell
Cathode—Grain
—Cr lacking region
Grain
(Cr . Fe)23C6
Cr lacking region
Grainboundary
5.Stress corrosion (SC Fracture)
i. Definition
应力腐蚀The destruction is caused
by both corrosive media and the
tensile stress action, this kind of
damage is called Stress Corrosion.
ii. Initiation Circumstances
Carbon steel and various kinds of Alloy steel (such as austenitic stainless steel) are in the media listed as following:
(1)High concentrated chloride solution above
80℃ (2)High temperature and pressure water at
150~300 ℃ (3)High temperature and concentrated caustic
solution
iii. Mechanism
Stage I: Breeding stage ( 孕育阶段 ?) The primary destruction
(mechanical crack) is formed in metallic
surface under the co-action of corrosion
and tensile stress.
Stage II: Corrosion crack’s extension stage Corrosive media dissolve the
passivation film in the cracks to form
anode with the film becoming cathode, the
electrochemical corrosion therefore occurs.
The crack extents rapidly under the co-action
of this corrosion and tensile stress.
Stage III: Breaking stage
iv. Prevention measure
(1)Decrease or clear up the stress concentration
(2)Select the stress corrosion resistant materials:
Two-phase stainless steel ——
austenite + small amount (about 5%) of Ferrite
such as: 1Cr18Mn10Ni5Mo3N 0Cr17Mn13Mo2N 0Cr21Ni5Ti
1.Uniform (General) Corrosion: i. Corrosion is over the whole metallic surface
ii. Effect and danger are small
iii. Remaining enough corrosion allowance in
designation can still assure the strength
and expected life of equipments
1.5.5 Types of metallic corrosion
2.Local Corrosion:
i. Corrosion is at the local region in metals
ii. Very dangerous
iii. Remaining the corrosion allowance in
designation has no effect.
iv. Categories of Local Corrosion
(1)Seam Corrosion
(2)Pitting Corrosion
For example:
the pitting corrosion of Cr-Ni stainless
steel in the media containing [Cl- ]
(3)Stress Corrosion
(4)Inter-crystalline Corrosion
For example:
the inter-crystalline corrosion of
Cr-Ni stainless steel under certain conditions
1.Selecting materials reasonably
1.5.6 Corrosion Resistant
Measures in Metallic
Equipments
2.Adding the lined protection
i. Metallic lining: stainless steel,
other metals(Cu Al Ti Cr Ni)
ii. Nonmetallic lining: plastics,
rubbers, enamelware, etc.
iii. Coating
iv. Adding corrosion
buffering agents
v. Electrochemical
protection
such as:
cathodic protection
+-
Cathodic Protection
Apparatuses
Mechanism of cathodic protection: 阴极保护The protected metallic devises are polarized into cathodes by the direct current (DC) from outer electrical power supply taking the auxiliary electrode as the anode. When the potential of cathode < that of anode, the corrosion will be prohibited.
1.Suitable Mechanical Properties: Strength, hardness, plasticity, etc.
2.Good Corrosion Resistance3.Economic and rational
1.6. Materials Selection of Chemical Equipments
1.6.1 General Principles
1.Pressure Vessels commonly use
Full-killed Steels
2.Common Alloy Steels are preferred
3.Q235- A and 16Mn can’t be used to
fabricate the vessels in which the
liquified petroleum gas is held.
4.The C content of Welded Vessels’
materials should be C < 0.24%.
1.6.2 Others
Austenite
A3
A1
Ferrite
Austenite+
Ferrite
Pearlite+
1667
1333
Tem
per
atu
re º
FIron-iron carbide equilibrium diagram
Percent carbon of weight
0 0.80.2 0.4 0.6
The lattice structure of steel varies from one
form to another as the temperature changes.
This is illustrated in the above figure. Between
room temperature and 1333ºF, the steel consists
of what s known as “ferrite and pearlite”.
Ferrite is a solid solution of a small amount of
carbon dissolved in iron. Pearlite, which is
shown in the figure, is a mixture of ferrite and
iron carbide. The carbide is very hard and
brittle.
In the previous figure between line A1
(lower critical temperature) and A3 (upper
critical temperature) the carbide dissolves
more readily into the lattice that is now
called “Ferrite and austenite”. Austenite is a
solid solution of carbon and iron that is
denser than ferrite.
Above line A3 the lattice is uniform
in property with the austenite the main
structure. The actual temperature for this
austenite range is a function of the carbon
content of the steel as shown in the figure.
Recommended