MMET 207 Tamu David Craig Exam 1

What are the types of indentors used in Rockwell hardness testing?

The indentor can be a tungsten carbide ball (for softer materials, Rockwell B and T scales) or a diamond cone called a brale (for harder materials, Rockwell C, A, and N scales). The 1/16-inch diameter ball is used for Rockwell B and T, and the 120° diamond cone is used for Rockwell C, A, and N.

How is Rockwell hardness testing done?

A minor load is first applied to seat the indentor. Then a major load is applied to create the indentation. The depth of the indentation is measured by the machine and converted on a dial to a hardness number. The minor load is typically 10 kg, and the major loads vary: 60, 100, or 150 kg for regular Rockwell; 15, 30, or 45 kg for superficial Rockwell.

What variations of Rockwell hardness tests exist?

Regular Rockwell scales (B, C, A) use major loads of 60, 100, or 150 kg. Superficial Rockwell scales (T, N) use lighter major loads of 15, 30, or 45 kg and are used for thin or soft materials. Rockwell B uses a 1/16-inch ball with 100 kg major load for soft steels and nonferrous metals. Rockwell C uses a diamond cone with 150 kg major load for hardened steels. Rockwell A uses a diamond cone with 60 kg major load for cemented carbides. Rockwell T uses a ball with 15, 30, or 45 kg for thin soft metals. Rockwell N uses a diamond with 15, 30, or 45 kg for hard thin sheet metals.

What are the minor loads in Rockwell testing?

The minor load is typically 10 kg, with the superficial load being typically 3kg. It is applied first to seat the indentor and establish a zero reference before the major load is applied.

What does major load mean in Rockwell testing?

The major load is the additional load applied after the minor load to create the actual indentation. It is the load that determines the depth of penetration and thus the hardness reading. Major loads for regular Rockwell are 60, 100, or 150 kg; for superficial Rockwell, they are 15, 30, or 45 kg.

What is the difference between regular and superficial Rockwell testing?

Regular Rockwell uses higher major loads (60, 100, 150 kg) and is used for thicker, larger parts. Superficial Rockwell uses much lighter major loads (15, 30, 45 kg) and is used for thin sheet metals, thin hardened layers, and small parts. The superficial test is more sensitive to surface conditions and is used where the regular test would penetrate too deeply.

How are hardness numbers standardized and indicated?

Hardness numbers are indicated with a number followed by the scale designation. Examples: 60 HRC means hardness of 60 on the Rockwell C scale; 85 HR15T max means maximum hardness of 85 on the Rockwell Superficial 15T scale; 185-240 HV100 means hardness of 185-240 using Vickers with 100 kgf load; 200 HB min means minimum Brinell hardness of 200. The letter code for hardness (H) is followed by the scale designation (B, C, etc.), and the measurement method is indicated (R = Rockwell, V = Vickers, K = Knoop, B = Brinell).

What are the physical properties of metals?

Physical properties are attributes that can be measured without altering or destroying the material. They include: density (mass per unit volume), thermal expansion (change in dimensions with temperature), thermal conductivity (ability to transmit heat), specific heat (energy required to raise temperature), electrical resistivity (opposition to electron flow), electrical conductivity (ability to conduct electricity), magnetic properties (ferromagnetism, permeability, hysteresis), color (how material reflects light), and optical properties.

What are the mechanical properties of metals?

Mechanical properties are characteristics displayed when force is applied and often require destruction for measurement. They include: tensile strength (maximum stress in tension), yield strength (stress at which plastic deformation begins), compressive strength (maximum stress in compression), shear strength (stress at which shear failure occurs), hardness (resistance to indentation), toughness (energy to fracture), ductility (ability to plastically deform), modulus of elasticity (stiffness), fatigue strength (cyclic stress resistance), creep strength (resistance to time-dependent deformation), impact strength (resistance to shock loading), and wear resistance.

Which test goes with each mechanical property and how does each work?

Tensile strength, yield strength, modulus of elasticity, ductility, and toughness are measured by the tensile test (pulling a specimen until fracture). Compressive strength is measured by compression testing. Shear strength is measured by shear testing (double shear for metals). Hardness is measured by indentation tests (Brinell, Rockwell, Vickers, Knoop, Shore Durometer). Impact strength is measured by Charpy Vee, Izod, or drop-weight tests. Fatigue strength is measured by cyclic loading until failure. Creep strength is measured by sustained load at elevated temperature. Wear resistance is measured by various tribological tests (pin-on-disk, block-on-ring, etc.).

What are the categories of mechanical properties?

Strength (tensile, yield, compression, flexural, shear, creep, stress rupture), Formability (% elongation, % reduction in area, bend radius), Stiffness (modulus of elasticity, flexural modulus, shear modulus), Toughness (impact strength, notch sensitivity, critical stress intensity factor), and Durability (hardness, wear resistance, fatigue strength).

What is stress and how is it calculated?

Stress is force distributed over an area. Formula: σ = F/A, where F = force and A = area. Units are psi (lb/in²) or Pa (N/m²) or MPa.

What is strain and how is it calculated?

Strain is the percentage size change of a material in a particular direction when subjected to a force. Formula: ε = ΔL/L₀, where ΔL = change in length and L₀ = original length. Strain is unitless (in/in or m/m).

What are the units of stress and strain?

Stress: psi (lb/in²), ksi (1000 psi), Pa, MPa, GPa. Strain: unitless (in/in, m/m, or %).

What does the size of the area under the stress-strain curve indicate about a metal?

The area under the stress-strain curve is a measure of toughness — the energy required to fracture a given volume of material. The larger the area, the tougher the material. The area under the elastic portion is resilience.

What are the key points on a stress-strain curve?

Proportional limit (where curve deviates from linearity), elastic limit (where permanent deformation begins), yield point (0.2% offset yield strength), ultimate tensile strength (peak stress, necking begins), fracture point (where specimen breaks). The slope of the linear portion is the modulus of elasticity.

What is 0.2% offset yield and how is it found?

The 0.2% offset yield strength is found by drawing a line parallel to the linear elastic portion of the stress-strain curve, offset by 0.2% strain (0.002 in/in) on the strain axis. The stress at which this offset line intersects the stress-strain curve is the yield strength. This is used because the exact yield point is difficult to determine precisely.

What is modulus of elasticity (E)?

E is the slope of the linear portion of the stress-strain curve. Formula: E = Δstress/Δstrain = σ/ε. It is a measure of stiffness — how much a material elastically deflects under load. It does not change with heat treatment. Units: psi or GPa. Steels: ~30×10⁶ psi (207 GPa); Aluminum: ~10×10⁶ psi (69 GPa); Plastics:

What is the standard gage length for tensile specimens?

The standard gage length is the original separation distance between fiducial marks on the reduced section of the tensile specimen. It is typically 2 inches (50 mm) for round specimens, or 4D (four times the diameter) for some standards. The exact gage length depends on the specimen size and standard being used.

What is the difference in stress-strain diagrams for ductile and brittle materials?

Ductile materials show a linear elastic region followed by a nonlinear plastic region with significant elongation before fracture (e.g., low-carbon steel, aluminum alloys). Brittle materials show very little or no plastic deformation — the curve is essentially linear until sudden fracture (e.g., glass, ceramics, cast iron). Ductile materials have a large area under the curve (high toughness); brittle materials have a small area (low toughness).

What is impact testing for?

Impact testing measures a material's ability to withstand shock loading — the energy required to fracture a given volume of material. It assesses toughness and notch sensitivity.

How is impact testing done?

A pendulum-type impacting machine (Charpy or Izod) strikes a notched specimen. The energy absorbed in fracturing the specimen is measured by the height the pendulum reaches after impact. Drop-weight tests use larger specimens and a falling weight.

What are the types of impact testing?

Charpy Vee (notched specimen struck from behind the notch), Izod (notched specimen struck at the free end), drop-weight/dynamic tear (large specimens), and instrumented impact tests on actual parts.

What is the difference in specimens for impact testing?

Notched specimens have a V-notch cut into them to prompt fracture at a specific location and to measure notch sensitivity. Unnotched specimens measure the material's inherent toughness without stress concentration. Notched and unnotched data cannot be directly compared. The standard notched and unnotched pendulum test specimens have a cross section of about 0.2 in².

What is DBTT?

DBTT stands for Ductile-to-Brittle Transition Temperature. It is the temperature below which a material (especially BCC metals like carbon steel) transitions from ductile to brittle behavior. The incorrect term "nil ductility temperature" (NDT) is sometimes used, but the better term is DBTT. It is determined from impact test data by plotting impact strength vs. temperature and finding the temperature where the material drops below a predetermined toughness value (e.g., 15 ft-lb for Charpy Vee).

How do you get information from an impact testing curve?

Plot impact strength (ft-lb or J) vs. temperature. The upper shelf is the high-toughness ductile region. The lower shelf is the low-toughness brittle region. The transition region is between them. The DBTT is the temperature at which the material transitions from ductile to brittle behavior. Above the DBTT, the material is tough; below it, the material is brittle.

What is fatigue strength and how is it tested?

Fatigue strength (endurance limit) is the maximum stress below which a material can theoretically endure an infinite number of stress cycles without failure. It is tested by repeatedly loading a specimen at a given stress level until it fails, then reducing the stress level and repeating. A test duration of 10 million cycles is usually considered infinite life. The result is an S-N curve (stress vs. number of cycles). For steels with tensile strength

What is creep strength and how is it tested?

Creep strength is the constant nominal stress that will cause a specified quantity of creep in a given time at constant temperature. It is tested by applying a constant load to a specimen at elevated temperature and measuring strain over time. The creep curve shows: (a) elastic strain, (b) primary creep (decreasing rate), (c) steady-state creep (constant rate), (d) tertiary creep (increasing rate leading to failure). For metals, creep strength is often expressed as the stress to produce 0.1% strain in 1000 h. Creep is not important for most ferrous metals below 800°F (426°C).

What are the factors involved in materials selection?

Chemical properties (composition, microstructure, crystal structure, corrosion resistance, chemical reactivity), Physical properties (density, thermal expansion, thermal conductivity, specific heat, electrical resistivity, magnetic properties, color), Mechanical properties (tensile strength, yield strength, compressive strength, shear strength, hardness, toughness, ductility, modulus of elasticity, fatigue strength, creep strength, impact strength), Dimensional properties (surface finish, size, shape, tolerances, camber, flatness), and Procurement/manufacturing considerations (available shapes, sizes, surface texture, manufacturing tolerances, cost, availability).

What are the terms in Figure 3-1 related to metals?

For metals: Chemical properties include composition, microstructure, phases, grain size, corrosion resistance (reactivity), inclusions. Mechanical properties include tensile/compressive properties, toughness, ductility, fatigue strength, hardness, creep resistance, shear strength. Physical properties apply equally to all systems. Procurement/manufacturing considerations include available shapes, available sizes, available surface texture, manufacturing tolerances.

What are limestone and coke used for in the blast furnace?

Coke serves as a source of heat and as a source of reducing gas (carbon monoxide) that chemically detaches oxygen atoms from iron oxide. Limestone acts as a flux or purifying agent to assist in removing impurities from the iron ore.

How does the blast furnace work and what does it produce?

Iron ore (pelletized), coke, and limestone are charged into the top of the blast furnace. Air is blown in to combust coke, producing CO₂, which reacts with excess coke to form CO (the reducing gas). The CO reduces iron oxide to metallic iron. The molten iron (pig iron) is tapped from the bottom, and slag (impurities) is tapped from the top. Pig iron is high in carbon (4-5%) and must be further refined to make steel.

What are the chemical formulas for steelmaking?

Production of reducing gas: C (coke) + O₂ (air) → CO₂; CO₂ + C (excess coke) → 2CO. Reduction of iron oxide: 2Fe₂O₃ (ore) + 3C (coke) → 4Fe + 3CO₂; Fe₂O₃ + 3CO → 2Fe + 3CO₂. Steel refining (removing carbon): C (in iron) + O₂ → CO₂ (or CO).

What is reduction in steelmaking?

Reduction is the process of removing oxygen from iron oxide (ore) to produce metallic iron. The oxygen is chemically detached from the iron by reaction with carbon monoxide and carbon from coke.

What is steel refining?

Steel refining means making steel from pig iron, scrap, or ore by removing impurities (carbon, silicon, sulfur, phosphorus, manganese) through oxidation. Oxygen is blown into the molten iron to combine with carbon and other impurities.

What is the difference between primary and secondary refining?

Primary refining is the initial steelmaking process (BOF or EAF) where carbon, manganese, sulfur, and phosphorus are reduced to desired levels. Secondary steelmaking (secondary refining) is done in the ladle after primary refining and includes processes like vacuum degassing, ladle stirring, ladle heating, and special refining processes (VAR, VIM, ESR, electron beam) to further reduce impurities, modify chemical composition, and improve properties for specialty steels.

What are the different types of furnaces used in steelmaking?

Basic Oxygen Furnace (BOF): Refractory-lined vessel that tips to pour. Charged with scrap, pig iron, fluxing agents, and hot metal. Oxygen lance reduces carbon, sulfur, phosphorus. Process time ~20 min. Capacity up to 350 tons. Produces ~60% of U.S. steel. Electric Arc Furnace (EAF): Heat from an arc between melt and graphite electrodes. Charged with scrap or solid pig iron. Oxygen lance refines. Can be shut down between uses. Capacity few to 300 tons. Used for alloy and specialty steels, minimills. Open Hearth Furnace: Obsolete in U.S. (last decommissioned 1991). Long, broad, shallow. Up to 450 tons. Took up to 10 h to refine.

What is an oxygen lance?

An oxygen lance is a device used to blow oxygen into the molten metal in a BOF or EAF to reduce carbon, sulfur, and phosphorus by oxidation.

Which furnaces require external heat?

BOF requires external heat only initially (hot metal provides heat). EAF uses electrical energy (external heat) to melt the charge. Open hearth used external fuel (gas or oil).

How is carbon content controlled in steelmaking?

Carbon content is controlled by the amount of oxygen blown into the melt (oxygen combines with carbon to form CO and CO₂). Alloy additions in the ladle can also adjust carbon content. Secondary refining processes like vacuum degassing can further reduce carbon.

What is continuous casting?

Continuous casting (concast) is a process where refined steel is poured into a ladle, then into a tundish, then into a water-cooled continuous mold. The steel exits the bottom of the mold as a solid skin with a molten core, which solidifies as it moves through support rollers. The strand is cut into designated lengths. It skips the ingot step and produces semis (billets, blooms, slabs, beam blanks) directly. Over 90% of U.S. steel is continuously cast.

What is vacuum degassing?

Vacuum degassing removes dissolved gases from steels by streaming molten metal from an upper vessel to a lower vessel in an evacuated chamber. It reduces carbon and oxides.

What is vacuum arc remelting (VAR)?

VAR involves casting steel into cylindrical ingots, welding a stub shaft to them, and remelting in a vacuum by establishing an arc between each ingot (electrode) and a water-cooled copper mold. Used for superalloys and extra clean steels. Very effective at removing inclusions.

What is electroslag melting (ESR)?

ESR is similar to VAR but without vacuum. A VIM or electric furnace melt is cast into remelt ingots, which become electrodes for arc remelting in a water-cooled copper mold. Purification occurs when melting metal passes through a molten flux (slag) that removes impurities. Used for tool steels and special-purpose steels.

What is vacuum induction melting (VIM)?

VIM melts solid scrap or liquid charges in a crucible heated by high-frequency induced currents in a vacuum. Produces convection current mixing. Ingots are cast in vacuum. Used for specialty alloys.

What is electron beam refining?

Molten metal is poured down a tundish into an ingot mold in a vacuum. An electron beam vaporizes impurities as the metal flows, removing them as vapors. Used to purify specialty alloys.

What is ladle stirring injection?

Bubbling argon from a lance through the melt or from a port in the bottom of the ladle. Promotes sulfur removal, deoxidation, and removal of dissolved gases.

What is ladle furnace heating?

Restores heat to the metal in the ladle after primary refining. Allows time for alloy additions and secondary refining processes.

What are the acronyms for special refining processes and what are they used for?

VAR (vacuum arc remelting) — superalloys, extra clean steels. VIM (vacuum induction melting) — specialty alloys. ESR (electroslag remelting) — tool steels, special-purpose steels. These processes are used for aerospace, tool steels, bearing steels, and other critical applications.

What are the grades of steel (killed, rimmed, semi-killed, capped)?

Killed steel: Strongly deoxidized (usually with aluminum or silicon). Molten metal lies quiet during solidification. Less prone to segregation and pipe. Continuous cast steels are always killed. Rimmed steel: Slightly deoxidized. Solidifies with an outer shell low in impurities and very sound. Retains good finish even after severe forming. Used for sheet and strip. Semi-killed steel: Partially deoxidized. Intermediate between killed and rimmed. Capped steel: Partially deoxidized, but the top is capped to prevent rimming. Used for some sheet applications.

Which grade is used in continuous casting and why?

Killed (deoxidized) steel is used in continuous casting because it solidifies quietly without gas evolution, preventing porosity and pipe. Continuous cast steels are always deoxidized or killed.

What are ingots and how are they made?

Ingots are cast shapes poured from a ladle into ingot molds. They are the traditional starting point for steel finishing. They can have defects like pipe (shrinkage cavity at top) and segregation (chemical composition variation). Less than 10% of U.S. steel goes into ingot; most is continuous cast. Large forgings are still made from ingots.

What are non-metallic inclusions?

Non-metallic inclusions are oxides, silicates, sulfides, or aluminas that form during melting and refining. They are separate phases in the steel. Their size and volume fraction affect fatigue life and toughness. They can be measured by acid etching (dirty steel shows pits) or microscopic examination. Inclusion ratings can be specified in purchasing specifications.

What are the different forms of semi-finished steel and what are they used for?

Billet:

What is an integrated mill vs a mini mill?

An integrated steel mill has at least one blast furnace, a coke plant, and various steel-refining processes. It makes a number of product lines and usually has annual capacities over 100 million tons. Located on waterways for ore transport. A mini mill has only one product line (e.g., structural shapes, rebar) and converts scrap in a BOF or EAF. Can be located anywhere. Mini mills are becoming more important.

What is the difference between plate and sheet?

Sheet is rolled steel primarily in the thickness range of 0.010 to 0.250 in. (0.25 to 6.4 mm) with a width of 24 in. (610 mm) or more. Plate is steel shapes over 0.18 in. (4.5 mm) thick over 48 in. (1.2 m) wide, or over 0.230 in. (5.8 mm) thick over 6 in. (152 mm) wide.

What is the difference between strip and bar?

Strip is rolled steel in the thickness range of 0.010 to 0.25 in. (6.25 to 6.4 mm) with a width less than 24 in. (610 mm). Bar is hot- or cold-rolled rounds, squares, hexes, rectangles, and small shapes. Round bars can be as small as 0.25 in. (6.4 mm); flats can have a minimum thickness of 0.203 in. (5.0 mm); shapes have a maximum dimension less than 3 in. (76 mm).

What do BCC and FCC mean and what are their characteristics?

BCC = Body-Centered Cubic: atoms at corners and one in center of cube. FCC = Face-Centered Cubic: atoms at corners and one in center of each face. BCC metals (e.g., alpha iron, carbon steel) are often brittle at low temperatures (poor impact strength at subzero temperatures). FCC metals (e.g., austenitic stainless steel, copper, aluminum) do not have a significant lowering of impact strength at low temperatures.

How is impact strength affected by crystal structure?

BCC metals (carbon steels) have a ductile-to-brittle transition temperature (DBTT) — impact strength drops significantly at low temperatures. FCC metals (austenitic stainless steel, copper, aluminum) strengthen slightly at low temperatures but do not have a significant loss of impact strength.

What is allotropic and what is its importance?

Allotropic means the ability of a material to change crystal structure. Iron is allotropic: delta iron (BCC, high temperature), gamma iron (FCC, intermediate temperature, austenite), alpha iron (BCC, room temperature, ferrite). This is important because the FCC structure can hold more carbon in solution than BCC, which is the basis for quench hardening of steel.

What are delta iron, gamma iron, and alpha iron?

Delta iron: BCC, stable at high temperatures (above 2552°F/1400°C). Gamma iron: FCC, stable at intermediate temperatures (1670-2552°F/910-1400°C), called austenite, nonmagnetic, can dissolve up to 2% carbon. Alpha iron: BCC, stable at room temperature, called ferrite, magnetic, can dissolve only about 0.02% carbon at room temperature.

How does solidification progress?

When molten metal cools, dendrites (treelike, three-dimensional structures) form from nuclei (impurities, mold wall protrusions, etc.). Dendrites grow until they meet, forming grains. The unit cells in each grain are all oriented in the same direction. When grains meet, they form grain boundaries. The first metal to solidify is purer than the last, causing segregation.

What are grains and grain boundaries?

Grains are three-dimensional crystals that bond with other crystals to make the volume of a polycrystalline material. Each atom in every grain is aligned with each other in three dimensions. Grain boundaries are the junctions where grains meet — regions of atomic disarray. They are higher-energy areas and can be anodic to the grains in corrosion.

What are equiaxed grains?

Equiaxed grains are grains that have the same shape in the x, y, and z directions. They form in the center of continuously cast strands, as opposed to columnar grains that form at the cold mold walls.

What is the difference between interstitial and substitutional atoms and how do they affect the crystal lattice?

Interstitial atoms are small atoms (like carbon) that occupy spaces between host atoms in the crystal lattice. They cause local distortion of the lattice. Substitutional atoms are larger atoms that replace host atoms in the lattice. They also cause distortion. Both create strains that impede dislocation motion and strengthen the metal.

What is the difference between cooling curves of alloys and pure metals?

Pure metals have a single melting point — the cooling curve shows a flat plateau at the melting temperature. Alloys (with multiple elements) melt over a temperature range — the cooling curve shows a slushy zone where the metal is part solid, part liquid (like sleet). The liquidus is the temperature above which all is liquid; the solidus is the temperature below which all is solid.

How do you determine % of an element from the points?

Use the lever law. Draw a tie line (isotherm) at the temperature of interest. The fraction of a phase present is the length of the tie line to the opposite side divided by the total length of the tie line. For example, if the tie line goes from 5% to 22% B and the alloy is 12% B, the fraction of alpha = (22-12)/(22-5) = 10/17 = 58.8%; liquid = (12-5)/(22-5) = 7/17 = 41.2%. In the example given: 40 points = 0.40% carbon (40 points = 40/100 = 0.40%).

What are liquidus, solidus, solvus, eutectoid, and eutectic?

Liquidus: The line on a phase diagram above which all is liquid. Solidus: The line below which all is solid. Solvus: The line showing the solubility limit of one element in another as a function of temperature. Eutectic: A composition with a single melting temperature (solid → liquid at one temperature), typically with a lamellar structure of two phases. Eutectoid: A solid phase with a single transition temperature to another solid (e.g., austenite → pearlite at 1333°F/723°C for 0.8% carbon steel).

Define carbon steel.

Steel with carbon as the principal hardening agent. All other alloying elements are present in small percentages: manganese ≤1.65%, silicon ≤0.60%, copper ≤0.60%, sulfur and phosphorus ≤0.05%.

Define alloy steel.

Steel with significant additions of any element other than carbon, but generally total alloy additions of less than about 5%. Used primarily for structural applications.

Define rimmed steel.

Slightly deoxidized steel that solidifies with an outer shell on the ingot which is low in impurities and very sound. Retains good finish even after severe forming.

Define killed steel.

Strongly deoxidized steel (usually by chemical additions to the melt). Molten metal lies quiet during solidification. Less prone to segregation and pipe.

Define galvanized and galvannealed.

Galvanized = zinc-coated steel products (zinc applied by hot dipping). Galvannealed = zinc-coated and heat-treated steel (heat treatment creates an oxide layer for better paint adhesion).

Define sheet, bar, coil, wire, shapes, plate, strip.

Sheet: 0.010-0.250 in. thick, ≥24 in. wide. Bar: hot/cold-rolled rounds, squares, hexes, rectangles, small shapes (

Define free machining steel.

Steels with additions of sulfur, lead, selenium, or other elements in sufficient quantity that they machine more easily than untreated grades.

Define drawing quality and merchant quality.

Drawing quality: hot- or cold-rolled steel specially produced or selected to satisfy elongation requirements of deep drawing operations. Merchant quality: steels with an M suffix intended for nonstructural applications; low-quality material.

Define commercial quality.

Steels produced from standard rimmed, capped, concast, or semikilled steel. May have significant segregation and composition variation. Not made to guaranteed mechanical property requirements. Most widely used grade.

Define H steels, B steels, pickling, temper rolling, temper, E steels.

H steels: identified by H suffix, made to guaranteed ability to harden to a certain depth. B steels: small boron additions as hardening agent (xxBxx). Pickling: acid removal of oxides and scale. Temper rolling: small roll reduction to eliminate stretcher strains and improve surface finish. Temper: amount of cold reduction in rolled sheet and strip. E steels: E prefix indicates electric furnace melted.

What is cold rolling vs hot rolling?

Hot rolling: rolling steel in the red-hot condition (above recrystallization temperature). Grains dynamically recrystallize, so the steel does not get harder. Produces soft, scaled, dull gray surface. Cold rolling: rolling steel at room temperature. Grains get squashed, steel work hardens (gets stronger and harder), better surface finish. Requires annealing between passes to prevent fracture.

What is dynamic recrystallization?

The formation of new, undeformed grains during hot working. The elevated temperature provides energy for dislocations to annihilate and new grains to form, preventing work hardening.

Which to choose for applications?

Hot-rolled: when softness, formability, and lower cost are needed; for structural shapes, plate, and applications where surface finish is not critical. Cold-rolled: when higher strength, better surface finish, and tighter tolerances are needed; for auto-body sheet, appliances, and precision parts.

Be able to use the Fe-Fe₃C phase diagram to identify what happens to steels as they solidify.

As carbon steel cools from the melt: (1) At the liquidus, austenite (gamma iron, FCC) begins to form. (2) At the solidus, all is austenite. (3) As cooling continues, ferrite (alpha iron, BCC) begins to form at the grain boundaries (for hypoeutectoid steels,

What is austenite?

Austenite (gamma iron) is the FCC crystal structure of iron with carbon in solid solution. It is soft, moderate strength, nonmagnetic. Stable at intermediate temperatures (1670-2552°F/910-1400°C).

What is cementite?

Cementite (Fe₃C) is a compound of iron and carbon with a stoichiometric composition of 6.67% carbon. It is hard and brittle.

What is pearlite?

Pearlite is a lamellar microstructure of alternating layers of ferrite and cementite. It forms at the eutectoid composition (0.8% carbon) when austenite slowly cools. It is the equilibrium room-temperature structure for carbon steels along with ferrite (for

What is ferrite?

Ferrite (alpha iron) is BCC iron with carbon in solid solution. It is soft, ductile, and magnetic. It can dissolve only about 0.02% carbon at room temperature.

What is martensite?

Martensite is a body-centered tetragonal (BCT) phase formed when austenite is rapidly quenched. Carbon atoms are trapped in the crystal structure, causing distortion. It is very hard and strong but brittle. It is the hardening phase in quenched steels.

How do you determine percent ferrite and pearlite given carbon content?

For hypoeutectoid steels (

What happens to a steel containing 70 points of carbon as it increases in temperature across the solidus line?

70 points = 0.70% carbon. As it crosses the solidus line (on heating), the steel becomes completely austenite (FCC). The pearlite and ferrite transform to austenite as the temperature rises through the eutectoid temperature (1333°F/723°C) and above. At the solidus, all phases have transformed to austenite.

How does grain size affect ductility?

Fine-grained steels generally have higher strength and better ductility/toughness than coarse-grained steels. Grain boundaries impede dislocation motion, so more grain boundaries (smaller grains) means higher strength. Fine grains also provide more uniform deformation and better toughness.

Define isotropic and anisotropic.

Isotropic: properties are the same in all directions (e.g., amorphous plastics, cast metals). Anisotropic: properties vary with direction of loading (e.g., reinforced plastics, rolled steel sheet, wood). Anisotropy in steel is caused by grain orientation from rolling.

Define annealing — what is the condition of the material after treatment, how is it done, temperatures and times, and how are mechanical properties affected?

Annealing is heating a steel to its austenitizing temperature (above the critical transformation temperature) and cooling slowly (furnace cooling at ~100°F/h or less) to prevent formation of hardened structure. After treatment, the material is soft, ductile, and stress-relieved with an equilibrium microstructure of ferrite + pearlite (or pearlite + cementite for high carbon). It is done to soften hardened steel, improve machinability, relieve internal stresses, and homogenize chemical gradients. Mechanical properties affected: decreased hardness and strength, increased ductility and toughness. Process anneal is below the transformation temperature.

What is austenitizing?

Austenitizing is heating steel to a temperature in the austenite region (above the critical transformation temperature, typically 1400-1700°F/760-927°C) to transform the microstructure to austenite (FCC). This is the first step in hardening, annealing, and normalizing.

What is martensite and how is it formed?

Martensite is a body-centered tetragonal (BCT) phase formed by rapidly quenching austenite. The rapid cooling prevents carbon from diffusing out, trapping it in the crystal structure and causing distortion. It is very hard and brittle. Formation requires: (1) heating to austenitizing temperature, (2) sufficient carbon content (≥0.6% for full hardening in plain carbon steels), (3) rapid quench.

What are the properties of steel after hardening?

After hardening (quenching), steel is very hard (up to 65 HRC), very strong, but very brittle. It has a martensitic microstructure. It must be tempered to improve toughness. The microstructure depends on carbon content: 0.2% C → ferrite + pearlite (not fully hardenable); 0.6% C → mostly martensite; 0.8% C → 100% martensite (with sufficient quench); >0.8% C → martensite + retained austenite + cementite.

How does hardening differ from annealing with respect to final steel properties?

Hardening produces a hard, strong, brittle martensitic structure by rapid quenching from the austenitizing temperature. Annealing produces a soft, ductile, stress-relieved equilibrium structure (ferrite + pearlite) by slow cooling from the austenitizing temperature. Hardening increases strength and hardness but decreases ductility; annealing decreases strength and hardness but increases ductility.

To obtain a HB, the decimal is removed from the indentation diameter measurement.

False. The Brinell hardness number (HB) is calculated from the diameter of the indentation — the decimal is not simply removed. The formula involves the load and the diameter of the indentation, and the result is a number that is read from a table or calculated.

Rockwell hardness testing has regular and superficial versions that cover a wide range of materials.

True. Regular Rockwell (B, C, A) uses higher loads for thicker materials; superficial Rockwell (T, N) uses lighter loads for thin materials.

The indenter for Rockwell hardness testing can be a tungsten carbide ball or a diamond cone.

True. The tungsten carbide ball (1/16-inch) is used for softer materials (B, T scales); the diamond cone (brale, 120°) is used for harder materials (C, A, N scales).

The major loads for Rockwell hardness testing are either 5-kg or 10-kg.

False. The minor load is 10 kg. The major loads for regular Rockwell are 60, 100, or 150 kg; for superficial Rockwell, they are 15, 30, or 45 kg.