Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.     

Low‐alloyed TRIP‐Steels with Optimized Strength,Forming and Welding Properties

To obtain the superior strength‐ductility‐balance of TRIP‐grades, a special chemical composition in combination with well adapted processing parameters are a prerequisite. Despite of their excellent formability performance in terms of drawability as characterized by high n‐ and elongation values, compared to mild steels TRIP‐grades are challenging in the press and the body shops. The high strength level in combination with the high work hardening of TRIP‐grades result in higher levels of spring back compared to mild steels and higher press forces are required. Furthermore, a higher sensitivity to failure for sharp bending radii and a deterioration of the formability of punched edges is reported for TRIP‐grades. While spring back can only be minimized by advanced forming processes supported by new simulation techniques with improved ability to predict spring back, the sensitivity to failure under special forming conditions can be influenced by optimizing microstructural features. Contrary to the forming behaviour, which is influenced significantly by the microstructure, the weldability is mainly governed by the chemical composition and the surface condition of the material. The high carbon content of TRIP‐grades compared to mild steels results in a higher hardening potential after welding. Additionally, a fracture behaviour untypical for mild steels after destructive testing of spot welds is sometimes observed for TRIP‐grades, which is assessed critically by some OEMs. In this work, after a discussion of the processing conditions, possibilities are demonstrated to improve the forming behaviour by an optimization of the microstructure and the spot weldability by adapting the chemical composition of low‐alloyed TRIP grades. First very promising results for TRIP‐grades with a minimum tensile strength level of 700 MPa are discussed.     

A Strategy of Avoiding Delayed Fracture in Ultrahigh Strength Automotive Steels by Nb Microalloying

Recently the automotive industry has been confronted with the phenomenon of delayed fracture.This phenomenon was not relevant in earlier years since the strength level of the steels was generally below a critical level.However,delayed fracture is not necessarily related to an absolute strength value but rather to microstructural features as well as pre-existing micro-damage in the material that are likely to occur in ultrahigh strength steels.Niobium microalloying in combination with appropriate processing can effectively help to improve the resistance against delayed fracturing in such steels.The paper outlines a strategy how to achieve this based on microstructural control and hydrogen trapping.     

Achieving High Strength and High Ductility in Friction Stir-Processed Cast Magnesium Alloy

Friction stir processing (FSP) is emerging as an effective tool for microstructural modification and property enhancement. As-cast AZ91 magnesium alloy was friction stir processed with one-pass and two-pass to examine the influence of processing conditions on microstructural evolution and corresponding mechanical properties. Grain refinement accompanied with development of strong basal texture was observed for both processing conditions. Ultrafine-grained (UFG) AZ91 was achieved under two-pass FSP with fine precipitates distributed on the grain boundary. The processed UFG AZ91 exhibited a high tensile strength of ~435 MPa (117 pct improvement) and tensile fracture elongation of ~23 pct. The promising combination of strength and ductility is attributed to the elimination of casting porosity, and high density of fine precipitates in an UFG structure with quite low dislocation density. The effects of grain size, precipitate, and texture on deformation behavior have been discussed.     

Hydrogen Embrittlement of Automotive Advanced High-Strength Steels

Advanced high-strength steels (AHSS) have a better combination between strength and ductility than conventional HSS, and higher crash resistances are obtained in concomitance with weight reduction of car structural components. These steels have been developed in the last few decades, and their use is rapidly increasing. Notwithstanding, some of their important features have to be still understood and studied in order to completely characterize their service behavior. In particular, the high mechanical resistance of AHSS makes hydrogen-related problems a great concern for this steel grade. This article investigates the hydrogen embrittlement (HE) of four AHSS steels. The behavior of one transformation induced plasticity (TRIP), two martensitic with different strength levels, and one hot-stamping steels has been studied using slow strain rate tensile (SSRT) tests on electrochemically hydrogenated notched samples. The embrittlement susceptibility of these AHSS steels has been correlated mainly to their strength level and to their microstructural features. Finally, the hydrogen critical concentrations for HE, established by SSRT tests, have been compared to hydrogen contents absorbed during the painting process of a body in white (BIW) structure, experimentally determined during a real cycle in an industrial plant.     

Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part I: Yield Strength

Low carbon Nb-Mo microalloyed steels show interesting synergies between the “micro”-alloying elements when high strength–high toughness properties are required. Strain accumulation in austenite is enhanced, and therefore grain sizes are refined in the final microstructures. The presence of Mo facilitates the presence of non-polygonal phases, and this constituent modification induces an increment in strength through a substructure formation as well as through an increase in the dislocation density. Regarding fine precipitation and its strengthening effect, the mean size of NbC is reduced in the presence of Mo and their fraction increased, thus enhancing their contribution to yield strength. In this paper, a detailed characterization of the microstructural features of a series of microalloyed steels is described using the electron-backscattered diffraction technique. Mean crystallographic unit sizes, a grain boundary misorientation analysis, and dislocation density measurements are performed. Transmission electron microscopy is carried out to analyze the chemical composition of the precipitates and to estimate their volume fraction. In this first part, the contribution of different strengthening mechanisms to yield strength is evaluated and the calculated value is compared to tensile test results for different coiling temperatures and compositions.     

Improved Cold Formability by Thermo‐Mechanical Rod Rolling

Usually, unalloyed medium carbon steels and 1% chromium steels for cold heading applications are produced by hot rolling, subsequent controlled cooling and soft annealing. It is shown that by thermomechanical rod rolling and slow air cooling at the loop cooling conveyor the tensile strength can be lowered to such an extent so that cost‐intensive soft annealing procedures are not required. The effects of the processing parameters on the microstructural evolution and final mechanical properties of steel grades 23MnB4 and 1% Cr‐steels like 32CrB4, 37Cr4 and 41 Cr4 are tested by a Gleeble machine. Using a microstructural process model, called CAROD, the entire rolling and cooling process has been optimized.     

Mechanisms Governing Fatigue, Damage, and Fracture of Commercially Pure Titanium for Viable Aerospace Applications

In this paper, the cyclic stress amplitude controlled high-cycle fatigue properties and final fracture behavior of commercially pure titanium (Grade 2) are presented and discussed. The material characterization was developed and put forth for selection and use in a spectrum of applications spanning the industries of aerospace, defense, chemical, marine, and commercial products. Test specimens were prepared from the as-received plate stock of the material with the stress axis both parallel (longitudinal) and perpendicular (transverse) to the rolling direction of the plate. The test specimens were cyclically deformed at a constant load ratio of 0.1, at different values of maximum stress, and the corresponding cycles-to-failure is presented. The cyclic fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode, the intrinsic features on the fatigue fracture surface, and the role of applied stress-microstructural feature interactions in governing failure. The intrinsic features on the fracture surface as a function of maximum stress and resultant cyclic fatigue life are discussed.     

An investigation of the mechanism of splitting which occurs in tensile specimens of high strength low alloy steels

A study has been conducted to investigate the metallurgical cause of splitting that is sometimes observed to accompany regular fracture in tensile and Charpy impact specimens. The susceptibility of a steel to tensile specimen splitting (TSS) has been shown to depend on both the composition as well as the processing of Cb (Nb) and V bearing high-strength, low-alloy steels. This study has revealed that Fe₃C, either as grain boundary precipitate or in pearlite of a particular distribution, was the microstructural feature which seemed to be responsible for the TSS in the steels investigated. The compositional and processing factors influenced the susceptibility of specimens to TSS by controlling the Fe₃C precipitation. There was no correlation between the susceptibility of a microstructure to TSS and its short transverse ductility. This result indicates that the occurrence of splitting in broken tensile or impact specimens is neither related to nor a good predictor of the probability of occurrence of the lamellar tearing that may occur during welding. A mechanism is presented which establishes the link between composition, processing, microstructure, splitting susceptibility and short transverse ductility.     

Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part II: Impact Toughness

The present paper is the final part of a two-part paper where the influence of coiling temperature on the final microstructure and mechanical properties of Nb-Mo microalloyed steels is described. More specifically, this second paper deals with the different mechanisms affecting impact toughness. A detailed microstructural characterization and the relations linking the microstructural parameters and the tensile properties have already been discussed in Part I. Using these results as a starting point, the present work takes a step forward and develops a methodology for consistently incorporating the effect of the microstructural heterogeneity into the existing relations that link the Charpy impact toughness to the microstructure. In conventional heat treatments or rolling schedules, the microstructure can be properly described by its mean attributes, and the ductile–brittle transition temperatures measured by Charpy tests can be properly predicted. However, when different microalloying elements are added and multiphase microstructures are formed, the influences of microstructural heterogeneity and secondary hard phases have to be included in a modified equation in order to accurately predict the DB transition temperature in Nb and Nb-Mo microalloyed steels.     

Effect of quench rate on microstructure and tensile properties of ALSL 4320 and 4340 steels

A study has been made of the effect of quench rate on the microstructure and tensile properties of two commercial AISI 4320 and 4340 steels having fully martensitic structures. The steels were quenched from various temperatures from 1323 to 1473 K, at two different quench rates using iced brine (fast quench treatments) and oil held at 373 K (slow quench treatments). Tensile properties of these steels, after double-tempering at 473 K with intermediate quenching and refrigeration, were determined at ambient temperature (293 K) using an Instron test machine. The microstructural changes accompanying these quench rates were examined by means of optical and thin-foil transmission electron microscopic techniques. In the 4320 steel with a relatively high Ms temperature, the slow quench treatments compared to the fast quench treatments increased both the 0.2 pct proof stress and the ultimate tensile strength at similar total elongation levels, regardless of the prior austenite grain size, while the strength data of the slowly quenched steels exhibited a large scatter as the prior austenite grain size increased. However, in the 4340 steel with a relatively low Ms temperature tensile properties were less sensitive to quench rate, while the slow quench treatments compared to the fast quench treatments increased slightly only the 0.2 pct proof stress. From microstructural results, it is suggested that the beneficial effect on the strength of the slowly-quenched steels is caused by a dispersion-hardening effect due to carbon segregation or fine carbide precipitation in the martensite during the quench(i.e., autotempering).     

Delayed Fracture Behavior of CrMo-Type High Strength Steel Containing Titanium

Generally ,thequenchedandtemperedlowalloysteelswithtensilestrengthabove 12 0 0MPaaresus ceptibletohydrogen induceddelayedfracture(HIDF) [1,2 ] .Extensivestudiesonthedelayedfracturebehaviorofhighstrengthsteelshavebeenperformedtheseyears,butthesatisfactorys…     

亚稳奥氏体和微合金元素对冷轧高强汽车钢性能的影响

介绍了亚稳奥氏体和微合金元素（Nb、V）对TRIP钢、TWIP钢和Q＆P钢等冷轧高强汽车钢性能的影响。主要包括调控冷轧高强汽车钢亚稳奥氏体组织的成分体系和连续退火工艺、亚稳奥氏体组织特征及测量方法,及其对高强汽车钢强塑积、拉伸曲线形状等性能的影响等。同时介绍了微合金元素（V、Nb）对冷轧高强汽车钢成形性、强度等级、延迟断裂等个性化性能的影响。     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel

The temperature- and strain-rate-dependent tensile behavior of hydrogen-charged low-alloy pressure vessel steel ASTM A508 C1.3 has been investigated. The fatigue crack initiation and propagation behavior of the steel in high-temperature water environments has also been evaluated. It was found that hydrogen played significant roles in both tensile and cyclic deformation processes, especially in the temperature and strain-rate region of dynamic strain aging (DSA). The presence of hydrogen resulted in a distinct softening in tensile strength and a certain loss in tensile ductility in the DSA region. Remarkable degradation in fatigue crack initiation and propagation resistance in high-temperature water environments was observed in the DSA strain-rate region. Typical hydrogen-induced cracking features also appeared on the corresponding fatigue fracture surfaces. The interactions between hydrogen and DSA in tensile and cyclic deformation processes are discussed as well as their combined effects on the environmentally assisted cracking (EAC) mechanism of pressure vessel steels in high-temperature water environments.     

Microstructure effects on tensile properties of tungsten-Nickel-Iron composites

Controlled processing of heavy alloys containing 88 to 97 pct W resulted in high sintered densities and excellent bonding between the tungsten grains and matrix. For these alloys, deformation and fracture behavior were studiedvia slow strain rate tensile testing at room temperature. The flow stress increased and the fracture strain decreased with increasing tungsten content. The tradeoff between strength and ductility resulted in a maximum in the ultimate tensile strength at 93 pct W. Microstructure variations, notably grain size, explain sintering temperature and time effects on the properties. During tensile testing, cracks formed on the surface of the specimens at tungsten-tungsten grain boundaries. The crack density increased with plastic strain and tungsten content. The surface cracks, though initially blunted by the matrix, eventually increased in density until catastrophic failure occurred. An empirical failure criterion was developed relating fracture to a critical value of the surface crack tip separation distance. Application of the model explains the effects of microstructural variables on tensile properties. Formerly Graduate Research Assistant at Rensselaer Polytechnic Institute.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Formability of High Strength Dual‐phase Steels

The application of ferritic‐martensitic dual‐phase (DP) steels has become an increasing trend in the automotive industry due to the possibility to achieve significant weight reduction and fuel efficiency with improved crash performance while keeping the manufacturing costs at affordable levels. In order to meet the different design requirements of individual auto‐body components, a wide variety of DP grades exhibiting different strength and ductility levels is currently industrially produced. Despite the numerous studies on the relationship between the mechanical properties and the microstructural characteristics of DP steels over the last decades, it is still a challenge to increase their formability at a constant strength level (or equivalently increasing the strength while maintaining a high ductility). One of the possibilities to increase strength is grain refinement. Ultrafine‐grained ferritic‐martensitic microstructures were produced by intercritical annealing of a cold‐rolled, pre‐processed dual‐phase steel. Ferrite mean grain sizes in the order of ~ 1.5 μm were obtained. The mechanical properties of these steels are studied, revealing the beneficial effect of grain refinement. Ultimate tensile strength above 800 MPa is achievable, while reaching remarkable high uniform and total elongations, which are only slightly affected by the martensite volume fraction. Moreover, the yield to tensile strength ratio can be adjusted between 0.4 and 0.5. Light and electron microscopy investigations, fracture profile and fracture surface analyses, hole expansion tests and additional ultramicrohardness measurements are used for the interpretation of the results and for the correlation of the mechanical properties and the formability characteristics with the microstructure of the steel.     

Microstructural engineering applied to the controlled cooling of steel wire rod: Part III. Mathematical model-formulation and predictions

In this final part of the study, a mathematical model incorporating heat flow, microstructural phenomena, and structure-composition-mechanical property relationships has been developed to compute the yield strength (YS) and ultimate tensile strength (UTS) of steel rod control cooled on a Stelmor line. The predictive capability of the model, in terms of temperature response, microstructural evolution, and strength of the rods, has been tested by comparison to measurements from an extensive set of laboratory and plant trials. Thus, the model has been shown to simulate the complex heat flow and microstructural phenomena in the steel rod very well, although improvements need to be sought in the characterization of the austenite-ferrite transformation kinetics and of pearlite interlamellar spacing. The latter variable has a significant influence on the strength of eutectoid steels. Nonetheless, the model consistently is capable of predicting the strengths of plain-carbon steel rods ranging from 1020 to 1080 to within ± 10 pet. Formerly Graduate Student, The University of British Columbia.     

Tensile Deformation Behavior of Fe-Mn-Al-C Low Density Steels

Room temperature tensile tests of Fe-Mn-Al-C low density steels with four different chemical compositions were conducted to clarify the dominant deformation mechanisms.Parameters like product of strength and elongation,as well as specific strength and curves of stress-strain relations were calculated.The microstructures and tensile fracture morphologies were observed by optical microscope,scanning electron microscope and transmission electron microscope.The tensile behavior of low density steel was correlated to the microstructural evolution during plastic deformation,and the effects of elements,cooling process and heat treatment temperature on the mechanical properties of the steels were analyzed.The results show that the tensile strength of steels with different cooling modes is more than 1 000 MPa.The highest tensile strength of 28Mn-12Al alloy reached 1 230 MPa,with corresponding specific strength of 189.16 MPa·cm~3·g~(-1),while the specific strength of 28Mn-10 Al alloy was 178.98 MPa·cm~3·g~(-1),and the excellent product of strength and elongation of 28Mn-8Al alloy was over 69.2 GPa·%.A large number of ferrite reduced the ductility and strain hardening rate of the alloy,while the existence of κ carbides may improve the strength but weaken the plasticity.Some fine κ carbides appeared in the water-quenched specimen,while coarse κ carbides were observed in the air-cooled specimen.High temperature heat treatment improved the decomposition kinetics of γ phase and the diffusion rate of carbon,thus speeded up the precipitation of fine κ carbides.The dominant deformation mechanism of low density steel was planar glide,including shear-band-induced plasticity and microbandinduced plasticity.