M. P. Guerrero-Mata

Study of the Tempcore Process for the Production of High Resistance Reinforcing Rods

The process known as Tempcore is used to produce high resistance rods by the formation of a surface layer of quenched and tempered martensite that surrounds a core made of ferrite and pearlite. Such a mixed structure is result of processing hot rolled rods through waters headers that reduce the temperature at the surface below that for the transformation into martensite. This structure is tempered by the heat flowing from the core of the rod, which transforms into ferrite and pearlite while the rod is in the cooling beds. Such processing produces a significant increase in yield and ultimate tensile strength, while maintaining adequate ductility. The economic advantages of this process are huge in comparison with those that require alloying elements or further metal working to improve mechanical properties. A series of experimental trials were carried out in a pilot plant in which parameters such as reheating temperature, water flow and processing time were varied to study their effect on the mechanical properties of carbon steel rods and on the structures formed in the bars. The study is being complemented by the thermal modelling by the finite element method.

Characterization of Friction Stir Welding on Aluminum

A series of linear and spot stir welding friction tests were carried out on aluminum samples of 1 mm in thickness and area of 100 mm x 27 mm. The tool rotating speed was varied from 2000 to 4000 RPM and the feeding rate from 45 to 67.8 mm/min. The temperature distribution during welding was measured by thermocouples inserted within the aluminum strips. The microstructure of the welded nuggets and the HAZ were analyzed by optical and scanning electron microscopy. Some samples were tested intension to measure the load carrying capacity of the welded bead. Preliminary analyses indicate that the temperature at the heat affected zone increases with the rotating speed.

High-Speed Quenching of High Carbon Steel

Medium and high carbon steels are usually quenched in polymer and oil in order to avoid cracking and distortion; however, recent studies have proved that it is possible to minimize cracking using water as a coolant of these steels by promoting extremely high cooling rates. By great agitation and velocity of quenchant, the vapor blanket is reduced or prevented during water quench, allowing uniform hardening of the surface. In this study, the cooling severity of a spray quenching system and a high-speed quenching chamber are studied. Cylindrical samples of AISI 304 stainless steel (20 mm in diameter and 100 mm length) were employed to characterize the cooling severity. Thermal data was acquired through K-type thermocouples placed in the sample at three positions, 1 mm below surface, mid-radius and at center of the specimen, connected to a data acquisition system. High thermal gradients were observed in both systems, being the high-speed chamber the severest cooling. The maximum cooling rate obtained at the surface was 470 and 300°C/s for the high-speed chamber and the spray system, respectively. In addition, 5160 spring steel samples were quenched for short times in both systems; the cooling was interrupted to avoid through transformation and to produce a case-core type microstructure. Different cooling times were used for the interrupted quenching to modify the martensite case thickness. No cracks were observed. Thermal results and microstructures are discussed.

Plastic deformation of low carbon steels

The plastic deformation of two steels was studied by means of axisymmetric compression tests carried out in a computer driven servohydraulic machine. The tests were conducted within the temperature range of 700–1100°C for steels that have similar carbon contents, but differed in other alloying elements. Little difference in strength between the steels was found when deformation took place in the austenite phase field, but the strength of the material varied as soon as ferrite was present. The critical temperatures were evaluated by means of thermal analyses, which were conducted by inserting a thermocouple within samples of the two steels; these temperatures were found to be close to those predicted by empirical equations.

Diffusion in Electrodes Used for Resistance Spot Welding of Galvannealed Steel

A study was carried out on Zr-Cr bearing copper electrodes used for resistance spot welding of galvannealed steel strips. One electrode exhibited a series of well-defined layers in which Zn diffused to form β- and γ-brasses; an external layer containing iron was detected in this electrode. Another electrode that exhibited a high degree of damage did not exhibited continuous Zn-diffusion layers in all places, moreover, the Fe-containing layer was either removed, or had it grown to a high extent in some places; the occurrence of Cu-rich particles embedded within the Fe containing layer was observed. Multiple cracks were observed within the γ brass layer in both electrodes. The difference in the observed behaviour of the electrodes can be attributed to a difference in the characteristics of the galvannealed coating of the strips, as the first electrode was used to weld strips in which the layer corresponding to the  phase was well developed, whereas the second electrode was used to weld strips with only an incipient layer. It can be concluded that growth of the  phase changes the thermophysical properties of the zinc coating, affecting the temperature profile during spot welding.

Characterization of Galvannealed HSLA Steels

Galvannealed coatings are obtained by heat treating galvanized steel strips and are widely used in the automotive industry due to their improved properties. The variables influencing coating formation on high strength low alloy steel (HSLA) were analyzed. The study was carried out on samples obtained from an industrial plant and on samples coated in a hot dip process simulator. The mechanical properties of the steel substrate were evaluated, and the morphology and Fe content in the various phases of the coating and its characteristics were analyzed and compared. It was found that the Fe-Zn phases can be controlled and performed varying the temperature and holding time during the annealing cycles, avoiding the formation of undesirable phases.

Modelling oxidation and decarburisation for steel stock reheating

AbstractA one-dimensional finite difference model has been developed to compute the magnitude and concentration profile of the decarburised layer found when high carbon steels are reheated in oxidising atmospheres. It is assumed in the model (based on the explicit formulation of finite differences) that the rate at which carbon diffuses in the steel depends on both temperature and concentration. The model divides a piece of steel into cells of equal size before the computation proceeds. Oxidation of the steel is included in the model by assuming that growth of the oxide layer follows a temperature dependent parabolic regime. The two models are coupled by calculating the depth of oxidation first; the new thickness of the steel is obtained and is divided into a new set of cells; the concentration at the centre of each cell is calculated by interpolating the concentration at the centre of two older elements. The rate at which carbon is lost from the steel is calculated by the carbon content at the oxide side...

Forming analysis of a tubular connecting bolt

Abstract A visioplastic analysis was conducted on cold-forged samples which were part of the production of connecting bolts. Forming is carried out starting with a cylindrical blank that is pierced in two indirect extrusion passes to produce the tubular part. The strain gradients within each piece were determined using the flow lines of the material, put in evidence by metallographic means, as internal markers. The visioplastic analysis was complemented with microhardness indentation tests made on the pierced specimens and with compression tests made on specimens machined from an undeformed blank. It was found that the strain distribution was heterogeneous; the higher values of the normal components were located at the tubular walls of the pieces, whereas the higher values of the shear and equivalent components were found close to the changes in geometry of the specimens, where the material flow has to change its direction. A good correlation between the values of microhardness and equivalent strain distribution was observed.