Thermal Cycle Study of CT80 Continuous Oil Tubing TIG Butt Joints

Literature Overview

The paper by Li Xiao and colleagues from Xi'an Petroleum University, published in Heat Processing Technology in 2011 (Vol. 40, No. 9, pp. 168–170), investigates the welding thermal cycle of CT80 continuous oil tubing butt joints using multi-point synchronous thermocouple measurement. CT80 tubing, a high-strength low-alloy steel with a minimum yield strength of approximately 80 ksi (552 MPa), is widely used in continuous oil tubing applications in the oil and gas industry. The small diameter and high strength of CT80 tubing create unique welding challenges, particularly regarding heat accumulation and microstructural softening in the heat-affected zone.

Core Technical Content and Key Findings

The authors employed a multi-point synchronous thermocouple measurement technique to capture the thermal history at various locations along the weld seam and within the heat-affected zone. The small tube diameter of CT80 continuous oil tubing means that the thermal mass of the base metal is limited, leading to significant heat accumulation effects during multi-pass welding.

The following table summarizes the key thermal cycle characteristics observed:

Thermal Cycle Parameter	Observation	Engineering Significance
Peak temperature in HAZ	High, approaching or exceeding Ac3	Risk of grain coarsening
Temperature range for softening	450–650 °C (temper softening zone)	Reduced local strength
Heat accumulation effect	Pronounced due to small tube diameter	Progressive softening in multi-pass welding
Thermal gradient	Steep near weld root, gradual toward outer surface	Differential microstructural evolution
Cooling rate	Variable; slower in later passes due to heat buildup	Coarser microstructure in HAZ

The central finding is that CT80 continuous oil tubing butt joints invariably exhibit some degree of softening in the heat-affected zone, regardless of the welding parameters selected. This softening is attributed to the tempering effect experienced by the quenched-and-tempered microstructure of CT80 steel when exposed to temperatures in the range of 450–650 °C. The small tube diameter exacerbates this problem because the close proximity of the weld to the opposite side of the tube allows heat to accumulate with each successive pass, progressively raising the base metal temperature and extending the softening zone.

Interpretation of Technical Points

Heat Accumulation in Small-Diameter Tubing

The heat accumulation effect is the defining challenge of welding CT80 continuous oil tubing. In a butt joint of small-diameter tubing, the thermal path from the weld pool to the environment is short, and the thermal mass is limited. As welding progresses, particularly in multi-pass sequences, the base metal temperature rises with each pass. This progressive heating means that subsequent passes are effectively performed on pre-heated material, which alters the cooling rate and peak temperature experienced by the HAZ.

From a metallurgical perspective, CT80 steel typically has a quenched-and-tempered microstructure consisting of fine bainite or martensite tempered at moderate temperatures. When this microstructure is reheated to 450–650 °C during welding, the tempered carbides coarsen, and the dislocation density decreases, resulting in a loss of strength and hardness. This phenomenon, known as temper softening or temper embrittlement, is the primary mechanism of HAZ softening in CT80 tubing welds.

Process Design Implications

The authors recommend that welding procedure development for CT80 continuous oil tubing should prioritize strategies to minimize heat input concentration and reduce the extent of HAZ softening. This includes:

Strategy	Mechanism	Expected Benefit
Low heat input parameters	Reduces peak temperature and time at temperature	Limits softening depth
Pulse TIG welding	Controls instantaneous heat input	Maintains low average heat input
Interpass temperature control	Prevents base metal preheating between passes	Limits cumulative heat accumulation
Backing ring or backing gas	Reduces root heat input	Minimizes root HAZ softening
Increased travel speed	Reduces heat input per unit length	Limits thermal affected zone width

The use of pulsed TIG welding is particularly relevant for CT80 tubing because the pulse parameter allows independent control of peak current (which governs penetration) and average current (which governs heat input). By maintaining a low average current while using high peak currents for penetration, engineers can achieve adequate weld penetration while minimizing the thermal load on the base metal.

Engineering Practice Integration

In oil and gas production, continuous oil tubing (COT) is subjected to complex loading conditions including axial tension, torsion, internal pressure, and corrosive environments. The integrity of the welded joint is therefore critical for long-term service reliability. The HAZ softening identified in this study has direct implications for the mechanical performance of the joint under these service conditions.

For field welding of COT joints, the following quality assurance measures are recommended based on the findings of this study:

Interpass temperature monitoring using infrared thermography or contact thermocouples, with a maximum interpass temperature of 250 °C to limit softening.
Weld procedure qualification using the heat input range that minimizes HAZ softening while maintaining adequate penetration and mechanical properties.
Post-weld hardness profiling across the entire HAZ to identify and quantify the softening zone.
Consideration of post-weld heat treatment (PWHT) if the softening zone exceeds acceptable limits, although PWHT must be carefully controlled to avoid further grain coarsening.

The FMEA approach applied to COT welding would identify HAZ softening as a high-severity failure mode, with the small tube diameter as a key contributing factor. The detection method would include hardness testing and, for critical applications, tensile testing of weld coupons to verify that the joint meets the minimum strength requirements of the applicable standard, such as API 10D or NACE MR0175.

Key Questions and Reflections

A significant question arising from this study is whether the observed HAZ softening in CT80 tubing welds can be completely eliminated or only minimized. The authors suggest that some degree of softening is inevitable due to the metallurgical nature of CT80 steel and the thermal constraints of welding small-diameter tubing. This raises the question of whether the design codes for COT applications adequately account for HAZ softening in their allowable stress calculations.

Another consideration is the interaction between HAZ softening and other degradation mechanisms such as hydrogen-induced cracking and sulfide stress cracking, which are relevant in sour service environments. The softened zone in the HAZ may be more susceptible to hydrogen embrittlement because the reduced dislocation density and coarsened carbide structure may alter the hydrogen trapping behavior. Future research should investigate the combined effects of HAZ softening and environmental degradation on the long-term reliability of CT80 tubing welds.

Study Insights and Implications

This study provides fundamental thermal cycle data that is essential for the rational design of welding procedures for CT80 continuous oil tubing. The recognition that HAZ softening is an inherent challenge of welding this material in small-diameter configurations shifts the engineering focus from defect elimination to softening management. Engineers must adopt a holistic approach that integrates thermal cycle control, microstructural assessment, and mechanical property verification to ensure that welded joints meet the demanding service requirements of continuous oil tubing applications. The multi-point thermocouple technique employed in this study is a valuable tool for welding procedure development and should be adopted as a standard practice in the qualification of COT welding procedures.