Toughness variations during the tempering of a plain carbon, martensitic steel

The aim of this paper is to present results of toughness measurements and fractographic observations made on a quenched and tempered 0. 6%, carbon steel and to use these as a basis for a discussion of the factors affecting "623K (350 °C) embrittlement" (one-step temper embrittlement).

The Mechanics of the drawing of polygonal sections from round at elevated temperature

Economic factors such as the rise in cost of raw materials, labour and power, are compelling manufacturers of cold-drawn polygonal sections, to seek new production routes which will enable the expansion in the varieties of metals used and the inclusion of difficult-to-draw materials. One such method generating considerable industrial interest is the drawing of polygonal sections from round at elevated temperature. The technique of drawing mild steel, medium carbon steel and boron steel wire into octagonal, hexagonal and square sections from round at up to 850 deg C and 50% reduction of area in one pass has been established. The main objective was to provide a basic understanding of the process, with particular emphasis being placed on modelling using both experimental and theoretical considerations. Elevated temperature stress-strain data was obtained using a modified torsion testing machine. Data were used in the upper bound solution derived and solved numerically to predict drawing stress strain, strain-rate, temperature and flow stress distribution in the deforming zone for a range of variables. The success of this warm working process will, of course, depend on the use of a satisfactory elevated temperature lubricant, an efficient cooling system, a suitable tool material having good wear and thermal shock resistance and an efficient die profile design which incorporates the principle of least work. The merits and demerits of die materials such as tungsten carbide, chromium carbide, Syalon and Stellite are discussed, principally from the standpoint of minimising drawing force and die wear. Generally, the experimental and theoretical results were in good agreement, the drawing stress could be predicted within close limits and the process proved to be technically feasible. Finite element analysis has been carried out on the various die geometries and die materials, to gain a greater understanding of the behaviour of these dies under the process of elevated temperature drawing, and to establish the temperature distribution and thermal distortion in the deforming zone, thus establishing the optimum die design and die material for the process. It is now possible to predict, for the materials already tested, (i) the optimum drawing temperature range, (ii) the maximum possible reduction of area per pass, (iii) the optimum drawing die profiles and die materials, (iv) the most efficient lubricant in terms of reducing the drawing force and die wear.

The behaviour of polymer quenchants

The internationally accepted Wolfson Heat Treatment Centre Engineering Group test was used to evaluate the cooling characteristics of the most popular commercial polymer quenchants: polyalkylene glycols, polyvinylpyrrolidones and polyacrylates. Prototype solutions containing poly(ethyloxazoline) were also examined. Each class of polymer was capable of providing a wide range of cooling rates depending on the product formulation, concentration, temperature, agitation, ageing and contamination. Cooling rates for synthetic quenchants were generally intermediate between those of water and oil. Control techniques, drag-out losses and response to quenching in terms of hardness and residual stress for a plain carbon steel, were also considered. A laboratory scale method for providing a controllable level of forced convection was developed. Test reproducibility was improved by positioning the preheated Wolfson probe 25mm above the geometric centre of a 25mm diameter orifice through which the quenchant was pumped at a velocity of 0.5m/s. On examination, all polymer quenchants were found to operate by the same fundamental mechanism associated with their viscosity and ability to form an insulating polymer-rich-film. The nature of this film, which formed at the vapour/liquid interface during boiling, was dependent on the polymer's solubility characteristics. High molecular weight polymers and high concentration solutions produced thicker, more stable insulating films. Agitation produced thinner more uniform films. Higher molecular weight polymers were more susceptible to degradation, and increased cooling rates, with usage. Polyvinylpyrrolidones can be cross-linked resulting in erratic performance, whilst the anionic character of polyacrylates can lead to control problems. Volatile contaminants tend to decrease the rate of cooling and salts to increase it. Drag-out increases upon raising the molecular weight of the polymer and its solution viscosity. Kinematic viscosity measurements are more effective than refractometer readings for concentration control, although a quench test is the most satisfactory process control method.

Corrosion mechanisms of duplex stainless steels in the petrochemical industry

The recent search for new sources of hydrocarbons has led to production from very severe environments which can contain considerable amounts of carbon dioxide, hydrogen sulphide, and chloride ions, combined with temperatures which can exceed 100°C. Oil and gas production from such wells requires highly corrosion-resistant materials. The traditional solution of using carbon steel with additional protection is generally inadequate in these very-aggressive environments. Duplex stainless steels (DSS) are attractive candidates because of their high strength, good general corrosion resistance, excellent resistance to chloride-induced stress corrosion cracking, and good weldability. Although duplex stainless steels have a very good reputation in both subsea and topsides pipework, it is recognized that the tolerance of these materials to variations in microstructure and chemical composition are still not fully understood. The object of this paper is to review the corrosion behaviour of duplex stainless steels in the petrochemical industry, with particular emphasis on microstructures and the effect of changes in chemical composition.

The influence of a coal gasifier atmosphere on fatigue crack growth rates in BS 4360 steel

Fatigue crack growth tests have been carried out in a number of gaseous environments in order to assess their effects on the crack propagation resistance of BS 4360 grade 50EE, a weldable structural steel. Crack growth rates at 25 °C are up to 20 times higher in hydrogen than in air, but there is no effect when hydrogen is present as a 30% constituent of a simplified product gas (SPG). Indeed, crack growth rates in such a mixture are slightly lower than those measured in air, being comparable with those observed in an inert environment. The other gases present in the SPG are CO, CO2 and CH4, and it is probable that the carbon monoxide is responsible for nullifying the embrittling effects of hydrogen, by preferentially adsorbing on to the surface of the steel and thus blocking hydrogen entry. Experimental observations suggest that oxygen has the same effect when small quantities are allowed to diffuse into a non-flowing hydrogen environment around a propagating crack. The results are encouraging in terms of the suitability of conventional structural steels such as BS 4360 for gas plant applications. The gas mixtures present in such an environment would not have the severe detrimental effects on fatigue crack growth resistance which result from the presence of 'pure' hydrogen. © 1993.

A novel Mo-W interlayer approach for CVD diamond deposition on steel

Steel is the most widely used material in engineering for its cost/performance ratio and coatings are routinely applied on its surface to further improve its properties. Diamond coated steel parts are an option for many demanding industrial applications through prolonging the lifetime of steel parts, enhancement of tool performance as well as the reduction of wear rates. Direct deposition of diamond on steel using conventional chemical vapour deposition (CVD) processes is known to give poor results due to the preferential formation of amorphous carbon on iron, nickel and other elements as well as stresses induced from the significant difference in the thermal expansion coefficients of those materials. This article reports a novel approach of deposition of nanocrystalline diamond coatings on high-speed steel (M42) substrates using a multi-structured molybdenum (Mo) - tungsten (W) interlayer to form steel/Mo/Mo-W/W/diamond sandwich structures which overcome the adhesion problem related to direct magnetron sputtering deposition of pure tungsten. Surface, interface and tribology properties were evaluated to understand the role of such an interlayer structure. The multi-structured Mo-W interlayer has been proven to improve the adhesion between diamond films and steel substrates by acting as an effective diffusion barrier during the CVD diamond deposition.