JOURNAL OF MATERIALS SCIENCE LETTERS 17 (1998) 1353±1356 Microstructural evolution of modi®ed 9Cr±1Mo steel V. S. DWIVEDI, B. K. JHA Research and Development Centre for Iron and Steel, Steel Authority of India Limited, Ranchi-834002, India It is well known [1, 2] that creep-rupture strength of a material is determined largely by the microstruc- tural stability, and this may include changes in the morphology of starting phases and precipitates, evolution of secondary precipitates and=or replace- ment of old precipitates by new ones, migration of interfaces, formation of cell walls and recovery of dislocation structure. Although the general aging behavior of low-alloy Cr±Mo steels is now well understood, very few studies are available on microstructural changes in modi®ed 9Cr±1Mo steels, particularly under stress condition. The purpose of the present work is to study the nature of microstructure evolution in specimens aged under stress (i.e., under creep conditions) of a 9Cr±1Mo steel (C±0.1, Mn±0.35, Cr±8.5, Mo±0.9, V±0.07, Nb±0.21). An industrial 9Cr±1Mo steel was procured in an as-rolled condition, which was subsequently forged to 30 mm diameter 3 300 mm length blanks. The sample blanks were normalized at 1333 K for 1 h and tempered at 1033 K for 6 h, followed by air cooling. Samples for the aging test were prepared from the above heat-treated blanks and were loaded in creep at temperatures between 873 to 973 K. Microscopy samples for unstressed aging conditions were taken from the thread positions of the corresponding creep load specimen. Scanning elec- tron microscopy (SEM) and transmission electron microscopy (TEM) were employed to study the microstructural details. The scanning picture in Fig. 1a shows that the original normalized and tempered (N  T) samples had a lath structure, which is con®rmed by the transmission micrograph in Fig. 2a. Fig. 1b is the scanning micrograph of the sample aged at 873 K for 15446 h. The identity of the lath structure is still retained with ®ne precipitation of carbides predomi- nantly at lath boundaries. Fig. 1c is from the same sample as 1b but from gauge portion crept at 110 MPa. Appearance of coarse carbide is depicted at some of the lath boundaries that still exist. However, most of the lath structure is now destroyed. It indicates that destruction of the lath structure and coarsening of the carbide is accelerated in the case of aging under stress. Thus, the apparent micro- structural stability of the modi®ed 9Cr±1Mo alloy inferred from normal prolonged aging studies may create misleading expectations about its performance under load at similar time±temperature conditions. 0261-8028 # 1998 Kluwer Academic Publishers Figure 1 Scanning electron micrographs: (a) normalized and tempered sample showing latch structure; (b) sample aged at 773 K for 15446 h, showing precipitation of ®ne carbides at lath boundaries; (c) sample crept at 110 MPa and 773 K for 15446 h, showing precipitation of coarse carbides; and (d) sample crept at 60 MPa and 973 K for 100 h, showing destruction of lath structure. 1353