Reversible temper embrittlement of rotor steels
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I. INTRODUCTION THE demand in recent years for increased power output per turbine set has meant the use of low alloy steel forgings for low pressure turbine and generator rotors with proof stresses up to 900 MN/m2 and weighing as much as 190 tonnes. The defect tolerance of these rotors must be such that they do not fracture catastrophically even at the very high stresses imposed during accidental overspeed. One of the many factors whichinfluence toughness is reversible temper embrittlement, a condition which manifests itself as a loss in toughness when a hardened and tempered low alloy steel is heated for prolonged periods in a critical temperature range below the tempering temperature. McMahonI has summarized the characteristics of this form of temper embrittlement. Since large forgings are normally slow cooled after tempering, through this critical temperature range, they are liable to be temper embrittled. If this embrittlement could be suppressed there wouldbe an improvement in toughness without a change in the mechanical properties. In addition, low pressure turbine discs wouldbe less susceptible to stress corrosion cracking.~ The present work is a study of the susceptibility t o classical reversible temper embrittlement of r o t o r s t e e l s currently used by the Central Electricity Gene r a t i n g Board. Eight different c o m m e r c i a l steels were chosen and the study included the effects of composition, cooling rate from tempering temperature and strength l e v e l and the kinetics of embrittlemerit. Those steels giving c a u s e for concern are indicated and the possibilities for r e m e d i a l action a r e explored. It is emphasized that the conclusions d r a w n from the present work are specific t o the samples tested,
G. M.SPINK is Section Head, CentralElectricityGenerating Board, Scientific Services Department, NorthEastern Region, Harrogate, England, Manuscript submitted March 9, I976. METALLURGICAL TRANSACTIONS A
cast t o cast variations in composition, melting, pouring, forging and heat treatment practice will all contribute t o differences in the susceptibility t o t e m p e r embrittlement of finished components manufactured t o the same m a t e r i a l s specification. 2. EXPERIMENTAL WORK The composition and t h e r m a l history of each m a t e rial is shown in T a b l e I. The d e g r e e of embrittlement was taken as the change in the Charpy F r a c t u r e Appearance Transition Temperature ( F A T T ) from the v a l u e obtained a f t e r water quenching from the temp e r i n g temperature. In the case of the disc steels Charpy specimens were machined t o BS 131:1959 such that t h e i r f r a c t u r e plane was that on which the m a x i mum tensile s t r e s s would operate in s e r v i c e ( i . e . the a x i a l - r a d i a l orientation). The test p i e c e is a r e c t a n g u l a r bar 10 mm x 10 mm x 55 mm long containing a central 60 deg V notch, 2 mm deep with 0.25 mm root radius, cut in one face, having its plane of symmetry perpendicular t o the longitudinal axis of th
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