Metallurgical Terms
N
N
Chemical symbol for nitrogen.
Na
Chemical symbol for sodium.
Nb
Chemical symbol for niobium.
Negative Segregation
(See Inverse Segregation)
Neumann Bands
Narrow bands differently oriented within a grain of ferrite. Ordinarily
these bands are formed only on deformation by impact, but in some alloys
(silicon ferrite, for example), and particularly at low temperatures, the bands
are formed more readily, as in ordinary cold-working processes. It is suggested
that the Neumann band is primarily a shearing or faulting movement operating
along the pre-existent planar disjunctions of the mosaic structure, and as a
secondary operation, twinning may be completed, as is known to be possible in
ferrite.
Ni
Chemical symbol for nickel.
Nitriding
A process for producing a hard surface on steel by heating in contact with
partly dissociated ammonia or other suitable medium for periods of 10 to 90
hours at a temperature of about 500˚C. A hard surface can be obtained on steels
containing elements such as Al, Cr, Mo, V and W, which form stable nitrides. The
original Nitralloy steel range between 0•50 and 0•20% carbon, with about 1•5%
chromium, 1•10 aluminium and 0•20 % molybdenum, but other grades have been
developed which contain no aluminium and are characterized by a case of lower
hardness and greater ductility and toughness. Together, the various types of
Nitralloy steel embrace, when nitrided, a range of surface hardness from
600-1100 diamond number.
Noble Metals
Metals such as gold and platinum which are resistant to corrosion by all but
the most powerful acids, such as aqua regia, i.e., a mixture of 3 parts
hydrochloric acid and 1 part nitric acid.
Nodular Iron
(See Cast Iron)
Non-Ageing Steel
Low carbon steel to which is added an element (usually aluminium, less
commonly titanium or vanadium) which stabilizes the active nitrogen or carbon to
prevent strain ageing.
Non-Destructive Testing
Any form of testing which does not result in permanent damage or deformation
to the part being tested. Examples are ultrasonic inspection, X-ray inspection,
gamma radiography, magnetic crack detection and dye penetrant inspection.
Non-Magnetic Steels
Austenitic steels, such as 14% manganese, 25 % nickel, and 18/8
chromium-nickel steels, under normal conditions.
Normalizing
Heating to, and, if necessary holding at, a suitable temperature, i.e.,
about 50˚C above the transformation range followed by cooling in still air so
that moderately rapid cooling occurs, the object being to relieve internal
stresses, refine the grain size, render the structure more uniform, and improve
the mechanical properties. It is used specially for tempered, and to produce a
more uniform structure than exists in untreated pieces.
Notch Impact Value
(See Impact Test)