ASTM A516 Gr60 vs Gr70 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
ASTM A516 Grades 60 and 70 are two of the most commonly specified carbon steel plates for fabrication of pressure-containing equipment, especially boilers, pressure vessels, and storage tanks. Engineers and procurement professionals often weigh trade-offs between strength, toughness, weldability, and cost when choosing between these two grades. Typical decision contexts include selecting a grade for higher operating pressure (favoring strength) versus prioritizing notch toughness at low temperature or improved weldability (favoring lower carbon content and ductility).
The principal distinction between A516 Grade 60 and Grade 70 is the required mechanical-strength level: Grade 70 is specified to meet higher minimum strength than Grade 60. Because these grades share a similar base chemistry and processing routes, they are commonly compared when designs require a balance of pressure capability, fracture resistance, and fabrication economics.
1. Standards and Designations
- Primary specification: ASTM A516 / ASME SA-516 — “Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service.”
- Associated codes: ASME Boiler & Pressure Vessel Code (Section II and VIII) when used in pressure vessels.
- Comparable international standards (use for cross-reference): EN 10028 series (pressure vessel plates), JIS G3115 (pressure vessel steel plates), and GB/T 3274/ 7131 variants in Chinese standards. Exact equivalence depends on required properties and testing (e.g., impact energy requirements).
- Classification: Both A516 Gr60 and Gr70 are carbon-manganese steels (non-alloy carbon steels). They are not stainless steels, tool steels, or high-alloy HSLA grades in the sense of significant microalloying; some commercial mills may add microalloying elements in small amounts to achieve target properties.
2. Chemical Composition and Alloying Strategy
Below is an illustrative table of typical composition ranges (wt%). These are representative commercial ranges for A516 plate grades; actual guaranteed chemistry must be confirmed on the mill test certificate for each heat.
| Element | Typical range or maximum (wt%) |
|---|---|
| C (Carbon) | 0.18 – 0.28 (Grade & mill-dependent) |
| Mn (Manganese) | 0.60 – 1.35 |
| Si (Silicon) | 0.10 – 0.35 |
| P (Phosphorus) | ≤ 0.035 (max) |
| S (Sulfur) | ≤ 0.035 (max) |
| Cr (Chromium) | trace – ~0.30 (if present) |
| Ni (Nickel) | trace – ~0.40 (if present) |
| Mo (Molybdenum) | trace – ~0.10 (if present) |
| V (Vanadium) | typically trace (if microalloyed) |
| Nb (Niobium) | typically trace (if microalloyed) |
| Ti (Titanium) | typically trace (if present for deoxidation) |
| B (Boron) | trace if used intentionally |
| N (Nitrogen) | low (ppm) |
Notes: - ASTM A516 does not require substantial alloy additions; steels are primarily carbon–manganese compositions. Some manufacturers apply microalloying (V, Nb, Ti) or controlled processing to optimize strength and toughness. - Higher strength (Gr70) is generally achieved by controlled chemistry plus thermo-mechanical processing or rolling practice rather than by heavy alloying.
How alloying affects behavior: - Carbon increases strength and hardenability but reduces weldability and ductility. Moderate carbon levels maintain a balance. - Manganese improves hardenability and tensile strength and promotes deoxidation but in excess can increase hardenability and affect toughness. - Silicon aids deoxidation; small amounts do not strongly change mechanical behavior. - Microalloying elements (V, Nb, Ti) refine grain size, promote precipitation strengthening, and can raise strength without proportionally decreasing toughness when used in controlled amounts.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As-rolled A516 plate most commonly exhibits a ferrite–pearlite microstructure in the as-produced condition. Differences between Grade 60 and Grade 70 are largely quantitative (carbon and Mn gradients, grain size, dislocation density) rather than qualitative. - Thermo-mechanical controlled processing (TMCP) can lead to finer ferrite grain sizes and some bainitic constituents, improving both strength and toughness compared with conventionally rolled plate.
Heat treatment responses: - A516 is normally supplied in the normalized as-rolled condition; full quench-and-temper is not typical for this specification because the product is intended for moderate-temperature service and welded construction. - Normalizing (heat above critical and controlled cooling) refines grain size and improves toughness; it is an accepted route to improve notch toughness without excessive alloying. - Quenching and tempering can achieve higher strength levels and different toughness profiles but is not commonly specified for standard A516 plate; such treatments change the product class and may impact weld procedures and notch toughness. - Thermo-mechanical routes (controlled rolling + accelerated cooling) are used by mills to deliver Gr70 properties with fine-grained microstructure and good low-temperature toughness.
4. Mechanical Properties
Quantitative limits in the specification vary by grade, thickness, and optional impact testing. Rather than asserting precise numeric minima (which must be extracted from the current ASTM/ASME spec and mill test reports), the relative comparison is summarized below.
| Property | A516 Grade 60 | A516 Grade 70 |
|---|---|---|
| Tensile Strength | Lower range (sufficient for moderate pressures) | Higher (designed for higher-pressure service) |
| Yield Strength | Lower minimum yield | Higher minimum yield |
| Elongation (ductility) | Comparable; often slightly higher ductility at same thickness | Slightly lower ductility due to higher strength, but still suitable for forming |
| Impact Toughness | Specified by Charpy V-notch requirements when required; good with appropriate processing | Can meet equal or higher toughness depending on processing; often specified for higher-pressure, thicker sections |
| Hardness | Slightly lower | Slightly higher |
Interpretation: - Grade 70 provides higher strength margins, enabling either thinner sections for the same pressure or higher allowable pressures for the same thickness. - Toughness is controlled by chemistry, thickness, and mill processing — a properly processed Gr70 can have equal or superior toughness to Gr60, but as strength increases, careful attention to notch toughness and impact testing is necessary. - Designers should consult actual mechanical property certificates and required impact testing (temperature, energy) for their application.
5. Weldability
Weldability considerations hinge on carbon content, hardenability, and residual alloying. Two common carbon-equivalent formulas used to estimate weldability are shown below and should be used qualitatively:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
and
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Lower values of $CE_{IIW}$ and $P_{cm}$ indicate easier weldability and lower risk of cold cracking; both A516 grades typically have low-to-moderate carbon equivalents. - Grade 70 may have slightly higher carbon equivalent than Grade 60 depending on chemistry and processing, so preheating and controlled interpass temperatures may be more commonly recommended for thicker sections or low ambient-temperature welding. - Post-weld heat treatment (PWHT) is infrequently required for most carbon steel vessel plates but may be mandated by design codes or service conditions; decisions should follow ASME code requirements and welding procedure qualifications.
6. Corrosion and Surface Protection
- A516 Grades 60 and 70 are non-stainless carbon steels; inherent corrosion resistance is limited to the base steel and environment. Corrosion protection strategies include coatings (paints, epoxies), galvanizing (hot-dip or zinc-rich primers), and cathodic protection for buried or immersed service.
- Stainless steel corrosion indices such as PREN are not applicable to A516 because it is not a corrosion-resistant alloy. For reference, PREN is calculated as:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
but this index applies only to austenitic and duplex stainless steels. - Selection of surface protection should consider operating fluid, temperature, and mechanical wear. For aggressive environments, consider stainless materials or lined/coated solutions rather than relying on plain carbon steel.
7. Fabrication, Machinability, and Formability
- Cutting: Both grades cut and flame-cut readily; plasma and laser cutting are common. Slightly higher strength in Gr70 may require marginally more force or tool wear.
- Bending/Forming: Gr60 is typically more forgiving for forming operations due to slightly greater ductility at equivalent thickness. Gr70 can be formed but may require larger bend radii or lower bend strain.
- Machinability: Neither grade is a high-machinability alloy; standard tooling and feed rates apply. Slightly higher strength in Gr70 can increase tool wear.
- Surface finishing (grinding, polishing) behaves similarly for both grades; pre- and post-weld treatments are similar.
8. Typical Applications
| A516 Grade 60 — Typical Uses | A516 Grade 70 — Typical Uses |
|---|---|
| Storage tanks, low-to-moderate pressure vessels, boiler parts where toughness is prioritized over maximum allowable pressure | Higher-pressure boilers and pressure vessels, tanks and reactors where higher allowable stresses or thinner sections are desired |
| Fabricated components where improved formability is helpful | Applications demanding higher design stress or weight savings through reduced thickness |
| Low-temperature service with appropriate impact testing if required | Thicker plates and higher-pressure service with controlled processing to maintain toughness |
Selection rationale: - Choose the grade whose strength and toughness combination meets the design stress, required safety margins, and impact-temperature requirements while minimizing cost and fabrication complexity.
9. Cost and Availability
- Cost: Grade 70 typically commands a modest premium over Grade 60 because of the higher strength and sometimes more controlled processing (TMCP). However, if Grade 70 permits a thinner plate to meet the same design, overall material cost and fabrication cost can be lower.
- Availability: Both grades are widely produced and available in standard pressure-vessel plate thicknesses and widths. Availability of specific thicknesses, surface finishes, and certified impact testing at low temperatures varies by mill; procurement should confirm lead times and test certifications.
10. Summary and Recommendation
Summary table (qualitative):
| Criterion | A516 Gr60 | A516 Gr70 |
|---|---|---|
| Weldability | Excellent (slightly easier) | Very good (may require more preheat for thick sections) |
| Strength–Toughness balance | Very good for general applications | Higher strength; can match toughness if processed correctly |
| Cost | Lower material cost | Higher material cost but may reduce overall weight/cost via thinner sections |
Recommendations: - Choose A516 Grade 60 if: - The design favors higher ductility and ease of fabrication. - Operating pressures and allowable stresses are moderate and permit the use of the lower-strength grade. - Cost control and simpler welding procedures are priorities. - Low-temperature toughness requirements can be met with Grade 60 processing.
- Choose A516 Grade 70 if:
- Higher minimum strength is needed to meet pressure or structural requirements, or if weight reduction via thinner plates is desirable.
- The designer requires higher allowable stress values for vessel design and is prepared to specify appropriate impact testing and possibly stricter welding controls.
- The mill can supply Gr70 with the required toughness (via TMCP or controlled rolling).
Final note: Both grades are appropriate for pressure-vessel construction when specified and furnished per ASTM A516 and the applicable code (ASME). For any critical selection, engineers should review the current ASTM/ASME specification, require mill test reports (chemical and mechanical), verify impact-test documentation at the required service temperature, and validate welding procedures for the chosen grade and thickness.