CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of 45 Steel Weld Overlay Deposited Metal
Literature Overview and Research Background
The paper published in the Journal of Xihua University (Natural Science Edition) in 2006 by Xie Jun, Zeng Zhichao, Zhong Yu, Qu Jinshan, and Zeng Qingbao investigates the microstructure and mechanical properties of deposited metal produced during weld overlay cladding on 45 steel, a medium carbon steel widely used in mechanical components and pressure vessel fabrication. This study is particularly relevant to engineers working in bimetal product manufacturing because 45 steel serves as a common base material for wear-resistant and corrosion-resistant cladding applications, and understanding the deposited metal behavior is essential for process qualification under standards such as NB/T 47014 and ASME IX.
The research was conducted in collaboration between Xihua University School of Materials Science and Engineering and Sichuan Atlantic Welding Materials Co., Ltd., which brings together academic rigor and industrial welding consumable expertise. This combination is significant because it ensures that the findings are not purely theoretical but grounded in actual production conditions.
Core Technical Findings
The fundamental challenge in welding on 45 steel lies in the high carbon content (0.42–0.50 wt%) and the resulting susceptibility to hard and brittle microstructures in both the heat-affected zone (HAZ) and the deposited metal. The study systematically examines how the deposited metal microstructure evolves under different welding conditions, with particular attention to the role of dilution from the base metal.
Microstructure Analysis
The deposited metal microstructure in 45 steel overlay welding is dominated by the interaction between the welding consumable composition and the dilution from the base metal. Key observations include:
| Microstructural Feature | Description | Engineering Significance |
|---|---|---|
| Acicular ferrite | Fine needle-like ferrite grains | Provides good toughness and crack resistance |
| Widmanstätten ferrite | Plate-like ferrite precipitating on prior austenite grain boundaries | Reduces toughness and increases brittleness |
| Martensite | Hard, brittle phase formed by rapid cooling | Increases hardness but severely reduces ductility |
| Pearlite | Lamellar ferrite-cementite mixture | Contributes to moderate strength and hardness |
| Bainite | Transition phase between pearlite and martensite | Offers balance between strength and toughness |
The degree of dilution from 45 steel into the deposited metal is a critical parameter. When dilution exceeds 30–40%, the effective carbon equivalent of the deposited metal rises significantly, promoting martensitic transformation during cooling. This is particularly problematic for low-hydrogen consumables such as E7018 (AWS classification) or equivalent Chinese standard consumables, where the deposited metal is designed for lower carbon equivalent compositions.
Mechanical Properties
The mechanical properties of the deposited metal are directly influenced by the microstructural composition. The study demonstrates that:
- Hardness: Deposited metal hardness typically ranges from 200 to 350 HV depending on cooling rate and dilution level. Higher dilution leads to increased hardness due to martensite formation.
- Tensile strength: Values between 550 and 700 MPa are observed, with higher values correlating to higher martensite content.
- Impact toughness: Charpy V-notch impact energy drops significantly when martensite content exceeds 40%, falling below 20 J at -20°C in some cases.
- Ductility: Elongation decreases with increasing hardness, indicating a trade-off between wear resistance and formability.
Dilution Effect
The dilution rate is perhaps the most important variable in this study. For 45 steel overlay welding:
| Process Parameter | Low Dilution (<20%) | Moderate Dilution (20–40%) | High Dilution (>40%) |
|---|---|---|---|
| Microstructure | Predominantly ferrite-pearlite | Mixed ferrite-martensite | Predominantly martensite |
| Hardness (HV) | 180–220 | 220–300 | 300–380 |
| Impact Energy (-20°C, J) | 60–100 | 30–60 | <20 |
| Crack Susceptibility | Low | Moderate | High |
The dilution rate is influenced by welding process parameters including current density, travel speed, electrode diameter, and number of passes. Single-pass welding tends to produce higher dilution than multi-pass welding because the first pass has direct contact with the base metal.
Engineering Practice Implications
From a practical standpoint, this research has several important implications for cladding operations on medium carbon steel base materials:
- Preheating requirements: Preheating to 150–250°C is recommended for 45 steel to reduce cooling rates and minimize martensite formation. For thicker sections (>25 mm), preheating temperatures may need to be increased to 250–350°C.
- Consumable selection: Low-hydrogen electrodes (H ≤ 5 mL/100g) are mandatory to prevent hydrogen-induced cracking. For overlay applications requiring specific corrosion or wear resistance, the consumable composition must be carefully matched to the desired deposited metal properties.
- Interpass temperature control: Maintaining interpass temperatures between 250–350°C helps control the thermal cycle and reduces residual stresses. Excessive interpass temperatures can lead to grain coarsening and reduced mechanical properties.
- Post-weld heat treatment (PWHT): Stress relief annealing at 580–650°C for 2–4 hours per 25 mm of thickness is recommended to reduce residual stresses and transform retained austenite.
- Multi-pass strategy: Using a multi-pass approach with a transition layer (such as a low-carbon stainless steel or low-alloy steel) between the base metal and the final overlay layer can effectively reduce dilution and improve the metallurgical compatibility of the joint.
Key Questions and Reflections
The study raises an important question about the optimal balance between deposited metal properties and process control. While higher dilution increases hardness, which may be desirable for wear-resistant applications, it simultaneously reduces toughness and increases crack susceptibility. This trade-off must be carefully managed in engineering practice.
Another critical consideration is the reproducibility of the deposited metal properties. In production environments, variations in operator skill, consumable lot-to-lot differences, and ambient conditions can all affect the final deposited metal quality. The study's findings should be used as a baseline for developing qualification procedures and in-process monitoring protocols.
For pressure vessel fabrication specifically, the weld overlay deposited metal must meet the requirements of applicable codes. Under GB/T 150 and NB/T 47002, the overlay weld must be qualified through procedure qualification tests (PQR) and welder performance qualification (WPQ). The mechanical properties reported in this study provide valuable reference data for establishing acceptance criteria.
Study Insights and Practical Recommendations
After careful analysis of this research, several practical recommendations emerge for engineers involved in cladding operations on medium carbon steels:
- Always perform a carbon equivalent calculation for the expected deposited metal composition, accounting for anticipated dilution levels.
- Conduct dilution analysis through optical emission spectrometry or wet chemical analysis on representative test pieces before full-scale production.
- Develop a systematic approach to process parameter optimization using orthogonal experimental design or taguchi methods to identify the parameter combinations that yield the desired deposited metal properties.
- Implement in-process monitoring of welding parameters (current, voltage, travel speed) to ensure consistency and traceability.
- Establish a non-destructive testing (NDT) protocol that includes both surface methods (MT or PT) and volumetric methods (UT or RT) appropriate for the overlay thickness and application requirements.
This research, while published in 2006, remains highly relevant to current engineering practice because the fundamental metallurgical principles governing deposited metal microstructure and properties have not changed. Modern welding technologies such as cold wire TIG, hot wire TIG, and plasma transferred arc cladding may offer improved control over dilution and thermal input, but the underlying metallurgical considerations remain the same. Engineers should use this study as a foundation for developing site-specific procedures and qualification tests that ensure the reliability and performance of weld overlay cladding on medium carbon steel base materials.