Annealing of strain-induced martensite to obtain micro/nanometre grains in austenitic stainless

Micro/nanometre grain sizes appear to improve the biocompatibility of austenitic stainless steel. In order to realise the reverse transformation (from strain-induced martensite) austenite structure control with micro/nanometre size, the influence of annealing parameters on the microstructure evolution and mechanical properties of 316L-Nb austenitic stainless steel were investigated. Furthermore, the role of Nb in the annealing process was also studied. The results showed that the closely 100% reversion transformation austenite structures were obtained in the samples after annealing at 850°C, where the grains with the grain diameter ≤500?nm accounted for 25% and the grains with the grain diameter >0.5?µm accounted for 75%. The micro/nanometre grain steel not only exhibited a high strength level but also exhibited a desirable elongation. Moreover, the Nb demonstrates a remarkable effect on grain-refining and a significant role in improving the stability of the microstructure.     

The bainite transformation behavior,the influencing factors and control strategy in ferritic‐pearlitic forged crankshafts with coarse grains

Microstructural characterization of the bainite in a ferritic–pearlitic forged crankshaft was carefully investigated. A Gleeble thermo‐mechanical simulator as well as a high resolution dilatometer were also used to analyze the effect of cooling rate on the bainite formation and the bainite transformation mechanism in steels with different austenite grain sizes. Results show that the fine structure of the bainite mainly consists of bainitic ferrite and martensite. No segregations are found where bainite forms. Bainite tends to form in the slower cooled inner part of the crankshaft with an austenite grain size exceeding 100 μm. The formation of bainite is mainly affected by the austenite grain size as well as the cooling rate in the crankshaft studied. As the austenite grain size increases, ferrite start, pearlite finish and bainite finish temperatures are decreased. More bainite forms when bainite finish temperature decreases. The critical cooling rate of bainite transformation is increased from 0.34 °C?s^‐1 to 0.44 °C?s^‐1, if the maximum austenite grain size is refined from 216 μm to 100 μm. For ferritic–pearlitic crankshafts, or other bulky products, the elimination of bainite can be achieved through austenite grain refinement.     

一种新型高强塑积异质冷轧中锰钢的力学性能

使用原位电子背散射衍射(EBSD)和球差透射电镜(ACTEM)等手段,研究了新型异质结构中锰TRIP钢在拉伸过程中微观组织的演变机制和力学性能。结果表明,在680℃退火后的实验钢中生成了多形貌、多尺度的异质奥氏体结构(颗粒状、块状、片层状奥氏体)和铁素体组织,其抗拉强度为1272 MPa,总延伸率为54.5%,强塑积高达69.3 GPa·%。在拉伸过程中C/Mn含量较低的颗粒状奥氏体先发生相变,而C/Mn含量较高的块状和片层状奥氏体在较大的应变范围内逐渐发生相变,从而导致高强度与高塑性的良好匹配。结果还表明,马氏体相变优先在奥氏体晶界/相界附近的区域形核。与晶粒尺寸相比,C/Mn元素对奥氏体稳定性的作用更重要。     

Effect of grain size on austenite stability and room temperature low cycle fatigue behaviour of solution annealed AISI 316LN austenitic stainless steel

Abstract

The present paper investigates completely reversed room temperature low cycle fatigue (LCF) behaviour of solution annealed austenitic stainless steel AISI 316L with two different grain sizes of 90 and 139 μm developed by solution annealing treatment at 1050 and 1150°C respectively and at six strain amplitudes ranging between ± 0·375 and ± 1·00%. Complete cyclic hardening has been observed for both the grain sizes. While fine grained steel shows an improvement in cyclic life compared with that of coarse grained steel for strain amplitudes ± 0·375 and ± 0·50%, and perfectly follows the Coffin–Manson (C–M) behaviour within the experimental domain, higher cyclic life with bilinear C–M behaviour is observed in the case of coarse grained steel at ± 0·625% strain amplitude and above. Optical microscopy of fatigue fracture surfaces reveals the formation of martensite on cyclic straining predominantly at higher strain amplitudes.     

Improvement of NiTi Shape Memory Actuator Performance Through Ultra‐Fine Grained and Nanocrystalline Microstructures

Ultra‐fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra‐fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well‐appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra‐fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400 °C for 600 s allows for the evolution of microstructures with median grain sizes of about 34 nm, while annealing at 600 °C for the same length of time results in median grain sizes of about 5 µm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity.     

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel.Based on the microstructure analysis,the texture evolution and martensite transformation behavior in the strain hardening process were studied.The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the con-tinuous hardening stage.As the strain increases,the texture of retained austenite evolves towards the F orientation{111}〈112〉,which is not conducive to martensite transformation.After the strain of 5％,the number of austenite grains with high Schmid factor orientations is gradually increased,and then signif-icantly reduced when the strain is over 10％due to the occurrence of martensitic transformation,which results in a high martensitic transformation rate.However,the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage.Moreover,because of the coordination deformation of austenite grains,strain preferentially spreads between adja-cent austenite grains.Consequently,the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite,leading to a differential contribution to each strain hardening stage.     

Evolution of surface deformation during fatigue of PH 13-8 Mo stainless steel using atomic force microscopy

The relationship between microstructure and nucleation of fatigue cracks in PH 13‐8 Mo stainless steel was explored with the use of atomic force microscopy (AFM) that allowed an accurate quantitative characterization of the surface features. Fully reversed strain‐ controlled fatigue tests were performed at 0.4 and 0.6% strain amplitudes, and the evolution of the surface deformation was observed at various fractions of life. At 0.4% strain amplitude, fatigue surface damage occurred first in the shape of streaks about 4 nm deep that formed at the interface between martensite laths and at prior austenite grain boundaries, and eventually coalesced to form crack nuclei. The increase in strain amplitude to 0.6% led to the formation of large extrusions, on average between 2 and 5 μm long with heights between 10 and 200 nm, which were the preferred crack nucleation sites.     

Dislocation configurations through austenite grain misorientations

In the present investigation, many strain controlled low cycle fatigue experiments of metastable austenitic stainless steel were performed at various total strain amplitudes under ambient temperature where the strain rate was kept constant. Dislocation cell developed due to strain cycling was measured through extensive analytical transmission electron microscopic investigation and the deformed austenite grains’ misorientation was measured through electron back scattered diffraction experiments. A strong connection has been established with the dislocation substructures’ configurations, the deformed austenite grains’ misorientation and the extents of induced phase transformation occurs while cyclic plastic deformation of metastable austenite at various total strain amplitudes. It has been investigated that with the increase in strain amplitude, dislocation cells are getting more uniform. It has also been found that with the increase in strain amplitude, dislocation cell size decreases drastically towards the higher strain amplitudes.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

Investigation of cold rolling variables on the formation of strain-induced martensite in 201L stainless steel

In this work, effects of cold rolling variables including strain, strain rate, strain path, initial austenite grain size and rolling temperature on the formation of strain-induced martensite in AISI 201L stainless steel are investigated. Cold rolling was carried out at −40, −10, and 25 °C with strain rates of 0.1–1.2 s⁻¹ and thickness reductions of 0–95%. The results showed that saturation strain of martensite formation during cold rolling at room temperature with the strain rate of 0.5 s⁻¹ was about 0.5. Increasing the strain, strain rate, and initial austenite grain size, decreasing rolling temperature, and the use of cross rolling resulted in an increase in the volume fraction of strain-induced martensite and a decrease in the saturation strain value. It was found that effect of decreasing rolling temperature and cross rolling was more effective on the formation of strain-induced martensite compared to other parameters, leading to a reduction of saturation strain from 0.5 to 0.28.     

奥氏体不锈钢晶粒细化对形变机制和力学性能的影响

利用相逆转变原理采用冷变形使得亚稳奥氏体转变为形变马氏体,采用不同温度和时间退火分别获得纳米晶/超细晶和粗晶奥氏体不锈钢。通过拉伸实验得到不同晶粒尺寸的奥氏体不锈钢力学性能,采用透射电镜观察形变组织结构并利用扫描电镜观察断口特征。结果表明:高屈服强度纳米晶/超细晶奥氏体不锈钢通过形变孪晶获得优良塑性;而低屈服强度的粗晶奥氏体不锈钢发生形变诱导马氏体效应,得到良好的塑性;两组具有不同形变机制的奥氏体不锈钢拉伸断口均为韧性断裂。形变机制由形变孪晶转变为形变诱导马氏体归因于晶粒细化导致奥氏体稳定性大幅度提高。     

Generating duplex microstructures by nitriding; nitriding of iron based Fe–Mn alloy

Nitriding of Fe–2 at-%Mn alloy at 650°C employing a nitriding potential of 0·05 atm^??1/2 resulted in a highly complex microstructural development as a function of depth below the specimen surface: a surface adjacent layer exhibiting an austenite–martensite duplex microstructure, followed by an intermediate region showing a ferrite–austenite duplex microstructure, and at even larger depths a region where an austenite layer covers the grain boundaries of the ferrite matrix grains. Development of this complex microstructure is attributed to the strong austenite stabilising effects of Mn and N. This work demonstrates the power of a relatively simple nitriding treatment to realise a highly complex microstructure known to be associated with strongly enhanced mechanical properties.     

New Optimum Extrusion Parameters of 9% Cr Ferritic Steel Tubes

The mechanism of austenite grain growth prior to forging of 9% Cr ferritic steels was analyzed. Pre-forging heating temperature and holding time had an obvious effect on austenite grain size and delta ferrite morphology. Hot compression experiments were conducted by using a Gleeble 1500D thermal–mechanical simulator. A hot upsetting test was performed on an HP01-500 oil press to verify the geometric dynamic recrystallization mechanism. New extrusion technology parameters of steel tubes were optimized by the Deform 2D software, and practical tests were conducted. Results indicated that the grains grew slowly at low temperatures (i.e., 1000°C and 1050°C). The kinetic curve of grain growth was close to the “S” type at 1100°C. The grains exhibited abnormal grain growth at high temperatures (i.e., 1150°C and 1200°C). The quantity of delta ferrites markedly increased at 1250°C. In addition, grains were refined by geometric dynamic recrystallization during the formation process. Mechanical properties of 9% Cr tube met standard requirements.     

Surface Nanocrystalline of Martensite Steel Induced by Sandblasting at High Temperature

The mechanical and tribological properties were conducted to investigate surface nanocrystalline of martensite steel with hardness of 5 GPa by using sandblasting technique at 500 °C. The average grain size of surface is 30 nm. In contrast, the average grain size of the martensite steel surface, which processed by sandblasting at room temperature and post‐annealing at 500 °C, is 112 nm. Fine grains and Fe₃C phase in situ form during sandblasting at 500 °C that is favor for producing nanoscale structure of the martensite steel. The nanostructure surface of the martensite steel has higher hardness and better wear resistance.     

Effect of Prior Austenite Grain Size on the Morphology and Mechanical Properties of Martensite in Medium Carbon Steel

In industrial application,unintentional manufacturing line troubles often consequence in heating raw materials excessively,in terms of either time or temperature.One of the effects of such occurrence is a product with a variation of prior austenite grain size,even if after the heat treatment the end result is the same,martensite.The variation of the prior austenite grain size is believed to vary the end results of the martensite.This undesirable variation includes the variation of fatigue resistance,impact strength,yield strength,hardness,etc.This research studies the effect of the prior austenite grain size on the morphology of the martensite microstructure.The results show that within the typical industrial application of temperature and holding time set up,as holding time or the temperature increases,the prior austenite average diameter increases.The block and packet sizes in the martensite also increase.The variation of mechanical property dependence on the grain size is indeed due to the different characteristics reflected in the martensite morphology.With respect to the same area,smaller grain has more blocks and packets,which agrees with higher dislocation density verified with transmission electron microscopic evaluation.     

Effect of prior austenite on reversed austenite stability and mechanical properties of low carbon medium manganese steel heavy plate

The effect of prior austenite on reversed austenite stability and mechanical properties of Fe‐0.06C‐0.2Si‐5.5Mn‐0.4Cr (wt.%) annealed steels was elucidated. With the decrease of austenitizing temperature from 1250 °C to 980 °C, the prior austenite changed from complete recrystallization to partial recrystallization, and the average austenite size was reduced. The volume fraction of reversed austenite was increased from 26.32 % to 30.25 % because of high density of grain boundaries and dislocations. The martensite transformation temperature of annealed steels was increased from ～115 °C to ～150 °C, and both of thermal and mechanical stability of reversed were reduced. There was no significant different in tensile properties, however, the impact toughness was enhanced from 100 J to 180 J at ?60 °C. The excellent impact toughness in annealed steel (austenitized at 980 °C) was obtained because of higher density of high misorientation grain boundaries, more volume fraction of reversed austenite and reduced segregation at grain boundaries.     

Cyclic behaviour of 12% Cr ferritic‐martensitic steel upon long‐term on‐site service in power plants

The strain‐controlled and stress‐controlled low‐cycle fatigue behaviour of served 12% Cr ferritic–martensitic steel is conducted at room temperature. Continuous softening is observed at both control modes, and the fitting results show that the fatigue properties of 12% Cr steel are not reduced significantly after 230 000 h service at 550 °C/13.7 MPa. Scanning electron microscopy has been employed to investigate the microstructure evolution after long‐term service. It is proved that the decomposition of martensite laths structure and the coarsening of carbides at grain/lath boundaries are the main reasons why the pipe bursts after 180 000 h service at 550 °C/17.1 MPa. The fracture under both control modes has been observed by using scanning electron microscopy, and it indicates coarse carbides along grain/lath boundaries are favourable sites for micro‐crack nucleation and the secondary cracks along the fatigue striations are formed by the connection of micro‐cracks nucleated during fatigue behaviour.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Effect of Microstructure Refinement on the Strength and Toughness of Low Alloy Martensitic Steel

Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a HalI-Petch relation with the yield strength. When the prior austenite grain size was refined from 199 μm to 6 μm , the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at 77 K improved more than 8 times, indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength. The fracture surfaces implied that the unit crack path for cleavage fracture is identified as being the packet.