Physical Metallurgy19 th Lecture

MS&E 410

D.Ast

dast@ccmr.cornell.edu

255 4140

Content

• Handout

• Lecture

• Auxiliary Material on Steel

http://www.tf.uni-kiel.de/matwis/amat/mw1_ge/index.html

(Contains a highly entertaining subsection on how metallurgy developed from the need to make a better swords)

Comments:

Iron age started ~ 1200 BC in the Near East, ~ 800 BC in Central Europe, ~ 600 BC in Britain.

Austenite

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 Katana

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.

Cast Iron

Si added

Easiest to make, lowest T

The surface film comes from the Si, that loves to make SiO2

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

End of Handout

A brief history of steel

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

http://www.tf.uni-kiel.de/matwis/amat/mw1_ge/index.html

Lecture

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

Yes

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

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.

ledeburite

HW 19-1

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.

Computed Fe-C phase diagram (Thermo-Cal) 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

Upper Bainite Bainite Pearlite

Temperature vs time (sec, then min)

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,

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 -2

or

1/v2

The phase separation requires diffusion over distance 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

= 2 D.K1(1/v)

where k1 is a constant

The spacing is inversely proportional to the driving force, G, which is proportional to T.

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

HW 19.2

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.

Some remarks about Martensite

• Martensite is “hard” because it contains and introduces lots of dislocations

• Annealing Martensite, called tempering, allows a “controlled way up” (in T) to interesting microstructures.

• When the C in Martensite changes during tempering into more stable forms of C (cementite, graphite), alloy elements such as Mo can react with C and make secondary, finely dispersed, carbides called alloy carbides

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

Cast Iron

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

Selection of cast iron

• Meeting specs

• Price

•Processing costs

Gray cast iron shrinks much less than malleable, hence has less feeder loss.

Malleable cast iron has excellent machinability

The End

Practical metallurgy : heat treatment for 440C SS for knifes

A high hardness level, a fine array of uniformly distributed primary alloy carbides, and an adequate matrix chromium content are the three most desired properties required to produce a knife with optimum properties. Ideally, a martensitic stainless steel grade for knife applications should, therefore, satisfy the following two fundamental requirements:

• The carbon content of the austenitic matrix has to be around 0.6 wt. pct. or higher in order to achieve the hardness of 63-64 HRC.• The chromium content of the austenitic matrix has to be at least 12 wt. pct. in order to ensure corrosion resistance. can be replaced with molybdenum with little or no negative consequences for corrosion resistance.)

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.

The calphad ternary Fe-Cr-C phase diagram, at 1000oCAISI 440C is a common martensitic stainless steel with 17% Cr and 1.075% C

The Carbon can be dissolved into or precipitate out as carbide particles (M23C6

- high Cr, or M7C3 - low Cr)

The austenite contains 11.7% Cr and 0.3wt% C. The carbide contains 30 at% Carbon

Inspection of this calculated phase diagram shows

• AISI 440C at 1000°C (1832°F) consists of austenite and M7C3

primary carbides. The austenite contains 11.7 wt%Cr and 0.3% C. • Upon quenching the martensite formed from the austenite will contain chromium-rich M7C3 primary carbides dispersed within it.

• Since the martensite has the chemical composition of the Austenite from which it forms, it will have 0.3% C and 11.7% Cr

• The martensite will not meet the specs for either hardness nor corrosion resistance.

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