A significant part of the energy in forging is used to break the interfacial junctions due to friction between the tool and the workpiece. The life of hot-forging tools is usually limited by complex interactive mechanisms under cyclic loading such as abrasive, adhesive and scaling wear, thermal and mechanical fatigue, and plastic deformation. This contribution deals with the wear mechanisms of the tempered martensitic X38CrMoV5 steel (AISI H11) under high-temperature and dry-sliding wear. The investigations are carried out with high-temperature pin-on-disc tests. The pin is cut from bars of X38CrMoV5 steel treated at 42 and 47 HRC. The disc is made of common steel (AISI 1018, XC18). Temperature of the disc ranges from 20 to 950 • C. Before the test starts, the disc is first pre-heated for 1 h. The experiments are performed under constant load and velocity. The friction coefficient decreases quasi-linearly with the rising disc temperature up to 800 • C. Over this temperature, it decreases drastically for the 42 HRC steel but remains linear for the 47 HRC steel. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) investigations have revealed that wear is essentially due to abrasion, plastic deformation and fatigue. Set of cracks due to contact rolling fatigue is observed on the pin and the disc. Those cracks are located on the transferred scale on the pin and on the oxide scale of the disc wear track. The cross-section observations of the pin have revealed a plastically deformed zone beneath the surface. In this sub-surface layer, the tempered martensitic microstructure seems to be more aligned due to friction and the plastic deformation.

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In a hot strip mill, the contact established between the hot strip and the work rolls in the first times of running has to be oxide on oxide to allow the strip to be pulled in the roll bite. The oxide scale formed on the roll is submitted to thermo-mechanical stresses and grows up. From a critical thickness, the scale spalls and causes some superficial damage to the rolls and to the strip.For the roll manufacturers as well as for the steel makers, it is essential to understand the influence of the creation and the growth of such a scale on friction in order to control the antagonist superficial damage and consequently to reduce the running cost of the mill.The present work aims to study the interaction between the oxides formed on a work roll grade and the coefficient of friction established with a strip steel usually rolled by this roll grade. A high temperature tribometer was set up in a pin-on-disc configuration. A previous part of study showed that friction was dependent on the nature of antagonist materials and the thermal transfer. We observed the establishment of a running-in period in the case of a metal-on-oxide initial contact between the pin and the disc which corresponds to the creation of an oxide layer on the pin.The mechanisms that allowed the formation of this scale were determined. SEM observations in conjunction with EDS analysis, both inside and outside the contact area on both antagonists, led to the development of a phenomenological model explaining the creation and the movement of oxide debris in the contact.

Mechanical engineering often uses fluid lubrication to limit friction. Even in the presence of a lubricant, metallic contact between the sliding surfaces may readily occur. In running-in, the oil film thickness can be so thin that contact arises at the summit of asperities, thus, increasing both the friction coefficient and wear. Such regimes are the so-called mixed or boundary lubrication. In these situations, the friction coefficient varies continuously and it is therefore, necessary to calculate the friction coefficient at any given moment.Given two rough parallel surfaces in a lubricated environment, a model is herein proposed whereby the friction coefficient is controlled by a single hydrodynamic parameter. This experimental analysis seeks to take into account a wide variety of factors influencing the conditions in which the contact operates: functional parameters (normal load, sliding speed, viscosity), the contact pair's mechanical properties (elastic modulus, Poisson's ratio) and surface microgeometry (as expressed by the standardised roughness parameters). This extended friction model predicts tribological behaviour in lubrication regimes: thus, by judicious choice of materials (according to the most appropriate mechanical properties and surface roughness) one can reduce the running-in period. To confirm the model, different material pairs have been tested. The tests have been conducted using a pin on disc apparatus in a 100 Neutral solvent (100 NS) oil at 20 • C. Experimental results totally confirmed the model.

The wear rate of sliding materials seems impossible to predict. It is clearly a function of substrate material, but we are learning that many surface events are also important. In particular, studies on the last 20 years have shown that substances develop between two sliding bodies have a strong influence on both friction and wear. These substances had become known as "third bodies". Whereas third bodies have been proven to be important, they have not been completely characterized. The third bodies have been studied, but it appears that the geometries of the sliding pairs are also important. This paper describes some experiments in which the composition of third-body materials are controlled by temperatures and in which the durability of third-body films are influenced by the geometry of contact between sliding bodies.

a b s t r a c tGiven the specific micro-structure of some steel grades, under tribological conditions the sub-surface material or sub-layers of sliding bodies are prone to cumulative cyclic plastic deformation, leading to the formation and emission of wear debris.In the present paper, a new wear model based on a cyclic ratchetting-type plastic deformation of subsurface material is proposed. It is considered that the debris is formed and the wear-loss occurs when the accumulated plastic deformation at sub-surface exceeds "a critical strain" or "rupture limit". The model takes into account the number of cycles or test duration, a characteristic thickness of the sub-layer dependent on tribological conditions and material properties, the shear rupture ductility and an average plastic strain increment. The average plastic strain increment is estimated by numerical simulation of pinon-disc friction. A very close correlation is found between the predicted and experimental wear heights versus time and/or versus the number of cycles.The wear investigations were carried out on a high-temperature pin-on-disc tribometer under dry friction conditions. Experiments were performed under constant load, speed and disc temperature for different durations. The steel grade involved was a tempered martensitic tool steel X38CrMoV5 (AISI H11). Wear mechanisms were investigated by Scanning Electron Microscopy (SEM) observations in surface and cross-section.

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An earlier brief phenomenological study (1) allowed the authors to divide the period of synchronization into three distinct stages of operation. Each stage corresponded to different types of friction. Expressions establishing estimations of pressure, axial stress, torque and coefficient of friction were made for the case of a smooth synchromesh ring. These all depend upon the film thickness and angular velocities whose variations in time are unknown. The dynamical study presented here permits the solution to this problem. The influence of circumferential and radial grooves on different parameters (distribution of pressure on the contact area, the load borne by the film, torque transmitted by viscous friction and coefficient of friction) is presented and a comparison of the efficiencies of these two types of grooves is made. This study allows the determination of the initial values necessary for the study of the second stage, which takes place in the presence of a variable friction.