Influence of isothermal aging onσprecipitation in super duplex stainless steel

The time-temperature-transformation （TTT） curve of the 00Cr25Ni7Mo4N duplex stainless steel was obtained with a Formastor-digital thermal dilatometer, and the influence of isothermal aging on o precipitation was studied by metallographic observation, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. The results show that the decomposition of ferrite phase is accompanied by the formation of σ phase at 750-1000℃, especially in the range of 800-900℃. The longer the aging time, the higher the amount of o precipi- tation. The area fxaction of various phases remains at a certain value upon the completion of ferrite deformation. The temperature of 850℃ is the most sensitive transaction temperature, the incubation time for the formation of o precipitation is less than 1 min, and aging for 20 min leads to the complete transformation of ferrite. The o phase is formed preferentially at the α/α/γjunction, and then grows along the α/α boundary in the matrix.

Sigma Phase Precipitation and Properties of Super-duplex Stainless Steel UNS S32750 Aged at the Nose Temperature

The nose temperature for σ-phase precipitation in super-duplex stainless steel （SDSS） UNS S32750 was evaluated by hardness method.Color-optical microscopy,scanning electron microscopy,energy spectrum analysis,impact and corrosion testing were carried out to investigate characteristics of microstructure and properties of the SDSS aged at the nose temperature.The experimental results indicate that the nose temperature of precipitation is 920℃ and aging at this temperature tiny σ phases can precipitate at phase interfaces or ferrite grain boundaries within 2min.Prolonging aging duration the amount ofσ-phase increases and a dual structure with σ and γ is obtained when aging for 120min.The precipitation ofσ-phase leads to severe deterioration in impact toughness （longitudinal/transverse direction） and corrosion resistance of SDSS.

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 y phase to 6 phase and growth modes of ~ phase differed. Variations in microstructures for as- cast SDSS2607 during heat preservation at 1 220 ~C indicated two kinds of transformations from y phase to 6 phase. In-situ observations of microstructural changes during the tensile process at 1 050 showed a mutual coordination between y and 6 phases. When the true strain increased, the mutual coordination between 7 and 6 phases was damaged. Subsequently, cracks nucleated at the ＂y/g interface. With the increase in temperature, the strength of as- cast 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.

An EBSD study of the micro structural development during annealing of a folded super duplex stainless steel sheet sample

Super duplex stainless steels(SDSS) show complex precipitation and transformation behavior during heat treatment processes,which affects both mechanical and corrosion properties. This report presents some data on the microstructures that develop after folding and subsequent precipitation during heat treatment of UNS S32750 SDSS sheet samples.The microstructural and textural changes have been followed using SEM/EBSD techniques.Upon folding,both a texture and strain gradient form in the folded/bent region,subsequent heat treatment at 845℃results in the ferrite phase to transform to sigma,austenite and chi phases.Transformation was found to be accelerated by strain.Complete transformation of the ferrite phase occurred within half the annealing time required in the unstrained regions.The local mis-orientations in the ferrite and austenite phases reduced during annealing,however,the reduction in the austenite was not very high and a significant amount remained even after the longest annealing time.The texture components that developed during the folding process remained unchanged even after one hour annealing at 845℃.The implication of these findings could have a bearing on the formation of sigma phase during welding of SDSS that may have residual stresses introduced during final processing.

Research on the properties and microstructures of 27Cr-7Ni-4Mo super duplex stainless steel

In this study, metallographic analysis and nanoindentation characterization were used to analyze the properties and microstructures of as-cast nitrogen alloyed 27Cr-7Ni-4Mo super duplex stainless steel （super-DSS）. The as-cast microstructure of the super-DSS was characterized by its ferrite and island-like austenite phases. During the solution annealing process, the austenite volume percentage of the steel decreased gradually with increased annealing temperature. As a main element, the chromium content in the ferritic and austenitic phases elevated slightly at first then decreased with increased annealing temperature. The chromium partition coefficient in the steel varied by around 1.0. The contents of nickel, another main alloy element, also increased in the ferritic and austenitic phases with increased annealing temperature, as did the nickel partition coefficient in the steel, which tended to be close to 1.0. The nanoindentation characterization results indicate that the hardness of the austenite phase is slightly greater than that of the ferrite phase. They were similar to each other within a certain temperature range from 1 050 ℃ to 1 100℃. This temperature range was consistent with the temperature range in which the content ratio of the two phases was close to 1： 1. We found the Young＇ s modulus of the ferrite phase to be greater than that of the austenite phase. With increased annealing temperature, the Young＇ s modulus of the ferrite phase decreased while that of the austenite phase remained almost unchanged.

Mechanical Properties for Circumferential Welding Applied to UNS S32750 Super Duplex Stainless Steel Using the GMAW Process with CMT Control

Super Duplex Stainless Steel has been studied for oil and gas industry applications since the 90 s, but their welding always is a technical issue. Then different methods were developed to meet all requirements presents in ASME standards, where most of are applied for lower production, when compared to other materials such carbon steel. This study accomplished circumferential welding experiments in base material (BM)—UNS S32750 Super Duplex Stainless Steel with a thickness of 3.68 mm. Welding was performed using GMAW (Gas Metal Arc Welding) process with CMT (Cold Metal Transfer) control and the aid of a Motoman robot and a turntable device, which were used to ensure the accurate positioning of the torch and that the welding has been fully implemented in the 1 G position, respectively. This process achieves higher production in relation other welding process usually applied for this material class. The joints which presented adequate dimensional results were, then, subjected to tensile and hardness Vickers tests. To avoid the lack of penetration problem, the welding was performed using the CMT process combined with pulsed arc, thus, resulting in full penetration and improved surface finish. The results have shown that the CMT procedure, combined with pulsed arc, led to an adequate superficial finishing, mechanical properties and corrosion resistance in accomplishment with the requirements presented in applicable standards.

Cutting Characteristics of Duplex Stainless Cast Steel X2CrNiMoN25-7-3

X2CrNiMoN25-7-3 duplex stainless steel is suitable for chloride-containing environments such as seawater because it has excellent pitting corrosion resistance. Duplex stainless cast steel is often used to accommodate the complexity of part geometry at joints. However, machining after casting is indispensable. This study evaluated the cutting characteristics of stainless cast steel. Adhesion of the cutting edge was weak at high cutting speeds, but tool wear was large. Diffusion and reactions between the work-pieces and the tool edge were investigated. Slight inter-diffusion and a reaction phase were observed, but the bond-ability was low.

超级双相不锈钢2507及其焊接工艺

超级双相不锈钢2507具有优良的抗腐蚀性能、较高的强度和相对低廉的价格,广泛应用于石油天然气开发开采等富含氯离子、CO2和H2S的恶劣腐蚀环境中。介绍了超级双相不锈钢2507的主要性能,讨论了2507钢的特点及其焊接技术。通过2507钢的焊接试验,并进行焊后力学、抗腐蚀性能试验以及相比例测量分析,得出了2507钢的焊接工艺。正确的焊材选择、保护气体配比以及焊接工艺参数是保证焊接接头同时满足力学和抗腐蚀性能的关键。

254SMo和2507超级不锈钢中的σ析出相

通过JMat Pro软件,计算预测254SMo超级奥氏体不锈钢(SASS)和2507超级双相不锈钢(SDSS)中σ相的热力学析出条件,并制定了不同的热处理制度;同时借助光学显微镜、扫描电镜等手段分析研究了经不同温度时效不同时间后σ析出相的形态及析出机理。结果表明:254SMo SASS在1020℃不同时效时间下,在晶界上析出富含Cr、Mo和低Ni的σ相。随着时效时间增加,σ相数量增多、尺寸增大,且呈短棒状、长条状和胞状等形态,并按照γ→σ方式析出;2507 SDSS在950℃不同时效时间下,在α/γ界面和α/α晶界处析出富含Cr、Mo和更低水平Ni的σ相。随着时效时间增加,σ相数量增多,向铁素体内部长大并最终呈网状分布在奥氏体基体上,且伴有二次奥氏体(γ_2)生成,其是按照α→σ+γ_2方式析出。

高温时效对2507超级双相不锈钢组织和性能的影响

对2507超级双相不锈钢在920℃进行了不同保温时间的时效处理,采用光学显微镜、扫描电镜、X射线衍射仪和能谱仪分析了不同时效状态下的组织演变规律,通过硬度试验和冲击试验研究了时效时间对2507超级双相不锈钢性能的影响。结果表明,920℃时效处理时,大量的σ相沿γ/α及α/α晶界析出,并向铁素体内部长大,其形成机理为铁素体共析转变成σ相和二次奥氏体γ2;在时效5 min内σ相的析出速率最快,随着时效时间的延长,σ相的含量增加,但析出速率逐渐变小;σ相的出现严重降低了超级双相不锈钢的冲击韧性,并且使其硬度明显增加,冲击功和硬度值的大小与σ相析出量有关,当920℃时效30 min时,σ析出相的含量接近于28%,对应双相不锈钢的冲击功和硬度值分别为6 J和376 HB。

SAF2507双相不锈钢与 Q235碳钢异种金属板材的焊接

将切割的Q235钢板和SAF2507双相不锈钢板分为4组,采用ZX7-400IGBT逆变式直流弧焊机和A302奥氏体不锈钢焊条进行焊接,并对焊前和焊后板材做微观组织分析及显微硬度分析。研究表明,在焊接电流为90A^130A的条件下,焊接熔合线附近的Q235钢一侧形成了脱碳层,SAF2507双相不锈钢一侧则形成了增碳层;随着焊接电流的增大,热输入也随之增大,焊缝长度变短,焊缝的余高增大,在熔合区的界面处发生碳迁移现象,使接头区域形成显著的硬度梯度。

超级双相不锈钢SAF2507焊接热模拟组织的耐点蚀性

采用Gleeble 3800热力模拟机对超级双相不锈钢SAF2507焊接热影响区组织进行模拟,研究了不同热输入条件下焊接热影响区显微组织和合金元素含量的变化,通过电化学循环伏安法评价了该组织的耐蚀性。研究结果表明,热输入由0.807 k J/mm增加至2.552 k J/mm时,模拟热影响区上奥氏体以晶内奥氏体、晶界奥氏体和魏氏奥氏体3种形貌析出,含量由40%增加到52%。各个热输入时奥氏体的PRE值均大于铁素体的PRE值,因此奥氏体含量的增加使热影响区组织的点蚀电位从1030 m V提高至1082 m V。但是继续增加热输入至2.965 k J/mm时,在两相交界处观察到有粒状的χ相析出。虽然该组织上奥氏体含量仍有增加,但是χ相的析出却导致组织的点蚀电位降至1065 m V。

层间温度对超级双相不锈钢SAF2507模拟热影响区组织转变规律及力学性能的影响

针对实际焊接中热影响区过于狭小而无法准确进行试验评价的问题,提出了一种研究焊接热影响区的试验方法。采用GLEEBLE3800型试验机进行焊接热模拟试验,研究了层间温度对SAF2507超级双相不锈钢热影响区(HAZ)显微组织演变和力学性能的影响规律。试验结果表明:当层间温度为150℃时,铁素体和奥氏体2相比例接近1∶1,而且没有魏氏体状奥氏体和金属间相析出;随着层间温度的升高,二次奥氏体和魏氏体状奥氏体从晶界或晶内析出,并不断增多、长大,使热影响区中奥氏体含量增加,硬度降低,冲击韧度升高;当层间温度升高至250℃时,有χ相析出;继续提高层间温度至300℃时,硬度增大,冲击韧度降低,经SEM和EDS鉴定,有脆性相σ析出。在实际焊接中为保持SAF2507超级双相不锈钢焊接接头中良好的双相比例和力学性能,焊接层间温度应不高于150℃。

2507双相不锈钢高温氧化行为

通过高温氧化试验(1200℃,保温5 min,氧化气氛A和B)模拟了2507双相不锈钢热轧初期的氧化过程,并应用扫描电子显微镜(SEM)、辉光放电光谱仪(GDS)、X射线衍射仪(XRD)和X射线光电子能谱仪(XPS)等手段,分析了不同气氛下氧化膜的组成和结构以及初期氧化特性。结果表明:氧化膜主要由Fe3O4、Cr2O3和尖晶石型(FeCr2O4、MnCr2O4等)等复合氧化物组成;气氛中氧含量的变化可改变氧化膜中氧化物结构、组成,铁素体优先于奥氏体氧化。

2507超级双相不锈钢的组织、性能及其焊接工艺

介绍了2507超级双相不锈钢的组织、性能特点及其焊接工艺。指出2507超级不锈钢焊接方法适应性较广，气体保护焊焊接效果较好，焊接热输入和冷却速率影响焊缝组织中铁素体和奥氏体相比例，焊接时，为保证焊缝组织中具有合适的相比例和良好的力学性能及其腐蚀性能，应该控制焊接热输入2～20kJ／cm之间，多道焊时道间温度控制在100℃以下，实际生产中通过调整焊接热输入及控制道间温度，可以得到合适的焊接接头组织及较好的性能。

2507双相不锈钢在含铁氧化菌冷却水中的腐蚀行为

为研究2507双相不锈钢在含铁氧化菌IOB冷却水溶液中的腐蚀行为,采用失重法、扫描电镜和能谱及电化学测试等方法,分析了2507钢在含体积比为0.5%、1%及2%IOB冷却水模拟溶液中浸泡30d后的腐蚀行为.实验结果表明:冷却水溶液中含IOB大大加速了2507双相不锈钢的腐蚀,但腐蚀程度仍属轻度腐蚀,平均腐蚀速率随着IOB浓度增加先升后降,1%IOB下达到最大.随着IOB浓度增加结瘤形体增大,但均未出现点蚀坑,点蚀电位高达约1.25V.该钢抗含铁氧化菌冷却水的点腐蚀能力优良.

2507超级双相不锈钢的热变形行为

用MMS-200热模拟实验机对2507超级双相不锈钢(/%:0.022C、0.58Si、25.35Cr、7.17Ni、4.05Mo、0.28N)12 mm热轧板在1 000～1150℃、应变速率0.01～10s^(-1)下进行了热压缩实验。实验结果表明,在应变速率一定的条件下,变形温度越高,2507超级双相不锈钢峰值应力越低;在变形温度一定的条件下,峰值应力随着应变速率的增加而增加。根据热变形方程计算得到压缩变形时的平均表观应力指数n=3.25,热变形激活能Q=460kJ/mol。基于实验数据构建了2507超级双相不锈钢在相应变形条件下的热变形方程。

热轧变形量对铝热法制备的2507双相不锈钢组织和力学性能的影响

对铝热反应制备的微纳结构2507双相不锈钢在1000℃下进行了变形量为40%、60%和80%的轧制处理.利用X射线衍射仪(XRD)、光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)研究其轧制态显微组织.采用万能拉伸试验机和布洛维光学硬度计测试其力学性能.测试结果表明:轧制过程中,奥氏体和铁素体沿轧制方向被拉长,且奥氏体向铁素体转变.随着轧制变形量的增加,纳米晶平均晶粒尺寸变化不大,但体积分数减小.1000℃下轧制变形量为40%、60%和80%后的屈服强度分别为232、284、456 MPa,抗拉强度分别为533、577、582 MPa,硬度分别为325、330、337 HV,延伸率分别为12.5%、11.1%和11.5%.

SAF2507与321管板异种金属焊接接头组织形态和相组织分布

为了解SAF2507和321异种金属管板焊接接头的相比例分布情况,按照ASTM标准对其焊缝截面各重要部位进行了铁素体含量检测。通过金相分析,发现两道焊缝组织形态区别很大,第1道焊缝基本为针片状或羽毛状奥氏体,第2道焊缝却成团块状奥氏体。

2507双相不锈钢在含硫酸盐还原菌冷却水中的腐蚀

采用电化学测试、失重法、扫描电镜及能谱分析等方法研究了2507双相不锈钢在含体积比为0.5%、1%、2%硫酸盐还原菌(SRB)冷却水模拟溶液中浸泡30d后的腐蚀行为。结果表明:当浓度低于1%SRB时,钢表面生长缓慢,对于钢腐蚀影响相比于未添加SRB的基本没变化。而浓度达到2%SRB时,钢表面生长活动明显旺盛,对于钢的腐蚀倾向性显著增加,相比于不添加SRB的钢腐蚀速率增长了4倍多,钢表面多处生成斑状黑褐色腐蚀产物FeS。但2507双相不锈钢在含SRB冷却水中腐蚀程度均属于轻度腐蚀,且点蚀电位高达约1.2 V。