贝氏体区等温时间对TRIP钢残奥及力学性能影响

为研究贝氏体区等温时间对热轧TRIP钢残余奥氏体和力学性能的影响,采用金相显微镜、X射线衍射、拉伸实验等方法对3种不同贝氏体区等温时间下制备的热轧TRIP钢进行分析.结果表明:随着贝氏体等温时间的延长,残余奥氏体量减少而残余奥氏体碳含量增加,残余奥氏体晶粒尺寸及残余奥氏体形貌变化不大;热轧TRIP钢的力学性能随着贝氏体...     

Hydrogen embrittlement susceptibility of microstructures formed in multipass weld metal for HT780 class steel

The effect of reheating by following passes on the hydrogen embrittlement of MAG weld metal for HT780 class steels has been investigated by using specimens subjected to simulated thermal cycles. The hydrogen-charged specimens exhibited transgranular quasi-cleavage fracture and intergranular fracture along prior austenite grain boundaries on slow strain rate tensile (SSRT) tests, depending on the reheated temperature and charged hydrogen content. The reduction in elongation of hydrogen-charged specimens became more significant when intergranular fracture occurred. When specimens in as-welded state and precedently reheated at coarse grained HAZ temperature of 1,623 K were reheated at a tempering temperature of 873 K, significant amount of intergranular fracture occurred at charged hydrogen contents above 3 ppm in spite of the decrease in hardness. The specimen reheated at 1,173 K showed no intergranular fracture even after receiving the reheating at 873 K at a hydrogen content of 6 ppm, suggesting the strong influence of the prior austenite grain size on the hydrogen-induced intergranular embrittlement. The measurement of hydrogen content desorbed from the hydrogen-charged specimen at room temperature suggested that the intergranular fracture caused by the reheating at 873 K was associated with an increase in susceptibility to hydrogen embrittlement of the prior austenite grain boundary itself rather than a decrease in the amounts of trapping sites such as dislocation and retained austenite.     

Effect of heat treatment on susceptibility of duplex stainless steel to embrittlement by hydrogen

Abstract

Slow straining of smooth tensile specimens of heat treated duplex stainless steel from a fabricated pipe has been used to assess susceptibility to embrittlement by hydrogen. The effect of the proportions of ferrite and austenite in the microstructure, produced by quenching after solution treatment at temperatures between 1000 and 1300°C, on the ductility was measured. Tests were carried out by either straining in a hydrogen atmosphere or in air after thermal charging in high pressure hydrogen. The measured susceptibility increases proportionately with increase in the amount of ferrite in the structure and reflects the role of austenite in arresting propagating cracks. However, there is little doubt that the amount of austenite presents greater dominance than its orientation in this respect.     

The effect of hydrogen on the bainite transformation

The effect of hydrogen on the upper bainite transformation in two silicon containing steels has been investigated. For comparison, isothermal transformation at the same temperature has also been performed in a helium atmosphere. In both Fe-0.2C-3Mn-2Si and Fe-0.4C-4Ni-2Si (nominal wt %) alloys it was discovered that the bainite reaction proceeds further towards completion when the transformation is carried out in a hydrogen atmosphere. This can result in the reduction or elimination of the martensite phase which forms from residual austenite upon quenching to room temperature. The resultant microstructure of specimens heat treated in hydrogen was a fine aggregate of upper bainitic ferrite and interlath retained austenite. This effect is discussed in terms of hydrogen interactions in the lattice undergoing bainite transformation via a displacive mechanism. Additionally, it is found that the stability of the retained austenite in the final bainitic microstructure is not markedly influenced by hydrogen.     

Crack propagation of secondary hardened alloy steels in gaseous hydrogen atmosphere

Abstract

The crack propagation behaviour of secondary hardened alloy steels having various Ni contents and a 18%Ni maraging steel was studied using modified compact tension specimens under 98–784 kPa hydrogen gas pressure p_H2 . The effect of Ni content and retained austenite was examined by comparing oil quenched specimens with those cooled to liquid nitrogen temperature. It was estimated from the effect of p_H2 on the crack propagation rate da/dt that the permeation of hydrogen from the crack tip surface decreased with increasing Ni content in the order 6 or 9, 13, then 18%Ni. It was also estimated that the hydrogen induced embrittlement of grain boundaries decreased with increasing Ni content in the order 6, 9, then 13%Ni and that the embrittlement was greater for the steel containing 18%Ni than for the steel containing 13%Ni. The effect of retained austenite was expected to suppress not the permeation of hydrogen, but the embrittlement of grain boundaries.

MST/757     

Effect of thermomechanical treatment on the susceptibility of structural steel to hydrogen embrittlement

It was established that high-temperature thermomechanical treatment (HTMT) produces a substantial reduction in the susceptibility of a medium-carbon steel 37KhN3A (tempered at a low temperature) to hydrogen embrittlement. An anisotropy of the ductility and the resistance to fracture of hydrogen-charged steel was observed after HTMT which points to a selective character of hydrogen charging and hydrogen embrittlement at the boundaries of the initial austenite grains. Torsion test results showed that hydrogen charging has practically no effect on the shear strength and affects only such structure-sensitive properties as tensile strength and ductility.     

Study on carbide-bearing and carbide-free bainitic steels and their wear resistance

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si?+?Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si?+?Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to strain-induced martensitic transformation from the retained austenite.     

Relative effect of C and Mn on strength-toughness of medium Mn steels

In order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.     

Mechanical and Transformation Behaviors of a C-Mn-Si-Al-Cr TRIP Steel under Stress

Transformation induced plasticity （TRIP） steels combine high strength and excellent ductility, making them suited for application in crash-relevant parts in the automotive industry. However, the high Si contents in the conventional TRIP steel will generate surface defects on the hot rolled strip, which is difficult to process in continuous galvanizing lines. In order to solve the above problem the TRIP steel with the addition of Al replacing majority of Si was designed. In the present paper, the volume fraction of various phases in a C-Mn-Si-Al-Cr TRIP steel was determined by metallographic examination and X-ray diffraction analysis, and the multi-phase microstructures were characterized using an atomic force microscope based on their height difference. Tensile tests were performed at different temperatures ranging from -40℃ to 90℃. The results show that transition temperature Ms^σ in the present TRIP steel cannot be determined due to its lower volume fraction of retained austenite, different from the conventional TRIP steel. While the yield stress and tensile strength at different temperatures are higher than those of the conventional TRIP steel, which is attributed to the addition of Cr. In order to evaluate the effect of martensitic transformation on the total elongation, the sample without retained austenite obtained by quenching in liquid nitrogen was carried out under tensile test. The results indicate that the elongation of the original sample containing 9% retained austenite is about 20% higher than that of the sample quenched in liquid nitrogen, which demonstrates that the retained austenite plays an important role in improving the elongation of the TRIP steel.     

X射线应力仪侧倾法测定板材的残余奥氏体含量

采用X射线应力仪侧倾法对某型板材的残余奥氏体含量进行了测定，研究了测试条件对测定结果的影响。结果表明：不同试样表面状态、不同测试方向和侧倾角度的单次测定结果之间存在较大的差异，因而单次测定结果不适合作为最终测定结果；就不同侧倾角残余奥氏体含量测定结果的平均值而言，试样表面状态对测定结果没有明显的影响．并4用X射线应力仪，采用多个测试方向、较大直径的准直管和多个侧倾角进行残余奥氏体含量测定，可以获得较为均匀一致的残余奥氏体含量平均值。     

Novel method for refinement of retained austenite in micro/nano-structured bainitic steels

A comparative study was conducted to assess the effects of two different heat treatments on the amount and morphology of the retained austenite in a micro/nano-structured bainitic steel. The heat treatments used in this work were two-stage bainitic transformation and bainitic-partitioning transformation. Both methods resulted in the generation of a multi-phase microstructure containing nanoscale bainitic ferrite, and/or fresh martensitic phases and much finer retained austenite. Both heat treatments were verified to be effective in refining the retained austenite in micro/nano-structured bainite and increasing the hardness. However, the bainitic transformation followed by partitioning cycle was proved to be a more viable approach than the two-stage bainitic transformation due to much shorter processing time, i.e. ～2?h compared to ～4 day, respectively.     

The effect of lattice defects induced by cathodic hydrogen charging on the apparent diffusivity of hydrogen in pure iron

The variation of apparent hydrogen diffuoivity with the applied current density and the types and amounts of hydrogen -induced damage caused by hydrogen during the cathodic charging of hydrogen in pure iron are investigated by thermal analysis techniques, and analysed in the light of theoretical models. Internal microcracks and microvoids are generated predominantly below 1 mA cm^–2 and are the major trapping sites of hydrogen in pure iron when charging hydrogen cathodically. Blisters on the surface of iron specimens are found tD be interconnected to the surface of the specimen through microcracks remaining in the vicinity of blisters. The peak temperature of hydrogen released from an internal microcrack or microvoid decreased as the applied current density is increased. The apparent diffusivity of hydrogen at 458 K decreases linearly with the reciprocal value of the square root of applied current density during cathodic charging. This implies that the amounts of internal microcracks or microvoids are linearly proportional to the lattice hydrogen solubility or the square root of the applied current density.     

Microstructural evolution of bainitic steel severely deformed by equal channel angular pressing

High Si bainitic steel has been received much of interest because of combined ultra high strength, good ductility along with high wear resistance. In this study a high Si bainitic steel (Fe-0.22C-2.0Si-3.0Mn) was used with a proper microstructure which could endure severe plastic deformation. In order to study the effect of severe plastic deformation on the microstructure and properties of bainitic steel, Equal Channel Angular Pressing was performed in two passes at room temperature. Optical, SEM and TEM microscopies were used to examine the microstructure of specimens before and after Equal Channel Angular Pressing processing. X-ray diffraction was used to measure retained austenite after austempering and Equal Channel Angular Pressing processing. It can be seen that retained austenite picks had removed after Equal Channel Angular Pressing which could attributed to the transformation of austenite to martensite during severe plastic deformation. Enhancement of hardness values by number of Equal Channel Angular Pressing confirms this idea.     

Effect of Tocopheryl Polyethylene Glycol Succinate on the Percutaneous Penetration of Minoxidil from Water/Ethanol/Polyethylene Glycol 400 Solutions

ABSTRACT

We described to achieve the local retention of minoxidil which has penetrated the skin with minimization of its absorption into the general circulation and elimination of local irritation induced by propylene glycol. The effect of tocopheryl polyethylene glycol succinate (TPGS) on the penetration flux of minoxidil and its retention in the skin from topical minoxidil formulations consisting of water, alcohol, and polyethylene glycol 400 was characterized by an experimental design of ten solvent formulations in this study. Results show that the addition of TPGS was only able to improve the solubility of minoxidil in those solvent systems containing higher proportions of water and PEG 400, and the extent of improvement was also more profound with the addition of TPGS at concentrations higher than 5%. For those solvent systems containing a higher fraction of alcohol, an insignificant change in minoxidil solubility with increasing added amounts of TPGS was noted even with the tendency to decrease the solubility of minoxidil with higher amounts of TPGS. Increasing the amount of TPGS added gradually increased the flux and the corrected flux from solvent formulations with a lower solubility parameter, but decreased those from solvent systems with a higher solubility parameter. With the addition of TPGS, solvent formulation F6 (alcohol:PEG 400 of 50:50) was demonstrated to be the optimal choice by having an improved local effect and a reduced systemic effect compared to the reference of 2% Regaine®. Tocopheryl polyethylene glycol succinate (TPGS) was mainly retained locally in the stratum corneum, and the amount was proportional to the increase in the amount of TPGS added to these ten solvent formulations.     

Effect of tocopheryl polyethylene glycol succinate on the percutaneous penetration of minoxidil from water/ethanol/polyethylene glycol 400 solutions

We described to achieve the local retention of minoxidil which has penetrated the skin with minimization of its absorption into the general circulation and elimination of local irritation induced by propylene glycol. The effect of tocopheryl polyethylene glycol succinate (TPGS) on the penetration flux of minoxidil and its retention in the skin from topical minoxidil formulations consisting of water, alcohol, and polyethylene glycol 400 was characterized by an experimental design of ten solvent formulations in this study. Results show that the addition of TPGS was only able to improve the solubility of minoxidil in those solvent systems containing higher proportions of water and PEG 400, and the extent of improvement was also more profound with the addition of TPGS at concentrations higher than 5%. For those solvent systems containing a higher fraction of alcohol, an insignificant change in minoxidil solubility with increasing added amounts of TPGS was noted even with the tendency to decrease the solubility of minoxidil with higher amounts of TPGS. Increasing the amount of TPGS added gradually increased the flux and the corrected flux from solvent formulations with a lower solubility parameter, but decreased those from solvent systems with a higher solubility parameter. With the addition of TPGS, solvent formulation F6 (alcohol:PEG 400 of 50:50) was demonstrated to be the optimal choice by having an improved local effect and a reduced systemic effect compared to the reference of 2% Regaine®. Tocopheryl polyethylene glycol succinate (TPGS) was mainly retained locally in the stratum corneum, and the amount was proportional to the increase in the amount of TPGS added to these ten solvent formulations.     

Evaluation of hydrogen effect on the fatigue crack growth behavior of medium-Mn steels via in-situ hydrogen plasma charging in an environmental scanning electron microscope

Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to ferrite(α)phase fractions and different γ(meta-)stabilities.Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope(ESEM).The in-situ hydrogen plasma charging increased the fatigue crack growth rate(FCGR)by up to two times in comparison with the reference tests in vacuum.Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum.For both materials,the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum.The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction,phase stability,yielding strength and hydrogen distribution.This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.     

Tensile behaviour of superelastic NiTi alloys charged with hydrogen under applied strain

ABSTRACT

The tensile behaviour of NiTi alloys is investigated after hydrogen charging during the austenite, half-transformation and martensite phases. The specimens are charged with different current densities and charging durations. During the tensile tests, the strain of the plateau transformation decreases due to hydrogen-induced residual martensite variants. This decrease becomes important when the charging happens during the martensite phase. Accordingly, the hydrogen ensures the stability of the phase in which the charging process occurs. Moreover, a heightening of transformation stress is noticed during the plateau. The transformation stress increases when the current density grows and the charging duration rises. This occurrence is caused by the interaction between the hydrogen and NiTi structures, where hydrogen delays the NiTi martensite transformation.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Grain-size and heat-treatment effects in hydrogen-assisted cracking of austenitic stainless steels

Hydrogen embrittlement of 304L and 316L types austenitic stainless steels has been studied by charging thin tensile specimens with hydrogen through cathodic polarization. Throughout this study we have compared solution-annealed samples, having various prior austenite grain size, with samples given the additional sensitization treatment. The results of the tensile tests while undergoing cathodic charging show that the additional sensitization treatment and coarse-grained samples together, lower the mechanical properties in both 304L and 316L types, and the sensitized steel is more susceptible to hydrogen-assisted cracking. However, the room-temperature yield and ultimate strengths, and the elongation of type 316L, were much less affected depending on the heat treatment and prior austenitic grain size. The fracture surfaces of the specimens tested while cathodically charged show considerable differences between the annealed and the sensitized specimens. The sensitized coarse-grained specimens were predominantly intergranular in both 304L and 316L types, while the annealed 316L type specimens fracture shows massive regions of microvoid coalescence producing ductile rupture and the annealed 304L type specimens fracture were primarily transgranular and cleavage-like. Sensitization seems both to facilitate the penetration of hydrogen along the grain boundaries into the steel and to introduce susceptibility to fracture along grain boundaries while refined grain size improves resistance regardless of the failure mode.     

Acicular ferrite transformation in deformed austenite of an alloy-steel weld metal

The effect of different amounts (5, 10 and 15%) of compressive deformation of austenite on the isothermal transformation of acicular ferrite in an alloy-steel weld metal has been investigated. It was found that prior deformation of austenite significantly enhanced acicular ferrite transformation. At the same isothermal transformation temperature, as a higher amount of prior deformation was applied, a greater quantity of acicular ferrite could be obtained and the size of acicular ferrite plates became much finer. These results implied that the effective nucleation sites of acicular ferrite increased with increasing amount of prior deformation. The other results also emphasized that the accumulated strain (due to prior deformation of austenite) could trigger acicular ferrite to nucleate on inclusions at high temperatures, where undeformed austenite remained stable. The acicular ferrite start temperature was found to be raised continuously by increasing the amount of prior deformation of austenite. Further evidence suggests that the application of deformation can boost the driving force for acicular ferrite formation. This phenomenon is similar to the case in which martensite forms under the influence of deformation.     

Hydrogen effects on phase transformations in austenitic stainless steels

The effects of hydrogen and stress (strain) on the phase transitions of a variety of stainless steels (316, 321, 347) were investigated. Hydrogen was introduced by severe cathodic charging at room temperature. X-ray diffraction was employed to reveal the transformations occurring in thin surface layers. After charging expanded ∈ phase is always present,α′ martensite content increases during ageing and the final content depends on the stability of the austenite. The broadening of diffraction peaks of austenite after cathodic charging is caused by nonuniform distribution of hydrogen. The state of hydrogen distribution in the steel and the relationship between internal stresses, surface cracking and phase transition is discussed.