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56 Cards in this Set

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classify steel according to carbon content. pros, cons and use?

Low-carbon steel: high ductility, low tensile strength, use: sheets for car body panel

mild steels: high ductility, lowtensile strength, use: buildings, bridges, pipes

medium-carbon steel: balances ductility and strength, good wear resistance, use: machinery, shafts, gears, large parts

high carbon steel: verry strong, use: springs, high strength wires, tools

higher carbon content in steel make the steel...:

ability to become harder and stronger with heat treatment

less ductile

reduces weldability

lower meltingpoint

three types of alloying content of steel, uses

microalloyed steels (HSLA), use: pressure vessels, ships, cars

lowalloyed steels, use: pressure vessels, piping, machinery components

highalloye steels, use: tools and stainless steel

what is plain carbon steel

plain carbon steel = mild carbon steel

most common steel because, low price and properties that are acceptable for many applications.

what happens with the microstructure when adding more carbon?

no carbon: only alfa islands

low carbon: alfa island and small strains of perlite, austenite transforms into pearlite when tempering gets lower than A3. Under A1, pearlite forms in grain bounderies

mild carbon: islands are getting smaller and more perlite¨

medium carbon: almost the same amount of perlite as alfa

high carbon steel:

under 0,76: a lot of perlite and strains of alfa

over 0,76: perlite islands and strains of cementite. More austenite transforms to pearlite which prevents dislocationgives higher tensile strength, yieldstrength and hardness but reducing ductility.


alfa-ferrite, alpha-iron. a term for pure iron.

BCC. the bcc is the structure that gives steel and cast irons their magnetic properties.

a classic exemple of ferromagnetic material (=strongest type of permanent magnetism)

The name given to the bcc crystal structure of iron that can occur as alpha or delta.

delta ferrite is the first bcc structur of the iron. immediately after solidification iron forms bcc, called delta ferrite. further cooling gives fcc called austenite or gamma. further cooling will give you another bcc called alpha


cementite = iron carbide = Fe3C

forms when the solubility of carbon in solid iron is exceeded.

extremely hard and brittle (like a ceramic material)

When properly dispersed, provides the strengthening in steels.


austenite=gamma-phase iron

the name given to the fcc crystal structure of iron and iron-carbon alloys.

non-magnetic, solid solution of iron.


To heat the iron or steel to a temperature at which it changes crystal structure from ferrite to austenite.


a hardening process to promte better mechanical properties. the metal is heated into austenite and then quenched to 300-375 c. annealed at this temp until the austenite turns to bainite or ausferrite.

Lower austenization temperature produces a more uniform distribution of austempered structure.

higher austenization temperature can produce a higher carbon content in austenite.


a metastable phase formed in steel and other materials by a diffusionless, athermal (no change in temp.) transformation

a very hard form of steel crystaline structure.

formed in carbon steel by rapid cooling of austenite at such high rate that carbon atoms do not have time to diffuse out of the crystal to form cementite. result: thee fcc austenite transforms to a highly strained bct form of ferrite.


Spheroidite forms when carbon steel is heated to a high temp under A1 (austenite to pearlite) for a long time (ish 30 h). result is spheres (or rods) of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel.


a two-phase lamellar microconstituent, containing ferrite and cementite, that forms in steels cooled in a normal fashion or isothermally transformed at relatively high temperatures.


a two-phase microconstituent, containing ferrite and cementite, that forms in steels that are isothermally transformed at reltively loe temperatures.


a heat treatment used to reduce the hardness of martensite by permitting the martensite to begin to decompose to equilibrum phases. this leads to increased tougness.

explain: A1, A3, Acm. also c and r that sometimes occures. ex, Ac1

A1: The temperature where austenite transform to pearlite during cooling. Eutectoid temperature.

A3: Where the austenite transform into ferrite when cooling down. Applies only on steels Approx. 0-0.76%% carbon. Higher carbon content => lower start temp.

Acm: Where the austenite starts to transform into cementite when cooling down. more carbon content => higher start temp.

c: ex: the Ac1 temp is that at which austenite begins to form during heating

r: Ex: the Ar1 temp is that at which all austenite has decomposed during cooling of a steel sample from the austenite phase field.


Body-centered cubic, one atom in the center of the unit cell and eight atoms in the corner. total 2 atoms per unit cell (1/8)*8+1=2.

at rommtemp. theses metals are bcc:




HCP is usually more brittle than bcc and fcc because hcp has a lower degree of symetri, gives fewer active slip. in general, hcp has lower ductility


Face-centered cubic, one atom on every side and every corner, total 4 atoms per cell, (1/2)*6+(1/8)*8=4.

At roomtemp. these metals are fcc:








FCC is likely to be more ductile than bcc and hcp.


body centered tetragonal, like a stretched bcc structure

bct have fewer active slip and is therefore usually lower ductility (more brittle)


hexagonal close packing, like a two layer "6-kant" with a atom in every corner and three atoms in the center inbetween the layers... so three layer...

6 atoms per unit cell







Hcp have a lower degree of symetry, this gives fewer active slip => lower ductility = more brittle

Application for low- medium- and high-carbon steel

Low-carbon steel: Sheets for car body panels

Medium carbon steels: machinery, shafts, gear

High carbon steels: springs, high-strength wire, tools

Commonly used heat-treated carbon steel?

low, medium, high


Low Carbon steel:

Process annealing (Glödning)

removes cold work

reduce residual(rester) stresses

is done below the A1 temp.

Annealing and normalization- dispersion strenghtning.

used to control the fineness of pearlit.

fist austenitizing => cool slowly in a furnace(ugn) => coarse pearlite=> normalization that allows the steel to cool more rapidley => fine pearlite.

Medium-/high-carbon steels

Quenching (snabbkylning)

Austenitizing => quench => martensite structure = a form of steel that passes a super saturated carbon content in a deformed BCC structure.

extremly hard and brittle => to brittle to use.

Quenching and tempering

Most common used heat treatment due the finel properties of steel can be pricesly determined by temperature and time of tempering.

after quench, reheat below eutectoid point => cold=>small amounts of spherodite forms => restores ductility and reduce hardness.

Annealing and normalizing

dispersion strenghtening

(a lot of phase changes make it hard for dislocations to move)

Warming until austenite is created, then cooling

allows steel to cool more rapidly => fine pearlite

The faster cooling => the more more pearlite (not to fast => martensite)

Annealing = lower cooling rate => coarser pearlite

Carbon steels are not alloyed, but still they contain small amounts of elements like Mn(manganese) and Si (silicon) - explain why!

The Mn, Si and Cu which are usual in steel comes from the manufactory. The carbon content of pig-iron (4% carbon) is reduced oxygen is used, which will produce carbon dioxide. But some oxygen stays in the furnace and creates slag which later on can initiate fractures. Therefore is Mn and Si used to remove the slag. The Cu (cupper) comes from re-cycled material.


hot metal (råjärn) is the molten iron in a blast furnace. this "hot metal" consits of 95 % iron, 4% carbon, silicon, manganese,sulfur and phosfor.

List some important reasons for alloying steels!

Provide solid solution strengthening for ferrite.

Cause the precipitation(armeringen i ferriten) of alloy carbides rather than that of cementite

Improve corrosion resistance and other special characteristics of the steel.

Improve hardenability, (how easy the steel form martensite). How easily we can form martensite in a thick section of steel that is quenched.

Improve wear resistance (nitrides and carbide makes the surface hard).

HSLA (improve toughness)

Retain strength at elevated temperatures.

Improve strength without lowering ductility and weldability.

Which are the most common alloying elements in steel?

Ni, Cr and Mo are frequently used to increase the hardenability in steels. A combination of different elements gives a stronger effect than increasing the amount of only one element.

List alpha and gamma stabilizers respectively and describe how these elements influence the critical temperatures in steels. Which alloying elements are carbide formers?

Austenite (Gamma) stabilizers:

C, Ni, N and Mn are stabilizing the austenite. With high content austenite can appear even in room temperature.

Ferrite (Alfa) stabilizers:

Cr, Si, Mo, W and V. More than 13% Cr makes the steel into alfa-phase up to total melting.

Carbide formers:

Cr, W, Mo, V, Ti, Nb. The hardenability increase with alloying elements, when cooling to martensite. Interstitial and substitutional hardening.

Describe microalloyed (HSLA) steels in general!

High-strength-low-alloy steels are low-carbon steels containing small amounts of alloying elements.

HSLA are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition.

Higher strength means less space needed (increased strength-to-weight ratios)

Which are the main alloying elements used in microalloyed steels (HSLA). How do the alloying elements influence the mechanical properties?

What are the mechanisms behind the high strength of microalloyed (HSLA) steels?

Main alloying elements used:

The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

Alloying elements influence of the mechanical properties:

Carbide and carbonitride forming elements (nb, V, Ti) for precipitation hardening and grain refinement. Smaller grain size improves both strength and toughness and lower the transition temperature. Increase of yield strength but reduced ductility( about 30-40%)

Explain HSLA grades

from 942X to 980X

Higer grade =>

higher Yield strength

higher tensile strength

more carbon and more alloying elements

What are the main areas of applications of HSLA?

Main areas of applications of HSLA:

Weathering steels. HSLA steels were developed primarily for the automotive industry to replace low-carbon steel parts with thinner cross-section parts for reduced weight without sacrificing strength and dent resistance. Typical passenger-car applications include door-intrusion beams, chassis members, reinforcing (förstärkning in SWE) and mounting brackets (stödskikt in SWE), steering and suspension parts, bumpers, and wheels(3).

Explain the main idea behind thermomechanical controlled processing of HSLA! What features of the microstructure are affected by this process and how does it influence the mechanical properties of the steel?

The main idea about the thermomechanical controlling process is to create new, smaller grains of ferrite or perlite. These grains should have greater strength, and still high toughness. When doing a hot rolling, the grains are normally growing. The purpose of doing that rolling is to increase the strength of the steel. But normally other mechanical property changes during that, for example the toughness and the ductility. We need to do some hot working to change that and compensate the decreased properties. Another way to change this dilemma is to use the thermomechanical controlling process. This is a process is made for alloyed steels, and not normal carbon steel. First we warm up the steel to a high temperature, but under the recrystallization temperature, and doing a warm working. The alloyed substances increase this temperature, so a warm work can easily be done without growing grains. Then we cool the steel down. Either slower, or faster, depending on how small we want the grains. Faster cooling leads to smaller grains. Then we have new, SMALLER grains (of perlite or ferrite), with higher strength, and still high toughness and ductility. This process could be done thanks to the alloyed substances.

Describe low-alloyed steels in general!

Which are the main alloying elements used and how do they influence the mechanical properties of the steel?

A low-alloyed steel is a steel with a higher percent of alloying elements than HSLA-steels, but still with a low percent. The steel is called a low-alloyed steel if the sum of all the alloying elements stays between 2% to 10.5%, when the steel becomes stainless. Normal alloying elements are (except carbon, silicon and mangan) chromium, nickel and molybdenum. They are the main alloying elements used in the steel.

All the alloying elements increases the hardenability for the steel, so martensite could more easily be created, without increasing the CEV to much, so the weldability decreases. The strength for these steels should increase, and get other specific properties depending on the type of the low-alloyed steel. Nickel could make the steel easier to be used in low temperatures. Chromium helps the steel to prevent corrosion. Together with molybdenum it makes the steel suitable in higher temperatures.

Give some examples of types of low-alloyed steels and their applications!

Chromium-molybdenum-steels: Which should consist of 0.5-9% Cr, 0.5-1% Mo and an amount of carbon under 0.2%, has an acceptable weldability. It has, thanks to the high amount of Cr, a higher resistance to corrosion. And, thanks to the molybdenum, it has a higher strength in high temperature conditions. This steel is commonly used in the oil/gas industries, steam power equipment, nuclear power plants and high temperature services.

Bearing steels: Which is used for ball bearings, and roller bearings. These has a lower amount of carbon (0.1-0.2%).

Nickel steels: Which contains a higher amount of nickel, normally around 2.5-3.5%. This makes it a good low temperature steel. It will, in low temperatures, have a higher strength, and good toughness. This steel shall be used in low temperature situations, like machinery in minus degrees situations.

Medium-carbon-ultrahigh-strength-steels: They have a higher carbon content, and are stronger steels. They are normally used in structural applications. A carbon content closer to the eutectoidic point gives the steel better opportunities for creations of springs and other steel constructions.

Explain how TTT (IT) diagrams can be used for selection of a correct heat treatment.

shows the cooling rate which is needed to create coarse pearlite, fine pearlite, bainite and martensite.

TTT (Time temperature transformation) shows to which temperature the austenite would be quenched and then hold on constant temperature to produce different kinds of material. Pearlite, bainite or martensite.

Explain how CCT diagrams can be used for selection of a correct heat treatment.

shows the cooling rate which is needed to create coarse pearlite, fine pearlite, bainite and martensite.

CCT (continuous cooling transformation) – Shows at which constant colling rate that must be done to reach the material that was searched for.

Describe Full annealing heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Full annealing: Hypoeutectoid steel: Austenitizing 30 degrees over A3 which creates 100% austenite.Hypereutectoid steel: 30 degrees over A1 which crates Austenite and cementite. This heating process prevent brittle cementite to be crated at the grain boundaries when later cooling, like if heating above Acm would be made.The annealing then creates coarse perlite. It is more ductile than fine perlite, lower strength, not that hard, lower toughness. This depends on the perlite where the layers of cementite and ferrite not so closed packed which makes it easier for dislocations to move.

Describe Recrystallization annealing heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Recrystallization annealing: Low carbon steels. This is used at cold worked pieces of metal which have high density of dislocations and greater grains. The recrystallization heal the steel to the microstructure which it had before it was cold worked. Dislocations disappear, the metal becomes soft and ductile and ready to be re-cold worked. Strenght is lower because of less dislocations, and toughness is higher. Good formability.

Describe Normalizing heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Normalizing: First austenizing 55 degrees above A3 and Acm, then cooling in air which creates fine perite (the diffusion is not fast enough to build coarse perlite). Harder, stronger, less ductile and higher toughness than coarse perlite.

Describe Stress relieving heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Stress relieving: Is done after cold work. Is almost the same as recrystallization process annealing, except not warming until new grains is created. The stress relieving is getting the dislocations in order in a kind of pattern. This releases internal stresses. The material becomes hard and brittle but not as much as after cold work. Its more brittle than recrystallization, higher strength(??) and higher toughness(??).

Describe Spheroidizing heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Spheroidizing: Is made on steels with high carbon content and much cementite. Heating 30 degrees below A1 in many hours. This creates spheres of cementite because of less thermodynamic surface (lower internal energies are needed). The matrix is ferrite. This increases the machinability of the steel. The properties are: less hard, lower strength, more ductile, lower toughness.

Describe Direct hardening heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Direct hardening: Heating the steel to austenite, and then cooling to martensite, bainite, fine pearlite or coarse pearlite. The properties depends on how fast the cooling is made. General the materials get harder, more brittle the faster it cools.

Describe Tempering heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Tempering: Is a process of heat threatening used to increase the toughness of iron based alloys, but is at the same time reduces the hardness of the metal. Heating below the A1 for some hours, and cooling in air.

Describe Martempering heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Martempering: Austeniitizing, then quenching under the martensite-start temp then holding the temp until the metal has become martensite. Hard, brittle. (The martensite is tempered at 100-600 degress for 1-2h. The structure changes from BCT to BCC and spheres of cementite is created like in the spherodizing process. Martemtempering is stress relieving. The process makes the material less hard than martensite, but more ductile, higher strength, and higher toughness.)

DescribeAustempering heat treatments regarding procedure, purpose, and influence on microstructure and mechanical properties (hardness, strength, ductility, toughness).

Austempering: Produces bainite. Austenitizing the steel and quenching to some temperature below the nose of the TTT curve, holding that temperature until all the austenite transforms to bainite. Property: Harder than pearlite, but not hard as martensite. Less ductile than pearlite. Bainite is more tough than martensite.

What is the difference between hypoeutectoid steel and hypereutectoid steel?

Hypo-eutectoid steel has less than 0,8% of C in its composition.It is composed by pearlite and α-ferrite.

Hyper-eutectoid steel has between 0.8% and 2% of C, composed by pearlite and cementite.

Eutectoid would be with a percentage of 0.8 and formed by pearlite, every percentage is of weight.Pearlite, cementite and α-ferrite are three microstructures of the Fe-C solution.

Pearlite is a two-phased, lamellar structure composed by alternating layers of cementite and α-ferrite.

The three microstructures are formed by cooling austenite.

the influence of heat treatment on final mechanical properties of a selected low alloy steel!

Hardening and tempering:

The higher tempering temperature the material becomes more ductile but the yield strength and hardness increase. This is because when quesnching the austenite becomes hard and brittle martensite. When tempering this material, the BCT-structure becomes BCC and carbide spheres (cementite) are formed. Higher tempering temperature forms bigger and less spheres which decrease the total face boundary area. Therefore the material becomes more ductile with higher tempering temperature

compere annealing and Normalisation:

The annealing leads to slower cooling which allow more diffusion to happen. More diffusion let the layers of ferrite and pearlite to be thicker. This leads to coarse pearlite which is pretty ductile, soft and not so strong. The normalization let the material to cool much faster which creates a finer pearlite because of less diffusion. The thin pearlite is harder, stronger and less ductile.

how does emount of alloying element of low alloy steels influence the mechanical properties after heat treatment?

1340 are more alloyed than 5140 by Chromium and Manganese. Therefore the Yield strength increases. Because of the alloying increasing the hardenability the material becomes martensitic much easier and there is harder, like 1340.More alloying improve strength without lowering ductility and weldability (Svetsbarhet in Swe). Plain carbon steels (low and high carbon content), increases its hardenability by alloying.

Give a definition of hardenability! What methods are used to measure hardenability?

Hardenability means how easy the austenitic becomes martensitic during cooling. If the metal allow slower cooling to form martensitic the hardenability is higher.Method to measure hardenability: The higer hardenability the nose of the TTT-diagram moves to the right and therefore the austenite creates martensitic easier. The Jominy test is a method to measure the hardenability(see askeland or PP). A piece of metal is warmed to martensitic and then quenched at one side. By doing a analyze of the piece of metal it is possible to see how the hardening had gone through it (the jominy distance). Higher austenitisation temperature will give a higher amount of alloying elements and carbon in solid solution, and therefore increase the hardenability.

Discuss how carbon content, alloying elements, grain size and austenitisation temperature will influence hardenability?

Smaller grainsize means that there are increased number of nucleation sites for ferrite and cementite (pearlite carbides), and therefore decrease the hardenability. This means: bigger grains leads to higher hardenability. Look at notes from class 3). A combination of different elements gives a stronger effect on hardenability than if on element increases its content. Lower carbon content increases the hardenability. The martensitic becomes softer because of lower carbon content and less carbide particles. Högre austenitiseringstemp gör att man löser upp kolet från karbidartiklarna vilket ökar martensitens härbarhet när man sedan kyler snabbt eftersom det då finns mer fritt kol som kan sätta sig i BCT-strukturen. Högre kolhalt och mer legeringsämnen förflyttar ttt-diagrammet mer åt höger vilket gör att härdbarheten ökar. Pga kolet sänks starttemperaturen för martensitiering men detta har liten inverkan jämfört med att ttt flyttas åt höger. Större korn leder till att det finns färre ställen som pearlite kan börja skapas på, därför ökar även större korn härdbarheten.

Discuss how carbon content, alloying elements, and austenitisation temperature will influence the hardness of the obtained martensite and the amount of retained austenite after quenching

With higher carbon content the hardenability decreases which means the cooling must be faster. The faster the cooling is, the more retained Austenite is created.Higher carbon and alloy content in solid solution in austenite leads to higher amount of retained austenite after quenching.Higher carbon content, and more alloys leads to more cementite and nitride particles, which makes the martensitic harder.Higher austenitation temperature may let diffusion to create bigger grains. Bigger grains increase the hardenability as mentioned in the question before.The carbon content have biggest influence of how hard the martensite become. With more alloying elements the alloying will create carbides nitrides with the carbon, therefore less carbon are able to build the hard BCT structure(martensite) which means that the martensite will be less hard with increased amount of alloying elements. Higher austenitisation temperature will unbrace the carbon from the carbide and nitride and therefore make more carbon able to create the BCT-structure. Which means higher temperature leads to harder martensite. Note that the martensite is discussed, not the material over all.

Describe the four tempering stages when tempering martensite at different tempering temperatures in carbon steels.

Stage 1: 80 – 250 degrees celsius. Precipitation of -carbides (Fe2.4C) as narrow laths or rodlets. The carbon content of martensite is reduced to about 0.25% - partial loss of tetragonality.

Stage 2: 200 – 300 degrees Celsius. – Reintained austenite decompose into bainite ferrite and cementite.

Stage 3: 250– 350 degrees Celsius. – Replacement of -carbides with cementite (Fe3C). Martensite losen tetragonality (becomes ferrite).

Stage 4: Above 300 degrees Celsius. Cementite coarsens and spheridisen.

What is the difference between primary and secondary carbides?

Carbides are hard particles that are wear resistant, but at the same time brittle and difficult to grind (Slipa in Swedish).

Primary carbides – Big particles. These are formed during the primary production stage and are large, being up to 40 microns in diameter. They are very stable, which means that they do not dissolve into the matrix during heat treatment.

Secondary carbides – Smaller particles. The secondary carbides structure is formed during hot rolling/forging and annealing of the steel. These carbides are small, the average size is about 0.5 microns in diameter. The small carbides contribute to good wear resistance, but without compromising sharpness and regrindability

What is temper embrittlement? When can it be observed and what are mechanisms causing the dramatic decrease in fracture toughness?

The impact toughness that you get depends on what temperature. The more carbon the alloyed steel contains the lower impact toughness the material gets. At the same time the material is less sensitive for different temper temperatures.With less carbon content (the left diagram), for example 0.2% the impact toughness will vary more depending on what temperature that is used.The temper

Irreversible temper embrittlement: 200- 4000c, carbon and alloy steels. It is belived to be connected to the decomposition of retained austenite into bainite. Can only reach the lower toughness once (during the first tempering). Can not be obtained by first tempering, then cooling and later on tempering to 200 – 400o.

Reversible temper embrittlement: At 450-650oc. Alloy steels with impurities such as P, Sb, Sn. Cosegregation of alloying elements and impurities former along austenite grain boundaries. Weak grain boundaries leads to intercrystalline fractures.