铌微合金化重载齿轮钢的疲劳性能

利用旋转弯曲疲劳试验方法研究了三种重载齿轮钢渗碳后的疲劳性能。结果表明,添加铌能够细化重载齿轮钢组织,提高渗碳层硬度,从而提高其疲劳强度。同时,疲劳裂纹在渗碳层沿原奥氏体晶界扩展,铌微合金化重载齿轮钢的晶粒细化,从而可以阻碍疲劳裂纹的扩展。此外,扫描电镜观察疲劳断口发现,重载齿轮钢渗碳后疲劳裂纹起源于基体或夹杂物,夹杂物尺寸越小,疲劳性能越好。     

Interphase and intergranular stress generation in carbon steels

Neutron diffraction spectra have been acquired during tensile straining of high and low carbon steels, in order to compare the evolution of internal stress in ferritic steel with and without a reinforcing phase. In low carbon steel, the generation of intergranular stresses predominates, while in high carbon steel similar intergranular stresses among ferrite grain families are superposed upon a large redistribution of stress between phases. Comparison is made to calculations using elastoplastic self-consistent and finite element methods.     

Recrystallization/precipitation behaviour in microalloyed steels

Microalloyed high-strength low-alloy (HSLA) steels contain additions of Nb, V, Ti, or in combination, in amounts of 0.01 to 0.1 weight percent to improve mechanical properties, which are strongly dependent on the thermomechanical interaction taking place in the course of rolling mill processes. The recrystallizatian of hat-twisted austenite has been investigated in a cylindrical specimen (f 6×50 mm) machined from hat rolled plates of 0,052 wt % Niobium microalloyed steel. Continuous and interrupted torsion test were carried out in the temperature range 1123 K to 1173 K after a solution treatment of 1.5 minutes at 1423 K and torque-twist data were analysed. The various methods were discussed for obtaining results from torsion tests. The effect of precipitation kinetics was appreciated by way of connection t_p/t_p(red), where t_p is the experimental measured time for the peak stress and t_p(red) is the newly defined reduced time. The softening ratio X and time t_0.05R for start of static recrystallization were established.

The correlation between precipitation and recrystallization is presented as a graphs for chosen requirements (temperature of austenitization, carbon and niobium content and strain rate). If temperature goes below 850°C, the restoration processes are hardly suppressed, both are limited by diffusion and Nb(CN) precipitation, which are extended dynamically in the range of strains rates 10⁻² to 1 s⁻¹.

In the present paper, an attempt is made to derive the PRTT diagram and to define all mathematical equations for describing recrystallization times t_0.05R, t_0.5R, t_0.95R and t_0.05P for the start of precipitation. In real metal forming processes such as the hot rolling of plates or strips the knowledge of these parameters and results is extremely important for the the correct microstructure and sheet quality to be obtained.     

The interphase precipitation reaction in HSLA steels

An in-depth study of the interphase precipitation reaction in model vanadium steels has shown that the reaction may not just be confined to HSLA steels, but may be part of a general class of banded microstructures which are common to both eutectoid and eutectic systems. A new mass transport theory has been developed in which the interphase precipitation reaction in Fe-C-V steels is treated as a generalized type of cooperative growth. In addition to predicting the spacings of sheets of interphase precipitates and the precipitate sizes in steels, this theory is providing new insights into the origin of banded structures occurring in eutectic systems at solid-liquid interface boundary velocities faster than those required for coupled growth, but slower than those required to produce the extended metastable solid solution.     

Study of molybdenum precipitation in steels using thermoelectric power measurement

The precipitation of molybdenum in iron has been investigated using thermoelectric power measurement. We found an increase of the thermoelectric power of iron with the molybdenum content and a coefficient of its influence has been determined. A correlation between secondary hardening due to the precipitation of Mo₂C carbides and thermoelectric power variations has been established.     

铌、钛碳氮化物在微合金钢中的回溶及析出行为

简述了钢中主要添加微合金元素Nb和Ti在钢坯加热过程中的回溶过程。阐明了钢板在轧制各阶段析出相的类型、晶体结构、分布规律,特别是应变诱导析出与再结晶的关系,同时,分析了时效硬化行为。讨论了微合金元素的选择及节约利用问题。     

Thermodynamic calculation for precipitation hardening steels and titanium aluminides

Thermodynamic calculation using Thermo-Calc was carried out to quantify the phase fraction and element partition in precipitation hardening steels and γ-TiAl based alloys. Calculations show that the precipitates formed in Fe–Ni–Al–Mo systems and PH13-8 stainless steel are NiAl phases, in accordance with atom probe microanalysis. Calculations of the phase constitution and element distribution in several titanium aluminides show again a good agreement with the experimental measurement from atom probe. In addition, Thermo-Calc calculation can help identify the existence of some phases which were not readily observed experimentally. It can be used as a guide in alloy design, especially the design of the γ-TiAl based alloys.     

Sigma-phase precipitation upon industrial-like heating of cast heat-resistant steels

In order to investigate the formation of sigma-phase in two cast heat-resistant steels of wide industrial use, two ferrous alloys containing 25.02Cr–4.26Ni–0.53C and 31.23Cr–6.08Ni-0.36C were prepared by casting techniques and then subjected to sigma precipitation by heating the alloys, for increasing periods of time, at a temperature which resembles the average thermal level usually encountered in service conditions of these materials. Initially, the phase nucleates along ferrite–austenite boundaries, but with more time sigma precipitates inside the ferrite grains. Furthermore, both alloys showed a behavior that resembles the well-known Johnson–Mehl–Avrami's mechanism, stated for a nucleation and diffusion-controlled growth process in metallic systems. Hence, the phenomenon reached asymptotic values of approximately 25 and 55 vol.% in the HC- and HD-grade, respectively, after 120 h heating at 1053 K. The experimental results may help in controlling the behavior of these materials when they are to be utilized in industrial applications at elevated temperatures.     

Thermodynamics of precipitation and textural development in batch-annealed interstitial-free high-strength steels

A systematic and detailed characterization of precipitates, as a function of steel composition, has been carried out in five batch-annealed interstitial-free high-strength steels. The precipitation behavior has been discussed from the point of view of thermodynamics as well as on the basis of the kinetics. An attempt has been made to explain the discrepancies between the thermodynamic prediction and the actual observations. Finally, the correlation between precipitation and textural evolution and deep drawability has been critically investigated.     

Modelling the kinetics of strain induced precipitation in Nb microalloyed steels

Strain induced precipitation is a key phenomenon that controls the microstructure evolution during the finish rolling stages of microalloyed steels. Extensive research has shown that the precipitation of Nb(CN) delays the onset of recrystallisation. This paper presents a model to describe the precipitation kinetics during isothermal holding following high temperature deformation in Nb-containing steels. The model is based on the assumption that heterogeneous nucleation of precipitates on dislocations and enhanced coarsening due to pipe diffusion are responsible behind the accelerated kinetics observed in strain induced precipitation. Results show a very good agreement between reported experimental observations and predictions of the present model for precipitate size and volume fraction evolution.     

Role of copper in precipitation hardening of high-alloy Cr-Ni cast steels

The mechanism of strengthening with second-phase particles that results from heat treatment, i.e., precipitate hardening, plays an important role in modern alloys. The strengthening effect of such particles can result from their coherence with the matrix, inhibition of dislocation slip, inhibition of grain boundary slip, as well as hampering recovery processes due to dislocation network pinning. The results of investigations into high-alloy Cr-Ni-Cu cast steels precipitate hardened with highly dispersed ε phase particles are presented within. The influence of heat treatment on changes in microstructure, mechanical properties, and morphology of fracture surfaces obtained under loading have been analyzed. It has been demonstrated that, with the appropriate selection of heat treatment parameters, it is possible to control the precipitation of the hardening ε phase and, thus, to change the final mechanical and functional properties.     

电工钢铸锭内析出相粒子特征及回溶行为

利用场发射电镜及能谱仪研究了取向电工钢薄、厚板坯铸锭中不同区域的析出相差异,确定了MnS、AlN及Fe3C的析出顺序和形貌特征。结果表明AlN以先析出的MnS为形核核心,形成粗大微米级的复合析出相。针状Fe3C既可以在复合析出相表面形成也可以在基体中单独出现,主要受冷却速度的影响。在1250℃保温1～2 h,AlN、Fe3C均可完全回溶,但部分MnS仍不回溶。MnS尺寸随着保温时间延长而增大。导致热轧板中沿轧向分布的粗大MnS,减弱了粒子钉扎力。     

The precipitation of copper in abnormal ferrite and pearlite in hyper-eutectoid steels

The precipitation of copper within abnormal ferrite and pearlitic phases in hyper-eutectoid Fe-C-Cu and Fe-C-Mn-Cu experimental steels has been examined, principally by transmission electron microscopy, in terms of morphology, mechanism and crystallography. Interphase precipitation of copper during austenite decomposition was found to be a primary precipitation mechanism. However, precipitation due to ageing was also observed in both abnormal and pearlitic ferrite. The latter case appeared either as uniformly distributed precipitates within the ferrite or in association with dislocations. Copper precipitates adopted either the Kurdjumov–Sachs or the Nishiyama–Wasserman orientation relationship with respect to the ferrite. Disc-shaped precipitates thought to form by ageing within abnormal and pearlitic ferrite develop on {001}_ferrite habit planes. Twinning observed within copper precipitates formed by interphase precipitation was attributed to a requirement to accommodate the growth of precipitates at the interphase boundary. Multi-twinned precipitates were found in pearlitic ferrite and may be evidence for an intermediate transitional state between b.c.c. and f.c.c. structure.     

不锈钢中硼微合金化的晶界偏聚析出行为

针对20多年来不锈钢中硼微合金化的研究,介绍了微量硼在不锈钢晶界上的偏聚析出机制,重点综述并讨论了微量硼的偏聚析出行为.     

Stress corrosion and hydrogen embrittlement of two precipitation hardening stainless steels

Abstract

Straining of smooth tensile specimens of two precipitation hardening stainless steels over a wide range of controlled potentials in natural sea water has revealed regimes of low ductility above about — 200 mV(SCE) and below — 600 mV(SCE). The apparent loss inductility in the anodic region was attributable to dissolution and was found to be associated with pitting and crevice corrosion effects that were exacerbated when straining was conducted in acidified sea water. The embrittlement observed at more cathodic potentials, which gave rise to transgranular or intergranular failure depending upon the material and its heat treatment, is believed to be due to hydrogen that is evolved at such potentials. The occurrence of embrittlement by hydrogen in these materials, to produce similar modes of failure, was confirmed by testing smooth specimens ajtercathodic charging and both smooth and precracked specimens in gaseous hydrogen. Cracking of precracked specimens in gaseous hydrogen is complicated by the presence of δ ferrite stringers in the microstructure, which can give rise to delamination perpendicular to the principal plane of fracture if the stresses ahead of the crack are not reduced significantly by hydrogen embrittlement.     

Processes of precipitation and intercrystalline corrosion in high-nitrogen Cr-Mn steels after isothermal annealing

Conclusions Susceptibility to ICC is the determining factor in selecting nitrogen-containing steels after technological operations that involve heating.The best combination of resistance to ICC and high mechanical properties of nickel-free Cr–Mn steels is obtained with 0.5–0.6% N.Institute of Metal Science and Technology of Metals, Bulgarian Academy of Sciences. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 13–15, May, 1981.