가압경수로 보수용접에 적용하기 위한 와이어 아크 적층 제조 과정에서의 열처리 효과에 대한 연구

Abstract

A wire-arc additive manufacturing process using the temper bead welding technique is used to replace post-weld heat treatment in the repair process of dissimilar material welded joints in pressurized water reactors. In this study, we focused on identifying the mechanism of heat treatment effect occurring in the wire-arc additive manufacturing process through metallurgical analysis.

For this purpose, a mock-up specimen to reproduce the welded joints of the actual pressurized water reactor was manufactured by wire-arc manufactured process with 14 layers and 146 passes. OM, SEM, and EBSD analysis were introduced to analysis the heat affected zone generated in wire-arc additive manufacturing process. And quantitative analysis of misorientation angle, grain size, phase fraction was performed using OIM software. For quantitative measurement of the phase fraction, the phase fraction of polygonal ferrite was measured by analyzing the grain orientation spread map, and the fractions of bainite and martensite were quantitatively measured by analyzing the image quality map. The consistency of the phase fraction measurement results with CALPHAD software simulation results was also evaluated. And micro-vickers hardness test and tensile test were to evaluate mechanical properties.

After additive manufacturing process, the prior austenite grain size decreased by 82% and the grain size also decreased by 85%. In grain boundary map, low-angle grain boundaries were decreased, and high-angle grain boundaries were increased, it is due to recrystallization which occurs as the fraction of polygonal ferrite increases. As a result of grain orientation spread map analysis, it was confirmed that fraction of polygonal ferrite increased from 5.4% to 31.2%. And in image quality map, the bainite fraction increased by 18.4% and the martensite fraction decreased by 44.2%. Micro-Vickers hardness measurement results showed that the hardness distribution was higher than the SA508 Gr.3 Cl.1 base material but lower than 380Hv the hardness value that required by the ISO 15614-1. In the tensile test, the tensile strength of the dissimilar weld joint specimen was measured at the intermediate value of the SA508 Gr.3 Cl.1 base metal and additive manufactured specimen. And the analysis of the tensile fractured surface of the dissimilar weld joint specimen showed that the crack occurred in the additive manufactured area.

To determine through what mechanism the microstructure and mechanical properties change during wire-arc additive manufacturing process, three types of specimens (1 layer, 2 layer, 3 layer overlayed) were manufactured and analyzed. The EBSD analysis results the prior austenite grain size tended to decrease as additive manufacturing process progressed. As a result of phase fraction to analysis the microstructure change, phase fractions of polygonal ferrite and bainite were increased and phase fraction of martensite was decreased. And the same tendency was shown when compared with the calculation results of CALPHAD software simulation results based on the heating and cooling rates measured through thermos-couples. In the additive manufacturing process, the microstructure of heat affected zone transformed to martensite matrix during first- and second-layer overlay process. And through repeated thermal cycles at temperature under AC3, the microstructure changes into various phases such as martensite, bainite and polygonal ferrite. As microstructure phase fraction change, the mechanical properties changes. As a result of micro-vickers hardness measurement, the average hardness value was measured as 395.8Hv in the first layer heat affected zone, 365.1Hv in second layer heat affected zone, and 320.9Hv in third layer heat affected zone. As additive manufacturing progresses, the hardness distribution tends to decrease, and this phenomenon is considered to be influenced by the change of microstructure phase fraction.

In this study, the effect of tempering in the wire-arc additive manufacturing process as an alternative to post-weld heat treatment was studied. As additive manufacturing progresses, the phase fraction in heat affected zone changed due to repeated welding heat input and directly affects the mechanical properties. Phase fractions in heat affected zone tended to decrease for martensite and increase for polygonal ferrite and bainite. The decrease in martensite and the increase in polygonal ferrite and bainite directly affect the mechanical properties of the heat affected zone, especially the hardness distribution. In additive manufacturing process, the average hardness tended to decrease and met the 380Hv required by the ISO standard. Through this study, the mechanism of tempering effect occurring during the wire-arc additive manufacturing process was investigated, and it was confirmed that the post-welding heat treatment process can be completely replaced. In addition, it has been proven that there is no problem in applying it to nuclear power plant maintenance and repair by satisfying the relevant ASME codes and ISO standards.