A516 Gr60 vs A516 Gr70 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
ASTM A516 is a family of carbon steel pressure‑vessel plates widely used for boilers, pressure vessels, and storage tanks. The common engineering choice between A516 Grade 60 and Grade 70 is a tradeoff between material cost, required strength, and the toughness/weldability behavior demanded by the service environment. Engineers, procurement specialists, and fabricators routinely weigh factors such as plate thickness, operating temperature, impact requirements, and welding constraints when selecting between these two grades.
The primary distinction between A516 Gr60 and A516 Gr70 is their strength/toughness balance: Grade 70 is the higher‑strength specification (hence often chosen where higher design stresses or reduced section thickness are required), while Grade 60 offers lower specified strength with generally slightly more favorable ductility and cost in typical practice. Because both grades come from the same specification family and share very similar chemistry and processing options, they are compared most often in pressure‑containing and welded plate fabrication contexts.
1. Standards and Designations
- Primary standard: ASTM A516 / A516M — “Standard Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower‑Temperature Service.”
- Related codes: ASME Section II and VIII reference material allowable stresses based on A516 grade and temperature. National or regional equivalents may exist for plate supply, but A516 is the primary industrial designation in North America.
- Other standards: EN, JIS, or GB standards may offer alternative pressure‑vessel steels (e.g., P265, S355, SS41), but direct one‑to‑one equivalence requires careful property matching.
- Material classification: A516 grades are carbon steel pressure‑vessel plates (non‑stainless, non‑tool, generally considered plain carbon or low‑alloy depending on mill additions). They are not high‑strength low‑alloy (HSLA) steels in the narrow modern sense, though mills may use microalloying and controlled rolling to meet mechanical targets.
2. Chemical Composition and Alloying Strategy
The chemical specification for A516 plates emphasizes low impurity levels with controlled carbon and manganese so that toughness, weldability, and predictable hardenability are achieved. The A516 specification sets limits that are applied across grades; mills use heat‑treatment and thermo‑mechanical control to reach grade mechanical properties.
Table: Typical composition limits for ASTM A516 plate (representative maximums; verify specific mill certificates)
| Element | Typical limit / comment |
|---|---|
| C (Carbon) | Low to moderate; controlled to limit hardenability and maintain weldability (typical max in industry practice is on the order of a few tenths of a percent) |
| Mn (Manganese) | Primary deoxidizer and strength contributor; limited to control CE and toughness |
| Si (Silicon) | Present as deoxidizer; typically limited |
| P (Phosphorus) | Kept very low to preserve toughness |
| S (Sulfur) | Kept very low to avoid hot shortness and poor toughness |
| Cr, Ni, Cu, Mo | Usually present only in small amounts if any — may be limited or used to improve hardenability/toughness in some mill practices |
| V, Nb, Ti, B | Not typically large additions; microalloying (Nb, V, Ti) may be used for grain refinement in some plates |
| N (Nitrogen) | Typically low and controlled |
Notes: - The specification concentrates on low P and S and on controlled carbon and manganese to maintain a balance of strength and weldability. Exact numeric limits differ with the full ASTM text and supplier mill certificates; always check the certificate of analysis for procurement and welding procedure qualification. - Alloying strategy: manufacturers rely primarily on controlled carbon and manganese plus thermomechanical rolling (and sometimes microalloying) to reach the grade‑specific strength while keeping hardenability low enough to preserve weldability and toughness.
3. Microstructure and Heat Treatment Response
- Typical microstructure: as‑rolled or normalized A516 plate microstructures are predominantly ferrite and pearlite. Controlled rolling and cooling can refine grain size and increase strength without significant alloying.
- Gr60 vs Gr70 processing: both grades are produced from the same steel chemistry spectrum; the difference in specified mechanical properties is usually achieved by processing temperature control, thermo‑mechanical rolling, and controlled cooling rates rather than large differences in alloy content.
- Heat treatment response:
- Normalizing/refinement: normalizing (above A3 then air cooling) refines grain size and can improve toughness. It is a common practice when higher toughness at lower temperatures is required.
- Quench & temper: not typical for A516 as‑delivered pressure‑vessel plates; quench & temper produces different microstructures (tempered martensite) and is used for higher‑strength pressure steels beyond A516 family.
- Thermo‑mechanical processing (controlled rolling): enables Gr70 properties in heavy sections without excessive carbon by refining ferrite grain size and producing fine pearlite distributions.
- Hardenability: relatively low compared to alloy steels; microalloying and CE control keep susceptibility to hard, brittle weld HAZ structures limited.
4. Mechanical Properties
Table: Comparative mechanical property characteristics (typical practice; always confirm with ASTM A516 and mill test report)
| Property | A516 Gr60 (typical) | A516 Gr70 (typical) | Comments |
|---|---|---|---|
| Tensile strength | Lower range compared with Gr70 (used for lower design stress) | Higher minimum tensile range than Gr60 | Grade number correlates with a higher minimum tensile requirement for Gr70 |
| Yield strength | Lower minimum yield than Gr70 | Higher minimum yield than Gr60 | Higher yield enables thinner sections for the same design load |
| Elongation (%) | Slightly higher (better ductility in many cases) | Slightly lower (as expected for higher strength) | Elongation depends on thickness and supplier practices |
| Impact toughness | Generally good; may be easier to achieve required toughness at low temperature for thinner sections | Good, but achieving specified low‑temperature impact requirements in thick sections may require extra processing | Charpy requirements are often invoked contractually and depend on thickness and specified test temperature |
| Hardness | Lower typical hardness | Slightly higher typical hardness | Both generally within machinable/hardness limits for pressure‑vessel steels |
Explanation: - Grade 70 is the stronger of the two grades and is often specified when higher allowable stresses or thinner plates are desired. Grade 60 is chosen where conservative strength is not required and cost or ductility are prioritized. - Toughness (resistance to brittle fracture) is influenced by chemistry, rolling, thickness, and any post‑processing (e.g., normalization). For low‑temperature service, impact testing to a specified temperature is commonly required regardless of grade.
5. Weldability
Weldability for A516 plates is generally good using common pressure‑vessel welding consumables and procedures, provided preheat, interpass temperature, and heat input are controlled for thickness. Key weldability considerations include the carbon level, CE (carbon equivalent), and the presence of microalloying elements.
Useful carbon‑equivalent expressions (interpret qualitatively; do not substitute for procedure qualification): - International Institute of Welding CE (example form): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - More comprehensive parameter for weldability: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - A lower carbon equivalent (CE) correlates with easier weldability and lower risk of hardened heat‑affected zone (HAZ) and cold cracking, thus reducing preheat requirements. - A516 grades are formulated and processed to keep CE relatively low. Grade 70, being higher strength, may have marginally higher CE in some mill practices, so thicker sections or certain welding conditions may require more conservative preheat and interpass control. - Always qualify welding procedures (WPS/PQR) for the actual plate thickness, grade, and service temperature. Use preheat and post‑weld heat treatment recommendations from codes (e.g., ASME) and supplier guidance.
6. Corrosion and Surface Protection
- A516 steels are non‑stainless carbon steels; corrosion resistance is typical of unalloyed carbon steel and requires protective systems for atmospheric, splash, or buried exposure.
- Common protective strategies: surface preparation followed by primers and topcoats, galvanizing (where compatible with service and fabrication), corrosion‑inhibiting linings for internal surfaces, or cathodic protection for buried or submerged structures.
- PREN (pitting resistance equivalent number) is relevant for stainless alloys only: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- PREN is not applicable to A516 grades because they are not stainless steels. Corrosion mitigation for A516 relies on coatings, linings, or alternate metallurgy (stainless or corrosion‑resistant alloys) where severe corrosion is expected.
7. Fabrication, Machinability, and Formability
- Cutting: plasma, oxy‑fuel, or laser cutting are common for plate; thermal cutting parameters and allowances must reflect thickness and grade.
- Forming/bending: both grades can be cold‑formed within limits. Higher strength of Gr70 reduces allowable bendability or increases required bend radius compared to Gr60 for a given thickness.
- Machinability: typical of low‑alloy carbon steels; higher strength (and slightly higher hardness) in Gr70 may marginally reduce machinability versus Gr60.
- Drilling, tapping, and thread forming follow standard practices for carbon steel; consider hardness and toughness when specifying post‑machining heat or grind treatments.
8. Typical Applications
| A516 Grade 60 — Typical Uses | A516 Grade 70 — Typical Uses |
|---|---|
| Low‑ to moderate‑pressure boilers, tanks, and vessels where design stresses are conservative and cost is a factor | High‑pressure vessels, thicker plates, or designs where higher allowable stress or reduced section thickness is required |
| Structural components in auxiliary plant equipment where ductility and ease of fabrication are prioritized | Pressure vessel shells, steam drum plates, and applications where higher minimum tensile/yield are specified by code or design |
| General fabrication where low‑temperature impact requirements are modest | Applications requiring higher strength with acceptable toughness after controlled processing; often used when weight or plate thickness savings matter |
Selection rationale: - Choose a grade to meet code allowable stresses, plate thickness limits, and any specified low‑temperature impact requirement. Fabrication approach, welding procedures, and coating/life considerations also drive the final selection.
9. Cost and Availability
- Cost: A516 Gr70 is typically modestly more expensive than Gr60 because of tighter processing control required to meet higher mechanical requirements and sometimes more demanding quality assurance. The premium varies with market, mill, and plate thickness.
- Availability: Both grades are standard and widely stocked by plate distributors in common thicknesses. Very thick plates or exotic thickness/chemistry combinations may have lead times; availability is a function of mill capability and regional inventory.
- Product forms: plate (standard), cut‑to‑size blanks, and sometimes normalized or test‑qualified versions are available. Custom impact testing at specified temperatures may add cost and lead time.
10. Summary and Recommendation
Table: Quick comparison
| Aspect | A516 Gr60 | A516 Gr70 |
|---|---|---|
| Weldability | Very good; slightly easier in some practices | Very good; may need slightly more preheat control in thick sections |
| Strength–Toughness balance | Lower specified strength, generally good ductility | Higher specified strength, still good toughness if processed/controlled |
| Cost | Lower (typically) | Higher (typically) |
Conclusions — practical guidance: - Choose A516 Gr60 if: - Your design allows the lower specified strength and you prioritize cost savings, slightly improved ductility, or easier forming for a given thickness. - Welding procedures and low‑temperature impact requirements are moderate and you prefer the most economical plate meeting the code. - Choose A516 Gr70 if: - Your design requires higher allowable stress, reduced plate thickness for weight/clearance reasons, or specific code requirements that call for the higher grade. - You need higher minimum tensile and yield values and are prepared to apply appropriate welding controls (preheat, interpass, and impact testing where required).
Final note: A516 grades are specified and procured within a code and contractual framework; always verify the exact chemical and mechanical limits, impact testing requirements, and manufacturing practice with the ASTM A516/A516M standard, the mill test report, and the governing design code (ASME or equivalent) before final material selection and WPS qualification.