Improvement Of Creep Rupture Strength Research Articles

Microstructure of P92 Steel for Main Steam Pipeline of Ultra Supercritical Units after 50,000 h Service

The microstructure of P92 steel used in the main steam pipeline of ultra-supercritical unit after 50,000 h service was studied by means of metallographic microscope, scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. The results show that the microstructure of P92 steel is still lath martensite after 50,000 hours of service at high temperature, but the martensite recovers. There are many large-sized M23C6 and Laves phases in the structure, and the aging phenomenon is obvious. The compound of bismuth is small in size and difficult to grow during service, which is beneficial to the improvement of creep rupture strength of P92 steel. The initial particle size of M23C6 and Laves phase is small, which is beneficial to improve the high temperature strength of the material. With the extension of service time, the size of M23C6 and Laves phase increases, which promotes the creep cavity and reduces the creep rupture strength of the material.

Microstructural Studies in Understanding the Role of Metallurgical Parameters on Creep Behaviour of Austenitic Alloys

Studies on creep behaviour of austenitic steels as a function of metallurgical variables like: (a) extent of cold work (b) trace alloying elements (c) choice of joining method indicate the need to understand their role through microstructural studies. As the materials evolve during creep exposure in terms of defect structure and precipitation behaviour, extensive electron microscopy studies were carried out to understand the influence of the above variables. It was observed that cold work shows an increase in creep life as it is varied from 10 to 24 % due to the enhanced prior defect structure providing a complex tangled dislocation structure when the material undergoes creep deformation. The addition of Ti results in enhanced nanoscale precipitation of TiC which was found to influence the creep strength up to 0.23 % of Ti. Employing modified TIG joining over the conventional TIG process it has been found less creep cavitation in the former joint which led to the improvement in creep rupture strength. The paper would discuss in detail of the microchemical variations, microstructural evolution in terms of defect structure and precipitation behaviour on the creep life.

Creep rupture strength of activated-TIG welded 316L(N) stainless steel

316L(N) stainless steel plates were joined using activated-tungsten inert gas (A-TIG) welding and conventional TIG welding process. Creep rupture behavior of 316L(N) base metal, and weld joints made by A-TIG and conventional TIG welding process were investigated at 923 K over a stress range of 160–280 MPa. Creep test results showed that the enhancement in creep rupture strength of weld joint fabricated by A-TIG welding process over conventional TIG welding process. Both the weld joints fractured in the weld metal. Microstructural observation showed lower δ-ferrite content, alignment of columnar grain with δ-ferrite along applied stress direction and less strength disparity between columnar and equiaxed grains of weld metal in A-TIG joint than in MP-TIG joint. These had been attributed to initiate less creep cavitation in weld metal of A-TIG joint leading to improvement in creep rupture strength.

Improvement of creep rupture strength of a liquid metal cooling directionally solidified nickel-base superalloy by carbides

The creep rupture strength of a liquid metal cooling directionally solidified nickel-base superalloy was improved by an inexpensive and effective way—carbide strengthening. The higher cooling efficiency of liquid metal cooling process results in finer MC eutectic carbides. The discrete distributed MC carbides derived from the decomposition as well as the undecomposed MC eutectic carbides were the main contributors to the improvement of the creep rupture strength especially in the case of the high temperature and low stress.

Suppression of Type IV fracture in welded joints of advanced ferritic power plant steels – effect of boron and nitrogen

The effect of boron and nitrogen on the grain refinement in HAZ during heating and on the Type IV fracture in welded joints has been investigated for P92 and 9Cr–3W–3Co–VNb steel containing boron and or nitrogen. Creep tests were carried out at 650°C (923 K) for up to about 3 × 104 h for base metals, simulated HAZ specimens and welded joints. The addition of boron of 47 to 139 ppm to 9Cr – 3W–3Co–VNb steel containing minimized nitrogen as low as 10–20 ppm suppresses the grain refinement in HAZ and hence suppresses the Type IV fracture in welded joints at low stresses. The addition of excess nitrogen of 0.06% causes the grain refinement during heating at around Ac3 temperature, in both conditions of no boron and high boron of 130 ppm. A combination of high boron of 130 ppm and low nitrogen of 80 ppm causes no grain refinement in HAZ during heating at around Ac3 temperature as well as significant improvement of creep rupture strength of base metal by fine MX nitrides without any formation of boron nitrides.

Advances in Physical Metallurgy and Processing of Steels. Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel.

The creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure are critically reviewed on the basis of experimental data. Special attention is directed to the following three subjects: creep mechanism of the ferritic steel, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use. The high Cr ferritic steel is characterized by its fine subgrain structure with a high density of free dislocations within the subgrains. The dislocation substructure is the most densely distributed obstacle to dislocation motion in the steel. Its recovery controls creep rate and rupture life at elevated temperatures. Improvement of creep strength of the steel requires a fine subgrain structure with a high density of free dislocations. A sufficient number of pinning particles (MX particles in subgrain interior and M 23 C 6 particles on sub-boundaries) are necessary to cancel a large driving force for recovery due to the high dislocation density. Coarsening and agglomeration of the pinning particles have to be delayed by an appropriate alloy design of the steel. Creep rupture strength of the high Cr ferritic steel decreases quickly after long-term use. A significant improvement of creep rupture strength can be achieved if we can prevent the loss of rupture strength. In the steel tempered at high temperature, enhanced recovery of the subgrain structure along grain boundaries is the cause of the premature failure and the consequent loss of rupture strength. However, the scenario is not always applicable. Further studies are needed to solve this important problem of high Cr ferritic steel. MX particles are necessary to retain a fine subgrain structure and to achieve the excellent creep strength of the high Cr ferritic steel. Strengthening mechanism of the MX particles is another important problem left unsolved.

酸化物分散強化型耐熱超合金のパルス通電焼結接合継手の機械的特性

In order to evaluate the mechanical properties of ODS superalloy joints bonded by pulsed electric-current sintering (PECS) technique, the tensile test at elevated temperature and creep rupture test were conducted for MA956 (Fe-based ODS alloy) and MA754 (Ni-based ODS alloy) joints. The ODS superalloys were bonded at 1323-1373 K for 0.9-64.8 ks in vacuum. The tensile properties at elevated temperature of completely densificated joints were almost comparable to those of base metal. However, the creep rupture lives of joints could be improved by the further holding at bonding temperature after the fully densification. The creep rupture lives of joint bonded for 64.8 ks reached about 70% of longitudinal directional life of base metal. TEM observations revealed that Cr2O3 inclusions existed on the prior particle boundary (PPB), and percentages of Cr2O3 inclusions on PPB were decreased with increasing the bonding time. The strengthen particles of oxide(Y2O3) were uniformly dispersed in the whole area of bonded interlayer without coagulating and coarsening. These results suggested that the improvement in creep rupture strength with bonding time was mainly attributed to the amelioration of the bonding strength of PPB.

Improvement of Creep Rupture Strength of High Cr Ferritic Steel by Addition of W.

The effect of W addition on creep rupture strength (CRS) of high Cr ferritic steel was evaluated with three kinds of steels; base steel of 9Cr-1Mo-V-Nb-N, 0.7%W addition to base steel, and 1.7%W addition with decreased No addition to 0.5%. The latter two steels have the same values of Mo+0.5W viz. Mo equivalent. Further, the reasons for the beneficial effect of W was analyzed. The CRS at 600°C for 1000 h increases with W addition at a rate of 35 MPa/%W. As the hardness remains almost constant during the creep test, the enhancement of CRS could not be attributed to the formation of precipitate or cluster during the creep test. The W addition was found to increase the partition of Nb to VN, resulting in the lattice expansion of VN and enhancing the CRS through coherency strain around VN. This effect accounts for about half of the increase of CRS by W addition.The W addition also causes the precipitation of a film-like Laves phase along the subgrain boundary. Inconsequently, Cr2C precipitates simultaneously. The Laves phase contributes to the other half of the CRS increase, possibly through suppressing the growth of subgrain during the creep test.

Improvement of Creep Rupture Strength of 9Cr-1Mo-V-Nb-N Steel by Thermo-Mechanical Control Process.

A 9Cr-1Mo-V-Nb-N steel was subjected to the thermo-mechanical control process (TMCP), more specifically the direct quenching and tempering process, and the influences of heating temperature (Ts) and finish-rolling temperature (Tf) on the mechanical properties, including the creep rupture strength (CRS), were examined. The results were analyzed by observation of the substructure and precipitates, then the reasons for the improvement in CRS were discussed.Raising Ts and lowering Tf improve CRS for separate reasons. Raising Ts augments the coherency strain around VN through lattice expansion owing to the increased solution of Nb to VN. This increase in coherency strain is the reason for the improvement of CRS. On the other hand, lowering Tf disperses VN more finely by serving dislocations as nucleation sites for VN, resulting in improved CRS through decreased interprecipitate distance.When TMCP is applied, thin disc-like (V, Nb)N which nucleates on dislocations becomes a dominant type of VN precipitation, instead of NbN/VN complex precipitate which prevails when the steel is normalized and tempered. Furthermore, successive rolling from sufficiently high temperature is important to avoid coarse precipitation of VN and to exert the precipitation hardening during tempering to its full extent.

Improvement of creep rupture strength of welded joints in 9Cr Mo V Nb steels (ASTM A387-Gr.91)

Improvement of creep rupture strength of welded joints in 9Cr Mo V Nb steels (ASTM A387-Gr.91)

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.

高Cr-Niオーステナイト鋼の高温強度

The present study was made to examine high temperature strength of high Cr-Ni austenitic steels. Creep rupture tests and short time tensile tests at 650, 800 and 1000°C were conducted for AISI type 309 (25Cr-12Ni), 310 (25Cr-20Ni) and 330 (15Cr-35Ni) specimens to determine the effect of alloying elements, solution treatment and micro-structure on high temperature strength.The results obtained are summarized as follows:(1) Differences of tensile properties and creep rupture strengths between those of three grades of austenitic steels tested are not clear. But grade 309 possesses the highest rupture strength for 1000h at 800°C, while grade 310 has the lowest strength, and the latter shows the highest elongation at any temperature.(2) As to the effect of alloying elements, C and N, which are interstitial atoms, have a remarkable effect for the improvement of high temperature strength, while substitutional atoms such as Cr, Si and Ni are only slightly effective for creep rupture strength.(3) Because of the transformations of ferrite into sigma phase during test, its very effect is not clear at 650 and 800°C tests, while detrimental effect is observed at 1000°C where the transformation does not occur. Elongation at rupture of specimens containing ferrite at 1000°C is higher than that of fully austenitic specimen.(4) Specimens containing 0.16% or more of C show a clear grain growth when solution treatment temperature is increased from 1000 to 1300°C while low C specimens do not show further grain growth over 1100°C. On the tensile properties at room temperature, the higher the solution temperature is, the lower is the strength and the higher ductility. But both of strength and ductility in short time tensile test at 650 and 800°C decrease with the increase of treatment temperature and remarkable embrittlement appears at 800°C.Creep rupture strengths of higher C steels are improved as solution treatment temperature raises up to 1200°C. Lower C specimens do not show definite improvement of creep rupture strength, while elongation decreases remarkably. In this respect, creep rupture strengths are influenced more by behavior of C and N than by grain size.(5) Type of fracture is transgranular at room temperature test and intergranular at 650°C and above, independent of steel grades and heat treatment temperatures.