Characterizing steel tube for hydroforming applications

With the increased use of tubular steel products, especially for hydroforming applications, it is important to be able to predict the performance of tube from sheet tensile tests. In the present study, two aluminum killed draw quality (AKDQ) steels and one high strength low alloy (HSLA) steel were evaluated. Tensile properties and plastic strain ratios were measured on sheet material in the longitudinal and transverse directions. Axial tensile tests were performed on material extracted from production tubes. Material from quasi tubes, which are strip material bent to the same curvature as the tubes but not welded or sized, was also tested. Residual stresses in the production and quasi tube were determined by displacement methods. A hydraulic burst test was performed on the production tubes to simulate a hydroforming operation. Effective strains resulting from tubemaking are calculated for two discrete operations: bending and sizing. For the production tubes, a linear relationship was found between a load factor (strength times thickness) and effective sizing strain. The relationship between load factor and residual stress was also linear. Predictions of the maximum pressure and the strain at instability during a hydraulic burst test are shown to compare favorably with experimental values, based on flat sheet properties and tubemaking strains. The prediction of the yield strength in the tube based on flat sheet properties is shown to be fairly accurate when the effective sizing strain is small compared to the effective bending strain.     

Preparation and electrochemical study of cerium–silica sol–gel thin films

Design and development of suitable multilayered systems for delaying corrosion advance in metals requires that both the alteration mechanisms of the metal and the behaviour and properties of the protective coatings be known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coatings can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared starting from an organic–inorganic hybrid system. The precursors were γ-methacryloxypropyltrimethoxysilane (MAP) and tetramethoxysilane (TMOS). Cerium nitrate hexahydrate in three different concentrations was added. Cerium salts may perform a similar protective effect to that carried out by the well-known lead oxides and chromium salts, even though in this case a negative environmental impact is not expected. Application of coatings upon pure zinc substrates and common glass slides were performed by spinning. Coated samples were heat treated at 40 °C for 6 days. Optical measurements (UV-Vis absorption and diffuse reflectance spectroscopies) pointed out that the coatings were colourless and transparent, reducing the diffuse reflectance of the metallic surface up to 60%. Optical and scanning electron microscopies (SEM) allowed observation of the texture and microstructure of the coated samples, both before and after the corrosion tests were carried out. Likewise, the remaining sols were kept to gelify at 60 °C for 4 days and then powdered to obtain suitable samples for analysing them by other characterisation techniques (Fourier transformed infrared, FTIR and differential thermal analysis, DTA). Electrochemical measurements were performed by impedance spectroscopy. This technique was used to clarify the anticorrosive protection role of cerium ions incorporated into the hybrid sol–gel network. The effect of cerium concentration on the impedance spectra was analysed, as well as the system behaviour against the corrosive medium (0.6 M NaCl aqueous solutions), as a function of exposure time. From the electrochemical point of view, the sol–gel films behave as a conversion coating on the metallic surface.     

Microstructure and properties of TiB/Ti-6Al-4V coatings produced with laser treatments

TiB/Ti-6Al-4V metal-matrix composite (MMC) layers were produced on Ti-6Al-4V substrates by laser cladding. A TiB₂/Ti powder mixture was used as a precursor to obtain a dispersion of TiB needles in the Ti alloy matrix, with the aid of an exothermic reaction between TiB₂ and Ti. A eutectic microstructure was obtained that consisted of an extremely homogeneous dispersion of TiB eutectic needles in the Ti alloy matrix, having a volume fraction as high as 0.33. Also, an equilibrium-like microstructure was found, consisting of a dispersion of both primary and eutectic TiB needles inside the Ti alloy matrix. An analysis of the geometry of the layers was performed and proved successful in determining the percentage of B. Further, it correctly predicted the variation of atomic B content as a function of laser power. The relative wear resistance coefficient, defined as the wear coefficient of the uncoated matrix divided by that of coating, shows an improvement by a factor as high as 1500 for the eutectic microstructure. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appeared on pp. 411–18 of the Proceedings.     

Superplastic response in Al-Mg sheet alloys

The ability to achieve large strains to failure coupled with extremely low flow stresses makes superplastic forming (SPF) an attractive option in the automotive industry for the manufacture of complex parts from aluminum (Al) sheet. However, a barrier to increased usage is the cost penalty associated with superplastic alloys, which are specially processed to have a small and stable grain size. In this article, high-temperature tensile tests are used to compare the superplastic performance of two different Al-Mg alloys that were specially processed for SPF with that of a conventionally processed Al-Mg alloy. The results of the tensile tests and optical microscopy are used to highlight the mechanisms that control deformation in each of these alloys under different test conditions. Failure in both types of materials was found to change from internal cavitation to external necking with increases in strain rate. The specially processed alloys experienced minimal grain growth or grain elongation during forming, and therefore it was assumed that deformation was controlled by grain boundary sliding. Contrary to this, the conventionally processed alloy experienced significant grain growth at the higher test temperatures, and hence it was concluded that deformation was at least partially controlled by some mechanism other than grain boundary sliding. The different deformation characteristics resulted in a different set of optimal forming conditions for the two types of materials. The SPF alloys displayed higher strains to failure at the slower strain rates and higher temperatures, while the conventionally processed alloy displayed higher strains to failure at the faster strain rates and lower temperatures.     

High-strength metastable beta-titanium alloys for biomedical applications

This investigation has shown that the strength of low-modulus metastable beta-titanium alloys can be increased by increasing their oxygen content and/or aging. Yield strength as high as 1,288 MPa along with reasonable ductility was obtained by aging Ti-35Nb-7Zr-5Ta-0.7O at 482°C for 8 h. Strengthening of these alloys is discussed in terms of ω-and α-phase precipitates. For more information, contact H.J. Rack, Clemson University, School of Materials Science and Engineering, Clemson, SC 29634-0907; (864) 656-5636; e-mail rackh@ces.clemson.edu.     

A comparative study of hard turned and cylindrically ground white layers

Compared with grinding, hard turning is competitive in many cases, with substantial benefits. However, hard turning applications are not preferred, due to the existence of the process-induced white layer on the component surface, which is often assumed to be detrimental to component life. Nevertheless, white layer properties have not been well understood or clearly defined, especially the properties of the white layer induced in hard turning as against grinding. A clear understanding of white layer properties will provide a solid physics basis for product performance analysis and useful data for process selection. In this study, benchmark hard turning and cylindrical grinding experiments were conducted to generate thick white layers for reliable measurement. It was found that the properties of white and dark layers by hard turning and grinding are fundamentally different in four aspects: surface structure characteristics, microhardness, microstructures, and chemical composition. A white layer is not untempered martensite in terms of retained austenite. Additionally, a thick white layer can be produced in grinding under certain conditions.     

Thermal analysis of laser surface transformation hardening—optimization of process parameters

This paper deals with the optimization of process parameters for maximum productivity (given by the product of scanning velocity and cross feed) in laser transformation hardening. The process parameters considered are laser beam power, P; laser beam diameter, D_b; and the heat intensity distribution, namely, normal, bimodal, or uniform. A thermal analysis of the laser surface transformation hardening of gears was conducted (based on Jaeger’s classical moving heat source method) by considering the laser beam as a moving plane (disc) heat source to establish the temperature rise distribution in the workpiece (gear) of finite width. In a recent investigation [Int. J. Heat Mass Transfer 44 (2001) 2845], the authors considered the case of a heat source with a pseudo-Gaussian (or normal) distribution of heat intensity. The analytical results were compared with the experimental results published in the literature. In laser heat treatment of steel, it is generally considered preferable to use a wider heat intensity distribution, such as uniform or bimodal, for it enables more uniform case hardening depth. In this paper, this model is extended to cover bimodal and uniform distributions and compared with the normal distribution. Scanning velocities for no surface melting and for a case hardening depth of 0.1 mm were determined for surface transformation hardening of AISI 1036 (EN 8) steel for a range of laser beam powers, P, laser beam diameters, D_b, and various heat intensity distributions. Since diffusion during the heat treatment (surface transformation hardening) process is a time dependent phenomenon, based on the literature review, an interaction time of 15 ms was taken as a basis. It is hoped that laser industry with adequate facilities available can validate the thermal analysis and subsequent optimization presented in this paper.     

Impact damage detection in carbon fibre composites using HTS SQUIDs and neural networks

A neural network-based data analysis tool, developed to speed the damage detection process for the NDE of impact damaged carbon fibre composites, is discussed. A feature extraction method utilising a gradient threshold search function and a feed forward neural network for pattern recognition were used to develop the system. Impact damaged carbon composite sample plates were scanned with an eddy current-based NDE setup using HTS SQUID gradiometers and double-D excitation coils. Detection of damage sites in data affected by noise spikes caused by environmental disturbances is demonstrated. Finally, a possible design for a future entirely automated scanning system is also introduced.     

Influence of chromizing treatment on the corrosion behavior of AISI 316 stainless steel in supercritical water oxidation

SCWO, sometimes referred to as hydrothermal waste processing, uses the solvating traits of water in its supercritical condition to effectively destroy liquid organic wastes. One major problem in the supercritical water oxidation process is corrosion, because all metallic tubes in the process are exposed to high temperature and high pressure as well as severe corrosive species such as Cl⁻, F⁻, S²⁻, and O²⁻. The presence of Cl⁻ when the pH of a solution is very low and the solution has excess oxygen causes active corrosion and metal loss by metal-chloride and/or oxychloride formation. This study performed a chromizing treatment on 316 stainless steel and immersion tests in supercritical water. Weight change of chromized steels and untreated steels was measured, and the chemical state and composition of oxide films on 316 stainless steel were investigated. On the basis of SCWO tests using distilled water, the oxide layer was found to be very thin and homogeneous and weight gain was observed regardless of testing temperature, while the chromizing treatment slightly reduced weight gain. In the case of SCWO tests using salt water, weight loss was observed regardless of testing temperature and its corrosion mode was pitting by chloride ion, while chromizing treatment greatly decreased the corrosion rate.     

The Kinetics and Mechanism of Thermal Oxidation of Electrolessly Deposited Ni–B and Ni–W–B Alloys

The kinetics and mechanism of thermal oxidation of electrolessly reduced Ni–B and Ni–W–B alloys are studied by precision thermogravimetry and x-ray photoelectron spectroscopy. It is shown that the oxidation is mainly determined by properties of boron oxides. At temperatures over 300°C, the oxidation is accompanied by a sublimation of boron oxides formed. Traces of water and hydrogen in freshly deposited alloys affect significantly both the kinetics of the alloys oxidation and sublimation of the oxides. The vitreous oxide film well adsorbs water vapor and can be readily removed from the sample surface.