Microfractography in failure analysis of cold forging dies

In this paper microfractographic features in fracture surfaces for tensile, fatigue, impact and three point bending of cold forging die steels with Rockwell C scale hardness number of 52–68 is presented. The emphasis is placed upon the stretched zone formation ahead of fatigue crack and the relation between the stretched zone width and fracture toughness of these cold forging die steels. Finally it is briefly described that the quantitative analysis for cold forging die failure can be possible by measuring the stretched zone width.     

Enhancing high-cycle fatigue properties of cold-drawn Fe–Mn–C TWIP steels

High-cycle fatigue properties of cold-drawn twinning-induced plasticity (TWIP) steel, a favored candidate for replacing fully pearlitic (FP) steels in wire applications, were investigated. The high-cycle fatigue tests were conducted on cold-drawn TWIP and FP steels that had comparable ultimate tensile strength for comparison. Fatigue strength of both TWIP and FP steels increased with the tensile strength, but the TWIP steel cold-drawn to a tensile strength of 1.5 GPa exhibited a very low fatigue ratio (a ratio of fatigue strength to tensile strength) which deviated far from the predicted linear relationship. Fracture surface analysis showed that crack initiation mainly occurred at the ferrite matrix in FP steels, while either at grain or twin boundaries in TWIP steels where a large density of dislocations piled up during cold drawing. In the case of TWIP steels, the presence of inclusions at grain boundaries led to high local stress concentration and caused early intergranular fatigue cracking as notch sensitivity increased with tensile strength. Subsequent annealing after cold-drawing effectively increased fatigue strength of TWIP steels. It was suggested that TWIP steel revealing both high tensile strength and excellent high cycle fatigue strength could be a promising alternative for replacing conventional FP steels.     

High-temperature properties and microstructural stability of hot-work tool steels

Indexable insert tools for machining operations are in service exposed to high temperatures and cyclic mechanical loads. Secondary hardening steels such as hot-work steels are commonly used for tools subjected to thermal exposure. However, these steels, highly alloyed with strong carbide forming elements as Cr, V and Mo, are generally difficult to machine and machining represents a large fraction of the production cost of a tool. Thus, the present study concerns the development of a new steel with improved machinability and meeting the requirements for high-temperature properties.Softening resistance of the THG2000 and QRO90 tool steels, commonly used in hot-work applications, and a newly developed tool steel MCG2006 with lower alloying content of carbide forming elements, was investigated by tempering and isothermal fatigue testing. Mechanisms of high-temperature softening of the tested tool steels were discussed with respect to their microstructure and high-temperature mechanical properties. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis.No difference in softening behaviour was found among the QRO90 and MCG2006 regarding hot hardness measurements. The THG2000 indicated some stabilization of the hardness between 450 and 550 °C and a considerable hardness decrease at higher temperatures.The short-time cyclic softening in isothermal fatigue was controlled by dislocation rearrangement and annihilation. The alloying composition of the steels presently tested had no influence on the dislocation density decrease.The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance. The QRO90 with greater molybdenum and lower chromium contents than in the THG2000 show the best resistance to softening among the tested grades at all temperatures. The MCG2006, leaner alloyed with the carbide forming elements and alloyed with 4 wt% nickel, has better temper resistance than THG2000 at higher temperatures and longer tempering times.     

Manifestation of Hydrogen and Low-Temperature Brittleness in Near-Threshold Cyclic Crack Resistance of Materials

We experimentally show that the realization of conditions of plane deformation at the tip of a fatigue crack is not sufficient for guaranteeing the unique dependence of the crack growth rate on the range of the stress intensity factor, which is explained by the effect of crack closure. We describe advantages and disadvantages of the effective range of the stress intensity factor as a parameter that determines the mechanical conditions for the propagation of a fatigue crack. We analyze the phenomenon of positive influence of strengthening factors (a decrease in the temperature of testing and hydrogenation) on the cyclic crack resistance of materials in a low-amplitude range of loading determined with regard for the effect of crack closure. The decrease in the crack growth rate and the increase in fatigue thresholds are intensified as the level of loading decreases and the ductility of materials increases. Differences in the influence of strengthening factors in low- and high-amplitude ranges of loading are explained by different mechanisms of fracture controlled by the shearing strength and the tensile strength, respectively. We give several examples of the mechanical behavior of materials that show the inversion of the influence of hydrogen on the resistance to fracture: fatigue fracture of smooth steel specimens in gaseous hydrogen, high-temperature corrosion fatigue of preliminary hydrogenated titanium alloys, and the influence of hydrogenation on the wear resistance of structural steels in the process of friction and cavitation and on the parameters of cutting of a tool steel.     

LOW- AND HIGH-CYCLE FATIGUE PROPERTIES OF VARIOUS STEELS SPECIFIED IN JIS FOR MACHINE STRUCTURAL USE

Abstract —Low-cycle fatigue properties were investigated on four carbon steels and five low alloy steels specified in JIS (Japanese Industrial Standard) for machine structural use, which are the most commonly used in Japan. Several different heats from each of several representative manufacturers were sampled so as to represent the average fatigue characteristics of current materials. The cyclic deformation behaviour of material was denned by comparing the monotonie yield stress on the extrapolated tensile work hardening curve with the cyclic yield stress in the cyclic stress-strain curve determined by incremental step test. The normalized ferrite-pearlitic steels cyclically hardened, while the quench-tempered martensitic were cyclically stable or softened. The S–N relations derived from the strain-controlled low-cycle tests were compared with the results obtained by load-controlled high-cycle tests. The extrapolated S–N curves based on the cyclic stress-strain curve predicted the fatigue strength in the high-cycle range to be stronger for cyclic-hardening steels, but weaker for cyclic-softening steels. The predicted S–N curves for stable steels coincided with the high cycle test data. The fatigue limit had a proportional relationship with cyclic yield stress, slightly depending on the cyclic deformation behaviour. On the other hand, the cyclic yield stress was found to exhibit a very good linear correlation with the monotonie tensile strength, independent of cyclic deformation behaviour. This explains the empirical law that the fatigue limit is approximately proportional to the tensile strength.     

Effect of retained austenite characteristics on fatigue behavior and tensile properties of transformation induced plasticity steel

Transformation induced plasticity steels are commonly used for automotive industry due to their high strength and high ductility. These steels achieve good balance of strength and ductility due to transformation of retained austenite to martensite during deformation. In this study, effect of retained austenite characteristics on fatigue and tensile property of conventional CMnSi steel is evaluated. Tensile and fatigue test were carried out at room temperature. After mechanical tests, fractography observations were carried out by scanning electron microscopy. All samples show reasonably high values of tensile strength and fatigue limit. Results of fatigue test show that fatigue performance of this steel improved by increasing volume fracture of retained austenite.     

Effect of mean stress on fatigue properties of 1800 MPa-class spring steels

Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in terms of stress amplitude. However, the fatigue strength degradation was less than that expected from a modified-Goodman line, and the best fit line was obtained by connecting the fatigue limit at zero mean stress to true fracture strength instead of tensile strength. This research also reviewed application of a power low to the stress ratio effect evaluation. In these results, the difference between the spring and conventional steels was negligible.     

结构钢S355N及S355N-Z25的低温CTOD断裂韧性及其da/dN研究

分别采用三点弯曲SE(B)与紧凑拉伸C(T)试样,通过-20℃的低温裂纹尖端张开位移CTOD断裂韧性试验及疲劳裂纹扩展速率da/dN试验,研究了S355N及S355N-Z25结构钢的裂纹启裂与扩展的抗力.研究结果表明:S355N钢抵抗裂纹启裂及前期扩展的能力比S355N-Z25钢强,但前者抵抗裂纹后期扩展的能力比后者弱...     

一种自动控制的液压式低温改性处理机

低温 (深冷 )处理作为一门新的工艺技术对高速钢、工具钢、模具钢、高碳高合金钢和轴承钢等材料均能明显的改善其力学性能 ,提高耐磨性与延长其使用寿命。在原真空室型低温处理装置的基础上 ,开发成功了一种操作简单、使用方便及性能可靠的自动控制液压式低温改性处理机。实现了低温处理工艺的自动化 ,增加了处理工艺的准确性 ,提高了金属材料改性处理工件的效果     

Competing roles of microstructure and flaw size

Historically, engineers have relied on macroscopic properties, e.g. hardness and tensile strength to predict fatigue limits as analytical tools to model the process did not exist. Consequently, many empirical modifications to the fatigue limit have been made to account for variables, e.g. surface roughness, state of stress, inclusion content, environmental effects, etc. A mechanistic model is proposed to quantify the effects of these parameters on the fatigue limit of metals, specifically steels. Fatigue resistance, i.e. the threshold condition of a non-propagating crack, is determined by two parameters: non-propagating defect or crack size; and the strength of the barrier to crack propagation. The concept of three defect types associated with three different flaw-dominated fatigue regimes is introduced. Furthermore, application of the model to fatigue mechanisms in high-strength steels, synergistic effects of surface finish and intergranular cracks, competition between surface and subsurface fatigue nucleation, tempering, and scatter in fatigue behaviour is demonstrated. The model can be implemented in material screening, selection and processing, as well as a guide for future material research and design. Overall, the model is proven as a simple and robust tool for qualifying and statistically quantifying material behaviour.     

A high-cycle fatigue accumulation model based on electrical resistance for structural steels

For high-cycle fatigue of metals, the DC electrical resistance is a more sensitive parameter to the initiation of micro-cracks during the irreversible fatigue damage accumulation process. This implies that the electrical resistance is a suitable parameter that can be consistent with the fatigue damage physical mechanism. The relation between the ratio of electrical resistance changes and the cyclic fraction of the fatigue specimen may reasonably represent deterioration in mechanical properties of structural steels during the high-cycle fatigue process. The high-cycle fatigue damage accumulation model based on electrical resistance for structural steels was proposed. The model was verified by some experimental data for three structural steels; normalized 45C steel, 20 Mn steel and 16 Mn steel, and good agreement was obtained. The corresponding fatigue lifetime on the basis of the electrical resistance model was also performed. The results show that the approach to fatigue lifetime prediction and failure based on the electrical resistance is a good non-destructive technique.     

Microstructure and mechanical behavior of porous sintered steels

The microstructure and mechanical properties of sintered Fe–0.85Mo–Ni steels were investigated as a function of sintered density. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young's modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young's modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure-based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels are discussed.     

Deep cryogenic treatment of AISI 302 stainless steel: Part II – Fatigue and corrosion

During the last decade, the interest about deep cryogenic treatment (DCT) is grown beyond its successful application on tool steels. The use of such cold treatment has shown positive effects on carburized steel fatigue life and some promising results were also noticed on stainless steels and on other materials. In this article, the DCT effects on fatigue and corrosion resistance of the AISI 302 austenitic stainless steel are analysed starting from the results of an extensive experimental campaign that was performed on both hardened and solubilized material conditions. The analysis includes an evaluation of the influence that the most important treatment parameters have on the final result. Considering their statistical significance at different reliability levels, the results show that the DCT can improve the fatigue behaviour of the solubilized AISI 302, while no important changes are noticed on the hardened material. Moreover, no difference was detected on the corrosion resistance of the cryotreated material, ensuring that such key-property for this class of steel is not compromised by the cryogenic treatment.     

Cyclic stress response and fatigue behavior of Cu added ferritic steels

Steels in which Cu content was altered from 0 to 1.5 mass% were subjected to various heat treatments to change the state of Cu. Concerning these respective steels, fatigue ratio by a stress controlled fatigue test and fatigue resistance by a strain controlled fatigue test thereof were compared. Furthermore dislocation substructure and surface defect during and after cyclic straining were investigated to clarify the effect of Cu on fatigue properties. The fatigue ratios at 2.0 × 10⁶ loading cycles of the Cu added steels after aged at 450 and 750 degrees C are 0.7, remarkably high as compared with those of the Cu added steels after aged at 550 and 650 degrees C, the Cu free steels and any other conventional steels whose fatigue ratio are approximately between 0.5 and 0.6. The fatigue resistance of the as-rolled Cu added steel maintains steady cyclic hardening until fracture. To the contrary the Cu added steel after heat treatment at 550 degrees C shows cyclic hardening to the peak stress and then shows a cyclic softening until fracture. The surface roughness of the Cu added steels after cyclic straining are relatively shallow compared with those of the Cu free steel. The internal substructure of the Cu free steel shows typical cell structure but those of the Cu added steels exhibit vein structure.     

Effect of toughness on low cycle fatigue behavior of pipeline steels

The effect of toughness on the fatigue behavior of pipeline steels was investigated, including the fatigue crack propagation rate and low cycle fatigue test under the loading condition simulating the actual operation of pipelines. The results indicate that the toughness can strongly influence the fatigue behavior of pipeline steels (i.e., the steels with high toughness possess high resistance to fatigue crack propagation and high tolerance of damage, which are much beneficial to obtaining a long life for line pipe structures).     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

EFFECT OF YIELD STRENGTH ON THE BASIC FATIGUE STRENGTH OF WELDED JOINTS

Abstract— The basic fatigue strength of welded joints in four steels having different yield strengths has been obtained by tests in which the maximum applied stress was held constant and equal to the yield strength, to simulate the tensile residual stress in real large-scale structures. In the long-life region superior properties occurred with a decrease in the yield strength. It is therefore suggested that both low yield strength steel, which can be produced by a thermo-mechanical control process without affecting the tensile strength, and steels or welding consumables which show a low transformation temperature, may have a high fatigue strength.     

Evaluation of local fatigue strength in spot weld by small specimen

It has been reported that high strength steel sheet cannot improve fatigue strength of components with a spot weld. The purpose of this study is to discuss the dominant factors on the fatigue strength of spot weld in order to clarify the reasons. A new fatigue testing technique is developed for a small specimen with a total length of less than 3 mm, and the local fatigue strength of heat‐affected zone (HAZ), which is the crack initiation site in the joint, in a mild steel sheet (270MPa‐grade) and a high strength steel sheet (590MPa‐grade) are evaluated by this technique. The fatigue strength of HAZ is almost equal in both steels although the tensile strength of the 590MPa steel is higher than that of the mild steel. The stress in the tensile‐shear spot‐welded joint under cyclic loading and the residual stress by the spot‐welding are evaluated by finite element analyses. The residual stress is tensile in both steels. However, the plastic deformation takes place in the joint of the mild steel and this releases the residual stress. On the other hand, the stress in the 590MPa steel is elastic and the residual stress decreases the allowable alternating stress. The stress under the condition of the experimental fatigue limit of the joint considering the residual stress coincides well with the fatigue limit diagram of HAZ, which means that the fatigue limit of the joint is determined by the fatigue limit of HAZ and the residual stress.     

Mechanical and corrosion behavior of plain low carbon dual-phase steels

Dual-phase steels are being used in automobile industries for last three decades. The mechanical properties of dual-phase steels can be altered by varying its martensite volume fraction. However, the benefits obtained in mechanical properties have to be viewed in light of other properties such as corrosion resistance. In this work, dual-phase steels with different volume fractions of martensite are obtained after thermal processing using different intercritical soaking times. The mechanical properties of dual-phase steels such as Vickers hardness and tensile properties are measured. Corrosion properties are evaluated using potentiodynamic polarization test and immersion test. It was observed that the tensile strength and hardness increased and ductility decreased with increase in martensite volume fraction. The corrosion rate for dual-phase steels is found to be lower than that for subcritically heat treated ferrite–pearlite steel. The higher corrosion resistance of dual-phase steels is explained on the basis of microstructural features.