Effects of Mn and Mo addition on high-temperature tensile properties in high-Ni-containing austenitic cast steels used for turbo-charger application

Jung, Seungmun,Jo, Yong Hee,Jeon, Changwoo,Choi, Won-Mi,Lee, Byeong-Joo,Oh, Yong-Jun,Kim, Gi-Yong,Jang, Seongsik,Lee, Sunghak. Elsevier Sequoia 2017 Materials science & engineering Structural materia Vol.682 No.-

<P><B>Abstract</B></P> <P>Since turbo-chargers require more excellent high-temperature properties to maintain their structures at further higher exhaust gas temperatures up to 1050°C, a 20wt%-Ni-containing austenitic cast steel (N20 steel) has been suggested as a promising candidate cast steel. However, this steel is very expensive because it contains a large amount of expensive Ni. In order to partly replace expensive Ni by inexpensive Mn and to improve high-temperature tensile properties in the N20 steel, three austenitic cast steels were fabricated by replacing 6wt% of Ni by 6.9wt% of Mn or by adding 2–4wt% of Mo. Thermodynamically calculated fractions of equilibrium phases (austenite, ferrite, and M<SUB>7</SUB>C<SUB>3</SUB> carbide) were matched with experimentally measured fractions, although they were somewhat overestimated. The N14 steel where 6wt% Ni was replaced by 6.9wt% of Mn did not contain any ferrite, and showed comparable or more excellent high-temperature tensile properties than those of the N20 steel, which indicated the successful replacement up to 6wt% Ni by Mn, together with alloying cost reduction of 10%. The Mo addition also favorably affected high-temperature properties because Mo worked for increasing both M<SUB>7</SUB>C<SUB>3</SUB> fraction and austenite matrix hardness. Simultaneously considering mechanical properties and alloying costs, therefore, these Mo-containing N14 steels can be fully adopted for high-performance turbo-chargers requiring excellent high-temperature properties.</P>

Interpretation of High-Temperature Tensile Properties by Thermodynamically Calculated Equilibrium Phase Diagrams of Heat-Resistant Austenitic Cast Steels

정승문,손석수,최원미,이병주,오용준,장성식,이성학 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.1

High-temperature tensile properties of three heat-resistant austenitic cast steels fabricated by varying W, Mo, and Al contents were interpreted by thermodynamically calculated equilibrium phase diagrams of austenite, ferrite, and carbides as well as microstructural analyses. A two-step calculation method was adopted to cast steel microstructures below the liquid dissolution temperature because the casting route was not an equilibrium state. Thermodynamically calculated fractions of equilibrium phases were well matched with experimentally measured fractions. Ferrites existed at room and high temperatures in both equilibrium phase diagrams and actual microstructures, which has not been reported in previous researches on austenitic cast steels. In the W2Mo1Al1 steel, 38% and 12% of ferrite existed in the equilibrium phase diagram and actual microstructure, respectively, and led to the void initiation and coalescence at ferrites and consequently to the serious deterioration of high-temperature strengths. The present equilibrium phase diagrams, besides detailed microstructural analyses, effectively evaluated the high-temperature performance by estimating high-temperature equilibrium phases, and provided an important idea on whether ferrite were formed or not in the heat-resistant austenitic cast steels.

Effects of Cr and Nb addition on high-temperature tensile properties in austenitic cast steels used for turbo-charger application

Jung, S.,Sohn, S.S.,Jo, Y.H.,Choi, W.M.,Lee, B.J.,Oh, Y.J.,Kim, G.Y.,Jang, S.,Lee, S. Elsevier Sequoia 2016 Materials science & engineering. properties, micro Vol.677 No.-

<P>In automotive turbo-chargers, excellent high-temperature properties are needed for retaining their structures at extremely high temperature of exhausted gas, and thus austenitic cast steels have been actively developed. In this study, high-temperature tensile properties of austenitic cast steels containing different W, Nb, and Cr were interpreted by deformation and fracture mechanisms related with carbides and austenite matrix. Replacement effects of W by Nb or Cr were also investigated because of expensive alloying cost of W. The high-temperature tensile test results revealed that the Cr-added steel containing many M7C3 carbides showed the higher strength than the Nb-added steel containing hard MC carbides. This was because the strength could be enhanced by distributing carbides having slightly lower hardness such as M7C3 carbides in the strengthened matrix as it could hold hard carbides, like in the Cr-added steel. The Cr-added steel (1 wt% W) also showed the higher strength and elongation at 900 degrees C than the basic-composition steel (2 wt% W), which indicated that the partly replacement of W by Cr was successfully achieved. (C) 2016 Elsevier B.V. All rights reserved.</P>

Carbide Formation and Matrix Strengthening by Nb Addition in Austenitic Stainless Cast Steels Used for Turbo-Charger-Housing Materials

Jisung Yoo,Won‑Mi Choi,Byeong‑Joo Lee,Gi‑Yong Kim,Hyungjun Kim,Won‑Doo Choi,Yong‑Jun Oh,Sunghak Lee 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.10

In order to increase the exhaust-gas temperature of austenitic stainless cast steels used for turbo-charger-housings up to1050 °C, the more excellent high-temperature strength should be obtained because it works for keeping a sustainability ofthe housing shape. Three austenitic stainless cast steels were fabricated in this study by adding Nb into the Ni/Mn-replacedconventional ASTM HK40 steel, and their high-temperature strength was investigated in relation with the carbide formationand matrix strengthening. The 1-wt%-Nb-added steel showed the enhancement in high-temperature strength over thenon-Nb-added steel. This indicated that the Nb addition was effective on the strength improvement because it raised boththe fraction of thermally-stable hard MC-type carbides and the austenite-matrix hardness. However, the 1.5-wt%-Nb-addedsteel showed the lower high-temperature strength than the 1-wt%-Nb-added steel, although it contained the more MC carbides. The more MC carbides reduced the matrix hardness, enlarged the hardness difference between carbides and matrix,and deteriorated the high-temperature strength. Our results demonstrate that the strengthened matrix as well as populatedcarbides due to the Nb addition are desirable for further improving the high-temperature strength.

니켈감량 Fe-22Cr-9.5Mn-3.3Ni 오스테나이트강 주조재의 고온 변형성

안상연,송재숙,홍순익 대한금속·재료학회 2017 대한금속·재료학회지 Vol.55 No.7

The hot deformation behavior of as-cast and homogenized Fe-22Cr-9.5Mn-3.3Ni stainless steel was investigated over a temperature range between 600~1,250 ℃. The deformability of the as-cast Fe-22Cr-9.5Mn-3.3Ni steel decreased with increasing temperature up to 850 ℃, and then increased at temperatures between 900 and 1200 ℃ caused by dynamic recrystallization at high temperatures above 900 ℃. At temperatures between 800 and 900 ℃, the discontinuous precipitation of Cr2N and an decrease in ductility were observed in the as-cast steel. The ductility minimum at 850 ℃ in the as-cast Fe-22Cr-9.5Mn-3.3Ni could be associated with cracking at the interface of the lamellar chromium nitrides. The presence of fatal cracking at 1,250 ℃ and the loss of hot ductility can be attributed to the partial melting at austenite grain boundaries at high temperatures.

비선형 초음파를 이용한 주조 오스테나이트 스테인리스강 열취화 손상 평가

이기영,김현명,김진겸,강성식,김종범 한국비파괴검사학회 2023 한국비파괴검사학회지 Vol.43 No.6

가압경수형원자로의 1차 계통의 가동 운전 환경에서 주조 오스테나이트 스테인리스강(Cast austenitic stainless steels, CASS)은 스피노달 분해가 발생하는 온도보다 낮은 온도임에도 불구하고, 가동 운전 환경에서장시간 노출되면서도 심각한 열취화 현상이 발생하고, 이에 따라 파괴인성이 감소하는 등 기계적 특성의 변화가 발생하는 것으로 알려져 있다. 따라서, 주조 스테인리스강의 열취화가 진행됨에 따라 파괴인성과 관련된 재료 물성 변화를 평가하기 위해 비선형 초음파 기법을 적용하였다. 이를 위해, 다양한 온도와 시간으로열처리한 2종류의 주조 오스테나이트 스테인리스강(CF8과 CF8M)을 준비하였고, 이 재료에 대하여 열취화에따른 초음파 비선형 파라미터와 강도의 변화 사이의 상관관계를 분석하였다. In the operational conditions of the primary system of a pressurized water reactor, cast austenitic stainless steels (CASS) experience pronounced thermal embrittlement, persisting even during prolonged exposure to the operational environment at temperatures below the threshold for spinodal decomposition. This phenomenon is recognized for inducing alterations in mechanical properties, notably a reduction in fracture toughness. Consequently, we employed nonlinear ultrasonic techniques to assess shifts in material properties associated with the progression of heat embrittlement in cast stainless steel. To facilitate this investigation, two variants of cast austenitic stainless steels (CF8 and CF8M) were heat-treated at various temperatures and durations. Subsequently, we conducted an analysis to establish correlations between ultrasonic nonlinear parameters and the changes in strength attributable to heat embrittlement.

주조 오스테나이트 스테인리스강 상경계 균열부 산화물 분석

최민재,김성우 한국압력기기공학회 2023 한국압력기기공학회 논문집 Vol.19 No.2

For the phase boundary crack found in the gasket made of cast austenitic stainless steel in the nuclear power plant, the oxide layers were analyzed through SEM and TEM. The results showed that cracks initiated and propagated along the austenite/δ-ferrite phase boundary, the propagation path was changed to penetrate the inside of the phase. The oxide layer located at the periphery of the crack along the phase boundary was identified as a complex multi-layered spinel structure, and Cr-rich carbides were also detected in the oxide. The cracks that propagated inside the austenite matrix were attributed to the presence of high external stresses and impurities.

Effects of V or Cu Addition on High‑Temperature Tensile Properties of High‑Ni‑Containing Austenitic Cast Steels Used for High‑Performance Turbo‑Charger Housings

Jisung Yoo,Won‑Mi Choi,Seok Su Sohn,Byeong‑Joo Lee,Gi‑Yong Kim,Hyungjun Kim,Won‑Doo Choi,Yong‑Jun Oh,Sunghak Lee 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.2

For maintaining high-performance turbo-charger housings at higher exhaust-gas temperatures, e.g., 1050 °C, a high-Ni(20 wt%)-containing austenitic steel (ASTM HK40 steel) has been actively modified. As a promising method, the solidsolutionhardening obtained from the V or Cu addition was utilized in this study. Five austenitic cast steels were made byadding V or Cu in the modified HK40 steel (N16 steel; 4%-Ni replacement by 4.6%-Mn), and the high-temperature propertyimprovement was explained by detailed microstructural evolutions coupled with thermodynamically calculated phase diagrams. The V- or Cu-added steels showed improvements in high-temperature properties over the N16 steel because the additionof V or Cu raised both austenite matrix hardness and volume fraction of M7C3carbide. Considering that only 3.6–4.8vol% of M7C3was present in the five austenitic cast steels, the strengths were affected more by the matrix hardness. Whenthe V or Cu content was quite high, however, the ferrite or Cu-coring is formed, thereby leading to the serious deteriorationof high-temperature properties. The V- or Cu-added steels within proper contents are applicable well to automotive turbochargerhousings requiring or emphasizing high-temperature tensile properties.

Applicability of nonlinear ultrasonic technique to evaluation of thermally aged CF8M cast stainless steel

Kim, Jongbeom,Kim, Jin-Gyum,Kong, Byeongseo,Kim, Kyung-Mo,Jang, Changheui,Kang, Sung-Sik,Jhang, Kyung-Young Korean Nuclear Society 2020 Nuclear Engineering and Technology Vol.52 No.3

Cast austenitic stainless steel (CASS) is used for fabricating different components of the primary reactor coolant system of pressurized water reactors. However, the thermal embrittlement of CASS resulting from long-term operation causes structural safety problems. Ultrasonic testing for flaw detection has been used to assess the thermal embrittlement of CASS; however, the high scattering and attenuation of the ultrasonic wave propagating through CASS make it difficult to accurately quantify the flaw size. In this paper, we present a different approach for evaluating the thermal embrittlement of CASS by assessing changes in the material properties of CASS using a nonlinear ultrasonic technique, which is a potential nondestructive method. For the evaluation, we prepared CF8M specimens that were thermally aged under four different heating conditions. Nonlinear ultrasonic measurements were performed using a contact piezoelectric method to obtain the relative ultrasonic nonlinearity parameter, and a mini-sized tensile test was performed to investigate the correlation of the parameter with material properties. Experimental results showed that the ultrasonic nonlinearity parameter had a correlation with tensile properties such as the tensile strength and elongation. Consequently, we could confirm the applicability of the nonlinear ultrasonic technique to the evaluation of the thermal embrittlement of CASS.

Classification of ultrasonic signals of thermally aged cast austenitic stainless steel (CASS) using machine learning (ML) models

김진겸,장창희,강성식 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.4

Cast austenitic stainless steels (CASSs) are widely used as structural materials in the nuclear industry. Themain drawback of CASSs is the reduction in fracture toughness due to long-term exposure to operatingenvironment. Even though ultrasonic non-destructive testing has been conducted in major nuclearcomponents and pipes, the detection of cracks is difficult due to the scattering and attenuation of ultrasonic waves by the coarse grains and the inhomogeneity of CASS materials. In this study, the ultrasonicsignals measured in thermally aged CASS were discriminated for the first time with the simple ultrasonictechnique (UT) and machine learning (ML) models. Several different ML models, specifically the Knearest neighbors (KNN), Support Vector Machine (SVM), and Multi-Layer Perceptron (MLP) models,were used to classify the ultrasonic signals as thermal aging condition of CASS specimens. We identifiedthat the ML models can predict the category of ultrasonic signals effectively according to the agingcondition