Oxidation and nitridation of alloy 800 H at a growing creep crack and for unstressed samples
- PDF / 1,849,853 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 204 Views
I.
INTRODUCTION
E X P O S U R E of Alloy 800 H to oxidizing atmospheres at temperatures around 1000 ~ leads to marked internal oxidation of A1 preferentially along grain boundaries. It~ It is supposed that surface creep cracks, generated by simultaneous mechanical stresses, are initiated at these alumina precipitates and also that further growth of the creep cracks may be assisted by this kind of internal grain boundary oxidation. 12,3m In air, besides internal oxidation, massive nitridation is observed in the vicinity of the surface creep cracks, lsl A marked change of the microstructure in the form of internal nitridation should also have an effect on the mechanical properties of the near surface regions, and it cannot be excluded that an influence on creep crack growth exists. Since there has been quite a number of open questions concerning the interaction of internal oxidation/internal nitridation and creep crack growth in Alloy 800 H at high temperatures and the role of the aluminum content of the alloy, the present work was initiated. In this paper, the results of investigations on the mechanisms and the course of oxidation and internal nitridation at a growing creep crack and (for comparison) in nondeformed specimens at 1000 ~ are reported. II.
EXPERIMENTAL
In order to investigate the influence of the A1 concentration on internal oxidation and nitridation, three batches of Alloy 800 H were prepared with A1 contents ranging from 0.02 wt pct to 0.34 wt pct, the latter being in the range of the commercial version. The compositions of M. WELKER, formerly with DECHEMA, is with Leonische Drahtwerke AG, Roth, Federal Republic of Germany. A. RAHMEL and M. SCH(YrZE are with DECHEMA, Postfach 97 01 46, Frankfurt, Federal Republic of Germany. Manuscript submitted July 8, 1988. METALLURGICAL TRANSACTIONS A
the three batches are given in Table I. This investigation concentrated on the batches with the lowest and the highest A1 concentration, i.e., batches 1 and 3. The material was solution annealed at 1150 ~ for one hour and then quenched in water. Grain sizes corresponded to ASTM classes 4 and 5. Two types of specimens were used for the investigations. Corrosion tests in the absence of applied mechanical stresses were run with cubes of 12-mm side length. The cube surfaces were polished with a 1200-grit paper. For investigations of corrosion at a growing creep crack, compact tension (CT) specimens according to ASTM E 813 were used I61 (Figure 1). In order to provide a state of approximately plane strain and to guide the growing crack perpendicular to the crack opening direction, side grooves of a depth of 0.125 Bo were introduced. The starter crack was prepared by a saw cut which was extended by fatigue at room temperature to a length of ao/W = 0.55 to 0.65 before crack growth tests at high temperature (ao = initial crack length before the test, W = 25.4 m m , and Bo = 12.7 mm). Corrosion tests without applied mechanical stresses were performed at 1000 ~ in air and N z / 5 pct H2 for up to 1000 hours. For the invest
Data Loading...
