마찰용접한 니켈기 초내열합금 Alloy718의 미소 비커스 경도의 통계적 성질에 관한 연구

Abstract

니켈기 초내열합금인 Alloy718의 동종 마찰용접재에 대한 미소 비커스 경도의 통계적 성질에 관한 연구를 수행하여 다음과 같은 결과를 얻었다. (1) 마찰용접재의 미소 비커스 경도의 평균은 모재부(BM)가 열영향부 (HAZ)보다 높게 나타났으나, 마찰용접 후열처리재의 경우는 열영향부가 모재부보다 높게 나타났다. 이는 마찰용접재의 경우 마찰용접열과 단조가열압력에 의한 영향으로 사료되며, 용접 후열처리재의 경우는 용접 후열처리에 의한 미시조직의 변화에 기인하는 것으로 사료된다. (2) 미소 비커스 경도의 변동계수(COV)는 마찰용접재와 마찰용접 후열처리재 모두 하중이 증가함에 따라 모재부와 열영향부 모두 감소하는 결과를 보였다. (3) Meyer 식에 의하여 압입치수효과의 영향을 평가한 결과 마찰용접재가 마찰용접 후열처리재보다 크게 나타났으며, 열영향부가 모재부보다 크게 나타났음을 알 수 있었다. 즉, 마찰용접재와 마찰용접 후열처리재의 모재부 및 열영향부는 각각 2.18, 2.24, 2.15 그리고 2.19의 n값을 나타냈다. (4) 표본수에 따른 변동계수의 영향은 열영향부가 모재부보다 크게 나타났으며, 표본수가 증가할수록 변동계수는 대체로 감소하는 경향을 보였다. (5) 경도치에 대한 확률분포는 Weibull 분포에 잘 따름을 알 수 있었다. 마찰용접재와 마찰용접 후열처리재의 모재부 및 열영향부는 하중의 증가에 따라 분포의 형상 및 척도 파라메터 모두 증가하였다.

In today's competitive world, all engineering systems are expected to perform satisfactory over the duration of their expected life span. No system will perform reliably unless it is designed specifically for "reliability". Due to the increasing complexity of modern engineering systems, the concept of reliability has become a very important factor in the overall system design. Reliability is important because a designer must design machinery and equipment that will work both in theory and in practice. In order to express the reliability of a system in quantitative terms, it is necessary to develop a mathematical model of the overall system and analyze its performance under realistic operating conditions. Reliability is an analytical problem that involves both statistical and engineering aspects. It must be given critical attention throughout the life of a system - including its development, design, production, quality control, shipping, installation, operation and maintenance. It requires the integrated application of many disciplines: statistics, probability, materials engineering, circuit analysis, mechanical design, structural analysis, production engineering and several other engineering disciplines. Reliability also requires sound organization for the discovery and correction of the cause of failure. In order to investigate the statistical properties of micro-Vickers hardness numbers of materials, a lot of experimental data are needed. It is difficult to know the micro-Vickers hardness characteristic of materials with data obtained by experiments and few of experimental observations. Therefore, analysis of the statistical properties and getting specific properties of the distribution data by gathering a lot of experimental data is the best way to know the micro-Vickers hardness characteristic of materials. Therefore, the aim of this paper is to investigate the statistical properties of micro-Vickers hardness (HV) for friction welded joints in Ni-base Alloy718. Several micro-Vickers indentations were made under same identical conditions. The micro-Vickers hardness tests of friction welded Alloy718 were performed at three different applied loads of 100, 200 and 300g on base metal (BM) and heat affected zone (HAZ), respectively. In the case of as-welded parts, the arithmetic mean of micro-Vickers hardness for BM materials is larger than that of HAZ for all applied loads. In the case of post weld heat treatment (PWHT) parts, the micro-Vickers hardness of HAZ is larger than the BM. The coefficient of variation (COV) for BM and HAZ both as-welded and PWHT is decreased by increasing with the applied load. The indentation size effect (ISE) was analysed using Meyer's law. The results showed that the Meyer index n increased by increasing with the applied load. The COV of micro-Vickers hardness for BM and HAZ both as-welded and PWHT is decreased by increasing with the sample size. The probability distribution of micro-Vickers hardness was well followed Weibull distribution. The distribution of micro-Vickers hardness was not found to be symmetric type. The shape parameter (α) and the scale parameter (β) are increased by increasing with the applied load, as both BM and HAZ.