SOME MECHANICAL CHARACTERISTICS OF NITROGEN STRUCTURAL STEELS

In this paper, test conditions and microstructural factors have been investigated for newly developed nitrogen steels at load rates and test temperatures suitable for future operating conditions. Static tests have been carried out at ε～ 10^?4 s^-1 and dynamic tests at ε～ 10² s^?1. Different test temperatures enable fracture mechanisms and fracture toughness variations to be studied. Anisotropy of some characteristics in the initial (non-heat-treated) state, together with heat treatment effects and structural changes under two load rates have been analysed. Fracture appearance transition temperature (FATT) values have been obtained from absorbed energy and fracture surface crystallinity values. Using yield strength and fracture toughness data, an estimate has been made of critical crack lengths for unstable fracture.     

Effect of Hydrogen on Cryogenic Mechanical Properties of Cr-Ni-Mn-N Austenitic Steels

1.IntroductionTheCr-Ni-Mn-Nausteniticsteelsarewidelyusedasstructuralmaterialsinthefieldsofcryogenicengi-neering,nuclearengineering,aeronauticaltechnique,chemical,petroleumandenergyindustriesetc.,be-causeoftheirhightoughnessandstrength,highre-sistancetocorrosion,lowmagneticpermeabilityandverygoodcryogenicmechanicalproperties.InCr-Ni-Mn-Nausteniticsteelsausteniteisstabilizedbyman-ganeseandnitrogeninplaceofpartofthenickel.SomestableCr-Ni-Mn-Nausteniticsteelspresentgoodhydrogen-resistantprope…     

Influence of material properties on dynamic fracture toughness of steels

Recent studies of plastic enclave formation at running brittle cracks were extended to account for the influence of crack tip boundary conditions on the temperature at which the enclaves start to develop. The En 2A and three other steels were used in the analysis. It was found that this temperature depends very strongly both on the magnitude and on the distribution of the stresses in the discrete crack tip zone. This suggests that the onset of enclave formation and the rate of their growth are governed by the balance of two sets of material characteristics. The first set consists of at least two parameters describing the microscopic fracture resistance which promotes enclave formation. The second set includes the macroscopic yield and flow properties which may make enclave formation more difficult in higher strength steels.These findings are related to the dynamic or crack arrest fracture toughness which is found to be derived from two different sources. One is connected with the microscopic plastic deformation of the fracturing metal in the crack tip zone and is present at all temperatures. The other is the result of enclave formation, it is present only at higher temperatures and is responsible for the energy transition. In contrast to the case of crack initiation, the dynamic fracture toughness depends not only on the microscopic fracture strength or strain but on the complete stress-displacement relationship of the weakened material which is governed by the microscopic fracture mechanism at the tip of a running crack. It is noted that the present results can be expected to be valid for all steels which fracture in the cleavage or quasi-cleavage modes.     

Identification of Materials Properties with the Help of Miniature Shear Punch Test Using Finite Element Method and Neural Networks

This paper describes an approach to identify the mechanical properties i.e. fracture and yield strength of steels. The study involves the FE simulation of shear punch test for various miniature specimens thickness ranging from 0.20mm to 0.80mm for four different steels using ABAQUS code. The experimental method of the miniature shear punch test is used to determine the material response under quasi-static loading. The load vs. displacement curves obtained from the FE simulation miniature disk specimens are compared with the experimental data obtained and found in good agreement. The resulting data from the load vs. displacement diagrams of different steels specimens are used to train the neural networks to predict the properties of materials i.e. fracture and yield strength. Two different feed forward neural networks have been created and trained in order to predict the Fracture toughness and yield strength values of different steels. L-M algorithm has been used in the networks to form an output function corresponding to the input vectors used in the network. The trained network provides the output values i.e., fracture toughness and yield strength of unknown input values, which are within in the range of data that is used for the training of network.     

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Abstract

Recently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of ≥1400 MN m^?2 but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.

MST/1413     

Fracture behavior of ultra-fine grained steel bar under dynamic loading

Ultra-fine grained steel bars were recently developed by thermo-mechanical controlled rolling with rapid cooling for increasing the strength of low carbon and low alloy steels. The developed steels are characterized by fine ferrite grains of less than 1 m and high strength as a result of grain refinement. However, their correlations between tensile properties and impact behavior are not well understood. In this paper, impact absorbed energy (E _p) and dynamic fracture toughness (J _Id) were used to evaluate the dynamic fracture behavior of the ultra-fine grained steels, and the fracture mechanisms were also investigated. For the ultra-fine grained steels, tensile stress-strain curve was shown to be correlated with the impact curve of load vs. time, and to be related to the dynamic fracture toughness. The steel with large ferrite grains, small ferrite grain colony and martensite was found to have a good combination of strength and toughness.     

Weibull master curves and fracture toughness testing Part I Master curves for quasi-static uniaxial tensile and bend tests

Three types of specimen-size-independent Weibull master curves, characterizing strength and failure of macroscopically homogeneous, brittle materials have been derived. These Weibull master curves are significant if an uniaxial tensile stress is applied to the investigated specimens, as, for example, in the case of quasi-static uniaxial tensile tests or, under some restrictions, in the case of quasi-static three- or four-point bend tests. In addition, the existence of three types of apparent fracture toughness master curves, which can be applied to any material undergoing brittle cleavage fracture such as ceramics, intermetallics, or structural steels at low homologous temperatures, has been established. Furthermore, the same is also valid for the specimen-size-independent Weibull master curves. The apparent fracture toughness master curves can be obtained, by performing fracture toughness tests, or simply by applying a mathematical transformation to the corresponding Weibull master curves, which have been evaluated from quasi-static uniaxial tensile or bend tests.     

Statistical assessment of notch toughness against cleavage fracture of ferritic steels

The work is an initial effort on adopting a statistical approach to correlate the fracture behavior between a notched and a fracture mechanics specimen. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness, and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch brittleness of ferritic steels according to a recently proposed local approach model of cleavage fracture. The temperature independence of the 2 Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the 2 Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with 2 different microstructures.     

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions. It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low-temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature. An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.     

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress-strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175-300 °C at a nominal strain rate of 3 × 10⁻³ s⁻¹. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.     

On the notch effect in load bearing capacity,apparent fracture toughness and fracture mechanisms of polymer PMMA,aluminium alloy Al7075-T651 and structural steels S275JR and S355J2

This paper presents an analysis of the notch effect in the load bearing capacity, the apparent fracture toughness and the fracture mechanisms of four different materials: polymer PMMA, aluminium alloy Al7075-T651, and structural steels S275JR and S355J2 within their corresponding ductile-to-brittle transition zone.Concerning the load bearing capacity and the apparent fracture toughness, a clear notch effect has been observed. In the case of the apparent fracture toughness, this has been adequately predicted here through the Theory of Critical Distances.As for fracture mechanisms, it is shown how these progressively change with the notch radius, from basically brittle ones in cracked conditions, to non-linear mechanisms observed for higher notch radii. This evolution has been observed in the four analysed materials and, in all cases, has justified the corresponding consequences of the notch effect on both the load bearing capacity and the apparent fracture toughness.     

Enhancement of fracture toughness in high strength steel by microstructural control

The development of new alloys with improved mechanical properties has been seriously hampered in the past by the inability of a metallurgist to relate quantitatively the variables of microstructure and fracture toughness. The emergence of a unified theory of fracture toughness in the past decade has done much to alleviate this difficulty. As a consequence of a recent interdisciplinary research effort involving both the disciplines of physical metallurgy and experimental fracture mechanics, we have been able to develop alloys with engineering properties superior to those of commercially available materials. This research has required the creation of new and unusual microstructures, utilizing a variety of thermal and thermomechanical processes. The quantitative relationships of mechanical properties (strength, ductility, work hardening, and fracture toughness) with composition and microstructure are discussed in detail for the newly developed TRIP steels. In the report of another development, it is shown how the fracture toughness of low alloy quenched and tempered steels with yield strengths over 200,000 psi can be improved by as much as 70 per cent by microstructural control. Lastly, the initial results of research on alloys intended for cryogenic service are described. The composition, heat treatment, microstructure and properties of an alloy having more than three times the toughness of the presently used alloys are discussed.     

The effects of functionally graded material structure on wear resistance and toughness of repaired weldments

To perform a long lasting, crack-free repair welding on ultrahigh strength steels, the filler metal must be chosen and applied properly. Avoiding several short-term repairs or replacements, the repaired weldment should reveal comparative characteristics such as wear resistance, toughness and hardness to base metal. In the present study, a novel functionally graded material have been introduced to obtain enhanced wear resistance and hardness at surface as well as improved fracture toughness at fusion line of repaired weldments. A comparative study of wear resistance of repaired weld metals has been carried out by pin-on-disk apparatus at 5 N normal load and 0.14 ms⁻¹ sliding speed. Fracture toughness of weld metal was also evaluated by charpy absorbed fracture energy tests and scanning electron microscopy fractograghs. The results show that by employing functionally graded layers, toughness was enhanced significantly while retaining the surface wear resistance.     

Regression-based CVN–KIC Models for hot work tool steels

Dies and tools used in hot metal forming (extrusion, forging, rolling, etc.) are exposed to high pressures, elevated temperatures, and thermo-mechanical fatigue. The most common mode of in-service die failure is fatigue fracture (brittle failure through crack propagation). Reliable determination of fracture toughness of the die material is thus critically important. However, as die steels have a combination of high-hardness and high-strength, and are used at elevated temperatures, standard plane-strain fracture toughness (K_IC) testing methods become impracticable. Alternate testing procedures such as the Charpy impact energy (CVN), together with empirical/semi-empirical correlations of K_IC to other data, are then more viable and economical. Experimental data (values of K_IC, CVN, and HRC) of H13 steels have been collected through an exhaustive literature search. This data set has been augmented through in-house experimentation: samples variously heat treated (different tempering temperatures and times, and both air-cooling and oil-quenching), and tested at different working temperatures. Linear and quadratic models are proposed for determination of fracture toughness, based on experimental (in-house) and published values of Charpy impact energy (CVN) and Rockwell hardness (HRC), both at room and at elevated temperatures.     

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

The fracture toughness of two experimental silicon-containing steels in the bainitic condition has been measured and related to the microstructural state of the steels. The optimum bainitic microstructure for high strength and high toughness combinations consists of bainitic ferrite and thin interwoven laths of retained austenite instead of cementite, this condition being achieved through the silicon addition to the steels. The thin films of retained austenite are thermally and mechanically stable and act to reduce the effective fracture grain size and also possibly help to blunt propagating microcracks; blockier volumes of retained austenite are unstable and hence not beneficial to toughness. The two experimental steels achieved strength and toughness values equal to, or better than, some commercial steels in the martensitic condition.

MST/528     

Modelling ductile fracture behaviour from deformation parameters in HSLA steels

In this work, an attempt is made to model the ductile fracture behaviour of two Cu‐strengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.     

Investigation of fracture and determination of fracture toughness of modified 9Cr–1Mo steel weld metals using AE technique

Three-point bending test and acoustic emission technique are used to determine the fracture toughness and fracture process of three types of modified 9Cr–1Mo steel weld metals. Scanning electron microscopy is used to accomplish fractography analysis of fracture specimens. Microstructure of weld metals is investigated using optical microscopy and transmission electron microscopy. The fracture process and factors which affect fracture of the 9Cr–1Mo steel weld metals in post-weld heat treated condition are studied. Experimental results show that the modified 9Cr–1Mo steel weld metals fracture by a quasi-cleavage mechanism at ambient temperature. The microstructure of the weld metals is composed of mainly tempered martensite with M₂₃C₆ precipitates. In weld metals, microcracks nucleate from non-metallic inclusions. Fractures develop very quickly when cracks started to propagate. Comparatively, in weld metals with low strength, microcracks initiate at a low stress, but propagation of cracks is limited by plastic deformation. In weld metals with high strength, microcracks nucleate at high stresses, but cracks propagate very quickly and lead to almost immediate fracture of the specimens. As a result, weld metals with the low strength have a higher fracture toughness, while weld metals with higher strengths has a lower fracture toughness.     

Vanadium high-strength low-alloy steels for low-temperature use

High-strength low-alloy (HSLA) steels having low impact transition temperature are possible substitutes for costlier 2 1/2% and 3 1/2% nickel steels. The effects of solid solution strengthening, grain size and precipitation in ferrite on the strength and toughness of low-carbon steels and the special advantages of vanadium as an alloying element in HSLA steels, are discussed. An investigation has been carried out with 1.5% manganese low-carbon steels containing vanadium in the range 0.12% to 0.29% and 0.013% to 0.017% nitrogen. Room temperature tensile and sub-zero temperature impact tests down to–100° C, and a metallographic study to determine the grain sizes and pearlite contents of the steels normalized at different temperatures, have been carried out. Calculations are made with empirical equations for yield and tensile strengths and the values obtained are compared with those experimentally observed. The solubility products of vanadium carbide and vanadium nitride are calculated and compared with available data to throw light on the mechanism of strengthening of the steels.     

Assessment of notched structural steel components using failure assessment diagrams and the theory of critical distances

When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. The proposal of this paper consists, on the one hand, in the application of the theory of critical distances for the estimation of the notch fracture toughness and, therefore, for the conversion of the notched situation into an equivalent cracked situation in which the material develops a higher fracture resistance. On the other hand, once the notch fracture toughness has been defined, the assessment is performed using the failure assessment diagram methodology, and assuming that the notch effect on the limit load is negligible. The methodology has been applied to 336 CT notched fracture specimens made of two different structural steels, covering temperatures from the corresponding lower shelf up to the upper shelf, providing satisfactory results and a noticeable reduction in the overconservatism derived from the analyses in which the notch effect is not considered.