ASTM A36 vs A572 Steel: Mechanical Properties, Uses and Selection Guide

ASTM A36 and ASTM A572 are the two most widely specified structural carbon and high-strength low-alloy (HSLA) steels in North American construction, bridgework, and industrial fabrication. Understanding their compositional differences, mechanical property envelopes, weldability characteristics, and appropriate application domains is essential for any structural materials engineer or fabricator working to AISC or AASHTO design criteria.

Specification Overview

ASTM A36: Standard Specification for Carbon Structural Steel

ASTM A36, first issued in 1960, covers carbon steel shapes, plates, and bars for structural applications. It is produced in the as-rolled condition and achieves its properties primarily through carbon and manganese additions. A36 is notable for a relatively wide carbon range (up to 0.26 wt% for plates under 20 mm), which historically made it the workhorse of structural fabrication but also imposes constraints on weldability for heavier sections. The tensile strength range of 400–550 MPa (58–80 ksi) reflects the variability inherent in the broad chemistry window.

ASTM A572: High-Strength Low-Alloy Columbium-Vanadium Structural Steel

ASTM A572, introduced in 1966, is a family of HSLA steels that achieves elevated yield strength through grain refinement and precipitation strengthening, principally via columbium (niobium) and/or vanadium microadditions, rather than through increased carbon content. This approach preserves weldability while delivering substantially higher strength. The standard covers five grades — 42, 50, 55, 60, and 65 — each designating the minimum yield strength in ksi. Grade 50 accounts for the overwhelming majority of production volume.

Chemical Composition

Source: ASTM A36/A36M-19 and ASTM A572/A572M-21. Values for shapes and bars may differ from plate; consult current standards for thickness-specific limits.

The Role of Microalloying in A572

The key metallurgical distinction between A36 and A572 lies in the microalloying additions in A572. Niobium at concentrations of 0.02–0.05 wt% strongly retards austenite recrystallisation during hot rolling by solute drag and by Nb(C,N) precipitate pinning of austenite grain boundaries. This produces a finer ferrite grain size in the final microstructure, raising yield strength by the Hall-Petch relationship. For more on the grain boundary physics underlying this strengthening mechanism, see the Grain Boundaries Guide.

Vanadium acts predominantly through precipitation hardening: V(C,N) precipitates form on dislocations and sub-grain boundaries during and after transformation, contributing 50–100 MPa of additional yield strength depending on nitrogen availability. Because both mechanisms are active at lower carbon levels than solid-solution or pearlite strengthening, A572 achieves its strength without the weldability penalties associated with higher-carbon A36 heats.

Mechanical Properties

Yield-to-Tensile Ratio Implications

The increasing Fy/Fu ratio with grade is an important design consideration, particularly for seismic and ductility-critical applications. A low Fy/Fu ratio (as in A36 at 0.55–0.70) indicates a large strain-hardening reserve between yield and fracture, which provides ductility and energy absorption capacity under overload. As yield strength rises toward tensile strength in the higher A572 grades, the plastic deformation capacity decreases proportionally. AISC 341 (Seismic Provisions) and FEMA 350 explicitly limit Fy/Fu to 0.85 for certain moment frame applications, which is why Grade 50 is preferred over Grade 65 for seismic force-resisting systems.

The martensite formation behaviour in these steels under rapid cooling is also relevant: higher-strength A572 grades, particularly 60 and 65, have a lower martensite-start temperature margin due to slightly elevated alloy content, which reinforces the importance of controlled heat input in welding.

Hardness and Machinability

A36 typically presents a Brinell hardness of 119–162 HBW in the as-rolled condition, while A572 Grade 50 runs slightly higher at 137–179 HBW. Neither steel is specified by hardness; the values emerge from the microstructure resulting from the as-rolled chemistry and cooling rate. Both steels machine readily with standard HSS or carbide tooling; the higher strength of A572 marginally increases cutting forces but presents no practical machinability penalty in structural fabrication contexts.

Microstructure of A36 and A572 in the As-Rolled Condition

Both A36 and A572 Grade 50, in their standard as-rolled delivery condition, exhibit a predominantly ferrite-pearlite microstructure. The primary metallurgical difference is grain size and pearlite fraction. A36, produced with higher carbon and no grain refinement additions, typically shows ASTM grain size numbers of 6–8 (mean grain diameter 22–45 μm) with 15–25% pearlite by volume at the 0.20–0.26 wt% C range. For a detailed treatment of pearlite colony morphology and growth mechanisms, refer to the Pearlite Colony Growth article.

A572 Grade 50, particularly when produced by controlled rolling (thermomechanical controlled processing, TMCP), achieves ASTM grain size numbers of 9–12 (mean grain diameter 8–16 μm). This grain refinement is the most thermodynamically efficient strengthening mechanism available to structural steel producers because it simultaneously increases both yield strength and low-temperature toughness — uniquely so among the strengthening mechanisms. The Hall-Petch equation quantifies this relationship:

Precipitation strengthening from V(C,N) and NbC contributes an additional 50–100 MPa in well-optimised A572 heats, bringing the total yield strength increment over A36 to the observed 95 MPa (from 250 to 345 MPa). The iron-carbon phase diagram and the eutectoid reaction are foundational to understanding why pearlite fraction and carbon distribution govern the baseline strength before microalloying additions take effect.

Weldability and Welding Procedures

Weldability of structural steels is governed primarily by the carbon equivalent (CE) and the hardenability of the heat-affected zone (HAZ). Both A36 and A572 Grade 50 are classified as readily weldable, but there are important differences in the mechanism and management of weld-related risks. For a thorough treatment of HAZ metallurgy and the associated transformation behaviour, see the HAZ Microstructure article.

Preheat and Interpass Temperature

Filler Metal Matching

For A36, E70XX electrodes (tensile strength 480 MPa / 70 ksi) are the standard choice and provide overmatching relative to the 400 MPa minimum tensile strength of A36. For A572 Grade 50, E70XX electrodes match the 450 MPa (65 ksi) tensile requirement of the base metal with adequate margin. For Grades 55 and above, E80XX or E90XX consumables may be required to ensure weld metal tensile strength does not undercut the base metal in single-pass or low-dilution situations. Procedure qualification per AWS D1.1 Section 4 governs in all cases.

Heat Treatment Behaviour

Neither A36 nor A572 is routinely heat-treated in structural fabrication — both are supplied in the as-rolled condition and designed to meet their specification in that state. However, fabricators and engineers encounter heat treatment in the context of annealing and normalising, post-weld heat treatment (PWHT), and hot forming.

Normalising

Normalising at 900–930°C (1650–1700°F) followed by air cooling will refine grain structure and homogenise composition in both steels. For A572, normalising may moderately reduce yield strength (typically by 20–40 MPa) because it dissolves and re-precipitates V(C,N) particles less efficiently than the controlled rolling process, and coarsens the grain structure relative to the TMCP condition. If normalised A572 is required, properties must be re-verified against the specification minimum.

Quenching and Tempering

Both A36 and A572 Grade 50 have insufficient hardenability for through-hardening in plate sections by quench and temper (Q&T). Thin sections (<10 mm) may achieve martensite near the surface, but the core transforms to bainite or ferrite-pearlite under practical quench rates. This is by design: the quenching and tempering process is not part of the delivery condition for either specification, and designers should not assume Q&T properties unless a separate Q&T specification (such as ASTM A514 or A517) is invoked. The bainite microstructure that forms under intermediate cooling rates in these steels has properties intermediate between the as-rolled and fully martensitic conditions.

Post-Weld Heat Treatment

PWHT by stress relief at 600–650°C (1112–1202°F) is applied to welded A36 and A572 fabrications when dimensional stability, residual stress reduction, or certain pressure vessel codes require it. For A572, the temperature ceiling for PWHT should not exceed 650°C for extended soaks, as prolonged exposure above this temperature can dissolve V(C,N) precipitates and reduce yield strength permanently. ASME BPVC Section VIII and AWS D1.1 both specify maximum PWHT temperatures and soak times calibrated to avoid this effect.

Industrial Applications and Selection Criteria

ASTM A36 Applications

A36 retains a presence in modern construction due to its wide availability, low cost, and predictable fabrication behaviour. Typical applications include secondary structural members (girts, purlins, bracing angles), base plates, anchor rods, miscellaneous connection plates, stairs and handrailing, and any application where the design is deflection-governed rather than strength-governed (i.e., where the structure is sized to limit deformation under service loads, not to resist ultimate stress). In these cases the 38% yield strength advantage of A572 Grade 50 provides no benefit because section size is already determined by stiffness requirements.

ASTM A572 Grade 50 Applications

A572 Grade 50 has displaced A36 as the standard specification for W-shape structural sections across virtually all major US mills. It is specified for primary moment frames, column members, girders, floor beams, and composite construction. The higher yield strength allows material savings of 25–35% by mass compared to A36-designed sections at the same load, which reduces dead load, foundation costs, and erection time. AASHTO M270 Grade 50 (essentially A572 Grade 50 with supplemental requirements) governs primary bridge girders, diaphragms, and floor beams.

Seismic Applications

AISC 341 (Seismic Provisions for Structural Steel Buildings) designates A572 Grade 50 as a pre-qualified material for special moment frames (SMF), intermediate moment frames (IMF), and eccentrically braced frames (EBF). The actual yield strength of the delivered material must be checked against the expected yield strength ratio (Ry) to ensure plastic hinge development assumptions remain valid. For A572 Grade 50, Ry = 1.1 per AISC 341 Table A3.1. A36 is also pre-qualified but rarely specified for seismic primary members in modern practice.

Offshore and Atmospheric Corrosion

Standard A36 and A572 have similar corrosion performance in atmospheric environments, both requiring protective coatings or galvanising for exterior exposure. When atmospheric corrosion resistance is needed without coating, ASTM A588 (weathering steel) or A242 should be specified instead. Both A36 and A572 may be ordered with a minimum 0.20 wt% copper content (the “Cu” supplemental requirement) for marginal improvement in atmospheric corrosion resistance, but this does not replicate the full weathering steel performance.

Practical Selection Guide

Related Standards and International Equivalents

Specification equivalence is never exact. EN S355 carries mandatory Charpy toughness sub-grades (J0, J2, K2) that A572 Grade 50 does not; S355 has lower maximum carbon (0.20 vs 0.23 wt%) and tighter composition control. For international project specifications, cross-reference mechanical properties, toughness requirements, dimensional tolerances, and marking requirements individually rather than assuming grade-for-grade substitution. The Charpy impact test is particularly important in this comparison because it governs which sub-grade of EN S355 is required for a given design temperature, while A572 leaves toughness entirely to supplemental ordering requirements.

For corrosion behaviour, both steels fall in the same general category; the detailed mechanisms are covered in the Corrosion Mechanisms and Pitting Corrosion articles. Hardness testing methods applicable to acceptance of both steels are reviewed in the Hardness Testing Methods guide.