High-Strength Structural and High-Strength Low-Alloy Steels

High-strength carbon and low-alloy steels have yield strengths greater than 275 MPa and can be more or less divided into four classes:

• As-rolled carbon-manganese steels
• As-rolled high-strength low-alloy (HSLA) steels (which are also known as micro alloyed steels)
• Heat-treated (normalized or quenched and tempered) carbon steels
• Heat-treated low-alloy steels.

These four types of steels have higher yield strengths than mild carbon steel in the as-hot-rolled condition. The heat-treated low-alloy steels and the as-rolled HSLA steels also provide lower ductile-to-brittle transition temperatures than do carbon steels.

These four types of high-strength steels have some basic differences in mechanical properties and available product forms. In terms of mechanical properties, the heat-treated (quenched and tempered) low-alloy steels offer the best combination of strength and toughness.

Structural Carbon Steels

Structural carbon steels include mild steels, hot-rolled carbon-manganese steels, and heat-treated carbon steels. Mild steels and carbon-manganese steels are available in all the standard wrought forms: sheet, strip, plate, structural shapes, bar, bar-size shapes, and special sections. The heat-treated grades are available as plate, bar and. occasionally, sheet and structural shapes.

Mild (low-carbon) steels are normally considered to have carbon contents up to 0.25% C with about 0.4 to 0.7% Mn, 0.1 to 0.5% Si and some residuals of sulfur, phosphorus, and other elements. These steels are not deliberately strengthened by alloying elements other than carbon; they contain some manganese for sulfur stabilization and silicon for deoxidation. Mild steels are mostly used in the as-rolled, forged, or annealed condition and are seldom quenched and tempered.

The largest category of mild steels is the low-carbon (<0.08% C, with <0.4% Mn) mild steels used for forming and packaging. Mild steels with higher carbon and manganese contents have also been used for structural products such as plate, sheet, bar, and structural sections.

High-strength structural carbon steels have yield strengths greater than 275 MPa and are available in various product forms:

• Cold-rolled structural sheet
• Hot-rolled carbon-manganese steels in the form of sheet, plate, bar, and structural shapes
• Heat-treated (normalized or quenched and tempered) carbon steels in the form of plate, bar, and occasionally, sheet and structural shapes.

The heat treatment of carbon steels, which typically attain yield strengths of 290 to 690 MPa, consists of either normalizing or quenching and tempering. These heat treatments can be used to improve the mechanical properties of structural plate, bar, and occasionally, structural shapes.

Normalizing involves air-cooling from austenitizing temperatures and produces essentially the same ferrite-pearlite microstructure as that of hot-rolled carbon steel, except that the heat treatment produces a finer grain size. This grain refinement makes the steel stronger, tougher, and more uniform throughout.

Quenching and tempering, that is heating to about 900^oC, water quenching, and tempering at temperatures of 480 to 600^oC or higher, can provide a tempered martensitic or bainitic microstructure that results in better combinations of strength and toughness. An increase in the carbon content to about 0.5%, usually accompanied by an increase in manganese, allows the steels to be used in the quenched and tempered condition.

Quenched and Tempered Low-Alloy Steel

Alloy steels are defined as those steels that:

1. contain manganese, silicon, or copper in quantities greater than the maximum limits (1.65% Mn, 0.60% Si, and 0.60% Cu) of carbon steel; or
2. that have specified ranges or minimums for one or more other alloying additions.

The low-alloy steels are those steels containing alloy elements, including carbon, up to a total alloy content of about 8.0%.

Except for plain carbon steels that are micro alloyed with just vanadium, niobium, and/or titanium, most low-alloy steels are suitable as engineering quenched and tempered steels and are generally heat treated for engineering use. Low-alloy steels with suitable alloy compositions have greater hardenability than structural carbon steel and, thus, can provide high strength and good toughness in thicker sections by heat treatment. Their alloy contents may also provide improved heat and corrosion resistance. However, as the alloy contents increase, alloy steels become more expensive and more difficult to weld. Quenched and tempered structural steels are primarily available in the form of plate or bar products.

Alloying Elements and Their Effect on Hardenability and Tempering. Quenched and tempered steels have carbon contents in the range of 0.10 to 0.45%, with alloy contents, either singly or in combination, of up to 1.5% Mn, 5% Ni, 3% Cr, 1% Mo, 0.5% V, 0.10% Nb; in some cases they contain small additions of titanium, zirconium and/or boron. Generally, the higher the alloy content, the greater the hardenability and the higher the carbon content, the greater the available strength. The response to heat treatment is the most important function of the alloying elements in these steels.

Microalloyed Quenched and Tempered Grades. Although fittings with 0.69% Mn and induction bends use quenching and tempering as a standard practice, mild steels (plain, low-carbon steels with less than 0.7% Mn) with microalloying additions of vanadium, niobium, or titanium are seldom used as quenched and tempered steels.
However, elements such as boron and vanadium are considered as substitutes for other elements that enhance hardenability. The titanium was added in order to form titanium nitride, thereby retaining an increased amount of vanadium in solution. This provided for a more efficient use of vanadium as a hardenability agent.

Some scientist investigated completely V-substituted variants of 4140 base series (0.4C-1Cr) with titanium additions, as well as partially V-substituted variants with and without titanium additions. These studies concluded that:

• Complete substitution of molybdenum by vanadium does not increase the hardenability over standard 4140 (0.20% Mo) even when all the vanadium is dissolved during austenitization
• Steels containing 0.1 to 0.2% V and 0.04% Ti are characterized by significantly increased hardenability (10 to 25% in D1) over standard 4140
• Microalloy combinations of V + Mo + Ti (~0.06-0.06-0.04%) provide very high hardenability with D1 being up to 60% greater than the D1, in standard 4140 with 0.20% Mo. This effect is completely absent in partially substituted steel without titanium.

The pronounced effect on hardenability of molybdenum-vanadium combinations without titanium as observed by Manganon in 4330 steels, can probably be reconciled with the third result of Sandberg in that the latter studied steels containing 0.06% Al, which would be expected to remove nitrogen to about the same extent as 0.04% Ti.

Quenched and tempered alloy steels can offer a combination of high strength and good toughness. In addition, quenched and tempered alloy steel plate is available with ultrahigh strengths and enhanced toughness. Enhanced toughness and high strength are achieved in the nickel-chromium-molybdenum alloys, which include steels such as ASTM A 543, HY-80, HY-100 and HY-130. These steels use nickel to improve toughness.

High-Nickel Steels for Low-Temperature Service. For applications involving exposure to temperatures from 0 to -195^oC, the ferritic steels with high nickel contents are typically used. Such applications include storage tanks for liquefied hydrocarbon gases and structures and machinery designed for use in cold regions. These steels utilize the effect of nickel content in reducing the impact transition temperature, thereby improving toughness at low temperatures. Carbon and alloy steel castings for subzero-temperature service are covered by ASTM standard specification A 757.

The 5% Ni alloys for low-temperature service include HY-130 and ASTM A 645. For steel purchased according to ASTM A 645 minimum Charpy V-notch impact requirements for 25 mm plate are designated at -170^oC for hardened, tempered, and reversion-annealed plate.

Double normalized and tempered 9% nickel steel is covered by ASTM A 353, and quenched and tempered 8% and 9% nickel steels are covered by ASTM A 553 (types I and II). For quenched and tempered material, the minimum lateral expansion in Charpy V-notch impact tests is 0.38 mm. Testing of typical tensile properties of 5% and 9% Ni steels at room temperature and at subzero temperatures shows that yield and tensile strengths increase as testing temperature is decreased. These steels remain ductile at the lowest resting temperatures.

Ferritic nickel steels are too tough at room temperature for valid fracture toughness (K_Ic) data to be obtained on specimens of reasonable size, but limited fracture toughness data have been obtained on these steels at subzero temperatures by the J-integral method. The 5% Ni steel retains relatively high fracture toughness at -162^oC and the 9% Ni steel retains relatively high fracture toughness at -196^oC. These temperatures approximate the minimum temperatures at which these steels may be used.