CLADDING TECHNOLOGY SHANXI CO., LTDThermal Cycle Analysis of CT80 Continuous Oil Tubing TIG Butt Welded Joints
Literature Overview
This research by Li Xiao, Shi Kai, Wang Hongduo, and Li Jie from the Key Laboratory of Materials Processing Engineering at Xi'an Petroleum University, published in Hot Working Technology (2011), investigates the thermal cycle characteristics of CT80 continuous oil tubing TIG butt welded joints. CT80 tubing is a specialized high-strength steel used in oil and gas extraction applications where deep penetration, high yield strength (minimum 80 ksi), and excellent resistance to sour service are required.
CT80 Material Characteristics and Welding Challenges
CT80 tubing is a high-strength low-alloy (HSLA) steel with a minimum yield strength of 552 MPa (80 ksi). Its microstructure typically consists of fine-grained ferrite with dispersed carbide particles, providing the combination of strength and toughness required for demanding oilfield service conditions.
Material Properties
| Property | CT80 Specification | Welding Implications |
|---|---|---|
| Yield strength | ≥ 552 MPa | High preheat requirement |
| Tensile strength | ≥ 620 MPa | Matching filler metal needed |
| Impact energy (25°C) | ≥ 27 J | HAZ toughness critical |
| Carbon equivalent | 0.40-0.50% | High hardenability, cracking risk |
| Typical thickness | 2.5-4.0 mm | Low heat input required |
TIG Welding Thermal Cycle Analysis
Thermal Cycle Characteristics
The researchers instrumented the weldment with thermocouples to capture the complete thermal cycle from base metal through weld metal. Key observations included:
| Thermal Cycle Parameter | Measured Value | Significance |
|---|---|---|
| Peak temperature (weld centerline) | 1200-1500°C | Controls microstructural transformation |
| Peak temperature (HAZ, 1 mm from fusion line) | 800-1000°C | Affects grain growth and phase transformation |
| Cooling rate (800-600°C) | 5-25 °C/s | Critical for hardness and cracking susceptibility |
| Dwell time above 800°C | 2-8 seconds | Influences grain coarsening |
| Number of thermal cycles (multi-pass) | 2-4 | Cumulative thermal damage |
Microstructural Zones and Their Properties
| Zone | Temperature Range | Microstructure | Hardness (HV) |
|---|---|---|---|
| Weld metal | > 1400°C | Fine-grained ferrite + acicular ferrite | 250-320 |
| Coarse grain HAZ | 1100-1400°C | Coarse ferrite + martensite/austenite | 350-450 |
| Fine grain HAZ | 900-1100°C | Fine ferrite + pearlite | 280-350 |
| Intercritical HAZ | 727-900°C | Slight grain growth | 250-300 |
| Base metal | < 727°C | Original microstructure | 280-320 |
Process Optimization Based on Thermal Cycle Data
The thermal cycle analysis directly informed welding procedure optimization:
Recommended Welding Parameters for CT80 TIG Welding
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Preheat temperature | 100-150°C | Reduces cooling rate below critical threshold |
| Interpass temperature | ≤ 250°C | Prevents excessive grain growth |
| Welding current | 120-180 A | Balances penetration with heat input |
| Travel speed | 150-250 mm/min | Controls cooling rate to acceptable range |
| Shielding gas | 98% Ar + 2% O₂ | Improves arc stability and wetting |
| Filler metal | ER80S-G or equivalent | Matches base metal strength |
Engineering Practice Implications for Pressure Vessel Fabrication
The thermal cycle analysis methodology presented in this study is directly applicable to welding of other high-strength steels used in pressure vessel fabrication, including:
- SA-516 Gr. 70 used for hydrogenation reactor shells
- SA-387 Gr. 22 used for high-temperature service vessels
- SA-333 Gr. 6 used for cryogenic service
The key engineering principle is that the cooling rate through the 800-600°C range must be controlled to prevent the formation of hard, brittle microstructures (martensite, bainite) that are susceptible to hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC).
For clad pressure vessels using CT80 or similar high-strength steels as the structural shell with stainless steel or nickel alloy overlay layers, the thermal cycle control becomes even more critical. The overlay layer must maintain its corrosion resistance properties while the structural layer must retain its mechanical integrity. This dual requirement necessitates careful selection of welding parameters that satisfy both material systems simultaneously.
Study Conclusions and Reflections
The thermal cycle analysis of CT80 TIG welded joints demonstrates that even relatively thin-walled tubing requires careful thermal management to achieve acceptable weld properties. The cooling rate through the critical temperature range is the single most important parameter governing weld joint quality. For engineers designing welding procedures for high-strength steel pressure vessels, this study reinforces the necessity of thermal monitoring during welding and the value of post-weld thermal cycle analysis for procedure validation. The integration of thermal analysis with metallurgical evaluation provides a comprehensive approach to ensuring that welded joints meet the demanding performance requirements of pressure vessel service.