Heat Treatment Effect on Microstructure and Mechanical Properties of Q345B Steel-Based Overlay Composite Plate

Literature Overview

This study by Chen Youming and colleagues from Hunan University of Science and Technology, supported by the National Natural Science Foundation of China (52174344), Hunan Provincial Natural Science Foundation (2021JJ30249, 2021JJ30250, 2023JJ30222), and Hunan Provincial College Student Innovation Training Program (S202210534024X), investigates the effect of heat treatment on the microstructure and mechanical properties of composite plates produced by overlay welding YD212 flux-cored wire onto Q345B steel substrate using CO2 gas shielded arc welding (GMAW-CO2). The study compares three heat treatment processes—quenching, normalizing, and quenching plus tempering—against the as-welded condition, providing a comprehensive evaluation of post-weld heat treatment options for overlay composite plates.

Core Technical Findings

All three heat treatment processes modify the microstructure of the fusion zone and overlay layer, and also affect the pearlite content in the substrate layer. Heat treatment increases the hardness of all layers to varying degrees, with normalizing providing the smallest improvement. The quenching plus tempering process produces the best overall mechanical properties, with significantly improved hardness and toughness in the substrate layer, markedly improved toughness in the fusion zone, excellent overlay layer hardness, and impact absorption energy 1.57 times that of the untreated composite plate.

The quenching plus tempering process emerges as the optimal choice, providing the best balance of hardness, toughness, and overall mechanical integrity across all layers of the composite plate.

Technical Interpretation: Heat Treatment Mechanisms

Quenching Effect

Quenching produces a martensitic transformation in the overlay layer and fusion zone, resulting in high hardness but potentially low toughness. In the substrate layer, quenching may partially transform the original ferrite-pearlite microstructure into martensite or bainite, depending on the cooling rate achieved. The rapid cooling rate during quenching suppresses the formation of equilibrium phases such as pearlite, leading to a non-equilibrium microstructure with high strength but reduced ductility.

Normalizing Effect

Normalizing involves heating the composite plate to a temperature above the Ac3 point (approximately 850-900 °C for Q345B) followed by air cooling. This process:

1. Homogenizes the microstructure: Dissolves non-equilibrium phases and promotes uniform grain structure.
2. Refines grain size: Recrystallization during normalizing produces finer grains compared to the as-welded condition.
3. Reduces residual stress: Partial relief of welding residual stress through thermal expansion and contraction.
4. Moderate hardness increase: The air cooling rate produces a microstructure with moderate hardness improvement.

The study notes that normalizing provides the smallest hardness improvement, which is consistent with the relatively slow air cooling rate that allows some equilibrium phase formation.

Quenching Plus Tempering Effect

The quenching plus tempering process combines the benefits of both processes:

1. Quenching: Produces high-hardness martensitic structure in the overlay and fusion zones.
2. Tempering: Relieves internal stresses, improves toughness, and stabilizes the microstructure.

The tempering step is critical for converting the brittle martensitic structure into tempered martensite, which offers a superior hardness-toughness balance. The study reports that the impact absorption energy is 1.57 times that of the as-welded condition, demonstrating the significant toughness improvement achieved through tempering.

Substrate Layer Microstructural Changes

The observation that heat treatment increases pearlite content in the substrate layer is noteworthy. This is attributed to the thermal cycle during heat treatment, which promotes the transformation of retained austenite or other non-equilibrium phases into equilibrium pearlite. The increased pearlite content in the substrate contributes to improved hardness and strength, as pearlite is harder than ferrite.