Effect of test temperature on fracture toughness of modified 9Cr–1Mo steel

Abstract

The quasi-static fracture behaviour (J–R curves) of modified 9Cr–1Mo (P91) steel was studied. The J–R curves were established at 298, 653, 823 and 893 K, and fracture toughness J_0·2 at 0·2 mm of crack extension was determined. The value of ～J_0·2 at 653 K was lower compared to that at 298 K followed by increases in J_0·2 values at 823 and 893 K. The decrease in J_0·2 at 653 K can be attributed to the influence of dynamic strain aging. At 893 K, a significantly higher (more than 200%) J_0·2 was observed, since plastic deformation of the net section, rather than crack growth, occurred in this condition.     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Microstructure and tensile properties of metal injection molding Co–29Cr–6Mo–0.23C alloy

The microstructure and tensile properties of a metal injection molding 0.23%C Co–Cr–Mo alloy (F75 alloy) were investigated. The as-sintered microstructure contains a significant amount of carbides, and is modified by solution annealing, the main effect being to reduce the amount of carbides. Ductility and ultimate tensile strength increase significantly, but yield strength decreases with solution annealing. Aging causes both intragranular and intergranular precipitation, which increases hardness and yield strength but decreases ductility excessively. In both as-sintered and solution-annealed conditions, the material displays noticeable work hardenability. By sintering at 1300 °C and solution annealing at 1220 °C, 440 MPa yield strength and 25% elongation at fracture are obtained.     

Optimization of hot working parameters for thermo-mechanical processing of modified 9Cr–1Mo (P91) steel employing dynamic materials model

Intrinsic workability of modified 9Cr–1Mo steel has been studied in a wide range of temperatures (1123–1373 K) and strain rates (0.001–10 s^?1). Using the experimental data obtained from isothermal hot compression tests, processing map at 0.5 true strain has been developed employing dynamic material model (DMM) approach. The activation energy map has been developed to substantiate the results obtained from processing map and to finalize the optimum processing parameters. Microstructural studies have been carried out to validate the domains of the processing map. The material shows localized deformation bands in the temperature range of 1150–1373 K at strain rates above 1 s^?1 and exhibits abnormally elongated martensite laths at higher temperature (1373 K) and lower strain rates (0.001–0.01 s^?1). The optimum domain for the hot deformation is found to be in the temperature ranges of 1250–1350 K and strain rate ranges 0.015–0.3 s^?1 with a peak efficiency of 38%. In this domain, apparent activation energy is found to be 400 kJ/mol. The microstructure of the specimens deformed in this region exhibits defect free equiaxed grains.     

Degradation in tensile and creep properties of 2.25Cr–1Mo steel by long term service in plant

Abstract

Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 10⁵ h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.     

Characterization of deformation instability in modified 9Cr–1Mo steel during thermo-mechanical processing

In this study, various existing instability criteria were employed to delineate the unstable flow regions in modified 9Cr–1Mo steel during hot deformation. Experimental stress–strain data obtained from isothermal hot compression tests, in a wide range of temperatures (1123–1373 K) and strain rates (10⁻³–10 s⁻¹), were employed to develop instability maps. The domains of these instability maps were validated through detailed microstructural study. It has been observed that Hart’s stability criterion, Jonas’s criterion and Semiatin’s criterion under-predicts the instability regions in the studied temperatures and strain rates regime. Gegel’s and Alexander’s criteria as well as Murty’s metallurgical instability criterion, on the other hand, found to over-predict the instability domains. The instability map developed based on Dynamic Materials Model criterion has been found to precisely predict the instability domains. This instability map revealed four major unstable domains. Microscopic examination in these domains revealed that the instability is manifested in the specimens either as localized deformation band primarily along one of the diagonal or inhomogeneous distribution of martensite lath in the prior austenite grains.     

Fracture characteristics of thermally exposed 9Cr–1Mo steel after tensile and impact testing at room temperature

The paper deals with the fracture analysis of thermally exposed 9Cr–1Mo ferritic steel after tensile and impact testing at room temperature. The temperatures of the thermal expositions were 580, 620, and 650 °C, respectively. The duration of annealing experiments varied from 500 to 5000 h. The influence of thermal expositions on Vickers hardness as well as tensile properties was found to be negligible. On the other hand, remarkable effects of the annealing on room temperature impact toughness were observed. Fracture behaviour of the 9Cr–1Mo steel is strongly affected by the presence of precipitates of secondary phases. Fracture surfaces of tensile samples indicate mainly transgranular dimple fracture mechanism. By contrast, the fracture mode of the samples after impact testing is more complex. It shows both – ductile dimple tearing as well as inter-lath decohesion.     

Hydrogen and temper embrittlement in 9Cr–1Mo steel

Abstract

The synergism between hydrogen embrittlement and temper embrittlement has been investigated in a 9Cr–1Mo martensitic steel. Measurements of tensile ductility were used to monitor the development of embrittlement with increasing hydrogen content in material as tempered and aged for up to 5000 h at 500 or 550°C. A detailed examination was made of associated changes in fracture mechanism, precipitate microstructure, and interfacial and precipitate chemistry. A strong interaction between hydrogen and temper embrittlement was observed. Both types of embrittlement in isolation reduced tensile ductility by promoting a ductile interlath fracture mechanism: ‘chisel fracture’. Hydrogen and temper embrittlement acted synergistically to reduce ductility further by the promotion of brittle intergranular fracture and transgranular cleavage. The dominant factor controlling the interaction was the precipitation of a brittle intermetallic Laves phase containing phosphorus in solution. Phosphorus segregated to interfaces was considered to make an important, but secondary, contribution to the embrittlement observed.

MST/791     

Effect of dynamic plastic deformation on microstructure and annealing behaviour of modified 9Cr−1Mo steel

The effect of dynamic plastic deformation on the microstructure of a modified 9Cr?1Mo steel has been investigated in comparison with the effect of quasi-static compression. It is found that the boundary spacing after dynamic plastic deformation is smaller and the hardness is higher than those after quasi-static compression. The microstructure after dynamic plastic deformation is however less stable than the microstructure after quasi-static compression. Annealing at 675 and 700°C leads to structural coarsening and recrystallisation in each sample, but with recrystallisation occurring faster in the sample annealed after dynamic plastic deformation. The lower thermal stability of the microstructure produced by dynamic plastic deformation is attributed to a higher driving force for recrystallisation in the dynamically deformed material.     

Serrated yielding in 9Cr–1Mo ferritic steel

Abstract

Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300–873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr–1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load–elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol^-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr–1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

Microstructure and compressive properties of NiAl–Cr(Mo)–Dy near eutectic alloy prepared by suction casting

Abstract

The NiAl–Cr(Mo) eutectic alloy doped with Dy was prepared by suction casting technique and its microstructure and mechanical properties were investigated. It is found that with the addition of Dy, the Ni₅Dy phase is formed along the NiAl/Cr(Mo) phase boundary in the intercellular region. By the suction casting method, the microstructure of the alloy get well optimisation which can be characterised by the fine interlamellar spacing, high proportion of eutectic cell area and fine homogeneous distributed Ni₅Dy phase. The compression test results reveal that the room temperature and high temperature mechanical properties of the suction cast alloy improve significantly, compared with the conventionally cast alloy.     

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.     

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Abstract

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R?=?0·1). Hold time was applied for a 10 min hold at the maximum stress (σ_max) and minimum stress (σ_min). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.     

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M₂₃ C₆ (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.     

Effect of modified austemper on tensile properties of 0·52%C steel

Abstract

The effect of a modified austemper on the tensile properties of 0·52%C steel has been studiedfor the purpose of developing the mechanical properties of upper bainitic steel. The modified austempering treatment involved intercritical annealing at 1018 K in the two phase region offerrite (α) and austenite (γ) followed by austempering at 673 K and subsequent water cooling. The results have been compared with those obtained from conventionally austempered steel, and quenched and tempered steel with a similar ultimate tensile stress. The modified austempered steel consisted of a mixed structure of upper bainite and 10 vol.-% ferrite in which ferrite appeared as layers along the rolling direction. The modified austempering treatment wasfound to significantly increase the product of ultimate tensile stress and total elongation, and also the notch tensile stress at 193 K. Conventional austenitising at 1173 K followed by subcritical annealing at 998 K in the two phase region of ex and y, and then austempering at 673 K and subsequent water cooling produced the same mixed structure of upper bainite and 10 vol.-% ferrite. However, this treatment yielded inferior mechanical properties to those obtained with the modified austempering treatment, independent of the test temperature. The results are described and discussed.

MST/3102     

Influence of precipitation and segregation on hydrogen embrittlement in aged 9Cr–1Mo steel

Abstract

A study is reported of temper embrittlement and hydrogen embrittlement in a series of model 9Cr–1Mo steel alloys in which the levels of silicon and phosphorus have been varied to separate the formation of the brittle intermetallic (Laves) phase from the segregation of phosphorus during aging. Phosphorus segregation was mildly detrimental to ductility properties, Laves phase formation was more detrimental, and their effects combined produced the most severe loss in ductility. Hydrogen effects were additive to those of aging. In unaged material without silicon enrichment, only M₂₃C₆ precipitates were detected, with little phosphorus segregation. With silicon enrichment, phosphorus segregation to lath and grain boundaries was enhanced. This enhancement increased the susceptibility of the materials to hydrogen embrittlement, promoting transgranular cleavage and chisel fracture. In aged material, the high phosphorus alloys showed some grain boundary segregation, but only limited interaction with hydrogen. In the high silicon alloys, the formation of Laves phase was most evident. This enhanced hydrogen embrittlement resulted in extensive chisel, transgranular cleavage, and some intergranular fracture. In the high silicon high phosphorus alloy, both Laves phase formation and phosphorus segregation were evident. This resulted in enhanced susceptibility to hydrogen embrittlement, producing intergranular fracture. Thus, silicon controls the susceptibility to hydrogen embrittlement in unaged alloy by promoting phosphorus segregation and in aged alloy by promoting Laves phase formation. In the aged alloy, segregation of phosphorus can enhance the effect of silicon.

MST/1785     

Microstructure and tensile properties of squeeze cast Al–Zn–Mg–Cu alloy

Abstract

It is well known that wrought aluminium alloys have tensile properties superior to those of the cast products. Wrought grade alloys cannot usually be produced by conventional casting processes to attain the same level of tensile properties. However, progress in casting methods in recent years has made it possible to produce wrought alloys by means of squeeze casting techniques. In the present study an Al–Zn–Mg–Cu alloy has been produced by squeeze casting. Tensile properties close to those of wrought products have been achieved by controlling the microstructure, pressure, and other processing parameters.     

Effects of heat input on microstructure and mechanical properties of Fe–2Cr–Mo–0.12C steel

ABSTRACT

Thermal simulated specimens with the heat inputs of 20, 50 and 80?kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at ?40°C) and hardness for the heat input of 50?kJ/cm were 102?J and 346?HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath.

This paper is part of a thematic issue on Nuclear Materials.     

Microstructure and mechanical properties of medium-carbon ferrite–pearlite steel microalloyed with vanadium

Abstract

Microstructural analysis and mechanical testing have been carried out on medium-carbon steels to which additions of vanadium in the range 0·075–0·6 wt-% were made. The steels were either continuously cooled or isothermally heat treated after austenitization. Vanadium carbide precipitation in the proeutectoid ferrite regions of the microstructure and, more unusually, also in the pearlitic ferrite lamellae, were identified by transmission electron microscopy. Moreover, in both ferrite phases the precipitates are aligned in rows, indicative of interphase precipitation at the austenite/ferrite transformation interface. These observations are discussed in terms of the various mechanisms that have been proposed for the interphase precipitation reaction. In the alloys studied the vanadium additions were found to increase the strength of the steels by up to 100%, but to reduce the ductility and notched impact resistance. The most useful combination of increased strength with reasonable ductility and impact toughness was achieved with an addition of 0·15 wt-% V. The vanadium additions contributed to a number of variations in microstructure and therefore in strengthening mechanisms, but the largest effect was the interphase precipitation strengthening of the ferritic phases. The highest strength levels were achieved in fully pearlitic microstructures with the pearlitic ferrite lamellae strengthened by interphase precipitation of the vanadium carbide.

MST/536