ASTM A105 Pipe Flange

This article focuses specifically on ASTM A105 flanges – their types, pressure classes, and selection criteria for piping systems. For detailed material properties and chemical composition of A105, please refer to our companion guide: ▶ [ASTM A105 Carbon Steel – Complete Material Guide].

A pipe flange is a vital component in the pipeline system, used to connect pipes and maintain their sealing. The material of the pipe flange directly determines its performance and service life. Among the many flange materials, ASTM A105 is one of the most widely used forged carbon steel flanges in industrial pipeline systems.

What is ASTM A105 Flange?

An A105 flange is a pipe connector made from ASTM A105 forged carbon steel, manufactured in accordance with ASME B16.5 (for sizes up to NPS 24) or ASME B16.47 (for large diameters NPS 26-60). It is designed for moderate and high-temperature pressure systems and is the most commonly used carbon steel flange material in oil, gas, petrochemical, and power industries.

Pressure Rating: Covers Class 150 to Class 2500 (ASME B16.5/B16.47).

Characteristics of A105 Material:

1. Good strength and hardness: A105 carbon steel has high yield strength and tensile strength, and can withstand greater pressure and load.
2. Excellent wear resistance: Due to its high carbon content, A105 material has good wear resistance and is suitable for various harsh working environments.

3. Good machinability: A105 carbon steel is easy to forge, cut, weld and other processing operations, and is convenient for making flanges of various specifications and shapes.

Why Choose ASTM A105 Pipe Flange?

Advantages:
1. High cost performance: The cost is significantly lower than stainless steel (such as A182 F316) or alloy steel flanges.
2. Reliable mechanical properties: tensile strength ≥485 MPa, yield strength ≥250 MPa, meeting most medium and high pressure requirements.
3. Convenient processing: easy to weld, drill and turn, suitable for quick installation.

4. Strong versatility: widely used in non-corrosive media systems such as oil, natural gas, electricity, and water supply.

Limitations:
1. Not corrosion-resistant: Anti-corrosion coating (such as galvanizing, epoxy resin) is required in humid or acidic environments.
2. Temperature sensitive: A350 LF2 should be used for extremely low temperatures (<-29°C), and A182 F11 should be used for high temperatures (>425°C).

3. Low hardness: Not suitable for high wear conditions (such as sandy media).

Common A105 Flange Types and Applications:

The table below summarizes the most common A105 flange types, their key features, and where they are used.

Flange Type	Key Feature	Typical Application
Weld Neck Flange (WN)	Butt-welded, high integrity, reinforced hub	High-pressure, high-temperature, critical services (Class 300-2500)
Slip-On Flange (SO)	Slips over pipe, fillet welded	Low-pressure, non-critical systems (Class 150-300)
Blind Flange (BL)	Solid plate, no bore	Pipe closure, system isolation, pressure testing
Threaded Flange (THD)	Screwed connection, no welding	Low-pressure, non-vibration, small-diameter systems (e.g., fire protection)
Socket Weld Flange (SW)	Pipe inserted and fillet welded	Small-diameter, high-pressure systems (NPS ≤ 4")

Applicable Temperature:

Parameter	Limit	Notes
Standard Temperature Range	-29°C to 425°C	Normal service range for A105.
Above 425°C	Graphitization risk	Use A182 F11/F22 (chrome-moly) instead.
Below -29°C	Impact test required	Use A350 LF2 for design temperatures below -46°C.

Note on A105 vs. A105N: 

The base A105 material is typically supplied in the as-forged condition. When normalized, it is designated as A105N – the normalizing heat treatment refines the grain structure, improving toughness and impact resistance, especially at low temperatures. For critical low-temperature or high-pressure services, many specifications require A105N over standard A105

Selection Considerations for A105 Flanges:

1. Media matching
For water, oil, gas (non-corrosive): A105 is suitable.
For corrosive media (acid, salt water): Use stainless steel (A182 F316) or apply internal anti-corrosion coating.

2. Temperature limit
Ensure service temperature remains within -29°C to 425°C.
For temperatures above 425°C, select chrome-molybdenum steel (A182 F11/F22).
For temperatures below -29°C, consult project specifications; A350 LF2 may be required.

3. Pressure rating
Select the correct Class (150, 300, 600, etc.) based on system design pressure and temperature using ASME B16.5 pressure-temperature tables.

4. Sealing face type
RF (Raised Face): Most common for general service.
FF (Flat Face): Used with cast iron flanges.
RTJ (Ring Type Joint): For high-pressure and high-temperature services requiring metal-to-metal sealing.

6. Flange type selection
Critical/high-pressure services → Weld Neck (WN) .
General low-pressure → Slip-On (SO) .
System isolation/closure → Blind (BL) .
Small-diameter high-pressure → Socket Weld (SW) .

Non-critical, non-welded connections → Threaded (THD).

Installation and Maintenance Tips:

Although A105 flanges have many advantages, the following matters should also be noted during use:

1. Inspection before installation: Check flange faces for scratches, cracks, or unevenness that could affect sealing.

2. Welding precautions: For weld neck flanges, control welding temperature and interpass temperature to avoid distortion or cracking.
3. Bolt tightening: Use the cross-symmetry method in at least three passes to ensure even gasket compression and prevent leakage.

4. Regular inspection: Periodically check bolting torque and flange face condition, especially in high-temperature or vibrating services.

FAQ:

Q1: When should I use a blind flange (BL) instead of other flange types?
A: A blind flange is used primarily to close off the end of a piping system, isolate a section for maintenance, or provide access for pressure testing. Unlike weld neck or slip-on flanges, it has no bore and is a solid plate. It is ideal for temporary or permanent pipe closures and is commonly used in high-pressure systems where a secure seal is required.

Q2: What is the difference between raised face (RF) and ring-type joint (RTJ) flange sealing faces?
A: The raised face (RF) flange has a small raised area around the bore that compresses a gasket to create a seal. It is the most common facing for general service. The ring-type joint (RTJ) flange uses a metal ring gasket that seats into a machined groove, creating a metal-to-metal seal. RTJ flanges are used in high-pressure and high-temperature services where a more robust, leak-proof seal is required, such as in oil and gas applications.

Q3: Can ASTM A105 flanges be welded directly to stainless steel pipes?
A: While technically possible, it is not recommended for corrosive services. Direct welding of carbon steel (A105) to stainless steel can create a galvanic corrosion risk in the presence of an electrolyte. Additionally, the weld area may become susceptible to cracking due to differences in thermal expansion and material properties. For such connections, it is best to use a transition joint or a flange adapter, or to match the flange material to the pipe material (i.e., use a stainless steel flange with a stainless steel pipe).

Conclusion

ASTM A105 pipe flange is an economical and practical choice for industrial pipelines. It offers reliable mechanical properties and good machinability, especially suitable for normal-temperature, non-corrosive, medium to high-pressure conditions. However, it must be used with caution in extreme temperature or corrosive environments – consider upgrading to alloy steel (A182 F11), low-temperature steel (A350 LF2), or stainless steel (A182 F316) as needed. Always consult the ASME B16.5 standard and project specifications for final selection.

Related Resources:

●  ASME B16.5 Flange Dimensions – Complete size reference for Class 150-2500 flanges.
●  ASTM A105 Carbon Steel Forgings Specification –Detailed material properties and chemical composition.

●  Common Material Types of Carbon Steel Flanges 

●  Pipe Elbow and Flange Connection