[en] The 9 Cr-1 Mo steel exhibits dynamic strain aging akin to substitutional alloys but the activation energy is consistent with carbon diffusion. (orig.)

[en] The use of the modified 9% Cr 1% Mo steel grade (grade 91) for the manufacturing of heavy products (forgings, plates) for pressure vessels is evaluated on the following aspects: (i) Possibility to produce heavy components (base material) with the required properties, (ii) conditions for welding without cracking, and (iii) availability of welding products and determination of their mechanical properties. Heavy plates with thickness ranging from 20 to 300 mm have been manufactured and tested. All mechanical properties (tensile, Charpy V notch impact, creep ..) have been obtained in conformity with the requirements of ASTM specifications and literature data on thinner materials. Some laboratory tests have been carried out to determine the sensitivity of this material to cracking phenomena during welding (cold cracking) or during the postweld heat treatment (reheat cracking). They show that this material does not present major welding difficulties in comparison with more conventional materials used for the manufacture of pressure vessels. To obtain high thoughness properties of the weld metal and low residual stresses, the postweld heat treatment temperature was found to be high (750 to 770 C). A significant modification of the weld metal deposit chemistry was found to be necessary to achieve the convenient level of impact properties, but correlatively, a decrease of its creep resistance may be expected for this very low Nb weld material. (orig.)

[en] The choice of various decomposition mechanisms of austenite in a 9Cr-1 Mo-0.07C steel under different rates of cooling has been studied. The techniques employed were electron probe micro-analysis, X-ray diffraction and electron microscopy. The observed morphological features may be explained based on the predominance of the two types of transformation, austenite → martensite and austenite → ferrite during cooling. In the steel used in this study, decomposition of austenite to proeutectoid ferrite was favoured at cooling rates less than about 2 Ks^-1. The mechanism by which the supersaturated proeutectoid ferrite relieves its excess solute concentration was also studied. A ''microstructural map'' has been proposed to predict the constitution at the end of any given cooling rate for 9Cr-1 Mo-0.07C steel. The choice of commercial treatment has been rationalized with respect to the resultant microstructural constituents. (Author)

[en] Dual phase (martensite + δ-ferrite) microstructures were developed in a modified 9Cr-1Mo steel, by austenitising at 1523-1623 K, followed by water-quenching. These duplex structures were thermally aged at 973 K for ageing periods varying from 30 min to 21 h. Morphological aspects of δ-ferrite phase and its response to age-hardening were studied by optical, scanning electron and transmission electron microscopy, X-ray diffraction, electron probe microanalysis and microhardness testing. It was observed that austenitizing at 1523 K produced fine, acicular δ-ferrite while the δ-ferrite formed by austenitising at higher temperatures (1573-623 K) were massive, irregular-shaped and banded. Moreover the presence of δ-ferrite caused an abnormally strong (110) reflection, observed in X-ray diffraction patterns of martensite plus δ-ferrite structures. This behaviour is thought to be due to development of (110) texture in δ-ferrite phase. Thermal ageing at 973 K caused age-hardening of δ-ferrite with a peak hardness attained after 3.6 ks of ageing. Electron microscopic results suggest that the observed hardening was caused by the formation of Fe₂Mo Laves phase. (orig.)

[en] Based on the studies of electrochemical stability of precipitates in 9Cr-1Mo steels, a method for selective separation and state analysis of Laves phase was established. By use of this method, precipitation behavior of Laves phase and carbides was examined and the effect of Laves phase on the mechanical properties of this steel after long-time aging was studied. The results are summerized as follows. (1) Laves phase extracted from 9Cr-1Mo steel was electrochemically less stable than carbides (M₂₃C₆), and was selectively decomposed and separated from carbides by controlled-potential electrolysis at +0.8 V vs. SCE in 10%acetylacetone-5%LiCl-methanol (secondary electrolysis). (2) The amount of Laves phase can be calculated from the total dissolved amount of Fe, Cr, Mo and Si after the secondary electrolysis. The amount of carbides (M as M₂₃C₆) can be analyzed from the total amount of Fe, Cr, and Mo in insoluble residues after the secondary electrolysis. (3) In the steels aged for 1000 h, Laves phase was found at temperatures between 550 and 625degC, the amount was maximum at 600degC, and increased with aging. (4) Precipitation of Laves phase decreased the Charpy absorbed energy of 9Cr-1Mo steel; when precipitated 0.5 mass% of Laves phase, about 120 J of Charpy absorbed energy decreased. (author)

[en] The size grain of the high temperature phase (austenite) can influence mechanical properties as well as some physical properties of 9Cr-1Mo martensitic steels. Automated image analysis makes it possible to quantify the size of an austenitic grain with respect to the heat treatment conditions of austenitization (time-temperature). The results obtained on a 9Cr-1Mo steel have made it possible to describe the growth kinetics of the austenitic grain of this steel. (authors). 21 refs., 14 figs

[en] A unified constitutive model incorporating dynamic strain aging effect was developed in order to describe inelastic behavior of Mod. 9Cr-1Mo steel adequately. In this constitutive model, an applied stress consists of a backstress, an overstress and an aging stress which shows negative strain rate dependency. Each stress component was measured by strain rate change tests and strain dip tests, and material constants were systematically determined based on these measurements. To evaluate the validity of the constitutive model, it was applied to analysis of inelastic deformation behavior under various loading conditions. It was found out that the constitutive model was successful in describing the inelastic behavior of the steel precisely. (author)

[en] In this paper, constitutive equations are presented for the nonisothermal small strain behavior of modified 9Cr-1Mo steel between 25degC - 600degC. The basic framework uses the superposition of nonlinear kinematic hardening rules as proposed by Chaboche and coworkes. A framework for accounting for isotropic softening is presented wherein this softening can be included in a drag stress, in a yield stress or as a decrease in the saturation level of kinematic hardening or some combination of these; a single isotropic softening equation is used as determined from experiments. The behavior of modified 9Cr-1Mo steel between 25degC - 600degC has been investigated by Swindeman, and Majors et. al. and many important features have been discussed. Here, model parameters are determined from available isothermal uniaxial data and some correlations with isothermal and nonisothermal tests are presented. (J.P.N.)

[en] The provisions of ASME B ampersand PV Code Case N-47 currently include reduction factors for creep strength and fatigue life of weldments. To provide experimental confirmation of such factors for modified 9 Cr-1 Mo steel, tests of tubular specimens were conducted at 538 degree C (1000 degree F). Three creep-rupture specimens with longitudinal welds were tested in tension; and, of three with circumferential welds, two were tested in tension and one in torsion. In each specimen with a circumferential weld, a nonuniform axial distribution of strain was easily visible. The test results were compared to an existing empirical model of creep-rupture life. For the torsion test, the comparison was based on a definition of equivalent normal stress recently adopted in Code Case N-47. Some 27 fatigue specimens, with longitudinal, circumferential, or no welds, were tested under axial or torsional strain control. In specimens with welds, fatigue cracking initiated at fusion lines. In axial tests cracks grew in the circumferential direction, and in torsional tests cracks grew along fusion lines. The test results were compared to empirical models of fatigue life based on two definition of equivalent normal strain range. The results have provided some needed confirmation of the reduction factors for creep strength and fatigue life of modified 9 Cr-1 Mo steel weldments currently under consideration by ASME Code committees. 8 refs., 5 figs

[en] The purpose of this paper is to report on transients in the fatigue crack growth rate which occur on change of either the range of the stress intensity factor, ΔK, or the R ratio, where R is the ratio of the minimum to the maximum stress in a loading cycle. In this study, the authors have made use of a 9Cr-2Mo ferritic alloy which when fatigue tested in vacuum at 538⁰C exhibits no dependency on R. It is also noted that the crack growth characteristics of 2 1/4 Cr-1 Mo and Modified 9Cr-1Mo steels are quite similar to those of the 9Cr-2Mo steel despite differences in monotonic properties. It is further noted that under these test conditions crack closure was not detectable by a crack-mouth LVDT gauge, and that the absence of closure is consistent with lack of sensitivity to the R level. A final point of interest is that the rate of fatigue crack growth under the stated test conditions is independent of the cyclic frequency in the range of 0.3 Hz to 30 Hz, an indication that creep does not play an important role in the propagation process