The fracture toughness and toughening mechanisms of wrought low carbon arc cast,oxide dispersion strengthened,and molybdenum-0.5 pct titanium-0.1 pct zirconium molybdenum plate stock

The high-temperature strength and creep resistance of low carbon arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened (ODS) molybdenum, and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have attracted interest in these alloys for various high-temperature structural applications. Fracture toughness testing of wrought plate stock over a temperature range of −150 °C to 1000 °C using bend, flexure, and compact tension (CT) specimens has shown that consistent fracture toughness results and transition temperatures are obtained using subsized 0.5T bend and 0.18T disc-CT specimens. Although the fracture toughness values are not strictly valid in accordance with all ASTM requirements, these values are considered to be a reasonable measure of fracture toughness. Ductile-to-brittle transition temperature (DBTT) values were determined in the transverse and longitudinal orientations for LCAC (200 °C and 150 °C, respectively), ODS (<room temperature and −150 °C), and TZM (150 °C and 100 °C). At test temperatures > DBTT, the fracture toughness values for LCAC ranged from 45 to 175 MPa√m, TZM ranged from 74 to 215 MPa√m, and the values for ODS ranged from 56 to 149 MPa√m. No temperature dependence was resolved within the data scatter for fracture toughness values between the DBTT and 1000 °C. Thin sheet toughening is shown to be the dominant toughening mechanism, where crack initiation/propagation along grain boundaries leaves ligaments of sheetlike grains that are pulled to failure by plastic necking. Specimen-to-specimen variation in the fraction of the microstructure that splits into thin sheets is proposed to be responsible for the large scatter in toughness values at test temperatures > DBTT. A finer grain size is shown to result in a higher fraction of thin sheet ligament features at the fracture surface. As a result finer grain size materials such as ODS molybdenum have a lower DBTT.     

低碳低合金钢的静力韧度与断裂韧度

采取静拉伸和平面应变断裂韧性测试方法,测量了S355,S275,Q345D,Q345E4种低碳低合金钢自室温至-100℃的基本力学性能,包括屈服强度、抗拉强度、静力韧度、断裂韧度以及韧脆转变温度,试图寻找4种金属材料静力韧度和断裂韧度之间的关系;通过对200多组样品的试验结果进行分析,发现4种金属材料在其各自韧脆转变温度之上的静力韧度Uk和断裂韧度J0.2BL存在线性关系。     

Relationship between tension toughness and fracture toughness of low carbon low alloy steel北大核心CSCD

Basic mechanical properties for four kinds of low carbon and low alloy steels, including S355, S275, Q345D, Q345E, were examined by means of tension toughness and plane strain fracture toughness tests at room temperature to -100 °C Test results of yield strength, tensile strengh, tension toughness, fracture toughness and ductile-brittle transition temperature for each steel were obtained for the sake of identifying the relationship between tension toughness and fracture toughness. More than 200 groups of samples were tested and analysed. It shows that there is a linear relationship between tension toughness Uk and fracture toughness J0.2BL for each steel at a tempearture which is greater than its own ductile-brittle transition temperature respectively. And this sastis-fied the requirement of the linear equation.     

A comparison of fracture behavior of low alloy steel with different sizes of carbide particles

The fracture behaviors of low alloy steels with similar grain sizes but different sizes of carbide particles were investigated using precracked and notched specimens. The results indicate that in precracked specimens (COD), steel with coarser carbide particles has a lower toughness than steel with finer carbide particles over a temperature range from –196 °C to – 90 °C. However, in notched specimens (four-point bending (4PB) and Charpy V), these two steels shows similar toughness at low temperature where specimens are fractured by cleavage without fibrous cracking. In the transition temperature range, the steel with coarser carbide particles conversely shows a little higher toughness due to the longer extension length of the fibrous crack. This phenomenon indicates that in precracked specimens, the second-phase particles play a leading role in cleavage fracture, while in notched specimens, the grain size dominates the fracture behavior.     

Effect of quenching temperature on microstructure and fracture toughness of high carbon steel

The effect of quenching temperatures on microstructure and fracture toughness of high carbon steel was investigated. Plane strain fracture toughness was tested with compact tension specimen. Microstructure and fracture morphology of KIC samples after quenching and tempering treatment were examined by scanning electron microscope (SEM).The results show that the residual carbides of steel in hardened state decreasea with the quenching temperature increasing and totally disappear after quenched at 920??;the grain size grows up obviously when the quenching temperature is more than 960??. The microstructure in high temperature tempered state is composed of residual carbides, precipitated carbides and ferrite matrix;plasticity decreases monotonically; the fracture toughness gradually decreases in the range from 800?? to 960??,and then almost invariant; the fracture type of KIC specimens is gradually changed from cleavage fracture to intergranular fracture. The main reason for the changes of fracture toughness has close relationship with the plasticity.     

Comparative fracture toughness of an ultrafine grained Fe-Ni alloy at liquid helium temperature

The fracture behavior of an Fe-12 Ni-0.25 Ti alloy, grain refined through thermal cycling was studied down to liquid helium temperature using a simple method of cryogenic fracture toughness testing. Comparison tests were also made with two other common cryogenic alloys. Ultrafine grained Fe-12 Ni-0.25 Ti alloy, which is 100 pct ferritic, behaved in a ductile manner similar to 304 austenitic steel. It was shown that the ductile-brittle transition temperature of the ferritic steel specimen can be suppressed below liquid helium temperature by grain refinement even in the presence of a sharp crack. Yield strength of 149 ksi with fracture toughness of 307 ksi√in. at liquid nitrogen temperature, and yield strength of 195 ksi with fracture toughness of 232 ksi√in. at liquid helium temperature were obtained.     

Fracture resistance of low-carbon alloy irons

The temperature dependence of the critical stress intensity factor and of the fracture energy were measured on six low-carbon iron alloys, one containing 0.002 wt pct C and five containing 0.02 wt pct C. Either Ni, P, Si, or Si and Mn were added to four of the five 0.02C irons in quantities typically found in ferritic steels. The fracture tests were conducted at rapid (but less than impact) speed of 1 ips on fatigue cracked, three-point bend beam specimens. Each alloy was tested over a temperature range of —195° to 24°C in both furnace-cooled and quench-aged states. Both alloying and heat treatment produced wide differences in the fracture resistance of these alloys. The quench-aged 0.002C iron and furnace-cooled phosphorus alloy failed by intergranular separation, whereas the remaining alloys exhibited cleavage fractures. With the exception of 0.002C iron, an alloy in the quench-aged condition had higher fracture toughness than the same alloy in the furnace-cooled state. The transition temperature, however, was influenced by heat treatment only in the plain carbon irons. In this case the transition temperature was independent of carbon content but the furnace-cooled specimen had a lower transition temperature than the quench-aged specimens. D. C. A. R. COX, formerly Exchange Scientist at the Naval Research Laboratory     

Subcritical intergranular crack growth rates and thresholds of Fe and Fe + Sb

The addition of 0.06 monolayers of antimony to the grain boundaries of iron with 0.3 monolayers of sulfur was found to have no effect on the fracture toughness or subcritical crack growth behavior at cathodic potentials. Tests were conducted using compact tension type samples tested in 1N H₂SO₄ at cathodic potentials of −0.6V (SCE) to −1.25V (SCE). The absence of any effect of antimony on the fracture toughness was related to iron being in a “minimum” fracture toughness condition such that further segregation of an embrittling element had no effect. Also, the subcritical intergranular crack growth threshold was found to decrease with increasing cathodic potential consistent with results reported by others for transgranular fracture of steels in gaseous hydrogen.     

Fracture toughness of aisi M2 high-speed steel and corresponding matrix tool steel

The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4 pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated. In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit 2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples is a maximum. The above described results were explained through changes in the microstructure and the fracture modes. CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI.     

Correlation of rolling condition,microstructure, and low-temperature toughness of X70 pipeline steels

Correlation of rolling conditions, microstructure, and low-temperature toughness of high-toughness X70 pipeline steels was investigated in this study. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot rolling, and their microstructures were varied by rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below −100 °C when the finish cooling temperature range was 350 °C to 500 °C, showing excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 pct, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture occurred at some finish cooling temperatures.     

Effect of prestrain on stretch-zone formation during ductile fracture of Cu-strengthened high-strength low-alloy steels

The effects of prestrain on the ductile fracture behavior of two varieties of Cu-strengthened high-strength low-alloy (HSLA) steels have been investigated through stretch-zone geometry measurements. It is noted that the ductile fracture-initiation toughness of both the steels remained unaltered up to prestrains of ∼2 pct, beyond which the toughness decreased sharply. A methodology for estimating the stretch-zone dimensions is proposed. Fracture-toughness estimations through stretch-zone width (SZW) and stretch-zone depth (SZD) measurements revealed that the nature of the variation of ductile fracture toughness with prestrain can be better predicted through SZD rather than the SZW measurements. However, for the specimen geometries and prestrain levels that were investigated, none of these methods were found suitable for quantifying the initiation fracture toughness.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Fracture toughness of the lean duplex stainless steel LDX 2101

Fracture toughness testing was performed on the recently developed lean duplex stainless steel LDX 2101 (EN 1.4162, UNS S32101). The results were evaluated by master curve analysis, including deriving a reference temperature. The master curve approach, originally developed for ferritic steels, has been used successfully. The reference temperature corresponds to a fracture toughness of 100 MPa√m, which characterizes the onset of cleavage cracking at elastic or elastic-plastic instabilities. The reference temperature, T ₀, was −112 °C and −92 °C for the base and weld materials, respectively. In addition, the fracture toughness is compared with impact toughness results. Complementary crack tip opening displacements (CTODs) have also been calculated. The toughness properties found in traditional duplex stainless steels (DSS) are generally good. The current study verifies a high fracture toughness for both base and weld materials and for the low alloyed grade LDX 2101. Even though the fracture toughness was somewhat lower than for duplex stainless steel 2205, it is still sufficiently high for most low-temperature applications.     

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (−190 to 200°C), although validJ _Ic measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1. An erratum to this article is available at .     

回火温度对高碳钢断裂韧性的影响

研究了回火温度对高碳钢断裂韧性及其他力学性能的影响。利用紧凑拉伸试样测量其平面应变断裂韧度,扫描电镜(SEM)观察回火后的组织演变规律及断裂韧度试样断口形貌。结果表明:随着回火温度的升高,马氏体逐渐分解消失,从过饱和α固溶体中析出的碳化物数量逐渐增多并发生聚集长大,强度和硬度下降,塑性、断裂韧性和冲击韧性上升。位错强化和固溶强化作用减弱是试验钢强度降低、韧性升高的主要原因。     

Microstructure and texture of hot-rolled Cb-Ti and V-Cb microalloyed steels with differences in formability and toughness

The relationship between microstructure and formability (also toughness) of industrially processed Cb-Ti and V-Cb steels with similar yield strength of about 600 MPa and total elongation of 20 pct was examined. The steels were processed under similar conditions and any variations in processing history were unintentional. Cb-Ti steels exhibited superior formability and toughness in relation to V-Cb steels. The improved formability and toughness of Cb-Ti steels is attributed to the cumulative contribution of relatively finer grain size, narrow ferrite grain size distribution, inherently more ductile behavior and microplasticity, the reduced intensity of {100}〈011〉 texture, and slightly higher intensity of the desired {332}〈113〉 texture.     

The improvement of cryogenic mechanical properties of Fe-12 Mn and Fe-8 Mn alloy steels through thermal/mechanical treatments

An investigation has been made to improve the low temperature mechanical properties of Fe-8Mn and Fe-12Mn-0.2 Ti alloy steels. A reversion annealing heat treatment in the two-phase (α+ γ) region following cold working has been identified as an effective treatment. In an Fe-12Mn-0.2Ti alloy a promising combination of low temperature (-196°C) fracture toughness and yield strength was obtained by this method. The improvement of properties was attributed to the refinement of grain size and to the introduction of a uniform distribution of retained austenite (γ). It was also shown that an Fe-8Mn steel could be grain-refined by a purely thermal treatment because of its dislocated α martensitic structure and absence of ε martensite. As a result, a significant reduction of ductile to brittle transition temperature was obtained. formerly with the Lawrence Berkeley Laboratory, University of California.