ER2594 超級雙相不鏽鋼之顯微組織演化特性研究

Abstract

本實驗所使用的材料為 ER2594 ( Fe-25Cr-8Ni-4Mo, wt.%) 超級雙相不鏽鋼，其具有優異的機械強度與抗腐蝕性能，因此被廣泛應用於沿海石化工業中。然而，為提升抗點蝕能力所添加的高含量鉻、鉬、鎢、氮等元素，易在熱處理與加工過程中形成二次析出物，導致材料性質劣化，甚至產生裂紋。。其中，σ相對性質影響最劇烈，為避免 σ 相析出，深入了解其生成與演化機制至關重要。因此，本研究透過原材（鑄錠）、均質化處理及恆溫相變熱處理等不同階段，探討σ相的生成與演化行為。 本研究在鑄錠的晶相觀察中，於三個不同區域（中心、0.5倍半徑、表面）取樣進行顯微組織的觀察，然而後續關於方位關係的分析以及熱處理，則聚焦於 0.5半徑位置來進行後續分析。均質化熱處理是使用高溫爐在 1250 °C 持溫 24 小時後水淬，以消除二次析出物；恆溫相變熱處理則是使用熱膨脹儀進行精準控溫與控時。所使用的分析方法包括OM、SEM-BSE、SEM-EBSD、IPF、Pole figure、TEM，針對析出物的成分、結構與和基地之間的方位關係進行分析。 在鑄錠結構中，透過晶相觀察可發現中心的最後凝固區域有大量σ相析出，利用Thermo-Cal模擬凝固流程以及液相在不同溫度下的成分變化可知在最後凝固區域由於鉻、鉬元素的偏析，所以有利於σ相析出。透過EBSD-phase mapping可知，σ 相與 γ₂ 是由肥粒鐵相分解而來；後續透過IPF以及Pole figure進行 δ、σ、γ₂ 三相之間的方位關係分析，發現 σ 相的析出機制可分成兩種：(1)以先生成的δ/γ2相界為成核點生長；(2)共析反應(δσ+γ2)，利用機制(1) δ/γ2呈現K-S或N-W的方位關係，而機制(2)中δ/γ2不具有K-S或N-W的方位關係來進行區分。 在均質化水淬快速冷卻後原本預期為完美雙相結構，但在TEM觀察中發現肥粒鐵相中會有細小的CrN析出，和基地具有B-N的方位關係且具有3種不同變體。此外，由於析出物和基地之間的晶格不匹配所產生失配差排，所以也有觀察到析出物附近有大量差排產生。 在恆溫相變中聚焦在σ相鼻端溫度不同持溫時間來觀察，發現χ相會先於σ相生長並逐漸轉換成σ相，且在持溫10分鐘時觀察到σ相是以共析反應生成，然而，持溫20分鐘時由於大量γ2生成，所以σ相主要是以δ/γ2相界為成核點生

Super duplex stainless steel ER2594 (Fe-25Cr-8Ni-4Mo, wt.%) possesses excellent mechanical strength and corrosion resistance. Owing to these superior properties, it is widely used in coastal petrochemical industries. However, to enhance pitting resistance, high amounts of chromium, molybdenum, tungsten, and nitrogen are added, which tend to promote the formation of secondary precipitates during heat treatment and processing. These precipitates can deteriorate the material properties and even lead to crack formation. Among them, the σ phase has the most detrimental effect. To prevent σ phase formation, a thorough understanding of its formation and evolution mechanisms is essential. Therefore, this study investigates the formation and evolution behavior of the σ phase through analyses conducted at three stages, including the as-cast ingot structure, homogenized structure, and isothermal heat treatment. In this study, samples were taken from three different regions of the ingot—center, half-radius, and surface—for optical microscopy (OM) analysis. However, subsequent analyses of orientation relationships and heat treatment effects focused on the half-radius region. Homogenization was conducted in a high-temperature furnace at 1250 °C for 24 h, followed by water quenching to eliminate secondary precipitates. The characterization techniques employed include optical microscopy (OM), scanning electron microscopy with backscattered electrons (SEM-BSE), electron backscatter diffraction (SEM-EBSD), inverse pole figure (IPF) mapping, pole figure analysis, and transmission electron microscopy (TEM). In the ingot structure, a significant amount of σ phase was observed in the center region of the ingot, which corresponds to the final solidification zone. Thermo-Calc simulations of the solidification sequence and compositional variations of the liquid phase at different temperatures indicated that segregation of chromium and molybdenum in the final solidification zone promotes σ phase precipitation. EBSD phase mapping revealed that both the σ phase and γ₂ phase originated from the decomposition of the δ-ferrite phase. Further analysis using IPF maps and pole figures showed that the orientation relationships among δ, σ, and γ₂ phases suggest two distinct precipitation mechanisms for the σ phase: (i) nucleation and growth at the δ/γ₂ interface, and (ii) eutectoid reaction (δ → σ + γ₂). In the former case, the orientation relationship between δ and γ₂ follows the Kurdjumov–Sachs (K–S) or Nishiyama–Wasserman (N–W) relationship. In the latter case, the orientation relationship between δ and γ₂ does not follow the K–S or N–W relationship. After homogenization and water quenching, a fully duplex microstructure was expected; however, TEM observations revealed fine CrN precipitates within the ferrite matrix, exhibiting a Baker–Nutting (B–N) orientation relationship, with three CrN variants identified. In addition, significant dislocation density was observed around the precipitates due to lattice mismatch with the matrix. During isothermal transformation, the study focused on various holding times near the nose temperature for σ-phase formation. It was observed that χ phase formed prior to the σ phase and gradually transformed into σ phase. After 10 minutes of holding, the σ phase was primarily formed via eutectoid reaction, whereas after 20 minutes, due to substantial formation of γ₂, the σ phase predominantly nucleated and grew at δ/γ₂ interfaces.