Alloy steel is frequently recognizable by its use. There are many varieties of alloy steel used in the manufacture of equipment. They have greater strength and durability than carbon steel, and a given strength is secured with less material weight. Manganese steel is a special alloy steel that is always used in the cast condition. Basic carbon steels are alloyed with other elements, such as chromium and nickel, to increase certain physical properties of the metal.
Nickel, chromium, vanadium, tungsten, molybdenum, and silicon are the most common elements used in alloy steel.
- Chromium is used as an alloying element to increase hardenability, corrosion resistance, and shock resistance. It imparts high strength with little loss in ductility.
- Nickel increases the toughness, strength, and ductility of steels, and lowers the hardening temperatures so than an oil quench, rather than a water quench, is used for hardening.
- Manganese is used to produce greater toughness, wear resistance, easier hot rolling, and forging. An increase in manganese content decreases the weldability of steel.
- Molybdenum increases hardenability, which is the depth of hardening possible through heat treatment. The impact fatigue property of the steel is improved with up to 0.60 percent molybdenum. Above 0.60 percent molybdenum, the impact fatigue property is impaired. Wear resistance is improved with molybdenum content above 0.75 percent. Molybdenum is sometimes combined with chromium, tungsten, or vanadium to obtain desired properties.
- Titanium and columbium (niobium) are used as additional alloying agents in low-carbon content, corrosion resistant steels. They support resistance to intergranular corrosion after the metal is subjected to high temperatures for a prolonged time period.
- Tungsten, as an alloying element in tool steel, produces a fine, dense grain when used in small quantities. When used in larger quantities, from 17 to 20 percent, and in combination with other alloys, it produces a steel that retains its hardness at high temperatures.
- Vanadium is used to help control grain size. It tends to increase hardenability and causes marked secondary hardness, yet resists tempering. It is also added during manufacture to remove oxygen.
- Silicon is added to obtain greater hardenability and corrosion resistance, and is often used with manganese to obtain a strong, tough steel.
High speed tool steels are usually special alloy compositions designed for cutting tools. The carbon content ranges from 0.70 to 0.80 percent. They are difficult to weld except by the furnace induction method.
High yield strength, low alloy structural steels (often referred to as constructional alloy steels) are special low carbon steels containing specific small amounts of alloying elements. These steels are quenched and tempered to obtain a yield strength of 90,000 to 100,000 psi (620,550 to 689,500 kPa) and a tensile strength of 100,000 to 140,000 psi (689,500 to 965,300 kPa), depending upon size and shape. Structural members fabricated of these high strength steels may have smaller cross sectional areas than common structural steels, and still have equal strength. In addition, these steels are more corrosion and abrasion resistant. In a spark test, this alloy appears very similar to the low carbon steels.
Alloy steel is much tougher than low carbon steels, and shearing machines must have twice the capacity required for low carbon steels.
Alloy Appearance test
Alloy steel appears the same as drop-forged steel.
Alloy Fracture test
Alloy steel is usually very close grained; at times the fracture appears velvety.
Alloy Spark test
Alloy steel produces characteristic sparks both in color and shape. Some of the more common alloys used in steel and their effects on the spark stream are as follows:
Alloys containing 1 to 2 percent chromium have no outstanding features in the spark test. Chromium in large amounts shortens the spark stream length to one-half that of the same steel without chromium, but does not appreciably affect the stream’s brightness. Other elements shorten the stream to the same extent and also make it duller. An 18 percent chromium, 8 percent nickel stainless steel produces a spark similar to that of wrought iron, but only half as long. Steel containing 14 percent chromium and no nickel produces a shorter version of the low-carbon spark. An 18 percent chromium, 2 percent carbon steel (chromium die steel) produces a spark similar to that of carbon tool steel, but one-third as long.
The nickel spark has a short, sharply defined dash of brilliant light just before the fork. In the amounts found in S. A. E. steels, nickel can be recognized only when the carbon content is so low that the bursts are not too noticeable.
The sparks given off during a spark test are straw colored near the grinding wheel and white near the end of the streak. There is a medium volume of streaks having a moderate number of forked bursts.
Alloys containing Manganese produces a spark similar to a carbon steel spark. A moderate increase in manganese increases the volume of the spark stream and the force of the bursts. Steel containing more than the normal amount of manganese will spark in a manner similar to high-carbon steel with low manganese content.
Alloys containing Molybdenum produces a characteristic spark with a detached arrowhead similar to that of wrought iron. It can be seen even in fairly strong carbon bursts. Molybdenum alloy steel contains nickel, chromium, or both.
Molybdenum with other elements
When molybdenum and other elements are substituted for some of the tungsten in high-speed steel, the spark stream turns orange. Although other elements give off a red spark, there is enough difference in their color to tell them from a tungsten spark.
Tungsten will impart a dull red color to the spark stream near the wheel. It also shortens the spark stream, decreases the size, or completely eliminates the carbon burst. Alloys containing 10 percent tungsten causes short, curved, orange spear points at the end of the carrier lines. Still lower tungsten content causes small white bursts to appear at the end of the spear point. Carrier lines may be anything from dull red to orange in color, depending on the other elements present, if the tungsten content is not too high..
Alloys containing vanadium produce sparks with a detached arrowhead at the end of the carrier line similar to those arising from molybdenum steels. The spark test is not positive for vanadium steels.