Physical Metallurgy 20 th Lecture

1. Physical Metallurgy20 th Lecture MS&E 410 D.Ast dast@ccmr.cornell.edu 255 4140

2. It is impossible to discuss all the various forms of steels and iron in Professor Bakers handout so I will concentrate on 3 examples a) low carbon steel. The most used one. It is cheap and works well - except that the yield strength is 30 KSI unless you want to do fancy heat treatments which cost too much to be worth the effort. b) Duplex stainless steel. It is expensive but beloved by the petroleum and chemical industry. You will see why c) Tool steels. These are very sophisticated steels, and highly specific. But the principle for all the same - stable carbides.

3. Classifications • Steels in the US (but not in other countries) go by AISI (American Institute for Steel and Iron) classification • Carbon steels and low alloy steels are designated by a four digit number, • first two digits indicate the alloying elements • last two digits indicate the amount of carbon, in hundredths of a percent by weight. • The UNS system is a letter plus 5 digits and not popular in the US, we will get to it later. • Tables exists that indicate what classification in one system (say ASTM) is closest to that in other (say SAE) but they are not exact. And yes, the military has MIL specs !

5. 18/8 means 18wt% Ni, 12wt% Cr. Ni expands the g field More AISI classifications Cheapest grade used in US cars. Cr provides corrosion resistance, absence of Ni makes it ferritic See App, Lec 19

6. In the ferritc SS you can tolerate 0.2% C as long as you have higher Cr as Cr binds C. Martensitic SS have higher C content to force transition to martensite

7. The austenitic SS all contain Ni (or Mn which is cheaper). The fcc phase has 12 glide systems, thus this SS are ductile. Also, do not embrittle at low T S is a controlled impurity as it sensitizes SS

8. Example 1: Low Carbon steel

9. Low Carbon Steel • Typical example 1020 • 1020 steels have all the same composition • Yet 1020 steels have different specs (and prices) - because mechanical properties and composition are not 1:1 linked (slags rolled in, surface finish, defect etc etc) P embrittles grain boundaries, and S was the downfall of the Titanic

10. The military, the ASTM, ASM, SAE, even USDA, they all have their own specifications. As have Swedes, Germans... UNS is Unified Number System. The letter G indicates Carbon Steels, the 5 digits its composition. Not widely adopted yet :-( The ASM used to be the American Society for Metals but now is the Materials Information Society… .

11. Applications of 1020 Steel Good steel for general purposes. Case hardening is packing the part into C and lead the C diffuse in. Heat and time required => both cost money. No matter how you hard, the piece will distort. After hardening, the only way to machine is grinding - an very expensive process compared to precision turning on a lathe

12. Heat Treatments HW question next page… may come back at final.

13. HW 20-1 What happens during tempering ? Why does the yield strength increase ?

14. More 1020 Data

15. Clarifying the Baker Table“Rephosporized” steel

16. Glossary Resulferized steel. Steel to which controlled amounts of S have been added to improve machinability Remember: Machining is to drive a crack in front of the cutting tool. It is not cutting as in cutting meat ! Rephosphorized : P is a very effective solute solution strengthening addition to Fe, but unfortunately, if not heat treated properly tends to wind up in GB. Rephosporized steel have low C contents. Hence a low yield stress. Given their high UTS they have good “deep draw ability” Mostly used in automotive side panels, hoods etc, as they are highly dent resistant once properly strained ( if designed to have about equal strain everywhere when stamped )

17. This is from a Japanese paper that I put on our website. The point is that you need to cool quickly to prevent the P from moving to grain boundaries. The steel is an ultra-low Carbon steel specifically developed for deep drawing automotive panels

18. Example Stainless Steels a) Duplex b) Stress Driven Martensitic transformation

19. A mixture of ferritic and austenitic SS. UNS 31803 Duplex (dual phase) stainless steel

20. Composition Note : Higher Cr than 18/8 Note : Lower Ni than 18/8 Note : Mo Note : NO (!) C, low S, P, N is added as an anti-pitting agent.

24. So what so good about it ?

27. Stress induced martensite in austenitic SS at -44 C The stress induced transition is the trick of memory shape alloys

28. On to an other SS in Professor Bakers table: 440C A ferritic SS with high C ideal for knives.

29. 440 C SS products If you like to cook, you want to have on of those !

30. Professional scissors for hairdressers

31. The inner and outer bearing cages in your dentists drill

32. Role of alloying elements, table and comments

33. Alloying Elements Read Bain’s Book free on line (see lecture 19)

34. Si shares a chemical similarity with C. In formulas that lump the influence of C and Si together, Si is counted 1/3 (I.e. C is three times as effective). It is major component in transformer steel P increases corrosion resistance. The famous iron pillar in Deli !

35. Cr forms Cr oxide at the surface but needs to be there at 12 wt%. Secondary hardening of Mo in steel is the formation of Mo carbide. Once formed very stable

36. Vanadium forms a very hard and stable carbide. As does Mo, W, and Co

37. VC precipitates in tempered martensitic Fe-C-V steel VC as an ultra hard surface film is used in some on some tools

38. Electrical Silicon Steel (A funny word!) Transformer steel contains up to 6% Si. The one the left has 3%. The main purpose of the silicon is to increase the electrical resistivity thus lowering eddy current losses. (Alloy scattering, Si has a lot charge !) The secondary purpose is to form a SiO2 layer on the surface that isolates on transformer sheet from the other.

39. Tool steels - interesting beasts…. 440C is a tool steel in the sense that it used for knifes, shears etc. But usually, tool steels implies high hardness and wear resistance at elevated temperatures. The higher the redhardness, the higher the feedrate. Time is money

40. We will only discuss Mo/Co and W/Co tool steels

44. Co is good for high redhardness. Total wt% of T.M. is around 25%

45. M stands for Moly, T for Tungsten (AISI classification)

46. The W,V, Cr can all be lumped together as W with weight factors, and so lets look at Fe-C-W The g field shrunk by a factor 5 !!!!!! Thus, the max solubility of C in austenite is now 0.4% ! This is lower the the C in the tool steels so the C must be in the form of transition metal carbides - which is what we want - if we can get the carbides into spherical shape so that the toughness is ok. The Co3C is very stable Wear: C can “burn away” at the surface. The Fe-C-W25 wt% systwm

47. Steel production and Consumption

48. In 2005 (From Prof Bakers Handout)

49. In 2006/2007 Oh, oh.. US is shrinking ! Not good !

50. The top steel companies in the world Largest company Mittal