Background

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.

		Main Alloying Elements
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