METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 2
However, closer observation will reveal a wavy motion of some of the
nickel streaks while the stellite streaks are all straight. Streaks
from these last three metals taper into extinction.
Identification of Sparks. An important aspect in spark testing
is a good set of specimens of known composition. These can serve as
a standard for learning to identify the various types of sparks and
When a spark
stream is studied carefully, it becomes apparent that it consists of
a large number of sparks of various lengths which look alike except
for size. A spark is understood to mean not only the main shaft or
carrier line, but also any burst and other details which may develop
from it. The details of a spark stream can often be seen better by
reducing the grinding pressure so there are fewer overlapping sparks.
The effects of alloying elements in the spark stream are important
because of their influence on the stream.
These effects are
discussed in the following subparagraphs.
(1) Wrought Iron with Carbon.
The spark stream of an alloy
containing about 0.08 percent carbon consists of long, moderately
bright carrier lines each of which thickens strongly at the end and
is followed by a fairly dull arrowhead separated by a small gap from
the rest of the spark. The sparks range in length all the way from 1
or 2 inches up to roughly 25 inches.
Manganese affects the spark stream in much the
same way as carbon and steel containing 1 or 2 percent of manganese
may be confused with carbon steel. When the manganese content rises
to 10 or 12 percent, the spark stream retains its brilliance and the
length of the stream drops to about 20 inches, as compared with 30
inches for carbon steels.
The distinguishing feature of the spark is a short
sharply outlined dash appearing near the end of the carrier just
before the fork.
In steels that contain 1 or 2 percent chromium
there will be nothing to indicate its presence. But, an 18 percent
chromium content produces a spark much like wrought iron, except that
it is only half as long.