The Default Forging Grade for Flanges, Fittings and Valve Bodies in Ambient and Moderate Temperature Service

What is ASTM A105?

A105 is a plain carbon steel forging with up to 0.35% carbon, 0.60-1.05% manganese and 0.10-0.35% silicon. Residual chromium, nickel, copper, molybdenum and vanadium are capped individually so the forging stays weldable and predictable. The strength floor is 485 MPa tensile and 250 MPa yield, higher than A234 WPB because forging and the higher carbon allowance give more margin.

The specification covers forgings for flanges to ASME B16.5 and B16.47, fittings to B16.11, and valve bodies to B16.34, in sizes from NPS ½ upward. A105 is supplied as-forged unless the order says otherwise. The normalised variant, A105N, is ordered where improved toughness or a finer grain size is wanted, and it is the usual choice for low-temperature flanges that cannot justify A350 LF2.

The grade is not intended for low-temperature service. ASME B31.3 stops at -29°C for A105, and there is no impact requirement in the base specification. Where the design temperature goes below that, or where a Charpy requirement exists at all, A350 LF2 is the correct specification and the cost difference is small.

Chemical Composition (Weight %)

A105 chemical requirements. The residual caps matter for welding: a forging that sits at the ceiling on chromium and molybdenum will behave differently from a clean heat.

A105 also limits the sum of Cu + Ni + Cr + Mo to 1.00% and adds a carbon equivalent check, typically 0.47 maximum, on forgings above a stated thickness. The CE limit is what keeps a heavy blind flange weldable without preheat beyond what a field crew can manage.

Mechanical Properties

A105 tensile requirements, with typical values for as-forged and normalised forgings. Hardness is capped, not just reported.

Reduction of area is also specified at 30% minimum, which matters for forgings because it detects internal soundness better than tensile strength does. A forging that meets strength but fails reduction of area usually has a forging or heat treatment defect rather than a chemistry problem.

Physical Properties

Same physical basis as A234 WPB, since both are plain carbon steel. Gasket and bolting selection usually governs the joint design, not the flange metal properties.

Heat Treatment, Welding & PWHT

As-forged and normalised supply

A105 is normally supplied as forged. A105N is normalised at 900-940°C and air cooled, which refines the grain and improves toughness. The N suffix is a separate ordering condition, not an option the mill chooses, so the purchase order has to specify it.

Post-weld heat treatment

PWHT follows ASME B31.3 for P-No. 1: required above 19 mm nominal thickness, at 593-649°C with one hour per 25 mm and a one hour minimum. Slip-on and socket-weld flanges welded to heavy pipe are the usual case where this is missed in the field.

Welding

Weld neck flanges are butt welded with ER70S-6 or E7018. Slip-on and socket-weld flanges use fillet welds, and the socket weld needs an expansion gap at the bottom of the socket, otherwise the weld root cracks as the joint cools.

Machining and galling

A105 machines well in the normalised condition. For large valves and spectacle blinds, stress relief after rough machining reduces distortion during final finishing, particularly on thin section bodies.

Service Limits & Corrosion Behaviour

Temperature envelope

ASME B31.3 lists A105 from -29°C to 427°C. It is an ambient and moderate temperature material: steam, water, oil, gas and process fluids inside that band.

Corrosion

No alloying element resists corrosion. Performance equals carbon steel: acceptable in dry gas, treated water and neutral hydrocarbons, poor in acids and chlorides. Internal coatings or a corrosion allowance come from the line class.

Low temperature

Below -29°C there is no code allowance and no impact requirement in A105. A350 LF2 covers -46°C, and LF3 covers -101°C. Impact-tested A105N is available but is not a substitute for a grade designed for the temperature.

Sour service

NACE MR0175 / ISO 15156 requires 22 HRC maximum, which A105 normally meets at 187 HBW. The requirement still has to be stated, because it adds hardness testing and chemistry verification.

ASTM A105 vs Similar Grades — Selection Guide

A105 and A350 LF2 have overlapping strength, which is why substitution is tempting. The difference is impact testing and heat treatment, and no amount of certificate wording converts an A105 flange into an LF2 flange.

Applicable Standards

HT PIPE Supply — ASTM A105 Product Forms

Flanges are supplied with EN 10204 3.1 certificates, heat numbers transferred to the flange face or rim, and dimensional inspection to B16.5 including facing finish and bolt circle.

Project Experience — ASTM A105

A105 forgings are supplied for refinery, chemical, water and utility piping where the design temperature stays inside the carbon steel band.

For projects that need both A105 and A350 LF2, we supply the two grades with distinct marking and separate certificates to prevent mixing at site.

FAQ — ASTM A105