API 20F Corrosion Resistant Stud Bolt

Corrosion Resistant Bolting for Oil & Gas Applications

API Specification 20F defines the qualification, manufacturing, testing, and documentation requirements for corrosion resistant bolting used in the petroleum and natural gas industry. It applies when referenced by API equipment standards such as API 6A and is aligned with high-reliability upstream applications.

Download API Specification 20F standard in PDF

1. Applicable Materials (Covered Grades)

API 20F applies to precipitation-hardened corrosion resistant alloys, including:

• Nickel-Based Alloys

• API 6A718

  • UNS N07718 (Alloy 718)

  • Fully wrought structure required

  • Microstructure strictly controlled per API 6A718

• Iron-Based Austenitic Alloys

• ASTM A453 Grade 660 Class D (660D)

  • Precipitation-hardened austenitic alloy

  • Used for high-temperature and corrosive environments

All requirements of the base material standards apply unless modified by API 20F, 

2. Bolting Specification Levels (BSL)

API 20F defines two quality levels:

🔹 BSL-2

• Enhanced requirements beyond ASTM A453 and API 6A718

• Mandatory chemical, mechanical, hardness, and basic NDE controls

🔹 BSL-3 (Highest Level)

• All BSL-2 requirements plus:

  • 100% surface NDE

  • 100% volumetric NDE (for ≥1 in / 25.4 mm)

  • Additional raw material verification

  • Stricter inspection and traceability

3. Heat Treatment Requirements

• Heat treatment shall comply with:

  • API 6A718 (for Alloy 718)

  • ASTM A453 Grade 660D

• Furnaces must be qualified to:

  • API 6A

  • SAE AMS 2750

  • SAE AMS-H-6875

• Heat treatment parameters must be fully documented:

  • Equipment type (batch or continuous)

  • Time & temperature

  • Cooling media

  • Calibration and control methods 

4. Mechanical & Chemical Testing

Chemical Analysis

• Per ASTM A751

• Minimum frequency: one test per heat

• BSL-3 requires additional product analysis verification

Mechanical Properties

• Per applicable material standard (API 6A718 or ASTM A453)

• Performed after all heat treatments

• BSL-3 requires retesting if mill tests are used

Typical tests include:

• Tensile strength

• Yield strength

• Elongation

• Reduction of area

5. Hardness Requirements

• Hardness testing is mandatory for each piece

• Test methods:

  • ASTM A370

  • ASTM E10 (Brinell)

  • ASTM E18 (Rockwell)

• Acceptance limits:

  • As defined in API 6A718 or ASTM A453 Grade 660D

• Results must be documented in the test report

6. Nondestructive Examination (NDE)

BSL-2

• NDE per referenced API or ASTM material specification

BSL-3 (100% Inspection)

• Surface NDE

  • Liquid Penetrant Testing (PT)

  • ASTM A962 S56 / S57

  • Each piece examined

• Volumetric NDE

  • Ultrasonic Testing (UT)

  • Required for:

    • Raw material

    • Bolting ≥ 1 in (25.4 mm)

  • Acceptance per API 6A PSL-3 criteria

7. Dimensional & Visual Inspection

• Dimensional compliance per ASTM A962

• Visual inspection per:

  • ASTM F788 (bolts, screws, studs)

  • ASTM F812 (nuts)

• BSL-3 requires 100% dimensional inspection

• Thread requirements:

  • UNR controlled-radius root threads

  • No undersized bolts or oversized nuts permitted

8. Positive Material Identification (PMI)

• Per ASTM E1476

• BSL-2: 10% sampling (100% if any failure)

• BSL-3: 100% PMI on all pieces

9. Marking & Traceability

Each bolting product shall be marked with:

• Manufacturer identification

• Heat / heat-lot number

• Material grade:

  • “6A718” or “660D”

• Specification level:

  • API 20F-2 (BSL-2)

  • API 20F-3 (BSL-3)

Bolting ≥ 1 in (25.4 mm) must be individually marked. Smaller sizes must maintain full container traceability

API 20F Bolting Types

This standard covers the following product forms, processes, and sizes:

• machined studs;
• machined bolts, screws and nuts;
• cold formed bolts, screws and nuts with cut or cold formed threads;
• hot formed bolts and screws <1.5 in. (38.1 mm) nominal diameter;
• hot formed bolts and screws ≥1.5 in. (38.1 mm) nominal diameter;
• roll threaded studs, bolts, and screws <1.5 in. (38.1 mm) diameter;
• roll threaded studs, bolts, and screws ≥1.5 in. (38.1 mm) diameter;
• hot formed nuts <1.5 in. (38.1 mm) nominal diameter;
• hot formed nuts ≥1.5 in. (38.1 mm) nominal diameter.