新型无Ni高N奥氏体不锈钢的低温变形行为

利用冲击实验、拉伸实验、XRD和TEM对2种不同N含量的无Ni高N奥氏体不锈钢低温变形行为进行了研究.结果表明,高N奥氏体不锈钢在低温下发生明显的韧脆转变和加工硬化现象.在实验材料的Mn含量水平内,提高Mn含量能够改善高N奥氏体不锈钢的低温塑性和韧性,降低其韧脆转变温度.18Cr-12Mn-0.55N钢在低温拉伸变形时会发生形变诱导马氏体相变,但马氏体转变量很少,降低温度对马氏体转变量无明显影响.形变诱导马氏体能提高高N奥氏体不锈钢的加工硬化能力,但降低了钢的低温塑性和韧性.加工硬化能力和层错能随温度的降低而降低是Re-Cr-Mn高N奥氏体不锈钢在低温下发生脆断的主要原因.     

氮强化高锰奥氏体钢热变形行为研究

利用Gleeble-3500热力模拟试验机在温度为1253～1423K,应变速率为0.1～10s-1的条件下对32Mn-7Cr-1Mo-0.3N奥氏体钢进行了热压缩变形试验,测定了其真应力-应变曲线,观察了变形后的组织.试验结果表明,流变应力和峰值应变随变形温度的降低和应变速率的提高而增大.真应变为0.6时,在1423K、应变速率在0.1～10s-1之间的试样均已发生完全动态再结晶;在1373K以下变形时,应变速率在0.1～10s-1之间,试样发生部分动态再结晶.动态再结晶晶粒尺寸随着变形温度的升高而增大,随着应变速率的升高而减小.32Mn-7Cr-1Mo-0.3N奥氏体钢的热变形激活能Q值为469.03kJ/mol,并获得热变形方程.     

应变速率对高氮奥氏体钢拉伸变形的影响

采用金相及透射电子显微镜对高氮奥氏体Fe-20Mn-19Cr-0. 6N钢在应变速率范围为3×10^-6～1 s^-1条件下的拉伸变形行为进行了研究。研究结果表明:N元素的固溶强化作用和促使位错平面滑移阻碍位错运动机制是高氮奥氏体钢的重要应变硬化机制,同时,随着应变速率的提升,这种强化机制不断提升,而应变诱导孪生机制不断削弱。随着应变速率的提升,高氮奥氏体钢的抗拉强度和屈服强度均呈逐步上升的趋势,断后伸长率则逐步下降。屈服强度提升超过60%,而抗拉强度提升仅10%。随着应变速率的提升,基体变形程度逐步下降,材料的位错密度和滑移带密度逐步下降。     

奥氏体钢低温断裂过程中网格位错的作用

利用扫描、透射电子显微镜,研究了22Mn-13Cr-5Ni-0.25N奥氏体低温用钢的77K8温度下准解理断裂与位错形态体系,结果表明,低温断裂过程中形成的网格状位错促进了准解理断裂。     

T45Al10Nb附带 金全片层组织韧脆转变机制的研究

研究了应变速率和温度对Ｔｉ４５Ａｌ１０Ｎｂ合金的屈服强度和延伸率的影响。结果表明：随着应经升高,合金的屈服强度升高而延伸率下降,由此得到韧脆温度ＴＢＤＴ随应变束率升高而升工计算出Ｔｉ４５Ａｌ１０Ｎｂ合金韧脆转变的激活能为３３０ＫＪ／ｍｏｌ。这一数值与ｒ－ＴｉＡｌ合金中原子的自扩散激活能（２９０ＫＪ／ｍｏｌ）相当,说明Ｔｉ４５Ａｌ１０Ｎｂ合金韧脆转变过程受扩散控制的形迹机制,即位错攀移控制,ＴＥＭ形     

应变速率对近全片层组织γ—TiAl合金韧脆转变温度的影响

采用拉伸试验，研究了不同应变速率下温度对具有近全片层组织的γ＋α２双相ＴｉＡｌ合金的屈服强度和延伸率的影响，得到ＴｉＡｌ合金韧脆转变温度随应变速率升高而升高的变化关系，随后确定ＴｉＡｌ合金韧脆转变激活能为３２４ＫＪ／ｍｏｌ。     

Fe—Cr—Mn（W,V）奥氏体钢的低温韧性

研究了Fe-Cr-Mn(W,V)低活性奥氏体钢的相稳定性对低温韧性的影响。结果表明,Fe-Cr-Mn(W,V)奥氏体钢存在韧－脆转变行为,应变诱发ε－马氏体和热诱发ε－马氏体的相互作用造成局部应力集中,导致粗晶粒亚稳奥氏体Fe-Cr-Mn(W,V）钢危性断理解,沿晶断裂抗力温度降低而减弱,发生沿晶断裂是高Mn钢断裂的重要特征。     

18Mn—18Cr—0.5N奥氏体护环钢热变形力学行为研究

采用Gleeble-1500热模拟试验机研究了18Mn-18Cr-0.5N奥氏体护环钢热变形时的力学行为。获得了18Mn-18Cr-0.5N钢的动态再结晶激活能及峰值应力σ_p、峰值应变ε_p与Zener-Hollomon参数Z间的关系式。     

高氮低镍奥氏体不锈钢的低温性能与组织稳定性

评估[Ni]当量相近、[Cr]当量相同、氮含量不同的奥氏体不锈钢的低温性能和组织稳定性,测试了含氮量分别为0.614%和0.529%的奥氏体不锈钢H1和H2的低温拉伸和低温冲击性能,利用扫描电镜和X射线衍射仪分别对两种试验钢的拉伸断口与冲击断口进行了形貌观察和组织检测,利用TEM分析拉伸断口的显微组织。结果表明：两种试验钢的抗拉强度与屈服强度随着试验温度的降低单调增加,伸长率和断面收缩率逐渐减小,氮含量增加提高材料的强度但降低塑韧性;低温冲击试验的结果是氮含量增加降低试验钢的冲击性能,但对韧脆转变温度影响不大,H1与H2试验钢的韧脆转变温度分别为－122 ℃与－123 ℃。XRD与TEM的检测结果均表明此两种试验钢均具有良好的组织稳定性,低温拉伸与冲击均未发生马氏体相变与氮化物析出。     

节镍型高氮奥氏体不锈钢热压缩行为研究

通过Gleeble-3800热模拟机对高氮节镍型奥氏体钢进行热压缩试验。研究了该钢种的高温变形与应变速率以及温度的关系。并通过数学模型建立了本构方程和参数Z的表达式,应用数学方法计算出变形表观激活能,为823.31 k J/mol。     

Brittle to ductile transition in nickel free high nitrogen austenitic stainless steels

Abstract

The brittle to ductile transition (BDT) in nickel free high nitrogen austenitic stainless steel was investigated. Falling weight impact tests at 176, 273 and 336 K revealed that Fe–25Cr–1·1N (wt-%) austenitic steel exhibits a sharp BDT in spite of being a face centred cubic alloy. The plastic deformation observed following the impact tests indicated that the BDT is induced by poor ductility at low temperatures, as is the case with ferritic steels. To measure the activation energy for the BDT, the strain rate dependence of the BDT temperature was examined using four-point bending tests. The BDT temperature was found to be weakly dependent on strain rate. Arrhenius plots of the BDT temperature against strain rate showed that the activation energy for the BDT of Fe–25Cr–1·1N steel is much higher than that of low carbon ferritic steels. The origins of this distinctive BDT and the large value for its activation energy in this high nitrogen steel are discussed in terms of the reduction in dislocation mobility at low temperatures because of the interactions between the glide dislocations and the solute nitrogen atoms.     

High-temperature oxidation and thermal cycling of aluminum-electroplated stainless steels

An alumina coating, produced from the oxidation of an aluminum-electroplated deposit, improved the oxidation resistance in air of a ferritic, AISI-type 446 stainless steel, Fe-24Cr-1.2Al containing 0.15% of mischmetal, and an austenitic AISI 321 stainless steel containing 0.53% Ti, at least up to 1100°C. In thermal-cycling tests from 1000°C to room temperature, the alumina coating was adherent on the ferritic and austenitic steels, for at least 1000 and about 700 cycles, respectively. The addition of rare earths to the ferritic steels and titanium to the austenitic, provided good adhesion between the coating and substrate. The porous nature of the coating was found to be very beneficial by causing the coating to be more resistant to thermal and growth stresses. Oxidation mechanisms are discussed in the light of results obtained from the thermogravimetric tests and metallographic observations by SEM-ED analysis.     

New stainless steels for sea-water applications. Part II: Comparison of corrosion and mechanical properties of laboratory and commercial ELI ferritic and superaustenitic stainless steels

This paper represents a follow-up to the first part of the work on new stainless steels for sea-water service. Four laboratory ELI (Extra Low Interstitial) ferritic stainless steels (types 25 Cr-4 Ni-4 Mo), two commercial ELI ferritic stainless steels (types 25 Cr-4 Ni-4 Mo? Ti and 26 Cr-2.5 Ni-3 Mo? Ti) and two highly alloyed austenitic stainless steels (types 20 Cr-25 Ni-4.5 Mo? Cu and 20 Cr-18 Ni-6 Mo? N) have been investigated. With a view to establish the performance of these new alloys in chloride containing environments, systematic electrochemical and laboratory exposure tests have been carried out to define how various factors affect its susceptibility to intergranular, pitting, crevice and stress corrosion. Tension tests were also performed. From the comparison of the localized corrosion resistance and mechanical properties it has been concluded that the laboratory Ti, Ti + Nb or Nb stabilized ELI ferritic stainless steels and the commercial type 25 Cr-4 Ni-4 Mo? Ti of analogous composition could be a valuable alternative to the more expensive highly alloyed stainless steel type 20 Cr-25 Ni-4.5 Mo? Cu which has been especially developed and already used for industrial sea-water applications.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Al含量对Fe-22Cr-25Ni含铝奥氏体耐热钢高温抗氧化性能的影响

以HR3C合金成分为基础,通过调控Cr、Ni含量和添加1.5%,2.5%和3.5% (质量分数) 的Al制备了Fe-22Cr-25Ni型含铝奥氏体耐热钢,并研究了合金的高温抗氧化性能。利用SEM、EDS和XRD对含铝奥氏体钢700、800和900 ℃氧化后的氧化膜组成、结构进行了表征。结果表明：22Cr-25Ni-2.5Al和22Cr-25Ni-3.5Al含铝奥氏体耐热钢在700和800 ℃下具有优异的抗高温氧化性能。氧化后表层形成了连续致密的Al₂O₃保护膜,提高了其高温抗氧化性能。3种耐热钢经900 ℃氧化时形成外层为Cr₂O₃和MnCr₂O₄的复合氧化层,且氧化层下存在Al₂O₃内氧化物和AlN析出相,不能对基体起到有效保护作用。     

The Sulfidation Behavior of Several Commercial Ferritic and Austenitic Steels

The sulfidation behavior of C-steel, 1Cr-0,5Mosteel, 12Cr-1Mo-0.25V steel, 18Cr-10Ni-Ti steel, thebinary alloys Fe-20Cr, Fe-25Cr, Fe-30Cr, and pure Cr wasinvestigated between 400 and 700°C in a94Ar-5H₂-1H₂S gas mixture. All steels sulfidize according tocomplex kinetics which, after a period with decreasingrate, can be approximated by a linear rate law. Thescale of the three ferritic steels consists of two layers, an outer outward-growing one of FeSwith traces of dissolved Cr and an inner, inward-growingone, which contains in addition to Fe the alloyingelements Cr and Mn. Most of the outer FeS layer is separated from the inner layer and can be splitinto several partial layers, the number increasing withincreasing sulfidation time and temperature. The scaleon the austenitic 18Cr-10Ni-Ti steel differs insofar as that of the ferritic steels as theouter FeS layer contains some Ni and that a third layerof the spinel FeCr₂S₄ is formedbetween the outer and the inner layer. This intermediatelayer is responsible for the lower sulfidation rate of this materialcompared with that of the ferritic steels. The scale ofthe binary Fe-Cr alloys is similar to that of theaustenitic steel. From AE-measurements it can be deduced that the separation of the outer FeSlayer occurs during isothermal sulfidation and isaccompanied by an increase in the AE event rate. Theseparation is a consequence of the formation and growth of pores in the region close to the inner/outerlayer interface and the development of compressivegrowth stresses in the outer FeS layer. While detachmentof the FeS layer on the ferritic steels was already observed at 400°C, the austenitic steelshowed a similar separation of the FeS layer only at600°C. The detached FeS layer is obviously rathergas tight. Differences in the sulfur partial pressure ofthe bulk gas and the gas in the cavity between theinner and separated outer layer lead to a reduction ofFeS at the inner surface of the detached FeS layer. TheFe ions and electrons, produced by this reaction, diffuse outward, forming new FeS on the outerFeS surface. This process not only shifts the detachedFeS layer continuously away from the core of thespecimen but offers also the possibility of healing cracks in the separated FeS layer. This scaledetachment does not stop scale growth. After scaleseparation the total sulfidation reaction consists of atleast seven partial reactions: phase-boundary reaction at the outer surface, diffusion of iron ionsand electrons outwards in the detached FeS layer,formation of H₂S at the inner surface of thedetached layer, gas diffusion in the cavity, formationof FeS on top of the porous inner layer, gas diffusionin the channels of the porous inner layer, FeS formationat the metal/scale interface. When the new FeS layer ontop of the porous inner layer exceeds a critical thickness, the detachment of the FeSlayer from the inner porous layer repeats. This processcan take place several times, leading to an outer FeSpartial scale, split into several layers, which are separated by relatively large cavities andkept together only locally by FeS bridges. The overallreaction rate is controlled by the phase-boundaryreaction at the outermost FeS surface.     

The Effect of Nb and Ti on Structure and Mechanical Properties of 12Ni-25Cr-0.4C Austenitic Heat-Resistant Steel after Aging at 900 °C for 1000 h

Austenitic heat-resistant steels are particularly suitable for applications where service conditions comprise high temperature. The demand for better performance has motivated developments in these steels. In this work, Ti and Nb were added to austenitic heat-resistant steels, Fe-12Ni-25Cr-0.4C, wt.% simultaneously. Microstructural changes were studied via scanning electron microscopy equipped with energy dispersive spectrum (EDS), optical microscopy, and x-ray diffraction (XRD) in as-cast condition and after aging in 900 °C for 1000 h. Mechanical properties were measured using tensile tests, impact energy, and Vickers hardness. It was observed that by formation of NbC and TiC, the level of fragmentation of the chromium carbides increased, as a positive aspect for mechanical properties. XRD and EDS results show increasing the amount of Ti can inhibit G-phase transformation.     

Codeposited chromium and silicon diffusion coatings for Fe-Base alloys via pack cementation

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation process is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designing specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated packs are chromizing, while NaF-activated packs deposit more Si with less Cr. The use of a dual activator (e.g., NaF+NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferritic coatings (150–250 microns thick) with a surface concentration of 20–35 wt.% Cr and 2–4% Si have been grown on AISI 1018, Fe-2.25 Cr-1.0Mo-0.15C, and Fe-0.5 Cr-0.5 Mo-0.2C steels using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica filler. Two-phase coatings (approximately 75 microns thick) containing 20–25 wt.% Cr and 2.0–2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF activator, and an alumina filler. The same pack chemistry allowed the diffusion of Cr and Si into the austenitic Incoloy 800 alloy without a phase change. A coated Fe-2.25 Cr-1.0 Mo-0.15 C coupon with a surface concentration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700°C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm²) with no scale spalling detected. Coated coupons of AISI 1018 steel, and Fe-0.5 Cr-0.5 Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmospheres at temperatures from 400 to 700°C and in erosion and erosion-oxidation testing in air at 650 and 850°C.     

Effects of deformation-induced martensite and grain size on ductile-to-brittle transition behavior of austenitic 18Cr-10Mn-N stainless steels

Effects of deformation-induced martensite and grain size on ductile-to-brittle transition behavior of austenitic 18Cr-10Mn-(0.3∼0.6)N stainless steels with different alloying elements were investigated by means of Charpy impact tests and microstructural analyses. The steels all exhibited ductile-to-brittle transition behavior due to unusual brittle fracture at low temperatures despite having a face-centered cubic structure. The ductileto-brittle transition temperature (DBTT) obtained from Chapry impact tests did not coincide with that predicted by an empirical equation depending on N content in austenitic Cr-Mn-N stainless steels. Furthermore, a decrease of grain size was not effective in terms of lowering DBTT. Electron back-scattered diffraction and transmission electron microscopy analyses of the cross-sectional area of the fracture surface showed that some austenites with lower stability could be transformed to α’-martensite by localized plastic deformation near the fracture surface. Based on these results, it was suggested that when austenitic 18Cr-10Mn-N stainless steels have limited Ni, Mo, and N content, the deterioration of austenite stability promotes the formation of deformation-induced martensite and thus increases DBTT by substantially decreasing low-temperature toughness.