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

Literature Overview

This 2023 study published in the Journal of Material Heat Treatment by researchers from Hunan University of Science and Technology and Inner Mongolia Baogang Steel Group investigates how post-weld heat treatment affects the microstructure and mechanical properties of Q345B steel-based overlay composite plates. Funded by the National Natural Science Foundation (52174344), Hunan Provincial Natural Science Foundation (2021JJ30249, 2021JJ30250, 2023JJ30222), and Hunan Provincial College Student Entrepreneurship Training Program (S202210534024X), this work addresses a critical manufacturing challenge in producing bimetal composite plates where the overlay layer must maintain performance after the necessary post-weld heat treatment.

Core Technical Points

Background and Engineering Context

Q345B is a low-carbon low-alloy structural steel widely used as a base material for pressure vessels, structural components, and heavy equipment. When overlay composite plates are fabricated — typically by welding a corrosion-resistant or wear-resistant alloy onto the Q345B substrate — the resulting weld joint often requires post-weld heat treatment (PWHT) to relieve residual stresses. However, PWHT can adversely affect the overlay layer microstructure, potentially degrading the very properties that justified the overlay application.

Microstructural Changes Under Different Heat Treatment Conditions

Key Microstructural Observations

The study identifies several critical microstructural transitions:

1. Carbide spheroidization: As PWHT temperature increases, blocky carbides in the overlay progressively spheroidize, reducing hardness but improving ductility
2. Phase transformation: At temperatures above 600°C, the martensitic matrix in the overlay transforms to bainite and then pearlite, fundamentally changing the mechanical character
3. Grain coarsening: Both overlay and HAZ experience grain growth with increasing PWHT temperature, with the HAZ being more susceptible due to its already refined grain structure from welding

Mechanical Property Evolution

The mechanical properties exhibit characteristic trends:

• Hardness: Decreases monotonically with PWHT temperature in both overlay and base metal
• Tensile strength: Overlay tensile strength decreases from approximately 950 MPa (as-welded) to 650 MPa (700°C PWHT)
• Impact toughness: Improves significantly with PWHT, particularly at the overlay-base metal interface where as-welded toughness may be critically low
• Bond strength: Maintains adequate levels (>15 MPa shear bond strength) across all PWHT conditions tested

Process-Property Trade-off Analysis

The Fundamental Dilemma

The core engineering challenge is the trade-off between:

• Residual stress relief: Requires adequate PWHT temperature (typically 550–650°C for Q345B per GB/T 150 or ASME VIII Div.1)
• Overlay property retention: Requires minimal PWHT temperature to preserve hard microstructure

Recommended PWHT Protocols

Based on the study findings, the following PWHT protocols are recommended depending on application requirements:

Quality Control and Inspection Requirements

Pre- and Post-PWHT Inspection Matrix

Defect Prevention Strategy

Using a PDCA approach:

• Plan: Establish PWHT parameters based on overlay composition and section thickness; define acceptable property ranges
• Do: Execute PWHT with precise temperature and time control; monitor heating/cooling rates (≤140°C/h for heating, ≤100°C/h for cooling above 400°C)
• Check: Perform comprehensive post-PWHT inspection including hardness, microstructure, and NDE
• Act: Adjust PWHT parameters for subsequent batches based on inspection results; maintain records for traceability

Engineering Practice Integration

In pressure vessel fabrication, the interaction between PWHT requirements and overlay performance is a frequently encountered design challenge. The study's findings have direct implications for:

1. Welding Procedure Specification (WPS): PWHT parameters must be explicitly defined in the WPS, not left to fabrication discretion
2. Material specification: Overlay consumables should be selected with consideration of post-PWHT microstructure, not just as-welded properties
3. Design calculations: Allowable stress values for the overlay layer in pressure vessel design should reflect post-PWHT properties, not as-welded values
4. Inspection procedures: Post-PWHT inspection should be mandatory, with acceptance criteria defined relative to the PWHT condition

Study Insights and Implications

This research provides valuable quantitative data on the PWHT sensitivity of overlay layers on Q345B substrates. The key insight is that the overlay layer undergoes more dramatic microstructural changes than the base metal during PWHT, primarily because the overlay's as-welded microstructure (typically martensitic or martensitic-bainitic) is thermodynamically unstable at PWHT temperatures.

From a practical standpoint, the study demonstrates that a 550°C PWHT can provide adequate stress relief (reducing residual stresses by 60–70%) while retaining approximately 85–90% of the as-welded overlay hardness. This represents a practical compromise that satisfies both code requirements for stress relief and the engineering need for overlay performance. Engineers designing overlay composite plates for pressure vessel applications should incorporate these findings into their design specifications, ensuring that the PWHT protocol is an integral part of the overlay qualification process rather than an afterthought. The work also highlights the importance of considering the complete thermal history — welding, PWHT, and any subsequent thermal exposure during service — when evaluating overlay layer performance throughout the equipment lifecycle.