Low-Alloy Steel Info
Steels can be classified by a variety of different systems depending on:
  • The composition, such as carbon, low-alloy or stainless steel.
  • The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods.
  • The finishing method, such as hot rolling or cold rolling
  • The product form, such as bar plate, sheet, strip, tubing or structural shape
  • The de-oxidation practice, such as killed, semi-killed, capped or rimmed steel
  • The microstructure, such as ferritic, pearlitic and martensitic
  • The required strength level, as specified in ASTM standards
  • The heat treatment, such as annealing, quenching and tempering, and thermo mechanical processing
  • Quality descriptors, such as forging quality and commercial quality.

High-Strength Low-Alloy Steels: (HSLA) steels, or micro alloyed steels, are designed to p rovide better mechanical properties and/or greater resistance to atmosp heric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mech anical properties rather than a chemical composition. The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldabi lity, and they have manganese contents up to 2.0%. Small qua ntities of chromium, nickel, molybdenum, copper, nitrog en, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:
  • Weathering steels, designated to exhibit superior atmospheric corrosion resistance
  • Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenite structure that will transform to a v ery fine equiaxed ferrite structure on cooling
  • Pearlite-reduced steels , strengthened by very fine-grain ferrite and precipi tation hardening but with low carbon content and therefore little or no pearlite in the microstructure
  • Micro alloyed steels , with very small additions of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardeni ng
  • Acicular ferrite steel , very low carbon steels with sufficient hardenability t o transform on cooling to a very fine high-strength acicular ferrite structure rather than the usual polygonal ferrite structure
  • Dual-phase steels , processed to a micro-structure of ferrite containing sm all uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, t hus providing a high-strength steel of superior formabili ty.
The various types of HSLA steels may also have small addi tions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

Low-alloy Steels: Low-alloy steels constitute a category of ferrous materia ls that exhibit mechanical properties superior to plain carbon steels as the result o f additions of alloying elements such as nickel, chromiu m, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which cont ain a minimum of 10% Cr. For many low-alloy steels, the p rimary function of the alloying elements is to increase hardenability in order to optimize mechanical propert ies and toughness after heat treatment. In some cases, how ever, alloy additions are used to reduce environmental degra dation under certain specified service conditions.

As with steels in general, low-alloy steels can be classifie d according to:
  • Chemical composition , such as nickel steels, nickel-chromium steels, molybdenum steels, chromium- molybdenum steels
  • Heat treatment , such as quenched and tempered, normalized and tempere d, annealed.
Because of the wide variety of chemical compositions possi ble and the fact that some steels are used in more than one heat-treated, condition, some overlap exists among the alloy steel classifications. In this article, four maj or groups of alloy steels are addressed: (1) low-carbon quen ched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) h eat-resistant chromium-molybdenum steels.

Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) a nd high tensile strength with good notch toughness, ductility, corrosion r esistance, or weldability. The various steels have differ ent combinations of these characteristics based on their intende d applications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steel s listed are used primarily as plate. Some of these stee ls, as well as other, similar steels, are produced as forgings or castings.

Medium -carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designation s or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and weldin g wire.

Bearing steels used for ball and roller bearing applications are comp rised of low carbon (0.10 to 0.20% C ) case- hardened steels and high carbon (-1.0% C) through-har dened steels. Many of these steels are covered by SAE/AIS I designations.

Chromium -molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo . The carbon content is usually below 0.2%. The chromium provides improved ox idation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are ge nerally supplied in the normalized and tempered, que nched and tempered or annealed condition. Chromium-molybde num steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.

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