P91 Steel Research Articles

A universal constitutive model for hybrid stress-strain controlled creep-fatigue deformation

On account of stress relaxation, the conventional strain-controlled creep-fatigue interaction (CCFI) tests make it difficult to obtain the creep-dominated data. To make up for this deficiency, a novel hybrid stress-strain controlled creep-fatigue interaction (HCFI) loading which is capable of controlling the ratio between fatigue damage and creep damage is developed. Besides, extensive comparisons of two CFI responses give better understanding of the newly proposed HCFI loading. Experimental results show that the cyclic creep behaviors under the two CFI loading conditions are obviously different. The creep strain for HCFI tests accelerates with cycle number and increases with test parameters significantly, while the creep strain decelerates with cycle number during CCFI loadings and it is not sensitive to dwell conditions. Besides, HCFI tests show a pronounced difference in failure life as the test parameters vary. These different phenomena challenge the current constitutive models, and thus, towards developing a simulation-based design methodology for high temperature components, a universal viscoplastic constitutive model based on Walker model is proposed, in which the self-adaptive hardening parameters and dynamic recovery factors are incorporated to capture the distinct cyclic deformation behaviors during HCFI and CCFI loadings. Good agreements between the experimental and simulated results verify the robustness of the proposed model in capturing various cyclic loadings, including strain-controlled cyclic loading (low cycle fatigue and CCFI), stress-controlled ratcheting-creep loading and HCFI loading, of P92 steel and Inconel 718 at different temperatures. Accordingly, apart from a loading profile able to generate wide ranges of CFI damage, this work also gives a universal constitutive model, which may provide a new solution for existing CFI design from loading waveforms to modeling methods.

Hot dip aluminization of 304L SS and P91 ferritic-martensitic steel – Comparison of interface morphology and growth kinetics of reaction zones

P91 ferritic/martensitic and AISI 304L austenitic stainless steels were hot dip aluminized (HDA) in 99.9% pure aluminum in a pit type vertical furnace at three different temperatures i.e. 750, 800 and 850 °C for 1 to 10 min. Evolution of interface morphology, type and distribution of phases and growth kinetics of the reaction zones were studied using a variety of experimental techniques. Though similar intermetallic compounds formed in both steels, their distribution, morphology and growth kinetics were distinctly different. In P91 F/M steel, aluminized surface adjacent to the substrate had wide Fe2Al5 layer with serrated morphology containing high volume fraction of 2–3 μm sized Cr rich intermetallic phases. Width of FeAl3 phase present adjacent to this layer did not show much variation with HDA conditions. Above FeAl3, on the surface, the pure Al top coat contained nonequilibrium phases. Compared to P91 steel, width of the aluminized layer in 304L SS was less (35 ± 5 μm vs 120 ± 15 μm at 800 °C-10 min) and the Fe2Al5 layer had a planar interface with the substrate. Optimum temperature-time combination to get thin, uniform, defect free surface aluminide layer in P91 F/M and 304L steels were found to be 750 °C-2 min and 800 °C-1 min respectively. Growth kinetics of surface aluminide layers in both steels were studied, kinetic parameters were determined and compared with information available in literature.

Some studies on dissimilar welds joint P92 steel and Inconel 617 alloy for AUSC power plant application

The Cr–Mo steel containing 9% Cr is generally preferred in producing high-temperature operating components (lower than 620 °C) like steam pipes and headers due to their outstanding combination of elevated temperature mechanical properties and creep strength. The components with an operating temperature of more than 620 °C (superheater and reheaters tubes) are mainly produced using stainless steel or Ni-based superalloy. Hence due to economic aspects of boilers, joining of P92 steel with Ni-based superalloy is inevitable in advanced ultra-supercritical (AUSC) plants. Thus the development of joining technology without diminishing the high-temperature mechanical properties of the weldments becomes essential. The present work introduces the microstructure evolution in gas tungsten arc welded (GTAW) dissimilar joint of P92 steel and Inconel 617 alloy produced by using the P92 filler. The microstructure study along weldments was examined using an optical microscope (OM) and field emission scanning electron microscope (FESEM). The mechanical properties of welded joint and joint integrity were determined by using the microhardness measurement, cross-weld tensile tests, and impact toughness test. The effect of the post-weld heat treatment (PWHT) on mechanical properties and microstructure evolution was also performed. The residual stresses were also examined using the blind hole drilling methods, and the effect of PWHT on the nature and magnitude of the residual stresses were also conducted. The microstructure observation showed the martensitic lath structure in the weld metal, which resulted in poor impact toughness (36 ± 5 J) and high hardness (458 ± 25 HV) of weld metal in the as-welded condition. A considerable increase in impact toughness (90 ± 4 J) and a decrease in hardness (371 ± 11 HV) of weld metal were observed after the PWHT. A wide region of the unmixed zone at the interface of weld metal and Inconel 617 was formed due to the difference in chemical composition and structure. The unmixed zone formation led to the poor tensile properties of the welded joint, and failure was observed both in P92 BM and at the interface of weld metal and Inconel 617 interface. The tensile tested sample failed from the interface region and showed the presence of Ti(C, N) particles, while secondary phase M23C6 particles were obtained from the tensile sample fractured from P92 BM. The impact test results also showed the variation in impact toughness along the weldments. The P92 BM was found the strongest zone, while weld metal was noticed as the weakest zone in terms of impact toughness. The variation in residual stresses was also measured along with the thickness of the plate, and the peak magnitude of the residual stresses was measured in the capping pass, which was compressive in nature.

A Physical Simulation of Heat Affected Zone of Creep Resistant Steel: Microstructure and Mechanical Properties Analysis

The structural integrity of ASTM P92 steel welded joints is of concern due to the Type IV cracking associated with the heat-affected zone (HAZ). Two HAZ treatments were done on P92 steel using a Gleeble® 3500 thermo-mechanical simulator at 900°C (intercritical HAZ) and 950°C (fine-grained HAZ). Followed by post-weld heat treatment (PWHT) of the samples in two sets:: a conventional PWHT at 760°C for 2 hours followed by air cooling; or re-austenitisation at 1050°C for 40 minutes then air cooling, followed PWHT. After conventional PWHT, the HAZ simulated at 950°C had the lowest toughness (108J) than the base metal (130J). After the heat treatments, samples that underwent a 900°C HAZ simulation had higher Charpy toughness (improved from 130-208 ± 6J) and lower hardness (decreased from 234.4-222.3HV0.5) than those at 950°C. The microstructures had lath martensite with differing precipitate densities depending on the thermal treatment. Keywords: P92 steel, heat affected zone, post-weld heat treatment, precipitates

Effect of Long-Term Aging on the Microstructural Evolution in a P91 Steel.

The precipitation and growth mechanism of the Laves phase and the coarsening behaviors of Laves phase, M23C6 and MX carbonitrides have been emphatically investigated in P91 steel at 625 °C under different aging conditions. After long-term aging at 625 °C (>1500 h), it was observed that the Laves phase grew rapidly in the region near the M23C6 carbide once precipitated, and further gradually completed the engulfment process until the M23C6 carbide particles disappeared. Furthermore, a new crystallographic orientation relationship between M23C6 carbides and Laves phase has been observed at 625 °C for 5000 h, which is {0001}Laves∥{111}M23C6, <111>Laves∥ <011>M23C6. The coarsening behaviors of Laves phase, M23C6 carbides and MX carbonitrides have been emphatically investigated, conforming to the existing ripening model of multicomponent alloys. The coarsening rates for the Laves phase, M23C6 and MX have values of ~32.2 (≥5000 h), 5.3 and 0.6 nm/h1/3, respectively.

Evolution of Precipitate Phases in Ferritic and Martensitic Steel P92 During Normalizing and Tempering

Evolution of Precipitate Phases in Ferritic and Martensitic Steel P92 During Normalizing and Tempering

Creep rupture ductility of ferritic creep resistant steels

ABSTRACT The creep rupture ductility of not only creep strength enhanced ferritic steels but also conventional 2.25Cr-1Mo steels was investigated, focusing on the remarkable decrease in ductility in the long term. The decrease was well described as a function of yield ratio of applied stress and appeared at about 50% of the 0.2% offset yield stress at the creep test temperature with decrease in the yield ratio. A trade-off relation between creep rupture strength and creep rupture ductility was found for Grade T/P23 steels. The similar trade-off relation between creep strength and ductility was observed also for conventional 2.25Cr-1Mo steels. A remarkable drop in creep rupture ductility of quenched and tempered 2.25Cr-1Mo steels was found in conjunction with a large drop in creep rupture strength. This remarkable drop was considered to be caused by microstructural changes due to the inhomogeneous progress of recovery of tempered martensitic microstructure. A modified heat treatment process with intermediate tempering on partially quenched dual phase microstructure was found to have a beneficial effect on creep rupture ductility for Grade P92 steel. This was considered to be the result of the reduction in residual stress introduced by martensitic phase transformation accompanied with shape change and volume expansion.

THOR®115 pipes in refinery furnaces

Summary Crude fired heater furnaces in refineries are adopting P5 and P9 steel grades, depending on temperature and feed characteristics. Process conditions, however, are characterised by temperatures up to 600°C, well above the conventional creep threshold, which eventually cause a considerable over-tempering of the material. At the same time, crudes being processed have become more‘critical’ in terms of sulphur content and TAN (total acid number). THOR®115 steel grade designed by Tenaris already showed very good corrosion resistance against oxidation and sulphidation. In the present work, THOR®115 and P9 have been analysed and compared, considering their static and creep resistance, together with oxidation and corrosion rate occurring on the external and internal surfaces. The output of the benchmark, resulting in a better performance of THOR®115 rather than P9, has been used to develop an economical comparison between the two materials and quantify the advantage to adopt THOR®115 in furnace applications.

Mapping of Creep Strain in Notched Specimens of Boron-Added P91 Steel

Mapping of Creep Strain in Notched Specimens of Boron-Added P91 Steel

Effect of Cr segregation on creep strength in grade 91 steels

ABSTRACT Several heats of T91 steel and P91 steel were evaluated, focusing on the distribution of segregation and its effect on creep strength. The difference between the maximum and minimum Cr content, ΔCr, at a constant distance (x) was estimated in the range of 1 mm along the wall thickness direction for T91 steels. The standard deviation of ΔCr was employed as an indicator of the degree of segregation. The standard deviation correlated with time to rupture at 650°C when the Cr content is relatively high. However, no clear correlation between them was observed when the Cr content is very low in the range of specification. In addition to Cr segregation, the Cr content itself should be considered as a cause of reduced creep strength when the Cr content is low. For P91 steel, the distribution of segregation was inhomogeneous as compared with T91 steel. For example, a strongly segregated area was confirmed at the position of 3/4 t from the outer surface of the pipe. On the other hand, no segregation band was observed in other areas of the same pipe.

Correlation between microstructure and nanomechanical properties of 9Cr– 1Mo ferritic martensitic steel through instrumented indentation technique

Instrumented indentation technique (IIT) has been used to probe the phase-specific nanomechanical properties of heat treated 9Cr 1Mo ferritic martensitic (P9) steel. P9 steel was subjected to austenitization treatment, followed by water quenching, normalizing, furnace cooling, and subsequently tempering. Quantitative TEM analysis showed systematic changes in microstructure with variation in cooling rate in aforementioned heat treatments. Instrumented indentation tests were carried out at room temperature with a spherical indenter for evaluating hardness and elastic modulus. The yield strength of the steel from the load-displacement curves of cyclic indentation tests has been derived using neural networking method and yield parameter calculations. The properties derived from IIT using these methods were also validated with tensile test results. This work aims at correlating the local mechanical response measured by IIT with microstructural changes due to different heat treatments, which would be useful to extract mechanical properties of materials with heterogeneous microstructure.

Influence of 650 °C Thermal Aging on Microstructure and Creep–Fatigue Behaviors of P92 Steel

Abstract In this paper, P92 steel was subjected to thermal aging treatment at 650 °C for 800 h, and then basic mechanical and creep–fatigue test was performed. The creep–fatigue cycle response trend is consistent before and after aging. Subsequently, microscopic observation shows that P92 steel after aging still has typical lamellar martensite and prior austenite grains. The thermal aging of 650 °C resulted in more precipitates of martensite lath, obvious lath boundary, coarsening of martensite lath, and decreased dislocation density. Furthermore, thermal aging results in the increase of precipitates (Laves phase) and martensite width of P92 steel. The fine Laves phase located on the grain boundary can effectively nail the grain boundary and play the role of precipitation strengthening. Besides, the Laves phase located on the dislocation has the effect of diffusion strengthening, which prevents dislocation slip and improves the creep–fatigue resistance of P92 steel. Finally, four creep–fatigue life model parameters of aging P92 steel were obtained according to the test, including strain range partitioning (SRP), strain energy partitioning (SEP), frequency separation life model (FSL), and strain energy density exhaustion model (SEDE). The prediction results of the four models fall within the double tolerance zone. The SRP and SEP are found to be conservative, while the FSL and SEDE are recommended herein due to their suitability of predicting creep–fatigue life of aging P92 steel.

Unraveling the Complexities of Deformation/Damage Incurred in P91 Steel Weld Joint During Creep–Fatigue Interaction Loading at 873 K

Unraveling the Complexities of Deformation/Damage Incurred in P91 Steel Weld Joint During Creep–Fatigue Interaction Loading at 873 K

In-phase thermomechanical fatigue studies on P92 steel with different hold time

Abstract The effect of hold time with 0, 20, and 40 s on in-phase thermomechanical fatigue (TMF) behavior and life of P92 steel is investigated in this study. TMF tests are carried out under mechanical strain control with strain amplitudes of 0.4 0.4 , 0.6 0.6 , and 0.8 % 0.8\text{\%} , and temperature range of 550–650°C which is closely relevant to the operating condition in power plant. TMF tests are performed in a mechanical strain ratio of R = − 1 R=-1 and cycle time of 120 s. The fatigue life variation follows the sequence of N f 0 s &lt; N f 20 s &lt; N f 40 s {N}_{\text{f}}^{0\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{20\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{40\hspace{.1em}\text{s}} for the same mechanical strain amplitude. In addition, the influence of hold time on fatigue life decreases with the increasing strain amplitude. A continuous softening can be observed from the cyclic stress response under all test conditions. Fractographic and microstructural tests indicate that the fracture surfaces are characterized by a multi-source cracking initiation and an oxidation phenomenon. Furthermore, a modified Ostergren model is used to predict the fatigue life and achieves a good predicted result.

Study on microstructure and mechanical behavior relationship for laser-welded dissimilar joint of P92 martensitic and 304L austenitic steel

A dissimilar laser beam welded joint of ferritic/martensitic P92 steel, and 304L austenitic stainless steel (ASS) has been examined in as-weld and post-weld heat treatment (PWHT) at 760 °C for 120 min for microstructure feature and mechanical aspects. The microstructure of the weld metal, interface, and heat-affected zone (HAZ) was characterized in detail, and it was observed that a great level of inhomogeneity exists in a dissimilar joint. In addition, the effect of the inhomogeneity in microstructure on mechanical properties, microhardness, and impact strength of the welded joint was discussed. The welded joint's mechanical properties met the USC boiler requirement for both as-weld and PWHT situations. The welded joint showed a decent combination of tensile strength and impact strength for both as-weld and PWHT conditions. The tensile results showed the failure from the 304L SS base metal, which ensured the acceptable strength of the weld metal for the laser welded joints. The part of the welded joint with relatively poor impact strength was the weld metal. Though, the Charpy impact toughness of the weld metal (89 J) was measured higher than the ASME standard value (>41 J) and EN 1599:1997 standard value (>47 J) in the as-weld situation. The impact strength of the weld fusion zone was 138 J in PWHT condition.

Understanding the relation between creep-fatigue fracture mechanisms and intergranular dislocation accommodation of a high chromium steel using nanoindentation characterization

Relying on nanoindentation technology, the effect of intergranular dislocation accommodation on fracture behavior for the P92 steel that had experienced creep-fatigue (CF) loading was studied. The results indicate that the fracture mechanism of P92 steel gradually changes from fatigue-dominating to creep-fatigue interactive damage with increasing dwell time. According to the variations of hardness (H), elastic modulus (E), creep resistance and strain rate sensitivity (m), a normalized damage indicator ln(H/E)/m is proposed to characterize the fracture mechanism. Thermally activated process of intergranular dislocation accommodation results in the formation of creep voids at GBs and a remarkable stress relaxation, leading to a creep-fatigue fracture mechanism for the CF specimens with long dwells. However, mechanically driven of dislocation accommodation contributes to enhanced grain size and lower hardness, resulting in a fatigue-dominated fracture mechanism.

Effect of Shot Peening on the Surface Oxidation of P92 Steel in the Steam Environment at 600 °C

Abstract The effect of shot peening on oxidation behavior of P92 steel was investigated under the steam environment of 600 °C. Evolution of microstructure, hardness and residual stress under different air pressure of shot peening were discussed. On the shot peened surface, a stable hematite outer layer was detected by using scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) analysis. The increased oxidation resistance of hematite layer was ascribed to the increased outward diffusion of Cr and the decreased inward diffusion of oxygen. In addition, the role of residual stress in the oxygen diffusion was discussed.

Effect of irradiation temperature on the nanoindentation behavior of P92 steel with thermomechanical treatment

The nanoindentation behavior of P92 steel with thermomechanical treatment under 3.5 MeV Fe13+ ion irradiation at room temperature, 400 and 700 °C was investigated. Pop-in behavior is observed for all the samples with and without irradiation at room temperature, while the temperature dependence of pop-in behavior is only observed in irradiated samples. The average load and penetration depth at the onset of pop-in increase as the irradiation temperature increases, in line with the results of the maximum shear stress. Irradiation induced hardening is exhibited for all irradiated samples, but there is a significant reduction in the hardness of sample irradiated at 700 °C in comparison to the samples irradiated at room temperature and 400 °C. The ratio of hardness to elastic modulus for all samples decreases with increasing penetration depth except for samples at 700 °C. With the increasing of irradiation temperature, the ratio of the irreversible work to the total work gradually decreases. In contrast, it increases for samples without irradiation.

Determination of Location-Specific Solidification Cracking Susceptibility for a Mixed Dissimilar Alloy Processed by Wire-Arc Additive Manufacturing

Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing.

Interfacial characteristics of P91 steel - Inconel 740H bimetallic structure fabricated using wire-arc additive manufacturing

Although bimetallic structures can offer unique solutions to engineering problems with varying functionality, joining two dissimilar alloys using additive manufacturing involves challenges such as cracking and segregation. In this work, bimetallic structures of Inconel 740H superalloy and P91 steel are processed using wire-arc additive manufacturing (WAAM) for the first time. P91 steel is directly deposited over 740H with a cooling time of 20 min in between the two depositions. During microstructure characterization, a large gradient zone with coarse grains spanning for ∼2200 μm is observed. Moreover, the microhardness of the gradient zone is the least in comparison with the P91 and 740H deposits. No deleterious phase formation is found; however, intergranular cracks are observed along the gradient zone. In order to deduce the reason for cracking, non-equilibrium solidification simulations are performed for the mixed compositions of P91 and 740H in the gradient zone. The reasons for crack formation in the gradient zone are the sudden change in the volumetric coefficient of thermal expansion due to the formation of MC carbides as well as the development of local strains from the thermal residual stresses. Therefore, altering the deposition sequence and introducing gradient layers with mixed composition between the P91 and 740H deposits will be beneficial in achieving defect-free builds. Through the integration of experiments and simulation, this work gains valuable insights into the processability of dissimilar alloy manufacturing using the WAAM technique.