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H
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Chemical symbol for Hydrogen. |
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Hard Metal Facing
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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. |
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Hard Metals
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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. |
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Hardenability
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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. |
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Hardening
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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
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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). |
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Heat
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In steel making terms this is often used to define the batch or cast
produced from a single melting operation. |
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Heat Treatable Alloy
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An alloy that can be hardened by heat treatment. |
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Heat Treatment
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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. |
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High Speed Steel
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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. |
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Holding
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The
portion of the thermal cycle during which the temperature of the
object is maintained constant. |
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Holding Temperature
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The constant temperature at which the object is maintained. |
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Holding Time
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Time for which the temperature of the object is maintained constant. |
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Homogenizing
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An annealing treatment at fairly high temperature designed to
eliminate or reduce chemical segregation. |
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Hooke's Law
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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. |
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Horizontal Batch Furnace
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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. |
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Hot Quenching
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Cooling in a medium, the temperature of which is substantially
higher than room temperature. |
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Hot Work
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The rolling, forging or extruding of a metal at a temperature above
its recrystallisation point. |
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Hydrogen
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An undesirable impurity if present in steel and a cause of fine
hairline cracks especially in alloy steels. Modern vacuum treatment
eliminates this problem. |
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Hydrogen Embrittlement
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The brittleness induced in steel by the absorption of atomic
hydrogen, most commonly from a pickling or plating operation. |
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Hypereutectic Alloy
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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. |
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Hyper-Eutectoid Steel
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A steel that contains more than 0.83% carbon which with appropriate
heat treatment consists of pearlite and cementite. |
