What Is the Difference Between A105 and A36 Carbon Steel Plate Type Flange Materials

Selecting the right material for a Carbon Steel Plate Type Flange is critical for piping integrity, safety, and cost management. Among the most common grades are ASTM A105 and ASTM A36. While both fall under the carbon steel family, they serve distinctly different roles in flange manufacturing. At Longan, we frequently guide engineers through this choice, as misapplication can lead to premature failure, leakage, or unnecessary expense. This blog breaks down the metallurgical, mechanical, and practical differences between A105 and A36 for Carbon Steel Plate Type Flange applications, helping you make an informed, code-compliant decision.

1. Material Specifications and Intended Service

The primary distinction lies in their governing standards and design intent.

• ASTM A105 is a specification specifically for forged carbon steel piping components, including flanges, fittings, and valves. It is intended for ambient- and high-temperature service in pressure systems. A Carbon Steel Plate Type Flange made from A105 is typically forged, not rolled from plate, which refines the grain structure and enhances directional strength.

• ASTM A36 is a structural steel grade, intended for general construction—beams, columns, and bridge components. While it can be cut from plate to manufacture a Carbon Steel Plate Type Flange, it lacks the strict pressure-retaining requirements of A105. Longan recommends A36 only for non-critical, low-pressure, or temporary connections where dimensional stability outweighs pressure integrity.

2. Chemical Composition and Weldability

• A105 has a higher carbon equivalent, which improves strength but requires preheating for thick sections to avoid hydrogen-induced cracking.

• A36 offers slightly better weldability in thin sections due to lower carbon, but its higher sulfur content reduces impact toughness—a critical factor for a Carbon Steel Plate Type Flange under cyclic loading.

Longan advises that for any welded Carbon Steel Plate Type Flange subject to code inspection (ASME B31.3), A105 is the mandated choice, while A36 may be acceptable for non-code utility stations.

3. Mechanical Properties – Side-by-Side Comparison

Although both have the same minimum yield strength (36 ksi), A105 provides higher tensile strength and guaranteed impact resistance. This makes a Carbon Steel Plate Type Flange in A105 suitable for hydrostatic testing and thermal cycling. Conversely, A36’s lower ductility at low temperatures means it is not recommended for outdoor cryogenic or winterized environments. Longan consistently observes that engineers overestimate A36’s performance in pressure systems—a costly oversight.

4. Manufacturing and Cost Implications

• A105 flanges undergo forging followed by normalizing or quenching-and-tempering, which eliminates internal voids and refines grain size. This process increases production cost but ensures reliability in a Carbon Steel Plate Type Flange up to 2,000 psi (depending on class).

• A36 flanges are flame-cut or sheared from structural plate, then machined. This is cheaper and faster, making them attractive for temporary spools, supports, or dummy flanges. However, they are not traceable to a heat code for pressure retention per ASME Section VIII.

Longan offers both grades but explicitly flags A36 for low-risk applications only. For permanent piping, we always recommend A105 with full material test reports (MTRs).

5. Common FAQs About Carbon Steel Plate Type Flange Materials

FAQ 1: Can I substitute A36 for A105 in a Class 300 Carbon Steel Plate Type Flange?

Answer: No. Class 300 flanges require a minimum tensile strength of 70 ksi and documented impact properties per ASME B16.5. A36 only guarantees 58–80 ksi, and its actual strength can fall below 70 ksi in plate form. Additionally, A36 lacks the forging integrity needed for raised-face or RTJ sealing surfaces under high bolt loads. Longan strictly prohibits this substitution in any pressure-containing service. If you attempt it, your system will likely fail hydrotest or experience gasket creep relaxation within months.

FAQ 2: How do I identify whether my existing Carbon Steel Plate Type Flange is A105 or A36 without a stamp?

Answer: Visual inspection is unreliable, but there are two practical methods. First, check for an ASTM stamp on the outer rim—A105 flanges are almost always marked with “A105” and a heat number. A36 plate flanges rarely carry such traceability. Second, perform a simple hardness test using a portable Leeb device: A105 typically reads 150–187 HB, while A36 reads 120–140 HB. However, the definitive method is to request an MTR from the supplier. Longan provides full traceability on every A105 Carbon Steel Plate Type Flange, including chemical and mechanical certification, so you never guess.

FAQ 3: Which material offers better corrosion resistance in a Carbon Steel Plate Type Flange for water service?

Answer: Neither grade offers inherent corrosion resistance—both are plain carbon steels and will rust in water. However, A105’s denser forged microstructure slightly reduces pitting initiation compared to A36’s plate surface. More importantly, A105 is often supplied with a protective zinc or epoxy coating for water service, while A36 is usually bare. For long-term potable or cooling water lines, Longan recommends A105 with a fusion-bonded epoxy coating. A36 should only be used if the system is dry or temporarily filled with inhibited water. Always account for corrosion allowance; A105’s higher tensile strength allows for a thinner wall, leaving more room for corrosion margin.

6. Decision Matrix – Which One Should You Specify?

Longan advises that when in doubt, default to A105. The incremental cost is marginal compared to the liability of a failed Carbon Steel Plate Type Flange in an active plant.

7. Practical Recommendation from Longan

For permanent, pressure-bound piping systems, Longan exclusively supplies A105 Carbon Steel Plate Type Flange products with normalized heat treatment and positive material identification (PMI). We reserve A36 for structural stools, lifting lugs, or temporary test blinds where pressure retention is not a factor. Always specify the service temperature, design pressure, and fluid corrosivity when requesting a quote—this allows our engineering team to validate the correct material grade for your specific duty.

Contact Us

Choosing between A105 and A36 for your Carbon Steel Plate Type Flange should never be a guess. Longan provides certified, traceable flanges with full documentation and technical support from order to installation. If you need a material recommendation, a custom bore size, or a rush delivery for turnaround projects, reach out to our flange specialists today. Let Longan be your reliable partner in every critical connection.