Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel

The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress.     

Creep fracture of dispersion strengthened low alloy ferritic steels

The kinetics of creep cavitation in ferritic steels has been modelled in terms of a continuum cavity growth process which accounts for the microstructural state of the material. Under conditions of uniaxial and multiaxial stressing it is shown that the presence of precipitate free zones (PFZ's) at the grain boundaries promotes cavity stability and enhances the intergranular cavity growth rate owing to an associated induced triaxiality. Accordingly the number of cavities and their specific rate of growth is shown to be maximised on transverse grain boundaries and increases as the matrix creep strength/PFZ creep strength ratio increases. In addition the model predicts that the multiaxial stress criterion for cavitation and rupture will be dependent upon the particular stress state imposed and the matrix creep strength/PFZ creep strength ratio. The applicability of the model is discussed in the light of observations on creep life and ductility under uniaxial and multiaxial stress. A method for strain assessment and residual life estimation on plant is proposed.     

Effect of aluminum on the stress rupture properties of Cr-Mo-V steels

A comparative evaluation of the stress rupture properties of two commercial Cr-Mo-V steels of similar chemistry and processing history has revealed marked differences in behavior between the two steels. In notch bar tests at 1100°F (593°C), one of the steels was found to be severely notch brittle, whereas the other did not show any tendency to notch failure, even in long time tests. In smooth bar tests, the notch brittle steel was characterized by extremely low values of rupture ductility and slightly enhanced creep and rupture strength compared to the notch ductile steel. In view of the fact that the only appreciable differences between the two steels were in terms of the aluminum content and the amount of the vanadium carbide precipitate, it is suggested that the presence of aluminum in solid solution in Cr-Mo-V type steels markedly reduces the rupture ductility by causing increased precipitation of vanadium carbide.     

Creep strengthening of low carbon grade type 316LN stainless steel by nitrogen

Nitrogen-alloyed 316LN stainless steel is used as a structural material for high temperature fast breeder reactor components. With a view to increase the design life of the components up to 60 years and beyond, studies are being carried out to develop nitrogen alloyed 316LN stainless steel with superior tensile, creep and low cycle fatigue properties. This paper presents the results from studies on the influence of nitrogen on the high temperature creep properties of this material. The influence of nitrogen on the creep behaviour of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt%. Creep tests were carried out at 923 K at stress levels 140, 175, 200 and 225 MPa. Creep rupture strength increased substantially with increase in nitrogen content. The variation of steady state creep rate with stress showed a power law relationship. The power law exponent varied between 6.4 and 13.7 depending upon the nitrogen content. Rupture ductility was generally above 40% at all the test conditions and for all the nitrogen contents. It was observed that the internal creep damage and surface damage decreased with increase in nitrogen content. Fracture mode was found to generally shift from intergranular failure to transgranular failure with increasing nitrogen content.     

Continuum damage mechanics modelling of circumferentially notched tension bars undergoing tertiary creep with physically-based constitutive equations

The paper reports the result of finite element computations, based on Continuum Damage Mechanics (CDM), carried out on circumferentially notched tension bars undergoing tertiary creep and failure. The material constitutive equations are physically based and generalised from thos applicable to polycrystalline nickel-based superalloys. The equations describe the stress level dependence of creep rate using a sinh function and two damage state parameters to model tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. The paper presents values of the computed lifetimes, normalised with respect to plain bar lifetimes at the same average applied stress across the notch throat, and determines their sensitivity to a wide range of material parameters. The parameters include those which determine: the relative strength of the two damage rate mechanisms; the power v, to which the stress-state sensitivity parameter (Σ₁/Σ_e) is raised for damage evolution due to grain boundary cavitation; and, their dependence upon the applied stress levels. The effects of the material parameters on the predicted transition from notch strengthening to notch weakening are rationalised analytically using a stress state modified Skeletal Point stress methodology. The predictive capability of this methodology was found to be extremely good in most circumstances but broke down seriously when the uniaxial ductility was reduced to approximately 1%.     

The effect of stress state on the hydrogen embrittlement of nickel

The ductility of nickel sheet subjected to in situ cathodic hydrogen charging has been investigated over a range of multiaxial stress states including uniaxial, plane-strain, and equibiaxial tension. The data show that the extent of ductility loss due to the presence of hydrogen increases as the stress state tends from uniaxial to equibiaxial tension. In all instances, the hydrogen embrittlement is characterized by intergranular fracture with failure occurring due to microcrack formation, microcrack link-up, and macrocrack growth. The increased susceptibility of nickel to intergranular hydrogen embrittlement with increasing biaxiality of stress state is shown to be a consequence of an enhanced rate of the link-up of strain-induced intergranular microcracks.     

Effect of multiaxial stresses on creep damage of 316 stainless steel weldments

The difference in creep strength between a base metal and a weld metal always produces a multiaxial stress state even if the macroscopic loading is uniaxial. In this study, weldments were formed between wrought 316 stainless steel and two types of 316 weld metals with slightly different creep properties and chemical compositions. Full-size 316 weldments, including base metal, heat-affected zone (HAZ) and welds, were creep tested at 650 °C. The multiaxial stress distributions in full-size 316 weldments were simulated by the finite element method (FEM). Three stress parameters, namely, the maximum principal stress (MPS), the yon Mises effective stress (VMS), and the principal facet stress (PFS), were used to correlate the local multiaxial stresses with local creep damage distributions and failure lifetime. Metallographic examination and creep rupture data showed that the PFS parameter gave the best prediction of the creep damage distribution caused by the multiaxial stresses in 316 weldments. This approach may have application in the design, life prediction, and in-service evaluation of weldments. Performed this work at Ohio State This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” sympsosium as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Experimental and numerical study on the relationship between creep crack growth properties and fracture mechanisms

Evaluation of creep crack growth properties taking microscopic aspects into account is effective for developing more accurate life prediction of structural components. The present study investigated the relationship between creep crack growth properties and microscopic fracture aspects for austenitic alloy 800H and 316 stainless steel. The growth rate of wedge-type intergranular and transgranular creep crack could be characterized by creep ductility. Creep damages formed ahead of the void-type crack tip accelerated the crack growth rate. Based on these experimental results, a three-dimensional finite element method (FEM) code, which simulates creep crack growth, has been developed. The effect of creep ductility on da/dt vs C* relations could be simulated based on the critical strain criteria. The diffusion of vacancies toward crack tip would accelerate the crack growth under creep conditions. The change of vacancy concentration during creep was computed for a three-dimensional compact-type (CT) specimen model by solving the diffusive equation under multiaxial stress field. The experimental results that crack growth was accelerated by creep damages formed ahead of the crack tip could be successfully simulated.     

Influence of Prior Fatigue Cycling on Creep Behavior of Reduced Activation Ferritic-Martensitic Steel

Creep tests were carried out at 823 K (550 °C) and 210 MPa on Reduced Activation Ferritic-Martensitic (RAFM) steel which was subjected to different extents of prior fatigue exposure at 823 K at a strain amplitude of ±0.6 pct to assess the effect of prior fatigue exposure on creep behavior. Extensive cyclic softening that characterized the fatigue damage was found to be immensely deleterious for creep strength of the tempered martensitic steel. Creep rupture life was reduced to 60 pct of that of the virgin steel when the steel was exposed to as low as 1 pct of fatigue life. However, creep life saturated after fatigue exposure of 40 pct. Increase in minimum creep rate and decrease in creep rupture ductility with a saturating trend were observed with prior fatigue exposures. To substantiate these findings, detailed transmission electron microscopy studies were carried out on the steel. With fatigue exposures, extensive recovery of martensitic-lath structure was distinctly observed which supported the cyclic softening behavior that was introduced due to prior fatigue. Consequently, prior fatigue exposures were considered responsible for decrease in creep ductility and associated reduction in the creep rupture strength.     

The effect of grain boundary orientation on creep and rupture of IN-738 and nichrome

Creep rupture specimens taken from directionally solidified ingots of IN-738 and Nichrome in which the grain boundaries were oriented longitudinally, transversely, and 45 deg to the stress axis have been tested over a range of temperature and stress. For both alloys, the ductility was appreciably higher in the longitudinal orientation; but in IN-738, the creep strength was higher in the other two orientations. The net effect on rupture life was small for the superalloy. The nichrome showed much greater scatter which was due partly to inhomogeneous deformation and local recrystallization at the higher temperatures. Because of the recrystallization, even the longitudinal specimens showed intergranular failure for nichrome. The microstructural features of intergranular cracking both internally and on the surface are documented. It is suggested that surface cracking may be an important contributory factor in leading to reduced life with decreased section size which is commonly observed in conventionally cast superalloys.