ASME SA 516 Gr 70 Hardness Values

ASME SA 516 Gr 70 Hardness Values

Introduction

ASME SA 516 Gr 70, equivalent to ASTM A516 Grade 70, is a widely used carbon steel plate specification for moderate- and lower-temperature pressure vessels in industries such as oil and gas, petrochemicals, boilers, and power generation. Known for its excellent weldability, strength, and toughness, this material is critical for applications requiring reliable performance under pressure and varying temperatures. One key property of interest to engineers and manufacturers is its hardness, which influences its machinability, wear resistance, and suitability for specific service conditions. This article explores the hardness values of ASME SA 516 Gr 70, including factors affecting hardness, typical ranges, measurement methods, and practical implications for industrial applications.

What is Hardness in ASME SA 516 Gr 70?

Hardness is a measure of a material’s resistance to deformation, typically assessed through indentation tests such as Brinell, Rockwell, or Vickers. For ASME SA 516 Gr 70, hardness is not explicitly specified in the ASME Section II Part A standard, as the focus is primarily on tensile strength (485–620 MPa), yield strength (≥260 MPa), and elongation. However, hardness is often evaluated in practice to ensure the material meets quality requirements, particularly for welding, forming, or service in corrosive environments like sour gas (per NACE standards).

Factors Influencing Hardness

Several factors affect the hardness of SA 516 Gr 70:

• Heat Treatment: SA 516 Gr 70 is typically supplied in the as-rolled condition, but it may be normalized (heated to ~900–950°C and air-cooled) for thicker plates (>38 mm) or specific applications (SA 516 Gr 70N). Normalization refines the grain structure, slightly increasing hardness while improving toughness.
• Chemical Composition: The carbon content (≤0.27%) and alloying elements like manganese (0.85–1.20%) and silicon (0.15–0.40%) influence hardness. Higher carbon or alloy content can increase hardness but may reduce ductility.
• Manufacturing Process: Hot rolling parameters, cooling rates, and any post-rolling treatments (e.g., stress relieving) impact the microstructure and, consequently, hardness.
• Service Conditions: Post-weld heat treatment (PWHT) or exposure to high temperatures may alter hardness, potentially softening the material by relieving residual stresses.

Typical Hardness Values

While ASME SA 516 Gr 70 does not mandate specific hardness values, industry data and material test reports provide typical ranges based on empirical measurements. Hardness is commonly reported in Brinell Hardness Number (HB), Rockwell Hardness (HRB), or Vickers Hardness (HV), depending on the testing standard used (e.g., ASTM E10 for Brinell, ASTM E18 for Rockwell).

Hardness Ranges

• As-Rolled SA 516 Gr 70:
  • Brinell Hardness (HB): ~130–180 HB
  • Rockwell Hardness (HRB): ~70–85 HRB
  • Vickers Hardness (HV): ~140–190 HV
• Normalized SA 516 Gr 70N:
  • Brinell Hardness (HB): ~140–200 HB (slightly higher due to grain refinement)
  • Rockwell Hardness (HRB): ~75–90 HRB
  • Vickers Hardness (HV): ~150–210 HV

These values are approximate and vary based on plate thickness, heat treatment, and specific mill practices. For instance, thinner plates (<25 mm) may exhibit slightly lower hardness due to faster cooling rates during rolling, while thicker plates or normalized material may trend toward the higher end of the range.

Comparison with SA 516 Gr 70N

Normalization in SA 516 Gr 70N results in a finer, more uniform microstructure, which typically increases hardness by 5–15% compared to the as-rolled condition. This is beneficial for applications requiring enhanced toughness or resistance to stress corrosion cracking, but it may necessitate adjustments in machining or welding parameters due to the slightly harder material.

Measurement Methods

Hardness testing for SA 516 Gr 70 typically follows standardized procedures:

• Brinell Hardness Test (ASTM E10): Uses a 10 mm diameter steel or tungsten carbide ball with a 3000 kgf load to create an indentation. Common for thicker plates due to its larger test area.
• Rockwell Hardness Test (ASTM E18): Employs a smaller indenter (e.g., 1/16-inch ball for HRB scale) with a 100 kgf load, suitable for thinner plates or surface testing.
• Vickers Hardness Test (ASTM E92): Uses a diamond pyramid indenter, providing precise measurements across a wide range of hardness values, often used for laboratory analysis.

In practice, hardness tests are conducted on mill test certificates (MTCs) or during quality control to verify material consistency. For critical applications, such as sour service (NACE MR0175/ISO 15156), maximum hardness limits (e.g., 22 HRC or ~237 HB) may be specified to prevent hydrogen-induced cracking.

Practical Implications of Hardness

The hardness of ASME SA 516 Gr 70 has several implications for its use in industrial applications:

• Machinability: Hardness in the range of 130–180 HB (as-rolled) is generally considered good for machining, allowing for efficient cutting, drilling, and forming. Normalized material (140–200 HB) may require slower cutting speeds or harder tools.
• Weldability: SA 516 Gr 70’s moderate hardness ensures excellent weldability with low-hydrogen electrodes. However, excessive hardness (e.g., in the heat-affected zone after welding) can increase the risk of cracking, necessitating PWHT to reduce hardness to safe levels (e.g., <237 HB for sour service).
• Service Performance: Moderate hardness provides a balance of strength and ductility, making SA 516 Gr 70 suitable for pressure vessels operating at temperatures down to -1°C (as-rolled) or -34°C (normalized, for 25 mm thickness). Higher hardness in normalized plates enhances resistance to fatigue and low-temperature brittle fracture.
• Corrosion Resistance: In corrosive environments, such as those with hydrogen sulfide, hardness control is critical. Normalized SA 516 Gr 70N often performs better due to its refined microstructure and lower residual stresses.

Industry Applications

The hardness properties of SA 516 Gr 70 make it ideal for:

• Pressure Vessels and Boilers: Used in refineries, chemical plants, and power generation for reliable performance under pressure.
• Storage Tanks: Suitable for storing liquids and gases at moderate temperatures.
• Piping and Fittings: Employed in high-stress piping systems where weldability and toughness are critical.
• Low-Temperature Applications: Normalized SA 516 Gr 70N is preferred for cryogenic vessels or environments requiring enhanced toughness.

Conclusion

The hardness of ASME SA 516 Gr 70 typically ranges from 130–180 HB (as-rolled) to 140–200 HB (normalized), influenced by heat treatment, composition, and manufacturing processes. While not explicitly specified in the ASME standard, hardness is a critical property for assessing machinability, weldability, and service performance. Normalized SA 516 Gr 70N offers slightly higher hardness and improved toughness, making it preferable for low-temperature or high-integrity applications. Engineers and manufacturers should refer to material test certificates and conduct hardness testing (e.g., Brinell, Rockwell, or Vickers) to ensure compliance with project specifications, particularly for critical applications like sour service. For precise hardness requirements, consulting the latest ASME Section II standards and supplier data is essential.

References

1. ASME Section II, Part A, SA-516/SA-516M, Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service.
2. ASTM E10, Standard Test Method for Brinell Hardness of Metallic Materials.
3. ASTM E18, Standard Test Methods for Rockwell Hardness of Metallic Materials.
4. NACE MR0175/ISO 15156, Petroleum and Natural Gas Industries – Materials for Use in H2S-Containing Environments.
5. Industry material test reports and technical datasheets for SA 516 Gr 70 and Gr 70N.