Nitriding Process and Nitriding Steels...

Nitriding Process and Nitriding Steels  According to DIN EN 10052:1994-01, nitriding is defined as the thermo-chemical treatment of a work piece in order to enrich the surface layer with nitrogen. Carbo-nitriding involves enriching the surface layer with nitrogen and carbon. The nitriding process, which was first developed in the early 1900s, continues to play an important role in many industrial applications. It often is used in the manufacture of aircraft, bearings, automotive components, textile machinery, and turbine generation systems. It remains the simplest of the case hardening techniques. The basic of the nitriding process is that it does not require a phase change from ferrite to austenite, nor does it require a further change from austenite to martensite. In other words, the steel remains in the ferrite phase (or cementite, depending on alloy composition) during the complete procedure. This means that the molecular structure of the ferrite (bcc) does not change its configuration or grow into the face-centered cubic (fcc) lattice characteristic of austenite, as occurs in more conventional methods such as carburizing. Also, since only free cooling takes place, rather than rapid cooling or quenching, no subsequent transformation from austenite to martensite occurs. Again, there is no molecular size change and, more importantly, no dimensional change, only slight growth due to the volumetric change of the steel surface caused by the nitrogen diffusion. What can (and does) produce distortion are the induced surface stresses being released by the heat of the process, causing movement in the form of twisting and bending. The purpose of nitriding is to enrich the surface layer of a work piece with nitrogen in order to increase the hardness in the surface. The process of nitriding takes advantage of the low solubility of nitrogen in the ferritic crystal structure...

Austenitic Manganese Steel...

Austenitic Manganese Steel The first austenitic manganese steel was developed in 1882 by Robert Abbott Hadfield. Hadfield had done a series of test with adding ferro-manganese containing 80 % manganese and 7 % carbon to decarbonised iron. Increasing manganese and carbon contents led to increasing brittleness up to 7.5 % manganese. At manganese contents above 10 % however, the steel became remarkably tough. The toughness increased by heating the steel to 1000 deg C followed by water quenching, a treatment that would render carbon steel very brittle. The alloy introduced commercially contained 1.2 % carbon (C) and 12 % manganese (Mn) in a ratio of 1:10. This composition is used even today, and the austenitic manganese steel is still known as Hadfield steel. The steel was unique since it exhibited high toughness, high ductility, high work hardening ability and excellent wear resistance. Because of these properties Hadfield’s austenitic manganese steel (AMS) gained rapid acceptance as a useful engineering material. Austenitic manganese steels have a proven high resistance to abrasive wear including blows and metal-to-metal wear, even though they have a low initial hardness. These steels are supposed to work harden under use and thus give a hard wear resistant surface, but it has been reported that these steels have a good wear resistance in components even without heavy mechanical deformation. Hadfield`s austenitic manganese steel is still used extensively, with minor modifications in composition and heat treatment, primarily in the fields of earthmoving, mining, quarrying, oil well drilling, steelmaking, railroading, dredging, lumbering, and in the manufacture of cement and clay products. Austenitic manganese steel is used in equipment for handling and processing earthen materials (such as rock crushers, grinding mills, dredge buckets, power shovel buckets and teeth, and pumps for handling gravel and rocks). Other...

Tool Steels

Tool Steels The term tool steel is a generic description for those steels which have been developed specifically for tooling applications. These steels are used for making tools, punches and dies etc. Tools used for working steels and other metals must be stronger and harder than the steels or the materials they cut or form. Normally tool steels are known for their distinctive toughness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red hardness). Some of the operations that tool steels are used for include drawing, blanking, mould inserts, stamping, metal slitting, forming and embossing, although their use is not limited to just these areas. The metallurgical characteristics of various compositions of tool steels are extremely complex. There are hundreds of different types of tool steels available and each may have a specific composition and end use. Tool steels are mainly medium to high carbon steels with specific alloying elements added in different amounts to provide it special characteristics. The carbon in the tool steel is provided to help harden the steel to greater hardness for cutting and wear resistance while alloying elements are added to the tool steel for providing it greater toughness or strength. In some cases, alloying elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness to it so that the tool retains its hardness and strength when it becomes extremely hot. Various alloying elements in addition to carbon are chromium (Cr), cobalt (Co), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and vanadium (V). The effect of the alloying elements on the properties of tool steels is as follows. Chromium –...

Wear Resistant Structural Steels...

Wear Resistant Structural Steels Wear is described as ‘the phenomenon of metal surfaces that are moving relative to each other getting worn out due to the surfaces scratching each other or due to metallic adhesion’. Wear resistance can be said to be the property in which such a phenomenon is difficult to occur. The properties of wear resistant steels enable them to resist wear, due to rubbing, impact or compressive loads from external agents such as cement, sand, stones etc., and are intended for use in equipment construction and for replacement of wearing parts. Numerous structures, such as dump bodies, materials handling equipment and crushing machines, for instance, are exposed to continuous, abrasive and impact wear, which is costly. As a solution, special structural steels have been developed that are highly resistant to wear and abrasion. Factors affecting wear resistance of steels There are four main factors which have considerable effect on the wear resistance of steels. These are (i) heat treatment, (ii) alloying additions, (iii) influence of carbon content, and (iv) effects of carbides, both primary and secondary. A big factor affecting wear resistance is ‘hardness’. In general, the wear resistance increases as the material becomes harder. There is a direct relationship between hardness and wear resistance. The resistance of a steel surface against wear is primarily a function of the ‘effective hardness’ resulting from the destructive action of the abrasive particles and depend on the strain hardening rate of the steel under the applied conditions. Factors affecting plastic deformation, such as grain size, recrystallization temperature, hardness, strain rate etc. also affect the wear of steels. Unlike single crystals which have free boundaries, the grains of a polycrystalline steel are influenced by their neighours during deformation, their constraining action on deformation is least...