Rolling contact fatigue behavior of sintered and hardened steels

Powder-metal-processed bearings and gears are finding increasing application because of their economical and technical advantages. The residual pores from the sintering operatives act as lubricant pockets and dampen sound and vibration. However, porosity also decreases the mechanical strength and reduces the life of components fabricated by powder processing relative to similar wrought components. The rolling contact fatigue behavior of sintered and heat treated steel rollers was investigated using a fatigue test machine designed and fabricated for that purpose. The powder-metal-processed and the wrought steel rollers that were tested had similar composition and hardness and were mated against wrought steel rollers of high hardness. The contact stress versus number of cycles to failure data showed that the wrought steel had a very high endurance limit under rolling contact fatigue compared to the sintered steels investigated. Rolling contact fatigue behavior was found to depend on the porosity present in the material. Large surface peeling failures and pitting type fatigue failures were observed in the sintered and hardened steels, while only pitting type failures were observed in the wrought steels     

Versuchs- und Rechendaten zur Dauerschwingfestigkeit von metallischen Werkstoffen unter mehrachsiger Beanspruchung

Experimental Data and Calculated Results about the Fatigue Endurance Limit of Metals under Multiaxial Alternating Load An extensive catalogue of present available experimental data about the fatigue endurance limit of metallic materials under multiaxial loading conditions and thereupon determined deviation ratios between experimental results (long life fatigue tests) and calculated values by five newer computation methods is demonstrated (a further showed statistical analysis of these devations indicates, that in relation to the other failure criterions the ?Quadratische Versagenshypothese”? QVH is preferably recommended for a reliable application). The tabulated data-catalogue totally includes 530 referenced loading cases (limited to various biaxial states of combined normal and torsional alternating stress with sinusoidal synchronous or out-of-phase amplitudes and superimposed mean stresses) with experimental results (probability of survival of P_S = 50%) on metallic materials (unalloyed and alloy steels, non-ferrous metals and wrought aluminium alloys, cast irons and sintered metals).     

The Morphology of Rolling Contact Fatigue Fracture of Hardened Steels

Rolling Contact Fatigue(RCF) is a cumulativedamage phenomenon when metals are subjectedto repeated contact stresses. The fomationof pitting on the contact surface is the resultof the rolling contact fatigue. The morphologiesof rolling contact fatigue fracture of the har-dened steels (86CrHoV7, 42CrMo) show that strongresemblance in fractuye mechanisms exists betweenrolling contact fatigue and uni-axial fatigue.Since fatigue striations are hardly observedin hardened steels under uni-axial fatigue,it is interesting to note that the state ofstress in rolling contact fatigue is more favor-able to ductile fractures than in uni-axialfatigue.     

Corrosion fatigue characterisation of sintered multiphase stainless steels

Abstract

Multiphase stainless steels are produced for their attractive properties of mechanical strength and corrosion resistance relative to their austenite–ferrite structure. The manufacture of these steel by powder metallurgy technology presents some advantages in terms of low cost and formability of complex shapes. Mechanical and corrosion resistances are not at the level of the wrought steels due to their porous nature. In this work the fatigue and corrosion fatigue behaviour of some sintered steels obtained by sintering from 316L and 434L base powders has been studied for characterisation and comparison. The sintered steels were fatigue tested in two different environments: air and NaCl aqueous solution. The tests performed indicate that the chemical and microstructral composition has no great influence on fatigue behaviour in comparison with the manufacturing technology (sintering). This is most evident in the more aggressive environment, like seawater, in which these steels could be advantageously used. The analysis of fracture surfaces using SEM microscopy shows a peculiar crack propagation characterised by cleavage, stress intensification due to porosity, and features of localised ductility on sintering necks and base powder particles.     

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.     

Untersuchungen Zur Festigkeit labormäßig hergestellter Silicium-Mangan-legierter Sinterstähle

Investigations on Mechanical Properties of Si? Mn-sintered steels Produced under Laboratory Conditions The influence of graphite additions on microstructure and behaviour under static and cyclic loading is studied for a newly developed sintered steel. For an optimum carbon content fatigue tests are carried out on notched and unnotched specimens. The results of these constant amplitude axial loading experiments are promising and show some advantages compared with the results of commercial sintered steels.     

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.     

Einfluß von Verfahrensparametern beim Fließpressen auf das Ermüdungsverhalten

Effect of Process Parameters in Extrusion on Fatigue Behaviour Fatigue test specimens were produced by forward extrusion and subsequent heading from conventional steels, P/M steels and AlMg4.5Mn. The strain, die-opening angle, temperature and in some cases the lubricant were varied. For purpose of comparation in some cases also specimens were prepared by machining. S/N curves or cyclic deformation curves were determined mainly by stress-controlled tests. For all materials the time strength was increased by cold forming whereby the effect of strain was stronger than that of the die-opening angle. The time strength of steel is less increased by warm (semihot) forming than by cold forming. The fatigue strength, however, is not necessarily increased by cold or warm forming. After cold forming the P/M steels contrary to conventional steels revealed mainly quasielastic deformation until fracture. After warm forming, however, P/M steels, due to the decrease of porosity, behaved almost like conventional steels. For AlMg4.5Mn specimens prepared by forming – probably due to worse surface quality – revealed a lower fatigue strength than those prepared by machining. Here also a feasible effect of the choice of the lubricant was stated.     

Maßnahmen gegen Schwingungsrißkorrosion an nichtrostenden Stählen

Methods to Combat Corrosion Fatigue Stainless Steels In chemical works substantial damage to moving components made of stainless steels is caused by corrosion fatigue. The very slight conditions for attack, which are sufficient to cause such corrosion, and the appearance of the surface of the fracture result in such damage frequently being attributed to purely mechanical causes and consequently being combatted with ineffective measures. Fracture analysis and remedial measures are discussed for a range of typical examples. Rotating bending fatigue tests are used to show that corrosion fatigue in the passive state of the steels can be suppressed by increasing the chromium content. Chloride ion contents up to 0.1% showed no appreciable effect. Finally the possible measures against corrosion fatigue are discussed.     

Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 2

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 1

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

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.     

Fatigue strength of mash seam welded joints

Reversed bending fatigue tests have been conducted using four series of mash seam welded joints obtained from the coupling of two different steels and plate thicknesses. Fatigue strength was evaluated and the effects of material property changes resulting from welding were studied. Fatigue strength of all series of the welded joints decreased slightly compared with that of the base steel. Type of steel and plate thickness in the welded samples exerted very little influence on fatigue strength. In the welded joints between steels with the same plate thickness, fatigue failure took place at a location away from the weld zone in the plate with the lower strength, while in the welded joints between plates of different thickness, failure occurred at the shoulder between the thin and thick plate, i.e. at the weld zone. Regardless of the type of steel and the plate thicknesses joined, fatigue strengths of the mash seam welded joints were slightly higher than those of the laser welded butt joints.     

预应变对不同含碳量钢材疲劳特性的影响

通过对非预应变15号、35号和45号钢及其各种预应变材料进行实验,研究了预应变量对不同含碳量钢材疲劳强度和疲劳裂纹发生的影响。实验结果表明:在相同的预应变预量的条件下,材料的疲劳极限随含碳量的增加而提高;当预应变量较低时,预应变材料的疲劳极限低于非预应变材料,含碳量越高降低的程度越明显;当预应变量较高时,材料的疲劳极限随预应变量的增加而恢复或提高。     

Crack initiation and propagation in Chromium pre-alloyed PM-steel under cyclic loading

Powder metallurgy processing of steels typically results in materials characterized by residual porosity, whose sizes and morphology, together with the microstructure, strongly affect the fatigue crack growth behaviour of the materials.Prismatic specimens were pressed at 7.0 g/cm³ density from Astaloy CrM powder and sintered under different conditions, varying the sintering temperature and the cooling rate after sintering.Optical observations allowed us to evaluate the sizes and the morphology of the porosity and the microstructural characteristics for all the investigated conditions. Fatigue tests were performed at R-ratio equal to 0.1 to investigate the threshold zone and to calculate the coefficients of the Paris law. All the tests were carried out according to the compliance method, and the crack length was evaluated during the tests. Moreover, K_IC tests were performed in order to complete the investigation.On both fatigue and K_IC samples, a fractographic analysis was carried out to investigate the crack path and the fracture surface features.The results show that the exponent of the Paris law is about 6.0 for 1120 °C sintered and about 4.7 for 1250 °C sintered materials. Interesting data have been also found for the threshold values.     

MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN DUPLEX STAINLESS STEELS

Abstract— Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.     

MICROSTRUCTURE AND FATIGUE PROPERTIES OF A WELDED DUPLEX STAINLESS STEEL

Abstract— In response to the increasing structural applications in duplex steels for welded structures, fatigue behaviour of a SAF 2304 grade duplex stainless steel was investigated, considering both the base metal and GTAW welded joints. Fatigue curves and fatigue limits under rotary bending fatigue were obtained. The study focused attention on the microstructural features of fatigue crack propagation of the two series of experiments, thereby permitting an evaluation of the tortuous crack path of welded joints and the mechanisms related to threshold microstructural barriers.     

Strain-amplitude dependent fatigue resistance of low-alloy pressure vessel steels in high-temperature water

Low cycle fatigue resistance of low-alloy pressure vessel steels was investigated in 561 K air and water over a wide strain amplitude range. It was found that fatigue resistance of the steels was enhanced in high-temperature water relative to high-temperature air under the low strain amplitude conditions (<0.3%) or in the high cycle regime (>2 × 10⁴ cycles), while it was remarkably degraded in high-temperature water under the higher strain amplitude conditions. Fatigue cracking and fractographic features suggested that effects of hydrogen be involved in the present corrosion fatigue process in high-temperature water. Possible environmentally assisted cracking mechanisms are discussed.     

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.