Microstructure and Properties of TA2/Co13Cr28Cu31Ni28/Q235 Pulsed TIG Welded Joints

Literature Overview

This study by Pei Longji, Hu Zhiyue, Qu Long, Jiang Shuying, and Zhang Junli from China University of Petroleum (East China), published in the Transactions of the Welding Institute of China in 2021, investigates the microstructure and mechanical properties of a triple-layer welded joint comprising TA2 titanium alloy, a Co13Cr28Cu31Ni28 interlayer, and Q235 carbon steel. The research was supported by the Shandong Provincial Natural Science Foundation (ZR2020ME013). This type of dissimilar metal joint is particularly relevant for oil and gas industry applications where corrosion-resistant titanium cladding is required on carbon steel pressure vessels and heat exchangers.

Core Technical Content

The welding of dissimilar metals such as titanium and carbon steel presents significant metallurgical challenges due to their vastly different physical and chemical properties. The direct welding of titanium and carbon steel is generally not feasible due to the formation of brittle intermetallic compounds (TiC, TiFe) and the potential for hydrogen embrittlement. The introduction of a nickel-based alloy interlayer (Co13Cr28Cu31Ni28) serves as a metallurgical buffer, reducing the direct contact between titanium and carbon steel and mitigating the formation of harmful phases.

The Co13Cr28Cu31Ni28 alloy is a complex composition containing 13% cobalt, 28% chromium, 31% copper, and 28% nickel, with the balance being other elements. This composition is designed to provide:

• Excellent corrosion resistance: The high chromium and nickel content provides resistance to a wide range of corrosive environments, including seawater and acidic conditions.
• Good weldability: The alloy can be welded to both titanium and carbon steel without the formation of brittle intermetallic compounds.
• Adequate mechanical properties: The alloy provides sufficient strength and ductility to accommodate the thermal strains generated during welding.

Microstructural Analysis of the Triple-Layer Joint

The microstructure of the triple-layer joint exhibits distinct regions that reflect the complex metallurgical interactions between the dissimilar metals:

The TA2/interlayer fusion zone is particularly critical. The titanium dissolves into the molten interlayer, and upon solidification, a Widmanstätten structure of acicular beta-phase within an alpha-Ti matrix forms. The high cooling rates associated with pulsed TIG welding promote a fine Widmanstätten structure, which provides good mechanical properties and avoids the formation of brittle intermetallic compounds.

The interlayer/steel fusion zone exhibits a martensitic structure due to the high carbon equivalent of the dilution. The retained austenite in this zone provides some ductility and helps to mitigate the brittleness of the martensite. However, excessive martensite formation can lead to cracking susceptibility, particularly under cyclic loading conditions.

Mechanical Properties and Bond Strength

The mechanical properties of the triple-layer joint are evaluated through several testing methods:

• Tensile strength: The joint achieves a tensile strength of 380-450 MPa, which is primarily governed by the Q235 steel side. The joint fails in the Q235 base metal or HAZ, indicating that the bond strength is adequate.
• Bond strength: The bond strength between the interlayer and Q235 steel is measured at 250-300 MPa, while the bond strength between the interlayer and TA2 titanium is 200-250 MPa. These values are well above the minimum requirements specified in relevant standards.
• Hardness profile: The hardness distribution across the joint shows a gradual transition from the TA2 side (150-170 HV) through the interlayer (250-290 HV) to the Q235 side (120-140 HV). The maximum hardness occurs in the interlayer/steel fusion zone (350-400 HV), which is a region of concern for cracking susceptibility.
• Fatigue resistance: The fatigue strength of the joint at 10^6 cycles is approximately 150-180 MPa, representing a fatigue strength ratio of 0.65-0.75 relative to the Q235 base metal. The fatigue crack initiation site is predominantly in the interlayer/steel fusion zone due to the high hardness and low ductility of the martensitic structure.

Pulsed TIG Welding Process Parameters

The pulsed TIG welding process is selected for this application due to its ability to control the heat input and minimize dilution between the dissimilar metals. The typical process parameters are as follows:

The pulsed mode allows for independent control of penetration (governed by the pulse current) and heat input (governed by the background current and pulse frequency). This is critical for dissimilar metal welding, where excessive dilution can lead to the formation of brittle intermetallic compounds.

Study Insights and Reflections

The triple-layer joint configuration of TA2/Co13Cr28Cu31Ni28/Q235 represents a practical solution for the corrosion-resistant cladding of carbon steel pressure vessels in the oil and gas industry. The key insight from this research is that the nickel-based interlayer effectively serves as a metallurgical buffer, preventing the formation of brittle Ti-Fe and Ti-C intermetallic compounds that would otherwise render the joint unusable. The pulsed TIG welding process provides the necessary control over heat input and dilution to achieve a sound joint with adequate mechanical properties. However, the martensitic structure in the interlayer/steel fusion zone remains a concern for fatigue performance, and post-weld heat treatment or the use of a pre-deposited interlayer plate may be considered to improve the long-term durability of the joint in cyclic loading applications.