Steel Numbering Sytem

A steel numbering system is used to identify the many types of steel. A steel’s numerical name usually consists of four numbers or digits. The first two digits refer to the alloy content. The last two digits (or three digits, in the case of a five- digits number) refer to the percentage of carbon in the steel. See Figure 1 below.

Figure 1: A steel’s name usually consists of four digits. It supplies information about the alloy content and percentage

 The steel numbering system is shown in Table 1 below . In 5147 steel, for example, the “51” identifies chromium as a key alloying element. In 2517 steel, the “25” indicates that there is an unusual amount of nickel in this steel. In 4718 steel, the “47” indicates that the amount of chromium, nickel and molybdenum in the steel is higher than average. In 1040 steel, the “10” means that the steel has very little alloy content except carbon. As shown by these examples, the first two numbers always give an indication of the alloy content in the steel.

Table 1:  AISI/SAE Steel Numbering System

Type	Steel Numerical Name	Key element
Carbon steels	10XX	Plain carbon, Mn 1.00% max
	11XX	Resulfurized free machining
	12XX	Resulfurized/rephosphorized free machining
	15XX	Plain carbon, Mn 1.00-1.65%
Manganese steel	13XX	Mn 1.75%
Nickel steels	23XX	Ni 3.50%
	25XX	Ni 5.00%
Nickel-chromium steels	31XX	Ni 1.25%, Cr .65-.80%
	32XX	Ni 1.75%, Cr 1.07%
	33XX	Ni 3.50%, Cr 1.50-1.57%
	34XX	Ni 3.00%, Cr .77%
Molybdenum steels	40XX	Mo .20-.25%
	44XX	Mo .40-.52%
Chromium-molybdenum steels	41XX	Cr .50-.95%, Mo .12-.30%
Nickel-chromium-molybdenum steels	43XX	Ni 1.82%, Cr .50-.80%, Mo .25%
	47XX	Ni 1.05%, Cr .45%, Mo .20-.35%
Nickel-molybdenum steels	46XX	Ni .85-1.82%, Mo .20-.25%
	48XX	Ni 3.50%, Mo .25%
Chromium steels	50XX	Cr .27-.65%
	51XX	Cr .80-1.05%
	50XXX	Cr .50%, C 1.00% min
	51XXX	Cr 1.02%, C 1.00% min
	52XXX	Cr 1.45%, C 1.00% min
Chromium-vanadium steels	61XX	Cr .60-.95%, V .10-.15%
Tungsten-chromium steels	72XX	W 1.75%, Cr .75%
Nickel-chromium-molybdenum steels	81XX	Ni .30%, Cr .40%, Mo .12%
	86XX	Ni .55%, Cr .50%, Mo .20%
	87XX	Ni .55%, Cr .50%, Mo .25%
	88XX	Ni .55%, Cr .50%, Mo .35%
Silicon-manganese steels	92XX	Si 1.40-2.00%, Mn .65-.85%, Cr 0-.65%
Nickel-chromium-molybdenum steels	93XX	Ni 3.25%, Cr 1.20%, Mo .12%
	94XX	Ni .45%, Cr .40%, Mo .12%
	97XX	Ni .55%, Cr .20%, Mo .20%
	98XX	Ni 1.00%, Cr .80%, Mo .25%

The last two digits (or three digits) of a numerical name for steel indicate the percentage of carbon in the steel. A two digit number represents hundredths of a percent. In 1040 steel, for example the “40” means there is 0.40% carbon in the steel. In the 1018 steel, the “18” indicates the the steel has only 0.18% carbon, thus it is a low steel. There is approximately 0.60% carbon in 8660 steel 0.60% carbon in 8660 steel which would make it medium-carbon steel.

When the carbon content of the steel is 1% or more, three digits are needed to describe the carbon content. For example 50100 steel contains 1.00%. See Table 2.

Table 2: Examples of steel names and carbon content

Indentification No.	Carbon Content
1040	0.40%
1018	0.18%
8660	0.60%
50100	1.00%

A steel numerical name provides much information about the alloy content of the steel. Indirectly, the number tells you about the quality, strength and corrosion resistance of the steel. For example 8622 steel has 0.22% carbon, 0.20% molybdenum, 0.50% chromium and 0.55% nickel. Molybdenum makes this steel strong at high temperatures. Chromium improves its corrosion resistance, while nickel improves the steel’s toughness.