Aging embrittlement and grain boundary
- PDF / 568,013 Bytes
- 6 Pages / 597 x 774 pts Page_size
- 14 Downloads / 242 Views
I.
INTRODUCTION
I T is well established that temper embrittlement of NiCrMoV steels is associated with the contents in impurities (mainly P and Sn), the effect of which is reinforced by manganese and silicon. A typical way of assessing the temper embrittlement sensitivity of a NiCrMoV heat is to determine AFATT, the shift of the 50 pct Charpy V fracture appearance transition temperature measured on (1) samples water-cooled after tempering and (2) samples "step-cooled" after tempering (cooling procedure: 540 ~ h + 525 ~ h + 500 ~ h + 475 ~ h + furnace cooling). Step cooling is more severe than cooling down at a rate of 10 ~ to 15 ~ h -i and enables safe comparisons between different materials to be made. Referring to References 1 and 2 makes it possible to appreciate the progress made in 20 years for improving the correlations between AFATT and the chemical composition. The respective influences of embrittling elements began to be quantified in 1972 to 1975. t3'41 A number of correlation formulae have been published since then. The "J factor," [51 J = (pct Mn + pct Si) (pct P + pct Sn) x 104
[1]
greatly helps in comparing different steel heats. The resuits obtained on 35 heats of 3.5NiCrMoV steels 16j allow the approximate relation (AFATT,
~
v=
J
[2]
to be proposed. The improvement of large forging toughness by reducing the contents in P, Sn, As, Sb ("clean" steels) and then in Mn and Si ("superclean" steels) is obviously the consequence of improved steelmaking practice, tT] More than 40 forgings for manufacturing electricity C. LEYMONIE, Chief Engineer, was head of the Testing and Research Center on Materials (CERM) GEC ALSTHOM until his death on September 3, 1991. G. THAUVIN is Head of the Metallurgy Department of this center. G. LORANG, Head of Research, is with the National Center of Scientific Research (CNRS), 94407 Vitry-surSeine, France. Manuscript submitted May 10, 1991. METALLURGICAL TRANSACTIONS A
generator shafts 1800 to 2000 mm in diameter (power: 1700 MVA) have been ordered by our company for about 15 years. The steels used can be considered as "clean," as defined above. Their quality has been progressively improved. As an indication, the chemical composition of the last 400-metric tonne forging delivered (1989) was 0.30 pct Mn, 0.06 pct Si, 0.003 pct S, 0.002 pct P, 0.002 pct Sn, 0.002 pct As, and 0.001 pct Sb. J is then equal to 15, a very low value. With such "clean" steels, the Charpy V FATT at core is less than - 2 0 ~ (impact value at FATT > 8 daJ. cm -2) for a 0.2 pct conventional yield strength of about 600 MPa. If the present situation can be deemed satisfactory as far as generator shafts are concerned, it is probably more difficult when steam turbine low-pressure (LP) rotor shafts are considered. The present maximum temperature of LP shafts is 320 ~ and no service embrittlement was ever observed when using "clean" steels. However, to design future machines, it is very useful to know the limit temperature at which "clean" NiCrMoV steels can operate without too high an embrittlement af
Data Loading...
