Steel Alloys

See also Iron and Iron & Steelmaking.

Chronology

Alloy steels are carbon steel with additives like nickel, chromium and vanadium to give the steel special characteristics.   There are hundreds of alloy steels in use today, each made to fulfill special functions.

Steel alloys can be divided into 4 main groups: Carbon Steel, Low Alloy Steel, High Strength-Low Carbon Steel, and Chromium-Molybdenum Steels.

Carbon steel is an alloy of iron and carbon with small amounts of manganese, sulfur, phosphorus and silicon.   High-carbon steel is very hard, so it is used for dies and cutting and drilling tools.   Low and medium-carbon are malleable enough to be rolled into sheets for plates that are welded or riveted together to form containers. ^{Metal n.p.}   Automotive bodies are made of die-pressed sheets of carbon steel.

Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.   Carbon steels are normally classified as shown below. ^{Metal n.p.}

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

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60 percent and the manganese from 0.60 to 1.65 percent.   Increasing the carbon content to approximately 0.5 percent with an accompanying increase in manganese allows medium-carbon steels to be used in the quenched and tempered condition. ^{Metal n.p.}

High-carbon steels contain from 0.60 to 1.00 percent C with manganese contents ranging from 0.30 to 0.90 percent. ^{Metal n.p.}

Low-alloy steels have mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum.   The total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.   The primary function of the alloying elements is to increase hardenability and to optimize mechanical properties and toughness after heat treatment.   Alloy additions are also used to reduce environmental degradation under certain specified conditions. ^{Key n.p.}

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties than conventional carbon steels.   They are designed to meet specific mechanical properties rather than a chemical composition.   The chemical composition of a specific HSLA steel may vary for different product thickness to meet mechanical property requirements.   The HSLA steels have low carbon contents (0.50 to ~0.25 percent C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0 percent.   Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used in various proportions. ^{Metal n.p.}

Carbon is the primary hardening element in steel.   Hardness and tensile strength increases as carbon content increases up to about 0.85% C.   Ductility and weldability decrease with increasing carbon. ^{Metal n.p.}

Manganese is beneficial to surface quality, especially in resulfurized steels.   Manganese contributes to strength and hardness, but less than carbon.   The increase in strength is dependent upon the carbon content.   Increasing the manganese content decreases ductility and weldability, but less than carbon.   Manganese has a significant effect on the hardenability of steel. ^{Metal n.p.}

Phosphorus increases strength and hardness and decreases ductility and toughness of steel.   The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels.   Phosphorus normally is kept to low levels, but higher phosphorus is specified in low-carbon free-machining steels to improve machinability. ^{Metal n.p.}

Sulfur decreases ductility and toughness, especially in the transverse direction.   Weldability decreases with increasing sulfur content.   Sulfur is found primarily in the form of sulfide inclusions.   Sulfur levels are normally controlled to low levels.   The only exception is free-machining steels, where sulfur is added to improve machinability. ^{Metal n.p.}

Silicon is one of the principal deoxidizers used in steelmaking.   Silicon is less effective than manganese in increasing as-rolled strength and hardness.   In low-carbon steels, silicon is generally detrimental to surface quality. ^{Metal n.p.}

Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%.   Weathering steels are sold having greater than 0.20% Copper.   Copper in significant amounts is detrimental to hot-working steels.   Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding.   Copper can be detrimental to surface quality. ^{Metal n.p.}

Lead is virtually insoluble in steel.   However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve machinability. ^{Metal n.p.}

Boron is added to steel to improve hardenability.   A very small amount of boron (about 0.001%) has a strong effect on hardenability.   Boron steels are generally produced within a range of 0.0005 to 0.003%.   Boron is most effective in lower carbon steels.   Whenever boron is substituted in part for other alloys, it is done only with hardenability in mind because the lowered alloy content may be harmful for some applications. ^{Metal n.p.}

Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength.   As a hardening element, chromium is frequently used with a toughening element such as nickel to produce superior mechanical properties.   At higher temperatures, chromium contributes increased strength.   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 to quenching. ^{Metal n.p.}

Nickel increases the hardenability and impact strength of steels.   For example, Permalloy (Ni 80%, Fe 20%) can be made highly magnetic and Invar (48% Ni, 50% Fe and small amounts of Cr, Mn, Si, C, Mg, Al) has low expansion under high temperatures. ^{Metal n.p.}

Molybdenum increases the hardenability of steel.   Molybdenum may produce secondary hardening during the tempering of quenched steels.   It enhances the creep strength of low-alloy steels at elevated temperatures. ^{Metal n.p.}

Aluminum is widely used as a deoxidizer.   Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size.   Aluminum is the most effective alloy in controlling grain growth prior to quenching.   Titanium, zirconium, and vanadium also are valuable grain growth inhibitors, but there carbides are difficult to dissolve into solution in austenite. ^{Metal n.p.}

Zirconium can be added to high-strength low-alloy steels to achieve improvements in inclusion characteristics.   Zirconium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending. ^{Metal n.p.}

Niobium increases the yield strength and, to a lesser degree, the tensile strength of carbon steel.   The addition of small amounts of niobium can significantly increase the yield strength of steels.   Niobium can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness. ^{Metal n.p.}

Titanium is used to retard grain growth and thus improve toughness.   Titanium is also used to achieve improvements in inclusion characteristics.   Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending. ^{Metal n.p.}

Vanadium increases the yield strength and the tensile strength of carbon steel.   Vanadium is one of the primary contributors to precipitation strengthening in microalloyed steels.   When thermomechanical processing is properly controlled, the ferrite grain size is refined and there is a corresponding increase in toughness.   The impact transition temperature also increases when vanadium is added. ^{Metal n.p.}

All microalloy steels contain small concentrations of one or more strong carbide and nitride forming elements.   Vanadium, niobium, and titanium combine preferentially with carbon and nitrogen to form a fine dispersion of precipitated particles in the steel matrix. ^{Metal n.p.}

Chromium-Molybdenum Steels: ^{Metal n.p.}

Stainless Steels are iron-base alloys containing chromium.   Stainless steels usually contain less than 30% Cr and more than 50% Fe.   They attain their stainless characteristics because of the formation of an invisible and adherent chromium-rich oxide surface film.   This oxide establishes on the surface and heals itself in the presence of oxygen.   Some other alloying elements added to enhance specific characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, and nitrogen.   Carbon is usually present in amounts ranging from less than 0.03% to over 1.0%.   Corrosion resistance and mechanical properties are commonly the principal factors in selecting a grade of stainless steel for a given application.   A typical stainless steel might be the following: Cr = 17 %, Ni = 12 %, Mo = 2.5 %, Si = 1.6 %, C = 0.1 %, rest = Fe. ^{Metal n.p.}

Other chromium steels are Inconel (Ni-Cr-Fe alloy containing 16% Cr, and 8% Fe) resists severe corrosion and high temperatures, the Hastelloys (Ni, 16% Cr, 15% Mo, rest Fe) resists corrosion and high temperatures, Nichrome ( 25% Fe, 15% Cr, rest Ni ) resists heating in wire.

Galvanized steel is steel coated with a thin layer of zinc to provide corrosion resistance.   It is used in automobile underbody parts, garbage cans, storage tanks, and fencing wire.   Sheet steel normally must be cold-rolled prior to being galvanized.

To 1790

Iron was used for heavy-load, durable construction, such as door and lock fasteners, farm implements, and machinery.   Steel required much hand work and tempering and was therefore very expensive.   It was used only for special applications, such as surgical, drilling, and surveying instruments and hand tools requiring great strength, such as saws, hammers, and drills.

1790-1799

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