Carbon Steel Unions Manufacturer & Exporter

Solitaire Overseas manufactures carbon steel unions dimensioned to MSS SP-83 and ASME B16.11, supplied in ASTM A105 forged material. A carbon steel union is a three-piece small bore fitting that lets a pipe run be disconnected and reconnected at a single point without rotating either connected pipe, unlike a coupling. Our carbon steel union range covers NPS 1/8 inch to 4 inch in threaded and socket weld end configurations across Class 2000, 3000 and 6000, with full mill test certification and dimensional inspection on every batch.

Types of Carbon Steel Unions

Threaded Union - NPT connections on both ends, Class 2000/3000/6000, for bolted-free disconnection of small bore threaded piping.
Socket Weld Union - socket bore connections on both ends, Class 3000/6000, welded into the line with a disconnect point at the center nut.
Ground Joint Union - features a lapped, metal-to-metal conical seat between the male and female ends for a leak-tight seal without a gasket.

Need forged elbows, tees or couplings to complete the small bore run? See our Carbon Steel Forged Fittings range, or reach out for a combined quote.

Technical Data

Specifications Chemical Composition (ASTM A105, %) Mechanical Properties (ASTM A105) Equivalent Grades Threaded vs Socket Weld

Carbon Steel Unions Specifications

Parameter	Specification
Standard	MSS SP-83 (forged steel unions); ASME B16.11 (general forged fitting dimensions/marking)
Material Specification	ASTM A105 (standard); ASTM A105N normalized on request
Size Range	NPS 1/8" to 4"
Class / Rating	Class 2000, 3000 or 6000 threaded; Class 3000 or 6000 socket weld
End Connection	NPT taper thread both ends (threaded union); socket weld bore both ends (socket weld union)
Manufacturing	Forged from bar stock; three-piece construction with male end, female end and center ring/nut
Fitting Types	Threaded union, socket weld union, ground joint (metal-to-metal seat) union
Seat Design	Ground joint metal-to-metal seat as standard; elastomer seat on special request
Certification	Mill Test Certificate EN 10204 3.1, 3.2 on request

Chemical Composition (ASTM A105, %) - Forged Grade

Element	A105 max/range
Carbon max	0.35
Manganese	0.60 - 1.05
Phosphorus max	0.035
Sulfur max	0.040
Silicon	0.10 - 0.35
Copper max	0.40
Nickel max	0.40
Chromium max	0.30
Molybdenum max	0.12
Vanadium max	0.08

Sum of Cu, Ni, Cr, Mo and V shall not exceed 1.00%; sum of Cr and Mo shall not exceed 0.32%. A105N denotes the normalized condition, offering improved impact toughness for low-temperature or critical service.

Mechanical Properties (ASTM A105)

Property	Requirement
Tensile Strength min	485 MPa (70 ksi)
Yield Strength min	250 MPa (36 ksi)
Elongation min (2 in.)	22%
Reduction of Area min	30%
Brinell Hardness	137 - 197 HBW

Equivalent Grades of Carbon steel Unions

ASTM Grade	EN	DIN	JIS
A105	P245GH / P250GH (EN 10222-2)	1.0460	SFVC1 (G3202)

Carbon Steel Unions Threaded vs Socket Weld

Criteria	Threaded Union	Socket Weld Union
End Connection	NPT taper thread	Socket weld bore, fillet welded
Installation	Fully removable, no welding required	Welded in place; center nut still permits disassembly
Typical Class	2000, 3000, 6000	3000, 6000
Leak Path Risk	Thread sealant dependent	Lower, due to welded end connections
Typical Use	Low pressure utility and instrument lines	Higher pressure or vibration-prone process lines

Applications of Carbon Steel Unions

Disconnection points in small bore utility and instrument piping
Compressor, pump skid and package unit piping requiring maintenance access
Instrumentation and gauge line connections
Fuel gas, air and water utility distribution piping
Any joint requiring disassembly without rotating the connected pipe runs

Frequently Asked Questions

What is a carbon steel union used for?

A carbon steel union allows a small bore pipe run to be disconnected and reconnected at a single point for maintenance, without needing to rotate either connected length of pipe, unlike a screwed coupling.

What is the difference between a threaded union and a socket weld union?

A threaded union connects to pipe with NPT taper threads on both ends and can be fully removed without welding, while a socket weld union is welded into the line and only the center nut is used to make or break the joint.

What standard governs carbon steel union dimensions?

MSS SP-83 sets the dimensions and pressure ratings for forged steel unions, used in conjunction with the general forged fitting requirements of ASME B16.11.

What material grade is used for carbon steel unions?

ASTM A105 forged carbon steel is the standard material for unions, with the normalized A105N condition available where improved impact toughness is required.

What size range is available for carbon steel unions?

Carbon steel unions are available from NPS 1/8 inch through NPS 4 inch, matching the general size range of ASME B16.11 forged fittings.

What is a ground joint union?

A ground joint union uses a lapped, metal-to-metal conical seat between the male and female halves to form a leak-tight seal without a separate gasket, and is the most common seat design for carbon steel unions.

Get a quote for carbon steel unions from Solitaire Overseas, or visit our Carbon Steel Pipe Fittings hub to compare unions against couplings and other forged fitting options.