Part of the metalworkers skill lies in the ability to identify various metal products brought to the shop. The metalworker must be able to identify the metal so the proper work methods can be applied. For Army equipment, drawings should be available. They must be examined in order to determine the metal to be used and its heat treatment (if required). If no drawing is available, knowledge of what the parts are going to do will serve as a guide to the type of metal to use.
TESTING OF METALS
Simple tests can be made in the shop to identify metals. Since the ability to judge metals can be developed only through personal experience, practice these tests with known metals until familiar with the reactions of each metal to each type of test.
This test includes such things as the color and appearance of machined as well as unmachined surfaces.
Some metals can be quickly identified by looking at the surface of the broken part or by studying the chips produced with a hammer and chisel.
This is a simple identification test used to observe the color, spacing, and quantity of sparks produced by grinding. It is a fast and convenient method of sorting mixed steels with known spark characteristics. This test is best conducted by holding the steel stationary and touching a high-speed portable grinder to the steel with sufficient pressure to throw a spark stream about 12 inches long. The characteristics of sparks generated by a spark grinding test are shown in Figure 2-7. These spark patterns provide general information about the type of steel, cast iron, or alloy steel. In all cases, it is best to use standard samples of metal when comparing their sparks with that of the test sample.
Figure 2-7. Spark test.
THE ROCKWELL HARDNESS NUMBER IS DETERMINED BY THE DEPTH OF THE IMPRESSION WHILE THE BRINELL HARDNESS NUMBER IS DETERMINED BY THE AREA OF THE IMPRESSION
Figure 2-8. Rockwell hardness tester.
One simple way to check for hardness in a piece of metal is to file a small portion of it. If it is soft enough to be machined with regular tooling, the file will cut it. If it is too hard to machine, the file will not cut it. This method will indicate whether the material being tested is softer or harder than the file, but it will not tell exactly how soft or hard it is. The file can also be used to determine the harder of two pieces of metal; the file will cut the softer metal faster and easier. The file method should only be used in situations when the exact hardness is not required. This test has the added advantage of needing very little in the way of time, equipment, and experience.
Rockwell Hardness Test
This test determines the hardness of metals by measuring the depth of impression which can be made by a hard test point under a known load. The softer the metal, the deeper the impression. Soft metals will be indicated by low hardness numbers. Harder metals permit less of an impression to be made, resulting in higher hardness numbers. Rockwell hardness testing is accomplished by using the Rockwell hardness testing machine (Figure 2-8).
Brinell Hardness Fest
Brinell hardness testing operates on almost the same principle as the Rockwell test. The difference between the two is that the Rockwell hardness number is determined by the depth of the impression while the Brinell hardness number is determined by the area of the impression. This test forces a hardened ball, 10 mm (0.3937 in) in diameter, into the surface of the metal being tested, under a load of 3,000 kilograms (approximately 6,600 lb). The area of this impression determines the Brinell hardness number of the metal being tested. Softer metals result in larger impressions but have lower hardness numbers.
Perhaps the best known numerical code is the Society of Automotive Engineers (SAE) code. For the metals industry, this organization pioneered in developing a uniform code based on chemical analysis. SAE specification numbers are now used less widely than in the past; however, the SAE numerical code is the basic code for ferrous metals Figure 29).
The SAE system is based on the use of four-or five digit numbers.
The first number indicates the type of alloy used; for example, 1 indicates a carbon steel.
Two indicates nickel steel.
The second, and sometimes the third, number gives the amount of the main alloy in whole percentage numbers.
The last two, and sometimes three, numbers give the carbon content in hundredths of 1 percent (0.01 percent).
The following examples will help you understand this system:
1- Type of steel (carbon).
0- Percent of alloy (none).
45- Carbon content (0.45-percent carbon).
TYPES OF STEEL
Free Cutting Manganese
Free Cutting Screw Stock
NICKEL CHROMIUM STEELS
1.25% Nickel: 1.00% Chromium
3.50% Nickel: 1.50% Chromium
3.00% Nickel: .80% CHromium
Corrosion and Heat Resisting
Chromium- Nickel- Molybdenum
46XX & 48XX
.60% to 1.10% Chromium
1.2% to 1.5% Chromium
Corrosion and Heat Resistant
7XXXX & 7XXX
Silicon Manganese Steels
Figure 2-9. SAE numerical code.
2- Type of steel (nickel).
3- Percent of alloy (3-percent nickel).
30- Carbon content (0.30-percent carbon).
7- Type of steel (tungsten).
16- Percent of alloy (16-percent tungsten).
50- Carbon content (0,50-percent carbon).
5- Type of steel (chromium).
0- Percent of alloy (less than l-percent chromium). 100- Carbon content (1-percent carbon).
A system similar to the SAE classifications for steel and alloys has been developed by the Aluminum Association (AA) for wrought aluminum and aluminum alloys.
This identification system of aluminum, as shown in Figure 2-10, consists of a four-digit number which indicates the type of alloy. control over impurities, and the specific alloy. The first number indicates the type of alloy. For example, 2 is copper, 3 is manganese, 4 is silicone, and so forth. The second number indicates the control that has been used. The last two numbers usually indicate an assigned composition. Thus, AA-2024 means:
2 - Type of alloy (copper).
O - Control of impurities.
24 - Exact composition (AA number 24).
Aluminum alloys vary greatly in their hardness and physical condition. These differences are called "temper," Letter symbols represent the different tempers, In addition to a letter, one or more numbers are sometimes used to indicate further differences. The temper designation is separated from the basic four-digit identification number by a dash; for example, 2024-T6. In this case there is an aluminum alloy, 2024, with a T6 temper (solution heat treated and then artificially aged). Figure 2-11 shows the numerals 2 through 10 that have been assigned in the AA system to indicate specific sequences of annealing, heat treating, cold working, or aging.