Abstract Scope |
Introduction
Dissimilar Metal Welds (DMWs) contain a gradient of physical and materials properties across the weld line. This gradient arises from the differences in chemical composition of the two joining species which tend to mix which each other to varying degrees at or near the fusion boundary. Mixing of materials can form microstructures (and therefore properties) that are distinct from that of the parent materials. This gradient of properties is thus a challenge to measure using traditional testing methods such as the cross weld tensile testing (CWTT). CWTT aided with digital image correlation (DIC) can detect the localized straining necessary to measure yielding of materials in different local regions of the weld. This allows for the calculation of yield stresses in various local regions across the weld line where the traditional CWTT using a physically mounted extensometer only give the global property of the weldment. This global property tends to represent the region containing the most plastic deformation which tells very little about the rest of the weldment. Using DIC during CWTT can aid to define the properties of other regions such as the heat affected zone which may be more important to determining in-service performance of the component.
Experimental Procedure
The DIC tensile test procedure resembles a typical CWTT but requires a specific sample preparation. First the samples are ground and polished to 1 micron and etched to reveal the microstructure of the fusion boundary and neighboring regions. Then half of the sample is masked (to protect this viewable microstructure) while the other half of the sample is painted with white and black paint to simulate the random speckle pattern necessary to conduct DIC. The sample is tested in tension to failure with a camera capturing images during testing. Then DIC results are broken down by region (weld metal, transition region, HAZ, and base metal) using virtual extensometers applied via a software package. Regions are identified through their etching on the sample next to the region of data capture. Using software packages such as excel or MATLAB, stress strain curves for the global and localized data can be analyzed and compared. The most important outputs from each test being yield strength of specific zones of interest across the welded joint, as well as approximate elastic modulus (if the data is sufficiently dense to calculate).
Results and Discussion
A series of DMWs, with weld metals overmatching and undermatching the base metals yield strength, from subsea piping systems used in the oil and gas industry were tested in this study. Such weldments usually encompass some form of low or mid alloy carbon steel welded to nickel alloy 625. For differing base materials (say F22 versus F65) there are distinct differences in the straining behavior of the weld metal near yield. This gives us our over and undermatching cases for both base materials. However, for both weldments, nickel alloy 625 can sufficiently strain harden under load to force failure in the base metal, regardless of undermatching / overmatching strength.
DIC aided CWTT were also performed on other weldments including a matching filler metal weld sample and samples with varying compositions along the weld profile (due to changing filler metals from root – cap passes). Most recently, the DIC aided CWTT was successfully performed on high energy density (HED) welds. In all cases the weld metal over / undermatching behavior is easily quantified and identified using the DIC aided CWTT.
Conclusions
Typical CWTT has a significant shortcoming when testing DMWs. The CWTT only outputs the global properties of the weldment that reflect the region of ultimate failure, where most strain will accumulate throughout the test. In DMWs, there are localized differences that can greatly impact performance. DIC aided CWTT can quantify weld metal matching condition as well as localized properties of regions of interest across the welded joint, such as the heat affected zone and weld metal dilution region. These regions are known to be of importance to the in-service performance of weldments in numerous environments.
The DIC aided CWTT is also beneficial for testing matching filler metal welds, which also contain mechanical property gradients. In addition, many industries require all weld metal tensile tests to prove overmatching weld metal strength. A single DIC aided CWTT can prove overmatching weld metal, while accounting for the effect of actual weld geometry and thermo-mechanical history on the weld metal properties. Therefore, the capabilities of DIC to quantify localized mechanical properties of weldments during CWTT deserve further attention. |