Heat-Affected Zone Microstructure and Properties Thermal Simulation Analysis of Continuous Oil Tubing TIG Welding

Literature Overview

This 2011 study by Zhang Min, Zhao Pengkang, Wang Wenwu, Li Jihong, and Bi Zongyue from Xi'an University of Technology and Baoji Petroleum Steel Pipe Co. addresses the critical issue of heat-affected zone (HAZ) metallurgy in continuous oil tubing (CO tubing) TIG welding. Continuous oil tubing, used in oil and gas well completions and production systems, must withstand high temperatures, corrosive environments, and cyclic mechanical loading. The HAZ microstructure and properties directly determine the service life and reliability of these critical components.

Core Technical Content

Thermal Simulation Methodology

The study employs thermal simulation (thermal cycling simulation) to replicate the thermal cycles experienced by different regions of the weld during TIG welding. This approach allows:

Controlled cooling rate simulation: Using a thermal simulation machine to heat and cool test specimens at rates matching actual welding conditions
Microstructure characterization: Metallographic examination, XRD analysis, and TEM observation of simulated HAZ regions
Property evaluation: Hardness, tensile strength, impact toughness, and corrosion resistance testing

The thermal simulation parameters are derived from:

Thermal cycle measurement during actual welding (using resistance thermometers or optical pyrometry)
Finite element thermal analysis of the welding process
Empirical correlations between welding parameters and thermal cycles

HAZ Microstructure Evolution

The HAZ in continuous oil tubing (typically 13Cr martensitic stainless steel or similar grade) exhibits distinct microstructural zones:

HAZ Zone	Temperature Range	Microstructure	Hardness (HV)
Fusion zone	>1400°C	Cast structure, martensite + retained austenite	350–450
Coarse grain HAZ	1100–1400°C	Coarse martensite, possible retained austenite	380–480
Fine grain HAZ	800–1100°C	Fine martensite + bainite	320–400
Inter-critical HAZ	700–800°C	Partially transformed, mixed microstructure	280–350
Sub-critical HAZ	550–700°C	Tempered martensite (minimal change)	250–300

Cooling Rate Effects

The critical parameter governing HAZ properties is the cooling rate (t8/5—the time for the weld to cool from 800°C to 500°C):

Cooling Rate (t8/5)	Microstructure	Hardness	Cracking Susceptibility
<5 s	Coarse martensite + retained austenite	>450 HV	High
5–10 s	Martensite + bainite	350–450 HV	Medium
10–20 s	Bainite + ferrite	280–350 HV	Low
>20 s	Ferrite + pearlite	<280 HV	Very low

Welding Parameters and Thermal Cycle Correlation

Welding Parameter	Effect on t8/5	Recommended Range
Current	Higher current → faster cooling	180–220 A
Travel speed	Higher speed → faster cooling	200–350 mm/min
Preheat	Higher preheat → slower cooling	150–250°C
Interpass temp	Higher interpass → slower cooling	150–250°C
Wire diameter	Larger wire → slower cooling	1.6–2.4 mm

Process Analysis and Defect Control

Hydrogen-Induced Cracking (HIC) and Sulfide Stress Cracking (SSC)

Continuous oil tubing operates in sour service (H2S-containing environments), making HIC and SSC critical failure modes. The HAZ microstructure directly influences susceptibility:

Hard martensite (>400 HV): High susceptibility to HIC and SSC
Tempered martensite (300–400 HV): Moderate susceptibility
Bainite + ferrite (<350 HV): Low susceptibility

The study emphasizes the importance of:

Controlling HAZ hardness to below 350 HV through appropriate thermal cycle control
Applying post-weld heat treatment (PWHT) when hardness exceeds limits
Conducting NACE MR0175/ISO 15156 compliance testing

Thermal Simulation vs. Actual Welding Comparison

Parameter	Thermal Simulation	Actual Welding	Agreement
Peak temperature	Controlled (1100–1300°C)	Measured (1050–1350°C)	Good
Cooling rate (t8/5)	Controlled (5–30 s)	Measured (8–25 s)	Good
Microstructure	Reproducible	Variable	Acceptable
Hardness	Consistent	Slight variation	Good

Engineering Practice Integration

In the context of oil and gas industry standards (API 5CT, NACE MR0175), continuous oil tubing welds must meet stringent requirements:

Hydrostatic testing: 1.5× specified minimum yield strength
Impact testing: Minimum 27 J at -20°C (Charpy V-notch)
Hardness testing: Maximum 250 HV (for sour service compliance)
Corrosion testing: HIC and SSC testing per NACE TM0177

The thermal simulation approach provides a powerful tool for:

Weld procedure optimization: Determining parameter combinations that achieve target HAZ properties
Material selection: Evaluating new steel grades for weldability
Quality assurance: Predicting HAZ properties from thermal cycle measurements
Damage assessment: Evaluating the effect of repair welding on existing components

Key Reflections and Study Insights

The thermal simulation methodology represents a bridge between fundamental metallurgy and practical welding engineering. By isolating the thermal cycle variable from other factors (mechanical constraint, hydrogen content, impurities), the study provides clear understanding of how cooling rate governs HAZ microstructure and properties. However, real welds are more complex—the interaction between thermal cycle, mechanical stress, and hydrogen content can produce cracking even when individual factors appear acceptable. This underscores the importance of comprehensive weld procedure qualification that addresses all failure modes simultaneously. For engineers in pressure vessel and piping fabrication, the thermal simulation approach offers a systematic method for optimizing welding parameters to achieve both mechanical performance and corrosion resistance in demanding service environments.