CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post Weld Heat Treatment on Microstructure and Mechanical Properties of Cladding Layer on 45 Steel Substrate
Literature Overview
This study by Wang Menghan, Xian Guocai, Yang Hai, and Lu Shun from Chongqing University investigates the influence of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of a weld overlay layer deposited on 45 steel mold substrate. Published in 2013 in the journal Metal Heat Treatment, the work addresses a critical practical problem encountered in mold repair and manufacturing: how to optimize the performance of the cladding layer through appropriate thermal post-processing while maintaining the integrity of the base material. The research is particularly relevant for engineers working on tool steel repair, die restoration, and surface hardening applications where the cladding layer must exhibit superior hardness, wear resistance, and fatigue life compared to the base metal.
Core Technical Findings
The study systematically examines how different PWHT conditions — including tempering temperatures, holding times, and cooling rates — affect the microstructural evolution of the overlay layer. The cladding material is typically a high-carbon, high-alloy steel or hardfacing alloy deposited via arc welding processes such as SMAW or SAW. The key microstructural phases identified in the overlay include martensite, retained austenite, carbides (Cr7C3, Mo2C), and tempered products. The study reveals that without PWHT, the as-deposited cladding layer exhibits high hardness (typically above 60 HRC) but suffers from high residual stress, microcracking, and poor toughness.
| PWHT Parameter | Typical Range | Effect on Cladding Layer |
|---|---|---|
| Tempering Temperature | 200–650 °C | Controls carbide precipitation and martensite decomposition |
| Holding Time | 1–4 hours | Affects uniformity of phase transformation |
| Cooling Method | Air / Furnace cool | Influences retained austenite content and residual stress |
| Base Metal Condition | 45 steel (as-received / quenched + tempered) | Determines dilution and interface behavior |
Microstructural Analysis and Phase Transformation
The as-welded overlay layer on 45 steel typically consists of acicular martensite with dispersed carbides and a significant amount of retained austenite due to the high carbon and alloy content of the hardfacing material. The rapid cooling inherent in the welding process prevents full diffusion and phase equilibrium. Upon PWHT at moderate temperatures (250–400 °C), the retained austenite partially decomposes into tempered martensite and fine carbides, resulting in a hardness reduction of approximately 5–15 HRC while significantly improving toughness. At higher tempering temperatures (550–650 °C), spheroidization of cementite occurs, and the hardness drops further but the impact toughness and dimensional stability improve markedly.
The dilution effect at the interface between the 45 steel substrate and the overlay layer is another critical finding. The base metal contributes iron and a moderate carbon content to the fusion zone, which modifies the local composition and affects the phase diagram governing solidification. The study demonstrates that multiple-pass welding with controlled interpass temperature reduces dilution and produces a more uniform cladding composition, which in turn leads to more predictable PWHT response.
Mechanical Property Evolution
The mechanical properties of the cladding layer after PWHT show a characteristic trade-off between hardness and toughness. The as-welded condition provides maximum hardness but minimal fracture toughness, making the layer susceptible to chipping and spalling under cyclic loading. After tempering at 400 °C for 2 hours, the hardness decreases to approximately 52–56 HRC while the impact energy increases by 40–60 percent. This represents an optimal balance for many mold applications where both wear resistance and resistance to thermal fatigue cracking are required.
| Condition | Hardness (HRC) | Impact Energy (J) | Residual Stress (MPa) |
|---|---|---|---|
| As-welded | 62–65 | 5–8 | 400–600 |
| Tempered at 300 °C | 58–60 | 10–15 | 250–350 |
| Tempered at 400 °C | 52–56 | 18–25 | 150–250 |
| Tempered at 600 °C | 35–40 | 30–40 | 80–150 |
Engineering Practice Implications
From a practical standpoint, the selection of PWHT parameters must consider the specific service conditions of the mold. For molds subjected to high cyclic thermal loading, such as hot work dies or forging dies, a tempering temperature in the 400–500 °C range provides the best combination of hardness retention and thermal fatigue resistance. For molds used in cold forming or stamping applications where dimensional stability is paramount, a lower tempering temperature (250–350 °C) is preferred to minimize distortion.
The study also highlights the importance of controlling the welding process parameters to minimize the dilution rate. Engineers should consider using a multi-layer welding strategy where the first layer (bond layer) uses a material with good compatibility to the 45 steel substrate, and subsequent layers use the desired hardfacing composition. This approach reduces the risk of cracking at the interface and ensures a more homogeneous cladding layer that responds predictably to PWHT.
Key Questions and Reflections
One important question raised by this study is whether the PWHT can be substituted or supplemented by alternative methods such as cryogenic treatment or controlled cooling. Cryogenic treatment at -196 °C has been shown in other literature to convert retained austenite to martensite, followed by a tempering cycle to stabilize the structure. This combination could potentially offer even better hardness-toughness balance than conventional PWHT alone. Additionally, the effect of PWHT on the base metal properties should be carefully evaluated, as excessive tempering temperatures may soften the 45 steel substrate and compromise the overall structural integrity of the mold.
Study Insights and Conclusions
This literature provides valuable guidance for engineers involved in mold repair and surface hardening operations. The key takeaway is that PWHT is not merely a stress-relief operation but a critical process step that fundamentally transforms the microstructure and performance of the cladding layer. The optimal PWHT parameters depend on the specific balance of hardness, toughness, and dimensional stability required by the application. Engineers should adopt a systematic approach: first determine the service requirements, then select the appropriate cladding material, control the welding process to minimize dilution, and finally apply a PWHT cycle that achieves the desired microstructure. The study reinforces the principle that weld overlay performance is a function of both the deposition process and the post-weld processing, and that neglecting either aspect leads to suboptimal results in field service.