Charpy V-notch Impact Energy Research Articles

Microstructure and Properties of Quenched-and-Aged Plates Produced from a Copper-Bearing HSLA Steel

For the first time in India, quenched-and-tempered (Q and T) plates of copper-bearing high-strength low-alloy (HSLA) steel have been commercially developed for naval structural applications. A 50 ton production heat was made through electric arc furnace (EAF)-vacuum are degassing (VAD) route and continuously cast into 170 mm thick slabs. These slabs were conditioned, reheated in walking-beam furnace and hot rolled in plate mill into plates of 10 to 16 mm thickness. The as-rolled plates were hardened through oil quenching and subsequently tempered (aged) at 630 C to achieve the combination of high-strength and good low-temperature impact toughness. The microstructures of heat treated plates showed fine acicular ferrite with grain sizes ranging between ASTM No. 9 and 10. From the standpoint of tensile properties, Q and T plates of all thicknesses exhibited significantly higher yield strengths than the minimum stipulated value of 552 MPa for HY-80/HSLA-80 steels. The elongation (22.20 to 26.00%) and reduction in area (62.12 to 67.62%) values achieved also exceeded the respective minimum requirements of 20 and 50% stipulated for such steels. The trend in variation of Charpy V-notch (CVN) impact energies at room temperature, {minus}18, and {minus}62 C not only showed significantly higher values than that stipulatedmore » for HY-80 and HSLA-100 steels at {minus}18 C, but also indicated that the CVN impact energies achieved (105.15 to 144.25 J) at {minus}62 C were higher than the estimated value of 90 J for HSLA-80/HSLA-100 steels at this temperature.« less

Carburisation of nickel-base alloys and its effects on the mechanical properties

The effect of carburisation on the ductility in terms of Charpy V-notch impact energy of five nickel-base alloys (alloy 45-TM, alloy 600H, alloy 617, alloy 601 and alloy 602CA) and three iron-nickel-chromium alloys (alloy 800H, alloy AC66, alloy DS) with different concentrations of aluminium, chromium and silicon was investigated in CH 4 - H 2 at a carbon activity of a c = 0.8 at 850°C and 1000°C. All chromia-forming materials suffer carbon pick-up, formation of internal carbides and a severe loss of ductility during exposure. The loss of Charpy V-notch impact energy after exposure is directly correlated to the carbon pick-up. The susceptibility to carburisation increases with increasing iron concentration and decreasing nickel concentration of the alloy. Silicon was found to decrease the carbon pick-up and the loss of ductility. Alumina-forming alloys do not suffer any significant ductility loss by carburisation.

Influence of inclusion characteristics on the formability and toughness properties of a hot-rolled deep-drawing quality steel

In industrial practice, variations in the steelmaking process may cause significant change in inclusion characteristics. During hot rolling of flat steel products, manganese sulfides, which are plastic at elevated temperatures, are elongated in the rolling direction. These elongated inclusions affect the formability properties, such as ductility, strain hardening exponent, average plastic strain ratio, critical strain represented by the forming limit diagram, and Charpy V- notch (CVN) impact energy as well as fracture behavior. The inclusion characteristics and microstructural features of three commercially produced hot- rolled deep- drawing quality steels were evaluated and their effects on formability and impact properties were investigated. All three heats were made in a basic oxygen furnace. Two heats were teemed into ingots while the other heat was argon purged and continuous cast. These heats were then processed into 3.10 mm thick strips with identical processing parameters. Manganese sulfide stringers were found to reduce the transverse ductility, whereas yield and tensile strengths remained virtually the same in all directions. The formability parameters were not significantly affected by small variations in inclusion characteristics. However, CVN impact energy and impact transition temperature data were observed to improve with steel cleanliness. The sulfide stringers were also found to adversely affect the impact energy, transition temperature, and fracture behavior in the transverse direction.

Heavy-section steel irradiation program summary

In light-water-cooled nuclear power plants, a steel reactor pressure vessel (RPV) forms the primary containment boundary for the nuclear fuel assembly. Since a failure of the RPV carries the potential of major contamination release and severe accident, it is imperative to safe reactor operation to understand and be able to accurately predict realistic failure models of the vessel material. A major known risk arises from irradiation-induced degradation of the RPV fracture resistance which occurs as a result of normally occurring neutron flux during the service life of the reactor. Better understanding of the irradiation degradation of the vessel material will provide better input to improve the engineering models vital for prediction of RPV integrity and safe service life. For this reason, the Heavy-Section Steel Irradiation (HSSI) Program has been established with its primary goal to provide a thorough, quantitative assessment of the effects of neutron irradiation on the material behavior, and in particular the fracture toughness properties, of typical pressure vessel steels as they relate to light-water RPVs. The program includes the direct continuation of irradiation studies previously conducted within the Heavy-Section Steel Technology Program augmented by enhanced examinations of the accompanying microstructural changes. Effects of specimen size, material chemistry, product form and microstructure, irradiation fluence, flux, temperature and spectrum, and postirradiation annealing are being examined on a wide range of fracture properties including fracture toughness ( K 1c and J 1c), crack-arrest toughness ( K 1a), ductile tearing resistance (d J/da), Charpy V-notch impact energy, dropweight nil-ductility temperature (NDT), and tensile properties. Models based on observations of radiation-induced microstructural changes using field ion and high-resolution transmission electron microscopy provide a firmer basis for extrapolating the measured changes in fracture properties to wider ranges of irradiation conditions. The principal materials examined within the HSSI Program are highcopper welds since their postirradiation properties are most frequently limiting in the continued safe operation of commercial RPVs. In addition, a limited effort Will focus on stainless steel weld overlay cladding, typical of that used on the inner surface of RPVs, since its postirradiation fracture properties have the potential for strongly affecting the extension of small surface flaws during overcooling transients. Results from the HSSI studies will be integrated to aid in resolving major regulatory issues facing the U.S. Nuclear Regulatory Commission which involve RPV irradiation embrittlement such as pressurized-thermal shock, operating pressure-temperature limits, low-temperature overpressurization, and the specialized problems associated with low upper-shelf welds. Taken together the results of these studies also provide guidance and bases for evaluating both the aging behavior and the potential for plant life extension of light-water RPVs.

Low temperature mechanical properties of Fe28Mn5Al1C alloy

The mechanical properties from 300 to 77 K of an Fe28.3Mn5.38Al1.04C0.24Si alloy were studied. It was found that this alloy possesses high strength, good ductility and high impact toughness. At 77 K, this alloy has 1112 MPa yield stress, 1536 MPa ultimate tensile strength, 28% total elongation and 72 J Charpy V-notch impact energy. After aging, the Charpy V-notch impact energy.at 300 K is much reduced, and also falls continuously with decreasing temperature. The impact energy is less than 10 J and the fracture surface becomes mainly intergranular at temperatures below 183 K.

A comparative study on evaluation of fracture toughness from charpy v-notch impact energy and reduction-in-area

The statistical scatter in fracture toughness of high strength materials obtained from the existing empirical correlations is compared with the scatter observed in the nominally identical fracture toughness tests. It is found that the correlation of fracture toughness with Charpy V-notch impact energy will be more useful for all practical purposes.

Fracture toughness of maraging steel from Charpy ?V? notch specimens

Compact tension (CT) and Charpy “V” notch (CVN) (impact and three-point bend) specimens of 18 Ni 1800 MPa maraging steel (parent metal and weldment) were used to determine plane strain fracture toughness (K IC) and CVN impact energy (CVNIE), respectively. Using an empirical equation,K IC-CVNIE correlation is attempted which could be advantageously utilized for routine quality control of inward material to effect savings in cost and time. Investigations reveal betterK IC-CVNIE correlation for tests using the precracked CVN specimens. Scanning electron microscopic (SEM) observations reveal good correlation between fractographic features and fracture toughness.

Improved mechanical properties of ultrahigh strength 0·4C–Cr–Mo–Ni steel through modification of sulphide inclusion shape and microstructural control

AbstractA new approach is suggested whereby the mechanical properties of commercially available 0·4C–Cr–Mo–Ni ultrahigh strength steel can be significantly improved whether the testing orientation is longitudinal or transverse. The new approach consists of combining the decreased hot rolling reduction treatment with a short time isothermal transformation treatment. This approach, compared with conventional heat treatment, resulted in significant improvements of the plane strain fracture toughness KIc at increased strength and of the Charpy V-notch impact energy, for either testing orientation. Steel treated using the new approach also had an improved combination of mechanical properties compared with steels treated using high austenitisation and controlled rolling, which are possible approaches to improving the values of KIc or other mechanical properties of steels. The favourable effect on the mechanical properties is discussed in terms of metallography, X-ray measurements, and fractography.MST/984

Improved mechanical properties of ultrahigh strength 0·4C–Cr–Mo–Ni steel through modification of sulphide inclusion shape and microstructural control

A new approach is suggested whereby the mechanical properties of commercially available 0·4C–Cr–Mo–Ni ultrahigh strength steel can be significantly improved whether the testing orientation is longitudinal or transverse. The new approach consists of combining the decreased hot rolling reduction treatment with a short time isothermal transformation treatment. This approach, compared with conventional heat treatment, resulted in significant improvements of the plane strain fracture toughness KIc at increased strength and of the Charpy V-notch impact energy, for either testing orientation. Steel treated using the new approach also had an improved combination of mechanical properties compared with steels treated using high austenitisation and controlled rolling, which are possible approaches to improving the values of KIc or other mechanical properties of steels. The favourable effect on the mechanical properties is discussed in terms of metallography, X-ray measurements, and fractography.MST/984

Fracture control in the transition-temperature region of structural steels

Abstract This paper describes the applicability of current research on the crack-tip opening displacement (CTOD) methodology to engineering approaches to fracture control in steel structures in the transition-temperature region. review of the four (not three) broad areas of fracture response that structural steels exhibit throughout their transition-temperature region is presented and the standard fracture toughness test methods applicable to each region are described. These four regions are the lower-shelf (linear-elastic K Ic type behavior), lower-transition (elastic-plastic behavior with no stable crack growth prior to brittle fracture), upper-transition (stable ductile tearing followed by brittle fracture),and upper-shelf (full fibrous ductile tearing). Correlations and relations between Charpy V-Notch impact energy results, K ID , K Ic , J-integral and CTOD test results are described. The importance of loading rate in understanding fracture control is demonstrated using the Standard Oil Development (SOD) wide plate test results of the early 1950s. Finally, actual CTOD test values, in combination with nonlinear finite element analyses, are used to predict the burst pressures of notched pressure vessels. The burst pressures computed agree to within 7% of experimental burst pressures in all five vessels analyzed.

Determination of Crack Arrest Toughness at High Temperatures Using Compact Specimens

In order to determine the influence of the test temperature on the crack arrest toughness, investigations were carried out on pressure vessel steels of various strength and toughness up to the upper-shelf of the Charpy V-notch impact energy (CV-energy). The tests were conducted on the basis of the proposed ASTM test method for ferritic steels with modified wedge-loaded compact specimens. The thickness ranged from 0.75–6.69 in. (19–170 mm), the in-plane dimensions from 3.94–15.75 in. (100–400 mm). The comparison with the reference curves of the American and German guidelines showed significant discrepancies at high temperatures.

Electron Microscopy Observations of Precipitation During Tempering of Vasco X-2M Steel and its Relation to Impact Toughness

ABSTRACTVasco X-2M is a low carbon tool steel which is being considered for critical gear applications. Since toughness is an important property for these applications, the Charpy V-notch impact energy was tested as a function of tempering temperature and found to drop considerably at 480C temper and above. A TEM investigation was carried out in order to better understand the results of this test. The precipitation sequence of carbides was examined along with their morphology, structure and orientation relation to the matrix. The precipitation of the intralath M3C carbides occurred on quenching from austenite. These carbides were then present in all tempered samples. M2C carbides were observed at 480C and 590C temper, and evidence for precipitation of these carbides was based on hardness data at 480C temper and TEM observations at 590C temper. After aging 200h at 590C, the ceitientite was observed to be in the process of transforming in an in-situ manner to an M7C3 carbide. The minimum in the Charpy impact energy was found to occur when both coarse M3C and a very fine dispersion of M2C were present in the microstructure.

The influence of the M23 (C,N)6 compound on the mechanical properties of type 422 stainless steel

Ductility and toughness of Type 422 stainless steel was shown to be strongly influenced by either the presence or absence of large grain boundary precipitates of the M23(C,N)6 compound. A lengthy, isothermal solution heat treat cycle combined with a longer, lower temperature precipitation heat treatment was used to exaggerate the M23(C, N)6 compound along the boundaries of large austenite grains. After a subsequent conventional heat treatment, the presence of this compound along the prior austenite grain boundaries substantially reduced the ductility and the Charpy V-notch impact energy. The primary fracture mode was intergranular failure along these prior austenite grain boundaries. This is a reversible process and this compound can be completely dissolved. The M23 (C, N)6 compound was removed from these prior austenite grain boundaries by another lengthy, isothermal resolution heat treatment. After a subsequent conventional heat treatment, the ductility returned to values which are near those found in as-received and heat treated material; the Charpy V-notch impact energy increased to values above that found in commercially processed Type 422 stainless steel. The fracture appearance returned to the more normal transgranular ductile dimple mode.

The evaluation of tempered martensite embrittlement in 4130 steel by instrumented charpy V-notch testing

Tempered martensite embrittlement (TME) was studied in vacuum-melted 4130 steel with either 0.002 or 0.02 wt pct P. TME was observed as a severe decrease in Charpy V-notch impact energy, from 46 ft-lb. at 200 °C to 35 ft-lb. at 300 °C in the low P alloy. The impact energy of the high P alloy was consistently lower than that of the low P alloy in all tempered conditions. Fracture was transgranular for all specimens; therefore, segregation of P to the prior austenitic grain boundaries was not a factor in the occurrence of TME. Analysis of load-time curves obtained by instrumented Charpy testing revealed that the embrittlement is associated with a drop in the pre-maximum-load and post-unstable-fracture energies. In specimens tempered at 400 °C the deleterious effect of phosphorus on impact energy became pronounced, a result more consistent with classical temper embrittlement rather than TME. A constant decrease in pre-maximum-load energy due to phosphorus content was observed. The pre-maximum-load energy decreases with increasing tempering temperature in the range of 200 °C to 400 °C, a result explained by the change in work hardening rate. Carbon extraction replicas of polished and etched as-quenched specimens revealed the presence of Fe2MoC and/or Fe3C carbides retained after austenitizing. Ductile crack extension close to the notch root was related to the formation of fine microvoids at the retained carbides.

Analyzing statistical variability of fracture properties

A recently developed statistical analysis is applied to the estimation of lower-bound Charpy V-notch impact energies. Scatter of the impact-energy-absorption data, at any given temperature, is described using a two-parameter Weibull distribution, with the parameters allowed to vary with temperature. Values for the parameters are obtained using standard linear-regression techniques. As a result, the analysis naturally yields the temperature dependence of the pth percentile of the impact-energy distribution. Data with both large and small scatter are analyzed using this procedure.

Improvement in lower temperature mechanical properties of 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel with the second phase lower bainite

A study has been made of the effect of the second phase lower bainite on lower temperature mechanical properties from ambient temperature (287 K) to 123 K of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel corresponding to AISI 4340. When 25 vol pct lower bainite, which appeared in acicular form so as to partition prior austenite grains, was associated with martensite at 473 K, it provided a better combination of strength and ductility than that achieved using 1133 K direct water quenching irrespective of the test temperature. With the lower bainite, notch tensile strength was dramatically improved over the temperature region studied about 2150 MPa even at 123 K; whereas, in the case of 1133 K direct water quenching, it remained at about 1700 MPa. Similar trends were observed in the relationship between the lower bainite and the Charpy V-notch impact energy at and above 238 K. The lower bainite also produced superior fracture ductility and notch toughness results with decreased temperature of testing as compared to those obtained using a y γ α′ ’ repetitive heat treatment for the same steel. The above beneficial effects of the second phase lower bainite on lower temperature mechanical properties are briefly discussed in terms of metallographic examinations, the law of mixtures, and so on.

Structure and mechanical properties of Fe-Cr-Mo-C alloys with and without boron

A study of the structure and mechanical properties of Fe-Cr-Mo-C martensitic steels with and without boron addition has been carried out. Nonconventional heat treatments have subsequently been designed to improve the mechanical properties of these steels. Boron has been known to be a very potent element in increasing the hardenability of steel, but its effect on structure and mechanical properties of quenched and tempered martensitic steels has not been clear. The present results show that the as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350 °C for 1 h. The properties above 500 °C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase in toughness when tempered above 500 °C, the boron-treated steel suffers a second drop in toughness at 600 °C tempering. Transmission electron microscopy studies show that in the 600 °C tempered boron-treated steel large, more or less continuous cementite films are present at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical properties at tempering temperatures above 500 °C are rationalized in terms of the effect of boron-vacancy interactions on the recovery and recrystallization behavior of these steels. Although boron seems to impair room temperature impact toughness at low strength levels, it does not affect this property at high strength levels. By simple nonconventinal heat treatments of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior to those of existing commercial ultra-high strength steels. It is also shown that very good combinations of strength and toughness can be obtained with as-quenched martensitic steels.

Mechanisms of tempered martensite embrittlement in low alloy steels

An investigation into the mechanisms of tempered martensite embrittlement (TME), also know as “500°F” or “350°C” or one-step temper embrittlement, has been made in commercial, ultra-high strength 4340 and Si-modified 4340 (300-M) alloy steels, with particular focus given to the role of interlath films of retained austenite. Studies were performed on the variation of i) strength and toughness, and ii) the morphology, volume fraction and thermal and mechanical stability of retained austenite, as a function of tempering temperature, following oil-quenching, isothermal holding, and continuous air cooling from the austenitizing temperature. TME was observed as a decrease in bothKIc and Charpy V-notch impact energy after tempering around 300°C in 4340 and 425°C in 300-M, where the mechanisms of fracture were either interlath cleavage or largely transgranular cleavage. The embrittlement was found to be concurrent with the interlath precipitation of cementite during temperingand the consequent mechanical instability of interlath films of retained austenite during subsequent loading. The role of silicon in 300-M was seen to retard these processes and hence retard TME to higher tempering temperatures than for 4340. The magnitude of the embrittlement was found to be significantly greater in microstructures containing increasing volume fractions of retained austenite. Specifically, in 300-M the decrease inKIc, due to TME, was a 5 MPa√m in oil quenched structures with less than 4 pct austenite, compared to a massive decrease of 70 MPa√m in slowly (air) cooled structures containing 25 pct austenite. A complete mechanism of tempered martensite embrittlement is proposed involving i) precipitation of interlath cementite due to partial thermal decomposition of interlath films of retained austenite, and ii) subsequent deformation-induced transformation on loading of remaining interlath austenite, destabilized by carbon depletion from carbide precipitation. The deterioration in toughness, associated with TME, is therefore ascribed to the embrittling effect of i) interlath cementite precipitates and ii) an interlath layer of mechanically-transformed austenite,i.e., untempered martensite. The presence of residual impurity elements in prior austenite grain boundaries, having segregated there during austenitization, may accentuate this process by providing an alternative weak path for fracture. The relative importance of these effects is discussed.

Temper embrittlement response and toughness of a rare earth treated Ni-Cr-Mo steel

Embrittled and unembrittled transverse Charpy V-notch impact energies are compared for three commercially-produced Ni-Cr-Mo steel plates. Two of the plates were made from ingots which had been treated by the addition of mischmetal during teeming. Although the mischmetal additions were found to be ineffective in altering the susceptibility of the steel to temper embrittlement, they have a significant effect on improving the transverse toughness. It was concluded that rare earth additions, because of their beneficial effect on toughness, may provide a convenient means of coping with the problem of temper embrittlement.