Electrode Strip Electroslag Weld Overlay of Inconel 600 on 15CrMo Forging

Literature Overview

This 2016 technical paper by Wang Qingguo from AVIC Liming Jinxin Chemical Machinery (Group) Co., Ltd., published in Petrochemical Equipment Technology, reports on the experimental investigation and industrial application of electrode strip electroslag welding (ESW) overlay of Inconel 600 on 15CrMo forgings. The study addresses the challenge of providing high-temperature corrosion and oxidation resistance to 15CrMo steel components used in high-temperature service environments, such as hydrogenation reactors and high-temperature heat exchangers.

Material System and Compatibility

15CrMo is a low-alloy steel containing 1.25-1.65% Cr and 0.40-0.55% Mo, designed for high-temperature service up to 550°C. It offers good creep strength and resistance to hydrogen attack but is susceptible to oxidation and corrosion at elevated temperatures. Inconel 600 (UNS N06600) is a nickel-chromium-iron alloy with excellent resistance to oxidation, carburization, and corrosion in high-temperature environments.

The coefficient of thermal expansion (CTE) mismatch between 15CrMo and Inconel 600 is relatively small (approximately 0.5 × 10⁻⁶/K), which reduces the risk of thermal fatigue cracking at the interface during thermal cycling.

Electrode Strip Electroslag Welding Process

Electrode strip electroslag welding (also known as strip cladding or ESW strip cladding) is a highly productive process for depositing thick overlay layers. The process uses a continuous strip of overlay material as the electrode, which is fed into the slag pool and melted by the heat of the slag and the electric arc.

Process Sequence and Control

The electrode strip ESW overlay process follows a carefully controlled sequence:

1. Base material preparation: The 15CrMo forging surface is machined to a smooth finish (Ra ≤ 6.3 μm) and cleaned to remove all contaminants. The surface is preheated to 200-300°C using induction heating.
2. Flux application: The specialized slag composition is applied to the prepared surface. The slag must have appropriate fluidity and coverage to ensure proper process stability.
3. Strip feeding: The Inconel 600 strip is fed at a constant speed into the slag pool. The strip thickness and feed rate must be precisely controlled to maintain a stable slag pool and consistent deposit thickness.
4. Welding execution: The welding current is applied, melting the strip and generating the slag pool. The torch assembly moves at a constant travel speed, producing a uniform overlay bead.
5. Post-weld heat treatment: The entire component is subjected to PWHT at 700-750°C for 2-4 hours to relieve residual stresses and homogenize the microstructure.
6. Inspection: The overlay layer is inspected using UT, MT, and hardness testing to verify quality and thickness.

Microstructural and Mechanical Analysis

The microstructure of the Inconel 600 overlay layer deposited by electrode strip ESW typically consists of equiaxed dendrites with a fine secondary dendrite arm spacing. The cooling rate in ESW is relatively low compared to arc welding processes, resulting in a more coarse microstructure than in GTAW or GMAW overlay. However, the low cooling rate also reduces the risk of cracking and promotes a more uniform composition.

The hardness of the overlay layer is slightly higher than the base material due to the presence of strengthening precipitates (Ni₃Nb, Ni₃Ti) in the Inconel 600 matrix. The HAZ hardness is elevated due to the formation of martensitic and bainitic phases, which can be mitigated by PWHT.

Defect Analysis and Process Optimization

Engineering Application and Performance

The Inconel 600 overlay layer on 15CrMo forgings has been successfully applied to components in hydrogenation reactors, high-temperature heat exchangers, and reactor internals. The overlay layer provides excellent resistance to high-temperature oxidation, sulfur corrosion, and hydrogen attack, extending the service life of the component by 3-5 times compared to unprotected 15CrMo steel.

The electrode strip ESW process offers several advantages for this application:

• High deposition rate (up to 10 kg/h)
• Thick overlay layers (up to 12 mm in a single pass)
• Low dilution rate (5-10%)
• Good process stability
• Suitable for large surface areas

The dilution rate is a critical parameter. A lower dilution rate ensures that the overlay layer retains its composition and properties. In electrode strip ESW, the dilution rate is typically 5-10%, which is acceptable for maintaining the corrosion resistance of Inconel 600. However, if the dilution rate exceeds 15%, the overlay layer may become susceptible to oxidation and corrosion.

Study Insights and Implications

This study demonstrates the successful application of electrode strip electroslag welding for overlaying Inconel 600 on 15CrMo forgings, providing a practical solution for enhancing the high-temperature corrosion resistance of critical components. The process parameters, including current, travel speed, and interpass temperature, must be carefully controlled to achieve a high-quality overlay with minimal defects. The post-weld heat treatment is essential for relieving residual stresses and improving the mechanical properties of the HAZ. The results of this study provide valuable guidance for the industrial application of Inconel 600 overlay on 15CrMo components in the petrochemical and chemical industries, where high-temperature service conditions demand reliable corrosion protection.