Steel Alloy

Table 4: Alloying elements and their properties in steel

Element	Effect
Aluminum	Ferrite hardener Graphite former Deoxidizer
Chromium	Mild ferrite hardener Moderate effect on hardenability Graphite former Resists corrosion Resists abrasion
Cobalt	High effect on ferrite as a hardener High red hardness
Molybdenum	Strong effect on hardenability Strong carbide former High red hardness Increases abrasion resistance
Manganese	Strong ferrite hardener
Nickel	Ferrite strengthener Increases toughness of the hypoeutectoid steel With chromium, retains austenite Graphite former
Copper	Austenite stabilizer Improves resistance to corrosion
Silicon	Ferrite hardener Increases magnetic properties in steel
Phosphorus	Ferrite hardener Improves machinability Increases hardenability

The American Iron and Steel Institute (AISI) in cooperation with the Society of Automotive Engineers (SAE) developed a standard number classification system for steel alloys. The Society of Automotive Engineers (SAE) has established standards for specific analysis of steels. In the 10XX series, the first digit indicates a plain carbon steel. The second digit indicates a modification in the alloys. 10XX means that it is a plain carbon steel where the second digit (zero ) indicates that there is no modification in the alloys. The last two digits denote the carbon content in points. For example SAE 1095 is a carbon steel where 95 points represent 0.95 % Carbon content. Carbon steels do not exceed 0.95% as further increases in carbon content are detrimental to steel properties. Alloy steels are indicated by 2XXX, 3XXX, 4XXX, etc. The (SAE) revised the percentages of the alloys to be used in the making of steel, retained the numbering system, and added letter prefixes to indicate the method used in steel making. If the prefix is omitted, the steel is assumed to be open hearth. Example: AISI C1050 indicates a plain carbon, basic-open hearth steel that has 0.50 % Carbon content.

Table 1: Steel classification letter prefix

letter	production process
Axxxx	Alloy steel produced by basic open hearth furnace
Bxxxx	Carbon alloy steel produced by Bessemer furnace
Cxxxx	Carbon alloy steel produced by basic open hearth
Dxxxx	Carbon alloy steel produced by acid open hearth
Exxxx	Alloy steel produced in an electric furnace with controlled atmosphere

Table 2: First digit for steel alloys

Number	Classification
1xxx	Carbon steels Low carbon steels: 0 to 0.25 % C Medium carbon steels: 0.25 to 0.55 % C High carbon steels: Above 0.55 % Carbon
2xxx	Nickel steels 5 % Nickel increases the tensile strength without reducing ductility. 8 to 12 % Nickel increases the resistance to low temperature impact 15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic properties. (Alnicometals) 25 to 35 % Nickel create resistance to corrosion at elevated temperatures.
3xxx	Nickel-chromium steels These steels are tough and ductile and exhibit high wear resistance , hardenability and high resistance to corrosion.
4xxx	Molybdenum steels Molybdenum is a strong carbide former. It has a strong effect on hardenability and high temperature hardness. Molybdenum increases the tensile strength of low carbon steels.
5xxx	Chromium steels Chromium is a ferrite strengthener in low carbon steels. Increases the core toughness Increases wear resistnace of the case in carburized steels.
61xx	Chrome Vanadium Steels
72xx	Tungsten Chrome Steel
81xx 86xx 87xx 88xx 93xx 94xx 97xx 98xx	Triple Alloy steels include Nickel (Ni), Chromium (Cr), and Molybdenum (Mo) These steels exhibit high strength and also high strength to weight ratio, good corrosion resistance.
92xx	Silicon-Manganese Steels

Table 3: Second digit for steel alloys

second digit	alloy content
10xx	plain carbon steel, Mn 1.00% max
11xx	sulfurized free machining
12xx	sulfurized/phosphorized free machining
13xx	Mn 1.75% max
15xx	Plain Carbon, Mn 1.00-1.65%
23xx	Ni 3.50%
25xx	Ni 5.00%
31xx	Ni 1.25%, Cr 0.65-0.80%
32xx	Ni 1.75%, Cr 1.07%
33xx	Ni 3.50%, Cr 1.50-1.57%
34xx	Ni 3.00%, Cr 0.77%
40xx	Mo 0.20-0.25%
41xx	Cr 0.50-0.95%, Mo 0.12-0.30%
44xx	Mo 0.40-0.52%
43xx	Chromium-Molybdenum Steel
46xx	Ni 1.82%, Cr 0.50-0.80%, Mo 0.25%
47xx	Ni 1.82%, Cr 0.50-0.80%, Mo 0.25%
48xx	Ni 3.50%, Mo 0.25%
50xx	Cr 0.27-0.65%
51xx	Cr 0.80-1.05%
50xxx	Cr 0.50%, C 1.00% min (three digit carbon)
51xxx	Cr 1.025%, C 1.00% min (three digit carbon)
52xxx	Cr 1.45%, C 1.00% min (three digit carbon)
61xx	Cr 0.60-0.95%, V 0.10-0.15%
72xx	W 1.75%, Cr 0.75%
81xx	Ni 0.30%, Cr 0.40%, Mo 0.12%
86xx	Ni 0.55%, Cr 0.50%, Mo 0.20%
87xx	Ni 0.55%, Cr 0.50%, Mo 0.25%
88xx	Ni 0.55%, Cr 0.50%, Mo 0.35%
92xx	Si 1.40-2.00%, Mn 0.65-0.85%, Cr 0-0.65%
93xx	Ni 3.25%, Cr 1.20%, Mo 0.12%
94xx	Ni 0.45%, Cr 0.40%, Mo 0.12%
97xx	Ni 0.55%, Cr 0.20%, Mo 0.20%
98xx	Ni 1.00%, Cr 0.80%, Mo 0.25%

Steel letter suffix

Another letter is the hardenability or H-value. Example: 4340H

General representation of steels:

Red Hardness: This property , also called hot-hardness, is related to the resistance of the steel to the softening effect of heat. It is reflected to some extent in the resistance of the material to tempering.

Hardenability: This property determines the depth and distribution of hardness induced by quenching.

Hot-shortness: Brittleness at high temperatures is called hot-shortness which is usually caused by sulfur. When sulfur is present, iron and sulfur form iron sulfide (FeS) that is usually concentrated at the grain boundaries and melts at temperatures below the melting point of steel. Due to the melting of iron sulfide, the cohesion between the grains is destroyed, allowing cracks to develop. This occurs when the steel is forged or rolled at elevated temperatures. In the presence of manganese, sulfur tends to form manganese sulfide (MnS) which prevents hot-shortness.

Cold-shortness: Large quantities of phosphorus (in excess of 0.12%P) reduces the ductility, thereby increasing the tendency of the steel to crack when cold worked. This brittle condition at temperatures below the recrystallization temperature is called cold-shortness.

Steel Alloy Classification