Strain-induced ε/α’-martensitic transformation behavior and solid particle erosion/cavitation erosion resistance of austenitic Fe-based alloys

Abstract

Strain-induced martensitic transformation was utilized for improving the solid particle erosion and cavitation erosion resistance in austenitic Fe-based alloys because it was expected that the strain-induced martensitic transformation might absorb the impact energy due to erosion. Thus, for this thesis, the effects of Mn and Ni concentration on the strain-induced ε/α’ martensitic transformation behavior and the correlation between this behavior and solid particle erosion/cavitation erosion resistance in Fe-12Cr-0.4C-xMn/Ni (x = 5, 7, and 10) alloys were investigated. First, to evaluate the effects of Mn and Ni concentration on the strain-induced ε/α’-martensitic transformation behavior, the investigation was divided into the following four different sections: (1) investigation of the kinetics of strain-induced α’-martensitic transformation, (2) measurement of stacking fault energy (SFE), (3) measurement of the volume fraction of ε-martensite, and (4) investigation of the effect of strain rate on the strain-induced martensitic transformation behavior. Results showed that both SFE and chemical driving force decreased as Mn concentration increased while the SFE increased and the chemical driving force decreased with increasing Ni concentration. At a high strain rate, adiabatic heating inhibited the γ→ε and γ→α’ martensitic transformation. Next, solid particle erosion and cavitation erosion tests were performed on Fe-12Cr-0.4C-xMn/Ni (x = 5, 7, 10) alloys. Results showed that, with decreasing the impact velocity, the solid particle erosion resistance of the alloys, in which the martensitic transformation actively occurred, gradually improved compared to that of the 10Ni alloy. This improvement could be attributed to an increase in the volume fraction of the martensite formed during the solid particle erosion test. On other hand, the 7Mn and 10Mn alloys, in which the γ→ε martensitic transformation occurred, exhibited better cavitation erosion resistance than the 5Mn alloy, in which the γ→α’ martensitic transformation occurred most actively. This was because the γ→ε martensitic transformation effectively delayed the erosion failure fracture during cavitation erosion and the increased absorption of cavity collapse energy. In this study, it seemed that the resistance to solid particle erosion and cavitation erosion was dependent on the work-hardening rate and erosion failure strain of the test alloys. As the work-hardening rate and erosion failure strain increase, the erosion resistance improves due to the increase in energy absorbed during erosion. Both the γ→ε and γ→α’ martensitic transformation augmented the work-hardening rate in the test alloys but the γ→α’ martensitic transformation played a key role. Moreover, it was thought that the γ→ε martensitic transformation greatly increased the erosion failure strain during cavitation erosion.|오스테나이트 철계 합금에서 고체입자침식(solid particle erosion)과 캐비테이션침식(cavitation erosion) 저항성을 향상시키기 위하여 변형유기마르텐사이트상변태(strain-induced martensitic transformation)를 이용하고자 하였다. 그 이유는 변형유기마르텐사이트상변태가 충격에너지를 흡수함으로써 침식에 의한 손상을 감소시켜줄 것으로 예상되기 때문이다. 따라서 본 논문에서는 Fe-12Cr-0.4C-xMn/Ni (x = 5, 7, 10) 합금에서 Mn과 Ni의 함량이 변형유기 ε/α’-마르텐사이트상변태 거동에 미치는 영향에 대하여 조사하였으며, 이 상변태 거동과 고체입자침식/캐비테이션침식 저항성 간의 관계에 대하여 고찰하였다. 먼저, Mn과 Ni 함량에 따른 변형유기 ε/α’-마르텐사이트상변태 거동을 평가하기 위하여 (1) 변형유기 α’-마르텐사이트상변태의 속도론, (2) 적층결함에너지(stacking fault energy)의 측정, (3) ε-마르텐사이트 분율의 측정, (4) 변형속도(strain rate)가 변형유기마르텐사이트상변태 거동에 미치는 영향과 같이 크게 네 부분으로 나누어 연구를 수행하였다. 이를 통하여 Mn 함량이 높아짐에 따라 실험 합금의 적층결함에너지와 화학적 구동력(chemical driving force)은 감소하며, Ni 함량이 높아짐에 따라 적층결함에너지는 증가하지만 화학적 구동력은 감소하는 것을 관찰하였다. 또한, 높은 변형속도 조건에서는 γ→ε과 γ→α’ 마르텐사이트상변태가 단열가열(adiabatic heating)에 의해 억제되었다. 다음은 Fe-12Cr-0.4C-xMn/Ni (x = 5, 7, 10) 합금에서 고체입자침식과 캐비테이션침식 실험을 수행하였다. 고체입자침식 실험 결과, 충돌 속도가 낮아짐에 따라, 변형유기 ε/α’-마르텐사이트상변태가 활발히 발생되는 합금의 고체입자침식 저항성이 점점 향상되었으며, 이는 변형유기마르텐사이트의 분율이 증가하였기 때문이다. 반면, 캐비테이션침식 실험 결과, γ→ε 마르텐사이트상변태가 발생되는 7Mn 합금과 10Mn 합금이 γ→α’ 마르텐사이트상변태가 가장 활발히 발생되는 5Mn 합금보다 더 우수한 캐비테이션침식 저항성을 지닌 것으로 나타났다. 이것은 γ→ε 마르텐사이트상변태가 침식에 의한 손상을 효과적으로 지연시켜주어, 공동 붕괴 에너지(cavity collapse energy) 흡수량이 늘어났기 때문이다. 본 연구에서, 고체입자침식과 캐비테이션침식에 대한 저항성은 실험 합금의 가공경화율(work-hardening rate)과 침식파손변형율(erosion failure strain)에 좌우되는 것으로 보인다. 가공경화율과 침식파손변형율이 증가할수록, 침식 과정 동안 합금의 에너지 흡수량이 늘어나, 침식 저항성이 향상된다. γ→ε과 γ→α’ 마르텐사이트상변태 모두 실험 합금의 가공경화율을 높이나 γ→α’ 마르텐사이트상변태가 더 큰 영향을 끼쳤던 것으로 보인다. 게다가, γ→ε 마르텐사이트상변태는 캐비테이션침식 동안 침식파손변형율을 크게 높였던 것으로 생각된다.; Strain-induced martensitic transformation was utilized for improving the solid particle erosion and cavitation erosion resistance in austenitic Fe-based alloys because it was expected that the strain-induced martensitic transformation might absorb the impact energy due to erosion. Thus, for this thesis, the effects of Mn and Ni concentration on the strain-induced ε/α’ martensitic transformation behavior and the correlation between this behavior and solid particle erosion/cavitation erosion resistance in Fe-12Cr-0.4C-xMn/Ni (x = 5, 7, and 10) alloys were investigated. First, to evaluate the effects of Mn and Ni concentration on the strain-induced ε/α’-martensitic transformation behavior, the investigation was divided into the following four different sections: (1) investigation of the kinetics of strain-induced α’-martensitic transformation, (2) measurement of stacking fault energy (SFE), (3) measurement of the volume fraction of ε-martensite, and (4) investigation of the effect of strain rate on the strain-induced martensitic transformation behavior. Results showed that both SFE and chemical driving force decreased as Mn concentration increased while the SFE increased and the chemical driving force decreased with increasing Ni concentration. At a high strain rate, adiabatic heating inhibited the γ→ε and γ→α’ martensitic transformation. Next, solid particle erosion and cavitation erosion tests were performed on Fe-12Cr-0.4C-xMn/Ni (x = 5, 7, 10) alloys. Results showed that, with decreasing the impact velocity, the solid particle erosion resistance of the alloys, in which the martensitic transformation actively occurred, gradually improved compared to that of the 10Ni alloy. This improvement could be attributed to an increase in the volume fraction of the martensite formed during the solid particle erosion test. On other hand, the 7Mn and 10Mn alloys, in which the γ→ε martensitic transformation occurred, exhibited better cavitation erosion resistance than the 5Mn alloy, in which the γ→α’ martensitic transformation occurred most actively. This was because the γ→ε martensitic transformation effectively delayed the erosion failure fracture during cavitation erosion and the increased absorption of cavity collapse energy. In this study, it seemed that the resistance to solid particle erosion and cavitation erosion was dependent on the work-hardening rate and erosion failure strain of the test alloys. As the work-hardening rate and erosion failure strain increase, the erosion resistance improves due to the increase in energy absorbed during erosion. Both the γ→ε and γ→α’ martensitic transformation augmented the work-hardening rate in the test alloys but the γ→α’ martensitic transformation played a key role. Moreover, it was thought that the γ→ε martensitic transformation greatly increased the erosion failure strain during cavitation erosion.