1. Metal Alloys: Their Structure & Strengthening by Heat Treatment
Part 2
Chapter 4

2. Heat treatment of ferrous alloys (4.7)
Heat-treatment techniques: the controlled heating and cooling of alloys at various rates
Phase transformations: greatly influence the mechanical properties
Strength
Hardness
Ductility
Toughness
Wear resistance

3. Iron-carbon system microstructural changes
Pearlite
Spheroidite
Bainite
Martensite

4. Pearlite Course pearlite Slow rate of cooling As in a furnace
Fine pearlite
High rate of cooling
As in air (fig.4.11)
Course pearlite
Slow rate of cooling
As in a furnace

5. Spheroidite
Pearlite is heated to just below the eutectoid temperature for a period of time
Example: a day at 700oC
Cementite lamellae transform into roughly spherical shapes
Higher toughness
Lower hardness
Can be cold worked
Spheroidites less conductive to stress concentration

6. FIGURE 4. 14 Microstructure of eutectoid steel
FIGURE Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700°C (1292°F). Magnification: 1000.

7. Bainite Very fine microstructure consisting of ferrite and cementite
Bainitic steel is stronger and more ductile than pearlitic steels at the same hardness levels

8. Martensite
When austenite is cooled at a high rate such as by quenching in water its FCC structure is transformed to BCT (body-centered tetragonal)
Hard
Brittle
Lacks toughness so limited in usefulness

9. FIGURE (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000.

10. Quench cracking
Internal stresses cause parts to undergo distortion or even crack during heat treatment
Distortion is the irreversible dimensional change of the part during heat treatment

11. FIGURE (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000.

12. TTT (time-temperature-transformation) diagrams
Allow metallurgists to design heat treatment schedules to obtain desirable microstructures
Fig.4.17a The higher the temperature or the longer the time, the more austenite that transforms into pearlite

13. FIGURE (a) Austenite-to-pearlite transformation of iron–carbon alloy as a function of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675°C (1247°F). (c) Microstructures obtained for a eutectoid iron–carbon alloy as a function of cooling rate.

14. Hardenability
Hardenability is the capability of an alloy to be hardened by heat treatment
Measures the depth of hardness obtained by heat treatment/quenching
Hardenability is not the same as hardness

15. FIGURE Mechanical properties of annealed steels as a function of composition and microstructure. Note in (a) the increase in hardness and strength, and in (b), the decrease in ductility and toughness, with increasing amounts of pearlite and iron carbide.

16. End-Quench hardenability test (Jominy Test)
Round test bar is austenized (heated to the proper temperature to form 100% austenite)
Bar then quenched at one end
Hardness decreases away from the quenched end of the bar
Quenching media
Water
Brine
Oil
Molten salts
Air
Caustic solutions
Polymer solutions
gases

17. FIGURE 4. 20 (a) End-quench test and cooling rate
FIGURE (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test. Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals, for better control of properties.

18. Precipitation hardening
Small particles of a different phase called precipitates are uniformly dispersed in the matrix of the original phase
Precipitates form because the solid solubility of one element in the other is exceeded
The alloy is reheated to an intermediate temperature and held there for a long time during which time precipitation takes place

19. Aging or Age Hardening
Because the precipitation process is one of time and temperature, it is also called AGING.
Age hardening is the property improvement of the material
Artificial aging is carried out above room temperature
Natural aging: some aluminum alloys harden and become stronger over time at room temperature

20. FIGURE The effect of aging time and temperature on the yield stress of 2014-T4 aluminum alloy. Note that, for each temperature, there is an optimal aging time for maximum strength.

21. Case hardening Hardening of the surface
Improves resistance to surface indentation, fatigue, wear
Gear teeth
Cams
Shafts
Bearings
Fasteners
Pins
Automotive clutch plates
Tools and dies

22. TABLE 4.1 Outline of Heat-treatment Processes for Surface Hardening

23. TABLE 4.1 (continued) Outline of Heat-treatment Processes for Surface Hardening

24. Annealing Steps Heat to a specific temperature range in a furnace
The restoration of a cold-worked or heat-treated alloy to its original properties
Increase ductility
Reduce hardness and strength
Modify the microstructure
Relieve residual stresses
Improve machinability
Steps
Heat to a specific temperature range in a furnace
Hold at that temperature (soaking)
Cooling in air or in a furnace

25. More about annealing
Normalizing-the cooling cycle is completed in still air to avoid excessive softness
Process annealing
Stress-relief annealing
Tempering
Austempering
Martempering
Ausforming