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

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

Iron-carbon system microstructural changes Pearlite Spheroidite Bainite Martensite

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

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

FIGURE 4. 14 Microstructure of eutectoid steel FIGURE 4.14 Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700°C (1292°F). Magnification: 1000.

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

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

FIGURE 4.15 (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.

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

FIGURE 4.15 (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.

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

FIGURE 4.17 (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.

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

FIGURE 4.19 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.

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

FIGURE 4. 20 (a) End-quench test and cooling rate FIGURE 4.20 (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.

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

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

FIGURE 4.22 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.

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

TABLE 4.1 Outline of Heat-treatment Processes for Surface Hardening

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

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

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