Microstructure and creep strength of welds in advanced ferritic power plant steels

Abstract

The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 10⁴ h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to A_C3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.     

Suppressing type IV failure via modification of heat affected zone microstructures using high boron content in 9Cr heat resistant steel welded joints

Abstract

Creep rupture strength at 923 K and microstructural evolution of welded joints have been investigated for high boron–low nitrogen–9Cr heat resistant steels developed at the National Institute for Materials Science (Japan). Welded joints were prepared from plates containing 47–180 ppm boron using gas tungsten arc welding and Inconel type filler metal, and showed superior creep properties to those of welded joints of conventional high chromium steels such as P92 and P122. No type IV failure was observed in the boron steel welded joints. A large grained microstructure was observed in the heat affected zone heated to Ac ₃ (Ac ₃ HAZ) during welding, whereas the grains are refined at the same location in conventional steel welded joints. The simulated Ac ₃ HAZ structures of the boron steels have a creep life almost equal to that of the base metal. Large grained HAZ microstructures and stabilisation of M₂₃C₆ precipitates are probable reasons for suppression of type IV failure and improved creep resistance of the boron steel welded joints.     

Relationship between loss of creep rupture strength and microstructure in the heat affected zone of heat-resistant ferritic steel

High-Cr heat-resistant ferritic steel used at elevated temperatures in the main steam lines of thermal power-generating plants is generally recognised to sustain greater loss of creep rupture strength in the HAZ of welded joints than the base metal in an effect known as HAZ softening.¹ This phenomenon is characterised by rupture in the fine-grained HAZ outside the coarse-grained region (type IV rupture), as shown in Fig. 1.     

Creep and damage investigation of advanced martensitic chromium steel weldments for high temperature applications in thermal power plants

Abstract

With the aim to increase base material creep strength and overcome the type IV cracking problem, a new design concept was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening through solid solution with precipitation strengthening by finely dispersed nitrides. In this work, uniaxial creep tests of the MARBN base material and welded joints have been carried out. The creep strength of the welded joints was analysed, and the evolution of creep damage was investigated. The creep tests of MARBN revealed increased strength of the base material of about +20% compared to the best commercially available 9Cr steel grade. At higher stress levels, the creep strength of crosswelds is between that of the MARBN base material and the conventional 9Cr base materials. Nevertheless, long term creep tests revealed a drop in creep strength of the MARBN welded joints. The underlying phenomena of crossweld creep behaviour are discussed in detail.     

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.     

Analysis of the Creep Behavior of P92 Steel Welded Joint

Different regions of heat-affected zone (HAZ) were simulated by heat treatment to investigate the mechanisms of the Type IV fracture of P92 (9Cr-2W) steel weldments. Creep deformation of simulated HAZ specimens with uniform microstructures was investigated and compared with those of the base metal (BM) and the weld metal (WM) specimens. The results show that the creep strain rate of the fine-grained HAZ (FGHAZ) is much higher than that of the BM, WM, the coarse-grained HAZ (CGHAZ), and the inter-critical HAZ (ICHAZ). According to the metallurgical investigation of stress-rupture, the FGHAZ and the ICHAZ have the most severely cavitated zones. During creep process, carbides become coarser, and form on grain boundaries again, leading to the deterioration of creep property and the decline of creep strength. In addition, the crack grows along the FGHAZ adjacent to the BM in the creep crack growth test (CCG) of HAZ.     

铁素体-奥氏体异种钢接头的界面组织及力学性能

为提高电站锅炉过热器铁素体－奥氏体异种钢焊接接头的高温蠕变断裂强度和服役寿命，设计了专用的镍基填充材料及组合焊接接头。通过对不同焊缝接头的高温蠕变力不试验，焊缝界面组织变化分析，碳扩散和热应力的测试分析，认为在高温低力的条件下，接头焊缝界面及热影响区碳元素扩散迁移和碳化物聚集是影响接头蠕变断裂强度的重要原因。     

Modelling creep rupture strength of ferritic steel welds

Abstract

Recent changes in the design of steam turbine power plant have necessitated the replacement of bolted flanges with welded joints. The design process therefore requires a knowledge of the creep rupture strength of the weld metal consumed in the welding process. This paper presents a method which can be used to estimate the creep rupture strength of ferritic steel weld metals, from a knowledge of the creep strength of wrought plates. The method is validated using published data.     

EFFECT OF CARBON MIGRATION ON CREEP PROPERTIES OF Cr5Mo DISSIMILAR WELDED JOINTS WITH Ni-BASED AND AUSTENITIC WELD METAL

In this paper, the effect of carbon migration on creep properties of Cr5Mo dissimilar welded joints with Ni-based (Inconel 182) and Cr23Ni13 (A302) austenitic weld metal was investigated. Carbon migration near the weld metal/ferritic steel interface of Cr5Mo dissimilar welded joints was analyzed by aging method. Local creep deformations of the dissimilar welded joints were measured by a long-term local creep deformation measuring technique. The creep rupture testing was performed for Cr5Mo dissimilar welded joints with Inconel 182 and A302 weld metal. The research results show that the maximum creep strain rate occurs in the decarburized zone located on heat affect zone (HAZ) of Cr5Mo ferritic steel. The creep rupture life of Cr5Mo dissimilar welded joints with A302 weld metal decreases due to carbon migration and is about 50％ of that welded with Inconel 182 weld metal.     

P92钢焊接接头蠕变本构关系

通过热处理方法模拟焊接热循环,在 3.53 K/s 的升温速度下制备 P92 钢焊接热影响区各微区的模拟组织试样,通过单轴蠕变试验获得 923K 下 P92 钢熔敷金属、热影响区各微区和母材组织不同应力水平下的单轴蠕变数据,用最小二乘法拟合得到 P92钢焊接接头各区域组织在稳态蠕变条件下的蠕变本构方程.结果表明,相比焊接接头其它微区组织,试验应力较低时,细晶区的蠕变应变率最大,因此在低应力下细晶区容易发生蠕变开裂;随着应力水平的增加,焊接接头和母材蠕变速率逐渐超过细晶区,蠕变断裂易发生在焊缝或母材上.

Abstract：

The welding heat thermal cycle was simulated through the heat treating method, and the simulated microstructures of the coarse grain heat-affected zone (CGHAZ), fine grain heat-affected zone (FGHAZ) and intercritical heat-affected zone (ICHAZ)of P92 steel were prepared by using a furnace at 3.53 K/s of heating rate. The uniaxial creep data of the base metal, deposited metal and simulated CGHAZ, FGHAZ and ICHAZ specimens were obtained through the creep tests under the temperature of 923 K, and the creep constitutive equations of each material were obtained too. The result shows that the creep strain rate of FGHAZ is higher than the other zones of the welded joint under the low stress. So the creep rupture is easy to occur in te FGHAZ. In addition, the creep properties are obviously different among the CGHAZ, FGHAZ and ICHAZ, which indicate the HAZ has the obvious material heterogeneity.     

IMPROVEMENT OF TYPE IV CRACKING RESISTANCE OF 9Cr HEAT RESISTING STEEL WELDMENT BY BORON ADDITION

Creep lives of high Cr ferritic heat resisting steel weldments decrease due to Type Ⅳ fracture, which occurs as a result of formation and growth of creep voids and cracks on grain boundaries in fine-grained heat affected zone (HAZ). Because boron is considered to suppress the coarsening of grain boundary precipitates and growth of creep voids, we have investigated the effect of boron addition on the creep properties of 9Cr steel weldments. Four kinds of 9Cr3WSCoVNb steels with boron content varying from 4.7×10-5 to 1.8×10-4 and with nitrogen as low as 2.0×10-5 were prepared. The steel plates were welded by gas tungsten arc welding and crept at 923K. It was found that the microstructures of HAZ were quite different from those of conventional high Cr steels such as P91 and P92, namely the fine-grained HAZ did not exist in the present steel weldments. Boron addition also has the effect to suppress coarsening of grain boundary carbides in HAZ during creep. As a result of these phenomena, the welded join     

EVALUATION OF CREEP PROPERTIES FOR 316FR STEEL THICK PLATE WELDMENT USING MINIATURE CREEP SPECIMEN

Evaluation of creep properties of the welded joint through taking local fluctuation of the mechanical properties into consideration is experimentally or analytically seldom carried out. The purposes of the present study are to examine the surface strain distribution in the weld metal of a full thickness welded joint specimen and subsequently to investigate the local variation in the properties of the all-weld metal part of the joint using miniature specimens. A welded joint was prepared for 316FR steel plates by gas tungsten arc welding process using Mod. 316L filler wire. Creep tests were conducted at 823K in air using full thickness large welded joint specimens, HAZ and all-weld metal miniature specimens. From the results obtained, it is concluded that the creep properties of multi-layer welded joints strongly depend on the location of specimen sampling.     

管线钢焊接接头H2S应力腐蚀试验分析

采用楔形张开加载(WOL)试样,对X56、X65管线钢不同焊接工艺下焊接接头的焊缝金属和热影响区在H2S介质中的抗应力腐蚀性能进行了试验研究.结果表明,全部平行试样都发生了少量的裂纹扩展,当应力强度因子K达到临界值(KISCC),裂纹不再扩展.在文中采用的焊接工艺下制备的X56钢和X65钢焊接接头,热影响区的抗H2S应力腐蚀性能略强于焊缝金属,且焊缝金属和热影响区性能均低于文献中介绍的母材性能,可以认为是焊缝金属和热影响区粗晶区的晶粒粗化、焊接缺陷、焊接残余应力和焊缝的高匹配等因素造成的.     

Observation of type IV cracking in welded joints of high chromium ferritic heat resistant steels

Abstract

Type IV cracking in welded joints of high Cr ferritic heat resistant steels at 923 K is investigated in the present study. First, internal pressure creep tests were performed to investigate the features of cracking at 923 K and at stresses of 108–167 MPa. Results showed that at low stresses cracks occurred in the fine grained heat affected zone (FGHAZ), showing low deformation, a vast amount of creep voids and short creep life: these were identified to be type IV cracks. However, as the stress was increased the crack locations moved further away from the fusion line, the creep void area density decreased, and the slope of the main crack ratio curve increased. At high stress, cracks occurred in the base metal and were identified to be normal cracks. Second, creep tests were carried out on one pass welded specimens to clarify the relationship between the creep property deterioration in the FGHAZ and the weld thermal cycle. Type IV cracking was repeated successfully and results showed good correspondence with the position of cracks among the softened region, the highest creep void area density and the fracture location. Both internally pressurised and one pass welded specimens showed that at low stress the FGHAZ was the weak zone and creep voids played an important role in type IV cracking.     

Analysis of factors affecting type IV cracking in welded joints of high chromium ferritic heat resistant steels

Abstract

In the present work, creep tests on single pass welded joints were carried out to investigate the factors affecting the deterioration of the creep strength of weldments of high Cr heat resistant steels. In creep tests at a temperature of 923 K and stresses of 90 and 120 MPa, creep fracture occurred in the fine grained heat affected zone (FGHAZ) and was identified as type IV cracking. It was found that a peak temperature between Ac₁ and Ac₃ in the FGHAZ caused the observed creep property deterioration. The factors identified as leading to the deterioration are a fine grain structure, low hardness, and large precipitates. Further investigations confirmed the influence of the fine grain structure as a critical factor decreasing the creep rupture time. The hardness difference between the FGHAZ and the base metal is insignificant, especially at 923 K. However, SEM investigations revealed that many large precipitates are distributed on the boundaries of the martensite laths and prior austenite grain boundaries. It is considered that the large precipitates also have a significant influence on the creep strength deterioration.     

超长寿命区间16Mn钢焊接接头疲劳性能

使用自行研制的TJU-HJ-Ⅰ型超声疲劳试验系统装置进行了16 Mn钢母材和焊接接头试件的超声疲劳性能试验,以探索其在超长疲劳寿命区间的疲劳行为.结果表明,无论母材还是焊接接头,其S-N曲线都是一条连续下降的曲线;在106～107循环周次范围内焊接接头不存在传统概念上的疲劳极限,载荷循环周次超过107甚至109以后,试件依然发生疲劳断裂.在超长寿命区间,焊接接头疲劳强度远低于母材的疲劳强度.使用现有疲劳极限数据去设计工作在超长寿命区间的焊接结构件是很危险的.     

P91钢焊接接头Ⅳ型裂纹力学控制参量有限元模拟

采用有限元方法对马氏体耐热钢（P91）焊接接头在温度为600℃、应力为80MPa下的最大主应力、von Mises等效应力、等效蠕变应变和应力三轴度进行了数值模拟.结果表明,在接头上下坡口交界处细晶热影响区（FGHAZ）两侧的最大主应力和von Mises等效应力很高,蠕变变形主要集中在FGHAZ,等效蠕变应变的最大值位于FGHAZ的底部.受焊缝和母材的强烈拘束,上下坡口交界处FGHAZ内应力三轴度最大,容易形成蠕变裂纹,导致接头蠕变开裂.数值模拟结果与试验获得的IV型裂纹产生、扩展结果一致.因此采用应力三轴度表征IV型裂纹开裂比较合理.     

P92钢焊接接头Ⅳ型蠕变断裂特性

在600-650℃,100-240 MPa对用埋弧自动焊工艺制备的P92钢焊接接头进行高温蠕变实验,采用OM,SEM和TEM等研究焊接接头的Ⅳ型蠕变断裂特性.结果表明,P92钢焊接接头的Ⅳ型断裂发生在高温和低应力条件下,存在一个临界Larson-Miller参数LMP和临界应力,它们的值分别约为35.5和120 MPa;Ⅳ型断裂部位的变形很小,位于靠近临界热影响区的细晶区,即加热峰值温度在Ac₃附近,该部位显微结构退化为铁素体等轴晶及蠕变过程中Laves相在晶界析出和长大是影响Ⅳ型断裂的主要因素,M₂₃C₆粗化的影响较小;焊接接头Ⅳ型断裂是一种晶界孔洞聚集型蠕变断裂,孔洞在粗大Laves相附近形核,可用损伤晶界上孔洞面积分数f或孔洞面积分数a作为发生Ⅳ型断裂的微观判据,它们在650℃时的临界值分别约为0.5%和1.2%.     

P92钢焊接接头Ⅳ型蠕变断裂特性

在600—650℃,100—240 MPa对用埋弧自动焊工艺制备的P92钢焊接接头进行高温蠕变实验,采用OM,SEM和TEM等研究焊接接头的Ⅳ型蠕变断裂特性.结果表明,P92钢焊接接头的Ⅳ型断裂发生在高温和低应力条件下,存在一个临界Larson-Miller参数LMP和临界应力,它们的值分别约为35.5和120 MPa;Ⅳ型断裂部位的变形很小,位于靠近临界热影响区的细晶区,即加热峰值温度在A_C3附近,该部位显微结构退化为铁素体等轴晶及蠕变过程中Laves相在晶界析出和长大是影响Ⅳ型断裂的主要因素,M₂₃C₆粗化的影响较小;焊接接头Ⅳ型断裂是一种晶界孔洞聚集型蠕变断裂,孔洞在粗大Laves相附近形核,可用损伤晶界上孔洞面积分数f或孔洞面积分数a作为发生Ⅳ型断裂的微观判据,它们在650℃时的临界值分别约为0.5%和1.2%.     

10Cr9Mo1VNbN/12Cr1MoV异种钢焊接接头的蠕变损伤及界面失效

针对马氏体耐热钢 10Cr9Mo1VNbN和珠光体耐热钢 12Cr1MoV ,采用TGS -9cb、H10Cr5Mo、TR31三种焊丝形成高、中、低匹配焊接接头 ,利用脉冲电流氩弧焊、高温加速模拟工况、力学性能试验及电子显微分析 ,研究了 10Cr9Mo1VNbN/ 12Cr1MoV异种钢焊接接头的力学性能、高温强度、界面蠕变损伤及失效特征。试验结果表明 ,焊前预热 2 5 0℃ ,焊后回火 75 0℃× 1h ,加速模拟运行 5 0 0h后 ,高匹配和中匹配接头的力学性能劣化明显 ,界面蠕变损伤及失效倾向较大 ;而低匹配接头的力学性能优于高匹配和中匹配接头 ,界面蠕变损伤较轻 ,失效倾向较小。因此 ,选用TR31焊丝作为10Cr9Mo1VNbN/ 12Cr1MoV异种钢焊接接头的填充材料较为合适。