Fracture Appearance Transition Temperatures Research Articles

Effect of welding heat input on microstructure and impact toughness in CGHAZ of X100Q steel

The coarse-grained heat-affected zones (CGHAZs) of X100Q steel were reproduced via simulating their welding thermal cycles with the varying heat input (Ej) from 10 to 55 kJ/cm in Gleeble3500 system. The microstructures were characterized, and the impact toughness was estimated from each simulated sample. The results indicate that the microstructure in each simulated CGHAZ was primarily constituted of lath-like bainite. With the decreased heat input and accordingly the lowered Ar3 (the onset temperature for this transition), the prior austenite grain and the bainitic packet/block/lath substructure were refined remarkably, and the impact toughness was enhanced due to the microstructure refinement. The bainitic packet was the microstructural unit most effectively controlling the impact properties in CGHAZ of X100Q steel, due to their close correlation with the 50% fracture appearance transition temperatures, their size equivalent to the cleavage facet and their boundaries impeding the crack propagation.

Hardening Embrittlement and Non-Hardening Embrittlement of Welding-Heat-Affected Zones in a Cr-Mo Low Alloy Steel

The embrittlement of heat affected zones (HAZs) resulting from the welding of a P-doped 2.25Cr-1Mo steel was studied by the analysis of the fracture appearance transition temperatures (FATTs) of the HAZs simulated under a heat input of 45 kJ/cm with different peak temperatures. The FATTs of the HAZs both with and without tempering increased with the rise of the peak temperature. However, the FATTs were apparently lower for the tempered HAZs. For the as-welded (untempered) HAZs, the FATTs were mainly affected by residual stress, martensite/austenite (M/A) islands, and bainite morphology. The observed embrittlement is a hardening embrittlement. On the other hand, the FATTs of the tempered HAZs were mainly affected by phosphorus grain boundary segregation, thereby causing a non-hardening embrittlement. The results demonstrate that the hardening embrittlement of the as-welded HAZs was more severe than the non-hardening embrittlement of the tempered HAZs. Consequently, a post-weld heat treatment should be carried out if possible so as to eliminate the hardening embrittlement.

An experiment-based model of combined hardening and non-hardening embrittlement in an interstitial free steel

The fracture appearance transition temperatures (FATTs) are evaluated for the samples of a Nb-stabilized and P-strengthened interstitial free (IF) steel. Based on the measurements, the combined effect of hardening and phosphorus grain boundary segregation on the embrittlement of the steel is investigated. Both hardening and P boundary segregation can raise the FATT of the steel, causing hardening embrittlement and non-hardening embrittlement, respectively. Meanwhile, the grain size influences both kinds of embrittlement, i.e., there is a synergistic effect between grain size and hardening or P boundary segregation on the FATT of the steel. With the aid of the Taylor expansion along with the experimental data, a combined hardening and non-hardening embrittlement equation is established, being expressed as FATT = 2.1Cp + 3.48σs − 22.36d−1⁄2 + 0.64(Cp − 14)(d−1⁄2 − 3.06) + 0.896(σs − 14)(d−1⁄2 − 3.06) − 13.7, where FATT is the fracture appearance transition temperature in °C, Cp is the phosphorus boundary concentration in at%, σs is the yield strength in 10 MPa, and d is the grain size in mm. A comparison of the calculated and measured FATTs is made, demonstrating that the calculated FATTs are well consistent with the measured ones.

Impact properties of low-alloy transformation-induced plasticity-steels with different matrix

The effects of matrix types on Charpy impact properties were investigated in Fe–0.2%C–1.5%Si–1.5%Mn (mass%) transformation-induced plasticity steels. The steels with annealed martensite and bainitic ferrite matrix possessed higher upper shelf Charpy impact absorbed energy than the steel with polygonal ferrite and martensitic matrix. In addition, the low ductile–brittle fracture appearance transition temperatures were achieved in annealed martensite and martensite types in comparison with those of other steels.

The Effects of Martensite Content on the Mechanical Properties of Quenched and Tempered 0.2%C-Ni-Cr-Mo Steels

Three martensite contents (approximately 35, 50, and 100%) were obtained in a SAE8822 steel by altering the quenching media and section size. Another variation in martensite content (approximately 80 versus 97%) was achieved by quenching a SAE8622 steel in the same section size. The impact toughness and fatigue properties were determined after tempering to various levels of monotonic strength. Toughness and strength-toughness combinations improved with increased as-quenched martensite contents at all levels of as-tempered ultimate tensile strength (UTS). Even at higher levels of yield strength (YS), increased martensite contents produced higher impact energies and lower fracture appearance transition temperatures. The cyclic YS was independent of martensite content (at the same level of UTS), even though the monotonic YS increased with martensite content. When fatigue test results were compared at a tensile strength of 1240 MPa (180 ksi), actual and predicted fatigue lives in the high cycle regime increased with martensite content, but low cycle fatigue resistance was relatively unaffected. Fatigue strength and UTS were directly related, and all the quenched and tempered steels exhibited cyclic softening.

Ｃｒ‐Ｍｏ‐Ｖ鋼タービンロータの使用中焼戻し脆化特性

Temper embrittlement is one of the typical material degradations of Cr-Mo-V steel high and intermediate pressure steam turbine rotors used at high temperature in thermal power plants, and so it is important to know the embrittlement characteristics of the turbine rotors serviced for a long term. For this purpose, FATTs (50% fracture appearance transition temperatures) at around the center hole of retired turbine rotors serviced for 150,000-240,000 h were investigated.It was found that Cr-Mo-V turbine rotors were embrittled at the estimated service temperature of 350-460°C, and the maximum shift in FATT (ΔFATT) occurred at around 400°C. This embrittlement has been well correlated with an embrittling factor K2 (K2=(2Si+Mn+Cu+Ni) · X, where X=(10P+5Sb+4Sn+As)). It is thus concluded that the reduction of impurities is effective to reduce the susceptibility to the emberittlement during the service.With this findings, the equation, which could estimate the amount of emberittlement ΔFATT of Cr-Mo-V steel steam turbine rotor, was obtained as a function of the embrittling factor K2, the service time and the service temperature. According to the equation, it was estimated that the embrittlement increased rapidly until about 100,000 h, but slowly after about 150,000 h because of approaching the saturation of temper embrittlement.

Effect of temper embrittlement and specimen size on Charpy impact testing of a Cr–Mo–V rotor steel

Full and sub­size Charpy V notch specimens from several locations of a high pressure–intermediate pressure Cr–Mo–V turbine rotor were tested. A comparison between full and sub­size impact energy data showed that the smaller specimens exhibited qualitatively similar behaviour, with a systematic reduction in fracture appearance transition temperatures (FATTs). The full and sub­size impact energy data were normalised against the specimen area and volume. The latter normalisation produced the closest match and the offset between the two data sets was described by a simple linear equation. The sensitivity of impact energy and FATT to specimen size was examined in samples possessing different degrees of temper embrittlement. It was found that the difference in FATT between full and sub­size specimens for embrittled samples was at least double that of de­embrittled samples. It is proposed that the observed specimen size/impact energy/FATT variations with degree of embrittlement arise from sensitivity of intergranular fracture to lineal specimen thickness, since fracture occurs predominantly through a two­dimensional network of grain boundaries.

Some material toughness properties of a series of turbine casing bolts removed from service after over 100 000 hours

The present study was aimed at examining the effects of temperature on both the Charpy and slow strain rate fracture toughness properties of a series of large steam turbine casing bolts which had been subjected to temperatures of around 500°C for some 122 000 h. Also, the various toughness properties were assessed in terms of the existing fracture toughness–Charpy correlations. The present CrMoV steel bolts had suffered varying degrees of Reverse Temper Embrittlement (RTE) during service and exhibited strong fracture toughness–temperature trends. Although no single fracture toughness–Charpy correlation effectively described the full toughness–temperature transition curves, it was evident that, for embrittled and partially embrittled bolts, the Wallin correlation best decribed the lower temperature portion of the curves. At the upper temperature region all embrittlement conditions exhibited reasonable agreement with the upper bound correlation of Barsom and Wolfe. Finally, it was shown that the fracture toughness transition temperatures, T K, and the Charpy Fracture Appearance Transition Temperatures (FATT) exhibited good agreement with the fracture toughness transition temperature being equal to the temperature at which the fracture toughness value was 3000 N/MM 3/2.

Ｓ１５ＣＫ同種摩擦圧接継手の延性‐ぜい性遷移挙動

In order to make clear the absorption energy, transition temperature and fracture behavior of friction welded joint, the instrumented Charpy impact tests have been carried out at various temperatures from 173K to 473K, using 0.15%C carbon steel specimens with butt joint. The main results obtained are as follows:(1) The energy transition curve of the weld interface and the heat affected zone shifted to the lower temperature side than that of the parent material. The absorption energy at the transition region was the highest at the weld interface.(2) The energy transition temperature was about 250K in both the parent material and the heat affected zone, and apporoximately 230K in the weld interface. These temperatures and the fracture appearance transition temperatures agreed in most cases.(3) The crack propagation energy in the weld interface at the lower position of the transition region was extremely larger in comparison with that of the parent material and the heat affected zone.(4) The impact bending strength of the weld interface, the heat affected zone and the parent material became maximum around the lower position of the transition region, having no corresponding relationship with the absorbed energy.

Comparison of Dynamic Tear and Charpy-V-Notch Impact Properties of Plate Steels

The increasing interest in the Dynamic Tear (DT) test for use in measuring the fracture resistance of steels in both quality control and research testing, invites a comparison to the commonly used Charpy-V-notch (CVN) impact test. The DT and CVN properties of 57 steel plates of various grades are compared in this survey. The investigation covered ASTM carbon steel grades A-36, A516, A537; ASTM alloy steel grades A533-B, AB42, A43; and other steels such as HY-130, ABS-E, and a C-Mn-Cb type. Graphical and statistical correlations were established between various DT and CVN properties including upper shelf energy, fracture appearance and mid-energy transition temperatures. These features were then compared to the nil ductility temperature (NDT). Certain of the correlations appear useful enough to allow casual prediction of DT behavior from CVN testing.

Grain-Boundary Segregation and Temper-Brittle Fracture in a Low-Alloy Steel

Conventional measurements of the degree of temperembrittlement suffered by a steel are made in terms of the shift of the notched-bar “fracture appearance” transition temperatures (FATT). Any fundamental explanation of temperembrittlement must be concerned with the lowering of grain-boundary cohesion by the segregation of impurity. elements to grain boundaries over a critical temperature range, typically 560–430° C. The use of transition shifts to estimate the kinetics of reduction of grain-boundary cohesion can be criticized on several counts. First, it is often the case that the unembrittled condition breaks by cleavage rather than by intergranular fracture at low temperatures, so that no measure of the unembrittled grain-boundary cohesion can be obtained. Secondly, even if both fractures are intergranular, the shift compares fracture events at different temperatures and there is no guarantee that the relative grain-boundary cohesive strengths do not change with temperature. Thirdly, the temper-brittle fracture processes at low temperatures are critically dependent, not only on grain-boundary properties but on the size and distribution of tempered carbides and on the yield strength of the matrix. Finally, though it may be possible to relate the “nil-ductility” transition temperature to the magnitude of the critical stress needed to propagate a crack nucleus along embrittled grain boundaries in the yielded region ahead of a notch, the more commonly used FATT embraces, by definition, both intergranular and ductile regions of fracture. If there were separate effects of impurity-element segregation on these two modes of fracture, e.g. by weakening carbide/ferrite matrix interface bonding in addition to weakening grain boundaries, it would prove to be almost impossible to extract any worthwhile quantitative information from the FATT.

Temper embrittlement in a Ni-Cr steel containing phosphorus as impurity

Phosphorus segregates to prior austenite grain boundaries in ferrite during embrittling treatments (below 1000°F) and contributes to temper embrittlement of a Ni-Cr-C-P steel. The phosphorus is confined to a width of 5 to 10A, at a very high concentration of approximately 5 wt pct (0.1 monolayer) in the vicinity of the grain boundaries; segregation of nickel is also observed to a lesser degree and spread over a distance somewhat larger than 50a. Nickel segregation to the grain boundaries appears to be of minor consequence either in terms of its own effect or in terms of its interaction with phosphorus. Fracture appearance transition temperatures of the various samples are directly correlatable to the phosporous content at grain boundaries through a linear plot. Deembrittlement of the steel is found to consist of reversing the boundary segregation and driving the phosphorus and nickel away from the prior austenite boundaries. The amount of phosphorus segregated at grain boundaries of the embrittled Ni-Cr-C-P steel is found to be compatible with an equilibrium segregation model.

Studies on Brittle Fracture of Steel in Slow Notch Bend Test

Slow notch bend tests with semi-killed, killed and HT 52 high tensile steels were performed. Fracture appearance transition temperatures Tii and energy transition temperatures are nearly the same, and transition temperatures in ductility criterion Ti, those in low energy level TrLE (10 kg-m/cm2) and those in reduction in thickness Trφ (4 %) and also nearly the same for three kinds of steels.Results of slow bend tests are compared with the results of Charpy and Tipper tests in same steels. As to fracture appearance transition temperature, slow bend test is most sensitive and Tipper test is the next.

Studies on the Effect of Residual Welding Stress on Brittle Fracture of Steel, Part I

A series of slow notch bend tests was performed to investigate the effect of residual welding stress on brittle fracture of steel.Chemical composition and mechanical properties of steel tested are shown in Table 1.Following 9 types of test specimens were prepared. (Fig. 1).(1) parent metal, as rolled, notched.(2) parent metal, prebent at room temperature as shown in Fig. 2a, notch side tension, permanent deflection 2. 5mm.(3) parent metal, prebent as shown in Fig. 2b, notch side compression, permanent deflection 2. 5mm.(4) welded specimen, as welded, notched.(5) welded specimen, similar to (2).(6) welded specimen, similar to (3).(7) welded specimen, pretensioned at elastic nominal stress of 25 kg/mm2, then notched.(8) do. pretensioned up to yield point. (9) do. pretensioned, permanent strain 2. 5% for 100mm gauge length.Fracture test procedure is the same as Van der Veen test.Temperatures were varied from 0° to -70°C. Fracture appearance are shown in Figs. 3-5.Depth of fibre are shown in Figs. 6-8, and fracture appearance transition temperatures Till are listed in Table 3. Among welded specimens, type (4) (as welded) has highest (TII = -18°C) and type (8) (pretensioned up to yield point) has lowest (TII=-38°C) transition temperature.Residual stress measurement was performed by resistance wire strain gauge. From the results of our experiment, we can conclude that residual welding stress has a pretty effect on brittle fracture of welded structure.