comparable to heat-treated steel. K-monel is used for instrument parts that must resist corrosion.

Inconel

This high-nickel alloy is often used in the exhaust systems of aircraft engines. Inconel is composed of 78.5% nickel, 14% chromium, 6.5% iron, and 1% of other elements. It offers good resistance to corrosion and retains its strength at high-operating temperatures.

METAL IDENTIFICATION

Many methods are used to identify a piece of metal. Identification is necessary when selecting a metal for use in fabrication or in determining its weldability. Some common methods used for field identification are surface appearance, spark test, chip test, and the use of a magnet.

SURFACE APPEARANCE

Sometimes it is possible to identify metals by their surface appearance. Table 1-3 indicates the surface colors of some of the more common metals. Referring to the table, you can see that the outside appearance of a metal helps to identify and classify metal. Newly fractured or freshly filed surfaces offer additional clues.

A surface examination does not always provide enough information for identification but should give us enough information to place the metal into a class. The color of the metal and the distinctive marks left from manufacturing help in determining the identity of the metal. Cast iron and malleable iron usually show evidence of the sand mold. Low-carbon steel often shows forging marks, and high-carbon steel shows either forging or rolling marks. Feeling the surface may provide another clue. Stainless steel is slightly rough in the unfinished state, and the surfaces of wrought iron, copper, brass, bronze, nickel, and Monel are smooth. Lead also is smooth but has a velvety appearance.

When the surface appearance of a metal does not give enough information to allow positive identification, other identification tests become necessary. Some of these tests are complicated and require equipment we do not usually have; however, other tests are fairly simple and reliable when done by a skilled person. Three of these tests areas follows: the spark test, the chip test, and the magnetic tests.

Figure 1-2. - Terms used in spark testing.

SPARK TEST

The spark test is made by holding a sample of the material against an abrasive wheel. By visually inspecting the spark stream, an experienced metalworker can identify the metals with considerable accuracy. This test is fast, economical, convenient, and easily accomplished, and there is no requirement for special equipment. We can use this test for identifying metal salvaged from scrap. Identification of scrap is particularly important when selecting material for cast iron or cast steel heat treatment.

When you hold a piece of iron or steel in contact with a high-speed abrasive wheel, small particles of the metal are torn loose so rapidly that they become red-hot. As these glowing bits of metal leave the wheel, they follow a path (trajectory) called the carrier line. This carrier line is easily followed with the eye, especial] y when observed against a dark background.

The sparks given off, or the lack of sparks, aid in the identification of the metal. The length of the spark stream, the color, and the form of the sparks are features you should look for. Figure 1-2 illustrates the terms used in referring to various basic spark forms produced in spark testing.

Steels having the same carbon content but differing alloying elements are difficult to identify because the alloying elements affect the carrier lines, the bursts, or the forms of characteristic bursts in the spark picture, The effect of the alloying element may slow or accelerate the carbon spark or make the carrier line lighter or darker in color. Molybdenum, for example, appears as a detached, orange-colored spearhead on the end of the carrier line. Nickel appears to suppress the effect of the carbon burst; however, the nickel spark can be identified