Physical Metallurgy 19 th Lecture

1. Physical Metallurgy19 th Lecture MS&E 410 D.Ast dast@ccmr.cornell.edu 255 4140

2. 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)

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

5. 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

6. 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.

7. Si added Cast Iron

8. Easiest to make, lowest T

9. 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

11. End of Handout

12. 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

23. Lecture

24. 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

26. 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 !

27. 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.

28. ledeburite

29. 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.

30. 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)

32. Upper Bainite Bainite Pearlite

33. 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)

34. 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.

35. The pearlite spacing goes as v a l-2 or l a 1/v2

36. 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

37. 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

38. 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

39. 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”

40. 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.

41. 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

42. What happens if you hold a quenched steel with a high fraction of martensite at 500oC for periods between 1 sec and 30 years ?

43. 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

44. Cast Iron We dealt with its forms (white, gray, mallable, ductile) previously, but repetition is the basis of building knowledge

46. 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

48. ASTM grad specification based on mechanical properties (as opposed for example, corrosion resistance)

49. Note that the addition of Si makes for good corrosion resistance. We discussed this before, it forms SiO2 at the surface

50. 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