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Unformatted text preview: 11.1 The two stages involved in the formation of particles of a new phase are nucleation and
growth. The nucleation process involves the formation of normally very small particles of the new phase(s) which are stable and capable of continued growth. The growth stage is simply the increase in size of the new phase particles. 1 1 _S For this problem, we are given: for the austenite—to—pearlite transformation, two values of y and two values of the corresponding times, and are asked to determine the time required for 95% of
the austenite to transform to pearlite. The first thing necessary is to set up two expressions of the form of Equation (10.1): and
then to solve simultaneously for the values of n and k. Rearrangement of Equation (10.1) and taking natural logarithms twice, leads to In{ln }=Ink+nlnt
1v The two equations are thus In {In [1 jig2]} = In It + n ln{12.6 s) In {In [1 jig8]] = In It + n ln{28.2 s) Solving these two expressions simultaneously for n and k yields n = 2.453 and k = 4.46 x 10—4.
Now it becomes necessary to solve for the value of t at which y = 0.95. Algebraic
manipulation of Equation (10.1) leads to an expression in which t is the dependent parameter 8.3 "111 — 0.95l
4 _[ 4.54 x10— “2.453 = 35.7 s il.15 Below is shown an isothermal transformation diagram for a eutectoid ironcarbon alloy, with timetemperature paths that will produce (a) 100% coarse pearlite (b) 100% tempered martensite
(c) 50% coarse pearlite, 25% bainite, and 25% martensite Eutectoid temperature 1400 1000 Temperature [“Ct
on
8
Temperature [‘Fl § mo 200 101 l 10 102 103 101 105
Time ts) 'l 'l.29 Two reasons why martensite is so hard and brittle are:
1) there are relatively few operable slip systems for the bodycentered tetragonal crystal structure
2} Virtually all of the carbon is in solid solution, which produces a solidsolution hardening effect. 1 1.40 For precipitation hardening, natural aging is allowing the precipitation process to occur at the ambient temperature; artificial aging is carried out at an elevated temperature. ...
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 Spring '10
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 Austenite, new phase, Tempering, Maraging steel, 0.95l

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