Objectives Flashcards
Describe an Ionic Bond
One or more electrons wholly transferred from an atom of one element to the atom of another element. +/- polarity change attracts. “Transfer of electrons”
Describe a Covalent Bond
A bond formed by shared electrons. One atom needs electrons to fill its outer shell so it shares those electrons with another atom. The electrons fill both atoms’ shells. “Sharing of electronics”
Describe a Metallic Bond
Atoms do not share or exchange electrons in order to bond. Approximately one electron per atom are free to move throughout the metal and interact with many of the fixed atoms.
Describe a Molecular Bond
A temporary weak charge when electrons of neutral atoms spend more time in one region of their orbit then another. “Van der Waals bond”
Describe a Hydrogen Bond
Similar to the molecular bond, occurs when hydrogen atoms are willing to give up an electron to atoms of oxygen, fluorine or nitrogen.
Describe an Amorphous type of solid
No regular arrangement of atoms or molecules.
-Exhibit properties of solids
-Have definite shape and volume
-Diffuse slowly
-Lack sharply defined melting points
-Solid but resemble liquids that flow slowly at room temperature
Examples: glass and paraffin
Describe Crystalline Solids
Atoms in a pattern that periodically repeat in a three-dimensional lattice.
Forces associated with chemical bonding cause this repetition to produce properties such as:
- strength
- ductility
- density
- conductivity
- shape
Describe Grain Structures
Arrangement of the grains in a metal - each grain has a crystal or lattice structure. Grain boundary is a region of misfit on interface between grains and is usually the diameter of 1-3 atoms. Grain size determines the properties of the metal. Smaller grain size = higher tensile strength, higher ductility. Larger grain size = good high temperature creep properties.
Describe the Body-Centered Cubic (BCC) lattice structure
Eight atoms at the corners of a cube and one atom at the body center of the cube. Examples: ferrite, chromium, vanadium, molybdenum, tungsten. High strength, low ductility.
Describe the Face-Centered Cubic (FCC) lattice structure
Eight atoms at the corners of a cube and one atom at the center of each of the faces of the cube. Examples: austenite, aluminum, copper, lead, silver, nickel, platinum, thorium. Lower strength, higher ductility than BCC metals.
Describe Hexagonal Close-Packed (HCP) lattice structure
Top and bottom layers contain six atoms at the corners of a hexagon with one atom in the center, middle layer contains three atoms. Examples: beryllium, magnesium, zinc, cadmium, cobalt, thallium, zirconium. Not as ductile as FCC metals.
Describe the Point Imperfections known as Vacancy Defects.
Missing an atom in a lattice position. Results from imperfect packing during crystallization-maybe due to increased thermal vibrations of atoms at higher temperatures.
Describe the Point Imperfections known as Substitutional Defects
Impurity at a lattice position (alloy material added to metal)
Describe the Point Imperfections known as Interstitial Defects
Impurity or atom at an interstitial site
Describe the “Edge” Line Imperfection
Extra row or plane of atoms in crystal’s structure.
Describe the “Screw” Line Imperfection
Tearing a crystal parallel to the slip direction (slip pattern similar to a screw thread)
Describe the “Mixed” Line Imperfections
Orientation of dislocations varies from pure edge to pure screw and at intermediate points. May possess characteristics of each
Describe Interfacial Imperfections
Large and occur over a two-dimensional area
Describe Macroscopic Defects (Bulk Defects)
Three-dimensional material defects introduced during material refinement or fabrication. Most caused by foreign particles called inclusions. Others include voids (gas pockets, shrinking, etc), cracks from work stress and welding or joining defects.
Describe the common characteristics of alloys
Stronger than pure metals.
Reduced electrical conductivity.
Reduced thermal conductivity.
Identify the desirable properties of type 304 stainless steel
Extremely tough (18-20% Cr & 8-10.5% Ni)
Resists most, but not all, types of corrosion.
Describe Strength
Ability of a material to resist deformation.
Maximum load that can be borne before failure is apparent.
Nominal stress is included when quoting a material’s strength.
Describe Ultimate Tensile Strength
The maximum resistance a material presents to fracture.
UTS = Max load ÷ Area of original cross section = PSI
Describe Yield Strength
The stress where a predetermined amount of permanent deformation occurs. The stress where plastic deformation occurs.
Describe Ductility
The ability of a material to deform easily on application of a tensile force.
The ability of a material to withstand plastic deformation without rupture.
The ability of a material to elongate in tension.
Describe Malleability
A metals ability to display large deformation or plastic response when subjected to compressive force
Describe Toughness
The work required to deform 1in³ of metal until it fractures.
The way a material reacts under sudden impacts.
- Shape of metal and the material composition determines toughness
Describe Hardness
Property of a metal that enables it to resist plastic deformation, penetration, indentation and scratching.
Describe Heat Treatment
A metallurgical process that changes properties of the metal. When hardness and tensile strength go up, toughness and ductility go down.
Describe Quenching
Faster cooling, smaller grain size = harder metal.
Cooling rate depends on method of cooling and size of the metal.
Uniform cooling prevents distortion.
Describe Annealing
Slow heating and soak, then slow cool. -Softens steel and improves ductility. -Stress relief.
-Refines grain structure.
Describe Coldworking
Plastic deformation in a particular temperature region over a particular time period such that the work hardening is not relieved.
-Lowered ductility.
Describe Hotworking
A process where metal deformation occurs above re-crystallization temperature, preventing work hardening from occurring.
Describe General Corrosion
Involving water and steel (or iron), results from chemical action where steel surface oxidizes forming iron oxide or rust.
Describe Galvanic Corrosion
To dissimilar metals with different electrical potentials are in electrical contact in an electrolyte. The larger the potential difference, the greater the galvanic corrosion rate.
Describe Stress Corrosion Cracking and how to prevent it
Intergranular attack corrosion that occurs at the grain boundaries of a metal under tensile stress. Cracks then occur at inter/trans-granular paths which leads to further corrosion.
To prevent:
- proper system & component design
- reduce stress
- remove hydroxides, chlorides, oxygen
- avoid stagnant areas & crevices
- use Inconel
Describe Chloride Stress Corrosion and how to prevent it
Intergranular corrosion that occurs in austenitic stainless steels under tensile stress in the presence of oxygen, chlorides and high temperatures.
To prevent:
- maintain low chloride ion content
- maintain low oxygen content
- use low carbon steels
Describe Caustic Stress Corrosion (CSS) and how to prevent it
Cracks initiate and grow along grain boundaries then extensive crack branching occurs along grain boundaries. Carbon steels are susceptible to CSS. High tensile stress from fabrication is the driving force. Inconel is known for getting CSS.
Describe Hydrogen Embrittlement
Another form of stress corrosion cracking, hydrogen or methane gas forms at grain boundaries building up internal pressure and rupturing in the form of tiny cracks causing steel to lose its strength and ductility.
Describe Fatigue Failure
A material’s tendency to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength. Thermal fatigue is the most common (cyclic changes in temperature).
To prevent:
- operate plant in a controlled manner
- heat up and cool down limits
- pressure limits
- pump curves
Describe Work-Hardening
Straining a material beyond the yield point.
Increasing stress produces additional plastic deformation, causing metal to become stronger and more difficult to deform.
-Makes a material stronger, but less ductile.
-Can be used to treat metal.
Describe Creep
At elevated temperatures and constant stress or load, material slowly deforms.
Rate of creep depends on stress and temperature.
To limit creep:
- stay within acceptable creep rate limits for component lifetime.
- creep becomes a concern at extremely high temperatures and pressures
