P. Psyllaki

Failure modes of liquid nitrocarburized and heat treated tool steel under monotonic loading conditions

The tensile properties and failure mode of heat treated and liquid nitrocarburized tool steels were studied. The tested steels are used as die and tool materials for plastic molds and punching/blanking dies, where wear resistance is required. In addition to intense friction, the main die block and other die components are subjected to tensile and repetitive stresses during operation (tension and fatigue loading). Therefore, hardness, tensile, and fatigue resistance are also critical quality parameters that contribute to material reliability and tool life. However, this study is an initial component of research and does not include fatigue data.

Anti-Wear TiC-Based Surface Layers Using Concentrated Solar Energy

The present study aimed to investigate the exploitation of Concentrated Solar Power (CSP) for the in-situ production of anti-wear TiC-based surface layers onto steel substrates. For this purpose, “solar” experiments were carried out at the installations of Plataforma Solar de Almería (Spain). TiC powder was pre-deposited onto carbon steel substrates that have been subsequently exposed to concentrated solar irradiation for various dwell time values. The temperature field developed at the specimens’ surface during solar exposure was recorded in real-time by the aid of an optical pyrometer. Microscopic observations revealed the characteristic microstructure features of the hardfacing surface layers obtained and preliminary dry sliding tests performed in a pin-on-disc apparatus, allowed to estimate their machinability using conventional cutting tools.

Progressive failures of components in chemical process industry: Case history investigation and root-cause analysis

Abstract The increased significance and value of the multi- and interdisciplinary failure analysis field arises from the benefits incurred in terms of insurance costs, quality improvement, and prevention or minimization of environmental and safety risks. Especially in the chemical industry sector, the preventive role of failure analysis is of paramount importance, since the criticality and risk of an unfolding failure event provides a potential of huge safety and/or environmental hazards (chemical accidents, explosions, environmental-type leakages, etc.). Progressive-type failures are considered as the most insidious damage mechanisms which are very often difficult to detect and predict. Aggressive service conditions prevailing in chemical process plants, such as elevated temperature and pressure and corrosive environments, stretch the safety operating limits of engineering materials and components at challenging levels of endurance and reliability. The current chapter presents the investigation methodology of characteristic failures of chemical and process plants’ components, based mainly on case history investigation approach. A brief introduction addresses the progressive failure modes, focusing mainly on creep and stress corrosion cracking (SCC) in special steel structural members. Next, the results of the investigation of two respective case histories are thoroughly analyzed and discussed. The evolution of creep damage is presented in the case of a steel pipe operating in a power plant together with the discussion of the influence of operation parameters. The SCC mechanism is addressed through the catastrophic failure of a stainless steel screen (grid) used in a naphtha production unit in petrochemical industry. Visual inspection, optical microscopy, and scanning electron microscopy coupled with energy dispersive X-ray microanalysis were used as the principal analytical techniques for the case history failure investigation.

Failure Mechanisms of an Automobile Clutch Assembly Cast Iron Pressure Plate

The present study is focused on the failure mechanisms of an automobile clutch disc/pressure plate assembly due to abnormal operation of a passenger vehicle. Terminal malfunction of the part took place about one year after an accidental overloading incident. This component operates under sliding friction conditions, during which the pressure plate is pressed against the clutch disc with normal load that varies in a non-predictable manner, depending on the driving conditions. Commonly, the clutch disc is considered as “consumable” part that can be replaced when malfunction occurs, whilst the metallic pressure plate is considered to remain intact. However, in this particular case, visual inspection, macro-examination and microscopic observation using principally light and scanning electron microscopy of such a cast iron pressure plate revealed its severe long-term progressive failure, through extensive deep grooving and oxidation of the surface and the development of subsurface shear cracks.

Tribological Behaviour of Gradient TiAlSiN Superhard Coatings

The present study addresses the influence of the gradient microstructure of nanocrystalline TiAlSiN coatings on their tribological behaviour. Cathodic arc deposition was applied to elaborate such coatings, with a total thickness of 3.5 μm, onto stainless steel substrates. Their microstructure has been characterised via Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) and has been reported in detail previously. Since the main application of TiN-based coatings is the enhancement of the anti-wear resistance of metallic substrates, this work is focused on the tribological performance of gradient TiAlSiN coatings under dry sliding conditions. For this purpose, tests were carried out in a ball-on-disc apparatus, using an alumina ball as counterbody. The influence of the sliding velocity on the evolution of the friction coefficient and the wear lifetime of the gradient coatings has been evaluated in comparison to those of TiN coatings of the same thickness, tested under the same experimental conditions. It was found that the gradient microstructure results in an increase of the coatings’ mean lifetime by an average factor of three.

Theoretical Study of the Oxidation Behavior of Precipitation Hardening Steel

The oxidation of precipitation hardening (PH) steels is a rather unexplored area. In the present work an attempt is made is made to estimate the kinetics of a PH steel. For this purpose specimens of the material under examination were isothermally heated at 850, 900 and 950° C for 15 hr. Kinetics was based on TGA results. During heating a thick scale is formed on the substrate surface, which is composed by different oxides. The layer close to the substrate is compact and as a result it impedes corrosion. The mathematical analysis of the collected data shows that the change of the mass of the substrate per unit area versus time is described by a parabolic law.

CRYSTAL NUCLEATION IN PLASMA DEPOSITED DLC COATINGS DURING ANNEALING

Diamond‐like carbon (DLC) films, hard carbon coatings, with unique physical and mechanical properties which approach those of natural diamond, such as high hardness, low coefficient of friction and chemical inertness. In several applications, heavy loads and high friction forces are generated and lead to local temperature increase. In such cases these coatings must be thermal stable and with enhanced high temperature oxidation resistance in order to be good candidates for wear protection of metallic components. In the present study a radio frequency plasma deposition system was used for the deposition of 2 μm‐thick amorphous DLC coatings onto AISI D2 substrates. The as deposited DLC covered samples were dense, homogeneous and well bonded to the substrate, while no cracks were observed. In order to study the thermal stability of the coatings’ DLC nature, in‐situ Transmission Electron Microscopic (TEM) observations were carried out during slow annealing of the specimen in the microscope vacuum chamber, as w...

Surface Modification of Nickel Foams by a Slurry Aluminizing Process

A novel slurry‐based process for aluminizing nickel foams while improving the mechanical properties and conserving the excellent ductility is reported. Cellular unalloyed nickel foams with 92% porosity and uniform pore size and distribution were used as a starting material. Several slurries of different compositions were examined to investigate the possibility of developing an aluminide‐nickel intermetallic coating on a Ni foam without considerably degrading the original ductile properties of the foam. The process temperature was varying from 400 to 850° C and the process holding time was ranging between 2h to 6h. Scanning electron microscopy with an energy dispersive X‐ray spectrometry and X‐Ray diffraction were applied to assess the effectiveness of the aluminizing process and determine both the optimum parameters of the procedure (slurry composition, holding temperature and time) and the concentration profiles across the coating cross‐section. The mechanical behavior of the aluminized Ni‐foams was eval...