| Alloy steels differ from carbon steels in that they have compositions that extend beyond the limits set for carbon steels. Usually this refers to constituents such as boron, carbon, chromium, manganese, molybdenum, silicon and vanadium. They also have chromium contents less than 4%. Steels with chromium contents of greater than 4% become classified as stainless or tool steels. As a general guide, an alloy steel will have: • Manganese content >1.65% • Silicon content >0.5% • Copper content >0.6% The American Iron and Steel Institute (AISI) naming system is one of the most widely accepted systems. Designations usually consist of a four digit number, but sometimes this extends to five. The first two digits indicate what the major alloying element is, while the last 2 or three indicate the carbon content in hundredths of a percent. Example: AISI 1340 is a manganese containing alloy steel with a 0.40% average carbon content. Table 1. Summarises AISI designations for alloy steels. | | | 13 | xx: 1.75Mn | Manganese | | 23 | xx: 3.50Ni | Nickel | | 31 | xx: 1.25Ni, 0.65-0.80Cr | Nickel-Chromium | | 40 | xx: 0.20-0.25Mo | Molybdenum | | 41 | xx: 0.50-0.95Cr, 0.12-0.30Mo | Chromium-Molybdenum | | 43 | xx: 1.82Ni, 0.50-0.80Cr, 0.25Mo | Nickel-Chromium-Molybdenum | | 44 | xx: 0.40-0.52Mo | Molybdenum | | 46 | xx: 0.85-1.82Ni, 0.20-0.25Mo | Nickel-Molybdenum | | 47 | xx: 1.05Ni, 0.45Cr, 0.20-0.35Mo | Nickel-Chromium-Molybdenum | | 48 | xx: 3.5Ni, 0.25Mo | Nickel-Molybdenum | | 50 | xx: 0.27-0.65Cr | Chromium | | 50 | xx: 0.50Cr, 1.00C | Chromium | | 51 | xx: 0.80-1.05Cr | Chromium | | 51 | xx: 1.02Cr, 1.00C | Chromium | | 52 | xx: 1.45Cr, 1.00C | Chromium | | 61 | xx: 0.60-0.95Cr, 0.10-0.15V | Chromium-Vanadium | | 81 | xx: 0.30Ni, 0.40Cr, 0.12Mo | Nickel-Chromium-Molybdenum | | 86 | xx: 0.55Ni, 0.50Cr, 0.25Mo | Nickel-Chromium-Molybdenum | | 87 | xx: 0.55Ni, 0.50Cr, 0.25Mo | Nickel-Chromium-Molybdenum | | 88 | xx: 0.55Ni, 0.50Cr, 0.20-0.35Mo | Nickel-Chromium-Molybdenum | | 92 | xx: 1.45-2.0Si, 0.65-0.85Mn,<0.65Cr | Silicon-Manganese | | 93 | xx: 3.25Ni, 1.20Cr, 0.12Mo | Nickel-Chromium-Molybdenum | | 94 | xx: 0.45Ni, 0.40Cr, 0.12Mo | Nickel-Chromium-Molybdenum | |