Microstructure and Properties of E-Grade Steel MAG Surfacing Layer

Literature Overview

The paper by Ding Yongfeng, Xu Wujiao, and Lei Fan from Chongqing University, published in Metal Heat Treatment (Volume 38, Issue 9, 2013, pp. 68-70), investigates the microstructure and mechanical properties of a surfacing layer deposited on E-grade steel using flux-cored wire with MAG (MIG/MAG) welding process. The study employs surfacing electrode 635E and systematically examines the fusion zone, heat-affected zone (HAZ), and surfacing layer through metallographic analysis, microhardness profiling, and chemical composition determination.

Core Technical Findings

The primary objective of this research is to evaluate the metallurgical compatibility and performance stability of a surfacing overlay applied to E-grade steel, which is a high-strength structural steel commonly used in heavy equipment and pressure vessel applications. The key findings can be summarized as follows:

The fusion zone exhibits good metallurgical bonding between the base metal and the surfacing material, indicating adequate wetting and interdiffusion.
The HAZ is predominantly composed of tempered martensite, which suggests that the base steel underwent austenitization during welding followed by tempering during subsequent post-weld heat treatment (PWHT).
The surfacing layer demonstrates good strength-toughness combination and stability after heat treatment.
Carbon migration occurred during both the surfacing and subsequent tempering processes, leading to the formation of a decarburized layer on the base metal side of the fusion zone and a carburized layer on the surfacing material side.

Detailed Analysis of Carbon Migration Phenomenon

Carbon redistribution at the fusion boundary is a critical phenomenon in surfacing operations, particularly when there is a significant difference in carbon content between the base metal and the surfacing material. The study reveals two distinct zones:

Zone Location	Microstructural Feature	Hardness Behavior	Root Cause
Base metal side of fusion zone	Decarburized layer	Significantly lower than base metal hardness	Carbon diffuses from base into surfacing material during high-temperature exposure
Surfacing material side of fusion zone	Carburized layer with carbide precipitation	Higher than base metal hardness	Carbon accumulates and forms carbides in the overlay material

The decarburized layer on the base metal side represents a potential weak zone that may be susceptible to cracking under cyclic or impact loading. The carburized layer on the overlay side, while harder, may introduce brittleness and reduce toughness in that region. Understanding this carbon redistribution mechanism is essential for optimizing surfacing parameters and post-weld heat treatment schedules.

Process Parameters and Metallurgical Implications

The use of flux-cored wire in MAG welding provides several advantages for surfacing applications:

The flux core delivers additional alloying elements to the weld pool, enabling precise control of the surfacing layer composition.
The gas shielding from the MAG process ensures a clean weld pool with minimal atmospheric contamination.
The relatively high deposition rate of flux-cored wire MAG is advantageous for building up thick surfacing layers economically.

The formation of tempered martensite in the HAZ indicates that the base E-grade steel has a high enough carbon equivalent to form hard martensite during rapid cooling, which is then softened by the tempering step. This is consistent with the typical microstructure of high-strength low-alloy (HSLA) steels after welding and PWHT.

Engineering Practice Integration

In practical surfacing operations on high-strength steels like E-grade steel, several considerations must be addressed:

Preheating temperature control is critical to prevent cold cracking in the HAZ. For E-grade steel with high carbon equivalent, preheating of 150-250°C is typically recommended.
The interpass temperature must be maintained to avoid excessive hardness buildup in multi-pass surfacing operations.
Post-weld tempering is essential to relieve residual stresses and reduce HAZ hardness to acceptable levels.
The decarburized zone identified in this study should be evaluated for its impact on fatigue performance in service applications.

Study Insights and Implications

This research provides valuable insights into the metallurgical behavior of surfacing operations on high-strength steels. The identification of carbon migration effects at the fusion boundary highlights the importance of considering the entire weld cross-section, not just the surfacing layer itself, when evaluating surfacing quality. The decarburized layer, while not always catastrophic, can reduce the effective fatigue life of the component at the critical fusion zone interface. Engineers should consider this when specifying surfacing operations for high-stress applications and may need to incorporate additional quality checks, such as microhardness mapping across the full weld cross-section, to ensure the decarburized zone does not exceed acceptable thickness limits.

The study also underscores the importance of PWHT in surfacing operations. Without proper tempering, the HAZ could remain in a hard, brittle martensitic state, which would significantly compromise the structural integrity of the component. The tempered martensite observed in this study confirms that the post-weld treatment was effective in achieving the desired HAZ microstructure.