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What Is Difference Between 4320 Rods & 5150?

The American Iron and Steel Institute, or AISI, is an organization with a mission of promoting the steel industry in the United States and North America. One of the products of that industry is alloy steel. It is carbon steel with chemical additives such as sulfur and phosphorus. These alloy steels carry AISI numbers such as 4320 and 5150 and are used to make products such as rods.

Names
- AISI 4320 is commonly known as nickel-chromium-molybdenum steel, and a rod made with this steel alloy would carry that name. AISI 5150 carries the name chromium steel. These names reflect the addition of chemicals that make them alloy instead of carbon steel.
Elements
- AISI 4320 consists of .17 to .22 percent carbon, .45 to .65 percent manganese, a maximum of .035 percent phosphorus and .04 percent sulfur. Other elements in 4320 include .15 to .30 percent silicon, .40 to .60 percent chromium, 1.65 to 2 percent nickel, and .20 to .30 percent molybdenum. The remainder of AISI 4320 is iron. In contrast, AISI 5150 contains all of the same elements with the exception of nickel and molybdenum. It has the same amounts of sulfur, phosphorus and silicon. It has more carbon, .48 to .53 percent, and more manganese, .70 to .9 percent. As expected by its name, chromium steel, it has more chromium, .7 to .9 percent. The remainder of AISI 5150 is iron.
Tensile Strength
- Tensile strength measures the force necessary to break a metal. The metric system measures it in MPa, or megapaschals, and the English system is psi, or pounds per square inch. A 4320 rod has a tensile strength of 579.2 MPa, or 84,005 psi. The 5150 rod is stronger. Its tensile strength is 675.7 MPa, or 98,001 psi. Both of these measurements reflect the alloy being annealed at 825 degrees Centigrade. Annealed means to heat and then cool the alloy to soften it so it can be molded into shapes.
Hardness
- The Brinell Hardness Test measures how hard a material is by subjecting it to indentation with a steel or carbide ball. The test results are given the letters HB. The 5150 has a greater hardness, with a 197, as opposed to the 4320, with a 163.

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