Thermal aging effect on 2205 duplex stainless steel

The effect of isothermal aging treatment on the mechanical and corrosion properties of 2205 duplex stainless steel was investigated by means of impact toughness test and micro-hardness measurement in combination with the critical pitting temperature( CPT) technique. The corresponding fractography of the steel was then observed after the impact toughness test. The results demonstrated that,at the critical temperature for precipitation of the sigma( σ) phase,e. g.,850 ℃,the impact toughness decreased rapidly and the micro-hardness increased gradually with increasing aging time. The CPT decreased from 61 to 15 ℃ as the aging time increased from 4 min to 8 h. In addition,optical microscopy,transmission electron microscope( TEM) and X-ray diffraction studies showed that the ferrite in the steel transformed into secondary austenite and σ phase.     

双相不锈钢热老化前后拉伸变形行为的电子背散射衍射表征

通过电子背散射衍射实验分析方法,研究变形量和热老化因素对双相不锈钢的拉伸性能、相边界、局部应变分布、重位点阵特殊晶界和取向分布的影响.研究结果表明:热老化后,双相不锈钢的强度提高,韧性降低;在大变形条件下铁素体晶粒内小角度晶界的数量和密度略有增加;热老化材料的铁素体的塑性变形和局部应变能力下降,大变形破坏初始奥氏体和铁素体以及Σ3孪晶边界的分布.     

A fractographic investigation of thermal embrittlement in cast duplex stainless steel

The fracture surface topography analysis (FRASTA) technique under development at SRI was applied to seek an explanation for severe thermal embrittlement observed in cast duplex stainless steel. By comparing topographic features of conjugate fracture surfaces, FRASTA showed that fracture in thermally embrittled cast duplex stainless steel occurs by microcrack initiation at delta-phase grain boundaries and at alpha-phase/gamma-phase interfaces, and by microcrack growth along these boundaries and interfaces. The critical crack tip opening displacement (CTOD) as measured from cross-sectional views generated by the FRASTA technique indicated a microcrack initiation toughness,J _Ic, of 287 KJ/m², in excellent agreement with measurements using conventional fracture mechanics procedures, and significantly less than the toughness of unaged material. Formerly with SRI International, Menlo Park, CA, is with the Department of Mechanical Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan 466.     

预应变对TRIP型双相不锈钢拉伸变形行为的影响

预变形通常作为工程构件服役前的关键预处理工艺,用以提高金属构件的力学性能.为了明确预应变对拉伸变形过程中力学行为和相变行为的影响,在INSTRON 8801试验机上进行了一系列准静态拉伸试验,研究了一种TRIP型双相不锈钢的拉伸变形行为及其预应变敏感性,同时结合TEM分析、EBSD分析和原位磁性测量讨论了微观机理.结果...     

Influence of carbon content and ferrite morphology on the sensitization of duplex stainless steel

The influence of ferrite morphology and carbon content on the intergranular corrosion behavior of 308 stainless steel was investigated using four wrought alloys and six weld deposited alloys. The four wrought alloys were heat treated at four different annealing temperatures to introduce four different amounts of ferrite. The annealed samples along with the weld deposited alloys were aged at temperatures ranging from 480 to 700°C for times varying between 15 min and 1000 h and then tested for intergranular corrosion susceptibility in acidified copper-copper sulfate solution. For a given carbon content there exists a critical amount and distribution of α-γ boundary area above which the alloy is immune and below which it is susceptible to intergranular corrosion. For amounts and distributions of α-γ boundary area less than the critical value two types of sensitization behavior are possible. First, there may be a sufficient amount and distribution of α-γ boundary area to insure rapid healing of the sensitized microstructure. Second, there may be an inadequate amount or distribution of α-γ boundary area to produce either immunity or rapid healing and the alloy behaves as a fully austenitic alloy regardless of the amount of ferrite present. A model is presented which describes as a function of carbon content the critical amounts and distributions of α-γ boundary area required for rapid healing and immunity to sensitization.     

冷轧硬化对超级双相不锈钢S32750析出相的影响

 为了优化不同厚度冷轧板加热工艺,避免有害相析出,研究了冷硬化对S32750板材析出相的影响。超级双相不锈钢S32750钢板经不同冷轧加工变形后,在850 ℃下进行热处理。利用SEM、EDS、XRD及TEM研究了不同冷轧总压下率对析出相的影响规律。结果表明,随着总压下率的增大,相同退火时间内,析出相增多,当总压下率大于50%时铁素体相几乎全部被析出相覆盖。析出相以σ相为主,伴随着很少量的Cr2N、χ相和γ2。σ相和χ相析出在α/γ相界和铁素体相内,Cr2N析出在α/γ相界处,γ2伴随着σ相析出在铁素体内。随着冷轧总压下率增加,晶体内的位错密度增大,析出数量增加。     

Influence of elastic and plastic anisotropy on the flow behavior in a duplex stainless steel

The load partitioning between two phases in a cold-rolled duplex stainless steel has been experimentally studied in situ by X-ray diffraction, for different loading directions. It was found that the load partitioning between the two phases is dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45-deg direction, more plastic deformation occurs in the ferritic phase. The strong crystallographic texture in the ferritic phase makes the material anisotropic, with a higher stiffness and yield strength in the transverse direction compared to the rolling direction. The measured texture was used as input to theoretical predictions of both elastic and plastic anisotropy. The plastic anisotropy was predicted by assuming intragranular slip as the main deformation mechanism. The predicted anisotropic material properties were then used in finite-element simulations to study the flow behavior of the material in different directions. The predicted flow behavior was found to be in good agreement with the experimentally observed load partitioning between the phases for loading in the rolling and transverse directions. However, the yield strength of the ferritic phase during loading in the 45-deg direction was found to be lower than what was predicted. The reason for this is the difference in slip characteristics in different sample directions, because of the morphological texture.     

加载方向对TRIP双相不锈钢板带拉伸性能的影响

 为了研究加载方向对一种TRIP型双相不锈钢板带力学性能的影响,利用拉伸试验机研究了加载方向与轧制方向分别成0°、45°和90°条件下试验钢板带的拉伸变形行为。利用EBSD、TEM、XRD等分析手段对比研究了不同加载方向下形变组织演化的特点及形变诱导马氏体相变动力学规律,探讨了作用机理。结果表明,试验钢表现出明显的各向异性,其中各方向塑性和抗拉强度(由高到低)的变化规律为0°＞45°＞90°,但屈服强度对加载方向不敏感。钢中奥氏体相发生了形变诱导马氏体相变,主要演化机制为γ→ε→α′,从而形成TRIP效应。0°加载有助于TRIP效应的发生与发展,而90°加载时,两相间的应变配分延迟了马氏体相变的进程,抑制了TRIP效应。通过回归分析分别建立了不同加载方向下形变诱导马氏体相变动力学模型,可实现各加载方向下不同变形阶段马氏体转变量的预测。     

超级双相不锈钢生产工艺实践

超级双相不锈钢S32760生产工艺流程为20tEAF→AOD→LF→30cm模铸。由于S32760成分设计中高铬、低磷、低硫、超低碳这一特点,冶炼后期钢水过氧化程度严重。为改善钢水洁净度、降低钢中氧质量分数,通过采取合理的脱氧造渣工艺,采用Al-Si-Ca复合脱氧,使钢中气体、夹杂物得到了有效控制,改善了钢水洁净度,成品钢中硫质量分数控制在0.002%以下,氧质量分数控制在0.004%以下。     

氮对SAF2205双相不锈钢组织及力学性能的影响

通过增加氮质量分数、降低镍质量分数研究了对SAF2205双相不锈钢组织及力学性能的影响。结果表明,增氮降镍后,当氮质量分数和镍质量分数在合适的范围(w(N)≤0.26%、w(Ni)≥5.940%)内时,试验钢的奥氏体相区扩大,奥氏体晶粒变小且均匀;随着镍质量分数降低,晶粒变得粗大,但冲击吸收功和强度均有所提升,其中A_(kV)由199.7增至232.5J,R_m由538提高到787 MPa,Rp0.2由484提升到692 MPa,此时钢的塑性指标降低较小。     

Microstructural changes during heating of super duplex stainless steel and its thermal deformation behavior

Microstructural changes during heating of highly alloyed Cr26Ni7 type super duplex stainless steel( SDSS2607) and its thermal deformation behavior were investigated. At different heating rates,the mechanism of phase transition from γ phase to δ phase and grow th modes of δ phase differed. Variations in microstructures for ascast SDSS2607 during heat preservation at 1 220 ℃ indicated two kinds of transformations from γ phase to δ phase.In-situ observations of microstructural changes during the tensile process at 1 050 ℃ showed a mutual coordination betw een γ and δ phases. When the true strain increased,the mutual coordination between γ and δ phases was damaged. Subsequently,cracks nucleated at the γ/δ interface. With the increase in temperature,the strength of ascast SDSS2607 decreased while its plasticity increased. Its thermoplasticity was poor,and the reduction in area of tensile specimens was less than 80%. When the deformation strain of hot compression increased,the stable deformation zone in the heat processing maps enlarged gradually. Moreover,the unstable deformation zones were extended.     

Influence of nitrogen addition on the microstructure and mechanical properties of duplex stainless steel weld metals

Duplex stainless steels find extensive applications in off‐shore, paper/pulp, refineries and petrochemical industries owing to their excellent combination of properties. Because of the thermal cycle experienced during the manufacturing/processing operations like welding, the weldment developed in this material is generally inferior in properties (mechanical and corrosion) when compared to the base material. The problem is more severe when the welding is done in the autogenous condition. The current work correlates the microstructure and property of UNS 31803 duplex stainless steel (material no. 1.4462) welded by autogenous GTA process with and without nitrogen in the shielding gas. It has been found that the nitrogen addition to the weld metal through the shielding gas mixture not only helps in achieving the proper phase balance but also improves the impact toughness of the resultant weld metal.     

铸坯取样位置对经济型双相不锈钢2101热塑性的影响

为了解大型铸锭在轧制过程中产生边裂的原因,通过对比铸坯中部和边部的成分、不同温度下相比例、两相硬度差等的变化规律,利用光学显微镜,扫描电子显微镜和电子背散射衍射观察分析试验钢的微观组织和断口形貌,分析了边部容易开裂的原因.结果表明,和中部相比,边部晶粒细小,且铁素体含量较多,但边部开裂更严重.这说明晶粒尺寸和相比例并不是影响使边部开裂严重的主要原因.而和中部比,铸锭边部试样两相硬度差较大,使两相在热变形过程中应变分配不均匀,容易在相界处产生应力集中,导致开裂.同时边部析出物较中部多,相界析出物的产生破坏了基体的连续性,容易在相界处产生显微裂纹,导致开裂.     

The 80-year history of duplex stainless steel

80 years has passed since duplex stainless steels were first produced and now they have developed into an integral series with the efforts on R & D and development of technology.In the recent decade, duplex stainless steels have been accepted by more and more customers and increasingly used. The first duplex grade produced in Sweden was 453E(26Cr-5Ni) in the 1930s,and then developed into 329.These two grades were characterized by high carbon content and called the first generation of duplex stainless steels.At that time,it was very difficult to add nitrogen into the steels and maintain the phase equilibrium,thus influencing the application properties,for example,intergranular corrosion post welding. One method to solve this problem is to alter the chemical composition,like adding nitrogen,etc.And that came to reality with the development of AOD and metallurgical theory of stainless steels.New series of duplex grades,called the second and third generations duplex,have successively emerged since the 1980s. These grades are characterized by high amounts of alloying elements,like chromium,molybdenum and nitrogen.Furthermore,super duplex stainless grades,like S32750,S32760 and S32707,were developed for various harsh service environments with their outstanding corrosion resistance and workability.These grades possess corrosion resistance corresponding to super austenitic grades,or close to nickel-base alloys, and are used in ocean-engineering,sea water desalination and oil industries,etc. And the application of duplex stainless steel is expending into other industries.For instance,453E is used in the pulp & paper industry.2205(S32205),a medium-alloyed grade,has become the most typical one in the duplex stainless steel family and widely used in many industries like pulp & paper,chemical and oil.New applications are emerging with better understanding of the duplex grades. Modern duplex stainless steels features most the corrosion resistance and strength,making them most cost-efficient in more and more projects. In this paper,the history of duplex stainless steels is recalled and reviewed from R&D,production to application,and latest grades like S82441 are also introduced.     

Room-temperature aging of manganese-alloyed high nitrogen duplex stainless steels

The room-temperature aging behavior of two duplex stainless alloys with different austenite stability was investigated. Both alloys readily aged at room temperature. Even for aging times as short as 30 seconds, the originally continuous yielding behavior becomes discontinuous upon reloading after prestraining. The magnitude of the stress increase due to aging was higher in the presence of strain-induced martensite, even though it was shown that aging also occurred in the austenite phase. The aging response was shown to be thermally activated, with increasing age hardening associated with increasing aging times. The results could be explained by the combination of aging phenomena in the bcc phases by interstitials and the aging by interstitial-vacancy complexes in the fcc phase, where the interstitials are thought to be immobile during the short aging times used and aging would occur due to short-range migration of vacancies instead.     

Influences of superheat and cooling intensity on macrostructureand macrosegregation of duplex stainless steel studied by thermal simulation

The influences of superheat and cooling intensity on macrostructure and macrosegregation of one new kind duplex stainless steel (DSS) were studied. Thermal simu...     

2205双相不锈钢中σ相析出规律的研究

 对原始组织为铸态和热轧态的2205双相不锈钢试样进行了不同条件下的固溶处理热模拟试验。通过对处理后的试样的显微组织进行观察和分析，得到了σ相析出规律以及原始组织对σ相析出规律的影响。     

镍含量对2205双相不锈钢焊接接头的影响

采用光学显微镜观察了不同镍含量下2205双相不锈钢焊接接头的金相组织,通过点腐蚀试验测量腐蚀速率,并采用电化学工作站对焊接接头的点蚀电位进行测量.结果表明,1.2 m m厚度规格2205双相不锈钢焊接接头为铁素体加奥氏体的两相组织,点蚀坑主要出现在热影响区和焊缝.镍含量高的样品金相组织中奥氏体含量高,对应点蚀电位也要高...     

节约型双相不锈钢组织演变规律的研究进展

节约型双相不锈钢是一种高性能、低成本的结构材料,拥有广阔的应用前景,有望广泛应用于核电、石化、造船、造纸和海水淡化等领域,但是其热加工制备困难,阻碍了大规模工业化生产。简要介绍了国内外双相不锈钢的研究现状,综述了双相不锈钢的组织性能演变、热变形规律及脆性相析出行为等。从组织演变规律入手,指出改善节约型双相不锈钢热塑性的重要性及可行性。