Flow Behaviour of Modified 9Cr–1Mo Steel at Elevated Temperatures

In this investigation, the flow behaviour of modified 9Cr–1Mo steel at elevated temperatures is reported. To understand the flow behaviour of the steel, tensile tests were performed at nominal strain rate of 3 × 10^?3 s^?1 and temperatures in the range of 300–823 K. The yield strength and ultimate tensile strength were found to decrease with increase in temperature with a plateau in a intermediate temperature regime (523–673 K). Serrations were also observed in the tensile curve at intermediate temperatures which caused plateau/peak in tensile behaviour of material and was a typical manifestation of dynamic strain ageing. An attempt was made to represent the flow behaviour of the material using different constitutive equations viz., Hollomon, Ludwik, Swift, Ludwigson and Voce. It was observed that the Voce equation could describe the experimental flow curve at different temperatures quite well. Instantaneous work hardening rate with respect to flow stress exhibited two stages of hardening especially at relatively lower temperatures.     

Effect of Thermo-mechanical Treatment on High Temperature Tensile Properties and Ductile–Brittle Transition Behavior of Modified 9Cr-1Mo Steel

Metallurgical and Materials Transactions A - In the present work, a modified 9Cr-1Mo steel is subjected to normalizing and tempering treatment with or without an intermediate rolling, which was...     

Hardfacing of a Modified 9Cr–1Mo Steel Component Using a Nickel-Base Alloy

A dashpot piston made of modified 9Cr–1Mo steel is hardfaced with NiCr-B alloy by the Plasma Transferred Arc (PTA) process. During initial trials, a large number of cracks were observed in the hardface deposit when hardfacing was carried out directly on the modified 9Cr–1Mo steel substrate using a preheat temperature of 723 K. Both the deposit and the martensitic structure formed in the heat affected zone of the substrate during deposition are hard and hence were unable to absorb the thermal stresses generated, leading to cracking. Subsequently, hardfacing trials carried out with an intermediate layer of 2 mm thick Inconel-625 alloy, were successful and deposits were crack-free. Use of a relatively soft Inconel-625 between the hardface deposit and the substrate reduced martensite formation in the substrate, and thus the cracking susceptibility of the deposit.     

Microstructure Evolution During Short Term Creep of 9Cr–0.5Mo–1.8W Steel

P92 steel (9Cr–0.5Mo–1.8W) was subjected to a heat treatment of 1050 °C/30 min/air cooling/780 °C/120 min/air cooling followed by 1080 °C/30 min/air cooling/740 °C/60 min/air cooling to obtain tempered martensite microstructure, for better creep strength. Stress rupture tests carried at 600 °C in the range of 250–350 MPa resulted in rupture times in the range of 200–3000 h. Straight line plot of stress rupture curve indicated no major change in deformation mechanism. Coarsening of precipitates and substructure development were the main reasons for microstructure degradation, consequently leading to reduced hardness of the sample. Gauge and grip portions of the same sample were sectioned to comparatively evaluate the effects of stress and aging. Gauge portion of 3000 h sample showed considerable change in the microstructure in terms of boundary migration, while that of grip portion hardly evolved. The ruptured samples exhibited predominantly ductile fracture with elongated cavities at higher rupture times.     

Initial Stage Oxidation and Surface Morphology of Modified 9Cr–1Mo Steel at High Temperatures in Air

Modified 9Cr–1Mo steel was oxidized in air at 550 and 750 °C for 25, 100, 250 and 500 h and the oxide scales formed were analysed. The surface morphology and the chemical state of the oxide scales were evaluated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The different exposure temperatures and time showed significant variations on the surface morphologies, the nature of oxide scale, and oxide constituents. The energy dispersive X-ray spectroscopic (EDS) analysis revealed the segregation of Mn at 750 °C even for short exposure time. Grazing incidence X-ray diffraction (GIXRD) patterns revealed the scales to be enriched with haematite and less intense magnetite peaks. Detailed XPS characterization indicated the presence of mixed oxides of iron (Fe), chromium (Cr) and manganese (Mn) in the oxide scales. The Fe–Cr spinel in the oxide scale offered resistance to oxidation of the steel, whereas Mn–Cr spinel was deleterious in nature as it promoted cracking and formation of blisters.     

Stability of the Transition Zones in a Steel–Vanadium Alloy–Steel Sandwich after Thermomechanical Treatment

A priority in atomic power today is to develop a new material for fuel-rod casings in fast-neutron reactors. A radiation- and corrosion-resistant three-layer composite based on vanadium alloy and stainless steel has been developed. This composite potentially meets the operational requirements on fuel-rod casings in very challenging operating conditions (high temperatures, radiation, and aggressive media). The performance of this material depends on the quality of the joint between the three layers, which is determined by the preliminary deformation and heat treatment. In the present work, the influence of tempering on the chemical composition, structure, and strength of the joint between the vanadium alloy and steel in the sandwich obtained by hot pressing a three-layer pipe blank at 1100°C is studied. The components of the pipe are 20Kh13 (Russian standard) steel for the external layers and V–4Ti–4Cr vanadium alloy in the core. The structure and chemical composition at the interfaces is investigated by optical and electronic microscopy, with X-ray spectral analysis. The strength of the steel–alloy bond is assessed in compressive tests of an annular three-layer sample with a cut; acoustic-emission measurements are employed. Pressing is found to form a transition zone of thickness 10–15 μm between the vanadium alloy and the steel, which is characterized by diffusional interaction and has a variable chemical composition. This zone consists of a series of solid solutions, without the deposition of brittle phases, and consequently the junction between the layers is strong. No pores, peeling, or defect are observed at the steel–alloy junction. However, in compressive tests of semiannular three-layer samples with a cut after hot pressing, a crack is formed in the steel layer at the tip of the cut. Annealing at 800°C improves the transition zone by increasing the thickness corresponding to diffusional interaction. Consequently, in mechanical tests, the sandwich behaves as a monolithic material, without cracking or peeling between the steel and the vanadium alloy.     

Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels

The influence of weld-simulated heat treatments of 9 to 12 pct steels is evaluated by a fundamental model for creep. The heat-affected microstructure is predicted by considering particle coarsening, particle dissolution, and subgrain coarsening. Particle coarsening is predicted for a multicomponent system, showing significant M₂₃C₆ coarsening in the bcc matrix. Dissolution simulations of MX and M₂₃C₆ are performed by considering a size distribution of particles, indicating that the smallest particles can be dissolved already at relatively low welding temperatures. Recovery in dislocation networks will take place due to the coarser particles. Creep rate modeling is performed based on the heat-affected microstructure, showing strength reduction of weld-simulated material by 12 pct at 1123 K (850 °C) and 30 pct at 1173 K (900 °C). The main cause of this degradation is believed to be the loss of the smallest carbonitrides.     

Retardation of Small Creep–Fatigue Crack in Gr. 91 Steel Through the Combined Effects of Stress Relaxation,Microstructural Evolution,and Oxidation

Metallurgical and Materials Transactions A - This investigation reports an unusual effect of hold time (up to 10 seconds) on retardation in the growth of creep–fatigue small cracks...     

Study of Dissimilar Header Welding Between 2.25Cr–1Mo Steel and 9Cr–1Mo Steel with 9018 B9 Electrode Under Various Conditions of Post Weld Heat Treatment

Headers are an integral part of the power plant equipment which serves as junction for receiving and distribution of fluid. Headers are routinely used in high temperature applications in which various combinations of steels are used to achieve weight and cost savings thus optimising the use of steels. This paper intends in studying the evolution of microhardness and microstructure in a dissimilar header fillet welding between the base materials 2.25Cr–1Mo steel and 9Cr–1Mo steel when welded using 9018 B9 electrode and a constant preheat of 220 °C. The post weld heat treatment (PWHT) is varied at temperatures from room temperature to 770 °C and the soaking duration is kept at 1 h. The changes in microstructure and microhardness are examined with the help of micrographs, electron dispersive spectrum and scanning electron microscopy analysis. As the PWHT temperatures changes, the variation in microstructure and microhardness becomes very much evident which is detailed out in this paper. Also, carbon migration phenomena and its relation with the PWHT temperatures has been studied in this paper.     

Role of Tungsten in the Tempered Martensite Embrittlement of a Modified 9 Pct Cr Steel

The effect of tempering on the mechanical properties and fracture behavior of two 3 pct Co-modified 9 pct Cr steels with 2 and 3 wt pct W was examined. Both steels were ductile in tension tests and tough under impact tests in high-temperature tempered conditions. At T ≤ 923 K (650 °C), the addition of 1 wt pct W led to low toughness and pronounced embrittlement. The 9Cr2W steel was tough after low-temperature tempering up to 723 K (450 °C). At 798 K (525 °C), the decomposition of retained austenite induced the formation of discontinuous and continuous films of M₂₃C₆ carbides along boundaries in the 9Cr2W and the 9Cr3W steels, respectively, which led to tempered martensite embrittlement (TME). In the 9Cr2W steel, the discontinuous boundary films played a role of crack initiation sites, and the absorption energy was 24 J cm^?2. In the 9Cr3W steel, continuous films provided a fracture path along the boundaries of prior austenite grains (PAG) and interlath boundaries in addition that caused the drop of impact energy to 6 J cm^?2. Tempering at 1023 K (750 °C) completely eliminated TME by spheroidization and the growth of M₂₃C₆ carbides, and both steels exhibited high values of adsorbed energy of ≥230 J cm^?2. The addition of 1 wt pct W extended the temperature domain of TME up to 923 K (650 °C) through the formation of W segregations at boundaries that hindered the spheroidization of M₂₃C₆ carbides.     

Microstructure Evolution in 9Cr Martensitic Steel During Long-Term Creep at 650℃

 Standardarized creep and rupture strength tests were conducted for commercial T91 martensitic heat-resistant steel at 650 ℃ and corresponding microstructure was characterized by BSED, TEM and EDS. The martensitic microstructure degenerated seriously during creep exposure, including martensitic substructure recovering, carbides coarsening, dissolving and precipitating. EDS analysis shows that the M23C6 carbides in different morphologies have dissimilar compositions. The rod/sheet like M23C6 particles within the matrix contain more additions, which might precipitate in situ while fine MX particles were re-solving. The high content of silicon in these rod/sheet like M23C6 carbides is probably related to self diffusion coefficient increasing for the exposed condition at 650 ℃ close to Curie temperature Tc. For those reasons, martensite substructure becomes unstable, and microstructure evolution is accelerated and leads to creep strength deteriorating severely.     

Pitting Corrosion Behaviour of Boron Added Modified 9Cr–1Mo Steel: Combined Effects of Alkali and Chloride Ions

Pitting corrosion behaviour of modified 9Cr–1Mo ferritic steel containing 100 ppm of boron was studied in deaerated 0.1, 0.2 and 0.5 M sodium hydroxide solutions containing 0.1, 0.2 and 0.5 M sodium chloride. The chemical compositions of the passive film developed at applied potentials were studied using XPS. E_pit values decreased with increase in chloride concentrations for the given concentration of alkali. The effect (of Cl^?) was less pronounced in higher alkali concentrations. Pit diameters were observed to be significantly higher in 0.1 M sodium hydroxide containing 0.1 M sodium chloride; with increase in alkali concentration pit diameters were found to be restricted to lower values even at higher chloride concentrations. Energy dispersive spectroscopic (EDS) studies showed that pits had nucleated at Al₂O₃ inclusions/matrix interface. XPS analysis revealed the presence of Fe₃O₄, Fe(OH)₃ at lower potentials (150 and 209 mV(SCE)) whereas Fe₂O₃, Fe(OH)₃ and FeO₂ ^2? at higher potentials (400 and 650 mV(SCE)).     

Effect of Heat Treatment on Delayed Fracture Resistance of High Strength Steel 30CrMnSi2NiNb

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Evaluation of Microstructural and Mechanical Behaviours of Tempered 2.25Cr–1Mo Steel Through Electromagnetic Characterization

The effect of tempering treatment has been investigated on water quenched P22 steel with the chemical composition of 0.13C, 0.24Si, 0.47Mn, 0.012P, 0.005S, 2.19Cr, 0.93Mo and balance Fe (all in wt%) within the temperature ranges of 650–900 °C. The microstructural, mechanical and magnetic properties of as-quenched and tempered steels have been investigated through optical and scanning electron microscopy, hardness and universal tensile testing, electromagnetic sensor (Magstar), respectively. The water quenched sample consists of fine martensitic structure with a hardness of 381 HV. With the progress of tempering, the martensite becomes coarse till 800 °C, decreasing the hardness of steel samples. The tempering at 700 °C results in martensite coarsening and precipitation of rod and globular shaped carbides; while a fraction of globular carbide is observed to increase in the matrix after 750 °C of tempering. Beyond 800 °C, the ferrite and bainite phases gradually form by replacing martensite, and the ferrite structure is prevalent after 900 °C. Due to microstructural changes, the magnetic properties are also affected as a function of tempering temperature. The coarsening of martensite causes the decrease in coercivity with increasing tempering temperature, leading to magnetic softening.     

A Comparative Study of Ratcheting Fatigue Behavior of Modified 9Cr–1Mo Steel and Inconel 617 Alloy at Homologous Temperature of 0.42

The modified 9Cr–1Mo steel and Inconel (IN) 617 superalloy are high-temperature alloys of prime importance in several industrial applications. Both of these are used at elevated temperature for various piping and tubing components and undergo asymmetric cyclic loading; therefore, it is highly essential to assess their resistance to accumulation of inelastic strain (ratcheting) under asymmetric stress cycling. In this study, ratcheting behavior of these alloys is compared at the homologous temperature of 0.42, under uniaxial asymmetric stress loading with positive mean stress. Deformation behavior of the tested samples is analyzed by transmission electron microscope. While there is homogeneous deformation in the modified 9Cr–1Mo steel, there is intersection of slip bands and presence of dislocation tangles in the IN 617 alloy. Fracture surface of the modified 9Cr–1Mo steel, tested under asymmetric cyclic loading, reveals typical ductile fracture resulting from tensile loading, whereas there is characteristic fatigue fracture of the IN 617 alloy.