Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high‐temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe? Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms – precipitation–hardening, grain refinement, and solid‐solution strengthening – were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high‐temperature strength about 40%, also after long‐term annealing. The contribution of the Laves phase precipitates on the high‐temperature strength is rather small due to their rapid coarsening.     

High temperature ductility and corrosion resistance property of novel tin-bearing economic 17Cr–xSn ferritic stainless steel

Abstract

A new family of resource-saving ferritic stainless steels, with the composition of 17Cr–xSn (x?=?0.1–0.4 wt-%) have been developed by examining the effect of Sn on high temperature ductility, the corrosion resistance properties and the passivation film structure. The results show that tin-bearing stainless steel obtains superb plasticity and there is no evidence of tin-containing precipitation in the fracture facets. High temperature brittleness at about 900°C is caused by the carbide or nitride precipitation along the grain boundary or inside. The precipitation in the grain boundary hinders the free movement of dislocation, thus increasing the strength and decreasing the toughness of steel. The polarisation curves and weight loss measurement indicate that the corrosion resistance of stainless steel is improved by adding small amount of Sn. The existence of Sn in passivation film suppresses the anodic dissolution of steels during long exposure to the aggressive solution, and the dense rust layer is thick enough to protect the steel and strongly inhabit the ingress of the solution.     

建筑维护系统用高耐蚀性铁素体不锈钢的研发与应用

研究了Mo、Nb、Ti对22.5%Cr铁素体不锈钢力学性能、耐腐蚀性能的影响,研究结果表明,Mo主要以固溶状态存在于铁素体不锈钢中,通过固溶强化提高了铁素体不锈钢的强度;在Ti/Nb双稳定铁素体不锈钢中,由于粗大TiN颗粒的形成和金属间化合物在晶界的析出,钢的韧性明显降低;在Nb单稳定铁素体不锈钢中,Nb以固溶状态和Nb(C,N)析出相存在于铁素体基体中,Nb(C,N)弥散分布于铁素体晶内,并作为σ相的形核质点,避免了σ相在晶界的析出,从而改善了钢的低温韧性。在此基础上研制出建筑维护系统用的高耐蚀性铁素体不锈钢B445R。测试结果表明,B445R在氯离子溶液中具有比316L更优越的耐点腐蚀性能,而且具有良好的成形性能和焊接性能,这些优点使其成为沿海地区建筑维护系统理想的装饰材料。     

Solidification and microstructure of a high alloy stainless steel

The steel considered in the present work can be classified as a 6Mo austenitic stainless steel, which has a much higher corrosion resistance than many other commercially available stainless steels. However, because of macrosegregation during solidification of ingot casting, the high alloy content in the steel, especially Mo, N, C, enhances precipitation of intermetallic phases such asa phase. Such precipitation mainly occurs in the center of the material and has normally no significant effect on mechanical properties or surface corrosion resistance. The introduction of modern production methods such as continuous casting of slabs for stainless sheet production has reduced the macrosegregation tendency and related precipitation. However, the microsegregation in a small scale may be enhanced as the higher cooling rate prevents the diffusion in the solidified region. In the present work, the continuous casting solidification process was simulated in a laboratory gradient     

超纯铁素体不锈钢脆性及形成特点对性能的影响

超纯铁素体不锈钢是一种具有良好耐蚀性能、加工性能和焊接性能的材料,主要应用于汽车行业、厨房用具和家用电器以及大型建筑的屋面设计等领域.主要研究超纯铁素体不锈钢脆性形成原因以及对性能的影响,总结了超纯铁素体不锈钢σ相脆性、475℃脆性和高温脆性等主要特征,并对其脆性形成的原因进行了详细分析,认为σ相,x相,α相,Lave...     

Nb对0Cr11铁素体不锈钢组织和性能的影响

 铁素体不锈钢与奥氏体不锈钢相比具有成本低、热膨胀系数低和耐应力腐蚀等优点,所以被广泛应用到汽车排气系统、家用电器和建筑等领域。研究了不同的Nb含量对铁素体不锈钢显微组织和力学性能的影响。研究结果表明：Nb有细化晶粒的作用,随着Nb含量的增加,晶粒的平均尺寸减小;由于合金元素Nb和Ti的加入,形成了TiN、NbC 和 Fe2Nb析出相,其透射电镜观察结果与Thermal calc计算结果一致;材料的抗拉强度和显微硬度随着Nb含量的增加而增加,这是由于Nb的固溶强化和析出强化共同作用的结果。     

Novel hafnium containing steels for power generation

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M₂₃C₆ carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M₂N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M₂₃C₆ grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.     

Influence of Aging and Thermomechanical Treatments on the Mechanical Properties of a Nanocluster-Strengthened Ferritic Steel

This study investigated the effect of aging and thermomechanical treatments on the mechanical properties of a nanocluster-strengthened ferritic steel, Fe-1.5Mn-2.5Cu-4.0Ni-1.0Al (wt pct). The effect of thermomechanical treatments on the microhardness and tensile properties were measured at room temperature and correlated with microstructural features. Cu-rich precipitates were characterized by transmission electron microscopy and were found to coarsen slowly during long-time aging. The microhardness measurements indicate a typical precipitation hardening behavior during aging at 773 K (500 °C). Tensile tests showed that thermomechanical treatments can improve the mechanical strength and ductility of the nanocluster-strengthened ferritic steel significantly compared with those without the treatments. Fractography results indicated that the high yield strength resulted from precipitation hardening makes the steel more susceptible to grain-boundary decohesion, which can be suppressed by grain refinement. Atmosphere adsorption and diffusion along grain boundaries were found to intensify brittle intergranular fracture, and this embrittlement can be avoided by vacuum heat treatment.     

Effect of Nb and Ti on second-phase precipitation and mechanical properties of 430 ferritic stainless steel

The influence of Ti and Nb on the microstructure,mechanical properties,and second-phase precipitation of 430 ferritic stainless steel was investigated.In addition to optical microscopy,transmission electron microscopy and X-ray diffraction analyses,tensile tests,and carbonitride extraction experiments were conducted to investigate the microscopic mechanisms.The results showed that the primary precipitates in SUS 430 ferritic stainless steel were Cr_(23)C_6,Mn_(23)C_6,and Cr_7C_3,and the primary strengthening mechanism was precipitation strengthening.When Ti was added separately,the main precipitates were TiC and TiN.However,coarse TiC adversely affected the mechanical properties of steel.When double-stabilized with Ti and Nb,coarse TiC was replaced by fine NbC.The type of precipitation was altered,and precipitation and solid solution strengthening occurred.Therefore,the tensile strength and plastic strain ratio(r-value) improved to 433.60 MPa and 1.37,respectively.     

节镍无磁不锈钢Cr18Ni6Mn3N的组织及性能

研究了节镍无磁不锈钢Cr18Ni6Mn3N的热轧及固溶后的力学性能和耐蚀性能,分析了其固溶和时效析出后的组织演变规律、冷变形过程中形变诱发马氏体相变及其磁性能.结果表明:该不锈钢的固溶组织为单相奥氏体,其力学性能和耐蚀性能均高于SUS304不锈钢;800℃保温4 h后,在晶界析出粒状氮化物,随着保温时间延长,逐渐沿晶界凸起片层状析出物并向晶内生长,保温20 h后,凸出的片层状析出物直径达20μm.冷轧压下率18.3%时尚未发现形变诱发马氏体组织,随着变形量增大,马氏体含量增多,磁导率上升,但与相同条件下的SUS304不锈钢相比,冷轧板固溶后相对磁导率可降至1.002,因此可用于低成本无磁不锈钢领域.      

Influence of thermal aging on the reactivity of duplex stainless steel surfaces

The annealing of large cast pieces in duplex stainless steel (SS) and the different heat cycles resulting from repairs involve significant structural changes characterized by carbide and intermetallic phase precipitation. This yields to lower local corrosion resistance in sea water due to changes in the local content of alloying elements. The precipitation of chromium carbide affects the resistance to the intergranular corrosion and the repassivation behavior. The eutectoidal decomposition of ferritic phase into regenerated austenite and in sigma phase (α → γ _r + σ) results in weakening the resistance to pit nucleation in synthetic sea water. In contrast, such precipitation will not have any significant effect when the treatment temperature is high enough to involve a rapid rehomogenization of depleted zones and ensure a self-healing.     

热处理温度对含钼铁素体不锈钢0Crl7Mo2S组织及性能的影响

不锈钢中添加钼元素可显著改善其耐点蚀能力,但中高铬铁素体不锈钢中添加钼元素后会促进组织中形成富铬、富钼类析出相,会在一定程度上降低材料耐点蚀性能;故制定合理的热处理工艺是生产含钼类铁素体不锈钢时的重要环节。以0Cr17MO2S钢种为例,研究了热处理温度750～1050℃对材料内组织、力学性能及析出相的影响。     

钒对18Cr-2Mo铁素体不锈钢性能的影响

 利用Thermo-Calc热力学软件、扫描电子显微镜、X射线衍射仪及透射电镜等分析方法,研究了不同钒含量对单铌稳定18Cr-2Mo铁素体不锈钢微观组织、力学性能和点腐蚀性能的影响规律。结果表明：添加钒后的析出相主要是NbC和VN,弥散分布于晶内和晶界,其中NbC的尺寸为100~250nm;随着钒含量的增加,试验钢的晶粒尺寸及屈服强度变化不明显,而冲击韧性及点腐蚀性能逐步得到改善和提高,当wV=0.36%时,钢的韧脆?湮露扔稍嫉?10℃下降至-30℃,点腐蚀电位由0.38V提高至0.42V。     

汽车尾气排放系统低温端用铁素体不锈钢开发

随着汽车工业的快速发展,以降低汽车自身重量和提高材料性能为目的的材料开发,得到了汽车部件制造厂商的重视.铁素体不锈钢因不含有贵重金属镍,明显降低材料成本,同时,铁素体不锈钢又具有比奥氏体不锈钢更好的抗点蚀、晶间腐蚀等性能,广泛用于汽车尾气排放系统中.介绍了宝钢超低碳、氮Cr17Mo汽车尾气系统低温端用铁素体不锈钢的开发过程.该钢种具备良好的力学性能、成型性能及耐腐蚀性能,可以满足汽车排气系统冷端部位的中管、消声器和尾管的使用性能要求.     

Stainless Steels for Exhaust Lines

Exhaust lines of cars have been strongly evolving during the last decade to meet the various requirements of the automotive industry and the new environment regulations. This development tends to favour stainless steel grades to replace cast iron for the exhaust manifold and aluminized low carbon steel for pipes and mufflers. Among the various stainless steel grades, the proportion of ferritic grade increases for the hot part of the exhaust system as its price is lower and more stable than austenitic or refractory grades, and as it exhibits a better resistance to cyclic oxidation. A large and rapid development of stainless ferritic grades has been based on Ti and Nb stabilization. Adding niobium presents many advantages for corrosion resistance, deep‐drawability, and mainly creep resistance at high temperature. The paper is focused on stainless steel with 14 wt% chromium, stabilized with Nb. Intermetallic precipitation is studied after a 100 h holding time at different temperatures (sag tests). It is shown that, depending on the test temperature and on the chemical composition of the steel, Fe₂Nb or Fe₂Nb₃ intermetallic particles can precipitate, which affects the creep resistance: Fe₂Nb₃ being more stable at higher temperature. Some impacts on the alloy design dedicated to exhaust line application are presented as a conclusion of the study.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.     

Effect of different mo contents on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of CD4MCU cast duplex stainless steels with different Mo contents of 0, 2, and 4 pct, respectively, were examined in the present study. The polarization and the in-situ slow-strain-rate (SSR) tests were conducted in a 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking (SCC) with different Mo contents. The addition of Mo, which is a strong ferrite stabilizer, affected the microstructure of the present alloy and, eventually, the tensile and corrosion behaviors in a complex manner. The tensile properties of CD4MCU cast duplex stainless steel, for example, were found to be determined by the volume fraction of hard ferritic phase, the presence of the second precipitates of soft austenitic phase in the ferrite matrix, and the shape of the austenitic phase. The addition of 2 pct Mo was detrimental to the corrosion properties of CD4MCU cast duplex stainless steel due to the significant increase in the volume fraction of ferritic phase. With the addition of 4 pct Mo, however, the resistances to pitting corrosion and SCC recovered to those of the specimen without Mo. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mo contents was discussed based on the micrographic and fractographic observations.     

热处理工艺对12%Cr铁素体不锈钢组织性能的影响

以12%Cr铁素体不锈钢为研究对象,研究了不同退火方式、退火工艺对12%Cr铁素体不锈钢热轧板和冷轧板组织、力学性能、耐腐蚀性能、成形性能的影响。结果显示,12%Cr铁素体不锈钢高温下存在铁素体—奥氏体相变过程,在双相区退火快冷后有马氏体组织生成。通过在热退过程中引入奥氏体相变和马氏体相变,可以显著细化最终产品晶粒,提高最终冷轧产品的强度、硬度,同时保持材料耐腐蚀性能不变。该工艺生产的产品可以适用于高强度的应用场合,打破了传统12%Cr铁素体不锈钢热轧后只能采用罩式炉退火的束缚,针对产品的最终用途可以采用不同的退火方式和退火工艺进行生产。     

稀土在铁素体不锈钢中的研究现状及发展前景

摘要：铁素体不锈钢具有“节镍资源型”特性及优良的综合性能,被广泛应用于轨道运输、汽车、家电和厨具等行业中。稀土作为战略性资源在不锈钢中的应用已有多年,国内外学者在这方面做了大量研究。归纳和总结了稀土对铁素体不锈钢组织和性能等方面的影响,并对其在钢中的作用机制和影响机制进行了简要分析。大量研究表明,稀土能将钢中不规则状且电子功函数较小的A12O3、MnS夹杂改性为电子功函数较大的球状稀土复合夹杂;此外,稀土能降低钢中晶粒尺寸,净化其晶界,对钢的凝固组织、退火组织等金相组织起到明显的细化作用,提高其耐蚀性能和力学性能。最后展望了稀土在不锈钢中的发展前景。