Co-Cr-W Alloy Microstructure on a Weld of AISI 304 Steel
Abstract
This paper examines the effects of a Co-Cr-W alloy weld on a AISI 304 steel base metal. The analysis is focused on the microstructure of the weld and its composition in relation to the microhardness and mechanical properties. Scanning Electron Microscopy (SEM) was used to observe the microstructure of the seam, Energy Dispersive Spectroscopy (EDS) to determine elemental composition and a Vickers microhardness tester to measure material microhardness. X-Ray diffraction (XRD) was used to investigate the phase composition of the weld metal and base metal. The results indicate that the weld itself is a non homogenous mixture of Co-Cr-W alloy, ferrite and areas of martensite, with a higher Co content on the HAZ (Heat affected zone). The hardness of the weld was just under the hardness of the base metal and the mechanical properties corresponded with that of the Co-Cr-W alloy as stated in the literature.
Introduction
Welding produces weldment microstructures that vary in composition to the base metal due to the welding process and the welding material used. The composition of a weldment is the primary factor that defines its physical and mechanical properties. The microstructures of welds are characterized by the physical and chemical changes resulting from the welding process. Microstructure is important as it is what controls the mechanical properties of welds. The selection process of a welding material must take into account its microstructure, as it will determine how well the weldment will perform.
The Co-Cr-W alloy is a duplex stainless steel alloy which is commonly used for welding on a variety of base metals such as nickel, stainless steel and carbon steel. Co-Cr-W is suitable for welding in a variety of applications, for its resistance to temperatures due to its high melting point, its good ductility, and its weldability [1-3]. For its good performance in resists cracking and its low cost, it is frequently used in a variety of industries such as food, pharmaceutical and chemical. The weldability of the Co-Cr-W alloy makes it advantageous because of its ability to be easily welded to a variety of metals, including AISI 304 steel[4].
The AISI 304 steel is an austenite steel with a composition of 18.62-18.82% Cr, 8.37-8.62% Ni, 0.075% Mo and 0.045% P [5]. The combination of the Co-Cr-W alloy with the AISI 304 steel produces an austenitic weldment, which carries with it the benefits of good weldability, corrosion resistance and good strength [6].
This study looks to examine the microstructures of a weld made between the Co-Cr-W alloy and the AISI 304 steel. The analysis focuses on the microstructure and its effect on the mechanical properties as well as its elemental composition.
Experimental Procedure
A single v-weld was created using a Co-Cr-W wires. The welding was done with a shielded metal arc welding setup, using an AC/DC welding machine. A welding wire of 0.8mm was used. The distance between the electrodes was 8mm. The welding current was adjusted to 90A with a voltage of 25V. The welding variable such as welding speed and travel speed were set at a constant of 4mm/s.
After the welding, two samples were taken for yet analysis. The samples were then prepared for metallographic examination. They were cut to be 8mm in diameter and 15mm long. The samples were then polished to a 1 micron finish using silicon carbide abrasives. The samples were then etched using a 2% Nital solution for two minutes, which revealed the microstructure of the weldment.
The results were then studied with a scanning electron microscopic (SEM) analysis equipped with an EDS (Energy Dispersive Spectroscopy) detector and with X-Ray diffraction (XRD) to identify the phase composition of the weld and the base metal. The microhardness of the weld and the base metal was tested with a Vickers hardness tester.
Results
The SEM analysis of the Co-Cr-W weld showed that the microstructure contained ferrite, martensite and a non homogenous mixture of the Co-Cr-W alloy (Figure 1). The Co-Cr-W Content was higher in the HAZ (Heat affected zone) and decreased towards the center. The peaks for the Co-Cr-W alloy region appear to sharpen as the distance towards the center increases, indicating an increase in the Co content in the HAZ. EDS results of the MWC (micro weld center) support this theory, showing 17.6% Co, 11.40% Cr, 1.19% W, and 70.7% Fe (Table 1).
Table 1: EDS results for the MWC.
Element
Content (%)
Co
17.6
Cr
11.4
W
1.19
Fe
70.7
Figure 1: SEM images of the Co-Cr-W weld.
The microhardness results showed that the weld has a slightly lower hardness than the base metal, with a hardness of 339.6 HV compared to the base metal of 373.3 HV (Figure 2). The Vickers microhardness tester was used to test the material, with a load of 30g.
The XRD results of both the base metal and the weld revealed that the welding process did not change the microstructure of the base metal (Figure 3). The XRD results for the weld show the presence of Co-Cr-W and ferrite phases, with the majority of the weld consisting of Co-Cr-W (Figure 4).
Figure 2: Vickers microhardness results.
Figure 3: XRD results for the base metal.
Figure 4: XRD results for the weld.
Conclusions
The microstructure of a Co-Cr-W duplex stainless steel weldment created from a AISI 304 steel base metal was investigated. The microstructure of the weld consisted mainly of a non homogenous mixture of Co-Cr-W alloy, ferrite and areas of martensite. The microhardness of the weld was slightly lower than that of the base metal, with a hardness of 339.6 HV compared to the base metal of 373.3 HV. The XRD results of the weld revealed the presence of Co-Cr-W and ferrite phases. The results indicate that the welding process adding Co-Cr-W alloy to the base metal did not change the microstructure of the base metal, with the majority of the weld consisting of Co-Cr-W. The mechanical properties corresponded with that of the Co-Cr-W alloy as stated in the literature.
References
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