Mechanical properties of weldment affected by various vibration frequencies

Abstract

Nickel alloy 600 has been widely used since 1960. In 1978 a serious defect of this material was discovered in a boiling water reactor at the Duane Arnold Atomic Center in the United States: the safe-end forging part at the circulation pump’s input exhibited instability after extended use. Upon failure analysis, IGSCC was found to be the cause of cracks in the safe-end [1]. Traditionally, nickel alloy 600 has been taken as the reference material. Yet, it has been found that stress and corrosion resistance of this material is significantly inferior to that of nickel alloy 690 in practical applications [1–5]. Accordingly, processing and application of the latter has begun to receive greater attention. Even though nickel alloy 690 strongly resists stress and corrosion, these problems cannot entirely be ignored [5, 6]. The basic solution to the above lies in reducing residual stress that develops from processing. Residual stress is generated by non-uniform thermal stress introduced by local heat absorption during the weld, together with unequal coefficients of thermal expansion in the base and filler materials. Residual stress is rapidly and unevenly amplified by local volume changed from the phase transformation of welded materials. Widespread methods of relieving residual stress include heat treatment and mechanical processing. Heat treatment is the most common and effective. However, heat treatment is limited by the size of the furnace relative to the workpiece and by the hardness decrease of the entire workpiece. The mechanical methods most often used to relieve residual stress involve vibration. Traditionally, vibration methods were applied postweld; normally the workpiece has cooled to near room temperature. In this case, energy required to drive dislocations is relatively high. However, if the vibrations can be complemented by heat energy, dislocation motion is facilitated; moreover, stress relief may be optimized [7]. In this paper, we report the effect of employing a vibration technique on the mechanical properties of weldments. To evaluate this stress relief technique, nickel superalloy 690 was welded while subjecting to synchronous mechanical vibrations. The base material used in this research was nickel alloy 690 plate, 150× 100× 8.7 mm. The filler wire was Inconel 82 wire of 1.2 mm diameter. The weld joint was a singleV butt. The V-angle was 80 degrees, the separation at the root was 2.0 mm, and the root’s height was 1.0 mm. Argon was purged from the bottom of the weld. The experimental plates were welded by semiautomated gas tungsten arc welding (GTAW or TIG). The weld parameters were weld current 100 A, voltage 12 V, travel speed 15 cm/min, and wire feed 30 cm/min for 4 passes. Additional parameters were: tungsten with 0.2% thorium rod, 2.4 mm diameter, tip 60◦; argon gas flow at backing 6 lpm; tail argon gas, 25 lpm; weld interpass temperature, 80 ◦C; and pre-weld cleaning with acetone. As the weld layers were built up, each bead’s surface was cleaned with a grinding wheel, and subsequently with a stainless steel wire brush. The resonant frequency of the system was found to be 58 Hz, as seen in Fig. 1. An oscilloscope was used for analyzing vibration harmonics. At 48 Hz, the amplitude of the high frequency small harmonics on the fundamental harmonics reached the biggest value, which was chosen as sub-resonant frequency. Consequently, subresonant frequency, 48 Hz, no vibration, 0 Hz, and resonant frequency, 58 Hz, were chosen for comparison.