| H | Chemical symbol for Hydrogen. |
| Hard Metal Facing | A method of increasing the wear resistance of a metal by the deposition of a hard protective coating. Alloys such as Stellite or a metallic carbide are most often used for the coating. |
| Hard Metals | A group of materials more commonly known as cemented carbides. They consist of mixtures of one or more of the finely divided carbides of tungsten, titanium, tantalum and vanadium embedded in a matrix of cobalt or nickel by sintering. Widely used for cutting tools where for many applications they have replaced conventional high speed steels. |
| Hardenability | The property that determines the depth and distribution of hardness when steel is heated to a given temperature and then quenched (more precisely it may be defined as an inverse measure of the severity of cooling conditions necessary to produce on continuous cooling a martensitic structure in a previously austenitized steel i.e. to avoid transformations in the pearlitic and bainitic ranges). The lower the cooling rate to avoid these transformations, the greater the hardenability. The critical cooling rate is largely a function of the composition of the steel. In general the higher the carbon content, the greater the hardenability, whilst alloying elements such as nickel, chromium, manganese and molybdenum increase the depth of hardening for a given ruling section. |
| Hardening | Increasing the hardness of steel by heat treatment. The steel is heated to a suitable austenitizing temperature, holding at that temperature for a sufficient time to affect the desired solution of carbon and other alloying elements, then quenching in a suitable medium, such as water, oil, air, polymer or molten salts to form martensite.
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| Hardness | The hardness of steel is generally determined by testing its resistance to deformation. A number of methods are employed including Brinell, Vickers and Rockwell. The steel to be tested is indented by a hardened steel ball or diamond under a given load and the size of the impression is then measured. For steel there is an empirical relationship between hardness and tensile strength and the hardness number is often used as a guide to the tensile strength, e.g. 229 Brinell = 772N/mm2 (50 tons/sq.in). |
| Heat | In steel making terms this is often used to define the batch or cast produced from a single melting operation. |
| Heat Treatable Alloy | An alloy that can be hardened by heat treatment. |
| Heat Treatment | A process where solid steel or components manufactured from steel are subject to treatment by heating to obtain required properties, e.g. softening, normalizing, stress relieving, hardening. Heating for the purpose of hot-working as in the case of rolling or forging is excluded from this definition. |
| High Speed Steel | The term `high speed steel' was derived from the fact that it is capable of cutting metal at a much higher rate than carbon tool steel and continues to cut and retain its hardness even when the point of the tool is heated to a low red temperature. Tungsten is the major alloying element but it is also combined with molybdenum, vanadium and cobalt in varying amounts. Although replaced by cemented carbides for many applications it is still widely used for the manufacture of taps, dies, twist drills, reamers, saw blades and other cutting tools. |
| Holding | The portion of the thermal cycle during which the temperature of the object is maintained constant. |
| Holding Temperature | The constant temperature at which the object is maintained. |
| Holding Time | Time for which the temperature of the object is maintained constant. |
| Homogenizing | An annealing treatment at fairly high temperature designed to eliminate or reduce chemical segregation. |
| Hooke's Law | This states that, "within the limits of elasticity the strain produced by a stress of any one kind is proportional to the stress." The stress at which a material ceases to obey Hooke's Law is known as the Limit of Proportionality. |
| Horizontal Batch Furnace | A versatile batch-type furnace that can give light or deep case depths, and because the parts are not exposed to air, horizontal batch furnaces can give surfaces almost entirely free of oxides. |
| Hot Quenching | Cooling in a medium, the temperature of which is substantially higher than room temperature. |
| Hot Work | The rolling, forging or extruding of a metal at a temperature above its recrystallisation point. |
| Hydrogen | An undesirable impurity if present in steel and a cause of fine hairline cracks especially in alloy steels. Modern vacuum treatment eliminates this problem. |
| Hydrogen Embrittlement | The brittleness induced in steel by the absorption of atomic hydrogen, most commonly from a pickling or plating operation. |
| Hyper-Eutectoid Steel | A steel that contains more than 0.83% carbon which with appropriate heat treatment consists of pearlite and cementite. |
| Hypereutectic Alloy | In an alloy system exhibiting a eutectic, any alloy whose composition has an excess of alloying element compared with the eutectic composition, and whose equilibrium microstructure contains some eutectic structure. |
