200, 020, and 002 X-ray peaks in tempered Fe-18 Ni-C martensites

Separate 200, 020, and 002 X-ray peaks were recorded for 0.0, 0.4, and 0.8 wt pct carbon (18 pct Ni) martensites after tempering between 25 and 500°C. The carbon bearing martensites studied here have been tempered initially enough to eliminate the “high tetragonality” 002 peak usually recorded for as-quenched martensite and the present results apply to tempered martensite only. The peak maximum is taken to determine the lattice parameter and the peak shape is recorded. At all carbon levels and after all tempering treatments, the “crd parameter is larger than or equal to the “a” or “b”. The relative enlargement is very small (0.08 pct) for the lowest carbon level and for any carbon level after severe tempering (500°C for 15 min). For the two higher carbon alloys tempered at temperatures below 400°C (for 15 min) the “c” parameter is significantly larger than the “a” and “b” and for the 0.4 wt pct C alloy the “b” is significantly smaller than the“a” whereas in the 0.8 pct C alloy the “b” is slightly larger than the “a”. Within experimental error the mean volume of the unit cell does not change during the tempering studied here and is nearly unaffected by the initial carbon content. This indicates that little (at most 0.1 wt pct) carbon is dissolved in tempered martensite. In the low carbon alloy the peaks are symmetric and sharpen symmetrically during tempering. In the higher carbon alloys the peaks are nearly symmetric and sharp after severe tempering. After less severe tempering the 002 peak is asymmetrically broadened toward lower9 values (higher lattice parameters) whereas the 200 and 020 peaks are asymmetrically broadened toward higher 0 values corresponding to lower lattice parameters. This collection of results is tentatively interpreted as being due to strains in martensite due to transformation induced substructure and precipitated carbides.     

Fatigue crack propagation in intercritically tempered Fe-9Ni-0.1C and Fe-4Mn-0.15C

Fatigue crack propagation was studied for two intercritically tempered cryogenic steels, Fe-9Ni-0.1C and Fe-4Mn-0.15C, at both intermediate (stage II) and low (stage I, near threshold) stress intensity ranges. Propagation rates were determined for varying intercritical tempering times corresponding to varying amounts of retained austenite and untempered martensite. The results show that the heat treatments that optimize impact fracture properties in the nickel steel are also beneficial with respect to the fatigue crack propagation rate in stage I, while no beneficial effect beyond that attributable to carbon redistribution was observed for stage II. For the manganese steel, heat treatments leading to increased concentrations of retained austenite also increased the threshold stress even though no improvement in fracture toughness was observed. To clarify the origin of this improved behavior, the fracture surface was analyzed by Mössbauer Spectroscopy and Auger Electron Microprobe. The Mössbauer results indicated that the retained austenite in the crack path is transformed to martensite as was earlier shown in this laboratory for Charpy specimens. Auger composition analysis suggested a tendency for a stage I crack tip to avoid the mechanically induced brittle untempered martensite in the Fe-Mn steel, while no such preference was observed for stage II.     

Effect of Heating Rate Before Tempering on Reversed Austenite in Fe-9Ni-C Alloy

 The alloy was reheated to 580 ℃ for tempering at rates of 2, 5, 10, 20, and 40 ℃/s, respectively, after quenching. The amount, distribution, and stability of reversed austenite were investigated by X-ray diffraction (XRD) and electron back scatter diffraction (EBSD). The microstructure and cryogenic impact energy were studied by scanning electron microscope (SEM), transmission electron microscope (TEM) and Charpy V-notch (CVN) tests. The results showed that when the sample was heated at 10 ℃/s, the volume fraction of reversed austenite exhibited maximum of 8%; the reversed austenite was uniform along all kinds of boundaries; the reversed austenite contained higher concentration of carbon which enabled it to be more stable. The cryogenic toughness of the alloy was greatly improved when heated at 10 ℃/s, as the fracture surface observation showed that it mainly fractured in ductile rupture mode, which was consistent with the results of cryogenic impact energy.     

The morphology and substructure of highly tetragonal martensites in Fe-7 pct Al−C steels

This investigation reports on an optical and electron microscope study of bct martensite formed in Fe-7 pct Al-1.5 pct C and Fe-7 pct Al-2.0 pct C alloys. In each case the martensite is plate-like containing \((112)[\bar 1\bar 11]\) transformation twins 100 to 200Å in width. The particular twin plane variant \((112)[\bar 1\bar 11]\) corresponds to the martensite habit plane variant (3, 15, 10)_F, which is predicted by the crystallography theory. The twins are uniformly spaced and extend completely from one martensite-austenite interface to the other as would be theoretically expected. The martensite plates are ideally lenticular in the 2 pct C alloy but those in the 1.5 pct C alloy frequently exhibit irregular interfaces which are attributed to impingement effects. All observations are in accordance with the phenomenological crystallography theory as applied to ferrous martensites with a {3, 15, 10}_F habit plane.     

On the substructure of athermal and isothermal martensites formed in an Fe-21 Ni-4mn alloy

Transmission electron microscopy and diffraction analysis have been made on the athermal and isothermal martensites formed in an Fe-21Ni-4Mn alloy, with particular attention to details of the martensite crystallography and substructure. In general, a variation in the martensite substructure has been found, comparing adjacent regions of near proximity in the same specimen, but nevertheless general conclusions can be drawn. Plates of martensite were observed after both isothermal and athermal treatment. At an early stage of growth, the isothermal martensite plates exhibit similar crystallographic and substructural features compared to those found in the athermally formed plates. In both cases, the overall macroscopic martensite habit plane is near (252)_f, but such macroscopic plates actually consist of many small plates (termed subplates) of the same orientation with a (121)_f habit plane. In the case of isothermal plates an apparent rotation of the habit plane to ward ( 111 )_f occurs as the plates thicken. This rotation was not observed for athermally formed plates. The observations made in this work suggest that temperature dependent relaxation of transformation strains may affect the growth and autocatalytic nucleation of martensite subplates, thus causing the microstructural differences occurring between isothermal and athermal martensite plates. Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Formerly with the Department of Metallurgy and Mining Engineering, University of Illinois     

The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy

As-quenched and tempered martensite in an Fe-0.2 pct C alloy were subjected to tensile testing and structural characterization by light and transmission electron microscopy. The light temper, 400°C-l min, did not change packet morphology, but did reduce dislocation density, coarsen lath size and cause the precipitation of carbides of a variety of sizes. The yield strength of the as-quenched martensite was strongly dependent upon packet size according to a Hall-Petch relationship, but tempering significantly diminished the packet size dependency, a result attributed to packet boundary carbide precipitation and the attendant elimination of carbon segregation present in the as-quenched martensite because of autotempering. Examination of thin foils from strained tensile specimens showed that a well-defined cell structure developed in the as-quenched martensite, but that the random distribution of jogged dislocations and carbide particles produced by tempering persisted on deformation of the tempered specimens. The authors were formerly Research Assistant and Professor, respectively, at Lehigh University, Bethlehem, PA.     

Stress-Assisted and strain-induced martensites in FE-NI-C alloys

A metallographic study was made of the martensite formed during plastic straining of metastable, austenitic Fe-Ni-C alloys withM _s temperatures below 0°C. A comparison was made between this martensite and that formed during the deformation of two TRIP steels. In the Fe-Ni-C alloys two distinctly different types of martensite formed concurrently with plastic deformation. The large differences in morphology, distribution, temperature dependence, and other characteristics indicate that the two martensites form by different transformation mechanisms. The first type, stress-assisted martensite, is simply the same plate martensite that forms spontaneously belowM _s except that it is somewhat finer and less regularly shaped than that formed by a temperature drop alone. This difference is due to the stress-assisted martensite forming from cold-worked austenite. The second type, strain-induced martensite, formed along the slip bands of the austenite as sheaves of fine parallel laths less than 0.5μm wide strung out on the {111}_γ planes of the austenite. Electron diffraction indicated a Kurdjumov-Sachs orientation for the strain-induced martensite relative to the parent austenite. No stress-assisted, plate martensite formed in the TRIP steels; all of the martensite caused by deformation of the TRIP steels appeared identical to the strain-induced martensite of the Fe-Ni-C alloys. It is concluded that the transformation-induced ductility of the TRIP steels is a consequence of the formation of strain-induced martensite. Formerly a graduate student at Stanford University     

The diffusivity of Ni in Fe-Ni and Fe-Ni-P martensites

The diffusivity of Ni in Fe-Ni and Fe-Ni-P martensite, , has been determined between 700 and 300 °C using electron microprobe (EMP) and scanning transmission electron microscope (STEM) techniques. Alloys of various bulk compositions (0 to 30 wt pct Ni, Fe) were homogenized in the single phase austenite (γ-fee) field and quenched to form martensite, α₂ (bcc). Appropriate alloys were tempered isothermally at 300 to 700 °C. The γ nucleated and grew in the parent α₂. The composition of the γ phase and the concentration gradients in the α₂ were measured with the EMP andJor STEM. In order to determine experimentally measured Ni concentration gradients were matched to Ni concentration gradients calculated by a simulation model. The calculated gradients were obtained by solving the appropriate form of Fick’s second law using the Crank-Nicholson numerical technique. The observed diffusivities varied with temperature. Above approximately 410 °C, while below 410°C, = (2.27 × 10⁻¹⁵) exp (− 10,600/RT) cm²/s. The effect of P is to increase the Fe-Ni diffusivities at any temperature by the factor (1 + 1.27C _p + 0.623C _p ² ) whereC _p is the amount of P (wt pct) in α₂. The discontinuous diffusion behavior of is attributable to the high dislocation density of the α₂. Above approximately 410 °C lattice diffusion is dominant while below 410 °C dislocation pipe diffusion is dominant. Formerly Research Assistant in the Department of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA     

Fe-18Cr-18Mn-C-Mo(Si)-N系相图垂直截面的热力学计算

利用Thermo-Calc软件和相关数据库对Fe-18Cr-18Mn-C-Mo(Si)-N合金系在氮气压力为100 kPa下随N含量变化的变温截面进行了计算.结果发现,随着C的质量分数从0增加到0.12%时,γ相区没有发生明显的变化,但对碳化物M23C6的析出产生了显著的影响,从而改变了高氮钢奥氏体化后的析出序列.当继续添加质量分数为2%的Mo时,γ/a γ相边界向右侧移动,不但存在碳化物M23C6还有碳化物M6C.当添加不同含量的Si时,γ相区明显缩小,而且碳化物M23C6的析出温度与成分也发生了很大的变化.Fe-18Cr-18Mn-O.09C-0.8Si-0.5N钢的奥氏体化温度及Cr2N相析出温度与该合金系的热力学计算结果符合,表明这些计算结果可为Fe-18Cr-18Mn-C-Mo(Si).N合金系的成分设计及热处理工艺提供依据.     

Deformation induced alpha and epsilon martensites in Fe-Ni-Cr single crystals

The orientation dependence of an applied tensile stress on the formation of specific α martensite variants of the Kurdjumov-Sachs (K-S) orientation relationship in Fe-15Ni-15Cr single crystals are presented. Test temperatures were 185, 242 and 273 K,i.e., aboveM _s. Quantitative volume fraction measurements of deformation induceda ande martensite were made on crystals with the [100]γ, [1•10]γ, [•1•12]γ and [•2•13]γ tensile axes, α-martensite was only observed after 5 pct strain at 185K in all orientations, but the total volume fraction ofa martensite varied from <0.002 to 0.07 for the [100]γ and [•2•13]γ tensile axes respectively. The distribution of the K-S variants was also found to be sensitive to the direction of the applied stress, and 80 pet of the α martensite present in crystals oriented for easy glide had the same K-S variant. Epsilon martensite was found in all specimens but occurred only in the (111) planes which had slipped. Glen Stone, formerly graduate student, University of California at Berkeley     

Strengthening Mechanisms in Nano Oxide Dispersion-Strengthened Fe-18Cr Ferritic Steel at Different Temperatures

Metallurgical and Materials Transactions A - The objective of the present work is to evaluate isothermal uniaxial compressive deformation behavior of nano oxide dispersion-strengthened (n-ODS)-18Cr...     

Nonlinear and nonlocal continuum model of transformation precursors in martensites

An intrinsic mechanism for explaining the origin of the transformation precursors observed above the transition in the parent phase of many materials undergoing a martensitic transformation is proposed. It is based on a nonlinear and nonlocal elastic continuum model for the elastic displacement field describing the parent-product deformation for the two-dimensional analog of a cubic-tetragonal transformation. By minimizing the Landau-Ginzburg free energy functional for the total elastic (strain plus strain gradient) energy a static, possibly stable, continuous, periodically modulated {110}/〈110〉 strain pattern is obtained which corresponds to alternating layers of more and of less transformed material, consistent with experimental observations. This pattern is stabilized by the balance between nonlinear and nonlocal elastic effects. Numerical application to In_1-x XT _x , alloys gives the minimum period of the modulation in the order of nanometers, in agreement with experimental observations.     

The effect of volume percent of fine γ’ on creep in Ds Mar-M200 + Hf

A correlation was found to exist between creep life at 1800∮F (1255 K) and the volume percent of uniformly dispersed fine (<0.5 μm) gamma prime (γ’) precipitate in direction-ally solidified (DS) Mar-M200 + Hf turbine blade castings. A three-fold increase in creep-rupture life was realized when the amount of fine γ’ increased from about the 30 pct level to approximately a 45 vol pct level. Heat treatment studies revealed that the amount of finey’ developed in this alloy is strongly dependent on solution temperature, especially in the range from 2160 to 2210∮F (1455 to 1483 K). Higher solution temperatures promote the development of greater amounts of fine y’ by more complete solutioning of the coarse γ’ and eutectic γ-γ’ constituents which subsequently reprecipitate as fine y’ upon cooling and aging. formerly Senior Materials Engineer, Pratt & Whitney Aircraft, is now with General Electric Co., Schenectady, NY. 12345.     

微合金化超纯Fe-18Cr-2Mo钢的冲击韧性及影响因素

超纯Fe-18Cr-2Mo不锈钢经合金化处理后性价比优良,在行业内受到越来越多的重视.研究了单独加Ti和Ti-Nb复合添加的超纯Fe-18Cr-2Mo不锈钢热轧板的冲击韧性及影响因素.结果表明,在wTi+ Nb/wC+N=9的条件下,Ti-Nb复合添加的钢韧脆转变温度(DBTT)较Ti单独添加的钢低35℃.含Ti钢中,3～5 μm的TiN析出物居多;Ti-Nb复合添加的钢中,1 ～3 μm的TiN析出物居多,此外含有200 nm左右的以TiC为核心的(NbTi)C复合相及100 nm左右NbC.低于DBTT时冲击,试验钢沿{100}晶面发生脆性解理断裂.细化TiN析出物,增加细小球形、椭球形的富Nb碳化物,同时降低碳在钢中的固溶量,有利于试验钢冲击韧性的提高.     

Corrosion fatigue in nitrocarburized quenched and tempered steels

In order to investigate the fatigue strength and fracture mechanism of salt bath nitrocarburized steels, specimens of the steels SAE 4135 and SAE 4140, in a quenched and tempered state, and additionally in a salt bath nitrocarburized and oxidizing cooled state as well as in a polished (after the oxidizing cooling) and renewed oxidized state, were subjected to comparative rotating bending fatigue tests in inert oil and 5 pct NaCl solution. In addition, some of the quenched and tempered specimens of SAE 4135 material were provided with an approximately 50-μm-thick electroless Ni-P layer, in order to compare corrosion fatigue behavior between the Ni-P layer and the nitride layers. Long-life corrosion fatigue tests of SAE 4135 material were carried out under small stresses in the long-life range up to 10⁸ cycles with a test frequency of 100 Hz. Fatigue tests of SAE 4140 material were carried out in the range of finite life (low-cycle range) with a test frequency of 13 Hz. The results show that the 5 pct NaCl environment drastically reduced fatigue life, but nitrocarburizing plus oxidation treatment was found to improve the corrosion fatigue life over that of untreated and Ni-P coated specimens. The beneficial effect of nitrocarburizing followed by oxidation treatment on cor-rosion fatigue life results from the protection rendered by the compound layer by means of a well-sealed oxide layer, whereby the pores present in the compound layer fill up with oxides. The role of inclusions in initiating fatigue cracks was investigated. It was found that under corrosion fatigue conditions, the fatigue cracks started at cavities along the interfaces of MnS inclusions and matrix in the case of quenched and tempered specimens. The nitrocarburized specimens, however, showed a superposition of pitting corrosion and corrosion fatigue in which pores and nonmetallic inclusions in the compound layer play a predominant role concerning the formation of pits in the substrate.     

A novel adrenaline derivative, AZ002, and its hypoglycemic action in yellow KK mice

A new device has been designed to provide anchorage for orthodontic tooth movement. It is a disk, textured and hydroxylapatite coated on one side, with an internal thread on the other side. It is placed on palatal bone and, after integration, can be connected to teeth for anchorage. This article reviews a dog study demonstrating unilateral tooth movement towards the "onplant" and a monkey study mimicking its use to anchor the molars for anterior retraction.