MATERIALS SCIENCE AND ENGINEERING I

Transcription

1 MATERIALS SCIENCE AND ENGINEERING I LECTURE COURSE 8 ANNEALINGS. HARDENING OF STEELS. HARDENABILITY. TEMPERING. THERMOCHEMICAL TREATMENTS. OTHER SURFACE TREATMENTS.

2 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Pearlite type structures = Ferrite + Fe 3 C 1. Coarse pearlite: lamellar (equilibrium), globular (globulization annealing) HB, A max.=12%, Rm = MPa; higher toughness for globular pearlite 2. Sorbitic pearlite: fine lamellar, obtained through the cooling of austenite max.280 HB, Rm over 800 Mpa Sorbite: oriented fine lamellar structure; obtained through the heating of martensite Rm over 850 MPa, optimum strength / toughness ratio 3. Troostite: very fine lamellar ~ 400 HB, highest strength for pearlite (Rm>900 MPa)

3 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Martensitic structures Martensite = oversaturated solid solution of C in Feα 1. Quenching martensite: white sheets (needles); Tetragonal structure, stressed; Highest strength (Rm > 1100 MPa), lowest toughness 2. Tempered martensite: black sheets (needles) Smaller stresses, higher toughness; obtained through the heating of quenching martensite (tempering)

4 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Bainitic structures (intermediate) Bainite = mechanical admixtures of oversaturated of C and carbides that did not reach the Fe 3 C stage 1. Upper bainite: resembling troostite; obtained isothermally at C; ~ 450 HB; 2. Lower bainite: resembling tempered martensite; obtained isothermally at C; ~550 HB

5 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Coarse lamellar pearlite Globular pearlite

6 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Troostite

7 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Martensites Quenching martensite in steel Tempered martensite in steel

8 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Globular pearlite Quenching martensite Troostite + Martensite Sorbite

9 STRUCTURES OBTAINED THROUGH HEAT TREATMENTS Lower bainite Upper bainite

10 ANNEALING

11 ANNEALING Annealings = HT characterized by slow cooling (most rapid in air) Stress relief annealing: for the relief of thermal stresses that resulted after hot processing (welding, casting etc.) C for steels, 2-6 hours, cooling in oven / sand Recrystallization annealing: for removing the cold hardened state C for steels Homogenization annealing: in order to remove dendritic segregation C for steels, oven cooling; generates overheated structures

12 ANNEALINGS Equilibrium annealing: in order to bring back the structure to the equilibrium state (in the diagram) For hyper-eutectoid steels: soft annealing Temperature: C over A3 / Acem; EXTREMELY SLOW COOLING! Normalization annealing: in order to obtain a fine / tough structure Temperature: not more than C over A3 / Acem; air cooling Spheroidize annealing: obtaining of globular pearlite (tougher and more machinable)

13 QUENCHING (HARDENING) Quenching = heating in order to produce the phase transformation ( P A for steels), followed by rapid cooling Steels: temperature with C over A3 (hypo-eutectoid) / A1 (hyper-eutectoid) Each cooling environment provides a cooling intensity - H H = 1 for water at 20 C; H < 1 for oil, Diesel oil,...; H > 1 for water with NaCl, water at 0 C,...

14 QUENCHING (HARDENING)

15 QUENCHING (HARDENING)

16 QUENCHING (HARDENING)

17 QUENCHING (HARDENING) Classical hardening: single environment Hardening temperatures Cooling rates for classical hardening Drawback: stresses thermal (between zones with different cross sections) structural (martensite displays the maximum volume)

18 QUENCHING (HARDENING) Special hardening methods: for removing the drawbacks of classical hardening (partially) - Time quenching 2 environments: water oil - Martempering maintaining for equalizing the temperature - Austempering for obtaining a bainitic structure + cryogenic quenching (for dimensional stabilization)

19 QUENCHING (HARDENING) Case hardening (surface hardening): only at the exterior of parts, that becomes hard and strong, while the interior remains tough High Frequency Quenching: Foucault currents are induced in the outer layer of the part; heating is performed through the Joule effect; The depth of hardened layer can be adjusted through the current frequency and inductor speed. Heating by the means of high frequency currents; 1 inductor; 2 alternating magnetic field; 3 part; 4 generator.

20 HARDENABILITY Hardenability = steel property to be deep hardened; it is determined by the hardening depth (depth of hardened layer) Hardening capacity hardenability; Hardening capacity = hardness after quenching

21 HARDENABILITY Hardening depth in a cylindrical part; a. cooling rates; b. layers in the cross section after quenching; c. variation of hardness in the cross section.

22 HARDENABILITY Semimartensitic layer = layer that contains 50% martensite (determined by the hardness) Hardening depth: up to the semimartensitic layer Determining method: Jominy end quench test hardness = f(distance from the quenched end) Hardenability curve Band of hardenability Hardenability curve Band of hardenability

23 HARDENABILITY D 0 real critical diameter = largest diameter of a part that is completely hardened in a real quenching environment; D - ideal critical diameter = largest diameter of a part that is completely hardened in an ideal quenching environment (H )

24 TEMPERING Tempering = heat treatment performed after hardening (martensitic structure), in order to obtain a more stable and less brittle structure 1. Low tempering: C Tempered martensite (hard, strong); for hardly wearing loaded parts, tools, etc 2. Medium tempering: C Tempered troostite (high strength and elasticity); for springs; 3. High tempering: C Sorbite; for parts that are loaded in a dynamic regime (gears, shafts, etc.) [ CALIRE + REVENIRE INALTA = IMBUNATATIRE ]

25 THERMOCHEMICAL TREATMENTS Thermochemical treatments (case hardening by diffusion) = surface treatments for introducing of certain chemical elements in the outer layer of parts; the name is given by the element (carburizing, nitriding, silicizing,...). Process in 3 stages : 1. Dissociation of environment obtaining of active atoms that take part to the subsequent processes NH 3 3H 2 + 2N* 2 CO CO 2 + C* 2. Adsorption adhesion of active atoms on the parts surface 3. Diffusion

26 THERMOCHEMICAL TREATMENTS Carburizing: increasing the carbon amount in the outer layer For steels with carbon content < 0.2%; Environment: (solid, liquid), gas, plasma; Temperature: C; Layer thickness: mm; Not a final treatment

27 THERMOCHEMICAL TREATMENTS Nitriding: increasing the nitrogen content in the outer layer Only for alloy steels with elements that form stable nitrides (Al, Mo, Cr, V, ); Environment: gas, plasma; Temperature: C; Layer thickness: mm (very hard, > 1000 HV); Final treatment

28 THERMOCHEMICAL TREATMENTS Nitrocarbiding: increasing simultaneously the C and N content in the outer layers of parts Advantages: unalloyed steels can be also treated; lower temperature than for carburizing; possibility of quenching directly after the treatment; higher layer thickness than for nitriding; higher hardness than for carburizing;

29 Other surface treatments - Shot peening - Surface cold hardening + changing of roughness

30 Other surface treatments - Chemical vapour deposition -

31 Other surface treatments - Physical vapour deposition -

32 Other surface treatments - Ionic implantation -

33 Other surface treatments - Nanostructured layers - ZnO Application: Optoelectronics Solar cells Ultrasensitive fiber optics Sensors Varistors SEM images of ZnO nanostructures annealed at different temperatures: (a) 450ºC; (b) 500ºC; (c) 550ºC; (d) 600ºC.

34 Glossary Martensita de calire / revenire = quenching / tempered martensite; Bainita superioara / inferioara = upper / lower bainite; Recoacere de detensionare = stress relief annealing; Recoacere de omogenizare = homogenization annealing; Recoacere de echilibru / înmuiere = equilibrium / soft annealing; Recoacere de globulizare a perlitei = spheroidize annealing; Călire întreruptă = time quenching; Călire în trepte = martempering; Călire izotermă = austempering; Călire criogenică = cryogenic quenching; Călire superficială = case hardening (surface hardening); Călibilitate = hardenability; Capacitate de călire = hardening capacity; Revenire (joasă, medie, înaltă) = (low, medium, high) tempering; Tratament termochimic = thermochemical treatment (diffusion case hardening);

35 Glossary Disocierea mediului = dissociation of environment; Adsorbţie = adsorption; Carburare = carburizing; Nitrurare = nitriding; Carbonitrurare = nitrocarbiding; Improscare cu alice = shot peening; Depunere chimica in vapori = chemical vapour deposition (CVD); Depunere fizica in vapori = physical vapour deposition (PVD); Implantare ionica = ionic implantation;