Texture development in dual-phase cold-rolled 18 pct Ni maraging steel
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I.
INTRODUCTION
MARAGING steels have been subjected to a great deal of research, in order to improve their mechanical and magnetic properties through thermomechanical processing.[1–9] Both the mechanical and the magnetic properties are important when the material is used as rotor in an hystresis motor that runs at very high speed. Cold deformation prior to aging improves the mechanical strength of the maraging steel. The magnetic properties are also improved by cold rolling as a result of textural changes in the material.[8] The rolling texture in the martensite matrix of maraging steel has been investigated by a number of researchers.[9–12] However, no work has been done on the effect of cold rolling in dualphase maraging steel. The second phase, i.e., the austenite, may be produced by two methods. First, by overaging the steel between 550 7C and 650 7C.[4] The austenite thus produced does not completely transform to the martensite phase by cold working. This retained austenite phase in the martensite matrix is known to deteriorate the mechanical properties of the alloy.[13] Furthermore, the hardening precipitates such as Ni3Ti and Fe2Mo also hinder the rolling process. Second, the austenite phase may be produced by repeatedly heating the samples between room temperature and above austenite finish, Af, temperature.[14] This austenite phase is mechanically unstable and can transform to martensite phase as a result of cold rolling. Furthermore, this process also eliminates the hardening precipitates. The aim of this work was to study the effect of cold rolling on the texture development and microstructure of 18 pct Ni 350-maraging steel containing 52 pct austenite phase in the matrix. II.
EXPERIMENTAL
The material used in the study was 18 pct Ni-350 maraging steel, whose chemical composition is shown in Table I. Samples of dimensions 20 3 10 3 7 mm were cut from the hot-extruded rod and were annealed under vacuum at a temperature of 850 7C for 1 hour. The samples were transferred into a preheated furnace at 800 7C for 5 minutes, Z. AHMAD, Senior Scientific Officer, M. FAROOQUE, Principal Scientific Officer, A. UL HAQ, Director, and A.Q. KHAN, Project Director, are with the Metallurgy Division, Dr. A.Q. Khan Research Laboratory, Rawalpindi, 46000 Pakistan. Manuscript submitted April 8, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
after which they were cooled in air. This process was repeated 10 times consecutively for each sample. Later, a laboratory scale rolling mill was used to reduce the thickness of the samples to 10, 20, 30, 40, 50, 60, 70, 80, and 90 pct in successive steps. Vickers hardness was measured at 20-kg load. The volume percentage of the austenite phase was determined by the X-ray diffraction technique,[15] in which the integrated intensities of (110), (200), and (211) peaks of the martensite phase were compared with the integrated intensities of (111), (200), and (220) peaks of the austenite phase. The X-ray measurements revealed that thermal cycling produced about 52 vol pct austenite (fcc
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