Thermal Cycle Characteristics of CT80 Continuous Oil Tubing TIG Butt Weld Joints

Literature Overview and Research Significance

Continuous oil tubing (COT), also known as continuous string tubing, is a critical component in deep well drilling operations. CT80 denotes a high-strength steel grade with a minimum yield strength of 552 MPa (80,000 psi) and a maximum hardness of 26 HRC, designed to withstand the extreme pressures, temperatures, and corrosive environments encountered in oil and gas wells. The tubing is typically manufactured from cold-drawn low-carbon or low-alloy steel and must be joined by welding when longer lengths are required for specific well applications.

This 2011 study by Li Xiao, Shi Kai, Wang Hongduo, and Li Jie from the Key Discipline Laboratory of Materials Processing Engineering at Xi'an Petroleum University investigates the thermal cycle process of TIG butt weld joints in CT80 continuous oil tubing. Understanding the thermal cycle is fundamental to predicting microstructural evolution, residual stress distribution, and mechanical property variation across the weld cross-section.

Base Material Characteristics and Welding Challenges

CT80 tubing is typically supplied in a cold-worked condition that provides high strength and hardness but limited ductility. The base material exhibits a yield strength of 552–620 MPa, an ultimate tensile strength of 620–700 MPa, and an elongation of 10–14 percent. The hardness ranges from 22–26 HRC in the cold-drawn condition.

The welding of CT80 presents several challenges:

• High strength requirement: The weld joint must retain at least 90 percent of the base material yield strength to maintain structural integrity during well operations.
• Low ductility of base material: The cold-worked microstructure has limited capacity for plastic deformation, increasing susceptibility to cracking.
• Thermal sensitivity: The high strength is derived from dislocation density and work-hardening, which are sensitive to thermal exposure above 300 degrees Celsius.
• Hydrogen embrittlement risk: High-strength steels are susceptible to hydrogen-induced cracking (HIC) and delayed cracking.

Thermal Cycle Analysis and Modeling

The study employed a combination of thermocouple measurements and finite element thermal simulation to characterize the thermal cycle in CT80 TIG butt weld joints. Thermocouples of type K were embedded at multiple positions across the weld cross-section: weld centerline, 1 mm from fusion boundary, 3 mm from fusion boundary, and in the base metal at 10 mm from the weld. The welding parameters used were: current 150–180 A, voltage 14–16 V, speed 5–7 mm/min, and heat input in the range of 1.0–1.5 kJ/mm.

The thermal cycle data revealed several key characteristics:

• Peak temperature: The weld centerline experienced peak temperatures exceeding 1800 degrees Celsius, while the HAZ peak temperature ranged from 800 to 1400 degrees Celsius depending on distance from the fusion boundary.
• Time above 800 degrees Celsius (t800): This parameter, critical for grain growth assessment, ranged from 0.5 seconds at the fusion boundary to 8–12 seconds at 3 mm from the weld.
• Time above 400 degrees Celsius (t400): This parameter, relevant for tempering and softening of the cold-worked base material, extended to 30–45 seconds at positions 5–8 mm from the weld.
• Cooling rate: The cooling rate from 800 to 500 degrees Celsius (CR800-500) at the fusion boundary was approximately 40–60 degrees Celsius per second, while at 3 mm from the weld it decreased to 8–15 degrees Celsius per second.

The thermal simulation using a three-dimensional finite element model with a moving heat source (double-ellipsoid model per Goldak) showed good agreement with experimental thermocouple data, with maximum deviations of less than 50 degrees Celsius in peak temperature and less than 2 seconds in t800 values. The model incorporated variable thermal conductivity and specific heat as functions of temperature, which is essential for accurate simulation of high-strength steel where these properties change significantly above 400 degrees Celsius.

Microstructural Evolution and Mechanical Property Distribution

The thermal cycle directly governs the microstructural evolution across the weld cross-section. In the weld metal, the rapid cooling rate (CR800-500 > 40 degrees Celsius/s) promotes the formation of acicular ferrite and fine grain ferrite, which provide good toughness and acceptable strength. The absence of coarse pearlite or bainite is attributed to the low carbon equivalent (CE) of the filler wire, typically ER70S-6 or equivalent with CE < 0.4.

In the HAZ, the thermal cycle produces distinct sub-zones:

• Coarse grain HAZ (CGHAZ): Located within approximately 0.5 mm of the fusion boundary, this zone experienced peak temperatures above 1200 degrees Celsius and rapid cooling, resulting in a mixture of acicular ferrite and upper bainite. The grain size increases from 15–20 micrometers in the base metal to 40–60 micrometers.
• Fine grain HAZ (FGHAZ): Located 0.5–2 mm from the fusion boundary, with peak temperatures of 900–1200 degrees Celsius, this zone shows a refined grain structure due to partial recrystallization. The microstructure consists primarily of fine ferrite and some pearlite.
• Intercritical HAZ (ICHAZ): Located 2–5 mm from the fusion boundary, with peak temperatures of 700–900 degrees Celsius, this zone experiences tempering of the cold-worked microstructure without full recrystallization. The dislocation density decreases, leading to softening.
• Tempered zone: Located beyond 5 mm from the weld, where peak temperatures are below 700 degrees Celsius. The cold-worked structure undergoes partial recovery, resulting in a moderate decrease in strength and increase in ductility.

The mechanical property distribution across the weld cross-section shows the expected pattern of softening in the HAZ:

The softening in the ICHAZ and tempered zone represents a significant reduction in strength, with the minimum hardness occurring at approximately 3–5 mm from the fusion boundary. This softening zone is a potential concern for fatigue crack propagation and pressure boundary integrity in well operations.

Engineering Practice and Welding Procedure Optimization

Based on the thermal cycle analysis, the study recommends the following welding procedure optimizations for CT80 tubing:

1. Heat input control: Maintain heat input in the range of 1.0–1.3 kJ/mm to minimize the width of the softening zone while ensuring full penetration. Higher heat inputs extend the tempered zone and increase the area of strength loss.
2. Preheating: A preheat temperature of 50–80 degrees Celsius is recommended to reduce the cooling rate in the CGHAZ and minimize the risk of martensite formation and cold cracking. Preheating above 100 degrees Celsius should be avoided to prevent excessive softening of the base material.
3. Interpass temperature: For multi-pass welds, the interpass temperature should be maintained between 100 and 150 degrees Celsius. This limits the number of thermal cycles in the HAZ while preventing excessive cooling rates.
4. Post-weld heat treatment (PWHT): A stress relief treatment at 550–600 degrees Celsius for 1–2 hours is recommended to reduce residual stresses and homogenize the microstructure. However, PWHT will further soften the base material, reducing the yield strength by 10–15 percent.
5. Hydrogen control: The filler wire should be low-hydrogen type (diffusible hydrogen content below 5 mL/100g), and the shielding gas should be dry argon with dew point below -40 degrees Celsius. Preheating and post-weld baking at 200–250 degrees Celsius for 1 hour are recommended to prevent hydrogen-induced cracking.

Summary and Study Reflections

This study provides a comprehensive understanding of the thermal cycle in CT80 continuous oil tubing TIG weld joints, linking thermal parameters to microstructural evolution and mechanical property distribution. The key engineering insight is that the softening zone in the HAZ, governed by the tempering of the cold-worked base material, represents the weakest region of the joint and must be carefully managed through heat input control. For engineers in the oil and gas industry, the findings emphasize that welding procedure qualification per ASME IX or API 934 must include evaluation of the softened HAZ region, not just the weld metal. The thermal simulation methodology presented is also valuable for predicting the effects of parameter variations without extensive coupon testing, thereby accelerating procedure development. In the context of pressure vessel fabrication, the same thermal cycle principles apply to the welding of high-strength low-alloy steel (HSLA) pressure boundaries, where control of the tempered zone is critical for maintaining design integrity. The study reinforces the fundamental principle that in welding high-strength steels, the HAZ often presents more challenges than the weld metal itself, and that thermal cycle management is the primary tool for controlling joint performance.