Zhuojin Pipe Fitting Co., LtdMicrostructure and Microhardness Study of Hardfacing Metal Deposited on 45 Steel
Literature Overview
This paper by Zhang Youyi et al. (2009), published in the Journal of Xihua University (Natural Science Edition), examines the microstructure and microhardness of hardfacing deposits produced on 45 steel substrates using two different shielded metal arc welding (SMAW) electrodes: D237 and D207. The study investigates how welding heat input and electrode alloy composition influence the resulting overlay microstructure, hardness, and the distribution of hard phases. This work is particularly relevant to engineers working on wear-resistant overlay applications in mining, cement, and material handling industries where 45 steel (equivalent to AISI 1045) is a common base material.
Core Technical Findings
The researchers identified three primary factors governing the microstructure and hardness of the hardfacing deposits:
- Welding heat input: Higher heat input leads to coarser microstructures and lower hardness due to increased dilution and slower cooling rates.
- Electrode alloy composition: The type and concentration of alloying elements in the electrode directly determine the hard phases formed in the deposit.
- Hard phase characteristics: The morphology, hardness, and distribution of carbides and other hard phases are the dominant factors controlling overlay wear resistance.
Comparison of Electrode Effects
| Parameter | D237 Electrode | D207 Electrode |
|---|---|---|
| Primary hard phases | Chromium carbides (Cr7C3, Cr23C6) | Chromium carbides with Mo, V carbides |
| Matrix microstructure | Austenite + martensite | Martensite + retained austenite |
| Typical microhardness (HV) | 800–1100 | 900–1200 |
| Grain refinement by Mo, V | Limited | Significant |
| Dilution sensitivity | Moderate | Moderate |
The study demonstrates that molybdenum (Mo) and vanadium (V) are particularly effective grain refiners in hardfacing deposits. These elements form fine, hard carbides (Mo2C, VC) that not only contribute directly to hardness but also act as nucleation sites for the matrix microstructure, promoting grain refinement. The presence of these fine carbides in a refined matrix microstructure creates a synergistic effect that significantly enhances the overall hardness and wear resistance of the overlay.
Welding Metallurgy Analysis
45 steel, with its medium carbon content (approximately 0.42–0.50% C) and ferrite-pearlite microstructure, presents specific challenges for hardfacing applications. The relatively high carbon content of the base metal leads to significant dilution effects during welding, which can alter the intended chemistry of the hardfacing deposit. The dilution rate is directly related to the welding heat input, which is a function of welding current, voltage, and travel speed.
The microstructure of the hardfacing deposit evolves through the following stages:
- Solidification: The weld pool solidifies with a columnar dendrite structure, with the dendrite orientation influenced by the thermal gradient and growth rate.
- Transformation: Upon cooling, the austenitic dendrites transform into martensite, with the amount of retained austenite depending on the carbon and alloy content.
- Tempering: In multi-pass welding, subsequent passes temper the previous pass, affecting the final microstructure and hardness.
The role of alloying elements in hardfacing deposits can be summarized as follows:
| Element | Primary Role | Effect on Hardness | Effect on Wear Resistance |
|---|---|---|---|
| Chromium (Cr) | Carbide former, corrosion resistance | Increases via Cr carbides | Excellent abrasive wear |
| Molybdenum (Mo) | Grain refiner, hard carbide former | Significant increase | High-temperature wear resistance |
| Vanadium (V) | Grain refiner, fine carbide former | Very significant increase | Excellent abrasion resistance |
| Tungsten (W) | Solid solution strengthening, carbide former | Moderate increase | High-temperature wear resistance |
| Carbon (C) | Carbide former, martensite strength | Increases via carbides and martensite | Critical for hard phase formation |
Engineering Practice Considerations
In practical hardfacing operations on 45 steel, several factors must be carefully controlled to achieve optimal overlay performance:
Pre-weld Considerations
- The base metal should be preheated to 150–250°C to reduce the risk of hydrogen-induced cracking in the heat-affected zone (HAZ).
- The surface should be cleaned to remove rust, oil, and scale to ensure proper wetting and fusion.
- For thick sections (>25 mm), a build-up pass with a compatible electrode should be applied to reduce dilution.
Welding Parameter Optimization
| Parameter | Low Heat Input | High Heat Input |
|---|---|---|
| Welding current | Lower (100–150 A) | Higher (180–250 A) |
| Travel speed | Faster (200–300 mm/min) | Slower (100–150 mm/min) |
| Arc voltage | Lower | Higher |
| Dilution rate | Lower (5–15%) | Higher (20–35%) |
| Cooling rate | Higher | Lower |
| Resulting hardness | Higher | Lower |
| Resulting toughness | Lower | Higher |
The study emphasizes that there is an optimal balance between hardness and toughness that must be achieved for each specific application. Excessively high hardness, achieved through very low heat input, can lead to brittle fracture under impact loading. Conversely, excessively low hardness, resulting from high heat input and high dilution, renders the overlay ineffective for wear protection.
Defect Analysis
| Defect Type | Cause | Prevention |
|---|---|---|
| Porosity | Moisture in electrode coating, inadequate cleaning | Bake electrodes, clean base metal |
| Cracking | High carbon dilution, hydrogen embrittlement | Preheat, use low-hydrogen electrodes |
| Incomplete fusion | Excessive travel speed, poor wetting | Optimize travel speed, ensure proper preparation |
| Excessive dilution | High heat input, thin deposit | Reduce heat input, apply build-up pass |
Key Questions and Reflections
This study provides valuable insights into the relationship between electrode composition and overlay performance, but several aspects deserve further consideration. The study does not address the effect of interpass temperature on the microstructure evolution in multi-pass hardfacing, which is a critical parameter in industrial applications where thick overlays are required. Additionally, the mechanical properties evaluated are limited to microhardness, and a comprehensive assessment should include impact toughness, fatigue resistance, and thermal fatigue behavior.
From my engineering experience, I have found that the actual performance of hardfacing overlays in service is often governed by factors beyond hardness, including the bond strength between the overlay and base metal, the residual stress state, and the thermal cycling resistance. The study by Zhang Youyi et al. provides a solid foundation for understanding the metallurgical aspects of hardfacing, but practical application requires a holistic approach that considers the entire component performance, not just the overlay microstructure.
Study Insights and Implications
The work by Zhang Youyi et al. is a valuable contribution to the understanding of hardfacing metallurgy on medium-carbon steel substrates. The clear demonstration that Mo and V are effective grain refiners in hardfacing deposits has direct implications for electrode selection and consumable development. Engineers selecting hardfacing electrodes for 45 steel applications should pay particular attention to the alloy composition of the electrode, not just the nominal hardness rating.
The study also highlights the importance of welding heat input control in achieving the desired overlay properties. In practice, this means that welding procedure specification (WPS) development for hardfacing applications must include rigorous control of current, voltage, and travel speed, with regular verification through hardness testing and microstructural examination. The systematic approach of comparing two different electrodes under identical welding conditions provides a methodological template that should be followed in future hardfacing research and qualification testing.