Carbon Steel Union — Factory Direct Supplier | Huaxia-Steel
A carbon steel union is a threaded pipe fitting that allows two pipes to be connected and disconnected without requiring pipe rotation or cutting. Unlike couplings that must be turned with the pipe, unions feature a three-piece design — a female end, a male end, and a central nut — enabling quick disconnection for maintenance, inspection, and equipment replacement. This makes unions essential components in piping systems where periodic disassembly is required.
Huaxia-Steel supplies forged carbon steel unions in sizes from 1/8″ to 4″ (DN6 to DN100), manufactured to ASME B16.11, MSS SP-83, and BS 3799 standards. All our unions are produced from high-quality carbon steel forgings with full material traceability and are available with NPT, BSPT, and BSPP thread types to suit international project requirements.
Product Overview
Our carbon steel unions are manufactured by hot forging A105 or A234 WPB carbon steel, followed by precision machining, threading, and rigorous pressure testing. The three-piece design consists of:
- Female end (nut end): Threaded internally, connected to one pipe and receives the male end.
- Male end (tailpiece): Threaded internally at the pipe end, with a ground seat face that mates with the female end.
- Union nut: Threaded externally to engage the female end, compressing the seat faces together to form a metal-to-metal or gasket seal.
The ground joint or seat design is the critical feature — it provides the leak-tight seal without requiring thread sealant on the union faces. For applications requiring frequent disassembly, we offer unions with integral seats (metal-to-metal) for high-temperature service and unions with PTFE or fiber gaskets for general service below 200°C.
Specifications and Dimensions
| Nominal Size | Thread Type | Pressure Class | A (Length, mm) | B (Hex, mm) | Approx. Weight (kg) |
|---|---|---|---|---|---|
| 1/8″ (DN6) | NPT / BSPT | 3000# / 6000# | 44 | 22 | 0.15 |
| 1/4″ (DN8) | NPT / BSPT | 3000# / 6000# | 48 | 25 | 0.20 |
| 3/8″ (DN10) | NPT / BSPT | 3000# / 6000# | 52 | 30 | 0.28 |
| 1/2″ (DN15) | NPT / BSPT | 3000# / 6000# | 57 | 35 | 0.40 |
| 3/4″ (DN20) | NPT / BSPT | 3000# / 6000# | 64 | 42 | 0.60 |
| 1″ (DN25) | NPT / BSPT | 3000# / 6000# | 76 | 50 | 1.00 |
| 1-1/4″ (DN32) | NPT / BSPT | 3000# / 6000# | 86 | 58 | 1.45 |
| 1-1/2″ (DN40) | NPT / BSPT | 3000# / 6000# | 95 | 65 | 2.00 |
| 2″ (DN50) | NPT / BSPT | 3000# / 6000# | 108 | 78 | 3.10 |
| 2-1/2″ (DN65) | NPT / BSPT | 3000# | 127 | 95 | 5.00 |
| 3″ (DN80) | NPT / BSPT | 3000# | 143 | 108 | 7.50 |
| 4″ (DN100) | NPT / BSPT | 3000# | 168 | 135 | 12.00 |
Note: Dimensions conform to ASME B16.11 for 3000# and 6000# classes. Dimensions may vary slightly between manufacturers. Contact us for exact dimensional drawings.
Available Material Grades
| Grade | Standard | Typical Application |
|---|---|---|
| ASTM A105 | ASME B16.11 | General service, high-temperature piping up to 425°C |
| ASTM A234 WPB | ASME B16.9 | Moderate temperature and pressure, good weldability |
| ASTM A106 Gr B | ASME B16.11 (when forged from pipe) | High-temperature service, seamless construction |
| Q235 / 20# Steel | GB/T 699 / GB/T 700 | Low-pressure utility piping, general industrial |
| SS400 | JIS G3101 | General structural piping, Southeast Asian markets |
| S235JR | EN 10025-2 | European market general service piping |
Manufacturing Standards and Quality
Our carbon steel unions are manufactured in compliance with the following international standards:
- ASME B16.11: Forged fittings, socket-welding and threaded — the primary standard for threaded unions up to 4″ NPS.
- MSS SP-83: Class 3000 and 6000 pipe unions, socket welding and threaded — provides detailed dimensional and pressure-temperature ratings.
- BS 3799: Specification for steel pipe fittings, screwed and socket-welding for the petroleum industry — UK/Commonwealth standard.
- ASME B31.1 / B31.3: Power piping / Process piping — design codes governing installation.
- ISO 9001:2015: Quality management system — all production is under ISO-certified quality control.
Quality Control Process:
- Chemical analysis (OES) on each heat of raw material
- Visual and dimensional inspection at forging, machining, and threading stages
- Hydrostatic pressure testing at 1.5× rated working pressure
- Thread gauging (NPT: L1 and L3 gauges per ASME B1.20.1)
- Hardness testing per ASTM A370
- Optional NDT: Magnetic particle testing for surface defects, ultrasonic testing for internal quality
Applications
Carbon steel unions are used across a wide range of industries where threaded piping requires periodic disconnection:
- Oil and gas: Instrument air lines, chemical injection lines, sampling connections, drain and vent connections on separators and tanks.
- Steam systems: Steam trap assemblies, strainer connections, heat exchanger isolation — A105 unions with metal-to-metal seats are preferred for steam service.
- Water treatment: Pump connections, filter housings, chemical dosing lines.
- Fire protection: Sprinkler riser connections, alarm valve trim, drain connections (must be UL/FM approved when used in listed systems).
- HVAC and building services: Chilled water, condenser water, heating hot water — valves, strainers, and equipment connections.
- Industrial gas: Compressed air, nitrogen, CO2 — moderate pressure gas distribution headers.
- Marine: Bilge and ballast systems, fuel oil transfer, freshwater systems.
Ordering Information
When requesting a quotation for carbon steel unions, please specify:
- Size (NPS or DN)
- Pressure class (3000# or 6000#)
- Material grade (A105, A234 WPB, etc.)
- Thread type (NPT standard; BSPT, BSPP available)
- Seat type (metal-to-metal ground joint or gasket type)
- Quantity required
- Any supplementary requirements: NDT, third-party inspection, special coating, specific certification
Packaging and Shipping
- Each union individually protected with plastic end caps
- Packed in plywood cases or pallets with waterproof wrapping
- Size markings, heat numbers, and material grade stamped on each fitting
- Shipping marks and handling instructions per buyer requirements
- Containerized shipment (FCL) or LCL consolidation available
- Shipping from Tianjin, Shanghai, or Ningbo ports
- Standard lead time: 2-4 weeks for stock sizes; 4-6 weeks for non-stock or large quantities
Why Choose Huaxia-Steel for Carbon Steel Unions?
- Direct from ISO-certified mills: We work exclusively with forged fitting manufacturers holding ISO 9001, PED, and API certifications.
- Complete documentation: Every shipment includes EN 10204 3.1 mill test certificates with chemical and mechanical test results traceable to heat numbers.
- Competitive pricing: Factory-direct supply eliminates distributor markups. We offer volume discounts for container-load quantities.
- Global shipping experience: Regular exports to Southeast Asia, Middle East, Africa, and South America. Experienced export documentation team.
- Quality assurance: Optional third-party inspection by SGS, Bureau Veritas, Intertek, or TUV arranged at cost. All shipments inspected before loading.
- Flexible order quantities: From trial orders of 100 pieces to container loads of 10,000+ pieces.
Get a competitive quotation today. Contact our sales team with your union requirements — size, pressure class, material, and quantity — and receive a detailed offer including price, lead time, and shipping terms within 24 hours.
