José Antonio Jiménez

Creep behavior of intermetallic FeAl and FeAlCr alloys

Abstract A study of creep behavior of several alloys based in the Fe Al and Fe Al Cr systems ranging in aluminium from 21.7 to 48 at.% was undertaken. The alloys were produced by induction melting and possess a coarse and equiaxial microstructure, with a grain size of about 500 μm. Compression tests at rates from 10 −5 to 10 −2 s −1 were conducted at temperatures ranging from 700 to 1000°C under a protective atmosphere of argon to minimize oxidation. Analysis of the stress-strain data revealed in the binary alloys a stress component of about 3 that suggest that creep is controlled by viscous glide of dislocations. For an aluminium content above 30 at.%, the activation energy for creep does not vary very much with the aluminium concentration and values ranging from 360 to 395 kJ mol −1 were obtained. On the other hand, the ternary alloys present an improvement in strength in the high temperature compressive creep. A stress exponent of 4–5 is observed in this case that suggest that creep is controlled by dislocation climb. An activation energy for creep of about 505 kJ mol −1 was deduced for the alloy containing 30 at.% Al and 10 at.% Cr.

Effects of carbide-forming elements on the response to thermal treatment of the X45Cr13 martensitic stainless steel

The effects of carbide-forming elements on the response to thermal treatment of the X45Cr13 martensitic stainless steel have been investigated. Heat treatments consisted of austenitizing for 60 s at temperatures ranging from 1000–1250 °C. The higher is the solution treatment temperature, the less M23C6 carbide is left out of solution in the austenite. As a result, the concentration of carbon and alloying elements in the martensite increases and, therefore, an increase in the hardness until a maximum value of 710 HV was found at austenitizing temperatures of 1120 and 1130 °C for the steels X45Cr13 and X45CrMoV14, respectively. At higher austenitizing temperatures, the presence of retained austenite was observed, which leads to a lowering of the hardness value. The higher amount of carbide-forming elements in the X45CrMoV14 determines an increase in retained austenite from 3 vol% to about 30 vol%. Thus, a drop in the hardness value from 710 to 680 and 585 for the steels X45Cr13 and X45CrMoV14, respectively, was found.

Microstructural characterization of an ultrahigh carbon and boron tool steel processed by different routes

Abstract A newly developed ultra high carbon and boron tool steel containing 0.8wt.%B-1.3wt.%C-1.6wt.%Cr was processed to obtain a fine grain microstructure following two routes. In the first route, several thermomechanical treatments of the as-cast material, including extensive warm rolling, were used to refine the microstructure. A microstructure consisting of large and small borocarbides in a ferritic matrix with a grain size of about 2 μm was obtained. In the second route, powder metallurgy techniques, including consolidation of rapidly solidified powders by extrusion and hot isostatic pressing (HIP), were used. The powders were produced by argon atomization and exhibit a dendritic microstructure, which remains unchanged after consolidation by HIP at temperatures up to 900 °C. The microstructure after consolidation by extrusion at 1050 °C is coarser and consists of spherical borocarbide particles, 2 μm in size, in a fine-grained ferritic matrix. In addition, the shear forces developed during the extrusion process improve the bonding between the powder particles. In comparison to the microstructures obtained by thermomechanical processing, the powder metallurgy material possesses a better homogeneity in the size and shape of borocarbide particles and a finer microstructure.

Splitting phenomena occurring in the martensitic transformation of Cr13 and CrMoV14 stainless steels in the absence of carbide precipitation

Previously unknown splitting phenomena were detected in the martensitic transformation of XCrl3 and XCrMoV14 stainless steels using high resolution dilatometric analysis. These splittings, which are denominatedMS0 in this article, indicate the martensitic subtransformation of areas of austenite rich in carbon and carbide-forming elements. In contrast to other types of splitting known until now, theMS0 occur in the absence of carbide precipitation during cooling. From the experimental results obtained in this study, it can be concluded that the splittings resulted from concentration gradients produced in the austenite as a consequence of the partial or total dissolution of M23C6 carbides during heating.

Mechanical properties of two ultrahigh carbon-boron tool steels

Abstract The microstructure and the mechanical behavior of two modified boron-containing tool steels were investigated at low and high temperatures. Both steels were processed by powder metallurgy methods, involving gas atomization and hot isostatic pressing at 180 MPa for 2 h at 900, 1000 and 1100 °C. The microstructure of the consolidated tool steels consist of a ferrite matrix and carboboride M23(C,B)6 particles. The 1wt.%C-1wt.%B tool steel contains, in addition, small particles of vanadium carbide and vanadium boride. An ultimate tensile strength of 300 MPa at 700 °C was obtained in the 1wt.%C-1wt.%B tool steel. At testing temperatures in the austenitic phase, a stress exponent of about 5 was obtained and the activation energy for creep was related to the activation energy for iron self-diffusion in austenite. This suggests that slip creep is the controlling deformation mechanism in the 1wt.%C-1wt.%B tool steel tested in this temperature range. At testing temperatures in the ferritic phase the creep rate was slower than that predicted by the slip creep equation. This was attributed to the presence of fine second phase particles. On the other hand, the 1.5wt.%C-0.5wt.%B tool steel showed an ultimate tensile strength somewhat lower than 300 MPa at 700 °C. At testing temperatures in the austenitic phase, a stress exponent of about 2 was obtained and the activation energy for creep was related to the activation energy for iron self-diffusion in austenite. This suggests that grain boundary sliding is the controlling deformation mechanism in the 1.5wt.%C-0.5wt.%B tool steel tested in this temperature range. A contribution from slip creep exists at testing temperatures in the ferritic phase.

Superplastic behavior of two ultrahigh boron steels

The high-temperature deformation behavior of two ultrahigh boron steels containing 2.2 pct and 4.9 pct B was investigated. Both alloys were processedvia powder metallurgy involving gas atomization and hot isostatic pressing (hipping) at various temperatures. After hipping at 700 °C, the Fe-2.2 pct B alloy showed a fine microstructure consisting of l-µm grains and small elongated borides (less than 1µm) . At 1100 °C, a coarser microstructure with rounded borides was formed. This alloy was superplastic at 850 °C with stress exponents of about two and tensile elongations as high as 435 pct. The microstructure of the Fe-4.9 pct B alloy was similar to that of the Fe-2.2 pct B alloy showing, in addition, coarse borides. This alloy also showed low stress exponent values but lacked high tensile elongation (less than 65 pct), which was attributed to the presence of stress accumulation at the interface between the matrix and the large borides. A change in the activation energy value at theα-γ transformation temperature was seen in the Fe-2.2 pct B alloy. The plastic flow data were in agreement with grain boundary sliding and slip creep models.

Relationship between microstructure and texture in Fe-25%Cr-5%Al ribbons produced by planar flow casting

The automotive industry has used for many years exhaust gas catalytic supports based on heat resistant Fe-Cr-Al ferritic stainless steels. The Fe-Cr-Al alloys are usually cast and then rolled into a foil of desired thickness. Planar flow casting (PFC) represents a cheaper alternative route for production of thick ribbons of about 100 {micro}m directly from the melt. On the other hand, it is well established that a solidification rate as high as 10{sup 5} K s{sup {minus}1} can be reached by means of PFC. This rapid solidification rate determines better properties of the alloy associated with the fine and homogeneous microstructure developed. The aim of the present work is to study the relationship between the microstructure and the texture in ribbons of a Fe-25% Cr-5% Al alloy produced by PFC with a thickness ranging from 40--180 {micro}m. All composition are given in weight percent.

Characterization and mechanical properties of ultrahigh boron steels produced by powder metallurgy

The present work is part of an investigation into the use of rapid solidification and powder metallurgy techniques to obtain iron-boron alloys with good mechanical properties. Two Fe-B binary alloys and two ultrahigh boron tool steels were gas atomized and consolidated by hot isostatic pressing (HIP) at temperatures ranging from 700 °C to 1100 °C to have a fine microstructure. Optimum properties were achieved for the binary alloys at low consolidation temperatures, since the solidification mi-crostructure from the original powders is eliminated and, at the same time, fine microstructures and low porosity are obtained in the alloys. At high temperatures and low strain rates, three of the four alloys exhibited low stress exponents, but only the Fe-2.2 pct B alloy showed tensile elongations higher than 100 pct. At low temperatures, only the Fe-2.2 pct B alloy deformed plastically. This alloy showed values of tensile elongation and ultimate tensile strength that were strongly dependent on testing and consolidation temperatures.

Superplastic behavior of thermomechanically processed Fe-0.8 B-1.3 C-1.6 Cr (wt%) alloy

Abstract The coarse microstructure of the as-cast Fe-0.8 B-1.3 C-1.6 Cr (wt%) alloy was refined by a thermomechanical processing consisting of rolling at 1050 °C (e = −2.8) and then rolling from 950 to 670 °C (e = −1). The final microstructure consists of very fine cementite particles and small, and some large, borocarbide particles in a fine-grained ferritic matrix. A stress exponent of about 2.5 at low strain rates and a maximum elongation to failure of 98% was observed at temperatures ranging from 660 to 720 °C.