Steel Alloys

8/8/2019 Steel Alloys

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Lincoln Machine, Inc. - P.O. Box 29798 - 6401 Cornhusker Highway, Lincoln NE 68529 (402) 434-9140 Page 1

Steel Alloys

Steel Alloys can be divided into five groups

Carbon Steels High Strength Low Alloy Steels Quenched and Tempered Steels Heat Treatable Low Alloy Steels

Chromium-Molybdenum Steels

Steels are readily available in various product forms. The American Iron and Steel Institutedefines carbon steel as follows:

Steel is considered to be carbon steel when no minimum content is specified or required forchromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium orzirconium, or any other element to be added to obtain a desired alloying effect; when thespecified minimum for copper does not exceed 0.40 per cent; or when the maximum contentspecified for any of the following elements does not exceed the percentages noted: manganese1.65, silicon 0.60, copper 0.60. Carbon steels are normally classified as shown below.

Low-carbon steels contain up to 0.30 weight percent C. The largest category of this class ofsteel is flat-rolled products (sheet or strip) usually in the cold-rolled and annealed condition. Thecarbon content for these high-formability steels is very low, less than 0.10 weight percent C, withup to 0.4 weight percent Mn. For rolled steel structural plates and sections, the carbon contentmay be increased to approximately 0.30 weight percent, with higher manganese up to 1.5weight percent.

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from0.30 to 0.60 weight percent and the manganese from 0.60 to 1.65 weight percent. Increasingthe carbon content to approximately 0.5 weight percent with an accompanying increase in

manganese allows medium-carbon steels to be used in the quenched and tempered condition.

High-carbon steels contain from 0.60 to 1.00 weight percent C with manganese contentsranging from 0.30 to 0.90weight percent.

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide bettermechanical properties than conventional carbon steels. They are designed to meet specificmechanical properties rather than a chemical composition. The chemical composition of aspecific HSLA steel may vary for different product thickness to meet mechanical propertyrequirements. The HSLA steels have low carbon contents (0.50 to ~0.25 weight percent C) inorder to produce adequate formability and weldability, and they have manganese contents up to2.0 weight percent. Small quantities of chromium, nickel, molybdenum, copper, nitrogen,vanadium, niobium, titanium, and zirconium are used in various combinations.


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Steel Alloys

Below is a list of some SAE-AISI designations for Steel (the xx in the last two digits indicate thecarbon content in hundredths of a percent)

Carbon Steels

10xx Plain Carbon

11xx Resulfurized

12xx Resulfurized and rephosphorized

Manganese steels

13xx Mn 1.75

Nickel steels

23xx Ni 3.5

25xx Ni 5.0

Nickel Chromium Steels

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

Chromium Molybdenum steels

41xx Cr 0.50-0.95 Mo 0.12-0.30

Nickel Chromium Molybdenum

steels43xx Ni 1.82 Cr 0.50-0.80 Mo 0.25

47xx Ni 1.05 Cr 0.45 Mo 0.20 0.35

86xx Ni 0.55 Cr 0.50 Mo 0.20

Nickel Molybdenum steels

46xx Ni 0.85-1.82 Mo 0.20

48xx Ni 3.50 Mo 0.25

Chromium steels

50xx Cr 0.27- 0.65

51xx Cr 0.80 1.05

Illustration of effect of Carboncontent on Steel Hardness

Effects of Elements on SteelSteels are among the most commonly used alloys. The complexity of steel alloys is fairlysignificant. Not all effects of the varying elements are included. The following text gives anoverview of some of the effects of various alloying elements. Additional research should beperformed prior to making any design or engineering conclusions.

Carbon has a major effect on steel properties. Carbon is the primary hardening element insteel. Hardness and tensile strength increases as carbon content increases up to about 0.85%C as shown in the figure above. Ductility and weldability decrease with increasing carbon.

Manganese is generally beneficial to surface quality especially in resulfurized steels.Manganese contributes to strength and hardness, but less than carbon. The increase instrength is dependent upon the carbon content. Increasing the manganese content decreasesductility and weldability, but less than carbon. Manganese has a significant effect on thehardenability of steel.

Phosphorus increases strength and hardness and decreases ductility and notch impacttoughness of steel. The adverse effects on ductility and toughness are greater in quenched andtempered higher-carbon steels. Phosphorous levels are normally controlled to low levels.Higher phosphorus is specified in low-carbon free-machining steels to improve machinability.



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Sulfur decreases ductility and notch impact toughness especially in the transverse direction.Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form ofsulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability.

Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective thanmanganese in increasing as-rolled strength and hardness. In low-carbon steels, silicon isgenerally detrimental to surface quality.

Copper in significant amounts is detrimental to hot-working steels. Copper negatively affectsforge welding, but does not seriously affect arc or oxyacetylene welding. Copper can bedetrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance whenpresent in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20%Copper.

Lead is virtually insoluble in liquid or solid steel. However, lead is sometimes added to carbonand alloy steels by means of mechanical dispersion during pouring to improve the machinability.

Boron is added to fully killed steel to improve hardenability. Boron-treated steels are producedto a range of 0.0005 to 0.003%. Whenever boron is substituted in part for other alloys, it shouldbe done only with hardenability in mind because the lowered alloy content may be harmful forsome applications. Boron is a potent alloying element in steel. A very small amount of boron(about 0.001%) has a strong effect on hardenability. Boron steels are generally produced withina range of 0.0005 to 0.003%. Boron is most effective in lower carbon steels.

Chromium is commonly added to steel to increase corrosion resistance and oxidationresistance, to increase hardenability, or to improve high-temperature strength. As a hardeningelement, Chromium is frequently used with a toughening element such as nickel to producesuperior mechanical properties. At higher temperatures, chromium contributes increasedstrength. Chromium is a strong carbide former. Complex chromium-iron carbides go into

solution in austenite slowly; therefore, sufficient heating time must be allowed for prior toquenching.

Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution inferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability andimpact strength of steels.

Molybdenum increases the hardenability of steel. Molybdenum may produce secondaryhardening during the tempering of quenched steels. It enhances the creep strength of low-alloysteels at elevated temperatures.

Aluminum is widely used as a deoxidizer. Aluminum can control austenite grain growth inreheated steels and is therefore added to control grain size. Aluminum is the most effectivealloy in controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are alsovaluable grain growth inhibitors, but there carbides are difficult to dissolve into solution inaustenite.

Zirconium can be added to killed high-strength low-alloy steels to achieve improvements ininclusion characteristics. Zirconium causes sulfide inclusions to be globular rather thanelongated thus improving toughness and ductility in transverse bending.


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Niobium (Columbium) increases the yield strength and, to a lesser degree, the tensile strengthof carbon steel. The addition of small amounts of Niobium can significantly increase the yieldstrength of steels. Niobium can also have a moderate precipitation strengthening effect. Itsmain contributions are to form precipitates above the transformation temperature, and to retardthe recrystallization of austenite, thus promoting a fine-grain microstructure having improvedstrength and toughness.

Titanium is used to retard grain growth and thus improve toughness. Titanium is also used toachieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to beglobular rather than elongated thus improving toughness and ductility in transverse bending.

Vanadium increases the yield strength and the tensile strength of carbon steel. The addition ofsmall amounts of Niobium can significantly increase the strength of steels. Vanadium is one ofthe primary contributors to precipitation strengthening in microalloyed steels. When thermomechanical processing is properly controlled the ferrite grain size is refined and there is acorresponding increase in toughness. The impact transition temperature also increases whenvanadium is added.

All microalloy steels contain small concentrations of one or more strong carbide and nitrideforming elements. Vanadium, niobium, and titanium combine preferentially with carbon and/ornitrogen to form a fine dispersion of precipitated particles in the steel matrix.

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Steel Alloys