John R. Bradley

Laser transformation hardening of iron-carbon and iron- carbon- chromium steels

The laser transformation hardening response of Fe-0.5C-0.8Mn and Fe-0.5C-0.8Mn-0.8Cr steels was examined. A 2 kW CO2 laser was used to scan the steel surfaces at various rates. Complete transformation of pearlite to austenite, and hence to martensite, occurred in the laser heated surface layer of the Fe-C-Mn steel. During equivalent heat treatment of the Fe-C-Mn-Cr steel, incomplete austenitization of the pearlite colonies left the cementite plates largely undissolved. However, the maximum surface hardness was approximately the same for both alloys. Comparison of calculated and measured hardened depths yielded values of the effective coupling coefficient of the laser beam to the steel which varied as a function of beam interaction time. Modeling the process allowed a dis-tinction to be made between the effects of alloying elements and of pearlite spacing upon the depth of complete austenitization. In this case, the effect of the difference in pearlite spacing between the two steels was negligible. In the alloy steel, Cr and Mn were strongly partitioned to the cementite before heat treatment, and remained so after laser processing. Incomplete austenitization of that steel is attributed to partitioning of alloying elements to the cementite and their retarding influence on the diffusion controlled dissolution kinetics of the alloyed carbide.

Improved yield of carbon fibers by pyrolysis of natural gas in stainless steel tubes

Carbon fibers are produced by pyrolysis of natural gas inside stainless steel tubes. Lengthening of unthickened precursor carbon filaments is normally catalyzed by loose metal or carbide particles generated in situ, primarily by fragmentation of the growth tube wall. More direct methods of supplying a greater number of catalytic particles were investigated in an effort to increase fiber yield. Growth of fibers on the inner surface of steel tubes could be enhanced by either: 1. 1) local melting of the surface in the presence of air, or 2. 2) wetting the surface with an aqueous ferric nitrate solution. Both methods increased the number of carbon fibers produced per unit area about 20-fold over as-received stainless steel tubes. Scanning electron microscopy and X-ray diffraction analyses showed that both procedures left the growth substrates covered with a rough, relatively thick layer comprised of a mixture of iron oxides. Precipitation of iron during reduction of the oxide mass is an important mechanism for providing an abundance of particles which catalyze carbon filament growth.

The structure of carbon filaments and associated catalytic particles formed during pyrolysis of natural gas in steel tubes

New information is presented on the structure and growth of carbon filaments formed in association with catalytic metal or metal carbide particles during pyrolysis of natural gas. The structures of the filaments and of the particles have been examined by transmission electron microscopy. The filaments are hollow, ~0.1 μm or less in diameter, and vermicular as opposed to straight. The graphite basal planes are frequently not parallel to the axis of the filament. Individual catalyst particles have been identified as (Fe,Cr)23C6 when pyrolysis occurs in a stainless steel tube and as γ-iron in a plain carbon steel tube.

On the influence of carbide formation upon the growth kinetics of proeutectoid ferrite in Fe-C-X alloys

In the preceeding paper, the growth kinetics of grain boundary ferrite allotriomorphs in Fe-C-Si, Fe-C-Mn, Fe-C-Ni, and Fe-C-Cr alloys are reported to be best described by the paraequilibrium model. Significant differences are still observed, however, between the experimentally measured kinetics and those calculated from this model. A TEM study was conducted on these alloys to ascertain whether any of these differences could be attributed to carbide precipitation. In the Fe-C-Mn and Fe-C-Cr alloys, where the measured growth kinetics are low, carbides precipitate on dislocations within the ferrite and are effectively absent, respectively; hence carbide precipitation cannot be responsible for the deviations in these alloys. In the low Ni, Fe-C-Ni alloy, where calculated and measured kinetics agree, carbide precipitation was again found on dislocations in the ferrite. Faster than calculated growth kinetics in the Fe-C-Si and the high Ni, Fe-C-Ni alloys, on the other hand, are attributed in part to carbide precipitation at austenite:ferrite boundaries.

Forming limit diagrams for AA5083 under SPF and QPF conditions

Forming Limit Diagrams (FLD’s) for AA5083 aluminum sheet were established under both Superplastic Forming (SPF) and Quick Plastic Forming (QPF) conditions. SPF conditions consisted of a strain rate of 0.0001/s at 500°C, while QPF conditions consisted of a strain rate of 0.01/s at 450°C. The forming limit diagrams were generated using uniaxial tension, biaxial bulge, and plane strain bulge testing. Forming limits were defined using two criteria: (1) macroscopic fracture and (2) greater than 2% cavitation. Very little difference was observed between the plane strain limits in the SPF and QPF conditions indicating comparable formability between the two processes with a commercial grade AA5083 material.

Laser transformation hardening of a high-purity iron-carbon-chromium alloy

Successful laser transformation hardening of steel surfaces requires that the absorbed laser energy is sufficient to austenitize the initial microstructure to a depth of 0.5 mm or more. Hardening is accomplished when rapid cooling by conduction of heat away from the surface causes transformation of the austenite layer to martensite. Heating and cooling rates of 10/sup 4/ K/s or greater are typical of the laser hardening process and the entire thermal cycle may be accomplished in less than 0.1 s. In an earlier study, laser surface hardening of commercial plain carbon and chromium alloyed steels was examined. It was shown that in the alloyed steel chromium enrichment of the cementite in the initial microstructure could prevent complete transformation of pearlite to asutenite during the very rapid laser heating cycle. However, interpretation of the results was complicated somewhat by the fact that manganese was also partitioned to the cementite. The purpose of this work was to conduct selected identical laser heating experiments on a high-purity Fe-C-Cr alloy to test the effect of chromium unequivocally, i.e., in the absence of manganese and other elements normally present in commercial steels.

Microstructure and magnetic properties of CO2 laser surface melted Nd-Fe-B magnets

Abstract The near-surface microstructure and magnetic properties of permanent magnets produced from hot-pressed melt-spun ribbons of a Nd 0.13 Fe 0.81 B 0.06 alloy were modified by CO 2 laser beam melting. Self-quenching of the molten surface layer at an average cooling rate of ≈10 4 K/s resulted in a stratified structure consisting of a zone of large columnar grains between thinner layers of equiaxed grains. The resolidified material was comprised mostly of the magnetically hard Nd 2 Fe 14 B phase dispersed interdendritically among primary α-Fe dendrites. A small volume fraction of a fcc Nd-rich phase was also present. A solidification texture in the columnar zone resulted in alignment of the c -axis of the Nd 2 Fe 14 B phase, the direction of easiest magnetization, parallel to the surface of the sample. The magnetic properties of the resolidified layer were anisotropic. The magnetization was oriented parallel to the surface, in accordance with the preferred orientation of the Nd 2 Fe 14 B phase. Low coercivity in all directions (≈140Oe) was attributed to the relatively large grain size of the Nd 2 Fe 14 B phase and also to the amount of ferrite present.

Local Thinning at a Die Entry Radius During Hot Gas-Pressure Forming of an AA5083 Sheet

Splitting at regions of local thinning below die entry radii is a critically important mechanism of failure in hot gas-pressure forming of sheet materials. Local thinning is controlled by sheet-die friction and die geometry, as well as sheet material properties. In this study, local thinning is investigated at a particularly severe die entry radius during hot forming of a fine-grained AA5083 sheet at 450°C. Particular emphasis is placed on the relationship between local thinning and sheet-die friction conditions. A simple analysis of the mechanics of this thinning phenomenon is presented. Finite element simulation results are presented for different sheet-die friction conditions. Sheet thickness profiles measured from parts produced in forming experiments using three different lubrication conditions are compared with predictions from simulations. Simulation predictions agree well with experimental data for the occurrence and location of thinning below a die entry radius. Additional insights into sheet-die friction for controlling local thinning and preventing premature necking failure are detailed.

Design, Construction, and Applications of the Uniform Pressure Electromagnetic Actuator

High velocity forming can lead to better formability along with additional benefits. The spatial distribution of forming pressure in electromagnetic forming can be controlled by the configuration of the actuator. A new type of actuator is discussed which gives a uniform pressure distribution in forming. It also provides a mechanically robust design and has a high efficiency for flat sheet forming. Key quantitative concepts are presented that help in the design of the system. Examples of uses of the actuator are then presented, specifically with regard to forming shapes and surface embossing. This paper emphasizes the approaches and engineering calculations required to effectively use this actuator.

Laser surface heating of Nd-Fe-B, Nd-Fe-Co-B, and BaO-6Fe2O3 permanent magnets

Abstract Hot-pressed Nd-Fe-B, epoxy-bonded Nd-Fe-Co-B, and BaO-6Fe 2 O 3 (ferrite) permanent magnets are heated by scanning their surfaces with an argon ion laser beam. The laser heating response of each magnet material is examined by measuring the depth of the heat-affected zone as a function of beam power and scan rate. An analytic heat transfer model is used to provide a convenient description of the laser surface heating process. The temperature distribution in the magnets is calculated to estimate the depth of the heat-affected zone as defined by the position of the Curie temperature isotherm. Agreement is good among the calculated and measured depths for all three permanent magnet materials.