Effect of Post Weld Heat Treatment on Interface Microstructure and Metallurgical Properties of Explosively Welded Bronze–Carbon Steel

Literature Overview

Explosive welding (EW) produces solid-state bonds between dissimilar materials through high-velocity impact, making it uniquely suitable for joining materials that are metallurgically incompatible in fusion welding processes. This study investigates the effects of post-weld heat treatment (PWHT) on the interface microstructure and metallurgical properties of explosively welded bronze–carbon steel bimetallic composites. The findings are directly applicable to the manufacture of bimetal pressure vessels and cladding systems where bronze provides corrosion resistance and carbon steel provides structural strength.

Core Technical Points

Explosive Welding Interface Characteristics

The as-welded explosive weld interface between bronze (typically CuSn8 or CuSn10) and carbon steel (Q235 or S235) exhibits a distinctive wavy bonding pattern resulting from the collision dynamics:

Feature	Description	Typical Dimension
Wave amplitude	Height of undulation	0.1–0.5 mm
Wave wavelength	Distance between peaks	0.5–2.0 mm
Interdiffusion zone	Thickness of mixed layer	5–15 μm (as-welded)
Bond ratio	Bonded area / total interface area	> 95% for qualified weld
Intermetallic phases	Cu₄Sn, Cu₆Sn₅, Fe₃Cu	Present in thin discontinuous layers

PWHT Effects on Interface Microstructure

Post-weld heat treatment induces significant changes in the interface region:

PWHT Condition	Temperature	Time	Interdiffusion Zone	Intermetallic Thickness	Bond Strength
As-welded (no PWHT)	—	—	5–15 μm	1–3 μm	280–320 MPa (shear)
Mild anneal	200°C	2 h	10–25 μm	3–8 μm	260–300 MPa
Moderate anneal	400°C	2 h	30–60 μm	10–20 μm	220–260 MPa
High anneal	600°C	2 h	80–150 μm	25–45 μm	150–200 MPa
Excessive anneal	700°C	4 h	200–400 μm	50–80 μm	80–130 MPa

The study clearly demonstrates that PWHT promotes interdiffusion and intermetallic compound growth at the interface, which initially refines the bonding but eventually degrades bond strength when excessive.

Metallurgical Property Evolution

Hardness: The bronze side softens with increasing PWHT temperature due to Sn diffusion into the copper matrix and relief of impact-induced strain hardening. The steel side shows minimal hardness change below 500°C but experiences significant softening above 600°C.
Ductility: Moderate PWHT (400°C, 2 h) improves ductility of the bronze layer by relieving residual stresses from the explosive welding process, while excessive treatment embrittles the interface through thick intermetallic layer formation.
Corrosion resistance: Moderate PWHT does not significantly affect corrosion resistance, but excessive intermetallic growth creates galvanic couples that accelerate localized corrosion at the interface.

Optimal PWHT Window

The study identifies an optimal PWHT window of 350–450°C for 1–2 hours, which:

Relieves explosive welding residual stresses (typically 200–400 MPa)
Maintains intermetallic layer thickness below 15 μm
Preserves bond strength above 250 MPa
Does not compromise corrosion resistance

Connection with Engineering Practice

In the fabrication of bimetal pressure vessels per GB/T 150 or ASME VIII Div.1, explosive welding is specified for applications requiring reliable corrosion resistance at the interface without the metallurgical incompatibility issues of fusion welding. The PWHT data from this study directly informs the selection of post-fabrication heat treatment parameters for vessels incorporating explosively welded components.

For hydrogenation reactors and chemical processing equipment where bronze-clad carbon steel is used for sulfuric acid or hydrochloric acid service, the PWHT temperature must be carefully controlled to avoid degrading the corrosion-resistant bronze layer. The study's data on intermetallic growth rates provides the quantitative basis for setting PWHT limits in welding procedure specifications.

Key Reflections and Study Insights

This study provides critical quantitative data on the PWHT sensitivity of explosive weld interfaces, which is often under-specified in fabrication standards. The key engineering insight is that the intermetallic layer acts as both a bonding mechanism and a potential failure initiation site, creating a fundamental trade-off that must be managed through PWHT control. I strongly recommend that fabrication specifications for explosively welded components include explicit PWHT temperature and time limits, supported by interfacial microstructural examination rather than relying solely on mechanical property testing. The study also highlights the need for standardized characterization methods for explosive weld interfaces, as current practices vary significantly between organizations. Future research should address long-term aging behavior at service temperatures to establish reliable life prediction models for explosively bonded components in sustained elevated temperature service.