MATERIALS & CONSTRUCTION PART A Flashcards
The overriding
considerations when selecting engineering materials for chemical process plant?
High temperature strength
Ability to resist corrosion
Who is responsible for recommending materials
that will be suitable for the process conditions?
Process Designer
Who must also consider the requirements of the
mechanical design engineer
Process engineer
The material selected by the process engineer must have?
- Sufficient strength
- Easily worked
What material should be selected?
Most economical that satisfies both process and mechanical requirements
The material selected will be a material that gives ?
lowest cost over the working life of the plant allowing for maintenance and replacement
What are other factors to be considered when selecting a material?
Product contamination
Process safety
The most important characteristics to be considered when selecting a material of construction are? (7)
Mechanical Properties
Effect of temperature on mechanical properties
Corrosion Resistance
Any special properties required
Ese of Fabrication
Availability in standard sizes
Cost
Under mechanical properties
Strength
Stiffness
Toughness
Hardness
Fatigue
Creep resistance
Measured through tensile strength
strength
Measured through elastic modulus (Young’s Modulus)
Stiffness
Measured through fracture resistance
Toughness
measured through wear resistance
Hardness
Under effect of temperature on mechanical properties particularly the effects of:
high temp
low temp
thermal cycling
Under ease of fabrication
Forming
Welding
Casting
Under Special properties required
Thermal conductivity
Electrical resistance
Magnetic properties
Under Availability in Standard sizes
Plates
Section
Tubes
What materials are satisfactory in all fabrication operations?
Nickel and Monel
Material with the highest annealing temp
Nickel
A measure of the basic strength of a material
Tensile Strength/Stress
the maximum stress that the material will withstand, measured by a standard
tensile test
Tensile Strength/Stress
Older name of Tensile strength/stress
Ultimate Tensile Strength (UTS)
the stress to cause a specified permanent extension
Proof stress
Metals/Alloy that has the highest tensile strength
Monel
Percentage of proof stress
0.1%
What codes are calculated from Tensile strength/stress
ASME Boiler and Pressure Vessel (BPV)
European EN 13445 Pressure vessel code
Hardest metal/alloy
Monel
Ability to resist bending and buckling
Stiffness
Function of elastic modulus of the material and the shape of the cross-sectional area of the member (the second moment area)
Stiffness
Slope=
Stiffness
The stress beyond which a material becomes plastic
Yield point
An indication the tendency for an element to return to its original form after being subjected to a force
Stiffness
Measures how much stress can be applied to an element before it deforms permanently or fractures
Strength
Measures a material’s resistance to surface deformation
Hardness
Associated with tensile strength
Toughness
Measure of the material’s resistance to crack propagation
Toughness
What ductile materials stop the propagation of crack by local yielding at the crack tip. They have crystal structure
Steel
Aluminium
Copper
The crystal structure of the ductile materials stop the propagation of crack by?
Local yielding at the crack tip
In this materials, the local yielding does not occur, that’s why they are brittle
Cast irons
Glass
T/F: Brittle Materials are strong in tension but weak in compression
F
Under ____ any incipient cracks present are closed up
Compression
Technique developed that allowed the use of brittle materials in situations where tensile stress would normally occur
Use of pre-stressed concrete and
Glass-fibre-reinforced plastics in Pressure vessel construction
Are induced to counteract external stresses
Internal stresses
Curve downward
Load deflection
Horizontal side to side
Tendons stressed
Horizontal to center
Prestress forces
Curve upward to center
Prestress deflection
Indication of a material’s ability to resist wear
Hardness
Hardness is measure using
Brinell Madness Test
An important property if the equipment is being designed to handle abrasive solids, or liquids containing suspended solids that are likely to cause erosion
Hadness
Hard to hardest
Lead
Pure aluminium
Copper
Soft brass
Hardened aluminium
Mild steel
Annealed chisel steel
White cast iron
Nitrided surface
Glass
Rhenium diboride
T/F: The higher the impression diameter the lower the Brinell Number
True
Is likely to occur in equipment subject to cyclic loading
Fatigue
Examples where fatigue occurs
Cracks in bicycle frame
Helical gear
Spring
Steering arm
Gradual extension of a material under a steady tensile stress over a prolonged period of time
Creep
Usually only important at high temperatures like with steams and gas turbine blades
Creep
For a few materials like ____ the rate of creep is significant at moderate temperatures
Lead
_____ Will creep under its own weight at room temperature and ___ must be supported at frequent intervals
Lead
Lead linings
Reported as the stress to rupture in 100,000 hours at the test temperature
Creep strength
____ and ____ of metals decrease with increasing temperature
Tensile strength and elastic modulus
____ and ____ of metals decrease with increasing temperature
Tensile strength and elastic modulus
Tensile strength and elastic modulus of metals ____ with increasing temperature
Decrease
Tensile strength of mild steel (low carbon, C<0.25%) is 450 Pa at 25°C falling to ___ @500°C
210 Pa
Young’s Modulus of Mild steel is 200,000 Pa at 25°C and falls to _____ at 500°C
150,000 Pa
SA-285 Plain Carbon Steel plate cannot be used to construct a pressure vessel with design temperature
> 900°F or 482°C
If any pressure vessel designed to for use above 900°F, what must be used
Killed steel or Alloy
Maximum allowable stress used in design is always based on the?
Design temperature
Are superior in sufficient strength at the design temperature that gives an economic and mechanically feasible wall thickness
Stainless steel than carbon steels
Will be important if the material is subjected to high stresses at elevated temperatures
Creep resistance
Special alloys that are used for high temperature equipment such as furnace tubes in environments that do not contain silphur
Inconel or incoloy
A these temperatures, metals that are normally ductile can fail in a brittle manner
Less than 10°C
Serious disasters have occurred through the failure of this at low temperatures
Welded carbon steel vessels
The phenomenon of _____ is associated with the crystalline structure of the metals
Brittle failure
Are more liable to brittle failure
BCC
For this low temperature equipment, austenitic steel (FCC) or aluminium (Hex) should be specified
Cryogenic plant
Liquefied - gas storage
Austenitic steel has
FCC
Aluminium alloys has ____ structure
Hexagonal lattice
Example of V-notch impact tests
Charpy test
Are used to test the susceptibility of materials to brittle failure
Charpy test
Is a complex phenomenon and is dependent on plate thickness and residual stress present after fabrication as well as the operating temperature
Brittle fracture of welded structure
General wastage of material
Uniform corrosion
Dissimilar metals in contact
Galvanic corrosion
Localized attack
Pitting
An electrochemical process
Metallic corrosion
Four components necessary to set up an electrochemical cell
Anode
Cathode
Conducting medium
Completion of the electrical circuit
Corroding electrode
Anode
Passive, non-corroding electrode
Cathode
Corroding fluid
Electrolyte
Cathodic areas can arise in many ways
Dissimilar metals
Corrosion products
Inclusions in the metals such as slag
Less well - aerated areas
Areas of differential concentration
Differential strained areas
The more or less uniform wastage of material by corrosion, with no pitting or other forms of local attack
Uniform corrosion
If the corrosion of a material can be considered to be Uniform, the life of the material in service can be predicted from experimentally determined?
Corrosion rates
Expressed as penetration rate in inches per year or mils per year
Corrosion rates
1 mill = inches?
10^-3
Can be expressed as weight loss in milligrammes per square decimeter per day
Corrosion rates
In here, the corrosion rate is measured by the reduction in weight of a specimen of known area over a fixed period of time
Corrosion testing
Of judging corrosion rate in mdd, it must be remembered that the penetration rate depends on ?
Density of the material
Corrosion rate of ferrous metals
100 mdd = 0.02 ipy
1 ipy = mm per year
25
What can be considered as an acceptable rate of attack will depend on the __ of the material, the ____ particularly as regards to safety and the ____ of the plant
Cost
Duty
Economic life
If the corrosion rate of inexpensive metals like carbon and low alloy steels less than 0.01 ipy, the more expensive alloy’s like brasses and aluminium will be
0.01/2
If the corrosion rate indicates only short exposures then the design engineer should
Allow for frequent inspection of the plant and periodic replacement of the affected equipment
Uniform corrosion affects in two ways
Reduces the on-stream factor
Increase the maintenance cost
Number of days of production per year
On-stream factor
T/F: usually the impact of the frequent shutdown and replacement is so negative that the use of more expensive alloys
With better corrosion resistance can be justified
T
What must be added to the minimum vessel wall thickness to comply with PV codes
Allowances for expected corrosion over the plant life
T/F: the corrosion allowance can be economically or mechanically prohibitive if the corrosion rate is high
T
T/F: the corrosion allowance should be atleast equal to the expected corrosion loss during the desired life of of the vessel
T
Corrosion depends on
Temperature
Concentration of corrosion fluid
Increase in temperature usually (not always) ____ the rate of corrosion
Increases
Corrosion rate will depend on other factors that are affected by temperature like
Oxygen solubility
As an example, the corrosion of mild steel in sulphuric acid, where the rate has unacceptably high in dilute acid and at concentrations above 70%, but is acceptable at intermediate concentration
Complex effects
- If dissimilar metals are placed in contact in an electrolyte, the
corrosion rate of the anodic metal will be ______, as the metal
______ in the electrochemical series will readily act as a _____
increased
lower
cathode
What is an example of metal that at certain conditions form natural protective film in oxidizing environments?
Stainless steel
The state of forming a natural protective film is called
Passive
” indicates the absence of the protective film
Active
Minor shifts in position in the series can be expected in other
electrolytes, but the series for _______ is a good indication of the
combinations of metals to be avoided.
sea water
- If metals that are widely separated in the galvanic series have to be
used together, they should be electrically _______ from each other,
breaking the conducting circuit
insulated
Alternatively, if _____ of the anodic material can be accepted,
the _______ of this material can be ________to allow for the
increased rate of corrosion.
sacrificial loss
thickness
increased
will depend on the relative areas of the anodic and
cathodic metals.
Corrosion rate
T/F: A high cathode to anode area should be avoided
T
are used to protect underground steel pipes.
Sacrificial anodes
highly localized corrosion that forms pits in the metal surface
Pitting
T/F: If a material is liable to pitting, penetration can occur prematurely and
corrosion rate data are not a reliable guide to the equipment life.
T
Causes of Pitting
Not uniform composition of metals
Impingement of bubbles
Aerated medium
Example of not uniform composition of metals
Presence of slag in welds
occurs during cavitation in pumps, which is an
example of erosion-corrosion
Impingement of bubbles
the oxygen concentration will be lower at the bottom of a
pit, and the bottom will be anodic to the surrounding metal, causing
increased corrosion and deepening of the pit
Aerated medium
When attack is caused by aerated medium, how can pitting be reduced?
Good surface finish
the preferential corrosion of material at the grain (crystal) boundaries
Intergranular Corrosion
In this corrosion, though the loss of material will be small, it can cause the catastrophic
failure of equipment
Intergranular corrosion
a common form of attack on alloys but occurs rarely with pure metals
Intergranular corrosion
- The attack is usually caused by a differential couple being set up between
impurities existing at the grain boundary. - Impurities will tend to accumulate at the grain boundaries after heat treatment.
Intergranular Corrosion
Classic example of intergranular corrosion
Weld decay of unstabilized stainless steel
caused by the precipitation of chromium carbides at the grain
boundaries in a zone adjacent to the weld, where the temperature has
been between 500–800°C during welding
Weld dacay of unstabilized stainless steel
can be avoided by annealing after welding
Weld decay
can be avoided by annealing after welding
By using low carbon grades
grades stabilized by the addition of tatnium or niobium
weld decay
Corrosion rate and form of attack can be changed if the material is under ?
Stress
Generally, the ____ will not change significantly within normal design stress values
Rate of attack
Stress corrosion cracking can occur with some combination of ____,____ and ___
Metal
Corrosive media
Temperature
The general name given to a form of attack in which cracks are produced that grow rapidly, and can cause premature, brittle failure of the metal
Stress corrosion cracking
Conditions necessary for Stress corrosion cracking to occur
Simultaneous stress cracking and corrosion
Specific corrosive substance particularly the presence of Cl-, OH- NO3- and NH4+ ions
Can cause cracking
Mild stress
Examples of mild stress
Residual stresses from fabrication and welding
Examples of corrosion cracking
Season cracking of brass cartridge cases
Caustic embrittlement of steel boilers
Stress corrosion cracking of stainless steels in the presence of chloride ions
How can stress corrosion cracking be avoided?
Selecting materials that are not susceptible in the specific corrosion environment
Relieving stress by post-weld heat treatment
The premature failure of materials in corrosive environments caused by cyclic stresses
Corrosion fatigue
T/F: even mildly corrosive conditions can markedly reduce the fatigue life of a component
T
Can occur in any corrosive environments and does not depend on the specific combination of corrosive Substance and metal
Corrosion fatigue
T/F: Materials with high resistance to corrosion must be specified for critical components subjected to cyclic process
T
The increased rate of attack caused by a combination of erosion and corrosion
Erosion Corrosion
If a fluid stream contains suspended particles or where there is high____ and ____, erosion will tend to remove the products of corrosion and any ____ and the rate of attack will be markedly _____
Velocity
Turbulence protective film
Increased
Can be used to prevent erosion Corrosion at the inlet to heat exchanger tubes
Plastic inserts
Corrosion is normally associated with
Aqueous solutions
Oxidation can occur in
Dry conditions
They will oxidize rapidly at high temperatures and their use is limited to temperatures below ___
Carbon and low alloy steels
480°C
Is the most effective alloying element to give resistance to oxidation, forming a tenacious oxide film
Chromium
____ is formed by Chromium to give resistance to oxidation
Tenacious oxide film
Should be specified to temperatures above 480°C in oxidizing atmospheres
Chromium alloys
Can be used up to 650°C
Type 304L stainless steel (18% Cr)
Is stabilized by niubiom and can be used up to 850°C
Type 347 Stainless steel
Can be used as along as sulphur is not present
High nickel alloys
A very common corrosive contaminant in gas processing, oil refining and energy conversion
Sulphur
Present in reducing environment as H2S
Sulphur
Causes sulphidation of metals
Sulphur
An example of material that must withstand sulphiding and oxidizing at high temperatures
Furnace tube
Can attack chromium oxide scale that protects the alloy
Sulphur
Causes breakaway corrosion particularly for high nickel alloys
Sulphur
Loss of ductility caused by the absorption of hydrogen in metal
Hydrogen embrittlement
Important when specifying steels for use in hydrogen reforming plant
Hydrogen embrittlement
Have a great resistance to hydrogen embrittlement than plain carbon steels
Alloy steel
A chart showing the suitability of various alloys steels for use in hydrogen atmosphere as a function of hydrogen partial pressure and temperature is given in?
National Association of Corrosion Engineers
Below ___ plain carbon can be used
500°C
To select the correct material of construction what must be clearly defined
Process environment to which the material will be exposed
In addition to .ain corrosive chemicals present, consider the following
Temperature
Pressure
pH
Trace impurities present
Aeration amount
Stream velocity & agitation
Heat transfer rates
Affects corrosion rates and mechanical properties
Temperature
Causes stress corrosion
Trace impurities present
Forms differential oxidation cells
Aeration amount
Causes erodion-corrosion
Stream velocity & agitation
Related to differential temperature
Heat transfer rates
Other sources of corrosion data
Rabald 1968
NACE 1974
Hamner 1974
Green and Perry 2007
Lai 1990
Schweitzer (2004,1989,2006)
DECHEMA Corrosion Handbook 2005
The ASM Handbook of Corrosion Data
Can be used for preliminary screening of materials likely to be suitable
Corrosion guides
T/F: If the published data indicate that a material is satisfactory, it is guaranteed that it will be suitable for the process environment being considered
F
T/F: Slight changes in process conditions or presence of unsuspected trace of jmpurities cannot change the rate of attack or nature of corrosion
F
Guides show clearly those materials that are manifestly ___
Unsuitable
If actual plant experience Is not available ___ and ___ under simulated plant conditions will help in the selection of sustaible materials
Pilot plant tests and laboratory corrosion tests
Perform preliminary tests like
Inserting coupons of different materials into an apparatus that resists corrosion before testing plant components
Reduce the likelihood of component failure and possible release of chemicals during testing
Preliminary tests
