MATERIALS SCIENCE AND ENGINEERING I

Transcription

1 MATERIALS SCIENCE AND ENGINEERING I LECTURE COURSE 7 THEORY OF HEAT TREATMENTS. HEAT TREATMENTS OF STEELS. TRANSFORMATIONS IN STEELS DURING HEATING. TRANSFORMATIONS IN STEELS DURING THE COOLING FROM AUSTENITE DOMAIN. TTT AND CCT DIAGRAMS.

2 THEORY OF HEAT TREATMENTS Heat treatments: technological processes - heating, maintaining at various temperatures and cooling under defined conditions improvement of certain properties by altering the structure Classification: 1. Position of HT in the manufacturing process Preliminary Intermediate Final 2. Mechanisms during the treatments simple HT Annealing, Quenching, Tempering thermochemical treatments thermophysical treatments Thermo-mechanical Thermo-magnetic

3 THEORY OF HEAT TREATMENTS -DIFFUSION- (Generally): Changing of the position of atoms / ions in solids, liquids or gases; Only for large groups of atoms (ions) Heterodiffusion determined by a concentration gradient Self-diffusion Mechanisms: Reciprocal replacing Through interstitials Cyclic Through vacancies Most of the transformations in materials - with diffusion (solidification, solid state transformations, recrystallization,...): Transformation with diffusion: requires a longer process time

4 THEORY OF HEAT TREATMENTS -DIFFUSION- Diffusion coefficient: number of atoms that diffuse through a unit surface that is perpendicular to the atoms flux during a second, for a unit concentration gradient D D 0 e Q a RT Types of diffusion: Surface Inter-granular Volume Qs < Q i < Qv Ds > Di > Dv

5 HEAT TRETMENTS APPLIED UPON STEELS Critical temperatures of steels - heating: Ac Ac 1, (Ac 2 ), Ac 3, Ac cem - cooling: Ar Ar 1, (Ar 2 ), Ar 3, Ar cem Ar 3, Ar cem : beginning of the separation of the pro-eutectoid phase (ferrite / sec. cementite) Ac 3, Ac cem : ending of the dissolving of the pro-eutectoid phase A 1 : eutectoid temperature (+ c / r = beginning / ending of dissolving / separation of the pro-eutectoid phase)

6 Transformations in steels during heating P-A transformation: for temperatures over Ac 1 (practically: C over A1)

7 Transformations in steels during heating Heredity of steels: tendency for the growth of austenite grains during heating Heredity fine (generally, alloyed steels) coarse (unalloyed steels) Grain Initial immediately after P A transformation Real Hereditary under the real practical conditions obtained under standard conditions (~930 C)

8 Transformations in steels during heating ; Heredity of steels a coarse heredity steel b fine heredity steel

9 Transformations in steels during heating ; Defects that occur during the heating of steels 1. Overheating = obtaining of a coarse structure (brittle) Overheated hypoeutectoid steels; a slowly cooled; b more rapidly cooled (Widmanstatten structure)

10 Transformations in steels during heating ; Defects that occur during the heating of steels Widmanstatten structures in hypereutectoid steels x 150 x 200 Widmanstatten structure in hypoeutectoid steel x 150

11 Transformations in steels during heating ; Defects that occur during the heating of steels 2. Oxidation = formation of oxides at the surface of parts; 3. Decarburizing = decreasing of the carbon content in the surface layer Layers: I completely decarburized (F) II partially decarburized III base material (P + F)

12 Transformations in steels during heating ; Defects that occur during the heating of steels 4. Burning = formation of coarse inter-granular oxide films as a consequence of heating close to the solidus line X 150

13 Transformations in steels during the cooling from the austenitic field Cooling: Isothermal sharp decreasing of temperature to a plateau of isothermal maintaining Continuous continuous decreasing of temperature to the room temperature with a cooling rate characteristic to the cooling environment

14 Transformations in steels during the cooling from the austenitic field Pearlitic transformation: A P For slow continuous cooling (maximum - air) or isothermal ( C) DIFFUSION Pearlitic structures: Ferrite + Cementite!!! Fineness degree increases with cooling rate = decreasing of the isothermal temperature In the order of increasing the cooling rate: Coarse lamellar pearlite (equilibrium) Sorbitic pearlite (fine) Troostite (finest pearlite)

15 Transformations in steels during the cooling from the austenitic field Martensitic transformation: A M Martensite = oversaturated solid solution Feα(C) resulted by the rapid cooling of austenite Tetragonal Hard, strong, brittle Characteristics of the transformation: Rapid (order of 10-7 s) WITHOUT DIFFUSION Chemical composition of M identical to A M is precisely oriented compared to A (habital planes) Irreversible for steels There exist alloys with reversible transformation Ex.: Shape memory alloys In a temperature interval Ms Mf (under 0 C) A is never transformed integrally in M - residual (retained)austenite

16 Transformations in steels during the cooling from the austenitic field Bainitic transformation: intermediate between pearlitic and martensitic transformations Bainite = mechanical admixture of carbon oversaturated ferrite and carbides that did nor reach the cementite stage (they do not display 3 Fe atoms for a C atom) Generally it is produced isothermally: Upper bainite at C Lower bainite at C resembling troostite resembling martensite (tempering)

17 TTT TRANSFORMATION DIAGRAM (time temperature transformation) Through isothermal cooling pearlitic structures (fineness degree increases for lower temperatures) bainitic structures Martensite can be obtained only through rapid cooling (quenching) Structures obtained through isothermal cooling of austenite

18 TTT TRANSFORMATION DIAGRAM (time temperature transformation) Analysis of transformation kinetics- through kinetic curves of transformation percent of transformed structure = f (time) for a certain temperature of isothermal cooling

19 TTT TRANSFORMATION DIAGRAM (time temperature transformation) TTT diagram determines the types of structures that are obtained at a certain temperature of isothermal cooling and after a certain time Drawing of the TTT diagram from the kinetic curves of transformation for various temperatures under A1 (eutectoid steel)

20 TTT TRANSFORMATION DIAGRAM (time temperature transformation) Lines of TTT diagram: beginning of transformation ending of transformation beginning of martensitic transformation (rapid cooling) Domains in the TTT diagram: austenitic (left of the beginning of transformation line) pearlitic (right of the ending of transformation line, over 500 C) bainitic (right of the ending of transformation line, C) transformation (between beginning ending of transformation lines) martensitic (+ residual austenite) (under Ms, only continuous cooling)

21 CCT TRANSFORMATION DIAGRAM (continuous cooling transformation) Through continuous cooling: pearlitic structures (fineness degree increases with cooling rate) martensite Structures obtained through the continuous cooling of austenite

22 CCT TRANSFORMATION DIAGRAM (continuous cooling transformation) CCT diagram for a eutectoid steel Comparison between TTT and CCT diagrams for a eutectoid steel

23 CCT TRANSFORMATION DIAGRAM (continuous cooling transformation) Critical cooling rates: Lower critical cooling rate (tangent curve to the ending of transformation line) = minimum rate at which martensite occurs in the structure Upper critical cooling rate (tangent curve to the beginning of transformation line) = minimum rate at which the whole structure is martensitic (+residual austenite)

24 Glossary Tratamente termice = heat / thermal treatments; Recoacere = annealing; Calire = quenching (hardening for steels); Revenire = tempering; Tratamente termochimice / termofizice = thermochemical / thermophysical treatments; Difuzie = diffusion; Punct critic = critical temperature; Graunte ereditar = hereditary grain; Grosolan (structura) = coarse; Supraincalzire = overheating; Decarburare = decarburizing; Racire izoterma / continua = isothermal / continuous cooling; Martensita = martensite; Bainita superioara / inferioara = upper / lower bainite; Curbă cinetică de transformare = kinetic curve of transformation; Diagrama TTT / TRC = TTT / CCT transformation diagram; Viteză critică superioară / inferioară = upper / lower critical cooling rate;