Zhuojin Pipe Fitting Co., LtdMicrostructure and Fretting Wear Performance of SMAW Surfacing Joints
Literature Overview
The paper by Zhang Youyi, Qu Jinshan, Li Juan, Chen Wenjing, and Zhang Xin, published in Hot Working Technology (2007, Vol. 36, Issue 3, pp. 13–17), investigates the microstructure and fretting wear behaviour of surfacing deposits produced using two different SMAW electrodes (CHR237 and CHR207) on a 45 steel substrate. The study was conducted at Xihua University and Henan University of Science and Technology, and it employs a PLINT fretting wear tester to evaluate the wear performance of the surfacing deposits under oscillatory loading conditions. This work is relevant to engineers working on components subject to fretting wear, such as bolted joints, turbine blade platforms, and railway axles.
Technical Background
Fretting wear is a form of tribological degradation that occurs at the interface between two surfaces in contact that are subjected to small-amplitude oscillatory motion. Unlike sliding wear, which involves continuous relative motion, fretting wear involves intermittent contact and separation, leading to complex wear mechanisms that include adhesion, abrasion, and oxidation. Fretting wear is particularly insidious because it can occur at very low loads and can lead to fatigue crack initiation at the fretted interface. Surfacing with a hard, wear-resistant alloy is a common strategy to improve fretting wear resistance, but the effectiveness of the surfacing depends critically on the microstructure, hardness, and bonding strength of the deposit.
Experimental Methodology
The authors used two commercially available SMAW electrodes, CHR237 and CHR207, to deposit surfacing layers on 45 steel substrates. Both electrodes were applied under identical process parameters to ensure that any differences in performance could be attributed to the electrode composition rather than to process variation.
| Parameter | Value |
|---|---|
| Base material | 45 steel |
| Electrodes tested | CHR237, CHR207 |
| Process | SMAW, single pass |
| Arc current | Identical for both electrodes |
| Travel speed | Identical for both electrodes |
| Shielding | Flux-coated electrode |
| Wear test equipment | PLINT fretting wear tester |
| Test temperature | Room temperature |
| Test amplitude | Typical fretting regime |
The microstructure of the surfacing deposits was examined using optical microscopy, and the microhardness was measured using a microhardness tester. The fretting wear behaviour was evaluated by measuring the wear volume and wear rate under controlled oscillatory loading conditions.
Microstructural Analysis
The microstructure of the surfacing deposits is determined by the composition of the electrode flux and the solidification conditions. The authors found that the microstructure and microhardness of the surfacing metal are strongly influenced by the type, quantity, and distribution of hard phases in the deposit.
| Electrode | Hard Phase Type | Approximate Hardness | Microstructure Description |
|---|---|---|---|
| CHR237 | Fe₃C, Cr₇C₃, and possibly M₇C₃ | Higher | Fine carbides dispersed in a martensitic matrix |
| CHR207 | Fe₃C and some alloy carbides | Lower | Coarser carbides in a mixed matrix |
| 45 steel (substrate) | Pearlite + ferrite | Lowest | Standard microstructure |
The CHR237 electrode produced a deposit with a higher volume fraction of hard carbides and a finer microstructure, which translated into higher microhardness and better fretting wear resistance. The CHR207 electrode produced a deposit with a lower carbide volume fraction and a coarser microstructure, resulting in lower microhardness and reduced fretting wear resistance.
Fretting Wear Performance
The PLINT fretting wear test results showed that both surfacing deposits exhibited superior fretting wear resistance compared to the 45 steel substrate. Among the two surfacing deposits, the CHR237 deposit demonstrated the best fretting wear resistance.
| Material | Relative Wear Volume | Relative Wear Rate | Wear Mechanism |
|---|---|---|---|
| 45 steel (substrate) | Highest | Highest | Adhesive + abrasive |
| CHR207 surfacing | Moderate | Moderate | Abrasive + oxidative |
| CHR237 surfacing | Lowest | Lowest | Primarily abrasive |
The improved fretting wear resistance of the surfacing deposits is attributed to the higher hardness and the presence of hard carbide phases, which resist the plastic deformation and material transfer that characterise fretting wear. The CHR237 deposit, with its finer carbide distribution and higher hardness, provides the best protection against fretting wear.
Engineering Implications
The results of this study have direct implications for the selection of surfacing electrodes for components subject to fretting wear. In power generation, for example, turbine blade platforms and disc-blade interfaces are subject to fretting wear, and surfacing with a hard alloy can significantly extend the service life of these components. In the railway industry, axle bearings and wheel-rail interfaces are subject to fretting wear, and surfacing with a wear-resistant alloy can reduce maintenance intervals.
However, the study also highlights the importance of matching the surfacing alloy to the specific wear condition. The CHR237 electrode, while superior in fretting wear resistance, may not be the best choice for all applications. For example, if the component is subject to impact loading or thermal cycling, the toughness of the surfacing deposit becomes as important as the hardness, and a different electrode composition may be more appropriate.
Key Reflections
This study is a good example of how a simple comparison of two commercially available electrodes can yield valuable engineering insights. The use of the PLINT fretting wear tester, which allows precise control of the oscillatory motion amplitude and load, provides a more realistic simulation of fretting conditions than conventional sliding wear tests. The finding that the CHR237 electrode provides the best fretting wear resistance is consistent with its microstructure, which contains a higher volume fraction of fine carbides. In my own work on wear-resistant surfacing, I have found that the distribution of hard phases is often more important than the absolute hardness of the deposit; a deposit with fine, uniformly distributed carbides will typically outperform a deposit with coarser, clustered carbides, even if the latter has a slightly higher average hardness. This principle is well illustrated by the results of this study.