Physical Metallurgy 19 th Lecture. MS&E 410 D.Ast [email protected] 255 4140. Content Handout Lecture Auxiliary Material on Steel http://www.tf.uni-kiel.de/matwis/amat/mw1_ge/index.html
Physical Metallurgy19 th Lecture
Iron age started ~ 1200 BC in the Near East, ~ 800 BC in Central Europe, ~ 600 BC in Britain.
Its phase field can be increased by adding Ni => nonmagnetic stainless steels.
Austenite comes from Sir William Chandler Roberts-Austen, a British metallurgist (1843–1902) who published the first phase diagram in 1897
Bainite Memory assist
Bainite interior, martensite surface, Tough and springy inside, very hard outside.
Bainite comes from E. Bain (Lect 18), US Steel Co, scientists.
Martensite comes from Adolf Marten 1850-1914, the director of the royal mechanical laboratory in Berlin.
Easiest to make, lowest T
Si in cast iron competes with C. Adding Si reduces the formation of cementite. Promotes tendency for C to come out of solution as as graphite
The surface film comes from the Si, that loves to make SiO2
End of Handout
It is nearly impossible to teach metallurgy without a historical context, so here is a brief history of steel from the website of a former postdoc of mine H.Foell
Ferrum (Lat) => Iron
Most countries have de facto legal specification* for ferrous alloys. I.e. 1020 Steel must have certain properties as defined by ASTM or SAE to be used in products. Otherwise you will be legally liable.
Each country has its own regulation..
* The specification is application dependent. The same steel may be speced by composition for chemical applications, and by mechanical properties of other. So there is considerable overlap
This is a famous case of a phase diagram, that strictly speaking is not an equilibrium diagram.
Which phase diagrams are supposed to be.
If it comes to graphite precipitation, just extend the right border all the way to the carbon end !
Ledeburite is the equivalent to pearlite, but unlike pearlite which form from a eutectiod, Ledeburite is formed, like the classical Pb-Sn eutectic structure, fro a liquid. It was discovered in 1882
It is named after the metallurgist Karl Heinrich Adolf Ledebur (1837-1916), professor of metallurgy at the Bergakademie Freiberg. He discovered ledeburite in 1882.
Ledeburite forms when the carbon content is between 2.06% and 6.67%. The eutectic mixture is 4.3% carbon, Fe3C:2Fe. Its melting point is 1147°C. At 4.3% carbon the metal becomes 100% ledeburite. Ledeburite is a phase mixture, of austenite and cementite (which is a catch all for carbites - there are different forms of cementitite.
1. You cool a 1% C steel to room temperature at a moderate rate (I.e no martensite forms).
A) What phases are present
B) What is the weight fraction of each phase
Note: Treat pearlite as single phase. Metallurgist do this all the time, even though it is a mixture of two phases, ferrite and cementite.
Look how well if fits
In the Appendix I put 4 slides that show how useful Calphad diagrams are in practical metallurgy.
The example is Stainless Steel for knifes, using Fe-Cr-C
Computed Fe-C phase diagram (Thermo-Cal)
Upper Bainite Bainite Pearlite
TTT diagram for isothermal transformation of steel W 1 (1% C)
A = austenite,
B = bainite,
P = pearlite
Ms = start of martensite transformation,
M50 = 50% M,
Temperature vs time (sec, then min)
Quenching in a liquid bath at 700°C; holding time 4 min. During this interval the C has separated out, partly as pearlite lamellae and partly as spheroidized cementite. Hardness 225 HV.
Quenching to 575°C, holding time 4 s. A very fine, closely spaced pearlite as well as some bainite has formed. Note that the amount of spheroidized cementite is much less than in the preceding case. Hardness 380 HV.
Quenching to 450°C, holding time 60 s. The structure consists mainly of bainite. Hardness 410 HV.
Quenching to 20°C (room temperature). The matrix consists of, roughly, 93% martensite and 7% retained austenite. There is some 5% cementite as well which has not been included in the matrix figure. Hardness 850 HV.
The pearlite spacing goes as
v a l-2
l a 1/v2
The phase separation requires diffusion over distance l, over a time that scales inversely with v (the faster the front moves) the less time.
Hence on expects from the x = 2SQRT(Dt) rule
l = 2 D.K1(1/v)
where k1 is a constant
The spacing is inversely proportional to the driving force, DG, which is proportional to DT.
A linear relationship is typical for a dissipative process.
Diffusion is a dissipative process
The TTT diagram is a kinetic (non equilibrium) phase diagram.
Martensite begins to form at 430 C, but does not complete to 100% at RT. Thus, there is retained Austenite left. Some C may deplete out into cementite at GB
Martensite is formed by quenching the steel into water or oil.
A problem of considerable practical importance is to know how deep the martensite forms.
A) Consult the course text book on line, or Google and find the simple test performed to assess this quantity (hint: It was developed by an engineer at the Crysler Automotive Co)
B) Find one alloy element the addition of which will increase the depth of “hardening”
This is the classic diagram to design heat treatments.
However, different parts of the structure will have different heat treatment histories as the interior (no matter what you use to quench) will stay hot longer.
What happens if you hold a quenched steel with a high fraction of martensite at 500oC for periods between 1 sec and 30 years ?
Aermet is the steel used in landing gears of jets.
The strength increase is alloy carbide formation.
Tempering requires high purity steels. Otherwise P or Sn will migrate to the grain boundaries and embrittle them. Aermet contains less than 0.003% P
We dealt with its forms (white, gray, mallable, ductile) previously, but repetition is the basis of building knowledge
Grey cast iron has high damping and lubricates well. Good corrosion resistance.
Nodular Cast Iron requires addition of Mg and is ductile akin ABS polymers (ask Ober)
White cast iron is hard, brittle and can only be machined by grinding. Great for hard surfaces. Depth to which it forms in metal molds controlled by Cr additions.
Malleable cast iron is white cast iron tempered at 1700 F. The cementite decomposes into ferrite and free carbon which forms small graphite particles or pearlite during cooling
ASTM grad specification based on mechanical properties (as opposed for example, corrosion resistance)
Note that the addition of Si makes for good corrosion resistance. We discussed this before, it forms SiO2 at the surface
Practical metallurgy : heat treatment for 440C SS for knifes
Isothermal sections of the calculated Fe-Cr-C ternary phase diagram are a good starting point when it comes to understanding the various trade-offs between the austenitization temperature selected for heat treatment and the resulting chemical composition of the austenitic matrix.
The composition plane for the Fe-Cr-C ternary phase diagram at 1000°C (1832°F) is shown in the next slide.. The carbon content is plotted along the horizontal axis and the chromium content along the vertical axis of the composition plane.
AISI 440C is a common martensitic stainless steel with 17% Cr and 1.075% C
The Carbon can be dissolved into g or precipitate out as carbide particles (M23C6 - high Cr, or M7C3 - low Cr)
The calphad ternary Fe-Cr-C phase diagram, at 1000oC
The austenite contains 11.7% Cr and 0.3wt% C. The carbide contains 30 at% Carbon
Increasing the Austenite hold temperature to 1100oC
The martensite contains 12.7% Cr and 0.53% C
Now meets specs for corrosion resistance and (almost) hardness
Tests show Rockwell 58-60