The tempering of FeNiN martensite

The tempering behavior of ternary iron-nickel-nitrogen martensitic specimens (∼13.5 at. pct Ni, ∼4.7 at. pct N; ∼4.9 N atoms/100 metal atoms) in the temperature range of 270 to 670 K was investigated by analysis of the corresponding changes in the crystalline structure (X-ray diffraction), volume (dilatometry), and enthalpy (calorimetry). At least three stages of structural change can be distinguished: (1) redistribution of nitrogen atoms, involving segregation and formation of nitrogen enrichments, occurs at temperatures up to 370 K, (2) precipitation of γ′-(Fe, Ni)₄N_1-x nitride takes place in the temperature range of 370 to 430 K, and (3) coarsening of the γ′ precipitates and decomposition of a part of the retained austenite occur at temperatures above 430 K. From a comparison with the tempering behavior of a binary iron-nitrogen martensitic alloy containing a similar amount of interstitials, it was concluded that the presence of nickel suppresses the development of the intermediate α″-(Fe,Ni)₁₆N₂ nitride and advances the precipitation of γ′ nitride.     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

Creep forging of an aluminum bronze

Abstract

Compression tests on a commercial three-phase aluminum bronze showed significant softening above the β transformation. This was not observed in either beryllium-copper or α/β brass above their transformation temperatures. Isothermal compression tests on the aluminum bronze showed some rate sensitivity and there was no significant change in either mechanical properties or microstructure after large strains at the elevated temperature. These characteristics are typical of superplastic alloys.

Closed-die forging experiments in which a thin tapered lip was backward extruded on a cylindrical billet have shown that the material forges adequately at low pressures at temperatures above the transformation. It was concluded that creep forging of this aluminum bronze gives good die filling and surface finish.

Résumé

On a observé un adoucissement significatif d'un bronze à l'aluminium commercial (contenant trois phases) lorsque déformé en compression au-dessus de la transformation β. Le phénomène n'a pas été observe dans le cuivre au beryllium et le laiton α+β déformés au-dessus de leur temperature de transformation. Des essais de compression isotherme sur le bronze à l'aluminium ont permis d'observer une certaine sensibilité au taux de déformation; après de grandes déformations à la température élevée, les propriétés mécaniques et la microstructure n'ont pas été modifiées appréciablement. Ce comportement est typique des alliages superplastiques.

Des essais de forgeage en matrice ferme, au cours desquels une mince lèvre effilée a été extrudée, d'une manière inverse, sur une billette cylindrique, ont montré que de faibles pressions sont suffisantes pour forger le matériau adéquatement lorsqu'on opère à des températures au-dessus du point de transformation. Les auteurs ont conclu que le forgeage de fluage permet de bien remplir la cavite et d'obtenir un bon fini de surface.     

The tempering of iron-nitrogen martensite; Dilatometric and calorimetric analysis

The aging behavior of iron-nitrogen martensitic alloys (0.8 to 7.0 at. pct N) between -190 °C and 450 °C was investigated by quantitative analysis of the corresponding changes in volume and enthalpy. Martensitic specimens were prepared by gaseous nitriding of pure iron in a mixture of NH₃ and H₂ and subsequent quenching in brine and liquid nitrogen. Both X-ray diffraction analysis and metallography (light microscopical analysis and microhardness measurement) were used for interpretation of the structural changes. Analysis of the transformation kinetics was achieved by employing a range of heating rates. At least five different stages of structural change could be distinguished, which were quantitatively analyzed in terms of their effects on volume and enthalpy: (1) transformation of retained austenite into martensite (between-160 °C and -40 °C); (2) segregation and “ordering” of nitrogen atoms (below 100 °C); (3) precipitation of incoherent α′ nitride (between 100 °C and 220 °C); (4) conversion of α′ nitride into γ′ nitride (between 220 °C and 290 °C); and (5) decomposition of retained austenite (between 240 °C and 350 °C). Differences with the tempering behavior of analogous iron-carbon martensites were discussed.     

The effect of the first and second stages of tempering on microcracking in martensite of an Fe-1.22 C alloy

The effect of tempering on microcracking in the plate martensite of an Fe-1.22 C alloy was investigated by isothermal heat treatments in the temperature range between 180 and 225°C. The second stage of tempering, followed by X-ray measurement of retained austenite, was confirmed to depend upon the diffusion of C in austenite, and the transformation product was found to consist of very closely spaced cementite lamellae in ferrite. Microcracking, despite the volume expansion that accompanies the transformation of the retained austenite, decreased only slightly with time during the second stage. The major decrease in microcracking occurred during the first stage, a result attributed to the plastic deformation that accompanies the dimensional changes caused by the reduction of the lattice tetragonality of the high carbon martensite in the first stage. Metallographic observations of surface relief and etching effects associated with martensite plates provided evidence of the first-stage plastic flow. The authors were formerly Research Assistant and Professor, respectively at Lehigh University, Bethlehem, PA.     

A theoretical analysis of the spinodal decomposition in Fe- C martensite during aging stage of tempering

A thermodynamic model of Fe-C martensite is proposed, which has a distinct physical meaning and is easily treated mathematically (high order derivation). When applying this model to the spinodal decomposition in Fe-C martensite during aging, the theoretical values (critical compositions, sizes of products) are in good agreement with experimental data. The miscibility gap of Fe-C martensite is set up. From this diagram, it is clear that spinodal decomposition may affect the later stages of tempering of medium and high carbon steels. Finally, the discussion shows that A1, A2 stages of aging are both spinodal decomposition, but to different extents.     

回火温度对高氮轴承钢碳化物演变及硬度的影响

对高氮马氏体不锈轴承钢进行直接淬火并重复2次深冷及不同温度的回火处理,采用光学显微镜、SEM电镜,TEM电镜和洛氏硬度计等,研究了不同回火温度下碳化物的演变规律与硬度变化的相关性,在不同回火温度下根据硬度出现先下降后上升再下降变化的趋势,对各回火温度下碳化物的尺寸区间分布频数、单位面积数量、平均尺寸及碳化物所占单位面积比进行了表征及分析,结果表明:随回火温度由150升高到500℃,碳化物的尺寸随回火温度的升高从0.39长大至0.62μm,碳化物为近球形M23C6型。150~300℃回火时硬度下降与基体脱溶有关;300~450℃回火时碳化物单位面积数量及所占面积分数都增加;回火至500℃时,细小碳化物聚集长大单位面积数量减少,碳化物所占面积分数减少。由此得出回火时硬度变化与析出碳化物的单位数量和其面积分数有关。     

TiC和VC在低碳马氏体钢回火中的析出和粗化

 利用维氏硬度计、OM、TEM对在600 ℃回火不同时间后的钛、钒微合金化马氏体钢的维氏硬度、微观组织及钢中析出相随回火时间的演变进行了研究,并采用现有计算方法对马氏体钢中析出相的析出动力学进行了计算。结果表明：两钢在600 ℃回火,随回火时间的增加,其硬度变化均呈现先下降后升高再下降的规律,且在回火1 h后,两钢出现峰值硬度,且回火过程中钛钢的硬度均大于钒钢。分析认为,前期硬度下降是由于位错密度的降低所致,而当MC相析出时起到沉淀强化作用,引起硬度上升并出现峰值,而回火时间更长时,由于MC相粗化及基体回复导致硬度再次下降。钛钢中由于析出相粒子析出动力学比钒钢的快,而其粗化速率却低于钒钢中析出相,因此钛钢在回火过程中沉淀强化效果及对基体回复抑制的作用更为明显,故而其回火时比钒钢的硬度高。     

The cracking mechanism of silicon particles in an A357 aluminum alloy

The cracking of Si particles in an A357 Al alloy has been investigated over a spectrum of stress and strain by varying aging strength and applying different tensile strains. The variation of the fraction of broken Si particles with stress, strain, and cleavage plane orientation has been obtained. The features of cracking reveal that cracking of Si particles is a very localized event. A dislocation pileup mechanism is the most probable one among all crack-initiation theories for explaining the behavior. Based on this mechanism, further deduction has been made to obtain the relationship between the fraction of broken particles and metallurgical factors. The present data, along with Gurlandrss and that of Lowet al., have been found to verify this relationship for the effect of stress, strain, and cleavage plane orientation.     

回火对00Cr15Ni6Mo2高强马氏体不锈钢组织和性能的影响

研究了一种高强马氏体不锈钢00Cr15Ni6Mo2在回火过程中的逆变奥氏体的体积分数变化及其对力学性能的影响.结果表明,在550 ～750℃之间回火,当回火温度达到670℃时,逆变奥氏体的体积分数达到峰值39％,该钢在670℃回火时的HV硬度为280,夏比V缺口冲击吸收能为187.5 J,也分别达到峰值.沿马氏体板条析出的逆变奥氏体,对提高该钢回火条件下低温冲击韧性具有重要作用.     

马氏体临界晶核的研究

应用Axiovert-25CA型金相光学显微镜和QUANTA-400环境扫描电镜对几种材料马氏体相变时的形核地点和形态进行了观察,发现马氏体不仅可在奥氏体晶粒内部形核,而且可在晶界、相界面等处形核.研究了马氏体晶核形成时的能量变化规律,计算了马氏体晶核的临界尺寸、形核功.马氏体临界晶核尺寸约为7～20 nm,形核功约为200～600 J·mol-1.     

电感耦合等离子体原子发射光谱法测定铝青铜中7种元素

采用硝酸(1+1)溶解样品,选择Pb 220.353 nm、Sn 189.927 nm、Si 251.611 nm、Zn 206.200 nm、Ni 231.604 nm、Mn 260.568 nm、Fe 259.939作为分析谱线,使用电感耦合等离子体原子发射光谱法(ICP-AES)同时测定了铝青铜中铅、锡、硅、锌、镍、锰、铁。试验探讨了铝青铜中基体元素对待测元素测定的影响,结果表明:通过基体匹配法绘制校准曲线消除了基体效应的影响。各元素的校准曲线线性相关系数均大于0.999;方法中各元素的检出限为0.9～20.8 μg/g。方法应用于铝青铜标准物质中铅、锡、硅、锌、镍、锰、铁的测定,结果的相对标准偏差(RSD,n=10 )在0.36%～4.0%之间,标准物质的测定值与认定值无显著性差异。按照实验方法对两个铝青铜QAl10-3-1.5产品中铅、锡、硅、锌、镍、锰、铁进行测定,加标回收率为90%～108%。     

The effects of hydrostatic pressure on the mechanism of tensile fracture of aluminum

The effects of hydrostatic pressures up to 600 MPa on the tensile fracture mechanisms of commercial aluminum and a free machining aluminum-copper alloy have been investigated. Tensile fracture of the commercial aluminum was found to occur by a mechanism leading to double-cup fracture at hydrostatic pressures up to 125 MPa with the pressure causing a progressive delay in the onset of void development. Above this pressure, void development was suppressed and all fractures occurred at chisel points. The chisel point fracture was found to involve the development of two opposing diagonal shear zones crossing through the neck center. Fracture of the aluminum alloy occurred by a cup-cone mechanism at pressures up to 300 MPa. Within this pressure range the natural strain to fracture increased linearly and the amount of void coalescence decreased. At higher pressures fracture of the aluminum alloy occurred entirely by a shear mechanism involving the development of void-sheets.     

The effects of hydrostatic pressure on the mechanism of tensile fracture of aluminum

The effects of hydrostatic pressures up to 600 MPa on the tensile fracture mechanisms of commercial aluminum and a free machining aluminum-copper alloy have been investigated. Tensile fracture of the commercial aluminum was found to occur by a mechanism leading to double-cup fracture at hydrostatic pressures up to 125 MPa with the pressure causing a progressive delay in the onset of void development. Above this pressure, void development was suppressed and all fractures occurred at chisel points. The chisel point fracture was found to involve the development of two opposing diagonal shear zones crossing through the neck center. Fracture of the aluminum alloy occurred by a cup-cone mechanism at pressures up to 300 MPa. Within this pressure range the natural strain to fracture increased linearly and the amount of void coalescence decreased. At higher pressures fracture of the aluminum alloy occurred entirely by a shear mechanism involving the development of void-sheets.     

The solubility of aluminum in cryolite-alumina melts and the mechanism of metal loss

The solubility of aluminum in cryolite-alumina melts has been determined in laboratory experiments by analyzing rapidly-quenched samples of the melt after equilibration with metal at temperatures between 960 and 1060°C. The solubility in pure cryolite increases from 0.085 at 1020°C to 0.12 wt pct Al at 1060°C. The addition of alumina decreases the solubility at 1020°C to 0.081 with 5 pct A1₂O₃ and to 0.073 wt pct Al in melts saturated with alumina. Quenched samples have been taken from operating 130 kA prebake cells at different heights above the metal pad, both in the center channel and beneath the anodes. Within about 10 mm of the cathode the metal content is close to the equilibrium value obtained in the laboratory but above this level it decreases rapidly. It is suggested that oxidation occurs in a central zone of the electrolyte. Mechanisms of metal loss and implications for current efficiency are discussed.     

不同pH值环境中镍铝青铜冷喷涂涂层腐蚀磨损性能

镍铝青铜合金是铸造大型舰艇螺旋桨的主要材料,海洋污损生物在合金表面附着引起的pH值变化会加速镍铝青铜合金的腐蚀磨损,威胁到舰艇安全航行.采用冷喷涂技术制备了较为致密的镍铝青铜涂层,厚度约300 μm,利用扫描电镜、光学显微镜观察了涂层的微观形貌,重点研究了涂层在pH值分别为3、7和11,质量分数为3.5%的NaCl溶液中的电化学腐蚀性能和腐蚀磨损行为.实验结果表明:pH值为3的环境中,镍铝青铜基体发生了选相腐蚀,表面疲劳裂纹主要分布在富Cu的α相上,涂层上的犁削沟槽加深发生了磨粒磨损,涂层耐腐蚀性能优于基体;pH值为7的环境中,基体发生了黏着磨损,表面有片层状金属剥落,涂层中的孔隙收容了大量磨屑避免了剧烈的三体摩擦,表面犁削沟槽较浅,涂层致钝电位高于基体,耐腐蚀性能变差.pH值为11的环境中,基体发生了表面疲劳磨损,涂层磨痕上有较深的犁削沟槽,沟槽内有微裂纹,涂层耐腐蚀性能优于基体.总体来说,在不同pH值环境中,涂层由于在冷喷涂过程中发生了冷加工硬化并且涂层上的孔隙收容了磨屑,导致涂层的耐腐蚀磨损性能增强.     

The self-accommodating character of the β1 copper-aluminum martensite

The phenomenological martensite theory is applied to the β₁ to β₁ martensitic transformation in Cu-Al. Crystallographic and morphological aspects of the resulting martensitic microstructure are discussed and verified with X-ray pole figures, which are combined with a single surface trace analysis based on the use of polarized light. From the analysis of the orientation of any single martensite plate related to that of one or more neighboring plates, it can be proved that the martensite microstructure in each former β-grain is composed of at most six self-accommodating martensite plate groups, each of which consists of four different martensite plate variants. Formerly with KUL, Leuven,