The Role of Material Composition in API 20E Bolting Performance

In the oil and gas industry, equipment integrity can make or break operations. Every component, no matter how small, must withstand extreme conditions—high pressures, corrosive environments, and dynamic stress cycles. One such component is bolting—the critical fasteners used to secure pipelines, valves, pressure vessels, and flanged joints.

To ensure bolting components are up to the task, the API 20E standard provides rigorous requirements for their design, manufacture, and testing. A central aspect of API 20E is material composition, particularly the inclusion and control of tungsten (W), chromium (Cr), molybdenum (Mo), and iron (Fe).

This article explains how these key elements contribute to the performance, reliability, and durability of API 20E-compliant bolts—and why understanding them is essential for professionals in petroleum and natural gas applications.

Why Material Composition Matters

Bolting used in critical applications doesn’t just need to be strong. It needs to be tough, corrosion-resistant, thermally stable, and durable over long lifespans.

The specific chemical composition of a bolt’s alloy steel determines how it will behave under different mechanical and environmental conditions. In API 20E bolting—especially under BSL-2 and BSL-3 (Bolting Specification Levels)—even slight deviations in composition can have significant effects on:

• Mechanical strength
• Corrosion resistance
• Creep resistance at high temperatures
• Fatigue life
• Weldability and machinability

Key Alloying Elements in API 20E Bolting

Let’s take a closer look at four critical elements used in alloy steels for API 20E bolting: tungsten, chromium, molybdenum, and controlled iron content.

1. Tungsten (W)

Role in Alloy Steel:

• Tungsten increases high-temperature strength and creep resistance.
• It stabilizes the microstructure of steel at elevated temperatures.
• It contributes to hardness and wear resistance, especially under dynamic loading.

Impact on Bolting Performance:
In deep drilling or offshore platforms, bolts often face extreme heat and continuous stress. Tungsten enhances the bolt’s ability to maintain strength and shape at high temperatures, reducing the risk of deformation or rupture under prolonged pressure.

Common Alloys with Tungsten:

• Some tool steels and high-performance fasteners used in API 20E BSL-3 applications incorporate tungsten to ensure performance in harsh conditions.

Downsides of Excess Tungsten:

• High tungsten content can make steel brittle and difficult to machine or weld.
• Careful balance is critical; API 20E limits and monitors this through precise chemistry controls.

2. Chromium (Cr)

Role in Alloy Steel:

• Chromium enhances corrosion resistance, especially against oxidizing agents.
• It also increases hardness, tensile strength, and hardenability.
• Chromium helps form a passive oxide layer on the steel surface, which protects against rust.

Impact on Bolting Performance:
Bolting in sour service (H₂S environments), offshore rigs, and subsea equipment faces continuous exposure to moisture, salt, and chemicals. Chromium-rich steels resist pitting, crevice corrosion, and general rusting, making them ideal for such conditions.

Common Alloys with Chromium:

• ASTM A193 Grade B8M (stainless) contains significant chromium.
• Chromium-molybdenum (Cr-Mo) steels such as AISI 4140, 4340, or F22 are commonly used under API 20E.

Optimal Range:

• Typically, 0.5% to 2.0% for carbon steels and 12–18% in stainless steels.
• Chromium content must be carefully balanced—too little, and corrosion resistance suffers; too much, and weldability decreases.

3. Molybdenum (Mo)

Role in Alloy Steel:

• Molybdenum improves strength at high temperatures, enhances creep resistance, and boosts corrosion resistance, particularly in acidic and chloride environments.
• It also improves toughness and hardenability.

Impact on Bolting Performance:
API 20E bolts are often used in applications involving high stress, cyclic loads, and corrosive service. Molybdenum allows these bolts to retain strength over time, particularly under conditions where chloride-induced stress corrosion cracking (SCC) is a concern.

Common Alloys with Molybdenum:

• Cr-Mo alloys like AISI 4130 and 4140 are widely used in API 20E bolting.
• Stainless grades like 316 (B8M bolts) benefit from Mo addition for marine and sour environments.

Synergy with Chromium:

• Molybdenum and chromium together create a robust corrosion-resistant matrix, making bolts more resilient to both pitting and uniform corrosion.

4. Controlled Iron (Fe) Content

Why Iron Needs Control:
While iron is the base element in all carbon and alloy steels, its quality and purity are crucial. Trace impurities or undesirable elements (like sulfur or phosphorus) in iron can weaken a bolt’s grain structure or promote intergranular corrosion and embrittlement.

Impact on Bolting Performance:
In API 20E bolting:

• Iron content must be high purity, with tight controls on tramp elements.
• Sulfur and phosphorus are typically restricted to <0.035%, as they can create weak grain boundaries and lead to hot shortness or hydrogen-induced cracking.

Effect on Weldability and Machinability:

• Controlled iron composition ensures predictable welding behavior—important when bolts are fabricated or attached in the field.
• It also ensures uniformity in machining and threading, reducing failure due to stress risers or misalignment.

Material Composition in API 20E Bolting: Typical Alloy Examples

Alloy	Chromium (Cr)	Molybdenum (Mo)	Tungsten (W)	Remarks
AISI 4140	~1.0%	~0.2–0.4%	—	Common Cr-Mo alloy for general bolting
AISI 4340	~1.0%	~0.25%	—	Stronger than 4140, higher toughness
ASTM A193 B7	~0.8–1.1%	~0.15–0.25%	—	Standard high-strength bolt grade
ASTM A193 B8M (316 stainless)	~16–18%	~2.0–3.0%	—	High corrosion resistance for marine and chemical exposure
High-performance tool steel	Variable	Variable	Up to 2%	Used in extreme temperature/pressure scenarios

How Material Composition Affects BSL Levels

As you move from BSL-1 to BSL-3 in API 20E, expectations for material quality, testing, and performance increase. This includes:

• Tighter chemical composition tolerances
• More advanced testing (e.g., microstructure, grain size)
• Greater resistance to environmental factors like H₂S, CO₂, and chlorides

For BSL-3 bolting, small variations in molybdenum or chromium can lead to rejection if they fall outside the specified range—even if the bolts seem structurally sound.

Testing and Certification of Material Composition

API 20E requires that all bolting materials undergo:

1. Positive Material Identification (PMI): Confirms elemental composition using X-ray fluorescence (XRF) or optical emission spectrometry (OES).
2. Mill Test Reports (MTRs): Documentation showing heat number, material chemistry, mechanical properties, and heat treatment.
3. Heat Lot Traceability: Each bolt must be traceable back to its batch or heat lot, ensuring quality control throughout the supply chain.