Q345R vs SA516 Gr70 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers and procurement teams commonly face a trade-off between cost, weldability, and mechanical performance when selecting pressure‑vessel steels. Q345R and ASTM A516 Grade 70 (SA516 Gr70) are two frequently compared grades for welded pressure equipment, storage tanks, and medium‑temperature vessels. Typical decision contexts include choosing a material for low‑temperature impact performance, meeting national code requirements (ASME vs. national standards), or optimizing cost and supply chain logistics.

The principal distinction between the two is their standards provenance and microalloying strategy: Q345R is a Chinese pressure‑vessel grade with HSLA characteristics and deliberate microalloying to raise yield strength, whereas SA516 Gr70 is an ASTM/ASME pressure‑vessel carbon steel specified for good tensile properties and predictable low‑temperature toughness under defined heat‑treat/test regimes. This makes them comparable for design but different in processing, weld procedure qualification, and regional availability.

1. Standards and Designations

• Q345R
• Major standard: GB/T 713 (pressure vessel steels) and related GB/T specifications in China.
• Classification: Carbon-manganese pressure‑vessel steel with microalloying (HSLA features).
• SA516 Grade 70 (A516 Gr70)
• Major standard: ASTM A516/A516M — used broadly in ASME Section II, Part A materials for pressure vessels.
• Classification: Carbon pressure‑vessel steel (conventional carbon–manganese steel with limited microalloying allowed).
• Related standards and cross-references often encountered: EN (e.g., P355 series), JIS, and other national standards — but Q345R and A516 Gr70 remain commonly compared because both are specified for welded pressure equipment.

Both are non‑stainless carbon steels intended for welded pressure‑retaining components; neither is a high‑alloy stainless material.

2. Chemical Composition and Alloying Strategy

Representative chemical composition ranges (wt%). These are typical ranges used in practice; exact limits must be verified from the controlling standard and material mill certificates.

Element	Q345R (typical, wt%)	SA516 Gr70 (typical, wt%)
C	0.10 – 0.20	≤ 0.27 (typical 0.12 – 0.27)
Mn	0.70 – 1.60	0.70 – 1.20
Si	0.15 – 0.50	0.15 – 0.40
P	≤ 0.025 – 0.030	≤ 0.035
S	≤ 0.020 – 0.030	≤ 0.035
Cr	trace – ≤ 0.30	trace – ≤ 0.30
Ni	trace – ≤ 0.30	trace – ≤ 0.30
Mo	trace – ≤ 0.20	trace – ≤ 0.20
V	trace – up to ~0.10 (microalloyed)	trace (occasionally microalloyed)
Nb (Cb)	trace – up to ~0.05 (microalloyed)	trace (occasionally present in small amounts)
Ti	trace	trace
B	trace	trace
N	trace	trace

Notes: - Q345R formulations often incorporate controlled microalloying (Nb, V, Ti) and tighter control of P/S to achieve higher yield and improved grain refinement without full quench‑and‑temper processing. - SA516 Gr70 is typically specified as a plain carbon–manganese pressure‑vessel steel with compositions optimized for toughness and weldability; some mills supply microalloyed variants, but the grade is defined by mechanical and impact acceptance criteria rather than a fixed microalloy package.

How alloying affects behavior: - Carbon and manganese primarily control strength and hardenability. Higher C increases strength but reduces weldability and toughness. - Microalloying elements (Nb, V, Ti) improve yield strength through precipitation strengthening and grain‑refinement, increasing toughness at a given strength level without large increases in carbon. - Low levels of Cr, Ni, and Mo (if present) increase hardenability and can affect toughness; these are generally low or absent in both grades.

3. Microstructure and Heat Treatment Response

Typical microstructures: - Q345R: delivered predominantly as normalized or as-rolled plate, with a ferrite–pearlite matrix refined by microalloy precipitates (Nb/Ti/V). Grain refinement and dispersion strengthening from microalloy carbides/nitrides increase yield strength while preserving ductility. - SA516 Gr70: typically delivered as normalized or as-rolled carbon–manganese steel with a ferrite–pearlite matrix; targeted heat‑treatment or controlled rolling provides ductility and impact resistance per ASTM acceptance.

Heat treatment response: - Normalizing (air cool from above critical temperature) refines grain size and improves toughness for both grades. - Quenching and tempering is not typical for pressure‑vessel plate of these grades — designed to meet mechanical and impact properties in the as‑rolled or normalized condition. - Thermo‑mechanical controlled processing (TMCP) is commonly applied to Q345R to produce higher yield strength and improved toughness through controlled rolling and accelerated cooling. - For both, post‑weld heat treatment (PWHT) is sometimes required by design codes depending on thickness, joint design, and service temperature to reduce residual stresses and temper the HAZ.

4. Mechanical Properties

Representative mechanical property ranges. Actual values depend on thickness, heat treatment, and certificate data.

Property	Q345R (representative)	SA516 Gr70 (representative)
Tensile strength (MPa)	~470 – 620	~485 – 620
Yield strength (MPa)	~345 (nominal design value)	~240 – 300 (min ~250 typical)
Elongation (A%)	20 – 25% (depending on thickness)	18 – 22%
Impact toughness (Charpy V-notch)	Specified at room or sub‑zero temperatures depending on spec; generally good with TMCP	Specified per ASTM A516 (often tested at −29°C/−20°F) with defined minimum energy
Hardness (HBW)	Typically < 200 HBW (depends on product)	Typically < 200 HBW

Interpretation: - Q345R is designed to provide a higher nominal yield (the “345” indicates 345 MPa yield class) through HSLA mechanisms, giving higher design strength for the same cross‑section. - SA516 Gr70 exhibits robust tensile properties with proven low‑temperature impact behavior when manufactured and tested to ASTM requirements; its lower yield eases forming and occasionally improves toughness in highly constrained conditions. - In practice, Q345R delivers higher yield for weight‑sensitive designs, while SA516 Gr70 provides a well‑characterized combination of tensile strength and verified impact toughness for ASME‑based fabrication.

5. Weldability

Weldability considerations hinge on carbon content, carbon‑equivalent (hardening propensity), and microalloying.

Common carbon equivalent formulas used qualitatively: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (more conservative): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both grades are considered weldable with common processes (SMAW, GMAW, SAW). SA516 Gr70, with generally higher allowed carbon ceiling, requires awareness of thickness effects and preheat requirements; A516 is widely used with established welding procedures and filler metals per ASME. - Q345R’s microalloying and potentially greater hardenability in thick sections may increase susceptibility to HAZ hardening and hydrogen‑induced cold cracking if not welded with appropriate preheat and interpass temperatures. Thus, welding procedure qualification for Q345R often includes attention to preheat, interpass temperatures, and consumable selection to match impact and tensile requirements. - Use of the above formulas helps determine need for preheat and PWHT: higher $CE_{IIW}$ or $P_{cm}$ implies more stringent preheat or lower cooling rates are advisable.

6. Corrosion and Surface Protection

• Neither Q345R nor SA516 Gr70 is stainless; corrosion resistance is that of carbon steel and both require surface protection in corrosive environments.
• Typical protections: coating systems (epoxy, polyurethane), primers, painting, and hot‑dip galvanizing (when compatible with pressure‑vessel requirements). For buried or seawater service, selection of linings, cathodic protection, or alloy upgrades is common.
• PREN (pitting resistance equivalent number) is for stainless alloys and is not applicable to non‑stainless carbon steels, but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ — PREN does not apply to Q345R or SA516 Gr70.

When to consider corrosion‑resistant alternatives: - Choose corrosion‑resistant alloys (stainless, duplex, or corrosion‑resistant claddings) when design conditions include aggressive media, chlorides, or long service without maintenance. Cladding SA516 or Q345R with stainless layers is sometimes used for pressure vessels to combine strength and corrosion resistance.

7. Fabrication, Machinability, and Formability

• Machinability: Both are standard carbon steels with similar machinability; slightly higher strength or microalloying in Q345R can increase tool wear marginally.
• Formability/bending: SA516 Gr70’s lower yield generally makes forming and cold bending easier for tight radii; Q345R’s higher yield demands larger bend radii or more force and careful control to avoid springback.
• Cutting and thermal processes: oxy‑fuel, plasma, and laser cutting are routine for both; microalloying in Q345R does not materially affect cutting but may require more attention to heat input during cutting to avoid HAZ embrittlement in heavy sections.
• Finishing: Standard grinding, polishing, and surface prep for coatings are common for both grades.

8. Typical Applications

Q345R — Typical Uses	SA516 Gr70 — Typical Uses
Pressure‑vessels and boilers in China/Asia where GB standards are used; heavy welded pressure parts requiring higher yield	Pressure vessels, boilers, storage tanks, and piping per ASME and other international codes; commonly used in North America and export markets
Thick‑plate welded structures where higher yield allows lighter sections	Fabricated tanks and boilers where verified low‑temperature impact toughness is required
Structural components in equipment that benefits from HSLA microalloy strength	General pressure‑retaining components and retrofits to ASME‑spec pipe/plate systems

Selection rationale: - Choose Q345R when higher design yield and weight savings are primary drivers, and procurement/fabrication are aligned with GB standards and suppliers. - Choose SA516 Gr70 when ASME/ASTM compliance, well‑documented weld procedures, and documented low‑temperature impact testing are required.

9. Cost and Availability

• Cost: Regionally dependent. Q345R is often cost‑competitive in China and Asia due to local production and common use. SA516 Gr70 is commonly available in the U.S., Europe, and many global markets; price competitiveness depends on local mill base, thickness, and order size.
• Availability: SA516 Gr70 has widespread availability in mill plates qualified to ASTM; Q345R is widely available in China and neighboring markets and can be sourced internationally from exporters. Lead times and certificate traceability should be confirmed for cross‑border procurement.
• Product forms: Both are supplied as plates, cut shapes, and sometimes as rolled coils; SA516 is more frequently specified for ASME boilers and pressure vessels, so supplier networks for certified plate are robust in those supply chains.

10. Summary and Recommendation

Metric	Q345R	SA516 Gr70
Weldability	Good; may need stricter preheat for thick sections due to microalloying/hardenability	Excellent; widely used with established ASME welding procedures
Strength–Toughness balance	Higher nominal yield (HSLA effect); good toughness with TMCP/normalizing	Balanced tensile and validated low‑temperature toughness per ASTM testing
Cost & Availability	Cost‑effective in Chinese/Asian markets; strong local availability	Widely available in ASME/ASTM supply chains; good global distribution

Recommendation: - Choose Q345R if you need higher nominal yield and want weight/section savings, are working within GB code or supplier base, and can control welding/preheat and acceptance testing to account for microalloying and thickness effects. - Choose SA516 Gr70 if you require ASME/ASTM compliance, predictable low‑temperature impact performance with established testing regimes, and broad global availability with familiar welding consumables and procedures.

Final note: Material selection should always be validated against the controlling design code, project welding procedure specifications (WPS/PQR), required impact testing temperatures, and mill certificates. When in doubt, review the exact chemical and mechanical limits on the mill certificate and qualify welding procedures with representative plate thickness and heat input consistent with the selected grade.