1、J Fail. Anal. and Preven. (2011) 11:611617DOI 10.1007/s11668-011-9482-8案例历史同行评审管板焊缝裂纹的裂纹 燃气 /蒸汽换热器维修提交：10 十一月 2010 / 2011 / 14 修订的形式：六月在线发表：2011 七月 6 ASM 国际 2011摘要严重的裂缝被发现在管子与管板焊缝开裂的启动燃气/蒸汽在气体厂热交换器操作期间。失效分析及修复过程是本文报道。几个月的运行修复后的修复是成功的。关键词热交换器管接头焊缝裂纹故障背景在这项研究的对象是一个修复焊接管管板接头上的裂纹的燃气/蒸汽在气体厂换热器。换热器是九个相同的管
2、壳式换热器，固定管换热器类型（1 壳 1 管通型），由同一制造商，与气体在管侧被冷却的壳侧蒸汽裂解过程。设计温度和壳侧压力分别为 653 LF（LC 345）和 1812 磅（127.4 公斤/平方厘米）。在换热器管束由 484 管固定在水平位置。冷拔无缝管是由 SA 213-t11 钢1.75。OD（ 44.45 毫米）9 7 BWG（4.57 毫米）9 21 英尺（6.4 米）LG 。管板是由 SA 182-f11 Cl2 钢 1.5。（38 毫米）厚。，归1600 LF 8 h，1325 锻炼 LF 8 h，用空气作为冷却剂。壳体的内径是 55.75。（1416 毫米）和由 SA 516
3、-70n 钢。该管的管板的填充材料是钨极氩弧焊焊丝 er80s-b2。S. AdullahRamsis Engineering, P.O. Box 28967, East Riffa, Bahrain e-mail: .bhH. M. Ezuber (&)College of Engineering, University of Bahrain, P.O. Box 32038, Isa Town, Bahraine-mail: hzubeireng.uob.bh换热器已被送到工地，在天然气厂，据报道，通过每一焊道的染料渗透剂检查后，25 psig的空气，在膨胀试验的肥皂，着色渗透探伤轧制后，着
4、色渗透探伤焊后热处理（热处理），和 3031 psi 的壳程水压试验。在煮，入口被暴露在 300 液晶的最高温度，而出口达到220 LC。九的换热器，泄漏已被发现在两个管板焊缝的两个换热器在启动的操作。裂缝是在入口目前，管与管板焊接。它通常是一种常见的做法在管子与管板的密封焊缝是用来补充胀管接头确保气密性 1，2 。管接头是常见的焊接时，管子管板是合适的材料 1 。在实践中，大多数是多道焊缝。焊接接头往往是热处理（预热和焊后热处理）使用前提高延展性和韧性，降低焊接应力 3 。填充材料成分应该是几乎相同的基体金属确保大学形成强度和耐热耐腐蚀 3，4 。应力腐蚀开裂是一种常见的故障模式在管子管板接
5、头。失败是不当的胀管主要与异常之间的间隙，管子与管板 5 ，拙劣的管子管板焊缝或缺陷在现场焊后热处理 6 。123612 J Fail. Anal. and Preven. (2011) 11:611617本研究的目的是确定故障原因，确定故障的机制，并给出一个解如何失败是可以避免的。失效分析试验包括目视检查和微观检查，SEM，力学性能测试（硬度），和非破坏性测试（PT）进行。基于失效分析结果和建议进行了修复手术。修复后的换热器一直有效地为多个月没有焊缝裂纹的艾莱依。修复程序失效分析在这篇文章中概述。检查现场目视检查对失败的换热器的现场检测表明，换热管与管板焊缝泄漏图 1 泄漏出现在管与管板焊接
6、（图 1）。使用铂的泄漏区密切调查显示许多裂纹在管与管板焊接。应力裂纹近似直线，直线和横向的类型（图 2 3）从管板与提取的样品和无故障报告，包含两个管子管板焊缝和它们之间的管板韧带，被送到实验室调查。结果结果如下：金相试验Examination on samples contained tube-to-tubesheet revealed visible cracks on most of the specimens surfaces. The cracks were through-wall and were circumferentially oriented. Observations
7、 on the cross sections of the welds are given in Fig. 3. The figure shows side-by-side images of the vertical cross sections of a tube-to-tubesheet weld. It is apparent from this figure that welds did not fully con-sume the chamfered area of the weld prep. Cracks were common in the areas where the w
8、eld preps were not con-sumed in the welding process. The initiation of the through-wall cracks took place in the area of the weld root in the tube-to-tubesheet joint and propagated in the weld metals. This side is submerged in the steam side of the tubesheet. Parallel cracks (Fig. 3, right) appeared
9、 to run side-by-side for much of the length of this through-wall crack. No cracks were found at the tube or tubesheet materials.SEMEDS AnalysisFigure 4 represents the EDS spectra for the chemical compositions of the tube metal, tubesheet base metal, and图 2 裂纹出现在管与管板焊接（左：横向裂缝；右：线性裂纹）J Fail. Anal. and
10、 Preven. (2011) 11:611617 613Fig. 3 Enlarged views on polished cross sections showing the through-wall crackthe deposited weld metal. The results presented in this figure clearly show that the chemical analysis of tube, tubesheet, and the deposited weld metal are virtually identical. The chemical co
11、mpositions are consistent with what be expected for the SA 213-T11, SA 182-F11 CL2, and ER 80S-B2 materials. Quantitative chemical analysis of the tube, the tubesheet, and the weld material is given in Table 1. The chemical composition of the materials com-plies with the manufactures specifications,
12、 and with ASTM specifications A387 Grade 11 (cover plate) and ASTM A182 (pipes), which are correct for the type of service sought 3. It is generally common to note that the carbon content of the filler metal is lower than that of the base metal. The average values of tensile test, yield, and elongat
13、ions given in Table 2 are typical values of the steel materials.Figure 5 is an overall SEM image of a polished cross section of a dialed sample. A through-wall crack in a deposited weld is observed. The crack propagation is quite straight and there is no evidence of ductile behavior at either end of
14、 the crack.MicrohardnessTables 3 and 4 show approximate median hardness values for various regions (tube-to-tubesheet: starting with the tube base material, passing through the HAZ, the weld root, another segment of the HAZ, and finally in the deposited wed metal) along the maximum measured hardness
15、. The weld metal and HAZ show much higher hardness than the base metal. Some interesting differences are systematically observed in hardness values of HAZ of the tube and tube sheet materials. Hardness of HAZ of the tube material is less than that of the tubesheet material. This denotes scanteffecti
16、veness of the stress relieving treatment 4. The high hardness in the interface between weld and the tube HAZ are reported to be related to microstructural embrittlement 6.Possible Root CauseThe results of the chemical analysis show that there are no apparent defects in the specification of the weld
17、material, the tube material, or the tubesheet material, i.e., no dis-crepancies with manufactures specifications were found in chemical composition. The diameter of the tubesheet hole for tubes has uniform and proper diameter through the whole thickness of the tube (i.e., normal clearance between th
18、e tube and the tube-sheet) and considered to be adequate and complies with the design specification. As a result, fabrication error has been ruled out as the root cause of the problem. Thermal calculation records confirm that the exchanger is operated in nucleate boiling mode. A film and/or vapor bl
19、anketing of tubes is not possible. The tube wall temperature is very close to boiler feed water (BFW)-saturated temperature or shell temperature. Based on these analyses, no excessive tube thermal expansion in relation to shell is expected. The vapor blanketing or film boiling is unlikely considered
20、 as a cause of the tube weld failures experiences. The root cause of the cracks is pos-sibly attributed to poor workshop in the tube-to-tubesheet welds. Less than adequate weld throat thickness appeared to be the factor that governed whether or not a particular tube-to-tubesheet weld remained intact
21、. This is possibly caused by incorrect welding condition (incorrect torch/gun angle and incorrect edge penetration). The incorrect weld design and fabrication (incorrect weld size and quality) of the weld joints thus results in an undetected lack of fusion123614 J Fail. Anal. and Preven. (2011) 11:6
22、11617Fig. 4 EDS results showing typical chemical compositions for tube and tubesheet materialsin welded joints. Although the exchanger underwent suc-cessful hydro test and further dye penetrant examinations before the start-up of the operation, the lack of fusion in these routine testings is usually
23、 not detected.The initiation and propagation of cracks appear to occur as a result of mechanical stress applied to the materialduring the commissioning of the exchanger, (during the start-up of the process, the boiling water coming out of the cycle at the shell side is possibly caused by thermal exp
24、ansion of the tubes). This in turn results in mechanical stress on the welding joints leading to the initiation and propagation of cracks at defective weld joints. The storage123J Fail. Anal. and Preven. (2011) 11:611617 615Table 1 Average chemical composition (wt.%) for tube, tubesheet, and filler
25、materials for the heat exchanger steels materialC Mn Si S P Cr Mo Ni Al Cu FeSA213-T11 0.10 0.452 0.65 0.018 0.009 1.23 0.49 0.07 0.022 0.17 Bal.SA182-F11CL2 0.11 0.54 0.57 0.003 0.01 1.13 0.50 0.14 0.032 0.15 Bal.ER80S-B2 0.09 0.55 0.48 0.006 0.012 1.35 0.55 0.15 Bal.Table 2 Mechanical properties f
26、or tube and tubesheet materials for the heat exchanger steels materialsAlloy Tensile test, psi 2% yield, psi Elongation, %SA213-T11 65,950 54,500 57.8SA182-F11CL2 71,600 40,200 34Fig. 5 SEM image of a polished cross section of the specimen showing the through-wall crack in deposited weld metalRemove
27、 all the cracks by machiningPT examinationSteam cleaning/ surface dryingPreheating 150-200oC(1)(3)(2)Preliminary welding 2nd pass welding(root pass)3rd welding passHeat Treatment (350oC/2hr + air cooling 70oC/hr)+ Rest for 48 hoursPostweld heat treatment 660oC/2hrsPT examinationTube expansionFinal P
28、T examinationTable 3 Approximate median and maximum hardness values for tube materialTube (left) Median, HV Maximum, HVWeld body 280 286HAZ 1 200 203Weld Root 280 203HAZ 2 190 205Base material 145 153Table 4 Approximate median and maximum hardness values for tubesheet materialTubesheet (right) Media
29、n, HV Maximum, HVWeld body 280 288HAZ 260 280Base material 185 221Hydro-testFig. 6 Flowchart of repair procedureof hydro test water (under a pressure) in the heat exchangers for a period of more than 6 months may play a role in the crack mechanism failure.Repair of the Tube-to-Tubesheet WeldmentsIn
30、order to eliminate the crack failures, the defective tube-to-tubesheet welds was removed and replaced with prop-erly designed and installed good quality welds.The repair procedure is shown in Fig. 6 and is described as follows:(i) All the tube-to-tubesheet welds were removed by machining. 123616 J F
31、ail. Anal. and Preven. (2011) 11:611617Fig. 7 Schematic diagram for preheating, welding range, and post-heating temperaturetime profileFig. 9 Schematic diagram for post weld heat treatment temperature time profileFig. 8 Schematic diagram for preliminary heating process(ii) PT examination was conduct
32、ed to insure that no cracks were developed during machining. (iii) Cleaning the surface using pressurized steam: This process was aimed to clean weld grooves area around the tubes and to remove all the traces of dye-penetrant residue. The cleaning process, however, was followed by drying process usi
33、ng compressed air. (iv) Preheating the surface at a temperature range of 150 (min)200 LC (max) as per ASME code B31.3, Fig. 7. The heating process was conducted using heating elements (ceramic fiber and insulation attached to mesh with stainless steel wire, Fig. 8). The preheat treatment is needed t
34、o prevent harden-ing and weld cracking. Preheating reduces stresses, limit, or temper martinstics areas, and reduce the amount of hydrogen retained in the weld 3. (v) Welding process (multipass welding). This process started by carrying out a preliminary welding (root pass). A filler material GTAW w
35、ire ER80S-B2 was used to obtain similar properties to the substrate. After conducting the preliminary welding process, the surface was subjected to heat treatment (HT) at 350 LC for 2 h followed by gradual air cooling with a rate of 70 LC/h (max). The surface was then left for 48 h to insure that no
36、 hydrogen damage is developed. Second- and third-pass welding proce- dure were followed as similar to the preliminary step (heating at 150200 LC, welding, HT at 350 LC, air cooling with 70 LC/h, followed by 48 h between each welding pass). Preheating along with heat treatment is required to minimize
37、 the detrimental effect of high temperature and severe thermal gradients in welding 7. (vi) Postweld heat treatment (PWHT) at 660 LC, soaking the metal at this temperature for 2 h, followed by air slow cooling with a rate of 70 LC/h as per applicable codes and clients specific request (Fig. 9). The
38、post heat treatment to a temperature of 600700 LC is required to soften the HAZ, remove the residual hydrogen, restore the toughness, stabilize the micro-structure of the weld metal, and reduce the welding stresses 3. In CrMo alloy steels, the weld transforms uniformly to martensite or bearlite. It
39、will then normally be subjected to a PWHT which has the effect of tempering and stress relieving the joint as a whole 8. (vii) Tube expansion was then applied as a method of sealing tube-to-tube sheet joints. (viii) Final PT examination was conducted to ensure no cracks were developed during the welding or expansion procedure. 123
