A36 vs A572 – Composition, Heat Treatment, Properties, and Applications

Introduction

ASTM A36 and ASTM A572 are two of the most widely specified structural steels in construction, fabrication, and heavy equipment. Engineers, procurement managers, and manufacturing planners typically weigh cost, required strength, weldability, and fabrication behavior when choosing between them — for example, whether to prioritize lower material cost and broad availability or higher yield strength and improved strength-to-weight.

The core distinction is that A36 is a conventional low‑carbon structural steel, while A572 (commonly specified as Grade 50 in many projects) is a high‑strength, low‑alloy (HSLA) structural steel family that achieves higher yield through controlled chemistry and microalloying. That difference drives tradeoffs in strength, toughness, weldability, and fabrication.

1. Standards and Designations

• ASTM/ASME:
• ASTM A36/A36M — Carbon structural steel.
• ASTM A572/A572M — High‑strength low‑alloy structural steel (grades 42, 50, 55, 60 are common; Grade 50 is most frequently compared with A36).
• EN: Equivalent concepts exist under EN 10025 (e.g., S235 ≈ A36, S355 ≈ A572 Grade 50), but direct equivalence requires thickness- and property-specific checks.
• JIS/GB: Japanese and Chinese standards have analogous structural steels, but chemical and mechanical requirements differ and must be cross-referenced.
• Classification:
• A36 — Carbon structural steel.
• A572 — HSLA structural steel (low alloy with microalloying elements in higher grades).

2. Chemical Composition and Alloying Strategy

The following table lists typical compositional ranges for the most relevant elements. Values are presented as typical limits or commonly encountered ranges for plate and structural shapes; actual mill certificates and the specific A572 grade must be checked for procurement.

Element	A36 (typical)	A572 Grade 50 (typical)
C	≤ 0.26 wt%	≤ 0.23 wt%
Mn	0.60–1.20 wt%	~0.70–1.35 wt%
Si	≤ 0.40 wt%	≤ 0.40 wt% (controlled)
P	≤ 0.04 wt%	≤ 0.04 wt%
S	≤ 0.05 wt%	≤ 0.05 wt%
Cr	trace	trace / controlled (may be present up to small amounts)
Ni	trace	trace
Mo	trace	trace (minor for some grades)
V	typically none	possible microalloy additions (0–0.15 wt%)
Nb (Cb)	typically none	possible microalloy additions (0–0.06 wt%)
Ti	typically none	possible microalloy additions (controlled)
B	not normally used	occasionally present in very small amounts
N	low, residual	low, controlled for HSLA processing

How alloying affects behavior - Carbon and manganese: raise strength but increase hardenability and, at higher levels, can reduce weldability and toughness. A572 typically controls carbon and increases Mn and beneficial microalloying to raise yield without large carbon increases. - Microalloying (V, Nb, Ti): small additions promote precipitation strengthening, finer grain size, and improved toughness without resorting to higher carbon — enabling higher yield strength with good ductility and weldability. - Elements such as Cr, Mo, and Ni are generally limited; A572 achieves strength mainly through HSLA mechanisms rather than heavy alloying.

3. Microstructure and Heat Treatment Response

Typical microstructures - A36: As‑rolled or normalized A36 typically exhibits a ferrite–pearlite microstructure with relatively coarse grains compared with HSLA steels. Strength derives mainly from carbon content and work hardening. - A572 (e.g., Grade 50): Processed to optimize strength and toughness, A572 often shows a finer ferrite–pearlite or ferrite with dispersed fine precipitates from microalloying elements. Thermomechanical control during rolling and controlled cooling refine grain size and disperse carbides/nitrides/ carbonitrides for precipitation hardening.

Heat treatment and thermal response - A36: Not commonly heat treated in structural use; normalization or stress relief may be used for improved toughness or to reduce residual stresses. A36 has limited response to quenching and tempering due to its nominal carbon level and intended use. - A572: Also typically supplied as rolled plate in the as‑rolled or thermomechanically processed condition. Higher grades can be produced using controlled rolling and accelerated cooling to obtain required yield strength with acceptable toughness. Quenching and tempering is not the normal commercial route for A572 structural products, but can be used for special applications.

Effect of normalizing / TMCP / Q&T - Normalizing can refine grain size and improve toughness for both steels, but A572 benefits more from thermomechanical controlled processing (TMCP) and microalloy precipitation for higher strength without large carbon penalties. - Quench & tempering will raise strength in principle but is not standard for these grades and changes weldability and residual stress expectations.

4. Mechanical Properties

The table below summarizes typical mechanical property requirements used for design and procurement. Actual values depend on grade, thickness, and purchase specification.

Property	A36 (typical)	A572 Grade 50 (typical)
Minimum Yield Strength	36 ksi (≈ 250 MPa)	50 ksi (≈ 345 MPa)
Tensile Strength	58–80 ksi (≈ 400–550 MPa)	65–85 ksi (≈ 450–585 MPa)
Elongation (in 50–200 mm gauge, depends on thickness)	~20% (minimum varies by thickness)	~18% (minimum varies by thickness)
Impact Toughness	Not universally specified; moderate at room temp; lower at cryogenic temps unless specified	Often available with specified Charpy requirements; generally better notch toughness when specified and processed
Typical Hardness (as‑rolled, approximate)	Lower than HSLA, variable with processing	Higher than A36 owing to higher yield and TMCP

Interpretation - A572 Grade 50 provides substantially higher yield strength than A36; tensile strengths overlap somewhat but A572 is higher on average. - Ductility and elongation are broadly similar when thickness and processing are controlled, but A572 can have superior toughness because of finer grain size and precipitate strengthening mechanisms. - The net effect is that A572 offers a better strength‑to‑weight ratio allowing lighter sections for the same load.

5. Weldability

Weldability depends on carbon content, hardenability (Mn, alloying), and microstructure. Two common indices used to assess weldability are the IIW carbon equivalent and the international Pcm.

Display of standard weldability indices: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation (no numerical substitution) - A36: Modest carbon and manganese give generally good weldability with common welding processes and consumables. Preheat and interpass temperature control are usually sufficient for heavy sections or low temperatures. - A572 (HSLA): Because alloying and controlled Mn can be higher and microalloying elements are present, hardenability increases slightly. However, modern A572 grades are engineered for good weldability: carbon is controlled, and microalloying levels are low enough that most structural welding procedures are suitable. For thicker sections, colder climates, or higher-strength A572 grades (e.g., 55, 60), preheat, controlled heat input, and appropriate filler metals should be specified. - Practical guidance: evaluate $CE_{IIW}$ or $P_{cm}$ for the specific mill certificate to decide preheat and post‑weld heat treatment needs. For critical welds or thick sections, perform procedure qualification (PQR) and consider impact testing.

6. Corrosion and Surface Protection

• Neither A36 nor A572 is stainless: both are plain carbon/HSLA steels and require corrosion protection for exposed environments.
• Common protection methods: hot‑dip galvanizing, shop or field painting, epoxy coatings, metallizing (zinc/Al), or weathering steel alternatives when appropriate.
• Use of stainless indices: PREN (pitting resistance equivalent number) is not applicable to A36 or A572 because they are not stainless steels. For reference, stainless evaluation uses: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Practical notes: A572’s alloying does not provide significant corrosion resistance; selection for corrosive environments should be based on coating and maintenance strategy rather than minor alloy content.

7. Fabrication, Machinability, and Formability

• Formability and bending: A36 is commonly used for bending, shaping, and forming; its more ductile as‑rolled microstructure yields predictable behavior. A572 (Grade 50) can be formed and bent but requires tighter control of bend radii and may need higher springback allowance due to higher yield.
• Cutting and machining: Both cut readily by standard methods (shearing, oxy‑fuel, plasma, laser) but A572 may be slightly harder on tooling; rates depend on thickness and heat input.
• Welding and post‑work: A572 often requires careful control of heat input during cold bending or local heating to avoid localized hardening; however, with correct procedures, it fabricates like A36 for most structural applications.
• Punching, drilling: Increased strength in A572 may demand more force or specialized tooling for hole fabrication; tool wear can be higher than with A36.

8. Typical Applications

A36 — Typical Uses	A572 (e.g., Grade 50) — Typical Uses
General structural shapes (angles, channels, I‑beams) for low‑ to moderate‑load buildings and equipment	Bridges, cranes, heavy frames, and applications where higher yield allows lighter sections
Baseplates, small to medium plates and components where cost sensitivity is primary	Fabricated steel plate where weight savings, higher allowable stress, or longer spans are required
Decorative structural elements, non‑critical welded assemblies	High‑strength bolted and welded connections, construction where code or design requires higher yield
Repairs and retrofit where matching to existing low‑strength steel is needed	Heavy machinery frames, truck bodies, and structural members subject to cyclic loading where improved toughness is desired

Selection rationale - Choose A36 when cost, ease of fabrication, and broad availability outweigh the need for high yield strength. - Choose A572 Grade 50 (or higher) when structural optimization, reduced member size, or specified higher yield and toughness are required.

9. Cost and Availability

• Relative cost: A36 is typically less expensive per ton than A572 Grade 50 owing to simpler chemistry and broader production volumes. A572 is more expensive per unit weight but can deliver lower total structure cost due to reduced material weight.
• Availability: A36 is universally available in a wide range of shapes, plates, and thicknesses. A572 Grade 50 is widely available but availability of specific thicknesses, plate sizes, and mill certifications may vary by region and product form.
• Product forms: both grades are common in plates, hot‑rolled structural shapes, and shapes fabricated to order. Lead times for A572 may be longer for specialty thicknesses or tight tolerances.

10. Summary and Recommendation

Summary table (qualitative)

Attribute	A36	A572 Grade 50
Weldability	Good (readily welded with standard procedures)	Good to Very Good (requires attention for thicker sections; evaluate CE/Pcm)
Strength–Toughness	Lower yield, acceptable toughness; simple processing	Higher yield and typically better toughness when processed; better strength‑to‑weight
Cost	Lower unit price, very available	Higher unit price, potential life‑cycle savings from reduced weight

Conclusion and practical choice guidance - Choose A36 if: - Project prioritizes lowest material cost and straightforward fabrication. - Structural demands are modest (design based on 36 ksi yield is adequate). - Broad availability, ease of welding without special procedures, and matching existing low‑strength parts are important.

• Choose A572 (commonly Grade 50) if:
• Higher yield strength is required to reduce section size, decrease weight, or meet code requirements.
• Improved toughness and fatigue/impact performance are needed in a structural context.
• You accept a modest premium per ton for material that can reduce fabrication or shipping costs and improve structural efficiency.

Final practical note: always consult the mill certificate and the full ASTM specification for the supplied lot (including thickness‑dependent requirements and any Charpy impact or supplementary requirements). For critical welded structures or low‑temperature service, perform weld procedure qualification and request the specific A572 grade and heat‑treatment/processing history on the purchase order so mechanical and toughness properties meet design needs.