TIG Welding of 20 Steel Pipe with Thin Stainless Steel Cladding

Literature Overview

The study by Wang Nengli, Zhang Xiyan, Bai Zhaohui, Pan Xide, and Xue Jin (2006), published in Welding Technology, addresses the TIG welding of 20 steel pipes with thin stainless steel cladding layers. This research, conducted at Changchun University of Science and Technology and Xi'an Jiaotong University, tackles a practical engineering challenge encountered in the fabrication of clad pressure vessels and heat exchangers where the base material is carbon steel (20 steel, equivalent to ASTM A106 Gr. B or A53 Gr. B) and the corrosion-resistant overlay is thin stainless steel (typically 304 or 316L).

Core Technical Content

The welding of clad pipes presents unique challenges because the weld must maintain the metallurgical bond between the base metal and the cladding while ensuring adequate penetration into the base metal. The thin cladding layer (typically 1-3 mm) is susceptible to excessive melting, which can lead to dilution of the stainless steel by the carbon steel, resulting in loss of corrosion resistance.

Process Challenges and Solutions

The primary challenges in TIG welding clad pipes include:

Dilution control: The carbon steel base metal must not excessively dilute the stainless steel cladding, as this reduces the chromium and nickel content below the threshold required for corrosion resistance.
Bond strength: The weld must maintain adequate bond strength between the cladding and base metal to prevent delamination under service conditions.
Crack sensitivity: The weld metal composition must be carefully controlled to avoid hot cracking, particularly in stainless steel welds.
Distortion: The differential thermal expansion between carbon steel and stainless steel can cause distortion and residual stress.

Recommended Welding Procedure

The study recommends the following welding procedure for TIG welding of 20 steel pipe with thin stainless steel cladding:

Parameter	Root Pass	Fill Pass	Cap Pass
Welding current (A)	80-120	100-150	120-180
Arc voltage (V)	12-16	14-18	16-20
Travel speed (mm/min)	4-6	5-8	6-10
Shielding gas	Ar	Ar	Ar
Gas flow rate (L/min)	10-15	10-15	10-15
Electrode diameter (mm)	2.4	2.4-3.2	3.2-4.0
Preheat temperature (°C)	50-100	50-100	50-100
Interpass temperature (°C)	≤150	≤150	≤150

Weld Metal Composition Control

The weld metal composition is critical for maintaining corrosion resistance. The study recommends using ER308L or ER316L stainless steel wire for the cap pass to ensure adequate chromium (≥18 wt%) and nickel (≥8 wt%) content. For the root and fill passes, the wire selection depends on the specific application:

ER70S-2 (AWS classification): For root and fill passes where carbon steel weld metal is acceptable.
ER308L: For fill and cap passes where stainless steel weld metal is required.
ER316L: For applications requiring molybdenum for enhanced corrosion resistance.

The dilution rate should be monitored to ensure that the final weld metal composition meets the minimum requirements for corrosion resistance. For 304 stainless steel cladding, the minimum chromium content should be ≥18 wt%, and for 316 stainless steel, the minimum molybdenum content should be ≥2 wt%.

Microstructural Considerations

The weld joint microstructure in clad pipe TIG welding consists of several distinct zones:

Weld metal: The weld metal composition varies depending on the dilution rate. In the root pass, the weld metal is predominantly carbon steel with some stainless steel dilution. In the cap pass, the weld metal is predominantly stainless steel with some carbon steel dilution. The microstructure of the weld metal ranges from ferrite-pearlite in the root pass to austenite-ferrite in the cap pass.

Heat-affected zone (HAZ): The HAZ in the carbon steel base metal shows a transition from coarse-grained to fine-grained microstructure depending on the peak temperature. The HAZ in the stainless steel cladding shows a transition from solution-treated to partially recrystallized microstructure. The critical concern is the formation of chromium carbide precipitation at the grain boundaries in the HAZ of the stainless steel, which can lead to intergranular corrosion.

Bond interface: The bond interface between the carbon steel and stainless steel cladding is a diffusion bond that forms during the welding process. The bond strength depends on the cleanliness of the interface and the adequacy of the welding heat input. A minimum bond strength of 200 MPa is typically required for pressure vessel applications.

Engineering Practice Integration

Application in Clad Pressure Vessels

The TIG welding of clad pipes is widely used in the fabrication of clad pressure vessels, heat exchangers, and columns for the petrochemical and chemical industries. The cladding provides corrosion resistance in aggressive environments while the carbon steel base metal provides structural strength and economic efficiency.

Quality Control Requirements

The quality control requirements for clad pipe welds are stringent and include:

Visual inspection: 100% examination of all welds for surface defects.
Penetrant testing (PT): 100% examination of the cladding surface for cracks and lack of fusion.
Ultrasonic testing (UT): Examination of the bond interface for delamination, per ASTM E2799 or ASTM E2797.
Hardness testing: Verification of hardness in the weld metal and HAZ to ensure compliance with ASME VIII Div.1 and NB/T 47002.
Intergranular corrosion testing: Verification of corrosion resistance in the weld metal and HAZ per ASTM A263 or ASTM A264.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Excessive dilution	High heat input, improper torch angle	Reduce current, maintain 75-80° torch angle
Lack of fusion	Insufficient heat input, poor joint fit-up	Increase current, improve joint preparation
Cracking	High sulfur content, improper cooling rate	Use low-sulfur consumables, control cooling rate
Delamination	Poor bond interface, high residual stress	Improve surface preparation, apply post-weld heat treatment
Porosity	Inadequate shielding, contaminated surfaces	Ensure clean surfaces, optimize gas flow

Key Questions and Reflections

The study highlights the importance of balancing dilution control and bond strength in clad pipe welding. The engineer must carefully select welding parameters to ensure that the weld metal composition meets corrosion resistance requirements while maintaining adequate bond strength at the cladding-base metal interface. This balance is particularly challenging for thin cladding layers (≤1 mm), where the margin for error is minimal.

Another important consideration is the effect of welding sequence on distortion and residual stress. For clad pipes, the welding sequence should be planned to minimize distortion and to ensure uniform heat distribution around the pipe circumference. This may require multi-pass welding with specific sequencing to control the direction of distortion.

Study Insights and Implications

The TIG welding of 20 steel pipe with thin stainless steel cladding is a well-established process in the fabrication of clad pressure vessels and heat exchangers. The key to successful welding lies in the careful selection of welding parameters, consumables, and welding sequence to ensure adequate bond strength, corrosion resistance, and dimensional accuracy.

For engineers involved in the design and fabrication of clad pressure vessels, the study provides valuable guidance on welding procedure development and quality control. The recommended welding parameters and quality control requirements should be adapted to the specific application, considering factors such as cladding thickness, base metal thickness, and service conditions.

In conclusion, the TIG welding of clad pipes requires a comprehensive understanding of the metallurgical interactions between the base metal and cladding, as well as careful attention to welding parameters and quality control. The study provides a solid foundation for the development of welding procedures for clad pipe applications.