Phase chemistry and precipitation reactions in maraging steels: Part III. Model alloys

This article describes studies of phase transformations during aging in a variety of model maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was also used during the course of the work. The composition and morphology of precipitates were compared in several maraging systems aged at different temperatures for different times to investigate the aging sequence. The APFIM results are compared with studies by other workers using different experimental techniques. In Fe-Ni(-Co)-Mo model alloys, ω phase and Fe₇Mo₆ μ phase have been found to contribute to age hardening at different stages of aging; no evidence was found for the existence of Mo-rich clusters in the as-quenched Fe-Ni-Co-Mo alloy. In a high-Si Cr-containing steel, Ti₆Si₇Ni₁₆ G phase and Ni₃Ti have been found to contribute to age hardening; reverted austenite was found after aging for 5 hours at 520 °C. In a Mn-containing steel, Fe₂Mo Laves phase and a structurally uncertain phase with a composition of Fe₄₅Mn₃₂Co₅Mo₁₉ have been found to contribute to age hardening. Formerly Graduate Student with the Department of Materials, Oxford University.     

Phase chemistry and precipitation reactions in maraging steels: Part II. Co-free T-300 steel

This article describes studies of phase transformations during aging in a commercial Co-free T-300 maraging steel. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was used to aid the interpretation of results. The composition and morphology of precipitates were compared in the same maraging system aged at different temperatures for different times to investigate the aging sequence. Ni₃Ti and Fe₇Mo₆ have been found to contribute to age hardening. The formation of the Fe₇Mo₆ phase took place at a much later stage of aging in the Co-free steel than in the equivalent Co-containing material. The matrix concentration of Mo was correspondingly higher at earlier stages of aging. No reverted austenite has been found during APFIM and transmission electron microscopy (TEM) examinations of specimens heat-treated at 510 °C, even after 360 hours of aging. Formerly Graduate Student with the Department of Materials, Oxford University.     

Phase chemistry and precipitation reactions in maraging steels: Part I. Introduction and study of Co-containing C-300 steel

This article introduces a series of studies of phase transformations in maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Hardness measurements, transmission electron microscopy (TEM), and thermochemical calculations were also used. The composition and morphology of precipitates in the commercial-grade C-300 steel were compared for different aging times at 510 °C to investigate the aging sequence. Both Ni₃Ti and Fe₇Mo₆ were found to contribute to age hardening. The decomposition starts with the formation of small Mo-enriched Ni₃Ti particles at very short aging times. The Fe₇Mo₆ phase forms at a later stage of aging. The matrix concentrations of both Ti and Mo were measured and were found to be low after standard aging conditions. The observation of the Fe₇Mo₆ μ phase is supported by thermochemical calculations. Austenite reversion has been found at the aging temperature, and its composition approaches the predicted equilibrium composition after 8 hours of aging. Formerly Graduate Student with the Department of Materials, Oxford University.     

Precipitation reactions and strengthening behavior in 18 Wt Pct nickel maraging steels

The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni₃Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni₃Ti (initially) and Fe₂Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis. Formerly with the University of Illinois, Formerly with the University of Illinois     

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}_γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ ε or faulted γ→ ά. The orientation relationships between the three phases γ, ε and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.     

Multiphase precipitation of carbides in Fe-C systems: Part I. Model based upon simple kinetic reactions

In this work, we present a theoretical scheme that allows us to follow the kinetics of the precipitation of carbides in the Fe-C system. Our scheme is based on reaction equations, which take into account all possible ways of atomic exchange between the different competing phases. Through the use of nonlinear regression methods, we have fitted the results from our model to those obtained from thermoelectric power measurements for the Fe-C system in the temperature range between 20 °C and 450 °C. By deconvolution of optimally fitted theoretical kinetics, we have deduced ranges for coexistence of the different carbides that formed and their activation energies. An adequate agreement between our results and those reported in the literature is obtained.     

Asking the right questions about leadership: Discussion and conclusions.

Five questions prompted by the articles in the American Psychologist special issue on leadership (January 2007, Vol. 62, No. 1., see records 2006-23492-001, 2006-23492-002, 2006-23492-003, 2006-23492-004, 2006-23492-005, and 2006-23492-006) suggest some new directions for leadership research: (1) Not do leaders make a difference, but under what conditions does leadership matter? (2) Not what are the traits of leaders, but how do leaders' personal attributes interact with situational properties to shape outcomes? (3) Not do there exist common dimensions on which all leaders can be arrayed, but are good and poor leadership qualitatively different phenomena? (4) Not how do leaders and followers differ, but how can leadership models be reformulated so they treat all system members as both leaders and followers? (5) Not what should be taught in leadership courses, but how can leaders be helped to learn? (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Multiphase precipitation of carbides in Fe-C system: Part II. Model based on kinetics of complex reactions

The precipitation of carbides in the Fe-C system aged at constant temperature is simulated considering two transformation kernels of complex reactions, involving different impingement and rate-time factors. The metastable and stable phase are identified by deconvolution of the experimental kinetics. The activation energy of carbides is found to depend, slightly, on the transformation kernel in which it is used.     

Correlation between precipitation reactions and bulk density changes in type 18-12 austenitic stainless steels

Dimensional changes in austenitic stainless steels exposed to fast neutron fluxes have been attributed mainly to void formation and thermal and irradiation induced creep. In this in-vestigation, immersion bulk density measurements were performed on variously preheated AISI Types 304, 316L and 316 stainless steels. The results show that the density changes accompanying sequential precipitation of the various carbide (M₂₃C₈, M₆C) and intermetallic phases (sigma, chi, eta) during thermal aging can be comparable to those attributed to void formation at low fast neutron fluences. Vacuum melted Type 316 stainless steel always den-sified with aging at 725°C or 810°C and exhibited higher rates of precipitation and recovery of cold work during aging than the equivalent air melted steel. The direction of the density change in the air melted material was dependent upon the prior thermomechanical treat-ments. This behavior can be rationalized on the basis of a higher impurity content in the air melted material. A mathematical model was constructed which successfully explains the variation of bulk density due to sequential precipitation in Type 18-12 austenitic stain-less steels.     

Hydrogen embrittlement of steels: IV. Delayed fracture during bending

The delayed fracture resistance of 35KhGM and 33KhM1F steels during hydrogen embrittlement has been studied. The results of a comparative analysis of the mechanical behavior of the materials during four-point bending and uniaxial extension analyzed using acoustic emission measurements and fractography demonstrate differences between the fracture mechanisms and kinetics of the steels.     

TTR-diagrams (time-temperature-reaction) for thermo-mechanical treatments of steels – Part II: Controlled hot-rolling and intercritical annealing of a cold-rolled HSLA steel and a maraging steel

Thermo-mechanical treatments of two steels starting by cold- or hot-rolling. Annealing in the (α + γ) field. Effect of the amount of deformation on grain size and shape. Maraging steel with duplex structure. Establishment of time-temperature-reaction diagrams (TTR-) Diagramms.     

Interactions between recrystallization and precipitation in hot-deformed microalloyed steels

The static recrystallization and precipitation characteristics of low alloy steels containing Nb were studied following high temperature compression in the austenite range. The recrystallization kinetics in the Nb-steels were slower than those in the plain carbon steels by approximately an order of magnitude when Nb was in solution. However, much greater retardation of recrystallization was observed in the Nb-steels when precipitates were formed. The initiation of both recrystallization and precipitation appeared to be localized in the early stage of their nucleation. Recrystallization initiated predominantely at the prior austenite grain boundaries. Boundaries were also the preferential nucleation sites for NbCN precipitation. Hence, the NbCN particles were distributed in a highly localized fashion, sometimes delineating what may have been the prior austenite grain or subgrain boundaries. The heterogeneous nature of the nucleation process for recrystallization and precipitation suggested that the critical factor which determined the retardation of recrystallization was the local pinning effect of precipitation. The local pinning force was estimated from the local distribution of precipitation and was found to be of magnitude comparable to that of the driving force for recrystallization. The precipitation pinning force increased in the beginning of precipitation, showed a peak in the intermediate stage, and finally decreased as particles coarsened and were distributed more uniformly.     

Austenite recrystallization and carbonitride precipitation in niobium microalloyed steels

The response of austenite to thermomechanical treatment is investigated in two series of niobium microalloyed steels. Optical and electron metallographic techniques were used to follow the austenite recrystallization and carbonitride precipitation reactions in these steels. The first series of steels contained a constant level of 0.05Nb, with carbon levels varying from 0.008 to 0.25 pct. It was found that a lower carbon concentration results in faster austenite recrystallization, due to a smaller carbonitride supersaturation, which leads to a reduced precipitate nucleation rate. The second series of steels was designed with a constant carbonitride supersaturation, by simultaneously varying the Nb and C concentrations while maintaining a constant solubility product. In these steels, the recrystallization kinetics increase as the volume fraction of Nb(C, N) is reduced and/or as the precipitate coarsening rate is increased. The volume fraction of carbonitrides increases as the Nb: (C+12/14 N) ratio approaches the stoichiometric ratio of approximately 8:1. The precipitate coarsening rate was shown to increase with increasing amounts of niobium remaining in solution in the austenite (i. e., “excess” Nb after precipitation). As expected, recrystallization proceeds more slowly at lower temperatures and after a reduced amount of deformation. An experiment to determine whether Nb atoms dissolved in the austenite could exert a significant solute-drag effect on the recrystallization reaction indicated that 0.20Nb in solution could reduce the rate of recrystallization compared to a Nb-free C-Mn steel. However, this solute effect was smaller than the retarding effect which 0.01Nb can have when it is precipitated in the form of carbonitrides on the austenite substructure after rolling.     

Slag-metal reactions during welding: Part II. Theory

A kinetic model is developed to describe the transfer of alloying elements between the slag and the metal during flux-shielded welding. The model accounts for changes in alloy recovery based on the geometry of the resulting weld bead. It also distinguishes compositional differences between single-pass and multiple-pass weld beads. It is further shown that the final weld metal oxygen content is directly related to the weld solidification time as well as the type of flux used.     

双相不锈钢中的相变:金相和磁性质研究

用金相和不同的无损磁试验对商业低合金双相不锈钢及超级双相不锈钢因热处理和冷加工产生的显微结构过程进行了研究。在双相不锈钢(DSS)和超级双相不锈钢(SDSS)中,亚稳态的铁素体由于热处理可以分解为σ相和二次奥氏体相。这些显微结构的改变显著影响着钢材的机械性能、耐腐蚀性能和磁特性。磁性测量使得破坏性实验中获取的谐频和机械性能存在定量关系。在低合金双相不锈钢中,当低含量的Ni被N和Mn取代后,可能导致奥氏体相的不稳定性,奥氏体相容易转变为亚稳态的马氏体相,而SDSS奥氏体会更加稳定。通过室温冷轧(10%～85%压下率)的方法进行固溶退火材料的塑性变形研究,2101材料仅考察了轧制条件,而冷轧2507样本还进行了热处理。冷轧样品用光学显微镜(OM)、扫描电子显微镜(SEM)、X-射线衍射(XRD)和磁试验进行了研究。还对双相不锈钢样品进行了三种不同的磁试验,以确定其磁性能。结果表明冷轧对晶粒的大小和形状有一定影响,磁测试结果还显示2101样品中存在马氏体。     

Weld thermal cycles and precipitation effects in Ti-V-containing HSLA steels

Particle dissolution, precipitation, and grain-coarsening effects were examined in the heat-affected-zone (HAZ) of submerged are welds deposited at 3 and 6 kJ/mm heat inputs on three Ti-V-containing steels. The three steels showed different responses to the thermal cycle of the weld. The Ti-bearing steel (0.014 pct Ti) showed little change in the TiN particle distribution on welding, in agreement with calculations which demonstrated very limited TiN solution during the weld thermal cycle. For the high-V, low-Ti steel (0.093 pct V,0.002 pct Ti), the V-rich particles in the baseplate dissolved, and Ti-rich precipitates formed during the heating leg of the cycle, while V-rich particles precipitated again on cooling. The third steel (0.014 pct Ti, 0.08 pct V) contained a wide distribution of particle compositions. On welding, the Ti-rich cuboidal precipitates did not dissolve, while the V-rich spheroidal precipitates did dissolve. No reprecipitation was observed on cooling in this steel. The different behaviors of the three steels were related to the dissolution of the precipitates in the baseplate and the available thermodynamic driving force for reprecipitation to occur on cooling.     

Interaction between deformation,recrystallization and precipitation in niobium steels

The present study was undertaken in order to establish the extent to which niobium (columbium) inhibits recovery and recrystallization of microalloyed austenite while present as solute on the one hand, and as carbonitride precipitates on the other. Three steels were used; the first was an HSLA steel containing 0.054 pct Nb, 0.05 pct C and 0.92 pct Mn. The second was prepared by treating the first in wet hydrogen at 1100°C so as to reduce the C and N content to about 10 ppm by wt. The third material was a plain carbon steel containing 0.055 pct C and 0.41 pct Mn. The isothermal recovery and re-crystallization of these materials, after an interval of hot working, was studied by means of interrupted compression tests. Samples were prestrained at 10^-2 and 10^-1s^-1 at 900, 950 and 1000°C to natural strains of 0.10 and 0.25, and held isothermally prior to reloading. The results obtained in this way, indicate that 0.56 pct of substitutional solute can give rise to an order of magnitude decrease in the rate of recrystallization. When precipitation of Nb(CN) takes place either during deformation or in strained aus-tenite the mean precipitate size is ∼ 20 nm. The presence of such particles inhibits both static recovery and recrystallization; the magnitude of the effect being dependent on the volume fraction of precipitate. When the volume fraction reaches about 0.02 pct, static recrystallization is completely suppressed for the present prestraining conditions. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.