Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

The influence of adhesive on the Al alloy in laser weld bonding Mg–Al process

Laser weld bonding is a new welding technology, being used to join Mg–Al alloys. The penetration depth of LWB Mg–Al joint was larger than that in simply laser welding joint in same welding parameters. The temperature at the edge of the Al fusion zone in LWB Mg–Al joint was higher than that in laser welding joint, which was measured through the thermal couples. The laser-introduced plasma in LWB Mg–Al process is observed by the high-speed camera, which is different from that in laser welding process. The surface temperature and state of the Al alloy were changed because of the addition of the adhesive, thus the laser absorptive of Al alloy was increased in LWB process, comparing with that in laser welding process; and the decomposition of the adhesive would make a depression in the Al fusion zone, which would be beneficial to the formation of keyhole welding in LWB Mg–Al joint.     

Effects of strain and strain aging on fracture toughness of C–Mn weld metal

Abstract

Specimens of both as-deposited and fine-grained, reheated, manual metal arc C–Mn weld metal have been subjected to various strain and static strain-aging treatments in an attempt to simulate the thermomechanical cycles imposed upon the root region of a multipass weld as subsequent passes are made. The toughness was then measured, as a function of severity of treatment, using a crack opening displacement test. The strain aging treatments are found to lower markedly the cleavage resistance of the as-deposited and reheated microstructures. Non-metallic inclusions within the crack-tip plastic zone are found to be active as cleavage initiation sites in both types of microstructure. While the general shift in toughness can be explained by considering the changes in flow properties brought about by the various treatments, the observed variations in size and distance from the crack tip of the initiating inclusions are thought to be responsible for the associated experimental scatter.

MST/157     

Zone-wise investigation of creep behaviour 9Cr–1Mo steel weld joints

Distinct regions such as weld metal, heat-affected zone (HAZ) and base metal of P9 steel weld joints fabricated by various welding processes were investigated using impression creep testing. Smaller prior austenitic grain size, lower density of precipitates and dislocations resulted in faster recovery and higher creep rate of HAZ in comparison to the weld and base metal. Compared to base metal, shielded metal arc weld (SMAW) and activated tungsten inert gas (A-TIG) weld of the P9 steel weld joints exhibited better resistance to creep and displayed higher activation energy due to their coarser prior austenite grain size. A-TIG HAZ exhibited superior creep properties compared to the SMAW and TIG HAZ due to the presence of higher number density of precipitates.     

Cryogenic stability of retained austenite in Fe–Cr–Ni weld metals obtained by laser welding

Fe–Cr–Ni alloy is the most potential substitute for Ni-based alloys as consumable in low-temperature nickel steel welding. In this study, six groups of Fe–Cr–Ni weld metals with different chemical composition were fabricated by single-pass laser welding. The volume fraction of retained austenite (RA) in the weld metals in as-welded condition increased from 0 to 30.2% with the increment of amounts of alloy elements (Cr, Ni and Mn). The thermal stability of RA was investigated by deep cryogenic treatment (DCT) to see whether sufficient RA can be maintained at low temperature. The results revealed that if the initial content of RA was?<??~?18%, RA would not transform into martensite after DCT. The surrounding martensite can hinder the transformation of RA, which plays a dominant role in the cryogenic stability of RA. The higher the strength/hardness of the surrounding martensite, the stronger the resistance to the transformation of RA. The carbon content of the surrounding martensite is the crucial factor affecting its strength/hardness. However, if the alloy elements amounts were too large, the thermal stability of RA would decrease and some of RA would transform to martensite after DCT. The newly formed martensite (fresh martensite) increased strain concentration of the weld metal, which enhanced the strain energy of martensite transformation and therefore restrained the further transformation of RA. The content of RA in the Fe-13.497Cr-7.249Ni-0.93Mn and Fe-15.548Cr-7.622Ni-0.961Mn weld metals after DCT dropped to?~?18%. The optimum of initial content of RA in the weld metals for low-temperature toughness is?~?18%.

     

Analytical investigation and numerical simulation of the stress–strain state in mechanically heterogeneous welded joints with a single-V butt weld

This paper aims to investigate the stress–strain state in mechanically heterogeneous welded joints with a single-V butt weld by an analytical model along with a numerical simulation. Analytical expressions for the stress–strain state in both the weld and the main material are proposed. In order to verify the proposed expressions, a numerical simulation of the stress–strain state in a mild welded joint with a single-V butt weld was carried out on the basis of the finite element method and the results were compared with the analytical solution obtained applying the proposed analytical model. EP-787 and JONI 13/45А steels were used for the weld while 15X2MFА steel was used for the main material in the analysed welded joints. A comparison of the analytical solution with the finite element analysis results showed a good agreement. The proposed equations could be used in design practice for calculations of the stress–strain state in welded joints.     

On the Correction of the Melting Curve of the Eutectic Co–C for the Effect of the Thermal Inertia of the Furnace

This paper considers the influence of the thermal inertia of the furnace on the shape of the melting curve of the eutectic Co–C. To this end, melting experiments have been performed in a uniform three-zone furnace, with an inherent substantial thermal inertia. The thermal inertia has been quantified by measuring the step-response of the furnace with the sample in its solid state, just below its melting temperature. From the analysis of the effect of the thermal inertia of the furnace, it turned out that during melting the temperature distribution within the furnace, surrounding the crucible, is bound to be in a non-stationary state. This provided the key to properly finalizing the correction to be applied. The shape of the corrected curve differs considerably from that of the curve, as measured, in that the former shows a flatter melting plateau, and a larger curvature on the way down to the solidus point. As regards the liquidus temperature \(T_{\mathrm{liq}}\)—of major interest in the characterization of the transition temperature of high-temperature fixed points—it is demonstrated that the thermal inertia of the furnace shows a kind of self-compensating mechanism. But the effects of the thermal inertia of the furnace on the parameters defining the Scheil fit, involved in the correction procedure, were considerable.     

Properties of coatings of the Al–Cr–Fe–Co–Ni–Cu–V high entropy alloy produced by the magnetron sputtering

It has been found that coatings from an Al–Fe–Co–Ni–Cu–Cr–V high entropy equiatomic alloy produced by the magnetron sputtering have nanocrystalline microstructures, are textured, and present a solid two-phase solution, which crystallizes in the bcc (a = 2.91 Å) and fcc (a = 3.65 Å) phases. The ion bombardment of a growing coating caused by the bias voltage (0–(–200) V), which has been applied to the substrate, decreases the growth rate of a condensate and affects its composition and structure. It has been shown that the composition of coatings deposited without an ion bombardment coincides with the target composition, whereas an increase of the ion bombardment intensity leads to the depletion of the coating composition in Al, Cu, and Ni and increase the microhardness. The anisotropy of the coating produced has been revealed.     

On the beat of the drum: improving the flow shop performance of the Drum–Buffer–Rope scheduling mechanism

One of the main elements of the theory of constraints is its Drum–Buffer–Rope (DBR) scheduling (or release) mechanism that controls the release of jobs to the system. Jobs are not released directly to the shop floor – they are withheld in a backlog and released in accordance with the output rate of the bottleneck (i.e. the drum). The sequence in which jobs are considered for release from the backlog is determined by the schedule of the drum, which also determines in which order jobs are processed or dispatched on the shop floor. In the DBR literature, the focus is on the urgency of jobs and the same procedure is used both for backlog sequencing and dispatching. In this study, we explore the potential of using different combinations of rules for sequencing and dispatching to improve DBR performance. Based on controlled simulation experiments in a pure and general flow shop we demonstrate that, although the original procedure works well in a pure flow shop, it becomes dysfunctional in a general flow shop where job routings vary. Performance can be significantly enhanced by switching from a focus on urgency to a focus on the shortest bottleneck processing time during periods of high load.     

Special features of the diamond crystallization in the Mg–Zn–B–C system

The diamond crystallization in the Mg–Zn–B–C system occurring in the diamond thermal stability region have been considered. The phase transformations, which take place during the preparation of the alloy–solvent for carbon and its structure, the diamond crystallization and properties of the resultant diamond crystals have been studied. The formation of the acceptor centers and inclusions in diamond crystals caused by the addition of boron into the growth system have been considered. It has been found that the use of the diamond powder produced in this system for abrasive machining surfaces of sapphire parts makes it possible to increase the machining efficiency and quality as compared with that of the powder produced in the Ni–Mn–C system.     

On the influence of preloading in the nonlinear viscoelastic–viscoplastic response of carbon–epoxy composites

On an ongoing research for the nonlinear viscoelastic response of composites and polymers, a study of the influence of preloading applied to composite laminates subjected to creep–recovery loading is performed. In cases where high stress levels are applied, this response becomes highly nonlinear and has to be taken into account when designing composite parts. A major problem encountered in the experimental investigation of the nonlinear viscoelastic behaviour is the mode of the initial applied loading and its effect in the overall viscoelastic response of the test sample. The damage that occurs due to the instantaneous application of the load leads to an additional viscoelastic/viscoplastic strain component. In order to investigate this effect as well as to compare different preloading modes, as far as viscoelastic/viscoplastic response is concerned, a test program was initiated and the experimental data were investigated in the current study. A preloading mode is applied in each specimen prior to the creep–recovery testing at different applied stress levels. Useful results concerning the effect of preloading in the time dependent response of the material are concluded. Variation of the values of viscoplastic strain in respect to the preloading mode is also of great concern.     

Influence of the Dispersion Hardening with Nanooxides on the Corrosion Resistance of High-Entropy Alloys of the Cr–Fe–Mn–Ni System in Lead Melts

Materials Science - We study the corrosion resistance of high-entropy alloys of the Cr–Fe–Mn–Ni system in lead melts at 480°С in testing for up to 1000 h. It is shown...     

The improvement of the amorphous environment of the germanate–tellurate glasses in the presence of the gadolinium ions

Glasses in the system xGd₂O₃·(100 − x)[TeO₂·GeO₂] with 0 ≤ x ≤ 50 mol% have been prepared from melt quenching method. In this paper, we investigated changes of the coordination numbers of germanium, tellurium, and gadolinium ions by investigations of FTIR, EPR, and UV–VIS spectroscopy. By analyzing the structural changes resulted from the IR spectra we found that the bending modes of [GeO₄] structural units and the deformed modes of the Te–O–Te linkages produce intercalation of the [GdO _n ] entities in the germanate–tellurate chain network and densification of the glasses by increasing the number of [GeO₆] structural units. EPR spectra of the studied samples reveal that the gadolinium ions play a role of network former. The UV–VIS spectra show broad UV absorption bands located in the 250–350 nm region. Their intensity increase with the increasing of Gd₂O₃ content showing that these stronger transitions can be due to the presence of the O=Ge bonds (n–π* excitations) of [GeO₅] structural units. The [GeO₅] structural units are more stable thermodynamically than their analogues and the [GeO₆] structural units produce the improvement of the amorphous character of these glasses.     

Transition in the creep failure mode of an alloy 800H weld at 850°C

Creep tests were conducted in air and helium on Alloy 800H welded with a high strength filler. Shorter tenn tests produced ductile parent metal failures, but long-tenn samples (>5000 h) failed near the weld with low ductility. Crack distributions suggested that this transition arose because restraint of the parent by the more creep-resistant weld caused triaxial tensile stresses to develop near the weld. At short times, these reduced local creep rates in the parent so that fracture occurred away from the weld. However, given sufficient time, restraint promoted diffusion-assisted cavitation near the weld. Implications for other weldments are discussed.     

Computational analysis of the thermal conductivity of the carbon–carbon composite materials

Experimental data for carbon–carbon constituent materials are combined with a three-dimensional stationary heat-transfer finite element analysis to compute the average transverse and longitudinal thermal conductivities in carbon–carbon composites. Particular attention is given in elucidating the roles of various micro-structural defects such as de-bonded fiber/matrix interfaces, cracks and voids on thermal conductivity in these materials. In addition, the effect of the fiber precursor material is explored by analyzing PAN-based and pitch-based carbon fibers, both in the same type pitch-based carbon matrix. The finite element analysis is carried out at two distinct length scales: (a) a micro scale comparable with the diameter of carbon fibers and (b) a macro scale comparable with the thickness of carbon–carbon composite structures used in the thermal protection systems for space vehicles. The results obtain at room temperature are quite consistent with their experimental counterparts. At high temperatures, the model predicts that the contributions of gas-phase conduction and radiation within the micro-structural defects can significantly increase the transverse thermal conductivity of the carbon–carbon composites.     

Calculation of the Free Energy of Fullerenes in the Gross–Neveu Model

Methods of field theory on Riemannian manifolds were used to study the Gross–Neveu model (incorporating the Hubbard model as a special case) at T 0. The generating functional of the Gross–Neveu model on a torus, S ² _r S ¹ , was obtained by the functional integration method. The model was regularized using the theory of zeta functions. Double sums were calculated using recurrent formulas. For the zeta function of the Dirac operator in the limit of s = 1, we obtained a polar singularity of the form 1/(s – 1), characteristic of the local limit of Green's function. The free energy density was computed as a function of the radius of the sphere r and inverse temperature using the Maple 6 pack. The results show no anomalies, indicating that there are no phase transitions to the principal order in 1/N. However, taking into account the kink–antikink configurations of the scalar field A(x) in calculations without the 1/N expansion may drastically change the phase structure of the model.