CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post-Weld Heat Treatment on Microstructure and Mechanical Properties of Overlay Layer on 45 Steel Mold
Literature Overview
Published in Metal Heat Treatment in 2013, this research by Wang Menghan and colleagues from Chongqing University examines the impact of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of a hardfacing overlay layer deposited on a 45 steel mold substrate. The study addresses a common industrial challenge: restoring or enhancing the mechanical properties of worn or damaged molds through overlay welding followed by appropriate heat treatment. The work is particularly relevant to mold repair operations where the overlay layer must achieve high hardness while maintaining good adhesion to the base material.
Core Technical Content
The 45 steel substrate, a medium-carbon steel with approximately 0.45 wt% carbon, is widely used in mold manufacturing due to its good balance of strength, toughness, and machinability. However, when overlay welding is performed on 45 steel, the thermal cycle can cause undesirable changes in the heat-affected zone (HAZ), including grain coarsening, martensite formation, and residual stress accumulation. The overlay layer itself, typically a high-carbon, high-chromium martensitic alloy, may contain retained austenite and brittle carbide networks that degrade its mechanical performance.
The study investigates several PWHT regimes, and the following table summarizes the key findings:
| Heat Treatment Condition | Overlay Hardness (HRC) | Overlay Toughness | HAZ Microstructure | Residual Stress |
|---|---|---|---|---|
| As-welded | 58–65 | Low | Coarse martensite | High (tensile) |
| Temper at 200 °C | 55–62 | Moderate | Tempered martensite | Moderate |
| Temper at 300 °C | 48–55 | High | Fine tempered martensite | Low |
| Normalizing + temper | 50–58 | High | Recrystallized grain structure | Low |
| Tempering at 400 °C | 35–45 | Very high | Spheroidized carbides | Very low |
The as-welded overlay layer typically exhibits a microstructure consisting of lath martensite, retained austenite, and M7C3 and M23C6 carbides. The hardness is high but the toughness is low, making the overlay susceptible to cracking under impact or thermal shock conditions. The HAZ in the 45 steel substrate shows a coarse-grained martensitic structure due to the rapid heating and cooling associated with welding, which significantly reduces the ductility and toughness of the base material near the weld interface.
Post-weld tempering at 200–250 °C is the most commonly recommended treatment for martensitic hardfacing overlays. At this temperature, the retained austenite begins to decompose, and the martensite undergoes tempering, resulting in a fine dispersion of carbides within a tempered martensite matrix. This treatment reduces the hardness by 3–8 HRC but significantly improves the fracture toughness by relieving internal stresses and reducing the brittleness of the martensitic structure.
Normalizing followed by tempering provides a more comprehensive solution, particularly when the HAZ in the base material requires restoration. Normalizing at 850–900 °C promotes austenitization and recrystallization of the HAZ, resulting in a finer and more uniform grain structure. Subsequent tempering at 600–650 °C achieves a balance between hardness and toughness in both the overlay and the HAZ.
Engineering Practice Implications
For mold repair applications, the selection of PWHT regime depends on the specific service conditions and the criticality of the component. For molds subjected to high impact loading, such as forging dies and extrusion tools, a normalizing and tempering cycle is preferred to ensure adequate toughness in both the overlay and the HAZ. For molds where maximum wear resistance is the primary concern, such as cutting tools and forming dies, tempering at 200–250 °C provides the best balance of hardness and acceptable toughness.
The residual stress relief achieved through PWHT is a critical consideration. Welding-induced residual stresses can cause distortion, cracking, and premature failure of the overlay. The tempering treatment effectively reduces residual stresses by allowing microstructural relaxation through diffusion and phase transformation. For large or complex mold geometries, a stress-relief annealing treatment at 550–650 °C may be necessary to prevent distortion.
The bond strength between the overlay and the 45 steel substrate is another important consideration. Post-weld heat treatment can affect the metallurgical bond by promoting diffusion bonding at the interface. A moderate tempering treatment enhances the bond strength by reducing the brittleness of the interface region and promoting the formation of a continuous, ductile transition zone.
Key Questions and Reflections
One important question raised by this study is the optimal sequence of heat treatment operations. Should the PWHT be performed immediately after welding, or is there a benefit to allowing the component to cool to room temperature first? In practice, allowing the component to cool naturally before heat treatment is generally preferred, as it allows for the full development of the as-welded microstructure and residual stress pattern, which can then be systematically modified by the heat treatment.
Another consideration is the effect of PWHT on the dimensional accuracy of the mold. Heat treatment can cause dimensional changes due to phase transformations and thermal expansion. For precision molds, the heat treatment cycle must be carefully designed to minimize distortion, potentially involving slow heating and cooling rates, or the use of controlled-atmosphere furnaces to prevent surface degradation.
The study also highlights the importance of understanding the interaction between the overlay alloy composition and the heat treatment response. Different overlay alloys respond differently to PWHT, and the optimal treatment parameters may vary depending on the specific composition. For example, overlays with higher chromium content may require higher tempering temperatures to achieve adequate toughness, while overlays with higher vanadium or niobium content may retain higher hardness after tempering due to the formation of stable secondary carbides.
Study Insights and Conclusions
This research by Wang Menghan and colleagues provides valuable practical guidance for the PWHT of overlay layers on 45 steel molds. The key insight is that PWHT is not merely an optional finishing step but a critical process parameter that can significantly influence the performance and service life of the overlay repair.
The recommended approach for mold repair is to first characterize the as-welded microstructure and residual stress state, then select a PWHT regime based on the specific service requirements. For most mold repair applications, a tempering treatment at 200–300 °C provides a good balance of hardness and toughness, while more severe service conditions may require normalizing and tempering for comprehensive microstructural restoration.
The work underscores the importance of integrating welding and heat treatment expertise in mold repair operations. A systematic approach that considers the base material, overlay composition, welding process, and service conditions is essential for achieving reliable and long-lasting repair results. The findings are directly applicable to industrial mold repair shops and foundries seeking to improve their repair quality and extend component service life.