Engineering Materials vol 2 Part 6 pot

Asa point of detail, when pearlite is cooled to room temperature, the concentration of carbon in the a decreases slightly, following the a/a + Fe3C boundary.. Hypoeutectoid means that th

Fig 11.3. Microstructures during the slow cooling of a eutectoid steel from the hot working temperature As

a point of detail, when pearlite is cooled to room temperature, the concentration of carbon in the a decreases slightly, following the a/a + Fe3C boundary The excess carbon reacts with iron at the a–Fe3 C interfaces to form more Fe 3 C This “plates out” on the surfaces of the existing Fe 3 C plates which become very slightly thicker The composition of Fe 3 C is independent of temperature, of course.

Fig 11.4. Microstructures during the slow cooling of a hypoeutectoid steel from the hot working temperature.

A 3 is the standard labelling for the temperature at which a first appears, and A1 is standard for the eutectoid temperature Hypoeutectoid means that the carbon content is below that of a eutectoid steel (in the same sense that hypodermic means “under the skin”!).

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Fig 11.5. Microstructures during the slow cooling of a hypereutectoid steel A cm is the standard labelling for the temperature at which Fe 3 C first appears Hypereutectoid means that the carbon content is above that of a eutectoid steel (in the sense that a hyperactive child has an above-normal activity!).

Fig 11.6 Room temperature microstructures in slowly cooled steels of different carbon contents (a) The

proportions by weight of the different phases (b) The proportions by weight of the different structures.

to A1 At A1 the remaining γ (which is now of eutectoid composition) transforms topearlite as usual The room temperature microstructure is then made up of primary α

+ pearlite If the steel contains more than 0.80% C (a hypereutectoid steel) then we get a

room-temperature microstructure of primary Fe3C plus pearlite instead (Fig 11.5).These structural differences are summarised in Fig 11.6

Mechanical properties of normalised carbon steels

Figure 11.7 shows how the mechanical properties of normalised carbon steels changewith carbon content Both the yield strength and tensile strength increase linearly withcarbon content This is what we would expect: the Fe3C acts as a strengthening phase,and the proportion of Fe3C in the steel is linear in carbon concentration (Fig 11.6a).The ductility, on the other hand, falls rapidly as the carbon content goes up (Fig 11.7)because the α –Fe3C interfaces in pearlite are good at nucleating cracks

Fig 11.7. Mechanical properties of normalised carbon steels.

Quenched and tempered carbon steels

We saw in Chapter 8 that, if we cool eutectoid γ to 500°C at about 200°C s−1, we willmiss the nose of the C-curve If we continue to cool below 280°C the unstable γ willbegin to transform to martensite At 220°C half the γ will have transformed to martensite.And at –50°C the steel will have become completely martensitic Hypoeutectoid andhypereutectoid steels can be quenched to give martensite in exactly the same way(although, as Fig 11.8 shows, their C-curves are slightly different)

Figure 11.9 shows that the hardness of martensite increases rapidly with carboncontent This, again, is what we would expect We saw in Chapter 8 that martensite is

a supersaturated solid solution of C in Fe Pure iron at room temperature would beb.c.c., but the supersaturated carbon distorts the lattice, making it tetragonal

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Fig 11.8 TTT diagrams for (a) eutectoid, (b) hypoeutectoid and (c) hypereutectoid steels (b) and (c) show

(dashed lines) the C-curves for the formation of primary a and Fe3 C respectively Note that, as the carbon content increases, both M S and M F decrease.

Fig 11.9. The hardness of martensite increases with carbon content because of the increasing distortion of the lattice.

Fig 11.10. Changes during the tempering of martensite There is a large driving force trying to make the

martensite transform to the equilibrium phases of a + Fe3 C Increasing the temperature gives the atoms more thermal energy, allowing the transformation to take place.

(Fig 11.9) The distortion increases linearly with the amount of dissolved carbon(Fig 11.9); and because the distortion is what gives martensite its hardness then this,too, must increase with carbon content

Although 0.8% carbon martensite is very hard, it is also very brittle You can quench

a 3 mm rod of tool steel into cold water and then snap it like a carrot But if you temper

martensite (reheat it to 300–600°C) you can regain the lost toughness with only amoderate sacrifice in hardness Tempering gives the carbon atoms enough thermalenergy that they can diffuse out of supersaturated solution and react with iron to formsmall closely spaced precipitates of Fe3C (Fig 11.10) The lattice relaxes back to theundistorted b.c.c structure of equilibrium α, and the ductility goes up as a result The

FeC particles precipitation-harden the steel and keep the hardness up If the steel is

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over-tempered, however, the Fe3C particles coarsen (they get larger and further apart)

and the hardness falls Figure 11.11 shows the big improvements in yield and tensilestrength that can be obtained by quenching and tempering steels in this way

Cast irons

Alloys of iron containing more than 1.7 wt% carbon are called cast irons Carbon

lowers the melting point of iron (see Fig 11.1): a medium-carbon steel must be heated

to about 1500°C to melt it, whereas a 4% cast iron is molten at only 1160°C This is whycast iron is called cast iron: it can be melted with primitive furnaces and can be castinto intricate shapes using very basic sand casting technology Cast iron castings havebeen made for hundreds of years.* The Victorians used cast iron for everything theycould: bridges, architectural beams and columns, steam-engine cylinders, lathe beds,even garden furniture But most cast irons are brittle and should not be used wherethey are subjected to shock loading or high tensile stresses When strong castings areneeded, steel can be used instead But it is only within the last 100 years that steelcastings have come into use; and even now they are much more expensive than castiron

There are two basic types of cast iron: white, and grey The phases in white iron are

α and Fe3C, and it is the large volume fraction of Fe3C that makes the metal brittle Thename comes from the silvery appearance of the fracture surface, due to light beingreflected from cleavage planes in the Fe3C In grey iron much of the carbon separates

Fig 11.11. Mechanical properties of quenched-and-tempered steels Compare with Fig 11.7.

* The world’s first iron bridge was put up in 1779 by the Quaker ironmaster Abraham Darby III Spanning the River Severn in Shropshire the bridge is still there; the local village is now called Ironbridge Another early ironmaster, the eccentric and ruthless “iron-mad” Wilkinson, lies buried in an iron coffin surmounted

by an iron obelisk He launched the world’s first iron ship and invented the machine for boring the cylinders

of James Watt’s steam engines.

out as elemental carbon (graphite) rather than Fe3C Grey irons contain ≈2 wt% Si: thisalters the thermodynamics of the system and makes iron–graphite more stable thaniron–Fe3C If you cut a piece of grey iron with a hacksaw the graphite in the sawdustwill turn your fingers black, and the cut surface will look dark as well, giving grey ironits name It is the graphite that gives grey irons their excellent wear properties – in factgrey iron is the only metal which does not “scuff” or “pick up” when it runs on itself.The properties of grey iron depend strongly on the shape of the graphite phase If it is

in the form of large flakes, the toughness is low because the flakes are planes ofweakness If it is in the form of spheres (spheroidal-graphite, or “SG”, iron) the tough-ness is high and the iron is surprisingly ductile The graphite in grey iron is normallyflaky, but SG irons can be produced if cerium or magnesium is added Finally, somegrey irons can be hardened by quenching and tempering in just the way that carbonsteels can The sliding surfaces of high-quality machine tools (lathes, milling machines,etc.) are usually hardened in this way, but in order to avoid distortion and cracking only

the surface of the iron is heated to red heat (in a process called “induction hardening”).

Some notes on the TTT diagram

The C-curves of TTT diagrams are determined by quenching a specimen to a giventemperature, holding it there for a given time, and quenching to room temperature(Fig 11.12) The specimen is then sectioned, polished and examined in the microscope.The percentage of Fe3C present in the sectioned specimen allows one to find out howfar the γ → α + Fe3C transformation has gone (Fig 11.12) The complete set of C-curves

Fig 11.12. C-curves are determined using quench–hold–quench sequences.

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can be built up by doing a large number of experiments at different temperatures andfor different times In order to get fast enough quenches, thin specimens are quenchedinto baths of molten salt kept at the various hold temperatures A quicker alternative

to quenching and sectioning is to follow the progress of the transformation with ahigh-resolution dilatometer: both α and Fe3C are less dense than γ and the extent of theexpansion observed after a given holding time tells us how far the transformation hasgone

When the steel transforms at a high temperature, with little undercooling, the pearlite

in the steel is coarse – the plates in any nodule are relatively large and widely spaced

At slightly lower temperatures we get fine pearlite Below the nose of the C-curve thetransformation is too fast for the Fe3C to grow in nice, tidy plates It grows instead asisolated stringers to give a structure called “upper bainite” (Fig 11.12) At still lowertemperatures the Fe3C grows as tiny rods and there is evidence that the α forms by adisplacive transformation (“lower bainite”) The decreasing scale of the microstructurewith increasing driving force (coarse pearlite → fine pearlite → upper bainite → lower

bainite in Fig 11.12) is an example of the general rule that, the harder you drive a transformation, the finer the structure you get.

Because C-curves are determined by quench–hold–quench sequences they can, strictlyspeaking, only be used to predict the microstructures that would be produced in a steelsubjected to a quench–hold–quench heat treatment But the curves do give a pretty good

indication of the structures to expect in a steel that has been cooled continuously For really accurate predictions, however, continuous cooling diagrams are available (see the literature of

the major steel manufacturers)

The final note is that pearlite and bainite only form from undercooled γ They never form from martensite The TTT diagram cannot therefore be used to tell us anything

about the rate of tempering in martensite

R Fifield, “Bedlam comes alive again”, in New Scientist, 29 March 1973, pp 722–725 Article on

the archaeology of the historic industrial complex at Ironbridge, U.K.

D T Llewellyn, Steels – Metallurgy and Applications, 2nd edition, Butterworth-Heinemann, 1994.

11.2 You have been given samples of the following materials:

(a) Pure iron

11.3 The densities of pure iron and iron carbide at room temperature are 7.87 and8.15 Mg m−3 respectively Calculate the percentage by volume of a and Fe3C inpearlite

Answers: α, 88.9%; Fe3C, 11.1%

··

700 600 500 400 300 200 100 0 –100

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low-(a) to improve the hardenability of the steel;

(b) to give solution strengthening and precipitation hardening;

(c) to give corrosion resistance;

(d) to stabilise austenite, giving a steel that is austenitic (f.c.c.) at room temperature.

Hardenability

We saw in the last chapter that carbon steels could be strengthened by quenching and tempering To get the best properties we must quench the steel past the nose of the C- curve The cooling rate that just misses the nose is called the critical cooling rate (CCR).

If we cool at the critical rate, or faster, the steel will transform to 100% martensite.* TheCCR for a plain carbon steel depends on two factors – carbon content and grain size

We have already seen (in Chapter 8) that adding carbon decreases the rate of thediffusive transformation by orders of magnitude: the CCR decreases from ≈105°C s−1for pure iron to ≈200°C s−1 for 0.8% carbon steel (see Fig 12.1) We also saw in Chap-ter 8 that the rate of a diffusive transformation was proportional to the number ofnuclei forming per m3 per second Since grain boundaries are favourite nucleationsites, a fine-grained steel should produce more nuclei than a coarse-grained one Thefine-grained steel will therefore transform more rapidly than the coarse-grained steel,and will have a higher CCR (Fig 12.1)

Quenching and tempering is usually limited to steels containing more than about0.1% carbon Figure 12.1 shows that these must be cooled at rates ranging from 100 to2000°C s−1 if 100% martensite is to be produced There is no difficulty in transforming the

surface of a component to martensite – we simply quench the red-hot steel into a bath

of cold water or oil But if the component is at all large, the surface layers will tend toinsulate the bulk of the component from the quenching fluid The bulk will cool moreslowly than the CCR and will not harden properly Worse, a rapid quench can createshrinkage stresses which are quite capable of cracking brittle, untempered martensite.These problems are overcome by alloying The entire TTT curve is shifted to theright by adding a small percentage of the right alloying element to the steel – usually

* Provided, of course, that we continue to cool the steel down to the martensite finish temperature.

Fig 12.1. The effect of carbon content and grain size on the critical cooling rate.

Fig 12.2. Alloying elements make steels more hardenable.

molybdenum (Mo), manganese (Mn), chromium (Cr) or nickel (Ni) (Fig 12.2)

Numer-ous low-alloy steels have been developed with superior hardenability – the ability to

form martensite in thick sections when quenched This is one of the reasons for addingthe 2–7% of alloying elements (together with 0.2–0.6% C) to steels used for things likecrankshafts, high-tensile bolts, springs, connecting rods, and spanners Alloys withlower alloy contents give martensite when quenched into oil (a moderately rapidquench); the more heavily alloyed give martensite even when cooled in air Havingformed martensite, the component is tempered to give the desired combination ofstrength and toughness

Hardenability is so important that a simple test is essential to measure it The Jominyend-quench test, though inelegant from a scientific standpoint, fills this need A bar

100 mm long and 25.4 mm in diameter is heated and held in the austenite field Whenall the alloying elements have gone into solution, a jet of water is sprayed onto oneend of the bar (Fig 12.3) The surface cools very rapidly, but sections of the bar behind

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Fig 12.3. The Jominy end-quench test for hardenability.

Fig 12.4. Jominy test on a steel of high hardenability.

the quenched surface cool progressively more slowly (Fig 12.3) When the whole bar

is cold, the hardness is measured along its length A steel of high hardenability willshow a uniform, high hardness along the whole length of the bar (Fig 12.4) This isbecause the cooling rate, even at the far end of the bar, is greater than the CCR; and thewhole bar transforms to martensite A steel of medium hardenability gives quite dif-ferent results (Fig 12.5) The CCR is much higher, and is only exceeded in the first fewcentimetres of the bar Once the cooling rate falls below the CCR the steel starts totransform to bainite rather than martensite, and the hardness drops off rapidly