Microstructure and Properties of Overlay Metal on Q345E Steel Substrate

Literature Overview

This 2016 paper by Li Fangzheng, Ding Chenggang, Xu Chang'en, Ma Bo, and Wu Hexiang, published in Hot Working Technology (Vol. 45, No. 1, pp. 232–234), investigates the microstructure and mechanical properties of overlay weld metal deposited on Q345E low-alloy steel substrate using two different consumables: RD-YD414(Q) and SHS9700U16. The paper is classified under TG455 and represents a systematic metallurgical study of overlay welding on low-alloy steel, with direct relevance to wear-resistant surface engineering applications.

Core Technical Content

Substrate Characterization

Q345E is a low-alloy structural steel widely used in heavy machinery, pressure vessels, and structural applications. Its key characteristics include:

The "E" designation indicates enhanced low-temperature toughness, making this steel suitable for cold-environment applications. The relatively high carbon equivalent requires careful welding procedure specification to prevent cold cracking.

Consumable Comparison

Microstructural Analysis

The paper provides detailed microstructural characterization of both overlay systems:

RD-YD414(Q) Overlay:

• Primary phase: Lenticular (plate-like) martensite, indicating rapid cooling from the austenite phase
• Secondary phases: Ferrite islands (from incomplete transformation) and bainite (from slower cooling regions)
• Carbides: M₇C₃ and M₂₃C₆ chromium carbides dispersed within the matrix
• The martensitic structure provides good toughness while maintaining moderate hardness

SHS9700U16 Overlay:

• Primary matrix: Ledeburite (iron-carbon eutectic structure)
• Cementite: Lenticular (plate-like) Fe₃C distributed throughout the matrix
• Boron carbides: B₄C particles providing extreme hardness
• Intermetallic compounds: Complex chromium-boron compounds contributing to wear resistance
• The high hardness is achieved through the combined effect of hard carbide phases and the ledeburite matrix

Wear Resistance Testing

The wear resistance results show that SHS9700U16 significantly outperforms RD-YD414(Q), which is consistent with the substantial hardness difference (63.5 HRC vs. 41.0 HRC). The wear mechanism for each overlay can be characterized as follows:

Engineering Practice Analysis

Selection Criteria for Overlay Consumables

The choice between RD-YD414(Q) and SHS9700U16 depends on the specific application requirements:

Welding Procedure Considerations for Q345E Substrate

Q345E steel requires careful welding procedure specification due to its carbon equivalent:

• Preheat temperature: 100–200°C (depending on thickness and joint configuration)
• Interpass temperature: ≤ 250°C
• Heat input: controlled to 0.5–1.5 kJ/mm to avoid excessive grain growth
• Post-weld heat treatment: stress relief at 550–620°C recommended for thick sections
• Consumable selection: low-hydrogen electrodes or flux-cored wire with controlled shielding gas

Dilution Effects

A critical factor in overlay welding is the dilution of the overlay material by the base metal. For Q345E substrate:

• Single-pass dilution: typically 20–40% depending on bead geometry and travel speed
• Multi-pass dilution: decreases with each subsequent pass as the overlay material becomes dominant
• The hardness values reported (41.0 HRC and 63.5 HRC) represent the final overlay surface, which has minimal base metal dilution

Study Insights and Reflections

This paper provides a valuable comparison of two fundamentally different overlay approaches: the tough martensitic system (RD-YD414(Q)) and the hard carbide-reinforced system (SHS9700U16). The metallurgical analysis is thorough and provides clear guidance for material selection based on service conditions.

From my perspective, the most important engineering insight is that hardness alone is not a sufficient criterion for overlay material selection. The RD-YD414(Q) overlay, despite its lower hardness, offers superior impact resistance and toughness, making it suitable for applications where the component experiences cyclic loading or impact events. Conversely, SHS9700U16, with its extreme hardness, is ideal for pure abrasive wear scenarios but would be inappropriate for impact-loaded applications.

The paper also highlights the importance of understanding the microstructure-hardness-wear resistance relationship. The SHS9700U16 overlay achieves its high hardness through a combination of ledeburite matrix, lenticular cementite, and hard boron carbide particles. Each of these phases contributes differently to the wear resistance: the cementite provides general abrasion resistance, while the boron carbides provide localized resistance to abrasive particle penetration.

One area that could benefit from further investigation is the long-term performance of these overlays under thermal cycling conditions. The Q345E substrate is often used in applications involving temperature variations, and the thermal expansion mismatch between the overlay and substrate could lead to cracking or spallation over time. A systematic thermal fatigue study would provide additional confidence in the long-term reliability of these overlay systems.

In summary, this paper serves as an excellent reference for engineers selecting overlay consumables for low-alloy steel substrates, providing both metallurgical depth and practical guidance for material selection.