Estudo de Caso Fracture Flash Butt

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    Case study

    Fracture analysis of U71Mn rail flash-butt welding joint

    Xuemei Yu a,b, Lichao Feng a,*, Shijie Qin c, Yuanliang Zhang a, Yiqiang He a

    a School of Mechanical Engineering, Huaihai Institute of Technology, Lianyungang 222005, ChinabJiangsu Province R&D Institute of Marine Resources (Lianyungang), Lianyungang 222001, Chinac School of Naval Architecture and Ocean Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

    1. Introduction

    Because of convenience, high efficiency, large volume and low transport cost, rail transport obtains high attention all over

    the world and gets very rapid development. Since the 1960s, the rail transport was developed toward the direction of high

    speed, high efficiency and large-capacity. Because of the high demands for the rail transport, the safety of the rail transport

    was directly threatened by the rail fracture accidents [13]. Since shock and vibration can be reduced effectively by

    continuously welded rail, which can ensure the safety and stability of the rail transport, it is widely applied at present.

    Continuously welded rail is laid by using the seamless welded rail. How to achieve an effective and stability welding is the

    key to the continuously welded rail laying. Practice shows that the rail welding joint is a weak link in the continuously

    welded rail, which directly affects the service life of the continuously welded rail. And most of the continuously welded railfracture comes from the fracture of welding joint[3,4]. For this reason, many researchers paid much attention to the failure

    fracture causes and preventive measures of the continuously welded rail [58]. It can be seen that, improving the mechanical

    properties of the welding joints, such as strength, toughness and so on, in order to avoid premature failure fracture is of vital

    significance for modern rail transport.

    In this paper, the U71Mn rail which was welded by flash-butt welding, broke at the welding joint after a period of service.

    The rail fracture surface was studied by analyzing the macroscopic and microscopic morphology and the chemical

    composition. The cause of the rail fracture was indicated and the preventive measures were proposed.

    Case Studies in Engineering Failure Analysis 4 (2015) 2025

    A R T I C L E I N F O

    Article history:

    Received 13 April 2015

    Received in revised form 7 May 2015

    Accepted 19 May 2015

    Available online 3 June 2015

    Keywords:

    Rail

    Welding joint

    Flash-butt welding

    Fracture analysis

    A B S T R A C T

    This paper mainly investigates the fracture problem of U71Mn rail flash-butt welding

    joint. Fracture surface morphology, microstructure and micro hardness are analyzed by

    using the scanning electron microscopy (SEM/EDS), the optical microscope (OM) and the

    micro Vickers hardness tester (Vickers-tester). The analysis results show that the welding

    joint is fatigue fracture, and the fracture surface morphology is the cleavage fracture

    characteristics. The metallographic morphology, inclusions and micro-hardness near the

    fracture surface are all in the normal levels. On the other side, the free solidification

    microstructure which extended from the outside to inside in thejoint of theleft side of the

    rail web and the rail head is the crack source of the rail welding joint fatigue fracture.

    Under the action of bending stress, the crack firstly generates in this area, and gradually

    extended to the rail web, to final fracture.

    2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC

    BY license (http://creativecommons.org/licenses/by/4.0/).

    * Corresponding author. Tel.: +86 518 85895330; fax: +86 518 85895326.

    E-mail address: [email protected](L. Feng).

    Contents lists available atScienceDirect

    Case Studies in Engineering Failure Analysis

    j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c s e f a

    http://dx.doi.org/10.1016/j.csefa.2015.05.001

    2213-2902/ 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

    http://dx.doi.org/10.1016/j.csefa.2015.05.001http://creativecommons.org/licenses/by/4.0/mailto:[email protected]://www.sciencedirect.com/science/journal/22132902http://www.elsevier.com/locate/csefahttp://dx.doi.org/10.1016/j.csefa.2015.05.001http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://dx.doi.org/10.1016/j.csefa.2015.05.001http://www.elsevier.com/locate/csefahttp://www.sciencedirect.com/science/journal/22132902mailto:[email protected]://creativecommons.org/licenses/by/4.0/http://dx.doi.org/10.1016/j.csefa.2015.05.001http://crossmark.crossref.org/dialog/?doi=10.1016/j.csefa.2015.05.001&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.csefa.2015.05.001&domain=pdf
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    2. Experimental results

    2.1. Macroscopic inspections

    Fig. 1 shows the rail fracture position and the crack propagation shape. As shown in Fig. 1, the rail fracture happens at the

    flash-butt welding joint, and crack propagation shape is irregular curved.

    The macroscopic morphology of the rail fracture surface is shown in Fig. 2. As shown inFig. 2, firstly, the rail head is

    slightly sunken, the rail web is rugged, and the rail bottom is relatively flat. Secondly, the fracture surface of the rail iscorroded at the different degrees. At the left side of the rail web, there exists a 100 mm 10 mm oxidation corrosion area,

    and a serious crescent-shaped corrosion area is found at the corner of the left side of the rail web and the rail head. And the

    fracture surface is fresh at the right side of the rail web. Another oxidation corrosion area can be found at the right side of the

    rail bottom. Thirdly, the fracture surface roughness in the serious corrosion area is smaller than that in the fresh fracture

    surface area in the rail web. Finally, the herringbone pattern at the both sides of the rail bottom corner points to the rail web

    and the radiation pattern of the rail head fracture surface also points to the rail web.

    2.2. SEM observations and EDS analysis

    In order to study the microstructure morphology of the rail fracture surface, the specimen is collected from the fractured

    rail which is shown in Fig. 2. The specimen is cleaned by ultrasonic wave in anhydrous alcohol, and then is observed by using

    the Hitachi S-570 scanning electron microscope.

    Fig. 3 shows the microstructure morphology and spectral analysis results of the different areas of the rail fracture surface.In Fig. 3(a), the serious corrosion crescent-shaped area (1# specimen) is primarily cleavage fracture microstructure

    morphology, in which many secondary cracks as well as a small amount of inclusions exist. From the analysis results, it can

    be seen that the inclusion mainly contains Si, Ca, C and O elements. In Fig. 3(b), at the corner of the left side of the rail web and

    the rail head (2# specimen) the microstructure morphology of the rail fracture surface have the characteristics of cleavage

    fracture, and free solidification structure morphology observed in the area extends from the edge welding joint to the

    internal. The analysis results show that the free solidification structure mainly contains Fe element. In addition, the

    observations suggest that the flat area (3# specimen) at the left side of the rail web and the instant fracture area (4#

    specimen) at the right side of the rail web are the cleavage fracture microstructure morphology.

    2.3. Metallographic morphology observations and micro-hardness tests

    After grounding and polishing, the microstructures closed to the edge of the welding joint fracture surface is observed via

    ZEISS optical microscope 80-DX (MC80-DX). Fig. 4 shows the metallographic morphology of the rail welding joint and

    [

    Fig. 1. Rail fracture position and crack propagation shape.

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    fracture surface edge. As shown in this figure, the distance between the 1# specimen welding joint edge and the fracture

    surface ranges from 3.0 mm to 5.0 mm. The distance between the 3# specimen welding joint edge and the fracture surface is

    about 3.8 mm. The distance between the 4# specimen welding joint edge and the fracture surface is about 1.8 mm. However,

    the welding joint morphology of the different specimens fracture surface are all normal pearlite and ferrite, and no abnormal

    morphology is observed.By using micro-hardness Vickers tester HV-5, the micro-hardness of the rail welding joint fracture surface edge of 1#, 3#

    and 4# specimens are measured, respectively. During the measure process, the load is 500 g and the holding time is 10 s.

    After testing 7 points of each specimen, the average is achieved. Table 1shows the micro-hardness (Hv) of the different

    fracture surface edges. From Table 1 it can be seen that the micro-hardness of the differentfracture surface edges ranges from

    299 to 325, which can further indicate that the morphology of the rail welding joint edge is pearlite and ferrite.

    3. Analysis and discussion

    From the analysis of the experimental results, it can be deduced that the failure mechanism of the rail welding

    joint is fatigue cracking due to the cycle loading when the rail was applied. One thing has to be noted here is that

    due to the material of the rail is made of high strength steel, therefore, it is not easy to see the beach marks in

    the crack areas. The relatively large corrosion area (100 mm 10 mm) indicates that it has been contacting with the air

    for a long time, so that the degree of the oxidation corrosion is large. For this reason, this is the area where the crack

    happens. The results show that, the metallographic morphology, inclusions and micro-hardness near the fracture

    surface are all at the normal levels. But, there is abnormal free solidification structure morphology at the corner of

    the left side of the rail web and the rail head, which extended from the fracture surface to the internal. Compared

    with the normal pearlite and ferrite morphology, the bonding strengths of the interfaces between the free

    solidification structure and the steel matrix is relatively low, so it is easy to generate the crack under the external load.

    [

    Fig. 2. Macroscopic morphology photo of the rail fracture surface.

    Table 1

    Micro-hardness of the rail welding joints fracture surface edges.

    Specimens 1# 3# 4#

    Vickers hardness (Hv) 324.1 299.1 312.7

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    It can be deduced that the crack source should be at the surface of the left side of the rail web and the rail head. The

    serious corrosion crescent-shaped area also proves that the crack happens in this area at first, and then propagates

    toward the rail web, until fracture.

    From the analysis results, it can be concluded that the free solidification structure defects were resulted from the

    upsetting process. During this process, due to the unmatched upsetting parameters, including the upsetting length,

    upsetting force, and upsetting speed, cannotguarantee the molten iron being completely extruded out, therefore, the molten

    part will be remained in the matrix to form the free solidified structure. The surfaces of the free solidified structure

    are relatively smooth, as a result, the interfaces between these solidified phase and the steel matrix becomes the weakpart with relatively low bonding strengths, and thus tend to form micro-cracks.

    [

    Fig.3. Microstructure morphology and spectralanalysis of the different areas of the railfracture surface (a) 1# specimencrescent-shaped corrosionarea, (b)

    2# specimen cornerareaof therailweb andrailhead, (c)3# specimenlightercorrosion flatarea, (d)4# specimen instantfractureareaof the rightside ofthe

    rail web.

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    4. Conclusions

    In the fracture surface of U71Mn rail, there exists obvious corrosion area, which indicates that the rail has been

    used for a long time. The fracture feature of the rail welding joint is fatigue fracture. At the corner of the left side

    of the rail web and the rail head, there is free solidification structure morphology, which is the crack source.

    During the process of straightening, transporting, laying and serving, under the action of bending stress the

    cracks propagate, until fracture. The free solidification structure is resulted from the unmatched upsetting length,

    upsetting force and upsetting speed. Therefore, this paper proposes that the technological parameters of the

    welding process should be controlled, and the welding inspection should be strengthened in order to ensure thewelding quality.

    [

    Fig. 4.Metallographic morphology of the rail welding joint and fracture surface edge (a) 1# specimen, (b) 3# specimen, (c) 4# specimen.

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    Acknowledgements

    This work was financially supported by a Project Funded by the Jiangsu Postdoctoral Science Foundation (1402129C),

    the Jiangsu Province Ocean Resource Development Research Institute Science Open Fund Project (JSIMR201317),

    the Six Talent Peaks Program of Jiangsu Province (2013-ZBZZ-032), the Natural Science Foundation of Jiangsu

    Province (BK20141250) and the Lianyungang Scientific Public Service Program (JC1307).

    References

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