JCOE Forming Method for LSAW Steel Pipes

LSAW steel pipes are crucial for transporting hydrocarbons under extreme conditions, thus requiring stringent mechanical and metallurgical properties. The JCOE forming process is a leading manufacturing technology due to its flexibility in handling high-strength, thick-walled pipes (typically 6-40 mm wall thickness and 16-60 inch diameter).

Unlike the UOE method (which relies on simultaneous forming in a U-shaped press followed by O-forming and flaring), JCOE employs a progressive, step-by-step forming method, reducing springback and residual stress.

This paper systematically evaluates the JCOE process for LSAW steel pipes by analyzing key parameters such as forming force, strain distribution, weld quality, and post-forming mechanical properties. Furthermore, it compares and evaluates JCOE with UOE and three-roll bending methods to highlight the advantages and limitations of each technology.

JCOE Forming Process for LSAW Steel Pipes

1. JCOE Process

The JCOE method comprises a series of continuous forming stages:

J-shaped: The edges of the sheet metal are pre-bent into a "J" shape using a hydraulic press. C-shape: The central portion is compressed into a "C" shape.

O-shape: The open "C" shape is closed into an "O" shape through gradual compression.

Expansion: Mechanical or hydraulic expansion machines ensure dimensional uniformity.

2. Key Process Parameters

The following parameters have a significant impact on the quality of LSAW steel pipes:

Parameter	Typical Range	Impact on Pipe Quality
Press force (J/C stage)	10,000–50,000 kN	Excessive force induces microcracks
Bending radius	1.5–3.0 × pipe diameter	Smaller radii increase strain hardening
Expansion ratio	0.8–1.2%	Over-expansion reduces yield strength
Welding speed	0.8–1.5 m/min	Higher speeds may cause lack of fusion
Preheating temperature	100–200°C	Reduces residual stress in HIC-prone steels

Finite element analysis (FEA) shows that JCOE exhibits a more uniform strain distribution compared to UOE, thus mitigating localized thinning. However, the stepped structure of JCOE introduces a slight ellipticity, requiring precise expansion control.

3. Producible Dimensions

Typically, the JCOE process can produce:

Outer Diameter: 406 mm – 1524 mm (16" – 60")

Wall Thickness: 6 mm – 40 mm

Pipe Steel Grade: API 5L X52 – X80 (some up to X100)

Single Length: 9 – 12.2 m

LSAW Steel Pipe Forming: JCOE vs. UOE vs. Three-Roll Bending

1. Mechanical Properties

API 5L X70 LSAW steel pipes manufactured using JCOE, UOE, and three-roll bending processes were compared.

Property	JCOE	UOE	Three-Roll Bending
Yield strength (MPa)	485–520	470–500	460–490
Tensile strength (MPa)	570–610	560–590	550–580
Elongation (%)	28–32	26–30	25–28
Impact energy (J, -20°C)	80–100	70–90	60–80

JCOE exhibits superior strength and toughness due to controlled work hardening and reduced heat-affected zone (HAZ) degradation.

2. Dimensional Accuracy and Residual Stress

UOE pipes have better roundness (deviation ≤0.5%), while JCOE LSAW steel pipes have an ellipticity of 0.8%–1.2% before expansion. However, after expansion, the roundness of JCOE LSAW steel pipes is comparable to that of UOE pipes (≤0.6%). X-ray diffraction residual stress measurements show:

JCOE: 200–250 MPa (compressive stress at weld)

UOE: 300–350 MPa (tensile stress in flange area)

Three-roll bending: 400+ MPa (non-uniform distribution)

The lower residual stress in JCOE pipes improves fatigue resistance, which is crucial for deep-water pipelines.

Microstructure and Weld Integrity

1. Grain Structure Evolution

Compared to UOE (ASTM 8-10), JCOE's progressive deformation refines the ferrite-pearlite grain size (ASTM 10-12).

2. Weld Properties

The submerged arc welding (SAW) process for JCOE pipes indicates:

Porosity: <1% (in contrast, UOE porosity is 1.5%–2% due to higher heat input)

HAZ Width: 1.2–1.5 mm (2.0–2.5 mm in UOE)

Hardness: 220–240 HV (consistent hardness at the weld)

Applications of JCOE-formed LSAW Pipes

LSAW pipes dominate in:

High-pressure natural gas transportation;

Deep-water oil pipelines (wall thickness >25 mm);

Arctic-class pipelines (excellent low-temperature toughness).

Metallurgical Transformations During JCOE Forming

1. Phase Evolution of High-Strength Steel

Thermomechanical cycling in JCOE alters the microstructure:

X70/X80 steel: Controlled deformation inhibits ferrite grain growth, promoting the formation of acicular ferrite (70-80% volume fraction) and dispersed M/A islands, thereby improving toughness.

X100/X120 steel: The combination of Nb/Ti microalloying and the strain rate (0.1–1 s⁻¹) of JCOE accelerates NbC precipitation, increasing the yield strength by 40–60 MPa.

2. Microhardness Distribution

Weld zone: 240–260 HV (SAW welding wire ER70S-6)

Heat-affected zone: 220–240 HV (tempered bainite)

Base metal: 190–210 HV (fine polygonal ferrite)

3. Resistance to Hydrogen-Induced Cracking (HIC)

Compared to UOE, JCOE's lower residual stress reduces susceptibility to hydrogen-induced cracking. NACE TM0284 testing shows: JCOE pipes: Crack length ratio (CLR) <5%, Crack thickness ratio (CTR) <2%; UOE pipes: CLR 8–12%, CTR 3–5%, due to higher tensile residual stress.

Summary

The JCOE forming process is one of the core technologies in modern LSAW steel pipe manufacturing. Its main advantages include: progressive forming reduces residual stress, suitability for thick-walled and high-strength pipeline steels, excellent weld quality and toughness, and excellent fatigue and HIC resistance.