Jose D. Villegas-Cardenas

Precipitation analysis of as-cast HK40 steel after isothermal aging

As-cast HK40 steel was aged at 700, 800, or 900°C for times as long as 2000 h. Microstructural characterization showed that the primary M7C3 carbide network contained a substantial content of manganese, in agreement with the microsegregation of manganese calculated by Thermo-Calc using the Scheil–Gulliver module. The dissolution of primary carbides caused the solute supersaturation of austenite and subsequent precipitation of fine M23C6 carbides in the austenite matrix for aged specimens. During prolonged aging, the carbide size increased with increasing time because of the coarsening process. A time–temperature–precipitation diagram for M23C6 carbides was calculated using the Thermo-Calc PRISMA software; this diagram showed good agreement with the experimental growth kinetics of precipitation. The fine carbide precipitation caused an increase in hardness; however, the coarsening process of carbides promoted a decrease in hardness. Nanoindentation tests of the austenite matrix indicated an increase in ductility with increasing aging time.

Effect of phase transformations on hardness in Zn-Al-Cu Alloys

Zn-Al-Cu alloys were prepared by melting of pure elements. The as-cast alloys were homogenized at 350 °C for 180 h. Both the as-cast and homogenized alloys were analyzed with X-ray diffraction and scanning electron microscope with Energy Dispersive X-Ray analysis. The hardness of alloys was determined using Rockwell “B” hardness. The X-ray diffractograms and scanning electron micrographs indicated the presence of several phases in the as-cast alloys. Some of them do not correspond to those shown in the equilibrium Zn-Al-Cu phase diagram. On the other hand, the homogenized alloys showed most of the phases present in the equilibrium diagram. The hardness of alloys increases with the increase in Cu content because of the presence of Cu-containing phases such as, the θ and τ’ phases.

Microstructure Characterization of Aged Heat Resistant Steels

The precipitation evolution was studied experimentally and numerically during aging of as-cast heat resistant steel at 700, 800 and 900 °C. Thermo-Calc result showed a good assessment of the non-equilibrium phases for as-cast HK40 steels in comparison to the experimental results. In contrast, the Time-Temperature-Precipitation diagram for the M23C6 precipitation calculated with PRISMA showed a good agreement with the experimental growth kinetics precipitation for the steel after aging at 700, 800 and 900 °C.

Carbide Precipitation in a Low Alloy Ferritic Steel

the precipitation evolution was studied during aging of 2.25Cr-1Mo steel at 550 °C. The as-received steel was aged in two different ways: the first one was the a continuous isothermal treatment at 550 °C for times up to 1000 h and the second aging was performed using isothermal aging cycles which consisted of aging at 550 °C for 60 min and then water quenched at room temperature. M23C6 and M6C precipitation occurred intragranular and intergranular in both types of aging. The coarsening of the carbides was observed to occur with the increase in aging time. Nevertheless, the growth kinetics coarsening was faster in the case of isothermal aging cycles. The hardness decreased with aging time in both cases; however, it occurred in a shorter aging time for the cyclic aging. Nanoindentation tests indicated the increase of ductility as the aging time increased.

Coarsening of Decomposed Phases in Cu-Ni-Cr Alloys

The coarsening process of the decomposed phases was studied in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys aged at 600, 700 and 800 °C for different times by transmission electron microscopy. The morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100= Cu-rich matrix directions as aging progressed. The variation of mean equivalent radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The linear variation of the density number of precipitates and matrix supersaturation with aging time, also confirmed that the coarsening process followed the modified LSW theory in both alloys. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the LSW theory. The coarsening rate was faster in the symmetrical Cu- 45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates.

Effect of homogenization process on the hardness of Zn–Al–Cu alloys

The effect of a homogenizing treatment on the hardness of as-cast Zn–Al–Cu alloys was investigated. Eight alloy compositions were prepared and homogenized at 350 °C for 180 h, and their Rockwell “B” hardness was subsequently measured. All the specimens were analyzed by X-ray diffraction and metallographically prepared for observation by optical microscopy and scanning electron microscopy. The results of the present work indicated that the hardness of both alloys (as-cast and homogenized) increased with increasing Al and Cu contents; this increased hardness is likely related to the presence of the θ and τ′ phases. A regression equation was obtained to determine the hardness of the homogenized alloys as a function of their chemical composition and processing parameters, such as homogenization time and temperature, used in their preparation.

Ostwald Ripening Process of Coherent β′ Precipitates during Aging in Fe0.75Ni0.10Al0.15 and Fe0.74Ni0.10Al0.15Cr0.01 Alloys

The Ostwald ripening process was studied in Fe0.75Ni0.10Al0.15 and Fe0.74Ni0.10Al0.15Cr0.01 alloys after aging at 750, 850, and 950°C for different times. The microstructural evolution shows a rounded cube morphology (Fe, Ni)Al β′ precipitates aligned in the ferrite matrix, which changes to elongated plates after prolonged aging. The variation of the equivalent radii of precipitates with time follows the modified Lifshitz-Slyozov-Wagner theory for diffusion-controlled coarsening. Thermo-Calc analysis shows that the chromium content is richer in the matrix than in the precipitates which causes higher hardness and coarsening resistance in the aged Fe0.74Ni0.10Al0.15Cr0.01 alloy.