temperature and thus the retained austenite content. Hence the cast steel was homogenized prior to austenite con- ditioning to produce a more homogeneous martensitic mi- crostructure. An SEM equipped with an energy dispersive spectrometer was used to analyze the as-cast microstructure of the base heat and heat 3 for the composition of the inter- dendritic regions. Cr and Nb enrichment of these interden- dritic regions as forecasted by thermodynamic analyses was experimentally confirmed (see Figure 5). Table 4 presents a standardless chemical analysis for the matrix and interden- dritic regions of the base heat and heat 3. The average Cr content in the alloy was 15.7 weight percent (see Table 1), whereas the concentration of Cr in the matrix (see Table 4) was nearly 16 to 17 weight percent and 20 to 23 weight per- cent Cr in the interdendritic regions. These values are higher than expected based upon the true alloy chemistry and are provided only to show the relative degree of segregation. Presence of a Nb-rich phase (white contrast in Figure 5[b]) was also observed in the interdendritic regions.
the Ms
The diffusion coefficient of Cr in steel was calculated using Equation 2. The interdendritic spacing, h, measured in the gating system of tensile bar (preheated ceramic shell mold) using SEM images (Figure 5) was 38µm ±9µm. The homog- enization time, t at different temperatures was estimated us- ing Equation 3, which is based upon a simple one dimen- sional random walk.
D = D0 t = h2
exp (- Q / RT) / 2D mol) are from recent publication of Laik et al.10 where: Do
The estimated homogenization times were approximately two hours and one hour for 1200°C (2190°F) and 1250°C (2280°F), respectively. Experiments were conducted at 1200°C (2190°F) for one hour and two hours, as well as 1250°C (2280°F) for one hour. All specimens were air- cooled after homogenization. The resulting microstruc- tures were characterized by optical microscopy. Figure 6 depicts the microstructure after homogenization and air- cooling of base heat specimens. Two parameters were ana-
(3.74 cm2
Equation 2 Equation 3
/s) and activation energy Q (62.97 kcal/
lyzed for the optimization of homogenization temperature and time: (1) minimization of Cr segregation and (2) the volume fraction and morphology of the undissolved ferrite pools. The volume fraction of ferrite was measured accord- ing to ASTM E562. The lowest amount of ferrite (5
Austenite Conditioning Treatment
The primary goal of austenite conditioning is to dissolve ε-Cu into the austenite. Hsiao et al.6
temperatures via dilatometry for wrought 17-4 PH steel as 725°C (1340°F) and 915°C (1680°F), respectively. The Fe-Cu phase diagram indicates6
and AC3 determined the AC1 that the solubility of
Cu increases to approximately 7 weight percent at 1040°C (1900°F) and the suggested austenite conditioning treatment for 17-4 PH is 1050°C (1920°F) for one hour.. Hence, aus- tenite conditioning was carried out at 1050°C (1920°F) for 1 hour. Formation of martensite as well as minimizing re- tained austenite on quenching are important for martensitic age-hardenable steels. The martensite start temperatures for the experimental heats were calculated to be 244°C (471°F) to 249 °C (480°F) as a function of weight percent alloy, us- ing equation 4. The volume percent retained austenite in martensitic steel is given9
by equation 5. Ms (in°C)=539–423[C]–30.4[Mn]–12.1[Cr]–7.5[Mo]
Vγ = exp [–0.011*(Ms where: Ms
quench bath temperature. is the martensite start temperature and Tq –Tq )]
Equation 4 Equation 5
is the
Experiments with three different quenching treatments: air- cooling (A), water quenching (W), and water quenching fol- lowed by liquid nitrogen quenching (W+N) were done on the base heat. The quenching treatment was investigated to determine a procedure that would produce a minimum
(a)
(b)
(c)
Figure 6. Optical micrographs of as-homogenized base heat specimens etched in Marbles reagent (a) one hour at 1200°C, 4.3% ± 0.9% ferrite (b) 2 hours at 1200°C, 1.4%±0.7% ferrite and (c) one hour at 1250°C, 3.5% ± 1% ferrite.
International Journal of Metalcasting/Spring 10 63
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