Effect of bainitic transformation on microstructure of Si-Mn steel

Several Si-Mn steels with similar Si and Mn levels and carbon contents, ranging from 0.25 to 0.75 wt %, were studied to determine the effect of bainitic transformation on the microstructure of Si-Mn steel. The microstructure was categorized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction. The results showed the existence of an optimum transformation time to produce the maximum content of retained austenite, though the retention of a large amount of retained austenite was encouraged as a result of bainitic transformation. The microstructure consisted of carbon-free upper bainite whose individual ferrite was separated by the thin-film type of retained austenite, while the blocky type of austenite was also found. The results also showed that carbide precipitation occurred in the residual austenite after the optimum time, which decreased the retained austenite content. The retained austenite stability is discussed in relation to the carbon content and morphology of the retained austenite.     

Effect of microsegregation on carbide-free bainitic transformation in a high-silicon cast steel

ABSTRACT

A high silicon cast steel was studied in the as-cast condition in order to characterise its solidification macrostructure and microsegregation. The steel, poured into 32?mm-keel-block-shaped moulds, has a coarse solidification structure and marked microsegregation, containing low-alloyed areas with a total alloy content (Cr?+?Mn?+?Si) of 2.3 wt-% and high-alloyed zones of 5.3 wt-%. The bainitic transformation behaviour at 300°C was studied at different austempering times. The bainitic reaction occurs at different rates within the specimen volume, because of its chemical heterogeneity. An austempering heat treatment leads to an inhomogeneous carbide-free bainitic microstructure with different phase amounts, morphologies and sizes. The heterogeneous distribution of sizes and chemical compositions of retained austenite is speculated to benefit mechanical properties.     

Effect of continuous-cooling transformation structure on mechanical properties of 0.4C-Cr-Mo-Ni steel

Commercially available 0.4C-Cr-Mo-Ni steel was studied to determine the effects on its mechanical properties of various microstructures produced by continuous-cooling transformation after austenitization. A good combination of strength and notch toughness was obtained independently of test temperatures (293 and 193 K) when the steel was austenitized at 1173 K and then continuously cooled at an average rate of 3.1 K s^–1 (expressed as the average cooling rate from 823 to 573 K) before final rapid cooling. The microstructure of the steel consisted of a mixed structure of martensite and 10–15 vol% lower bainite, which appeared in acicular form in association with the martensite. Slower cooling had a detrimental effect on the mechanical properties of the steel; the microstructure of this steel consisted of a mixed structure of martensite and upper bainite, which appeared as masses in the matrix. As the average cooling rate increased, the lath size and internal stringer-carbide size in the upper bainite were larger, and retained a somewhat increased austenite content.     

Si对团球状共晶体奥氏体—贝氏体钢中贝氏体相变的影响

在热力学，动力学的基础上，从贝氏体铁素体在奥氏体贫碳区切变机制出发，研究了Ｓｉ对团球状共晶体奥－贝钢中贝氏体相变的影响，并提出Ｓｉ促使奥氏体奥碳区的形成，有利于贝氏体切变的新观点。     

Effects of hydrogen on the properties of bainitic steel crossing

The microstructures on the damaged surfaces of two served bainitic steel crossings were investigated using optical microscope and scanning electron microscope (SEM). The tensile properties of the bainitic steel with different contents of hydrogen were measured. The results showed that the plasticity of the bainitic steel, such as the reduction of area and elongation, decreases sharply with the increase of hydrogen content. There was a critical content of hydrogen without the hydrogen embrittlement for the commercial bainitic steel used for crossing, which was 7 × 10^?5 wt%. When the content of hydrogen in a bainitic steel was lower than the critical value, during the used process of the crossings, the wear failure appeared during the early stage; however, the fatigue spalling appeared in the end of the process. When the content of hydrogen was higher than the critical, brittle fracture was responsible for the failure of the crossing in a short time during use.     

变形温度对贝氏体钢组织性能的影响

为合理制定Si-Mn-MO系无碳化物贝氏体钢的生产工艺,利用GLEEBLE-3800热模拟试验机,在真空条件下开展了变形温度对贝氏体钢组织性能影响的热模拟试验.利用光学显微镜、透射电镜等设备,采用力学性能测试、微观组织观察等技术分析手段,对热模拟试样进行了组织观察和硬度检测分析,绘制了Si-Mn-MO系无碳化物贝氏体钢不同变形温度的动态CCT曲线,得出了变形温度对其组织和硬度的影响规律.结果表明,变形温度越低,无碳化物贝氏体钢的相变温度越低,组织越细小,先析铁素体越易析出,越有利于提高贝氏体钢的强硬性和韧塑性.     

Acceleration of bainitic transformation by introducing AlN in medium carbon steel

Shortening the austempering time for bainitic transformation is beneficial for the application of high-performance bainitic steel, which has attracted lots of attentions from materials researchers. It is interesting to find that adding nitrogen, ～210?ppm, in a medium carbon steel can effectively accelerate the bainitic transformation process. Analysis results reveal that the heterogeneous nucleation sites provided by AlN particles should be mainly responsible for the accelerated bainitic transformation.     

Microstructure and mechanical properties of a low carbon carbide-free bainitic steel co-alloyed with Al and Si

A low carbon, low alloy steel has been investigated for producing low carbon carbide-free bainitic microstructure by co-addition of alloying elements of aluminum and silicon. The influence of heat treatment process on microstructure, impact toughness as well as tensile properties was investigated by light optical microscopy, transmission electron microscopy, X-ray diffraction and mechanical property tests. The results demonstrate that the co-addition of aluminum and silicon in the investigated steel plays an effective role in suppressing the precipitation of cementite. A desired microstructure consisting of mainly fine-scale carbide-free bainitic ferrite and thin film-like retained austenite located between the ferrite laths was obtained and accordingly an excellent combination of toughness, ductility and strength was achieved by optimized heat treatments, i.e. by isothermal treatment at 320 °C for ∼84 min or more. The microstructure-mechanical property relationships are discussed.     

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10?nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.     

Continuous cooling transformation behaviour and bainite formation kinetics of new bainitic steel

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025?K?s^?1, and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction (f_b), and the average energy is about 136.7?kJ?mol^?1. The local Avrami exponent mainly lies between 0.5 and 3 in a wide f_b range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

X120管线钢的相变组织及性能研究

为适应将来石油及天然气高效输送的需要,以及高级别管线对力学性能及焊接性能的要求,采用一种成分为0.05%C-2.01%Mn-0.30%Mo-0.48%Ni-0.35%Cu-0.076%Nb-Ti-B的试验材料,用于开发X120管线钢的试验研究.在Gleeble 1500热模拟机上,采用在奥氏体再结晶区和未再结晶区分别变...     

A carbide-free bainitic steel with high-ductility by dynamic transformation during coiling process

A new hot-rolled alloy of carbide-free bainite has been proposed for heavy trucks frames. It involves dynamic bainite transformation during the prolonged cooling of the coiling process after hot-rolling. The new hot-rolled Fe-0.5C-2.9Mn-2.3Al (in wt-%) steels exhibit tensile strength of 1110?MPa with a combined total elongation of 35%. The alloy has been re-heat-treated with one step and two steps isothermal bainitic transformation to study the effect of microstructure evolution and microstructure-property relationship. The transformation-induced plasticity (TRIP) effect of retained austenite has been discussed and revealed as the main contributor for the high ductility of bainitic steels. The formation of the cracks at fracture regions after tensile deformation, which is associated with the austenite stability and morphology, has been observed and discussed.     

Effect of electromechanical hardening on the mechanical properties of carbon steel

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 26–30, May–June, 1989.     

Effect of substitution of Si by Al on the microstructure and mechanical properties of bainitic transformation-induced plasticity steels

The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity(TRIP) steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3 rd generation advanced high strength steels(AHSS). It is shown from the dilatometry study,microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than450℃ whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450℃.Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450℃ while Si-added steel has a better strength-elongation combination at temperatures lower than 450℃. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite(MA) blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.     

机械滚压对A473M钢疲劳性能的影响

采用机械滚压对A473M马氏体不锈钢轴套材料进行表面处理,研究滚压工艺对其力学性能的影响。采用SEM、白光干涉仪、X射线衍射仪、显微硬度计、EBSD、拉伸试验机和疲劳试验机分别对试样表面形貌、表面粗糙度、残余应力、显微硬度、拉伸性能和疲劳性能进行系统表征。结果表明:滚压加工试样表面的粗糙度明显降低,仅为车削加工的1/5;滚压加工在材料近表面引入残余压应力,其值最高可达946 MPa,沿深度方向逐渐减小,残余压应力层深度约为200μm,表面硬度提高30%左右,硬度影响层深度可达200μm;抗拉强度、屈服强度和伸长率分别提升了40%,22%和8%,疲劳寿命由基体材料的5.4×10^4周次提高到1×10^7周次。采用滚压加工后材料的力学性能明显提升,疲劳寿命显著增加。