Alloy steel

Properties

Alloy steels variously improve strength, hardness, toughness, wear resistance, corrosion resistance, hardenability, and hot hardness. To achieve these improved properties the metal may require specific heat treating, combined with strict cooling protocols.

Although alloy steels have been made for centuries, their metallurgy was not well understood until the advancing chemical science of the nineteenth century revealed their compositions. Alloy steels from earlier times were expensive luxuries made on the model of "secret recipes" and forged into tools such as knives and swords. Machine age alloy steels were tool steels and stainless steels.

Because of iron's ferromagnetic properties, some alloys find important applications where their responses to magnetism are valued, including in electric motors and in transformers.

Low-alloy steels

Material science

Alloying elements enable specific properties. As a guideline, alloying elements are added in lower percentages (less than 5%) to increase strength or hardenability, or in larger percentages (over 5%) to improve corrosion resistance or temperature stability.

The alloying elements tend to form either solid solutions, compounds or carbides.

• Nickel is soluble in ferrite; therefore, it usually forms Ni₃Al.
• Aluminum dissolves in ferrite and forms Al₂O₃ and AlN. Silicon is also soluble and usually forms SiO₂•M_xO_y.
• Manganese mostly dissolves in ferrite forming MnS, MnO•SiO₂, but also carbides: (Fe,Mn)₃C.
• Chromium forms partitions between the ferrite and carbide phases in steel, forming (Fe,Cr₃)C, Cr₇C₃, and Cr₂₃C₆. The type of carbide that chromium forms depends on the amount of carbon and other alloying elements present.
• Tungsten and molybdenum form carbides given enough carbon and an absence of stronger carbide forming elements (i.e., titanium and niobium), they form the carbides W₂C and Mo₂C, respectively.
• Vanadium, titanium, and niobium are strong carbide-forming elements, forming vanadium carbide, titanium carbide, and niobium carbide, respectively.

Eutectoid temperature

Alloying elements can have an effect on the eutectoid temperature.

• Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may form at room temperature.
• Carbide-forming elements raise the eutectoid temperature and stabilize ferrites.

Microstructure

The properties of steel depend on its microstructure: the arrangement of different phases, some harder, some with greater ductility. At the atomic level, the four phases of auto steel include martensite (the hardest yet most brittle), bainite (less hard), ferrite (more ductile), and austenite (the most ductile). The phases are arranged by steelmakers by manipulating intervals (sometimes by seconds only) and temperatures of the heating and cooling process.

Transformation-induced plasticity

TRIP steels transform from relatively ductile to relatively hard under deformation such as in a car crash. Deformation transforms austenitic microstructure to martensitic microstructure. TRIP steels use relatively high carbon content to create the austenitic microstructure. Relatively high silicon/aluminum content suppresses carbide precipitation in the bainite region and helps accelerate ferrite/bainite formation. This helps retain carbon to support austenite at room temperature. A specific cooling process reduces the austenite/martensite transformation during forming. TRIP steels typically require an isothermal hold at an intermediate temperature during cooling, which produces some bainite. The additional silicon/carbon requires weld cycle modification, such as the use of pulsating welding or dilution welding.

In one approach steel is heated to a high temperature, cooled somewhat, held stable for an interval and then quenched. This produces islands of austenite surrounded by a matrix of softer ferrite, with regions of harder bainite and martensite. The resulting product can absorb energy without fracturing, making it useful for auto parts such as bumpers and pillars.

Three generations of advanced, high-strength steel are available. The first was created in the 1990s, increasing strength and ductility. A second generation used new alloys to further increase ductility, but were expensive and difficult to manufacture. The third generation is emerging. Refined heating and cooling patterns increase strength at some cost in ductility (vs 2nd generation). These steels are claimed to approach nearly ten times the strength of earlier steels; and are much cheaper to manufacture.

Intermetallics

Researches created an alloy with the strength of steel and the lightness of titanium alloy. It combined iron, aluminum, carbon, manganese, and nickel. The other ingredient was uniformly distributed nanometer-sized B2 intermetallic (two metals with equal numbers of atoms) particles. The use of nickel avoided problems with earlier attempts to use B2, while increasing ductility.