A516 Gr60 vs Gr70 – Composition, Heat Treatment, Properties, and Applications

Introduction

ASTM A516 Grade 60 and Grade 70 are two of the most commonly specified pressure‑vessel carbon steels worldwide. Engineers, procurement managers, and manufacturing planners frequently choose between them when designing boilers, heat exchangers, storage tanks, and other welded pressure-retaining equipment. Typical trade‑offs that drive the selection include required strength vs. fabrication ease, cost vs. performance, and weldability vs. thickness‑dependent toughness.

The principal engineering distinction between the two grades is that Grade 70 is specified to provide higher minimum strength than Grade 60 while maintaining comparable ductility and impact properties when produced and heat‑treated to the applicable specification. Because both grades are covered by the same ASTM/ASME specification (A516/A516M), they are commonly compared in design and procurement as interchangeable options when structural strength or thickness‑dependent toughness must be balanced against fabrication and cost.

1. Standards and Designations

• Primary specification: ASTM A516 / ASME SA‑516 (Pressure Vessel Plates, Carbon Steel, for Moderate‑ and Lower‑Temperature Service). Grades in this spec include 55, 60, 65, and 70; Grades 60 and 70 are most commonly used.
• International equivalents and related standards:
• EN: No exact one‑to‑one EU standard, but comparable pressure‑vessel steels include EN 10028 series (e.g., P235GH, P265GH) and EN 10025 variants for structural steels depending on application.
• JIS/GB: Local pressure‑vessel and boiler codes specify comparable grades; designers should map mechanical and toughness requirements rather than assume direct equivalence.
• Material classification: Both A516 Gr60 and Gr70 are plain carbon/mild carbon steels with controlled alloying and are considered carbon steels (not stainless, not tool steels, and not high‑strength low‑alloy (HSLA) in the strictest sense, though some microalloying may be present).

2. Chemical Composition and Alloying Strategy

The A516 specification focuses on ensuring adequate tensile and impact properties through controlled carbon, manganese, and impurity limits rather than by extensive alloying. Specific composition limits are set by ASTM A516/A516M; manufacturers often supply detailed mill certificates.

Table: Typical compositional elements — qualitative presence | Element | Typical presence and role (A516 Gr60 / Gr70) | |---|---| | C (Carbon) | Low to moderate. Grade 70 is typically specified to achieve higher strength and therefore may have slightly higher carbon or microalloying than Grade 60. Carbon controls base strength and hardness but reduces weldability and toughness at high levels. | | Mn (Manganese) | Moderate. Mn increases hardenability and strength and helps deoxidation; controlled to balance toughness and weldability. | | Si (Silicon) | Low. Used as deoxidizer; typically limited. | | P (Phosphorus) | Trace (max limits). Kept low to avoid embrittlement. | | S (Sulfur) | Trace (max limits). Kept low; may be intentionally minimized for toughness and weld quality. | | Cr, Ni, Mo, V, Nb, Ti | Typically very low or absent in standard A516 chemistry. Some mills may add controlled microalloying (e.g., V, Nb, Ti) in small amounts to refine grain size and increase strength without large carbon increases—especially in Grade 70 variants. | | B (Boron) | Not typical in A516; when present in trace amounts, affects hardenability. | | N (Nitrogen) | Trace. Controlled for controlled properties and grain‑refining in some processes. |

Note: Exact numerical limits and maximums for C, Mn, P, S, and other elements are provided in ASTM A516/A516M and mill certificates. The table above describes the alloying strategy: modest carbon and controlled Mn for strength, tight impurity limits for toughness, and minimal added alloying so that weldability remains favorable.

How alloying affects performance: - Strength: Carbon and manganese are the principal contributors to tensile and yield strength. Microalloying (Nb, V, Ti) can increase strength through grain refinement and precipitation strengthening with less carbon penalty. - Corrosion resistance: Neither grade is stainless; alloying here does not provide meaningful corrosion resistance — surface protection is required. - Hardenability: Mn and carbon increase hardenability; slight increases in these elements in Grade 70 make it slightly more prone to hardening in the HAZ under fast cooling.

3. Microstructure and Heat Treatment Response

Typical microstructures for A516 steels are ferrite + pearlite (or bainite depending on composition and rolling/thermal history). Because A516 steels are intended for moderate‑temperature pressure service, they are normally supplied in the normalized or as‑rolled condition to ensure uniform microstructure and adequate toughness.

• Grade comparison:
• Gr60: Microstructure generally ferrite with dispersed pearlite; controlled rolling and normalizing yield a fine ferritic‑pearlitic matrix optimized for toughness.
• Gr70: Similar base microstructure but may contain slightly higher pearlite fraction or finer precipitates if microalloyed; this yields higher yield and tensile strength without significant compromise in ductility if properly processed.
• Heat treatment response:
• Normalizing/refinement: Normalizing after hot‑rolling produces a uniform fine grain structure in both grades, enhancing toughness.
• Quenching & tempering: Not typical for A516 final products used in pressure vessels; would convert microstructure to martensite/tempered martensite and produce higher hardness and strength but is not a standard route for A516 pressure steels.
• Thermo‑mechanical rolling (controlled rolling): Used by some mills to achieve higher strength and toughness through grain refinement and precipitation control; beneficial for Grade 70 to achieve higher strength targets without damaging toughness.

4. Mechanical Properties

Rather than providing specific numeric values (which are set by the applicable standard and depend on thickness, testing direction, and heat treatment), the following table summarizes the relative mechanical behaviors most relevant to design and procurement.

Table: Comparative mechanical property tendencies | Property | A516 Grade 60 | A516 Grade 70 | Notes | |---|---:|---:|---| | Yield strength (relative) | Lower | Higher | Grade 70 is specified to provide a higher minimum yield strength than Grade 60. | | Tensile strength (relative) | Lower | Higher | Grade 70 typically has higher minimum tensile strength. | | Elongation / ductility | Comparable | Comparable | When supplied per spec and for comparable thicknesses, elongation requirements are similar. | | Impact toughness (relative) | Similar | Similar | Charpy impact energy requirements are set by the spec and manufacturing conditions; both grades can achieve comparable toughness if produced correctly. | | Hardness | Comparable (Grade 70 may be slightly higher) | Slightly higher | Grade 70 can be modestly harder due to higher strength; hardness remains moderate compared with quenched steels. |

Explanation: The design difference is strength: Grade 70 is intended for higher design stress. The toughness and ductility requirements in ASTM A516 ensure that both grades meet minimum impact energy values for specified service temperatures, so toughness is not necessarily compromised by selecting Grade 70 when the plate is produced per specification.

5. Weldability

Weldability for A516 steels is generally good because of relatively low carbon and controlled levels of other alloying elements. However, as strength (and implicitly carbon equivalent) increases, weldability constraints tighten.

Useful carbon equivalent and weldability predictors: - International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (for predicting preheat requirements in complex alloys): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both A516 Gr60 and Gr70 have relatively low $CE_{IIW}$ and $P_{cm}$ compared with high‑strength alloy steels, which means general arc welding processes (SMAW, GMAW, SAW) are readily applied. - Grade 70 may have a slightly higher $CE_{IIW}$ owing to its higher carbon/mn or microalloying level; therefore, for thick sections or when using high heat input procedures, modest preheat and controlled interpass temperatures are more often recommended for Gr70 to avoid HAZ hardening and cold‑crack susceptibility. - Use of low‑hydrogen consumables, post‑weld heat treatment (when specified), and adherence to WPS/qualification practices are common for both grades on pressure equipment.

6. Corrosion and Surface Protection

• Neither A516 Gr60 nor Gr70 is stainless; they will corrode if exposed to corrosive atmospheres or liquids. Corrosion protection strategies include coatings (epoxy, polyurethane), painting systems, cathodic protection, and hot‑dip galvanizing (subject to thickness and surface preparation constraints).
• PREN (pitting resistance equivalent number) is applicable for stainless alloys and not relevant to A516 carbon steels. For illustration only: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable for A516 grades because Cr, Mo, and N are present at trace or zero levels and do not confer localized corrosion resistance.
• Selection rationale: Choose surface protection based on operating environment (atmospheric, seawater, chemical exposure) rather than grade — both grades require similar corrosion control measures.

7. Fabrication, Machinability, and Formability

• Cutting: Plasma, oxy‑fuel, and flame cutting are commonly used; Gr70’s slightly higher strength can require modestly more power but does not change process selection.
• Machinability: Both grades machine similarly; Grade 70’s slightly higher strength and hardness can increase tool wear marginally.
• Forming and bending: Grade 60 is marginally more formable because of lower yield strength; Grade 70 requires higher forming forces and tighter bend radii considerations. For cold forming operations at tight radii, Grade 60 is often preferred when strength is not the primary constraint.
• Finishing: Surface preparation, grinding, and edge conditioning follow standard practices; when post‑weld heat treatment is required for pressure service, allowance should be made in fabrication sequencing.

8. Typical Applications

Table: Common uses for each grade | A516 Grade 60 | A516 Grade 70 | |---|---| | Low‑ to moderate‑pressure boilers and vessels where cost and formability matter | Higher pressure boilers, storage tanks, and pressure vessels where higher allowable stress is required | | Heat exchangers and fabricated components that require easier forming or bending | Vessels and piping subjected to higher stresses or thinner safety margins where the higher yield is beneficial | | Structural applications where toughness at moderate temperature is required but maximum strength is not critical | Retrofits or repairs where matching the original higher‑strength material is necessary |

Selection rationale: - Choose Grade 60 when fabrication complexity, bending/forming, or slightly lower cost is the priority and design stress permits lower yield. - Choose Grade 70 when the design requires higher allowable stresses, thinner sections for the same strength, or when the code or customer specifies Grade 70 for the service.

9. Cost and Availability

• Cost: Grade 60 is typically slightly less expensive than Grade 70 on a per‑ton basis because of lower processing to achieve strength targets; however, cost differences are often modest and depend on market conditions and supply chain logistics.
• Availability: Both grades are widely available in plate mills and distributors in common plate sizes and thicknesses for pressure vessel manufacture. Availability by thickness, surface finish, and plate width varies by local mills; Grade 70 may be more commonly stocked for high‑pressure applications in some regions.

10. Summary and Recommendation

Table: Quick comparison | Metric | A516 Grade 60 | A516 Grade 70 | |---|---:|---:| | Weldability | Very good | Very good (slightly more attention for thick sections) | | Strength–Toughness balance | Moderate strength with good toughness | Higher strength while maintaining toughness when produced per spec | | Cost | Lower (often) | Higher (often) |

Concluding recommendations: - Choose A516 Grade 60 if you need easier forming/bending, slightly lower material cost, or the design allowable stresses are satisfied by the lower yield strength. It is well suited to moderate‑pressure vessels, components requiring significant fabrication, and projects where fabrication speed and bendability are priorities. - Choose A516 Grade 70 if your design requires higher minimum yield and tensile strengths (allowing thinner sections or higher working pressures) while still meeting the toughness criteria in the specification. Grade 70 is preferred for higher‑stress pressure vessels, thicker plates where strength margin is critical, or when code or client requirements call for the higher grade.

Final note: Always confirm exact chemical limits, mechanical minima, and toughness requirements from the current edition of ASTM A516/A516M and from the mill’s material certification. For welded pressure equipment, follow applicable design codes (ASME Section VIII or local regulations), qualified WPS procedures, and preheat/post‑weld heat treatment practices determined by calculated $CE_{IIW}$ / $P_{cm}$ and by thickness and service temperature.