Carbon steel is the workhorse grade for industrial flanges. It covers the majority of the catalog and the majority of installed service. Inside that one word category, however, are two specifications that look similar on paper and behave very differently in the field. ASTM A105 is the general purpose forged carbon flange grade. ASTM A694 is a family of high-yield carbon grades engineered to mate to API 5L pipeline pipe. They overlap in chemistry, they part ways on mechanical properties, and they belong on different line items.
This walks through what A105 actually is, what each A694 grade actually delivers, where the two specifications govern, and how to recognize when a spec has called for the wrong one. The result is not academic. Putting A105 on a Class 600 pipeline service that wants A694 F60 is a real failure mode. Putting A694 on a Class 150 utility air header is just expensive.
What A105 Actually Specifies
ASTM A105 is the standard specification for carbon steel forgings for piping applications. The grade has a single mechanical property set: minimum yield strength of 36 ksi (250 MPa), minimum tensile strength of 70 ksi (485 MPa), minimum elongation of 22 percent, and minimum reduction of area of 30 percent. The chemistry is moderately specified, with carbon capped at 0.35 percent, manganese in the 0.60 to 1.05 range, and limits on phosphorus, sulfur, silicon, copper, nickel, chromium, molybdenum, and vanadium. The carbon equivalent is capped at 0.47 when phosphorus and sulfur are each at 0.025 or below.
A105 is normalize-capable but not normalize-required. Most stock A105 ships as-forged with mill heat treatment that targets the property minima. The grade is not impact-tested by default. It is not pressure-rated at low temperature. ASME B31.3 limits A105 to a minimum design metal temperature of minus 20 degrees F without impact testing. Below that, the spec wants A350 LF2 with charpy data at the design temperature. A105 is also not hardness-controlled for sour service. NACE MR0175 sour service applications want A105 with supplementary hardness testing at 187 HB maximum, and most stock A105 will not meet that without rework.
Where A105 lives is on standard B16.5 and B16.47 Series A forged flanges in Class 150 through Class 2500, on socket weld and threaded fittings under B16.11, and on long-weld-neck and reducing flanges. It mates correctly to API 5L Grade B pipe, which has 35 ksi minimum yield. It does not mate correctly to API 5L X-grade pipe, which has higher yields starting at 42 ksi for X42 and climbing through X80. When the pipe gets stronger than the flange, the flange becomes the weak link. That is the failure mode A694 was specified to prevent.
The A694 Family
ASTM A694 covers carbon and alloy steel forgings for pipe flanges, fittings, valves, and parts for high-pressure transmission service. The grade is structured as a family. The grade designation is the letter F followed by the minimum yield strength in ksi: F42, F46, F48, F50, F52, F56, F60, F65, F70. F42 has a minimum yield of 42 ksi. F60 has a minimum yield of 60 ksi. F70 sits at 70 ksi minimum yield with a tensile strength of 85 ksi minimum. The chemistry varies by grade and is more tightly controlled than A105, with explicit limits on carbon, manganese, silicon, nickel, chromium, molybdenum, copper, niobium, vanadium, and titanium.
A694 is normalize-and-temper or quench-and-temper heat treated to develop the elevated yield. The heat treatment is not optional. The grain structure that results carries the higher strength while keeping toughness adequate for transmission pipeline service. Charpy V-notch impact testing is required on A694 grades F52 and above at a test temperature specified by the purchase order, with minimum energy values that scale with the grade. F65 and F70 typically test at minus 50 degrees F or lower depending on the project location. Hardness is also controlled, with maximum hardness for sour service applications running 248 HV10 (about 235 HB) or lower depending on the NACE annex called out.
Where A694 lives is on MSS SP-44 pipeline flanges and on B16.47 flanges in transmission pipeline service. MSS SP-44 covers steel pipeline flanges in sizes 10 inch through 60 inch at API class ratings 150, 300, 400, 600, 900, and 1500. The mating pipe is API 5L X-grade, with X42 through X70 covering the most common combinations. A694 F42 mates to X42, F52 to X52, F60 to X60, F65 to X65, F70 to X70. The match is intentional. Flange yield meets or slightly exceeds pipe yield at every grade in the system.
Where the Two Standards Govern
ASME B16.5 dimensions and B16.47 dimensions are the same whether the flange is A105 or A694 of the same nominal pressure class. A 12 inch Class 600 raised face weld neck flange has the same outside diameter, bolt circle, bolt count, hub thickness, and raised face dimensions regardless of carbon grade. That dimensional sameness is what allows specifications to call either grade onto the same drawing. The difference is metallurgical and mechanical, not dimensional.
Pressure-temperature ratings are a different matter. A105 in Class 600 holds 1440 psi at 100 degrees F. A694 F60 in Class 600 also holds 1440 psi at 100 degrees F by the standard B16.5 rating tables. The standard rating numbers do not change with the higher yield. Where the higher yield buys design margin is in pipeline practice, where the actual pipeline operating pressure is often calculated against the pipe yield directly and the flange is required to match that yield. A pipeline operating at 1100 psi based on X60 pipe wall thickness wants a flange whose specified minimum yield is at least 60 ksi, which is A694 F60, not A105.
ASME B31.4 and ASME B31.8 are the governing codes for liquid hydrocarbon pipelines and gas transmission pipelines respectively. Both codes drive flange selection through pipe matching. A B31.8 gas transmission line operating at 1100 psi on X60 pipe with a design factor of 0.72 wants flanges qualified for the service. MSS SP-44 in A694 F60 is the qualified product. ASME B16.5 in A105 is not the qualified product even though the dimensional class number reads the same.
Specifying Across Grades
The first thing to confirm on any flange line item is the mating pipe grade. If the pipe is API 5L Grade B (35 ksi yield), A105 is the correct flange grade. If the pipe is API 5L X42, X46, X52, X56, X60, X65, X70, or X80, the flange grade should be the matching A694 F-grade or one level above. Mismatch in either direction creates a failure mode. Flange weaker than pipe is the obvious one. Flange much stronger than pipe is the less obvious one, where the bolt-up stress field in the flange transfers into the pipe wall and creates local strain concentration at the weld.
The second thing to confirm is the impact requirement. If the design minimum temperature is minus 20 degrees F or warmer, A105 with no impact testing is qualified. If the design minimum temperature is colder, A350 LF2 (carbon, low-temperature, impact-tested) is the right product, and A105 should not be substituted. A694 above F52 carries default impact requirements, but the test temperature must be specified on the purchase order. Field crews calling out 'A694 F60' without a test temperature will get whatever the mill last produced, which may or may not be qualified for the design.
The third thing to confirm is sour service and NACE compliance. NACE MR0175 covers materials for oil and gas service that contains hydrogen sulfide above 0.05 psi partial pressure. The standard limits hardness for steels in contact with sour environments. A105 and A694 can both meet the hardness limits with supplementary testing, but neither does it by default. The flange purchase order has to call out NACE compliance explicitly and require hardness testing on every heat lot. Without that callout, the supplier ships standard product and the sour service requirement is not met.
When to Push Back on a Spec
A spec that calls A105 on a Class 600 or higher MSS SP-44 pipeline flange is almost always wrong. The flange should be A694 of a grade matching the pipe. Push back, ask for the pipe spec, and confirm the grade against the API 5L grade. A spec that calls A694 F70 on a small bore Class 150 utility line is also almost always wrong, just expensively wrong. The high-yield grade has no value at Class 150 pressure ratings and adds material cost, lead time, and a probably-unnecessary charpy test record. Push back, ask what the engineer was matching to, and offer A105 with the correct standard testing.
A spec that calls A105 for a minus 40 degree F design temperature is wrong. The right grade is A350 LF2 with charpy impact testing at the design temperature or colder. Substituting A105 because it is in stock is a code violation. A spec that calls A694 F52 for a sour service line without specifying NACE hardness testing is incomplete. The flange will likely ship over-hardness and fail post-weld hardness mapping. Push back, ask for the hydrogen sulfide partial pressure, and require NACE supplementary testing on the purchase order.
These calls are part of the job. Carbon steel grades look interchangeable on a one-line bill of materials, and they are not. Call (281) 484-8325 with the line spec or send the pipe class and mating pipe grade by email and we will confirm which carbon grade belongs on the flange order before it ships. Catching the substitution at the order stage is much cheaper than catching it at receiving inspection or, worse, after the line is in service.