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

Introduction

ASTM A36 and ASTM A572 are two of the most commonly specified structural steels in construction, fabrication, and heavy industry. Engineers, procurement managers, and manufacturing planners often weigh cost, weldability, formability, and required mechanical performance when choosing between them. Typical decision contexts include: (1) whether higher yield strength justifies a premium material cost and potential changes to welding or forming procedures, and (2) whether improved toughness or reduced weight (through thinner sections) is required for a given design.

The practical distinction between these grades comes down to mechanical performance achieved by different alloying and processing strategies: A36 is a traditional carbon structural steel with relatively simple chemistry and predictable behavior, while A572 is a high-strength, low-alloy (HSLA) family produced to deliver higher yield and often better toughness through controlled alloying and thermomechanical processing. Engineers compare them because both serve the same structural roles but with different trade-offs in strength, ductility, weldability, and cost.

1. Standards and Designations

• ASTM/ASME:
• ASTM A36/A36M — Carbon structural steel (commonly referred to as A36).
• ASTM A572/A572M — High-strength low-alloy structural steel (available in multiple grades such as 42, 50, 55, 60, 65; Grade 50 is most common).
• EN: Roughly analogous specifications exist in European standards (e.g., S275 for lower-strength general structural steel; S355 and HSLA grades for higher strength), but direct one-to-one equivalence is not exact.
• JIS/GB: Japanese and Chinese standards have comparable structural steels, but users must compare mechanical requirements and chemistry rather than rely on direct name equivalence.
• Classification: A36 — carbon structural steel; A572 — HSLA (high-strength, low-alloy) structural steel.

2. Chemical Composition and Alloying Strategy

Table: Typical composition (wt%) — general guidance. Exact chemical limits depend on the specific ASTM subclauses and the A572 grade selected; consult mill certificates for procurement-critical tolerances.

Notes: - A36 is essentially a plain carbon structural steel with limited intentional alloying. A572 is a family of HSLA steels where controlled additions of microalloying elements (V, Nb, Ti) and tighter control of Mn and Si enable higher yield strengths and improved toughness without greatly increasing carbon equivalent. - The exact composition of A572 varies by grade (42, 50, etc.) and by mill; procurement should specify grade and request material test reports (MTRs).

How alloying affects properties: - Carbon raises strength and hardenability but can reduce weldability and toughness when high. - Manganese increases hardenability and tensile strength and promotes deoxidation. - Microalloying elements (V, Nb, Ti) refine grain size and produce precipitation strengthening that increases yield strength without large increases in carbon. - Small additions of Cr, Mo, Ni (when present) increase hardenability and high-temperature performance but are uncommon in standard A36/A572.

3. Microstructure and Heat Treatment Response

• A36: Typical as-rolled microstructure is ferrite with pearlite—coarse-grained compared to HSLA steels. A36 is generally supplied in the hot-rolled condition. It is not intended for significant heat treatment; properties are achieved in the as-rolled state. Normalizing can refine grain size and slightly improve strength and toughness.
• A572: Produced with controlled chemistry and often with thermomechanical rolling or accelerated cooling to produce a finer ferrite-pearlite or acicular ferrite microstructure and beneficial precipitates (carbonitrides of V, Nb, Ti). This refined microstructure provides higher yield strength and better toughness compared with plain carbon steels of similar thickness.
• Heat treatment response:
• Normalizing: both grades can be normalized to refine grain size; A572 typically responds better because microalloy precipitates control grain growth.
• Quench and tempering: not typically applied to A36 or standard A572 in structural practice; if high-strength quenched & tempered steels are required, different specifications are used.
• Thermo-mechanical processing (TMCP): A572 may be produced by TMCP to exploit controlled rolling and cooling to maximize strength and toughness without heavy alloying.

4. Mechanical Properties

Table: Typical mechanical properties (mill condition; values represent common minimums or ranges — check ASTM spec and mill test report).

Interpretation: - A572 (especially Grade 50) provides clearly higher minimum yield strength than A36, enabling design weight reductions or smaller sections for the same load. - Ductility (elongation) is often comparable, though higher-strength materials sometimes exhibit slightly lower elongation; modern A572 processing often preserves good toughness and acceptable ductility. - Toughness (low-temperature impact resistance) is frequently better controlled and specified for A572 applications, particularly when used in critical structures.

5. Weldability

Weldability depends on carbon content, carbon equivalent (hardenability), and presence of microalloying elements. Two commonly used empirical parameters are shown here for interpretation.

• IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• International Pcm (for more detailed prediction of weldability and cold cracking susceptibility): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - A36 typically has a low carbon equivalent and is widely regarded as readily weldable with standard processes (SMAW, GMAW, FCAW). Preheat and interpass temperatures are generally modest except for very thick sections or hydrogen-sensitive service. - A572 grades, despite higher strength, are engineered to remain weldable. Microalloying in controlled amounts increases strength without large increases in $CE_{IIW}$. However, higher-strength grades, thicker sections, or increased manganese and microalloy content can raise hardenability and require more careful welding procedures (preheat, controlled heat input, low-hydrogen consumables) to avoid crack susceptibility. - Practical advice: For critical structures, specify post-weld heat treatment (if required), control preheat and interpass temperature, and confirm weld procedures with procedure qualification records (PQRs).

6. Corrosion and Surface Protection

• Neither A36 nor standard A572 are stainless steels. Both require surface protection for corrosive environments.
• Common protective strategies:
• Hot-dip galvanizing (zinc coating) for long-term atmospheric corrosion resistance.
• Shop primer, field painting, or specialized industrial coatings for severe environments.
• Cladding or duplex systems if required for aggressive conditions.
• PREN (Pitting Resistance Equivalent Number) formula for stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to A36 and A572 because their chemistries lack the chromium/molybdenum/nitrogen content that confers stainless behavior. For corrosion-critical applications, choose stainless or corrosion-resistant alloys rather than relying on galvanizing/coatings alone.

7. Fabrication, Machinability, and Formability

• Cutting: Both steels are readily oxy-fuel cut, plasma cut, or laser cut. Higher-strength A572 may require slightly different cutting parameters due to higher tensile strength.
• Forming and bending: A36, with its lower yield and simple microstructure, is generally easier to form and bend with less springback. A572, because of its higher yield, will show more springback and may require adjustments (e.g., higher bending force or tighter tooling radii). Cold forming of higher-grade A572 should be validated.
• Machinability: Both are machinable with standard tooling; A572 may be slightly more abrasive to cutting tools depending on microalloy precipitates.
• Fastening: Bolt and weld design must account for higher strength when using A572 — e.g., bolt pretensioning and bearing calculations.

8. Typical Applications

Selection rationale: - Choose A36 when cost, ease of fabrication/welding, and availability for common thicknesses are driving factors. - Choose A572 when higher yield strength, controlled toughness, and potential weight savings are priorities.

9. Cost and Availability

• Cost: A36 is typically less expensive by unit weight than A572 because of simpler chemistry and more widespread production. A572 commands a premium for higher-strength grades and tighter processing control.
• Availability: A36 is ubiquitously available in a wide range of shapes and plate thicknesses. A572 (especially Grade 50) is widely available but less ubiquitous than A36 in some lower-volume product forms and thicknesses. Lead times can vary by region and product form (plate, coil, wide flange).
• Procurement tip: Specify exact grade, product form, and any supplemental requirements (impact testing temperature, surface condition, coating) to avoid pricing and lead-time surprises.

10. Summary and Recommendation

Table: Quick comparison

Conclusions: - Choose A36 if: - The structural application is routine and does not require high yield strength. - Ease of fabrication, broad availability, and lowest material cost are priorities. - Welding and forming simplicity are important and no special toughness is needed.

• Choose A572 if:
• Higher yield strength is required (e.g., Grade 50 offers a distinct design advantage).
• You want potential weight or section-size reductions while maintaining good toughness.
• The project can accommodate slightly tighter welding and fabrication controls and tolerates a higher material unit cost for lifecycle or performance benefits.

Final note: Always specify the exact ASTM grade, product form, thickness, and any supplemental requirements (impact testing temperatures, coating, or welding procedures) and require mill test reports (MTRs) with material deliveries. For critical designs, perform welding procedure qualifications and consult mill/steel supplier data sheets to align chemistry and mechanical properties to the intended use.