Effect of fast cooling on microstructure and toughness of heat affected zone in high strength offshore steel

E690 is a newly developed high strength high toughness steel for offshore structures. The effect of different cooling modes on the microstructure and toughness of the heat affected zone in E690 weldment were investigated in this work. The outcome of microstructural examinations and mechanical tests showed that fast cooling immediately after submerged welding can reduce the width of heat affected and coarse grained zones, as well as improving the low temperature impact toughness. It was also shown that reduction in the grain size of the parent austenite and having lower amounts of retained austenite within bainitic structure are the main causes of the observed improvement.     

热处理对含铬低碳钢组织与性能的影响

研究了低碳含铬钢950℃淬火后650℃回火,不同的保温时间下,Cr对低碳合金钢调质后性能的影响,并和不加Cr相同成分的钢作了对比。研究发现,当保温时间以2.5 min/mm计算时,加Cr钢的力学性能低于不加Cr同成分的钢,金相组织观察发现Cr能够有效提高低碳合金钢的淬透性,淬火后即使钢板的心部也能获得贝氏体组织,为后续的回火做好组织准备。然而在回火后容易形成M23C6及M7C3碳化物,其稳定性较差,容易聚集长大,且形态为条状,拉伸时容易弯曲破裂,严重破坏钢板的连续性,降低钢板的强度及塑性。对不同回火保温时间的含Cr钢的二次相粒子析出行为研究发现,以1 min/mm计算保温时间时,没有发现含Cr碳化物的析出,但随着保温时间的增加,Cr钢中的含Cr碳化物快速析出,并迅速长大,尤其是以3 min/mm计算保温时间下,含Cr碳化物尺寸已经达到了300 nm。因此对含Cr低碳合金钢,必须采用合适的调质工艺,适当缩短回火保温时间,以降低碳化物的聚集长大,避免回火后强度和塑性的同时降低。     

焊接热循环对09MnNiDR钢热影响区低温韧性的影响

利用Gleeble-3800研究了焊接热循环对09MnNiDR钢焊接热影响区粗晶区（CGHAZ）和中间临界再热粗晶区(IRCGHAZ)低温韧性的影响. 结果表明,热输入为15 kJ/cm、层间温度为150 ℃时,CGHAZ组织形态为板条状马氏体+下贝氏体,下贝氏体的存在限制了马氏体的生长,提高了低温韧性,而IRCGHAZ继续保持了CGHAZ的组织. ?70 ℃冲击试验中,IRCGHAZ相比于CGHAZ具有较好的低温冲击韧性,热输入为15 kJ/cm、层间温度为150 ℃时,冲击吸收能量最高为65 J. 根据热模拟结果,采用焊接热输入15 ~ 22 J/cm、层间温度为150 ℃的工艺参数对09MnNiDR钢进行焊接,?70 ℃冲击试验中热影响区冲击吸收能量值为101 J,冲击断口存在大量的等轴韧窝,具有较好的低温韧性;?70 ℃拉伸试验屈服强度为477 MPa、抗拉强度607 MPa、断后伸长率为28.5%,表现出较好的强度和塑性;硬度试验结果表明母材、焊缝和热影响区硬度依次增大,且没有软化现象.     

Effect of Ni addition on toughness and microstructure evolution in coarse grain heat affected zone

A Gleeble simulation was performed on low carbon steels to investigate the effect of Ni addition on toughness and microstructure evolution in coarse grain heat affected zone from low heat input welding. Ni addition increased the impact toughness at ?20 °C and the prior austenite grain size.Microstructures of the low Ni and high Ni steel show noticeable differences in crystallography. Lath-structures in the two can be defined as crystallographic packet. In low Ni steel the packet consisting of acicular ferrite and lower bainite has all the three Bain groups uniformly distributed; in high Ni steel the packet consisting of only lower bainite has a dominant Bain group within it, making the other two groups subsidiary. The closely linked M/A particles were found only in low Ni steel by tracking the orientation information. It is able to facilitate the fracture through the link-up of the micro-cracks initiated by each M/A particle. In the going direction of the link-up there is no high angle boundary which can deflect the propagation of crack, thereby in this case the high angle boundary in low Ni steel is relatively ineffective. The mechanism of the toughness improvement brought by Ni addition should be ascribed to the relatively uniform transformation to final microstructure, which reduces the appearance of closely linked M/A.     

含铜时效钢焊接粗晶区组织与韧性分析

研究了含铜时效钢焊接粗晶区的组织与韧性,并测定了含铜时效钢焊接粗晶区连续冷却转变曲线(SHCCT曲线).结果表明,冷却速度对含铜时效钢焊接粗晶区的组织和韧性有显著影响.随着冷却速度的增加,粗晶区的组织由粗大的粒状贝氏体转变为细小的板条贝氏体;当t8/5≤40s时,焊接粗晶区无塑性转变温度低于-50℃,可获得良好的低温韧性.当t8/5＞40s时,焊接粗晶区冲击韧度显著降低,大尺寸M-A组元的增多是导致含铜时效钢韧性恶化的重要原因.     

碳化钒析出对X80管线钢焊接热影响区韧性的影响

通过Gleeble-3500热模拟机对钒(V)含量分别为0.06%和0.02%的X80管线钢进行了峰值温度为1350℃和750℃的焊接热模拟试验,利用硬度计和光学显微镜(OM)分别分析了焊接热模拟试样硬度和试样中马氏体-奥氏体岛状组织(M-A)的数量、尺寸和形貌。利用透射电镜(TEM)对母材、粗晶区和部分相变区中粒子析出进行了表征。结果表明,与0.02%含V量X80管线钢相比,0.06%含V量的X80管线钢由于V含量较高,在受到多道次焊接热循环影响时,部分相变区有较大量碳化钒(VC)粒子析出,明显阻碍位错运动,使其强度明显提高的同时脆性增大、塑性变形难以进行,导致其焊接热影响区韧性下降。     

Effect of carbon content on creep rupture strength and microstructure in heat affected zone of heat resistant ferritic steel: alleviation of decrease in creep rupture strength in heat affected zone of heat resistant ferritic steel

Abstract

The effect of the carbon content on the creep rupture strength and the microstructural change in weld heat affected zone (HAZ) during high temperature services was investigated in order to alleviate the decrease in the creep rupture strength in HAZ of heat resistant ferritic steels. The test specimens were prepared from 9Cr–3Co–3–W–V, Nb ferritic steel plates with carbon content ranging from 0˙005 to 0˙1%. After the simulated HAZ thermal cycle treatments at the peak temperature of 1273 K was applied on these specimens, creep rupture tests, aging tests and microstructural examinations were conducted. As a result, it was clarified that the creep rupture time of simulated HAZ became longer and the decrease in the creep rupture strength in HAZ was alleviated by decreasing the carbon content. Then the mechanism to explain the effect of the carbon content was discussed from a viewpoint of the growth of precipitates, such as M₂₃C₆ and MX, during long term heating.     

热模拟微钙C-Mn钢焊接热影响区的组织及其对韧性的影响

分别研究了微钙C-Mn钢中分布的弥散的热稳定的第二相CaO粒子对焊接热影响粗晶区(CGHAZ)奥氏体晶粒的细化效果和CGHAZ显微组织中的M-A组元对冲击韧性的影响.研究结果表明:热稳定的第二相CaO粒子对焊接CGHAZ奥氏体晶粒的细化效果显著;当冷却速度较快时M-A组元主要以长条状分布,使材料的韧性降低,当冷却速度慢时M-A组元为颗粒状CGHAZ韧性较低,当中等冷却速度时M-A组元由长条状向颗粒状过渡CGHAZ韧性较好.     

回火温度对低碳贝氏体管线钢低温韧性和组织的影响

通过力学性能测试和微观组织分析,研究了回火温度对低碳贝氏体X80管线钢组织及低温冲击韧性的影响。结果表明,低碳贝氏体X80管线钢在300 ℃回火2 h后达到最佳强韧性匹配,屈服强度在625 MPa,-40 ℃夏比冲击功Akv为315 J,冲击断口呈现明显的韧性断裂形貌,-60℃夏比冲击功Akv也达到了268 J。低碳贝氏体管线钢轧态组织以粒状贝氏体为主,经过300 ℃回火2 h后,组织与TMCP状态基本相似,仍保持粒状贝氏体组织,但是MA组元略细小;经过600 ℃回火2 h后,贝氏体出现粗化,并且出现多边形铁素体组织。低温韧性的改善是由于回火处理过程中富碳残留奥氏体发生转变,M/A 组元由岛状转变为点状及细条状,粒状贝氏体晶间细化的M/A组元更好的阻碍了裂纹的扩展。     

双道次激光电弧复合焊热影响区微观组织与冲击韧性

采用焊接热模拟技术制备了低合金高强钢双道次激光电弧复合焊热影响区的均匀化组织试样,研究了二次峰值温度对热模拟试样微观组织和韧性的影响. 结果表明,未转变粗晶区为粗大的板条马氏体,晶粒尺寸在84 ~ 98 μm之间. 超临界再热粗晶区为细小的板条马氏体,晶粒尺寸为15.7 ~ 19.2 μm. 临界再热粗晶区为晶界和亚晶界分布有块状M-A组元的板条马氏体. 亚临界再热粗晶区组织为板条马氏体,晶粒尺寸在79 ~ 88 μm之间. 示波冲击试验结果表明,临界再热粗晶区试样抵抗裂纹形成能力最低,临界再热粗晶区和未转变粗晶区试样抵抗裂纹扩展能力最差.     

V - N钢焊接粗晶区的组织和韧性研究

在不同焊接工艺下对V钢、V—N钢和V—Ti—N钢三种钢的焊接粗晶热影响区的组织和韧性进行了研究。利用Gleeble3500模拟粗晶区的焊接过程，将锻后试样重新加热到峰值温度1350℃后给以不同的热输入量并以相应的t8/5冷却速度进行冷却。结果表明热输入量高时容易得到粒状贝氏体和晶界上的侧板条铁素体组织。随着热输入的降低会出现大量的多边形铁素体和晶界铁素体，并且明显长大。对于含氮量较高的V—N钢来说，容易形成马氏体-奥氏体岛，这种组织降低了粗晶区的韧性。在高氮的情况下添加另一种微合金化元素Ti，钛可以提高HAZ的相变温度，使铁素体和贝氏体连续冷却转变曲线的鼻点左移，细化奥氏体晶粒，促进铁素体和贝氏体的形核，改善粗晶区的韧性。     

低合金高强钢激光电弧复合焊热模拟热影响区组织与冲击韧性

采用焊接热模拟技术制备了低合金高强钢激光电弧复合焊热影响区的均匀化组织试样,并结合示波冲击试验和组织精细化表征技术分析了热模拟试样组织与韧性之间的关系.?结果表明,热模拟粗晶区、细晶区组织为板条马氏体,临界区组织为马氏体和晶界碳化物,亚临界区组织为回火马氏体;峰值温度对热模拟试样裂纹形成功影响不大,对裂纹扩展功具有较大...     

铌对高强结构钢大热输入焊接热影响区组织和性能的影响

采用气电立焊和热模拟试验研究了铌对610MPa级高强结构钢大热输入焊接热影响区组织和性能的影响。结果表明,在100kJ／cm的大热输入条件下,铌对实际接头热影响区宽度的影响不明显,其均为7．5～8mm,但铌可加大粗晶区宽度约1．5min,同时不含铌钢粗晶区韧性均高于含铌钢。热模拟试验时在峰值温度1200℃以上,不含铌钢韧性远高于含铌钢;低于1200℃时,随峰值温度降低两者差值逐渐缩小。高的峰值温度下碳氮化铌溶解增加,固溶铌可通过对界面运动的溶质拖曳效应阻碍铁素体长大动力学,促进粗大板条贝氏体的形成：铌的添加不利于大热输入条件下高强结构钢的焊接性能。     

热处理工艺对高碳铬轴承钢组织和性能的影响

研究了不同热处理工艺(淬回火、贝氏体等温淬火、马氏体预淬火后贝氏体淬火和贝氏体变温淬火处理)对典型高碳铬轴承钢GCr15的组织以及硬度、抗拉强度、冲击功和耐磨性等力学性能的影响,对比分析了不同热处理工艺的微观组织和性能的关系;为了更好地分析下贝氏体短时处理和尺寸稳定性,采用膨胀法进行了试验研究;利用XRD分析,研究了不同热处理工艺后残留奥氏体的量。研究表明,与常规淬回火相比,得到贝氏体组织的处理过程可以得到组织和性能的最佳匹配;贝氏体处理后,试样综合性能优于淬回火处理且尺寸稳定性好;马氏体预淬火处理和贝氏体变温处理在提高钢综合性能的同时又能不同程度缩短贝氏体转变时间。     

Observation of twinned martensite in weld heat affected zone of Grade 100 low carbon microalloyed steel

Abstract

The microstructural changes caused by a low nominal heat input of 0.5 kJ mm^-1 in the coarse grained heat affected zone (CGHAZ) of Grade 100 microalloyed steel were investigated. Microhardness measurements suggested that the CGHAZ was martensite of maximum theoretical hardness for the carbon content of the steel. The bulk of the CGHAZ was lath martensite containing none of the small and few of the intermediate sized Nb precipitates responsible for strength and grain size control in the steel plate. Twinned martensite was unexpectedly observed in local areas of the CGHAZ. The formation of twins, which are normally seen in steels with a higher level of carbon, is explained by a combination of the rapid heating rates, high peak temperatures, precipitate dissolution and dispersion, and rapid cooling rates.     

高强度桥梁钢焊接粗晶热影响区中粒状贝氏体对韧性的影响

采用热模拟试验机模拟高强度桥梁钢Q460q不同焊接工艺参数t8/5条件下的CGHAZ（粗晶热影响区）,并分析了CGHAZ的显微组织及组织中的粒状贝氏体对韧性的影响。结果表明,当t8/5〈60 s时,组织中板条贝氏体和粒状贝氏体相互交错,且粒状贝氏体内的脆性长条状M-A组元较少,使韧性保持较高的水平。当t8/5〉100 s后,晶粒粗大,组织中先共析铁素体和粒状贝氏体量不断增加,M-A组元尺寸变大从而使裂纹扩展中的有效晶粒尺寸粗大,导致韧性显著下降。     

板条M/B组织对低碳马氏体钢强韧性的影响

采用ANSYS有限元模拟、彩色金相技术并利用背散射电子衍射(EBSD)的方法分析板条M/B混合组织对10CrNi5Mo低碳马氏体钢强韧性的影响。结果表明,特厚板厚度方向各部位冶金质量差别很小。淬火冷却条件下,钢板表面获得单一马氏体组织,1/4部位和心部得到板条M/B混合组织。板条M/B混合组织较单一马氏体组织具有更小的亚结构和更长的大角晶界是造成心部获得优良低温韧性的原因。板条M/B混合组织能显著提高10CrNi5Mo钢的低温韧性,降低韧脆转变温度,但对强度影响不明显。     

热输入对Q500CF钢板焊接热影响区组织和冲击性能的影响

通过埋弧焊试验和Gleeble热模拟试验研究了热输入对Q500CF钢热影响区(HAZ)组织和冲击性能的影响.埋弧焊结果表明,热输入为15～ 50 kJ/cm时,HAZ的-20℃冲击吸收功大于等于146 J;HAZ中熔合线(FL)处冲击吸收功最低且随热输入增大而减小.组织观察表明,随热输入增加,粗晶区组织由15 kJ/cm时的板条贝氏体(LB)和粒状贝氏体(GB)转变为50 kJ/cm时的GB组织;临界粗晶区在晶界上出现了大量l ～7 μm的M-A组元,导致低温冲击韧性恶化.Gleeble热模拟结果表明,热输入为50 ～ 70 kJ/cm时,粗晶区GB组织粗化并导致该区冲击韧性恶化.因此为确保多道焊焊接接头HAZ低温冲击韧性,焊接热输入应限制在15～50 kJ/cm之间.