Carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels
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I.
INTRODUCTION
B E T W E E N 1950 and 1970 a new grade of high strength steel was developed for the low pressure rotors and discs of large steam turbines. 1 This NiCrMoV steel had to be of sufficient strength to support the turbines, which at that time were being redesigned to larger sizes, and also had to have sufficient deep-hardenability to provide the proper microstructure in the center of a large forging. One problem which plagued the development of this steel was temper embrittlement. 2 10Temper embrittlement occurs when tempered, low alloy steels are slowly cooled through or isothermally heated in the temperature range of 400 to 575 ~ In this temperature range impurity elements such as phosphorus, tin, and antimony will segregate to the grain boundaries. This segregation weakens the grain boundaries and raises the ductile-to-brittle transition temperature of the steel to the point that low energy intergranular fracture can
N. BANDYOPADHYAY, formerly with the University of Pennsylvania, Department of Materials Science, Philadelphia, PA, is now with the BOC Group, Inc., Group Technical Center, 100 Mountain Avenue, Murray Hill, NJ 07974. C.L. BRIANT and E. L. HALL are with General Electric Company, Research and Development Center-K1, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted August 6, 1984. METALLURGICAL TRANSACTIONS A
often occur at room temperature or above. 11,12This susceptibility to brittle fracture is very undesirable in a rotor material. For example, the Hinckley Point plant failure has been diagnosed as resulting from a small corrosion crack that formed in a keyway and was followed by rapid intergranular fracture through a severely temper embrittled steel.13 The research on temper embrittlement of these rotor steels during their development period clearly showed that phosphorus, tin, antimony, and sulfur could cause intergranular embrittlement.3-8 Of these elements, phosphorus appeared to be the most commonly encountered in commercial forgings. It was also shown that the degree of embrittlement caused by phosphorus probably depended on the total alloying content of the steel and on the microstructure. 5'6'8'14 However, these latter points were not sufficiently developed to provide firm conclusions; nor was the microstructure of these steels after various heat treatments examined in great detail. In recent years two new analytical techniques have become available which have allowed examination of microstructural changes in alloys in more detail. One of these is analytical electron microscopy and the other is Auger electron spectroscopy. The former technique allows one to analyze precipitate chemistry with great accuracy; the latter allows one to analyze elemental segregation to grain boundaries. VOLUME 16A, MAY 1985--721
There have now been studies which have employed one or both of these techniques to study microstructural
changes (i.e., precipitation or segregation) in NiCr ~5-21 and CrMo 22-26 steels and to relate these changes to such processes as embrittlement or corrosion (see
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