MTLS 4L04 Steel Section. Lecture 6

Transcription

1 MTLS 4L04 Steel Section Lecture 6

2 Tempering of Martensite To get around the problem of the brittleness of the Martensite, Martensite is heat treated at elevated temperatures ( C) to precipitate some of the carbon. This is referred to as Tempering, but it is a precipitation reaction (just like in Al-4Cu or Mg-9Al). Martensite contains a high number of dislocations and interfaces, there are many heterogeneous nucleation sites for carbides (usually Fe 3 C). Electron Micrograph of Tempered martensite. Tempering was carried out at 594 C (1100 F) The small particles are cementite; the black is α-ferrite Resolution =9300 x

3 Soft Zone in Nd:YAG Laser Weld of DP980

4 Microstructure Evolution with Tempering Temperature Highly supersaturated structure (i.e. martensite) Free Energy segregation of C to the lattice defects ( C) Formation of transient carbides (80-250C) Decomposition of the retained austenite Transition to rod-shaped cementite ( C) Segregation of alloying elements and impurities cementite spherodization Formation of alloy carbides ( C) Temperature Recovery and recrystallization Ferrite + stable carbide (Cementite or alloy carbide)

5 Basic Phenomena Occurring during Tempering Highly supersaturated structure High tendency for more stable atomic arrangements (lower energy) Redistribution of the carbon and segregation to the lattice defects such as dislocations. Precipitation of the transient carbides such as epsilon (ε) carbide and eta (η) carbide (more important in high carbon steels). -> occurs between C, maximum precipitation at ~150C. Decomposition of the retained austenite into martensite, lower bainite, or ferrite and cementite (usually happens in high carbon steels). -> between C, C. Conversion of the transient carbides epsilon (ε) into the rod-shaped cementite. -> between 260C- 360C. Segregation of alloying elements and impurities (can lead to the temper embrittlement) Spherodization of the rod-shaped cementite (reduction of the surface energy) Formation of alloy carbides (in alloy steels containing Ti, Cr, Mo, V, Nb and etc.) -> above 400C Recovery and recrystallization. Further coarsening of the microstructure (growth of cementite and other carbides)->above 450C

6 Effect of Tempering on Mechanical Properties

7 Effect of Tempering on Mechanical Properties of Low Alloy Steel

8 Effect of Tempering on Mechanical Properties of High Carbon Steel Note: this precipitation decreases the strength of the alloy (i.e. precipitation softening) but gives the alloy some ductility and toughness. A compromise is found where toughness and ductility are OK but the strength has not decreased too much. 70 Rockwell C hardness Time (min)

9 Effect of Tempering on Mechanical Properties Tempering at low T, (a) Driving force for nucleation of Fe 3 C is very large (b) Gives fine dispersion of Fe 3 C (c) Yield strength remains high, ~1000MPa Tempering at high T, (a) Carbide distribution becomes coarser (b) Strength decreases (c) Elongation/ductility increases The tempering conditions need to be chosen carefully for one s exact required combination of properties. Courtesy of C. Hutchinson, Monash University

10 Effect of Alloying Elements on Tempering Nowadays, quenched and tempered steels often contain significant alloy additions: Nb, Ti, V, W, Cr, Mo, etc. These elements are added in order to precipitate alloy carbides during tempering: Cr 23 C 6, WC, VC, NbC, TiC. Alloy carbides are very strong intermetallic particles with high melting points. Elevated temperature stability of tempered martensite containing alloy additions is significantly improved Why? In any structure containing a fine distribution of particles, the coarsening of the particle distribution results in a degradation in mechanical properties. For Fe-C systems, coarsening of the Fe 3 C requires only diffusion of C which occurs very quickly because it diffuses interstitially. If the carbide particles are instead alloy carbides (WC, VC, etc.), coarsening is controlled by diffusion of the W, V, etc. This occurs substitutionally and is ~10 6 time slower than C. Courtesy of C. Hutchinson, Monash University

11 Microstructure of Tempered DP Steel DP steel isothermally tempered at 650 C for 1.5Hrs: SEM (a) decomposition of the prior M phase, (b) precipitated carbides predominantly located at the grain boundaries, and TEM (c) carbide precipitation within the tempered structure and (d) recovery of the lath boundaries. SAD pattern in (d) indicates bcc structure (a) of tempered matrix. V.H. Baltazar Hernandez, et.al, Metall. Mater. Trans. A, 42(10), 2011,

12 Tempered HAZ in DP Steel Non-isothermally tempered Martensite in DP(Medium CE) Steel Schematic of tempered Martemsite evolution V.H. Baltazar Hernandez, et.al, Metall. Mater. Trans. A, 42(10), 2011,

13 Cementite Growth vs. Martensite Softening V.H. Baltazar Hernandez et. al, Metall. Mater. Trans. A, 42(10), 2011,

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15 Bainite in Steels B S Edgar Bain ( ) Worked for US Steel in Pittsburgh B f

16 Schematic of Bainite Formation

17 Microstructure of Upper and Lower Bainite Bainite High T Low T

18 Upper Bainite under TEM Fe 3 C α Laths or plates of ferrite nucleate and grow from the austenite grain boundaries. The laths are very thin (much thinner than lamella in pearlite) Bainite in Steels - 2nd Edition, H. K. D. H. Bhadeshia,Published by the Institute of Materials,March 2001

19 Shape change in the adjacent austenite during formation of Bainite plates Bainite in Steels - 2nd Edition, H. K. D. H. Bhadeshia,Published by the Institute of Materials,March 2001

20 Mechanical Properties of Bainite Bainite is very much intermediate between (α+pearlite) steels and martensitic steels Mixed microstructure (pearlite + Bainite)-.. Hardness drops... why?

21 MechanicalProperties ofbainite Properties improve as the microstructure is refined (just like decreasing ferrite grain size)

22 Commercial Bainitic Steels These took a long time to be used industrially. Why? TTT Curve for Fe-0.8C-0.8Mn (wt. %) TTT Curve: Low C steel

23 Commercial Bainitic Steels-contd Required new scientific knowledge! How do we shift the top of the TTT curve (ferrite and pearlite) but not the bottom (bainite) to achieve 100% bainitic steels on cooling. Method: Careful control of alloying element effects allowed this to be achieved: Especially additions of B and Mo

24 Commercial Bainitic Steels

25 Current industrial use of Bainite Today, bainite is mostly used as a constituent in multi-phase advanced steels e.g. high strength-high elongation steels (HSLA steels) containing: ferrite, pearlite, bainite, martensite, retainined austenite TRIP steels have some carbide-free bainite. Dual-phase steels where the majority of the structure is carbide-free bainite

26 [113] Garcia-Mateo, C., Caballero, F. G. and Bhadeshia, H. K. D. H. (2003). ISIJ Int., 43 Carbide-Free Bainite Si is added to the steel to supress the formation of carbide TEM image of Figure 18 TEM micrograph of Fe- 0.98C- 1.46Si- 1.89Mn- 0.26Mo Cr- 0.09V (wt%), transformed at 200 oc for 15 days. The dark phase represents the austenite whereas the white phase is the bainitic ferrite