Effect of inadequate heat treatment on creep strength of 12Cr–Mo–V steel

Abstract

Solution treatment of 12Cr–Mo–V steels below the specified temperature range leads to the development of spheroidized microstructures with dramatically reduced creep resistance. This is known to have resulted in the premature service failures of superheater tubing. Compositional and mechanical property checks currently specified in the relevant standards may not be sufficient to reveal deficiencies. Steels with Cr–Ni equivalents at the uppermost extreme of the range possible within the compositional limits of the tube steel standards show enhanced susceptibility to both δ– and α–ferrite formation. The significance of this is discussed in relation to creep strength, with particular reference to the differences between the effects of α- and δ-ferrite.

MST/147     

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.     

Intergranular cavity damage and creep fracture of 1Cr–0·5Mo steels

Abstract

The factors controlling the intergranular fracture of three 1Cr–0·5Mo steels, tested at 550°C, have been examined. Failure results from the nucleation and growth of grain-boundary cavities. It is shown that creep life is dependent on the maximum principal stress, and that variations in the rupture properties of the steels are controlled by their susceptibility to nucleate intergranular cavities. Increasing the metalloid element content and, in particular, increasing the austenitizing temperature from 930 to 1300°C resulted in an increase in the cavity nucleation rate and a concomitant decrease in the rupture life. The cavity nucleation rate was found to be dependent on the maximum principal stress and when this dependence is used in conjunction with a simple cavity diffusion growth model the stress-state dependence of rupture life and the effect of residuals and austenitizing temperature on fracture properties could be predicted. These results are discussed in terms of the material and fabrication factors and service conditions that designers and operators of high-temperature plants must consider so that the plant may be operated safely and efficiently.

MST/81     

Influence of microstructure on hydrogen embrittlement behaviour of 2·25Cr–1 Mo steel

Abstract

The hydrogen embrittlement of Cr–Mo steel has been studied. The effects of tempering temperature on the hydrogen diffusivity and hydrogen assisted cracking, without external stress, are investigated. Hydrogen permeation and trapping, for the various microstructural conditions, were measured using electrochemical equipment. The microprecipitate distribution in the steel was observed using transmission electron microscopy. The steel, which was heat treated to give a variety of microstructures, was cathodically hydrogen charged and the critical microstructural sites for hydrogen induced cracking examined. Cracks initiated by the charging treatments were found to nucleate at MnS interfaces. Hydrogen diffusivity and trapping are strongly dependent on the tempering parameters.

MST/1947     

Combined effects of post-weld heat treatment and aging on alloy 800/2·25Cr–1Mo steel joint

Abstract

The combined effects of post-weld heat treatment (PWHT) and aging on the interfacial microstructure and tensile properties of alloy 800/2·25Cr–1Mo steel transverse weld specimens, welded using Inconel 182, were studied to determine the optimum PWHT temperature for the joint. In the present study, the joints were subjected to PWHT for 1 h at 948, 973, 998, and 1023 K, followed by aging at 873 K for 100–5000 h. The aging treatment at 873 K is intended to provide an accelerated simulation of service exposure at 773 K. The results of the present work show that the optimum PWHT temperature for the joint investigated is 973 K.

MST/1474     

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.     

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.     

Effect of W–Mo balance on long-term creep life of 9Cr steel

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture t_r exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in t_r with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe₂(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening.     

Decarburization in 0·5Cr–0·5Mo–0·25V steel and its effects on creep and microstructure

Abstract

Many high–temperature creep tests are performed on low–chromium, ferritic steels in an uncontrolled atmosphere. Examination of creep rupture specimens of 0·5Cr–0·5Mo–0·25V steel tested in air has shown that decarburization accompanies oxidation and is an important factor in accelerating the failure of creep tests in air. Similarly, pre-aging in air reduces the creep life more than pre-aging in a capsule. There is also evidence that decarburization is accelerated during straining. Measurements of surface carbon contents in 10 mm thick blocks heat treated in air at 600–700°C have given an apparent activation energy for decarburization of about 250 kJ mol^?1, at least twice that for carbon diffusing in ferrite. However, this value is still below that for creep, so the influence of decarburization on creep life is expected to increase at lower temperatures. Structural observations are discussed in relation to loss of carbon and are related to creep behaviour. Secondary precipitation was observed after low-temperature treatments in aged encapsulated specimens, but not in specimens aged in air. This is attributed to the loss of carbon in the air aged specimens, which also showed a decrease in the M₃C content. The iron content of M₃C particles depends on carbon content as well as aging time.

MST/40     

Effects of heat treatment processes on microstructure and properties of 2·25Cr–1Mo–0·25V HSLA steels

Abstract

In the present paper, the effects of the heat treatment processes with two conditioning treatments and four quenching–tempering processes on the mechanical properties of 2·25Cr–1Mo–0·25V high strength low alloyed (HSLA) steel are investigated. The results show that the conditioning treatments have obvious effects on the low temperature impact energy but little effect on the tensile strength. The elevation of the final austenitising temperature increases the strength, whereas it results in the decrease in the low temperature impact energy due to the coarse microstructure. The results of the fracture surfaces analysis further make sure that the fracture surfaces of tensile specimens all exhibit ductile characters with a lot of dimples. However, the fracture surfaces of impact specimens exhibit two typical fracture characters, i.e. the ductile and brittle fracture surface corresponding to the fine and coarse microstructures respectively. In addition, the elongation and reduction in area seem to be insensitive to the heat treatments. Meanwhile, the impact fracture mode is more sensitive to the grain size and not to the low temperature impact energy.     

Scale growth on 2·25Cr–1 Mo steel

Abstract

A 2·25Cr–1 Mo steel was oxidised at 600°C in dry flowing oxygen for periods of up to 100 h. The change in the structure of the oxide scale with time was studied thermogravimetrically and microstructurally by use of SEM and EDX analyses. It was shown that an oxide layer of Fe₃O₄ nucleated and spread laterally between layers of α-Fe₂O₃ and a doped spinel oxide.

MST/931     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Effect of microstructure on appearance of near-threshold fatigue fracture in Cr–Mo–V steel

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

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.     

Influence of post-weld heat treatment on tensile properties of modified 9Cr–1Mo ferritic steel base metal

Abstract

The paper presents the influence of post-weld heat treatment (PWHT) on tensile properties of modified 9Cr–1Mo ferritic steel base metal. Tensile tests at room and elevated temperatures (300–873 K) were performed on specimens in normalised and tempered condition as well as with additional PWHT (993 K for 1 h; 1013 K for 1 h and 1033 K for 1 h). The yield and ultimate tensile strengths decrease gradually up to intermediate temperatures followed by a rapid fall at high temperatures in all heat treatment conditions. At intermediate temperatures, the steel exhibited ductility minima, serrated flow, negative strain rate sensitivity on flow stress and peak in the average work hardening rate. The influence of additional PWHT is reflected in a systematic and gradual decrease in both the yield and tensile strength values with increasing PWHT temperature from 993 to 1033 K for 1 h. However, there has been no appreciable change in ductility values as well as the fracture mode in PWHT conditions compared with those observed in normalised and tempered condition. Comparison of strength values in PWHT conditions suggested that the strength values remained higher than the average values specified in the French Nuclear Design Code, RCC-MR.     

Effect of heat treatment on mechanical properties of Co–29Cr–6Mo–0·33C superalloy

Abstract

Investment cast Co–29Cr–6Mo–0·33C alloy tensile test specimens were subjected to heat treatment at 1230°C under argon atmosphere for different times to ascertain what enhancement of tensile properties could be achieved by such processing. Improvements in tensile properties were observed in the 2 h annealed specimen. Also, tempering of this specimen at different times shows that tensile properties and hardness increase and exceed those of the ASTM specification. However, elongation decreases during tempering. The annealed specimen has better corrosion resistance in comparison with as cast specimen.     

Influence of heat treatment on creep of a Mn–N stabilised austenitic stainless steel

Creep at 700 °C/196 MPa and 900 or 925 °C/27.4 MPa of 21Cr–4Ni–9Mn austenitic stainless steel is determined as a function of the heat treatment. The heat treatment variation involves altering the solution heat treatment cooling rate from water quenching to cooling at 6 or 4 °C/min causing: serrated grain boundaries versus planar grain boundaries, coarser intergranular carbides, and discontinuous precipitation of grain boundary reaction zones. Water quenching causes improved creep resistance. Creep fracture and cracking is intergranular. Coarse intergranular carbides and grain boundary reaction zones cause premature void formation and cracking, this damage leading to an accelerating creep rate and lowering creep resistance of the more slowly cooled conditions. During creep, grain boundary serrations, which may otherwise contribute to improved creep, are eliminated. Determining the individual influence of grain boundary serrations on creep requires a detailed investigation of various heat treatment parameters to prevent concurrent formation of grain boundary reaction zones and serrations.     

Effect of σ-Phase on the creep properties of Cr25Ni20 stainless steel

It is concluded from above that effect of σ-phase on creep properties depends on particle size and distribution. Fine σ-phase particles dispersively precipitated along grain boundaries and within grains increase creep resistance and rupture strength, having general characteristics of dispersion hardening.