Metallurgical Terms
H
H
Chemical symbol for Hydrogen
Hair Line Cracks
(See Flakes)
Hammer Forging
(See Forging)
Hard Facing
A method of improving wear resistance by suitable application of a hard
protective coating to a metal. Examples are the deposition by welding of
Stellite or metal carbide.
Hard Metals (Cemented Carbides)
Powdered carbides of tungsten, tantalum or titanium, cemented into solid
masses by mixing with powdered cobalt or nickel, then compressing and sintering.
Used for cutting tools, wire-drawing dies and parts subjected to heavy wear or
abrasion.
Hardenability
The property that determines the depth and distribution of hardness induced
by quenching, i.e., it represents the resistance to transformation. It is
generally expressed in relative terms and is related to the critical cooling
rate, i.e., the rate at, or above which is wholly martensitic structures are
formed. This critical cooling rate, or hardenability, is largely a function of
composition, although steels of apparently similar composition can have
different harden-abilities and the same cast of steel may show fairly wide
variations. There is a limit to the section size which can be completely
hardened upon quenching. Plain carbon steels are shallow hardening, and alloy
additions increase the depth of hardening. The Jominy Test is a method of
assessing hardenability.
Hardenability Curve
(See Jominy Test)
Hardening
Increasing the hardness by heat treatment. This usually implies heating to a
temperature slightly above the cricial range, i.e., for hypo-eutectoid steels
above the Ac2 point and for hyper-eutectoid steels above the Ac1 point,
maintaining at that temperature until diffusion is complete, and by quenching in
water, oil or air, cooling at a rate sufficiently rapid to prevent or retard the
austenite-pearlite
transformation, and to form a martensitic or bainitic structure.
Hardening Crack
(See Quenching Crack)
Hardenite
A term, now obsolete, applied by Arnold to the solid solution of carbon in
iron in a eutectoid steel, i.e., containing 0•83 % carbon.
Hardness
Signifies in general, resistance to deformation. It is usually measured by
determining the resistance to indentation, as in the Brinell, Vickers, Rockwell
and Shore tests. The values of hardness obtained by the different methods are to
some extent related to each other, and to the ultimate tensile stress of
non-brittle metals. In Moh's scale comparative hardness is determined by testing
against ten standard minerals: (1) talc, (2) gypsum, (3) calcite, (4) fluorite,
(5) apatite, (6) orthoclase, (7) quartz, (8) topaz, (9) corundum, (10) diamond.
Thus, a mineral with "hardness 5" will scratch or abrade Auorite, but will be
scratched by orthoclase.
Hardometer
An instrument for measuring hardness by the indentation method.
Hatfield Time Yield
A short time creep test criterion. The specimen placed under the time-yield
stress should not show an extension exceeding O •50 % of the gauge length in the
first 24 hours, and during the next 48 hours should show no further extension,
within a sensitivity of measurement of 1/10,000th inch on a 2 inch gauge length,
which is approximately equivalent to a rate of creep of a millionth of an inch
per inch per hour during this period.
Heading
An upsetting process used to form rivet, screw and bolt heads from wire or
rod.
Heat
(a) A form of energy which when given to a body raises its temperature.
(b) The steel melting operation from the charging of raw materials to the
tapping of the molten metal.
(c) The batch of steel produced by a single melting operation.
(d) In forging, the period of working before reheating becomes necessary.
(e) The amount of forging stock heated up at one time.
Heat Treatment
A process in which steel in the solid state is taken through one or more
temperature cycles for the purpose of obtaining certain desired properties.
Heating for the sole purpose of hot-working is excluded from the meaning of this
definition.
Heyn's Reagent
An etching reagent containing 10% copper ammonium chloride in water.
High Frequency Induction Furnace
This is essentially an air transformer in which the primary is a
water-cooled spiral of copper tubing, and the secondary the metal being melted.
Currents at a frequency above about 500 cps are used to induce eddy currents in
the charge, thereby setting up enough heat in it to cause melting. The
electrical forces operating in the high frequency furnace keep the charge moving
rapidly in a vertical plane, and the steel is, therefore, effectively stirred.
This stirring ensures homogeneity in the melt and prevents gravitational
segregation. The slag being a bad conductor of electricity is not heated by
induction and, therefore, depends almost entirely upon its intimate contact with
the metal for maintenance of its temperature. Partly because of this it is
difficult to refine steel as in the arc furnace and the process is, in effect,
one of re-melting. In this respect it is analogous to the crucible process,
although, by generating the heat in the` charge, the possibility of
contamination of the metal from furnace gases is avoided.
High Speed Steel
A high alloy steel, capable of retaining a considerable degree or hardness
at a low red heat, and hence can be used in high speed cutting operations. It
usually contains 12-22 % tungsten, 4% Chromium, up to 1% Carbon and varying
amounts of other elements (vanadium, cobalt, etc.). Part or all of the tungsten
may be replaced by half its weight of molybdenum.
Hobbing
(a) A process for generating gear teeth by using a rotary tool with cutting
teeth set in the form of a helix. The hob and the work piece rotate so that the
teeth of the hob and the teeth cut in the gear are in mesh.
(b) The formation of an impression in a soft die by means of a hard master die.
The soft die is subsequently hardened and used to produce replicas of the master
die.
Hollow Forging
A method of producing hollow bodies, e.g., steel tubes or pressure vessels,
in which a comparatively small hole, trepanned or pierced in a solid forging, is
ex-
panded on a mandrel under a forging press or on a becking bar.
Homogenizing
A process of heat treatment at high temperature intended, by means of
diffusion, to decrease 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 producing it" The stress at
which a material ceases to obey Hooke's Law is known as the limit of
proportionality.
Hot Metal Process
See Basic Steel & Open Hearth Process)
Hot Quenching
A process of cooling in a medium, the temperature of which is substantially
higher than atmospheric.
Hot Shortness
Brittleness metal at certain ranges above normal room temperature. It is
manifested by loss in ductility and a liability to crack under stress at that
particular temperature.
Hot Tear
A crack formed in cast metal as a result of contraction at or just below the
solidus. It is characterized by being comparatively wide and having blunt
extremities.
Hot Top
(See Feeder Head)
Hot Work
The mechanical working, by rolling, forging or extruding, of a metal or
alloy at a temperature above its re-crystallization point. This temperature
varies according to the composition of the material.
Hyper-Eutectoid Steel
Steel with carbon content higher than eutectoid composition. With
appropriate heat treatment the typical structure is cementite and pearlite.
Hypo-Eutectoid Steel
Steel with carbon content lower than eutectoid composition. Typically in the
annealed condition the structure is ferrite and pearlite.
Hysteresis Curve
(See Magnetic Hysteresis Loop)