Fatigue behavior of a nial precipitation hardening medium carbon steel

The fatigue behavior of an Fe-0.30C-4.48Ni-l.32Al steel tempered to give three different microstructures of the same ultimate tensile strength has been investigated by light and electron microscopy, low and high cycle fatigue tests, X-ray line broadening and stress relaxation measurements. The three different heat treatments produced the following structures: I) a conventional quenched and tempered microstructure with a high density of dislocations and elongated carbides, II) a microstructure of high dislocation density, coarse carbides and fine coherent NiAl precipitates and III) a highly tempered micro-structure with a recovered dislocation substructure, coarse carbides and fine coherent NiAl precipitates. In low cycle, strain controlled fatigue cyclic softening in Treatment I was accompanied by a rearrangement of the dislocation substructure and a reduction in both the internal stress and lattice microstrain. Treatment II, which remained cyclically stable during the initial portion of the fatigue life, showed little change in the internal stress and dislocation density and showed a slight increase in lattice microstrain. Treat-ment III, which initially cyclically hardened, exhibited a rise in internal stress, lattice microstrain and dislocation density. The behavior of Treatments II and III is attributed in part to the presence of the fine NiAl precipitates which appear to reduce the tendency of the transformation induced dislocation substructure to rearrange itself into a cell structure during fatigue. In high cycle, stress controlled fatigue Treatment II showed the best fatigue resistance and Treatment I the worst. Improvement in life was attributed to improved resistance to crack initiation. Formerly Graduate Student, Marquette University,     

Fatigue behavior of a precipitation hardening Ni−Al−Cu medium carbon steel

The fatigue behavior of an Fe-0.3 wt pet C-4 wt pet Ni-1 wt pet Al-1 wt pet Cu precipitation hardening steel was investigated in three different heat treated conditions which give similar tensile strengths but different microstructures. One heat treatment produced a lightly tempered lath martensite having fine carbides and a high dislocation density. The other two heat treatments produced highly tempered martensite with coarse carbides, fine intermetallic precipitates and a relatively low dislocation density. The steel in the lightly tempered condition showed marked softening on strain cycling while the highly tempered conditions resulted in both hardening and softening. The lightly tempered structure had better low cycle fatigue resistance but the two highly tempered structures had better high cycle resistance. The dislocation substructure in the lightly tempered steel rearranges itself and accommodates plastic strain during cyclic deformation while the substructure in the highly tempered structures containing fine precipitates resists rearrangement. This difference is suggested as the reason for the differences in behavior. The three conditions show little variation in their resistance to fatigue crack propagation. However, the highly tempered, precipitate containing structures were much more resistant to fatigue crack initiation in notched specimens.     

Precipitation behavior in a medium carbon,ti-v-n microalloyed steel

The precipitation behavior of a medium carbon steel microalloyed with Ti, V, and N has been studied by analytical transmission electron microscopy in the as-cast and isothermally heat-treated states, as well as at different stages in the thermomechanical processing of the steel. Mixed (Ti,V) nitrides were found in all the structures, but there was no evidence for mixed carbonitride formation. The Hillert-Staffansson model was used to predict the composition of the nitrides as a function of tem-perature. Upon prolonged aging many of the precipitates became “fragmented” and were no longer single crystals. At the same time, the volume fraction of precipitates dropped, while their average Ti content increased. Possible explanations for this unexpected behavior are discussed in this article.     

低碳钢中纳米尺寸碳化物的相间析出行为

采用扫描电镜和透射电子显微镜对低碳Ti-Mo系的热轧板进行了组织分析,同时对其中的纳米粒子析出行为进行了研究.强化机理分析表明析出强化对于屈服强度的贡献值可达291 MPa.随着卷取温度的降低,纳米粒子相间析出的排间距会减小,相间析出的排间距与其在铁素体中形核点位置有一定的离散值,但基本上呈一定的固定值.α/γ界面的观察和采用不同理论的计算结果表明相间析出的产生主要与α/γ界面的台阶形成有关,相间析出的排间距大小由台阶高度、晶界扩散系数、等温温度、台阶面迁移速率等决定.     

含钛中碳钢中钛氧化物析出热力学分析

以含钛中碳钢为研究对象,从热力学的角度分析了含钛中碳钢中钛氧化物析出行为,结果表明:浇注温度为(1535±10)℃且钢中溶解氧含量大于0.003％(质量分数,余同)时,低钛中碳钢中的溶解Ti可与钢中的溶解氧反应生成Ti3O5、Ti2O3,生成TiO2、TiO夹杂的可能性较小;高钛中碳钢中的溶解Ti可与钢中的溶解氧反应生...     

High cycle fatigue behavior of gas-carburized medium carbon Cr-Mo steel

High cycle fatigue properties of gas-carburized 4140 steel were assessed to compare with those of 8620 steel which is widely used as a carburizing steel. Fatigue limit was evaluated associated with microstructure, case depth, and distribution of retained austenite and compressive residual stress near the surface. Test results indicated that the reheat quenching method of 4140 and 8620 steels produced a reduction in grain size, retained austenite level, and compressive residual stress at the surface and an increase in fatigue limit. The fatigue limit of direct-quenched 4140 steel shows substantially lower value than that of direct-quenched 8620 steel due to larger grain size of direct-quenched 4140 steel. However, the fatigue limit of reheat-quenched 4140 steel is greatly improved and is comparable to the reheat-quenched 8620 steel. This is attributed to the larger reduction ratio in grain size and deeper case depth of reheat-quenched 4140 steel as compared to direct-quenched and reheat-quenched 8620 steels.     

Isothermal precipitation behavior of copper sulfide in ultra low carbon steel

Copper and sulfur are typical residual elements or impurity elements in steel.Sufficient removal of them during steelmaking process is difficult for copper and costly for sulfur.Utilization of copper and sulfur in steel, especially in steel scrap,has been an important issue for a long period for metallurgists.Copper and sulfur may combine to form copper sulfide,which may provide a prospect to avoid the detrimental effects of copper and sulfur in steel.Unfortunately the formation mechanism of copper sulfide in steel has not been completely clarified so far. In the present paper,solution treatment of samples containing copper and sulfur are firstly performed at 1623 K for 2.7×10~3 s followed by quenching into water.The samples are then isothermally heat-treated at 673 K,873 K, 1073 K,1273 K and 1373K for different time followed by quenching into water again.The size,morphology, constituent and crystallography of sulfide precipitates in these samples are investigated by scanning electron microscope(SEM) and TEM equipped with EDS.Fine copper sulfides(less than 100 nm) are observed to coexist with silicon oxide in samples even isothermally heat-treated at 1 373 K for 1.44×10~4 s;Film-like copper sulfides are generally observed to co-exist with iron sulfide in all samples;Plate-like copper sulfides are observed especially in sample isothermally heat-treated at 1 073 K for 1.44×10~4 s.The formation mechanisms of these copper sulfides have been discussed.     

Modeling the flow behavior of a medium carbon microalloyed steel under hot working conditions

The constitutive equations for the flow behavior of a commercial 0.34 pct C-1.5 pct Mn-0.7 pct Si-0.083 pct V-0.018 pct Ti microalloyed steel were determined. For this purpose, uniaxial hot compression tests were carried out over a wide range of strain rates (10⁻⁴ to 10 s⁻¹) and temperatures (1123 to 1423 K). In combination with models developed in the literature, the experimental results permit the flow stress of the present steel to be predicted within ± 5 pct. It is shown that the classical constitutive equations must be modified to take the grain size into account, particularly when the latter is below 30 μm.     

Low-cycle fatigue properties of microalloyed medium carbon precipitation hardening steels in comparison to quenched and tempered steels

Monotonic and cyclic stress strain curves and strain fatigue-life curves of a normalized carbon steel Cf 53 N, two quenched and tempered steels Ck 45 QT, 34 CrMoS 4 QT and three microalloyed medium-carbon precipitation-hardening steels 27 MnSiVS 6 + Ti BY, 38 MnSiVS 5 BY and 44 MnSiVS 6 + Ti BY, have been evaluated. Similar strain hardening was observed in the monotonic tensile tests whereas different hardening or softening was found under cyclic loading conditions. QT steels reveal pronounced cyclic softening over the entire strain range investigated, the ferritic pearlitic steels show only a slight decrease in the cyclic proof stress and cyclic hardening at larger strains. Strain fatigue-life curves result in a common scatterband of all steels investigated with the microalloyed steels 27 MnSiVS 6 + Ti BY and 44 MnSiVS 6 + Ti BY lying at the upper limit. Crack initiation probability of the microalloyed medium-carbon precipitation-hardening steels in the low-cycle fatigue range is equivalent or lower than for the normalized carbon steel and the QT-steels.     

Fatigue anisotropy in cross-rolled, hardened medium carbon steel resulting from MnS inclusions

Anisotropy of forged steel components is especially adverse when it concerns rotationally symmetric components. Manganese sulfides (MnS) in steels may be desired for their improvement of machining properties; however, they also deteriorate fatigue behavior. A quantification of the effect of MnS on anisotropy is necessary to find an optimum for component dimensioning. To isolate the influence of MnS on anisotropy only, high cleanness of the test material is required. The test material in the current investigation was molten in a vacuum furnace to high-cleanness composition. Materials with two different S levels were produced to detect variations in anisotropy according to amount, shape, and distribution of the MnS inclusions. The two batches were cross-rolled to plates with a deformation ratio of 4.5. The MnS phase constitutes, upon forging or rolling, pancake-shaped inclusions. In the case of cross-rolling, an in-plane rotational symmetry of the inclusions could be created. The shape and size of these inclusions are essential for the mechanical behavior of the material. Push-pull fatigue testing was performed in longitudinal (in plane) and short transversal directions relative to the rolling plane. The results showed strong anisotropy of the fatigue behavior with inferior performance in short transverse directions where the principal stress is perpendicular to the flattened inclusions. The anisotropy was somewhat more pronounced for the high-S material, resulting from a different fatigue crack growth mechanism.     

Structure-property changes during hardening and tempering of new ultra high strength medium carbon low alloy steel

Abstract

A medium carbon low alloy steel, electroslag refined, modified AFNOR 15CDV6, has been developed for satellite launch vehicle and related applications. Conventionally processed (without electroslag refining) mostly bainitic AFNOR 15CDV6 (with 0·15 wt-% carbon and ~ 3·5 wt-% other alloying elements) has a yield strength of ~ 800 MPa. Electroslag refining, coupled with increased carbon (0·29 wt-% carbon, but no change in percentage of other alloying elements), increased the yield strength to about 1300-1400 MPa, without sacrificing ductility. The microstructure of the modified grade was martensitic. Martensite in the as hardened state was mostly in the form of laths, although ~20% plate martensite was also observed. Until 150°C tempering, no noticeable loss of tetragonality was observed, while the unit cell parameter c/a ratio dropped to almost 1 after 300°C tempering. The interesting observation at 150°C tempering was the predominant presence of fine rodlike ? carbide, which may also explain the increased yield strength. Tempering above 150°C converted the ? carbide to cementite, relatively thicker precipitates of similar morphology. At higher tempering temperatures, no evidence of spheroidisation of cementites was noted. The highest tempering temperatures of 500 and 600°C resulted in two marked changes in the microstructure: the appearance of M₂₃C₆ type (Cr, Fe and Mo bearing) carbides, and the appearance of, in some regions of the microstructure at least, a relatively 'recovered' lath structure. Misorientation among adjacent laths, nearly constant at 8-9° until 450°C tempering, increased noticeably, to 13 and 16°, after the respective tempering temperatures of 500 and 600°C.     

Investigation on microstructure and properties of a combined precipitation hardening ultrahigh strength steel

The microstructure and properties of a combined precipitation hardening ultrahigh strength steel with nano-sized carbides and intermetallics were studied systematically.The results show that after tempering at 300℃lots ofε-carbides are precipitated in the martensite,the strength rises and the toughness falls slightly.After tempering at 430℃,much coarser cementite lamina are precipitated in martensitic laths,which causes the impact toughness falls to the minimum value.With temperature further increasing the cementites are dissolved and M₂C carbides,β-NiAl intermetallics and reverse austenite begin to precipitate.The tensile strength and yield strength achieve the peak value at 470℃,490℃respectively.The tested steel achieve a tensile strength of 2 120 MPa,a yield strength of 1 950 MPa and impact energy of 54 J/cm² after optimum tempering at 510℃.When tempering temperature is above 530℃the M₂C carbides and reverse austenite is coarsening.After tempering at 560℃the reverse austenite reaches the maximum volume fraction in present work.     

Fatigue behavior of maraging steel 300

The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed (1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain range from 0.1 to 20 pct. The S-N curves were determined from 10 to 10⁷ cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’ theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10^-7 and 10^-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions in the aged alloy.     

plane-strain work hardening and transient behavior of interstitial-free steel

Stress transients resulting from abrupt changes in strain path have been shown to be important to subsequent formability. In order to investigate whether these transients result from strain aging or related interstitial effects, two-stage experiments were performed on Armco interstitial-free steel After a prestrain in plane-strain tension, the material was strained in uniaxial tension in the direction of zero initial extension. The stress-strain curve in plane strain was found to deviate markedly from that predicted by usual plasticity theory (Hill’s theory withM = 2.0). Comparison of monotonie curves from uniaxial and plane-strain tension using a newly-developed, self-consistent calculation suggested that IF steel follows Hill’s new theory with constantM ≈ 2.9. After the change from plane strain to uniaxial tension, positive stress transients (flow stress exceeds the monotonie flow stress) were measured. This form of transient agrees with ones measured for other steels. It therefore appears that the origin of the transient phenomenon is independent of interstitial content, and that static strain aging is not the mechanism by which these stress transients occur. Formerly A. E. Browning, Graduate Research Associate, Department of Metallurgical Engineering, The Ohio State University     

Dynamic precipitation and solute hardening in a titanium microalloyed steel containing three levels of manganese

The effect of increasing the manganese concentration on the dynamic precipitation kinetics of TiC was investigated in three microalloyed steels containing 0.1 wt% Ti and 0.5, 1.1 and 1.6% Mn. The lowest manganese level was associated with a carbon concentration of 0.06%, while the remaining two steels contained 0.1% C. Compression tests were carried out in the strain rate range from 10⁻⁴ to 10⁻¹ s⁻¹ at testing temperatures of 925, 975 and 1025°C. The dependence of the peak strain on strain rate and composition was established at each of the three temperatures and the dynamic precipitation kinetics (PIT curves) of TiC were determined in this way. The nose of the PTT curve for the dynamic precipitation of TiC in austenite containing 0.1% Ti, 0.5% Mn and 0.06% C is located at about 4s and 1000°C, and is estimated to advance to approximately l s when the carbon concentration is increased to the standard level of 0.1%. By contrast, the nose position for steels of equivalent titanium and carbon concentration (i.e. 0.1% for both elements) shifts to the vicinity of 6 and 7s respectively when the manganese concentration is increased to 1.1 and 1.6%. This phenomenon is discussed in terms of the influence of manganese on the activity coefficient of carbon and on the diffusivity of titanium. The increase in the yield strength and the solute retardation of dynamic recrystallization resulting from the addition of 0.1% Ti are also considered. These effects are compared to the influence of other microalloying additions, such as molybdenum, niobium and vanadium.     

Hundred years of precipitation hardening

About 100 years ago, precipitation hardening was discovered and the first structural aluminum alloys with considerable strength were developed. Our present understanding of the phenomenon in based on the application of dislocation theory as well as reaction kinetics, especially the role of metastable phases and lattice defects in nucleation from supersaturated solids. The prerequisites for optimum precipitation hardening can be precisely defined. However, the progress in the development of new alloys has been slow in the recent years. The original alloy is still in use today.     

热轧逆相变退火中锰钢的疲劳性能

 为了研究循环载荷对含奥氏体钢微观组织和力学性能的影响,对逆相变处理中锰钢进行了疲劳性能研究,采用SEM、EBSD、XRD等表征了微观组织,采用单轴拉伸试验表征了疲劳前后的力学性能。研究结果表明,在应力比R为-1的试验条件下,逆相变中锰钢中值疲劳极限为378 MPa,疲劳极限与抗拉强度之比σ-1/R_m为0.48;中锰钢超细晶粒尺寸特征有利于阻碍二次疲劳裂纹扩展,亚稳奥氏体的转变行为受应力幅影响较大;在中值疲劳极限的应力水平下,亚稳奥氏体和单轴拉伸性能受循环载荷影响不大。     

45钢中夹杂物析出行为的热力学研究

针对氧化物冶金技术,从具有较好诱发晶内铁素体形核的夹杂物着手,介绍了诱发形核最为有效夹杂物的热力学和动力学研发进展,系统分析了Ti-Al-O-N系、MnS、VC、VN、V(C,N)析出的热力学条件,从而得出该体系中夹杂物析出时机.并通过试验验证了理论分析的结果,为通过控温控制夹杂物析出尺寸、数量和分布提供依据.     

中碳钢高温力学和冶金行为

采用Gleeble 1500热模拟机对CSP生产的SS400、Q235B和Q345B钢的热塑性进行了研究.结果发现,所研究的钢存在两个低塑性区,即凝固脆性温区(T_m~1 310℃)和低温脆性温区(850~725℃).试样断口金相和成分分析表明:产生凝固脆性温区的原因主要是高温下枝晶间有害元素S、P和O富集形成液膜;产生低温脆性温区的原因主要是奥氏体晶界出现铁素体薄膜以及细小AlN析出造成连铸坯的塑性降低.根据研究结果,提出了改善钢的热塑性防止铸坯裂纹的工艺建议.