Lecture week 11 Annealing, Stress Releiving, Normalizing, Hardening

Lecture week 11 Annealing, Stress Releiving, Normalizing, Hardening

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Annealing, Stress Releiving, Normalizing, Hardening, and Tempering of Steel Chapter 10
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Heat Treatment In the process of forming steel into shape and producing the desired microstructure to achieve the required mechanical properties, it may be reheated and cooled several times.
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Steps for all HT (anneals): 1. Heating 2. Holding or “soaking” 3. Cooling Time and temperature are important at all 3 steps
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(Stress-relief)
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Full Annealing heats the steel to a temperature within the austenite (FCC, γ) phase region to dissolve the carbon. (50 deg.F above A 3 -A cm line) The temperature is kept at the bottom of this range to minimize growth of the austenitic grains. Then, after cooling ferrite ( α ) and cementite structures will be fine as well
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Resulting microstructure: For low-medium carbon steels – coarse pearlite and ferrite It is easily machined
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Why hyperetectoid steels are annealed intercritically? To prevent formation of brittle cementite network on the grain boundaries This is undesirable condition if machining is to be done Annealing is performed at temperatures between the critical lines A 3,1 -A cm
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Spheroidizing – improving machinability Used on steels with carbon contents above 0.5% Applied when more softness is needed Cementite transforms into globes, or spheroids These spheroids act as chip-breakers – easy machining Performed by heating to just below A 3,1 line, holding there (about 20h.or more) and then slowly cooling
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Normalizing Allows steels to cool more rapidly, in air Produced structure – fine pearlite Faster cooling provides higher strength than at full annealing
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Process Annealing – 3 stages Recovery (stress-relief anneals) Recrystallization (process anneals) Grain Growth
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