Metallurgical Terms
M
McQuaid Ehn Grain Size Test
A method of assessing austenitic grain size which was first developed to
determine the cause of soft spots on case hardened steels. A specimen of steel
from which all scale has been removed is carburized in a solid medium for 8
hours at 925˚C. The site of the pearlite crystals, which indicates the size of
the original austenite grain, is easily seen by the surrounding membranes of
excess Fe3C (cementite). The grain size is measured at 100 diameters and
compared with standard charts. The figures range from No.1, very coarse, to
No.8, very fine.
Macrograph
A low power reproduction, usually by photography, of the coarse structure
of, for example, a metal as shown by a polished and, as a rule, etched section
or surface.
Macrostructure
The general crystalline structure of a metal or alloy and the distribution
of impurities as seen on polished and etched surfaces, either by the naked eye
or under magnifications of less than 10 diameters.
Magnesia (MgO)
Important basic refractory, the main constituent of calcined magnesite. Used
for furnace hearths and linings.
Magnetic Change Point
The temperature at which iron becomes non-magnetic on heating (Ac2) and
conversely that at which it becomes magnetic on cooling (Ar2). This temperature,
also known as the Curie or magnetic transformation point, in pure iron is 770˚C.
Magnetic Crack Detection
The part to be examined is magnetized either by passing a heavy current
through it or by making it the core of a coil through which a heavy current is
passed. Cracks, or non-magnetic phases such as inclusions, cause the magnetic
flux to break the surface thus forming free magnetic poles. When the part is
sprayed with a suspension of iron oxide particles, they collect at the free
poles and thus show the presence of defects.
Magnetic Flux (φ)
A term used to denote the amount of magnetism induced in a piece of magnetic
material when it is placed in a magnetizing field.
Magnetic Flux Density (B) (Magnetic Induction)
The normal magnetic Bur induced by a magnetizing force H, measured in lines
or gauss per sq cm.
Magnetic Hysteresis Loop
A closed figure formed by plotting the magnetizing force (H) against the
magnetic flux density (B) when the former is taken through a complete cycle
after initial magnetization, the magnetic hysteresis loss being proportional to
the area of this loop.
Magnetic Induction
(See Magnetic Flux Density (B)).
Magnetostriction
The change in dimensions of magnetic materials brought about by
magnetization or conversely the changes in magnetic properties brought about by
strain.
Malleability
The property which enables a metal to be mechanically deformed under
compression, as in hammering or rolling into thin sheets without cracking. (See
Ductility).
Malleable Cast Iron
(See Cast Iron)
Malleablizinp.
A process of producing malleable cast iron.
Maraging Steel.
The name given to a group of high nickel alloy steels which after solution
treatment consist of martensite which can be age hardened by a low temperature
treatment. Because the steels have low carbon content the martensite initially
formed is soft and can be readily machined. Ageing takes place without
significant change in dimensions so that little or no finishing is required
subsequently.
Martempering.
In this method of heat-treatment, the ideal is to quench the part at such a
rate that it reaches the Ms temperature in the fully austenitic condition. After
equalization of temperature, the part is cooled slowly, so that the temperature
gradients are negligible and the resulting thermal stresses are at a minimum. In
practice, this ideal is only approached in so far as it is necessary to prevent
cracking, reduce distortion and to obtain a given hardness. As the Ms point is
usually in the region of 250˚C, the quenching must be done in a salt or metal
bath. The quenching and time available for the equalization of temperature
limits the section size that can be successfully martempered to somewhat less
than that permissible with conventional quenching. Similarly, the minimum carbon
or alloy content for a given section is higher. The outstanding advantage of
martempering is the reduction of thermal stresses and consequently the
prevention of cracking and the minimizing of distortion.
Martensite.
A microconstituent of steel characterized by its acicular structure.
Essentially a non-equilibrium condition of alpha iron formed directly from under
cooled austenite. It is produced typically when steel is cooled from the
hardening temperature at a speed greater than its critical cooling rate so that
the transformation of austenite occurs at 400˚C or below. It is the hardest of
the decomposition products of austenite and is brittle but when tempered becomes
softer and tough.
Martins-Siemen Process
(See Open Hearth Furnace)
Mass Effect
This term is commonly employed to signify the effect of size and shape
during heat treatment, since it is the rate of cooling of a piece of steel which
determines the properties resulting from a hardening or quenching process.
Matrix
As used in metallography the term applies to the principal constituent in
which the other constituents ate embedded.
Maximum Stress
(See Tensile Strength).
Meehanite
A trade name applied to certain pearlitic cast irons in which the molten
metal has been treated with calcium silicide resulting, it is claimed, in
increased tensile strength.
Melting Point (Fusion Point).
The temperature at which a solid begins to liquefy. Pure metals, eutectics
and some inter-metallic constituents melt at a specific temperature. Alloys
generally melt over a range.
Mereast Process
(See Precision Casting)
Mesnager Notch.
(See Charpy Test)
Metal-Are Welding
(See Welding)
Mf
(See Transformation Temperature)
Mg
Chemical symbol for magnesium
Microhardness
Hardness as measured with an indenter under the application of a low load,
usually between 1 gram and 200 grams. Owing to the small size of the impression,
the hardness of an individual phase in a microstructure can be determined.
Micron
A unit of length equal to a millionth of a metre (0•001mm). It is used for
expressing minute distances and is denoted by the Greek letter μ
Mischmetal
An alloy consisting largely of rare earth metals produced by the
aluminothermic reduction of purified rare earth oxides. The major constituent is
cerium, with lanthanum the second most important element. These two generally
for approximately 90 % of the alloy.
Mixed Crystals
An obsolete term synonymous with Solid solutions.
Mixer
A large furnace used as a reservoir for molten pig-iron coming from the
blast furnace. The product of several furnaces is thus mixed, and the
composition can be regulated by making suitable additions. It is used in
connection with hot-meta steelmaking and direct casting of pig iron.
M.m.f
Magneto-motive force.
Mn
Chemical symbol for manganese.
Mo
Chemical symbol for molybdenum.
Modulus of Elasticity
(See Young's Modulus)
Modulus of Rigidity
In a torsion test, the ratio of the unit shears stress to the displacement
caused by it per unit length in the elastic range. This modulus corresponds to
the modulus of elasticity in the tension test.
Mohs' Scale
(See Hardness)
Mend Gas.
The gas produced by passing air and a large excess of steam through
incandescent coal.
Monel Metal.
An alloy containing typically 67 % nickel, 30 % copper, 1•4 % iron, 1%
manganese, and 0•15% carbon.
Mottled Iron
Medium silicon pig irons in which about half the total carbon is in the form
of graphite, and the remainder in the combined form. Graphitization commences
from a series of centres and the graphite then exists in star-shaped masses
which give rise to the characteristic mottled fractures. This material is hard,
brittle and practically un machinable.
Mould
(a) In the foundry, the cavity of desired shape, usually of sand, into which
molten metal is poured.
(b) (See Ingot Mould).
Ms Point.
(See Transformation Temperature).