Microstructure and Properties of Co-Based Alloy Cladding on Cast Iron Surface

Literature Overview

This 2010 study by Liu Meng, Cui Lishan, Jiang Daqiang, Jiang Xiaohua, Zheng Yanjun, Gao Wei, and Yu Longwen from China University of Petroleum (Beijing), the Energy Materials Microstructure Laboratory, and Liaoning Petrochemical University investigates the microstructure and properties of cobalt-based alloy cladding deposited on cast iron surfaces. Funded under the National Science and Technology Major Project "Safe and Efficient Development Technology Research for High-Sulfur Gas Reservoirs" (sub-project: "Research on High-Sealing Underground Gas Production Tools," project code 2008ZX05017-01-01), this work addresses the corrosion and wear challenges in oil and gas production equipment operating in high-sulfur environments.

Service Environment and Material Challenges

The high-sulfur gas reservoir environment presents unique challenges for production tools:

Cast iron, while offering good castability and machinability, has poor corrosion resistance in high-sulfur environments. The graphite flakes in gray cast iron create galvanic cells with the iron matrix, accelerating localized corrosion. Additionally, the graphite flakes act as stress concentrators, promoting crack initiation and propagation. The cladding layer must therefore provide both corrosion resistance and mechanical integrity.

Co-Based Alloy Selection and Design

Cobalt-based alloys are widely used for corrosion and wear-resistant cladding due to their excellent high-temperature strength, oxidation resistance, and corrosion resistance. The study investigated several Co-based alloy compositions:

The cobalt-based alloys were selected based on the following design criteria:

1. High chromium content (12-16%): Provides passive film formation and general corrosion resistance.
2. Tungsten or molybdenum addition (4-6%): Enhances high-temperature strength and wear resistance through carbide formation.
3. Silicon addition (1.5-2.5%): Promotes SiC carbide formation for abrasive wear resistance.
4. Low sulfur and phosphorus: Minimizes sulfide stress corrosion susceptibility.
5. Austenitic or martensitic matrix: Balances toughness and hardness.

Cladding Process and Parameters

The cobalt-based alloy was deposited on the cast iron surface using gas tungsten arc welding (GTAW) with powder feeding (plasma transferred arc, PTA) and also using oxy-acetylene flame welding. The process parameters were:

The GTAW/PTA process provided better control of dilution and microstructure, while the oxy-acetylene process offered higher deposition rates but with greater dilution and potential for microstructural degradation.

Microstructure Analysis

Base Metal (Cast Iron)

The gray cast iron substrate exhibited a typical microstructure consisting of:

• Pearlite matrix: Ferrite-cementite lamellar structure with hardness 200-250 HB
• Graphite flakes: Type A graphite with size 20-50 μm
• Ferrite regions: Present at grain boundaries and near graphite flakes
• Nodular graphite: Some specimens contained nodular graphite (ductile iron)

Dilution Zone

The dilution zone between the cast iron base metal and the Co-based overlay exhibited a complex microstructure:

The dilution zone is the critical region for corrosion resistance, as the transition from Co-rich to Fe-rich microstructure creates a gradient in corrosion potential. This potential gradient can drive galvanic corrosion at the interface.

Overlay Layer Microstructure

The Co-based overlay layer exhibited a fine-grained austenitic microstructure with a high density of M7C3 and M23C6 carbides (Cr7C3, Cr23C6) and some SiC particles. The carbide volume fraction was 15-25%, providing excellent wear resistance. The matrix contained 8-12% retained austenite, which provided good toughness and thermal shock resistance.