Sture Hogmark

Wear mechanisms and tool life of high speed steels related to microstructure

Abstract The wear of high speed steel cutting tools during milling, twist drilling, bandsawing and power hacksawing has been studied. Five major wear mechanisms were identified: (1) edge chipping and blunting; (2) abrasion; (3) mild adhesive wear; (4) severe adhesive wear; (5) continuous wear. The relative intensities of these five mechanisms were found to be determined by the particular combination of cutting parameters, work material and cutting operation being used. In contrast, a variation in the chemical composition or in the microstructure of the high speed steel tool material generally did not change the dominant wear mechanism. However, the tool material properties determine the resistance against the operating wear mechanism and, therefore, they determine the tool life. In this paper a discussion on how the cutting conditions affect the relative intensities of the different wear mechanisms is presented. Also, results from comparative tool material testing and metallographic analysis of worn tools are combined to give a qualitative description of the microstructural parameters that promote wear resistance against each individual wear mechanism. Taken together, this information can be used to provide guidelines for selection of the optimum high speed steel grade in a specific application.

Correlation between groove size, wear rate and topography of abraded surfaces☆

Abstract Surface profiling or scanning electron microscopy of worn surfaces always reveals a spectrum of wear scar sizes. Each size interval contributes to a certain fraction of the total wear rate and the ranking of different materials with respect to wear resistance may differ between different wear scar sizes. Knowledge of the correlation between, for example, the surface area fraction covered by different wear scar size intervals and their contribution to the wear rate is of interest for materials selection based on results from different wear tests and selecting test parameters for the simulation of a desired application. For this reason a numerical model for abrasive wear has been developed which provides a correlation between the resulting surface topography and the groove sizes of individual grooving events. Numerical examples are given for the different geometries of the abrading tip.

Metallographic aspects on wear of special brass

Abstract The influence of alloying with aluminium, manganese, silicon, tin and iron on the wear properties of special brass has been investigated, mainly in the region of severe wear. The surface layer developed during dry sliding against steel was characterized with respect to structure, composition and hardness. Irrespective of original structure and composition, a superficial extremely fine-grained layer of α phase is formed. Oxidation of β-phase-promoting elements causes the transition to the α phase. The grain size of this α phase is approximately 0.1 μm; the layer is much harder than the bulk material and has very special wear properties. Beneath there is a second layer mainly consisting of β phase with a fine grain size (2–5 μm) and an increased hardness relative to the bulk material. Additions of solution- and precipitation-hardening elements like tin, aluminium, manganese, silicon or iron have a positive effect on the resistance against wear, both in the mild and the severe regime. A comparison is made with a service-tested synchronizing ring with respect to the wear mechanisms and the resulting surface layer.

Force and Energy Measurements During Controlled Grooving—A Basic Study of Abrasive Wear

Basic studies of abrasive wear have been performed by controlled grooving in a modified impact tester equipped with a cemented carbide tip. Specimen holders were constructed to permit normal and tangential force measurements during grooving and to enable quick-stop tests. The grooving energy is read directly from the standard pendulum meter or integrated from tangential force curves. A series of metals were studied by single-tip grooving and the grooving energy was plotted versus weight loss W within a large W interval. Mettallographic studies reveal characteristic friction layers in the groove bottom and walls and also show that the development of these layers is governed by the mechanisms of chip formation. A particular purpose of this work is to find relations between internal structure and microhardness profiles on one hand and grooving forces/energy and wear resistance on the other. There are indications that the specific grooving energy e = E/W can be used to predict abrasive wear resistance under w...

Mechanisms of Material Removal During Erosion of a Stainless Steel

Solid particle erosion of an austenitic stainless steel was studied utilizing various metallographic techniques. Examination of single impacts on a polished surface resulted in a semiquantitative crater classification. It was, however, found that material removal generally involves the interactive effect of several cumulative impacts. Consequently, topography and internal structure of the target surface layer after multiple impacts were investigated. In particular, preeroded targets were reexamined after additional single impacts. The strength of the surface layer was estimated by a simple tape experiment, which also supplied information of the size and morphology of presumptive wear debris. Two major erosion mechanisms were distinguished: 1. Cutting erosion. Detachment of crater lips by cutting action of one or several impacts. 2. Deformation erosion. Detachment of material by surface fragmentation due to multiple cumulative impacts. Surface layer hardness and ductility are the most important material pr...

Adhesive mechanisms in the wear of some tool steels

Abstract The adhesive wear properties of a number of tool steels have been investigated, with special emphasis on the formation and action of prows. The test pieces were worn by dry sliding in a pin-on-ring machine. A special specimen preparation technique made it possible to study the internal structure of wear fragments and prows as a function of depth under the external surface. It was found that prows form in successive steps. The initial step has been reported earlier for pure metals and low carbon steel. A detailed explanation of material deterioration in adhesive wear has been worked out. The dominating mechanism was found to be an abrasive dead zone action, rather than the shearing off or destruction of prows. Investigations by X-ray and electron microscope techniques revealed extensive transformations between the austenite and martensite phases, an extremely fine grain size and a redistribution of carbide and oxide particles during the formation of prows. Correspondingly high microhardness values, up to 1600 HV 0.15N , were measured in the transformed material. Distortion of the retained austenite lattice contributes to the high hardness.

Prediction of gouging abrasion resistance of steel by pendulum grooving and other laboratory test methods

Abstract A comparative investigation is made of different laboratory test methods with respect to their ability to guide in the selection of materials resistant to gouging abrasive wear. The evaluation is based on the wear of excavator bucket teeth during loading of wet blast stone. Five different steel grades were included, ranging from quenched and tempered carbon steel to tool steel. The specific energy consumption during single-pass pendulum grooving proved to give the best correlation with the field test. It was also found that the pendulum test produced friction layers typical of gouging abrasion. Among the test materials a 5.3 wt.% Cr tool steel proved to have the best wear resistance followed by a quenched and tempered carbon steel grade.

An experimental method for studies of intermittent cutting at small cutting depths

Abstract Most devices for metal cutting experiments are designed to simulate continuous cutting at relatively large cutting depths. However, there is also a need for techniques to study the more complex situations prevailing in other important cutting operations like milling, sawing, hobbing, shaper cutting and grinding. These operations are characterized as being intermittent and having a relatively small and varying cutting depth per edge. In order to supply an experimental set-up for basic studies of chip formation and cutting forces under these conditions a new method for single stroke, single edge metal cutting has been developed. The experiments are performed in a modified Charpy pendulum which offers force measurement and accurate selection of cutting speed and feed in the ranges typical of many intermittent cutting operations. The equipment is also provided with an excellent quick-stop mechanism to aid in chip formation studies. The test method is described in detail and examples of metallographical and scanning electron microscopical studies of quick-stopped samples as well as registrations of specific thrust and cutting forces are presented.

Intermittent metal cutting at small cutting depths—2. Cutting forces

Abstract Cutting forces in intermittent metal cutting at small cutting depths were investigated by single edge experiments. Single cutting strokes were performed in a modified Charpy pendulum tester which offers cutting and thrust force measurement and accurate selection of cutting speed and feed in ranges typical for many intermittent high speed steel (HSS) tool operations. The cutting performance of a number of double rake HSS edges, with primary rake angles ranging from +20° (“parrot bill”) to −60°, all with a preground 0.1 mm flank length were tested in two steel grades (one plain carbon and one austenitic stainless). Some of the edge geometries were tested also in TiN coated condition. The relative performance of the different edges was investigated with respect to specific cutting and thrust forces. The influence of cutting length and depth, edge micro geometry, TiN coating and cutting speed is discussed specifically. Among the most important observations were: — The cutting and thrust forces at a fixed cutting depth may change significantly during the short (25–30 mm) cuts. — The chamfer formed by a double rake geometry with negative primary angle increases the forces. — For these chamfered tools the forces increase linearly with the projected flank length. TiN coating increases rather than reduces the forces during these short cuts. The relationships between the varied parameters and chip formation phenomena like dead zone formation, chip curl and surface finish were presented in part 1 of this paper.

A high temperature test rig for sliding and rolling wear

Abstract New equipment for wear testing at elevated temperatures has been constructed. Test samples in the shape of blocks or rollers are pressed against a rotating disc (500 mm in diameter) which can be heated to 900°C. The periphery velocity is continuously variable between 0 and 50 m s −1 . When testing rollers, the angle between the disc and the roller axis can be varied from 0 to 5° to obtain a desired interfacial sliding at the contact surface. It is possible to load and unload the test specimen periodically and to record the frictional force. The ability of the test to reproduce the wear behaviour of guide rollers used in hot rod rolling mills was evaluated for two materials, tool steel (AISI D2) and Ferrodur, a sintered steel containing titanium carbides. Stationary guides of one grey cast iron grade were also included in this comparison. The specimens have been tested against a disc of structural steel (AISI 1045). For a particular set of test parameters it was found that the surface structure of the laboratory specimens very much conforms with that of the service case aimed at. Typical wear characteristics are microabrasion by oxide particles, thermomechanical surface cracking and adhesion of the hot material to the specimen surface. Parameters like contact pressure, rolling and sliding distance between the guide and rod material are extremely difficult to control in a hot rod rolling mill. Therefore this test equipment is expected to be of great value for the selection and development of new materials in mill guides and for similar applications in sliding or rolling contacts at elevated temperatures.