Download Classes of materials

Materials
and
Fabrication Selection
Chapter outline:
Classes of materials (categories of materials)
Materials selection criteria
Material properties
Commonly used material of construction
Materials fabrication steps
Economic selection of materials
Analysis of failure for metals (types of failure, finding primary
cause of failure and initiate corrective action to prevent repetition).
Classes of materials
Materials selection criteria
The most important characteristics (criteria) to be considered
when selecting a material of construction are:
1. Mechanical and physical properties
2. Corrosion resistance
3. Ease of fabrication—forming, welding, casting
4. Availability in standard sizes
5. Cost
1. Mechanical and physical properties
a. Strength–tensile strength;
b. Stiffness:
c. Toughness–fracture resistance;
d. Hardness–wear resistance;
e. Fatigue resistance;
f. Creep resistance.
Tensile Strength (UTS)
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.
Stiffness
the ability to resist bending and buckling. It is a function of the
elastic modulus of the material and the shape of the cross-section of
the member.
Toughness
The ability to absorb impact energy.
It is a measure of the material’s resistance to crack propagation or
measure of ductility.
Hardness
is an indication of a material’s ability to resist wear or scratching. 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 cause erosion.
Fatigue
Failure which is likely to occur in equipment subject to cyclic
loading; for example, rotating equipment, such as pumps and
compressors, and equipment subjected to temperature or pressure
cycling.
Creep
Creep is the gradual extension of a material under a steady tensile
stress, over a prolonged period of time. It is usually important only at
high temperatures.
2. Corrosion resistance
The conditions that cause corrosion can arise in a variety of ways.
it is convenient to classify corrosion into the following categories:
1. Uniform corrosion;
2. Galvanic corrosion–dissimilar metals in contact;
3. Pitting–localized attack;
4. Intergranular corrosion;
5. Stress corrosion;
6. Erosion–corrosion;
7. Corrosion fatigue;
8. High-temperature oxidation and sulfidation;
9. Hydrogen embrittlement.
Acceptable Corrosion Rates
ipy = inch per year
mpy = mills per year ( 1 mill = 10-3 inch)
SELECTION FOR CORROSION RESISTANCE
In order to select the correct material of construction, the process
environment to which the material will be exposed must be clearly
defined. In addition to the main corrosive chemicals present, the
following factors must be considered:
1. Temperature—affects corrosion rate and mechanical properties;
2. Pressure;
3. pH;
4. Presence of trace impurities
5. The amount of aeration—differential oxidation cells;
6. Stream velocity and agitation—erosion-corrosion;