Ferrous Metals and Alloys Flashcards
What is the difference between alloys and compounds?
Elements in alloys have chemical affinities to create mixtures of different ratios that can enhance certain properties of the elements.
Elements in compounds share chemical bonds to form new molecules with unique properties.
What is a solid solution and what are the two types?
A type of alloy where an element is dissolved into another to form a single-phase structure.
- Substitutional: atoms of solvent are replaced in its unit cell by dissolved element
- Interstitial: atoms of dissolving element fit into vacant spaces between base metal atoms in lattice structure
What are Intermediate phases?
Occur in alloys when amount of the dissolving element exceeds the solid solubility limit of the base material. Has chemical composition in-between those of pure elements
Phase diagram
Shows the relationship between the temperature (y axis) and composition (x axis) of an alloy
Binary phase diagram
When the alloy consists of two elements. Also known as an equilibirum diagram.
Inverse lever rule
Determines the proportion of each phase at a given temperature and composition.
See Ferrous Metals 1 for equations.
What are ferrous metals and alloys?
Ferrous metaks are based on iron, with the most common alloys being iron and carbon. Ferrous meteruals make up 70%-85% of all structural and mechanical members (by weight).
What is the carbon content of pure iron?
<0.008% C
What is the carbon content of steel?
> = 0.008% C and <2.11% C
What is the carbon content of cast iron?
> = 2.11% C and <6.7% C
What are the three phases of “pure iron” on an iron-carbon phase diagram?
𝜶-phase or ferrite (below 912°C)
𝜸-phase or austenite (between 912 °C and 1394 °C)
𝜹-phase (between 1394°C and 1538°C)
Ferrite
Takes place below 912°C. When lower than ~600°C, ferrite typically has <0.008%C.
Ferrite has a BCC crystal structure and is soft and moderately ductile.
Austenite
Takes place between 912°C and 1394°C.
Austenite has a FCC crystal structure and is ductile. It works well for hot-working procedures.
Cementite
Fe3C is an intermediate phase that is hard and brittle.
Pearlite
Is made up of ferrite and cementite when heated above the austenite zone and then cooled.
Has a relatively slow cooling rate and when air cooled has a fine grain structure. When furance cooled it is coarser.
Eutectic position
Is the melting temperature at a unique composition where two or more elements freeze or melt at the same time.
Is lower than the melting point of all components.
What is the eutectic position for an Fe-C phase diagram?
For an Fe-C phase diagram the eutectic position is at 4.3% carbon and 1130°C.
Eutectoid position
Is the “cooling temperature” at a unique composition where a single-phase solid turns into a multi-phase solid.
What is the eutectoid position for an Fe-C phase diagram?
For an Fe-C phase diagram the eutectoid position is at 0.77% carbon and 723°C. Austenite cools down into ferrite and cementite.
At room temperature under equilibirum conditions, iron-carbon alloys form a two-phase system at carbon levels even slightly above zero.
Martensite
Heated above the critical temperature (austenite zone) and then cooled.
Has a very fast cooling rate and forms BCT micro-structures. Is very hard and brittle.
Cools too fast for cementite to form.
Bainite
Heated above the critical temperature (austenite zone) and then cooled.
Has a medium cooling rate and forms very fine micro-structures of ferrite and cementite.
In between pearlite and martensite in terms of hardness.
Annealing
Involves heating the steel to above 800°C for longer than an hour and then letting it slowly cool in a furnace.
This process forms pearlite with coarse grains, good ductility, low hardness, and no residual stresses.
Normalizing
Involves heating the steel to 800°C for at least an hour and then cooling it by air (faster cooling rate than annealing).
This process forms fine pearlite with better mechanical properties than annealed steel.
Quenching
Involves heating the steel above the critical temperature (austenite at 912°C) and then cooling it quickly by submerging it in water, saltwater, or oil.
Is a non-equilibirum process and prevents the formation of ferrite or cementite.
Bct martensite is formed with high strength and hardness, but is very brittle.
Low carbon steels can not be hardened through quenching.
Tempering
Involves heating a martensitic material to 300°C or higher to precipitate the carbides out through slow air cooling.
This converts the micro-structures to bcc and relieves resiidual stresses and improves ductility and toughness. It may reduce hardness and strength.
Spheroidization
Involves holding pearlite at 700°C for a day.
Improves toughness by transforming the lamellar shape into a spherical one.
Is a way of hardening low carbon steels.
Decarburizing
Involves a loss of carbon from the surface as a result of heat treatment or hot working in a medium that reacts with the carbon.
This is not a good things and can be avoided by working in an inert atmosphere/vavccum that does not react or a neutral salt bath.
Hardenability
The capacity of steel to reach its maximum hardness by subjecting it to heating and quenching (to form martensite).
Is also the the depth to which a component can be hardened as well as the severity of the quench required.
Jominy test
Used to evaluate the hardenability of materials.
Case hardening
Aka carburization is the process of introducing carbon to the surface of a low carbon steel or iron. Increases surface hardness to increase wear resistance and fatigue strength.
Time-temperature transformation curve
Shows how cooling rates affect the transformation of austenite into the various phases of ferrite and cementite and of martensite.
To interpret the curve start at time=0 in the austenite region. See pg.29 of ferrous metals and alloys 1.
What are some design considerations for heat treatment?
To avoid cracking, distortion, residual stress, and non-uniform properties take these considerations into account:
No sharp corners, uniform thickness, and smooth transitions between different cross sections. Similar to casting parts.
AISI Numbering System
Plain carbon steels are specifies by a four digit numbering system: 10XX.
The 10 indicates that the steel is plain carbon, and XX indicates the percent of carbon in the hundrendths of percentage points.
E.g. 1020 steel contains 0.2% C.
Low-carbon steel
<0.3% C
Has a lower yield strength and good ductility. It can be found in common industrial products such as nuts, bolts, sheets, and plates.
Medium-carbon steel
0.3-0.6%C
Moderate strength and ductility. Can be found in machinery, and automotive and agricultural equipment parts (gearls, axles, cranshafts, etc.).
High-carbon steel
> 0.6% C
Has high hardness, strength, and wear resistance but is relatively brittle. It is usually tempered to improve toughness. It can be found in cutting tools, music wires, and springs.
Boron (B) as an alloy
Improves harnedability. Some improvement in machinability and formability.
Carbon (C) as an alloy
Improves hardenability, strength, hardness and wear resistance.
Reduces ductility, weldability, and toughness.
Chromium (Cr) as an alloy
Improves toughness, hardenability, wear, corrosion resistance, and high temperature strength.
Acts as a ferrite stabilizer in ferritic stainless steel.
Lead (Pb) as an alloy
Improves machinability.
Causes hot shortness (brittle when at melting point).
Manganese (Mn) as an alloy
Improves hardenability, strength, abrasion resistance, and machinability.
Deoxidizes the molten steel and reduces hot shortness.
Decreases weldability.
Molybdenum (Mo) as an alloy
Improves hardenability, wear resistance, toughness, elevated temperature strength, creep resistance, and hardness.
Minimizes temper embrittlement.
Nickel (Ni) as an alloy
Improves strength, toughness, corrosion resistance, and hardenability.
Acts as an austenite stabilizer in austenitic stainless steel.
Silicon (Si) as an alloy
Improves strength, hardness, corrosion resistance, and electric conductivity.
Decreases magnetic hysteresis loss, machinability, and cold formability.
Tungsten (W) as an alloy
Improves strength and hardness at elevated temperatures.
Vanadium (V) as an alloy
Improves, strength, toughness, abrasion resistance, and hardness at elevated temperatures.
Inhibits grain growth during heat treatment.
Stainless steel
Highly alloyed steels to provide high corrosion resistance.
The principle alloying element is chromium (at least 15%). It provides a thin impervious oxide film in an oxidizing atmosphere which protects the surface from corrosion.
Nickel is another alloying ingredient used some compositions to increase corrosion protection.
There are three types: Ferritic, Austenitic, and Martensitic.
Ferritic stainless steel
400 series, chromium only
Has a high chromium content (up to 27%) as chromium acts as a ferrite stabilizer.
It is magnetic, has good corrosion resistance, low ductility, and is the cheapest stainless steel.
It can’t be harded by heat treatment, but can be marginally hardened by cold working.
Its use ranges from kitchen utensils to jet engine components.
Austenitic stainless steel
300 series, 18% chromium and 8% nickel
Nickel acts as an austenite stabilizer.
It is non-magnetic, highly corrosion resistant, very formable, and can be mirror polished, machined, and welded.
It can’t be harded by heat treatment, but can be marginally hardened by cold working.
This type of stainless steel costs twice as much as the ferritic ones and is used to manufacture chemical and food processing equipment, as well as machinery parts taht require high corrosion resistance.
Martensitic stainless steel
500 and 400 series
Has a low chromium content (~12%), no nickel, and a high carbon content making it heat treatable.
It has high strength, hardness, and fatigue resistance, but has lower corrosion resistance due to the lower Cr and increased C.
This type of stainless steel costs 1.5x as much as the ferritic ones and is used in products like cutlery and surgical instruments.
Tool and die steels
Specially designed for use as cutting tools, dies, and molds with high strength, hardness, hot hardness, impact toughness, and wear resistance.
The main alloying elements include Mo, W, Cr, and V.
T,M
High speed steels are formulated for wear resistance and hot hardness for use as cutting tools in machining operations.
H
Hot-working tool steels are intended for hot-working dies in forging, extrusion, and die casting.
D
Cold-work tool steels are intended for cold-working operations such as sheet metal presswork, cold extrusion, and certain forging operations.
W
Water-hardening tool steels have high carbon with few or no other alloying elements. They can only be hardened by fast quenching in water.
S
Shock-resistant tool steels are intended for use in applications where high toughness is required such as sheet metal shearing, punching, and bending operations.
P
Mold steels are used to make molds for molding plastic or rubber
L
Low alloy tool steels are generally reserved for special applications
Free-machining steels
Carbon steels formulated to improve machinability. Include S, Pb, Sn, Bi, Se, Te, and P.
Lead is used less frequently nowadays due to environmental and health concerns.
Maraging steels
Low carbon alloys with high amounts of nickel (15-25%0 and lesser proportions of Co, Mo, and Ti. Cr is sometimes added for corrosion resistance.
These steels are strengthened by precipitation hardening. In their unharded state they are very processable.
Interstitial-free steels
Extremely low carbon (0.005%C). is a result of alloying with Nb and Ti taht combine with C and leave the steel almost free of intersitial C atoms.
Very ductile and is used in deep drawing operations in the automotive industry.
Cast iron
Main alloying elements are carbon (2.1-4%) and silicon (1-3%).
Carbon is found in the form of graphite as silicon promotes the decomposition of cementite into 𝜶-ferrite and graphite.
Weak in tension and brittle, but the graphite gives it good dampening properties.
Different heat treatments can make different microstructures (ferrite, pearlite, or martensite)
White cast iron
Verd hard and wear resistant, but aslo very brittle because of the presence of cementite (Fe3C) instead of graphite.
Malleable iron
Formed by annealing white iron in an atmosphere of carbon monoxide and carbon dioxide. This puts the graphite in clusters, making it similar to ductile iron.
ASTM (American Society for Testing Materials) Cast Iron Specifications
Gray iron: Class 20, 30, 40, 50, 60
The number is the tensile strength (ksi)
Ductile iron: Grade 60-40-18
The three numbers are tensile strength (ksi), yield strength (ksi), and % elongation
Malleable iron: 32510
Is the yield stength (psi) with % elongation tagged onto the back. E.g. 325X100psi (yield strength) + 10 (% elongation)
