Carbide Formation and Matrix Strengthening by Nb Addition in Austenitic Stainless Cast Steels Used for Turbo-Charger-Hou
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Carbide Formation and Matrix Strengthening by Nb Addition in Austenitic Stainless Cast Steels Used for Turbo‑Charger‑Housing Materials Jisung Yoo1 · Won‑Mi Choi1 · Byeong‑Joo Lee1 · Gi‑Yong Kim2 · Hyungjun Kim2 · Won‑Doo Choi3 · Yong‑Jun Oh3 · Sunghak Lee1 Received: 17 May 2019 / Accepted: 5 August 2019 © The Korean Institute of Metals and Materials 2019
Abstract In order to increase the exhaust-gas temperature of austenitic stainless cast steels used for turbo-charger-housings up to 1050 °C, the more excellent high-temperature strength should be obtained because it works for keeping a sustainability of the housing shape. Three austenitic stainless cast steels were fabricated in this study by adding Nb into the Ni/Mn-replaced conventional ASTM HK40 steel, and their high-temperature strength was investigated in relation with the carbide formation and matrix strengthening. The 1-wt%-Nb-added steel showed the enhancement in high-temperature strength over the non-Nb-added steel. This indicated that the Nb addition was effective on the strength improvement because it raised both the fraction of thermally-stable hard MC-type carbides and the austenite-matrix hardness. However, the 1.5-wt%-Nb-added steel 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 populated carbides due to the Nb addition are desirable for further improving the high-temperature strength. Keywords Turbo-charger housing · Austenitic stainless cast steel · Nb addition · MC carbide · Matrix strengthening · Hightemperature tensile strength
1 Introduction Austenitic stainless cast steels have been used as turbocharger-housing materials because they have excellent hightemperature properties including thermal-fatigue life and strength for sustaining themselves above very high exhaust gas temperature, e.g., 900 °C. They contain considerably large amounts of Ni, Cr, and W for high austenite stability, corrosion resistance, and carbide formation [1–10]. In
* Sunghak Lee [email protected] 1
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 37673, Republic of Korea
2
Research and Development Center, Key Yang Precision, Gimcheon 39537, Republic of Korea
3
Department of Advanced Materials Engineering, Hanbat National University, Daejeon 34158, Republic of Korea
order to further increase the exhaust gas temperature up to 1050 °C, the more excellent high-temperature strength should be obtained because it plays a role in keeping a sustainability of the housing shape at high temperatures. A high-Ni austenitic stainless steel, ASTM HK40 steel (0.4C–25Cr–20Ni–1Mn–1.2Si-low(P,S) (wt%)) [11–13] which has been widely used in petro-chemical applications, is proposed as a promising turbo-charger-housing material because it ca
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