Torsional deformation-induced gradient hierarchical structure in a 304 stainless steel

After applying torsion to cylindrical 304 stainless steel samples, a gradient structure along the radial direction was obtained. It was found that the volume fr...     

Ferromagnetic properties of deformation-induced martensite transformation in AISI 304 stainless steel

Ferromagnetic properties of plastically deformed AISI 304ss have been studied using magnetic hysteresis and Barkhausen emissions methods. The present study has been concentrated on low volume fraction of martensite, i.e., below 58 pct, as compared to the available literature for a higher percentage of martensite. In measured materials, the coercivity increased with deformation and had a tendency to go toward saturation value. A linear increase in remanence with the deformation was observed. A large number of small amplitude of Barkhausen emissions were found at low percentage of martensite, indicating that magnetization rotation took place within a small region. However, large amplitude Barkhausen emissions were observed with the increase of deformations. Angular variation of Barkhausen emissions indicated the formation of rolling texture within the materials. A model has been proposed to explain the results. At the initial stage, small martensite clusters are formed, which grow with the deformation, and the intracluster exchange interaction becomes predominant. With the increase of deformation, martensite volume fraction increases. In this process, existing clusters grow and new clusters are formed. As a result, martensite clusters come closer and intercluster exchange interaction becomes important.     

Fatigue of stainless steel in hydrogen

The fatigue crack growth rates of two austenitic stainless steel alloys, AISI 301 and 302, were compared in air, argon, and hydrogen environments at atmospheric pressure and room temperature. Under the stresses at the crack tip the austenite in type 301 steel transformed martensitically to a’ to a greater extent than in type 302 steel. The steels were also tested in the cold worked condition under hydrogen or argon. Hydrogen was found to have a deleterious effect on both steels, but the effect was stronger in the unstable than in the stable alloy. Cold work decreased fatigue crack growth rates in argon and hydrogen, but the decrease was less marked in hydrogen than in argon. Metallographic, fractographic, and microhardness surveys in the vicinity of the fatigue crack were used to try to understand the reasons for the observed fatigue behavior.     

Influence of strain-history on deformation-induced austenite transformation in 304 stainless steel sheet

In the present investigation experiments were carried out to find strain history effects on deformation-induced austenite transformation in a metastable stainless steel sheet. The aim of this work was to obtain information on a final amount of martensite formed during γ → α’ transformation under various strain paths. All tests were performed at room temperature and at 0°C. Relationships of volume fraction α’ martensite vs true plastic strain X_M = f(ε) are presented and analysed.     

Fatigue crack propagation in austenitic stainless steel weldments

The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures.     

Fatigue fracture mechanism maps for a type 304 stainless steel

Fatigue fracture mechanism maps at room temperature and 573 K for a type 304 stainless steel were constructed by correlating the crack propagation rate with information obtained on the fracture surface. Depending on the crack propagation rate, ranging from 1 × 10⁻⁶ to 1 × 10⁻¹¹ m/cycle, three types of fracture surfaces were observed. One was a striation region; the second was a “featureless” fracture region, which appeared rough under scanning electron microscope (SEM) observation; and the third was crystallographic fracture region, which appeared smooth under SEM observation. The area fractions and the indexes of the fracture surfaces were quantified and identified by the etch-pit method. From the results, crack initiation and propagation mechanisms were cleared and fatigue fracture mechanism maps were constructed. The maps may be useful for investigating the cause of the fatigue failure accident of structures made of type 304 steels.     

Decomposition of ferrite in commercial superduplex stainless steel weld metals; microstructural transformations above 700 °C

The microstructural stability at temperatures above 700 °C of weld metal of type 29Cr-8Ni-2Mo-0.39N and weld metal of type 25Cr-10Ni-4Mo-0.28N has been compared. Multipass welding was employed using the gas tungsten arc welding technique with a shielding gas of Ar+2 pct N₂. The quantitative assessment of the intermetallic phase was performed using automatic image analysis in the light optical microscope (LOM). Detailed microanalysis was also performed using scanning and transmission electron microscopy. A computer program developed by the authors was used to calculate a continuous cooling-temperature (CCT) diagram on the basis of the experimentally determined time-temperature-transformation (TTT) diagram. Thermodynamic calculations for estimating phase stabilities and for interpreting experimental observations were performed. It was found that weld metal of type 29Cr-8Ni-2Mo-0.39N was microstructurally more stable than weld metal of type 25Cr-10Ni-4Mo-0.28N. A lower molybdenum concentration and a higher nitrogen concentration in the former alloy could explain the higher stability with respect to the intermetallic phase. The higher nitrogen concentration also provides a rationale for the higher stability against the formation of secondary austenite in weld metal of type 29Cr-8Ni-2Mo-0.39N. This effect, which is associated with a lower thermodynamic driving force for precipitation of secondary austenite during multipass welding, can be explained by nitrogen-enhanced primary austenite formation.     

Cyclic deformation of superduplex stainless steels at intermediate temperatures

Cyclic deformation of a superduplex stainless steel at temperatures ranging from room temperature to 475 °C was evaluated for two different strain amplitudes. Cyclic hardening-softening response and the corresponding substructural features within each constituent phase of the alloy were characterized. Experimental evidence, such as abnormal cyclic hardening, inverse strain rate sensitivity (SRS), and serrated flow, reveals the existence of dynamic strain aging (DSA) in the studied temperature range. Substructural evolution suggests that DSA induces changes in the distribution of plastic strain between austenite and ferrite. In the case of tests performed at 475°C, there exists a significant influence of thermal embrittlement too. A. GIRONèS, formerly Doctor, with the Department of Materials Science and Metallurgical Engineering Universitat Politècnica de Catalunya, 08028 Barcelona, Spain.     

Fatigue crack propagation in austenitic and ferritic stainless steel single crystals

The orientation-dependence of fatigue crack propagation rates in single crystals of austenitic and ferritic stainless steels has been investigated at low stress levels on standard fracture mechanics specimens. The stress intensity factor required for a crack growth rate of 10⁻⁶ mm/cycle may vary by up to 50% according to the crystal orientation. In general, relatively high crack growth rates are associated with macroscopically plane fracture surfaces for both the f.c.c. and b.c.c. crystal structures. The crystallographic conditions for the formation of macroscopically plane fracture surfaces at low stress levels do not always correspond with the predictions of the original alternating slip model. In particular, for the ferritic steel, only those orientations for which two 〈111〉 slip directions are normal to the crack front and symmetrically inclined to the crack plane allow the formation of macroscopically plane fracture surfaces. The macroscopic appearance of the austenite fracture surfaces depends upon the crystal orientation and the crack growth rate: at low stresses, plane fracture surfaces are developed for most of the orientations tested. A model of crack propagation by alternating shears, in which the shears are accommodated by multiple slip processes at the crack tip is proposed to explain these results.     

Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate the relevance of crack growth data in predicting fatigue life data.     

Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Metallurgical and Materials Transactions A - Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is...     

Laws of the deformation-induced structural transformation in bainitic steel

The evolution of the defect and carbide subsystems of steel with a bainitic structure during deformation by compression is quantitatively analyzed by transmission electron diffraction microscopy and X-ray diffraction. The strain dependences of the parameters of a dislocation substructure and a carbide phase are determined, and the possible causes of stages in their changes are discussed.     

基于声发射监测的316LN不锈钢的疲劳损伤评价

疲劳裂纹的萌生与扩展容易导致压力容器及管道的严重疲劳失效.因此就设备的安全可靠性而言,非常有必要对疲劳裂纹扩展过程进行监测,并对疲劳损伤程度进行评估.本文针对316LN不锈钢材料进行疲劳实验研究,利用直流电位法测量实验中的裂纹长度,得到了材料的疲劳裂纹扩展曲线.利用声发射技术对疲劳裂纹扩展过程进行监测,通过声发射多参数分析对疲劳损伤状态进行评价,同时建立了声发射参数与线弹性断裂力学参数之间的关系,并进行寿命预测.研究表明:声发射能够对316LN不锈钢的疲劳裂纹损伤进行有效评估,声发射累积参数如累积计数、累积能量和累积幅值曲线上的转折点标志着疲劳裂纹进入快速扩展阶段,这可以为工程人员提供失效预警;声发射波形和频谱分析表明,噪声信号的幅值较小且信号持续时间较长,信号包含的频率成分比较复杂,而裂纹扩展信号是突发型信号,衰减较快,信号频率主要集中在80~170 kHz范围内;声发射计数率、能量率和幅值率与应力强度因子幅度以及疲劳裂纹扩展速率之间呈线性关系,裂纹长度预测结果与实测值接近.本研究工作对于工程结构的疲劳失效预警和剩余寿命预测具有重要意义.     

粉末注射成形奥氏体不锈钢的疲劳和冲击强度

本文的目的是研究粉末性能对粉末注射成形奥氏体不锈钢的力学性能(特别是疲劳和冲击强度)的影响。试样用混有聚酰氨粘结剂系的水雾化(WA)和气雾化(GA)粉末制成。注射坯在空气中脱粘，而后在不同温度、不同保温时间下真空烧结。粉末性能对烧结体的密度、显微组织和力学性能有显著影响。孔和析出物显示出Ostward时效，而孔和析出物的长大满足Lifshitz-Wagner方程。WA和GA粉末试样的疲劳极限分别约为300MPa和310MPa。它们的疲劳强度略低于常规锻材。烧结体的冲击值随密度增加线性增大。     

Investigation into the stress-strain curves of deformation-induced ferrite transformation in a low carbon steel

A series of tests of deformation-induced ferrite transformation ( DIFT) in a low carbon steel were carried out by the Gleeble-3500 hot simulation machine at a temperature range of Ae 3 -Ar 3 . The overall stress-strain curves during DIFT can be divided into three typical types: "double-humped","single-humped"and "transitional". The peaks exhibited in the curve are involved with deformation-induced transformation which happened in grains or at the grain boundaries. According to the stress-time curve and strain-time curve,strain capacity dramatically postponed the strain-induced transformation,which leads to the start of the transformation right ahead of the finish of deformation and the majority of the ferrite transformation process mainly happened after the deformation. Deformation-induced transformation is a metadynamic transformation process with dynamic nucleation.     

Mechanical properties, microstructural stability and kinetics of σ-phase formation in 29Cr-6Ni-2Mo-0.38N superduplex stainless steel

A time-temperature-transformation (TTT) diagram, with respect to the formation of intermetallic phase in the range from 700 °C to 1000 °C, has been assessed by point counting for a recently developed 29Cr-6Ni-2Mo-0.38N superduplex stainless steel (SDSS). Using a computer program previously developed by the authors, a continuous cooling-transformation (CCT) diagram was calculated from the TTT diagram, assuming that the transformation can be described by an Avrami-type equation. Controlled cooling experiments were made to confirm the validity of this computer model. Mechanical testing showed that the minimum acceptable impact toughness of 27 J corresponded to 5 vol pct of intermetallic phase. A comparison of impact toughness to hardness showed that toughness was a very sensitive measure of intermetallic phase formation, while hardness was insensitive and showed no significant increase until the material was catastrophically brittle. The thermodynamic computer program Thermo-Calc was used to predict microstructures and compare microstructures obtained after aging for 72 hours. It was found that this program could be used in a qualitative manner for predicting microstructural changes at various temperatures, but was unable to predict variables such as dissolution temperature and volume percentage with precision. The intermetallic phase was found to consist entirely of tetragonal σ phase with a P4₂/mnm crystal structure. The absence of χ phase as a precursor of σ phase provided an explanation for the delayed formation of intermetallic phase, as compared to that of more molybdenum-rich SDSSs.     

Numerical simulation of deformation-induced segregation in continuous casting of steel

The deformation-induced macrosegregation in continuous casting of steel has been simulated using a finite-volume scheme. For that purpose, a two-dimensional heat-flow computation was first performed in a Eulerian reference frame attached to the mold, assuming a unique solidification path, i.e., a unique relationship between temperature and enthalpy. This gave the stationary enthalpy field in the longitudinal section of the slab. On the other hand, bulging of the slab between two rolls was calculated in the same section, assuming plane-strain deformation and using the ABAQUS code. The Lagrangian reference frame was attached to the slab, and the rolls were moved at the surface until a stationary, bulging deformation profile was reached. The bulging of the surface was then used as an input condition for the calculation of the velocity and pressure fields in the interdendritic liquid. Using a fairly simple hypothesis for the deformation of the solid skeleton, the mass conservation and Darcy equations were solved in a Eulerian reference frame. This calculation was performed in an iterative loop, within which the solute conservation equation was also solved. At convergence and using the enthalpy field, this calculation allowed to obtain the temperature, the volume fraction of solid, and the average concentration fields, in addition to the fluid velocity and pressure. It is shown that the positive centerline segregation of carbon in the slab is well reproduced with this model. The effects of shrinkage and soft reduction were also investigated.     

Fatigue crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel

The crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel was studied by rotating bending fatigue test with smooth cylindrical specimens and notched specimens (theoretical stress concentration factor Kt=3). The fatigue limit and S- N curve of bearing steel were measured by up- and- down method and group method, respectively. The fractures of the specimens were observed by scanning electron microscopy. The results show that the cracking type of the smooth specimens is single source initiation. The crack source is surface defects and subsurface inclusion. The surface defects are surface roughness, persistent slip band and machining dent, while the subsurface inclusion is Al2O3- CaO- MgO- SiO2 composite inclusion. The fatigue limit of notched specimens is significantly decreased. The cracking type of the notched specimens is multi- source initiation. The notch sensitivity factor qf of bearing steel is 1. 18. The fatigue failure of the smooth specimens is transferred from the surface roughness with high stress amplitude to the persistent slip bands, the machining dents and the inclusions with low stress amplitude. The fatigue crack initiation life accounts for more than 94. 1% of the whole fatigue life.