Lower Shelf Energy Research Articles

EFFECT OF TANTALUM ON THE IMPACT RESISTANCE OF 12% CR STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT

The effect of tantalum on the impact resistance of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from -40 to +120°C with determining the brittle-ductile transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the entire fracture toughness versus temperature curve is 30-50 J/cm2 higher over the entire temperature range, including the upper and lower shelf energies, and the brittle-ductile transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller initial austenite grain size, a larger average size but lower density of M23C6 carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.

Impact toughness of a 12% Cr martensitic steel: Conventional heat treatment vs. thermo-mechanical processing

The impact toughness together with determination of the ductile–brittle transition temperature for the 12 %Cr steel with the low N and high B contents subjected to conventional heat treatment and thermo-mechanical processing have been investigated. The thermo-mechanical processing includes the interim hot forging and long-term annealing at 1050 °C with the aim to dissolve the delta-ferrite and coarse W-rich particles, as well as to decrease the particle number density of M23C6 carbides along the lath boundaries and to precipitate V-rich MX phase. The temperature dependence of Charpy energy curves including upper and lower shelf energy is found to be shifted in a side of higher energies. The ductile–brittle transition temperature decreases from 70 °C to (45–50)°C.

Detection of embrittlement in low alloy steels due to thermal aging by small punch test

The applicability of miniaturised small punch tests (SPT) to capture thermal aging degradation of two commonly used low alloy steels (LAS) viz. Mn-Ni-Mo and Cr-Mo-V steels, was investigated. This was done by comparing the SPT results to the results of the conventional Charpy V-notch (CVN) tests, fractographic analysis and results of chemical etching test. Both the steels were subjected to accelerated thermal aging at 450 °C (8400 h for Mn-Ni-Mo steel and 5600 h for Cr-Mo-V steel). For both LAS, conventional CVN tests showed reduction in the impact energy and fractographic analysis indicated increase in the percentage brittle fracture with thermal aging. Fractographs indicated intergranular (IG) brittle fracture for the Mn-Ni-Mo steel and transgrannular (TG) brittle fracture for the Cr-Mo-V steel. This was corroborated by the chemical etching tests that showed preferential attack at prior austenitic grain boundaries (PAGBs) for Mn-Ni-Mo steel and conspicuous absence of such IG attack for Cr-Mo-V steel. SPT could clearly capture the embrittlement due to thermal aging for both the LAS as investigated by the conventional CVN testing. However, SPT could not establish the ductile to brittle transition temperature (TSP) as the lower shelf energy was not reached even at the lowest possible test temperature (-196 °C). SPT has been successful in capturing the indications of embrittlement due to thermal aging for both the LAS i.e. it is able to show the loss of ductility and increase in the fraction of brittle fracture. However, the use of SPT, at least in the present form, as an alternative to the most commonly used CVN test, for the materials studied is not recommended.

Tensile and Charpy impact properties of an ODS ferritic/martensitic steel 9Cr–1.8W–0.5Ti–0.35Y2O3

A 9Cr-ODS ferritic/martensitic steel with a composition of 9Cr–1.8W–0.5Ti–0.35Y2O3 was fabricated by mechanical alloying and hot isostatic pressing, followed by hot rolling. Tensile properties were measured at room temperature (23°C) and 700°C in the rolling direction (LT) and the transverse direction (TL). The ultimate tensile strength (UTS) of the as-rolled samples in both directions reached 990MPa at 23°C, and still maintained at 260MPa at 700°C. The tensile strength and elongation of the rolling direction was greater than that of the transverse direction. The Charpy impact was tested from −100 to 100°C in the LT direction. The lower shelf energy (LSE) was more than 65% of the upper shelf energy (USE). The total absorbed energy was separated into the energies for crack initiation and propagation. The propagation energy was always higher than the initiation energy in the range of temperatures tested. The ductile-to-brittle transition temperature (DBTT) of the rolled 9Cr ODS evaluated by an absorbed energy curve was about 0°C. However, the high LSE and the fracture surface that still contained dimples at lower shelf indicated good toughness of the as-rolled 9Cr ODS steels at temperature of −60°C.

Low temperature thermal aging of austenitic stainless steel welds: Kinetics and effects on mechanical properties

Abstract Austenitic stainless steel welds in components used in light water reactors are susceptible to thermal aging embrittlement at reactor operating temperature of around 300 °C after a long service life. In this study, low temperature aging embrittlement of types 304L and 316L stainless steel welds with 10% ferrite was investigated on the basis of changes in mechanical properties and microstructure after aging up to 20,000 h at 335, 365 and 400 °C. Spinodal decomposition and G-phase precipitation in the ferrite was observed after aging which lead to embrittlement in the material. In contrast to the small effect on tensile properties, the impact toughness was significantly degraded after aging. Charpy impact test of the aged samples showed decrease in upper-shelf and lower-shelf energy and increase in ductile brittle transition temperature. Large increase in the microhardness of ferrite phase was observed with no change in austenite hardness. The embrittlement in 316L weld was higher compared to 304L weld for similar aging condition. The kinetics of aging embrittlement was established based on Arrhenius relationship. A constant activation energy was determined for 304L weld in the temperature range 335–400 °C, however, 316L weld showed different activation energy values in each temperature range.

Effects of cold work and phosphorous on the ductile to brittle transition behavior of F82H steels

The effects of cold work and phosphorous on ductile to brittle transition behavior were investigated in order to evaluate the effects of hardening and intergranular degradation on the ductile to brittle transition temperature (DBTT) of F82H steel. Yield strength and ultimate tensile strength are increased by increasing the level of cold work. Phosphorous additions resulted in a small increase in the strength of F82H. The DBTT after cold work is shifted to high temperature and upper shelf energy (USE) is decreased. The shift of DBTT and the reduction of USE increase with the cold work level. F82H as-received and cold worked steel fractured in a cleavage mode at temperatures of lower shelf energy (LSE) region. The DBTT is shifted to higher temperatures with increasing phosphorus additions accompanied by the reduction of the USE. Although the hardening is small, a significant embrittlement was observed in the steels added with phosphorus, which was accompanied by intergranular cracking at temperatures in LSE region.

Effects of Chemical Composition on the Impact Properties of A533B Steels

In order to estimate the long life integrity of vessel steels with considering various material compositions and irradiation conditions, it is necessary to understand physical mechanisms of the degradation of mechanical properties. In this research, chemical composition effects were investigated for Reactor Pressure Vessel Steels (RPVS) to apply small specimen test technique to surveillance test method. All specimens used in this study were machined from the A533B plate material, which are standard, low Mn, high Cu, high P, and high Cu and high P steels. Tensile strength is increased by phosphorous and copper additions. Charpy tests were carried out at temperature from 73 K to 473 K. The ductile to brittle transition temperature (DBTT) is shifted to higher temperatures with phosphorus additions accompanied by the reduction of the upper shelf energy (USE). The fracture mode of P-added A533B steels at temperatures in the lower shelf energy (LSE) region is intergranular cracking. Test results were discussed in view of the differences on elements of Cu, Mn and P.

Prediction of embrittlement during aging of nuclear grade AISI 304 stainless steel TIG welds

Simulated weldments of AISI grade 304 stainless steel having a ferrite content of 4–6% with three levels of nitrogen ( 0·03, 0·08, and 0·11 wt-%) were prepared using a modified elemental implant technique. From these weldments, subsize Charpy impact specimens were prepared and subjected to aging treatment at different temperatures, 623–748 K, and for different times, 1000–5000 h. Impact toughness curves for these aged samples were generated by testing at various temperatures from 77 K to 300 K. From the impact curves the upper shelf energy (USE) and lower shelf energy (LSE) were determined. It was observed that both USE and LSE decreased with aging time at all temperatures. Nitrogen seems to offer a beneficial effect as far as impact toughness is concerned, as both USE and LSE values increased with increasing nitrogen content. The worst aging conditions were identified as 748 K, 2000 h at the lowest nitrogen level of 0·03 wt-%. An empirical relation connecting the aging temperature, aging time, and nitrogen content to the LSE was developed, which can be used to predict the time for embrittlement at a given nitrogen level and aging temperature.

Mechanical properties of an ultrafine grained C–Mn steel processed by warm deformation and annealing

The mechanical properties of an ultrafine grained 0.2%C–Mn steel, processed by large strain warm deformation and subsequent annealing, have been investigated. The microstructure consists of spheroidized cementite particles in an ultrafine ferrite matrix (average grain diameter ≈1.3 μm). The steel shows an improved combination of strength and toughness when compared with corresponding coarse grained specimens. The reasonable ductility of the steel can be attributed to the finely dispersed cementite particles, which also effectively increase the work hardening rate by the accumulation of geometrically necessary dislocations in their vicinity. The lower shelf energy is significantly higher and the ductile-to-brittle transition temperature is lower in the ultrafine grained steel than in comparable coarse grained specimens. This may be due to the joint effect of grain refinement and delamination in the ultrafine grained steel processed by large strain deformation. The delaminations lead to a decrease in triaxiality of the stress state in the impact test samples. The upper shelf energy is slightly reduced in the ultrafine grained steel, which can be attributed to the effect of delamination.

Material inertia and size effects in the Charpy V-notch test

The effect of material inertia on the size dependence of the absorbed energy in the Charpy V-notch test is investigated. The material response is characterized by an elastic-viscoplastic constitutive relation for a porous plastic solid, with adiabatic heating due to plastic dissipation and the resulting thermal softening accounted for. The onset of cleavage is taken to occur when a critical value of the maximum principal stress is attained over a critical volume. Plane strain dynamic analyses are carried out for geometrically similar specimens of various sizes with all parameters adjusted so that a quasi-static analysis would predict a size independent response. Sizes ranging from 1/4 to 16 times the ASTM standard size are analyzed and two sets of material properties are considered. No size effect is seen below a critical specimen size. Above this limit, a monotonic increase with specimen size is found for the normalized lower shelf energy (LSE) and the normalized upper shelf energy (USE) for both sets of material properties. The ductile-to-brittle transition temperature (DBTT) is found to increase monotonically with specimen size for one set of material properties, but a non-monotonic variation is found for the other set of material properties.

Thermal Aging Embrittlement of Tungsten-Alloyed 9%Cr Ferritic Steels and Electrochemical Evaluation

Thermal aging embrittlement of newly developed W-alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92) was investigated. In order to clarify the controlling factor of embrittlement, Charpy impact tests were carried out, and characterization of precipitates was determined through use of materials that had been aged under various conditions. Drops in the upper and lower shelf energies and an increase in the fracture appearance transition temperature (FATT) induced by pre-aging were found to be closely associated with an increase in the amount of precipitates, such as the M23C6 type carbide and the Laves phase. As the result, the drop in impact toughness was found to be uniquely correlated with an increase in the area fraction of the Laves phase. In order to develop a nondestructive procedure for detecting thermal aging embrittlement, the changes in electrochemical properties of KA-STBA29/KA STPA29 induced by pre-aging have been investigated, and the electrochemical polarization technique has been applied to the pre-aged materials. Experimental results on electrochemical polarization measurements revealed that the peak current density “Ip” which appears at a specific potential during potentiodynamic polarization curve measurements in 1N-KOH solution increases linearly with the degree of embrittlement as evaluated by impact absorbed energy at 0°C. This increase in Ip was correlated with an increase in selective dissolution volume of the Laves phase. Consequently, the Charpy impact absorbed energy, along with the degree of thermal aging embrittlement, can be nondestructively estimated by the electrochemical technique.

A review of the ESB small punch test data on various plant components with special emphasis on fractographic details

The present paper was aimed at reporting the experience within the ESB regarding small punch testing of various plant components including steam drums, rotors and turbine bolts. It was shown that a reasonable relationship existed between the Charpy FATT and the small punch transition temperature, T SP. The T SP value coincided with a small punch energy level of some 0.75 J while the upper and lower shelf energies were 1.2 and 0.2 J, respectively. A detailed fractographic study revealed that the upper and lower shelf regions corresponded to fully ductile and brittle failure respectively. Furthermore, the extent of ductile fracture increased with temperature. Indeed, is was found that the T SP value corresponded with around 25% ductile fracture and a ductile crack depth of some 120 μm. A reasonable relationship was shown to exist between the Charpy FATT and the small punch energy value at liquid nitrogen temperatures.

Ｗ強化型９％Ｃｒフェライト系耐熱鋼の熱時効ぜい化と電気化学的手法によるその評価

Thermal aging embrittlement of newly developed W alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92) has been investigated. In order to clarify the controlling factor of the embrittlement, Charpy impact tests and determination of precipitates have been carried out using materials aged under various conditions. The decrease in the upper and lower shelf energies and the increase in the fracture appearance transition temperature (FATT) due to thermal aging were caused by the precipitation and coarsening of Laves phase. The decrease in the impact toughness was found to be uniquely correlated with the increase in the area fraction of Laves phase. In order to develop a non-destructive technique for detection of the thermal aging embrittlement, changes in electrochemical properties of KA-STBA29/KA STPA29 due to thermal aging have been investigated. The electrochemical polarization technique has been applied to thermally aged materials. Experimental results on electrochemical polarization measurements revealed that the peak current density “Ip” which appeared at a specific potential during potentiodynamic polarization measurements in 1N KOH solution linearly increased with the degree of embrittlement evaluated by the impact energy at 0°C. The increase in Ip corresponded to the increase in selective dissolution of Laves phase. As a consequence, the degree of thermal aging embrittlement can be estimated by the electrochemical technique.

The effects of double austenitization on the mechanical properties of a 0.34C containing low-alloy Ni-Cr-Mo-V steel

This article considers five different microstructures of a tempered martensitic 0.34C, 3Ni-1.3Cr-0.4Mo-0.1V steel through various heat treatments, including double austenitization (DA) treatments, and how the impact toughnesses are influenced by microstructure. Of the four mechanisms considered to explain the beneficial effect of DA treatment, the roles of retained austenite, grain-boundary embrittlement by impurity segregation, and matrix flow stress are discounted. The 50 pct fracture appearance transition temperature (FATT) of this steel is found to be dependent on both the grain size and the carbide dissolution. The conventionally treated steel contains mainly platelike M3C carbides. The DA treatment helps to dissolve VC carbides and coarsen and spheroidize M3C carbides in favor of the precipitation of short rodlike M7C3 carbides with a lower aspect ratio. The improvement of impact toughness (upper shelf energy, ductile-to-brittle transition temperature (DBTT), and lower shelf energy) by DA treatment, explained in detail, is attributed to a change of this material’s tensile and work-hardening behavior affected by a variation of carbide morphology.

鉄筋コンクリート用棒鋼のシャルピー衝撃特性に及ぼす諸因子の影響

The reinforcing steel bars which are one of the structural rolled steels for construction can be classified into two types as blast furnace reinforcing steel bars and electric furnace reinforcing steel bars. Electric furnace reinforcing steel bars are utilized for about 85 per cent of all reinforcing steel bars at present.In this study, firstly, the Charpy impact characteristics of the above-mentioned two types of material SD35 were investigated.Secondly, the correlations between the Charpy impact characteristics and various factors such as chemical compositions, rolling conditions, and optical microstructures were studied using the multiple regression analysis method.As a result, the apparent differences in Charpy impact characteristics were found between the two types of bars. That is to say, the electric furnace steel bars had energy transition curves or fracture transition curves with more gentle slopes, lower shelf energies (vEshelf), and higher energy ransition temperatures (vErE) or higher fracture transition temperatures (vTrs) than the blast furnace steel bars. Therefore, it has been realized that these differences in characteristics depend on Mn, Cr and N Contents as well as on reduction ratio. It has also been realized that vTrE and vTrs shift to lower temperatures as the ferrite perlite structures become lamellar.

Effects of the constituents on the mechanical behavior of low carbon steels

Various microstructural constituents were obtained in the present research by suitable control of alloy compositions and processing variables. The alloys used in this study were Fe/1.5Mn/0.06C and Fe/1.5Mn/0.06C/0.05Nb steels. These were hot rolled and directly quenched from the finish rolling temperature of 800/sup 0/C. The as-hot-rolled Fe/1.5Mn/0.06C alloy was two-phase annealed at 835/sup 0/C and directly quenched. Experimental details of thermal and thermo-mechanical treatments are described elsewhere. Round tensile specimens of 12.7 mm gauge length and 3.2 mm gauge diameter were used. Impact tests were done on the 3/4 subsize Charpy specimens in the longitudinal direction. The ductile-brittle transition temperature were taken at the temperature corresponding to the midpoint of the upper and lower shelf energies.