Boride Formation Induced by pcBN Tool Wear in Friction-Stir-Welded Stainless Steels
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- Park, S.H.C., Sato, Y.S., Kokawa, H. et al. Metall and Mat Trans A (2009) 40: 625. doi:10.1007/s11661-008-9709-9
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The wear of polycrystalline cubic boron nitride (pcBN) tool and its effect on second phase formation were investigated in stainless steel friction-stir (FS) welds. The nitrogen content and the flow stress were analyzed in these welds to examine pcBN tool wear. The nitrogen content in stir zone (SZ) was found to be higher in the austenitic stainless steel FS welds than in the ferritic and duplex stainless steel welds. The flow stress of austenitic stainless steels was almost 1.5 times larger than that of ferritic and duplex stainless steels. These results suggest that the higher flow stress causes the severe tool wear in austenitic stainless steels, which results in greater nitrogen pickup in austenitic stainless steel FS welds. From the microstructural observation, a possibility was suggested that Cr-rich borides with a crystallographic structure of Cr2B and Cr5B3 formed through the reaction between the increased boron and nitrogen and the matrix during FS welding (FSW).
Friction stir welding (FSW) has been widely studied and commercially used in low-softening temperature material structures since it was invented nearly 15 years ago.[1−16] The feasibility studies of FSW on high-softening temperature material (HSTM) were also made relatively early. Some initial feasibility studies of FSW on 12 pct Cr alloy and low carbon steel were demonstrated by The Welding Institution (TWI). Thereafter, FSW feasibility was examined in several types of HSTMs such as ferritic steels,[18,19] stainless steels,[20–24] and heat-resistant steels. Transverse tensile specimens failed in regions corresponding to the base material (BM), and their transverse tensile properties were governed by the BM properties in most of FS-welded steels; i.e., yield and ultimate tensile strengths of the weld were comparable to those of the BM.
However, the research on FSW of HSTMs such as steels representing most of the welded structures is still limited, compared to that of LSTM. One of the major causes of the limited studies on HSTM is the lack of suitable welding tools. The tool must resist physical and chemical wear, possess sufficient mechanical strength at elevated temperatures, and effectively dissipate the heat carried to the tool during the welding process. For initial tool materials for HSTM, W-series alloys such as W alloys and WC-Co alloy have been used.[17,21] The FSW of mild steel using Mo-based alloy tool has also been demonstrated. However, tool wear was inevitable. The previous study reported the changes in tool dimensions arising from both rubbing wear and deformation of the tool. The greatest changes in tool dimensions occurred during the initial plunging stage of the tool. Recently, a new tool made of polycrystalline cubic boron nitride (pcBN) that appears capable of meeting these requirements, and especially the wear resistance, has been developed. Development of a tool material with excellent properties offers a pathway to more active studies on FSW of steels.
Several studies have demonstrated the potential of pcBN tool for ferrous materials.[22–26] A refined microstructure formed in the stir zone (SZ), and adequate mechanical properties were achieved in the welds, suggesting that pcBN is one of the promising tool materials for HSTM. Unfortunately, wear and damage of pcBN tools still occur. It is important to examine the wear behavior of the pcBN tool during FSW, because the tool wear affects the mechanical and corrosion properties of the welds.
In the present study, the fundamentals of pcBN tool wear during FSW were investigated in stainless steels. The FSW was applied to ferritic (type 430), duplex (type 329J4L), and austenitic (types 304, 316L, and 310) stainless steels using pcBN tool. The microstructures were observed by electron microscopy. Their microstructural evolution and the boride formation induced by pcBN tool wear in austenitic stainless steel were examined in this study.
2 Experimental procedure
2.1 Materials and Process Details
Materials and the Chemical Composition Used in the Present Study
2.2 Evaluation of Tool Wear
When pcBN tool wear occurs during FSW, both boron and nitrogen content should increase in the weld because the pcBN consists of boron and nitrogen. Following FSW, the nitrogen contents of the SZ and BM were measured in an inert gas atmosphere by a fusion gas chromatographic analyzer produced by LECO1 (model TC-436DR oxygen and nitrogen analyzer). The samples of about 0.5 g for nitrogen content analysis were carefully cut from only the SZ using an electrical-discharge machine (EDM). The samples were then cleaned by filing away the surface and rinsing them with acetone for 300 seconds using an ultrasonic cleaner. At least three samples were analyzed for both the SZ and BM. The average value was used for the nitrogen contents of each region.
2.3 Microstructural Observation
Cross sections perpendicular to the welding direction were observed using optical microscopy (OM). Specimens for OM were etched electrolytically in a solution of 10 pct oxalic acid + 90 pct water with a power supply set to 30 V for about 10 seconds. Elemental analysis using an electron probe microanalyzer (EPMA) was carried out for particles found on the cross section. Thin disks with a diameter of 3 mm were cut from the various locations of the weld using an EDM, and then the electron-transparent thin sections were electrolytically made by twin-jet polishing in a 10 pct perchloric acid + 90 pct ethanol solution. These were observed at 200 kV with a JEOL2-2000EXII transmission electron microscope (TEM) and a Hitachi HD-2000 (Tokyo, Japan) scanning transmission electron microscope (STEM) equipped with an energy-dispersive X-ray (EDX) spectroscopy analysis system, using a 0.5-nm electron probe with a spatial resolution of 1.0 nm. The particles in the advancing side of the SZ were identified with both a selected area electron diffraction (SAED) pattern and STEM-EDX.
3 Results and discussion
3.1 Wear of pcBN Tool
Analysis of the nitrogen content was conducted to investigate the wear behavior of pcBN tool in five types of stainless steels including ferritic (type 430), duplex (type 329J4L), and austenitic (types 304, 316L, and 310) stainless steels. The typical regions analyzed are drawn in Figure 2(e) as white squares. The nitrogen contents of the advancing side of the SZ were compared with those of the BM and the retreating side of the SZ. The advancing and retreating sides are notated as SZ-AS and SZ-RS throughout the article, respectively.
3.2 Factor Governing Wear Behavior of pcBN Tool
3.3 Boride Formation Induced by pcBN Tool Wear
It should be noted that nitrogen was not detected in most of the Cr-rich borides, although the SZ-AS exhibited about 2 to 5 times higher nitrogen than the austenite matrix of the BM, as shown in Figure 4. This suggests that the nitrogen comes from the pcBN during FSW but exists in the matrix after FSW. Because the solubility of nitrogen is higher in austenite than in ferrite, the nitrogen from the pcBN would dissolve into the austenite matrix during the boride formation. Therefore, tribochemical reaction arising from differences in the solubility of nitrogen as well as the flow stress would also be one of reasons why the nitrogen pickup during FSW is shown more significantly in the austenitic steel welds.
Nonoxide ceramic borides generally have strongly negative free energies of formation, giving them excellent stability under many conditions. Because the available thermodynamic data on Cr borides are limited, it is difficult to calculate the standard free energy of the boride formation. It has been reported that the standard free energy of Cr5B3 boride formation is much lower than that of BN at 1500 K (1227 °C). This temperature is roughly equal to the welding temperature during FSW of austenite stainless steels in the present study. A previous study on Cr boride formation has shown that Cr2B and Cr5B3 borides can coexist at temperatures between 1273 K (1000 °C) and 1573 K (1300 °C) and that Cr2B is thermodynamically more stable than Cr5B3. These data suggest that Cr-rich borides with crystallographic structure of Cr2B and Cr5B3 possibly form by the reaction between BN and austenite matrix during FSW. As mentioned previously, high nitrogen solubility in austenite may help these reactions during FSW.
The analysis of nitrogen content and the Cr-rich borides shows that the increase in boron and nitrogen content was mainly found in the SZ-AS in austenitic stainless steels. This microstructural heterogeneity in the SZ could be attributed to the material movement from the retreating side to the advancing side at the trailing edge of the tool during FSW. The material undergoes large shear deformation along the pin surface toward the rotating direction during stirring.[31,36,37] The Cr borides and BN compounds from the tool are simultaneously captured into the material. The shear deformation probably moves most regions to the advancing side at the trailing edge of the tool, and then the regions accumulate around the SZ-AS. This would be one of the possible reasons why the higher boron and nitrogen contents are detected in the SZ-AS.
The present study clarified that the Cr-rich borides are formed in the SZ-AS in austenitic stainless steel FS welds when the pcBN tool severely wears during FSW. Because the Cr-rich borides consume the Cr, the Cr-depleted zone, whose width is generally a hundred and several tens of nanometers, would be created in the vicinity of the Cr-rich borides, as well as the sigma phase. The Cr-depleted zone was not observed in the EPMA map, which is attributed to the fact that the spatial resolution of EPMA is usually larger than 1 μm. Coexistence of the Cr-rich boride and sigma phase would cause the strong contrast consisting of the corroded grain boundaries and pits in the SZ-AS on the cross sections (Figure 2). This result implies that suppression of the pcBN tool wear is a requirement to produce the high-quality welds without the preferentially corroded SZ-AS in austenitic stainless steels. However, the pcBN tool having the much higher wear resistance (an MS80 pcBN tool) has been recently developed, and the degree of the tool wear occurring during FSW of austenitic stainless steels is remarkably reduced. The effect of the pcBN tool grade on the tool wear will be reported in a separate article.
The FSW was applied to five types of ferritic, duplex, and austenitic stainless steels, and interstitial pickup and B-rich phase evolution were investigated in their welds. The largely increased nitrogen content was detected in the advancing side of the SZ in austenitic stainless steel FS welds. It was shown that the level of boron and nitrogen increases in the SZ of austenitic stainless steel FS welds. A possibility was suggested that tool wear during FSW can be attributed to high flow stresses in austenitic stainless steels. The increase boron and nitrogen in the SZ resulted in the formation of Cr-rich borides, of between 100- to 1000-nm diameter, with a crystallographic structure of Cr2B and Cr5B3 through the reaction between the boron and nitrogen and the matrix during FSW.
The authors sincerely thank Professors K. Ikeda, K. Maruyama, T.W. Nelson, C.D. Sorensen, and Z.J. Wang, for many helpful suggestions and advice, and Mr. A. Honda, Mr. M. Doi, and H. Matsumoto for there technical assistance. Financial support from the Japan Ministry of Education, Culture, Sports, Science and Technology for the Promotion of Science with a Grant-in-Aid for Young Researchers and for the Global COE Program in Materials Integration International Center of Education and Research at Tohoku University is gratefully acknowledged.