Alloy steel

Alloy steels divide into two groups: low and high alloy. The boundary between the two is disputed. Smith and Hashemi define the difference at 4.0%,^[1] while Degarmo, et al., define it at 8.0%.^[2] Most alloy steels are low-alloy.

The simplest steels are iron (Fe) alloyed with (0.1% to 1%) carbon (C) and nothing else (excepting slight impurities); these are called carbon steels. However, alloy steel encompasses steels with additional (metal) alloying elements. Common alloyants include manganese (Mn) (the most common), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si), and boron (B). Less common alloyants include aluminum (Al), cobalt (Co), copper (Cu), cerium (Ce), niobium (Nb), titanium (Ti), tungsten (W), tin (Sn), zinc (Zn), lead (Pb), and zirconium (Zr).

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 developed as improved tool steels and as newly available stainless steels. Alloy steels serve many applications, from hand tools and flatware to turbine blades of jet engines and in nuclear reactors.

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

Alloying elements are added to achieve specific properties in the result. The alloying elements can affect multiple properties—flexibility, strength, formability, and hardenability.^[4] 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 achieve properties such as corrosion resistance or extreme temperature stability.^[2]

• Manganese, silicon, or aluminum are added during steelmaking to remove dissolved oxygen, sulfur and phosphorus.
• Manganese, silicon, nickel, and copper are added to increase strength by forming solid solutions in ferrite.
• Chromium, vanadium, molybdenum, and tungsten increase strength by forming second-phase carbides.
• Nickel and copper improve corrosion resistance in small quantities. Molybdenum helps to resist embrittlement.
• Zirconium, cerium, and calcium increase toughness by controlling the shape of inclusions.
• Sulfur (in the form of manganese sulfide), lead, bismuth, selenium, and tellurium increase machinability.^[5]

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

• Nickel is soluble in ferrite; therefore, it forms compounds, usually Ni₃Al.
• Aluminum dissolves in ferrite and forms the compounds Al₂O₃ and AlN. Silicon is also soluble and usually forms the compound SiO₂•M_xO_y.
• Manganese mostly dissolves in ferrite forming the compounds MnS, MnO•SiO₂, but also forms 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 & 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.^[6]

Alloying elements also have an effect on the eutectoid temperature of the steel.

• Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may be obtained at room temperature.
• Carbide-forming elements raise the eutectoid temperature; these elements are known as ferrite stabilizing elements.^[7]

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.^[9]

TRIP steels transform under deformation from relatively ductile to relatively hard under deformation such as a car crash. Such 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 requirements requires weld cycle modification, such as the use of pulsating welding or dilution welding.^[10]

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 beginning to be adopted. Refined heating and cooling patterns increase both 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.^[10]

• Dual-phase steel
• HSLA steel
• Microalloyed steel
• SAE steel grades
• Reynolds 531

1. ^ Smith & Hashemi 2001, p. 393.
2. ^ ^{a b} Degarmo, Black & Kohser 2007, p. 112.
3. ^ Smith & Hashemi 2001, p. 394.
4. ^ "What Are the Different Types of Steel? | Metal Exponents Blog". Metal Exponents. 2020-08-18. Retrieved 2021-01-29.
5. ^ Degarmo, Black & Kohser 2007, p. 113.
6. ^ Smith & Hashemi 2001, pp. 394–395.
7. ^ Smith & Hashemi 2001, pp. 395–396.
8. ^ Degarmo, Black & Kohser 2007, p. 144.
9. ^ Johnson, Jr, John (2024-08-05). "New forms of steel for stronger, lighter cars". Knowable Magazine. doi:10.1146/knowable-080524-1.
10. ^ ^{a b} Hickey, Kate (2021-06-23). "Transformation Induced Plasticity (TRIP)". AHSS Guidelines. Retrieved 2024-08-21.

• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2007), Materials and Processes in Manufacturing (10th ed.), Wiley, ISBN 978-0-470-05512-0.
• Groover, Mikell P. (February 26, 2009). FUNDAMENTALS OF MODERN MANUFACTURING: Materials, Processes, and Systems. John Wiley & Sons, Inc.
• Smith, William F.; Hashemi, Javad (2001), Foundations of Material Science and Engineering (4th ed.), McGraw-Hill, p. 394, ISBN 0-07-295358-6