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Failure analysis of a mast column
Engineering Failure Analysis 10 (2003) 317–323 www.elsevier.com/locate/engfailanal
Failure analysis of a mast column Jong-Hoon Leea,*, Wee-Do Youa, Jin-Woo Kimb, Jae-Ho Choib, Byung-Hak Choeb a
Korea Institute of Machinery and Materials, Kyungnam, Changwon 641-010, South Korea b Department of Metallurgical Engineering, Kangnung National University, Kangnung kangwon-do 210-702, South Korea Received 12 August 2002; accepted 14 October 2002
Abstract This paper presents the failure analysis of a low carbon steel mast column which failed in service. Optical microscopy was performed to evaluate the basic microstructure and crack propagation of the as-received material. The fracture surface was examined in a scanning electron microscope. Detailed microscopic studies have indicated that the failure was due to the presence of martensite in the center of the column. The cracking may be caused by the hardened martensite, which is induced by centerline segregation of carbon or Mn during cooling after hot rolling. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Centerline segregation; Martensite; Brittle fracture; Thermal stress; Hardness testing
1. Introduction The basic function of the mast column is to support a roof plate above it. The mast columns are originally produced from a hot-rolled bar of SM490A steel as a kind of low carbon Mn steel. The columns had a total length of 20 m, with width and thickness of 1.2 m and 0.1 m, respectively. Centerline segregation during continuous casting and hot rolled processing in steel making may have a great influence on the failure mechanism of the column. Moreover, the segregation of solute atoms like C, Mn and so on in the centerline of a hot rolled bar can cause martensite. A crack can be easily initiated and propagated in the martensite structure because its structure is too hard to be deformable under the thermal stress of the column during cooling down after hot rolling. This paper describes the detailed metallurgical investigations carried out on the fractured or cracked mast column sample, which included visual examination, optical microscopy, scanning electron microscopy and chemical composition analysis.
* Corresponding author.
1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00078-X
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2. Experimental procedure 2.1. Visual examination Visual examination of the mast column revealed a straight line crack in the middle of bar, which is called a ‘‘centerline crack’’, and a flat fracture surface was also observed as shown in Fig. 1. In the fracture surface of Fig. 1, the dark area shows the pre-existing and the bright area is a fractured surface. All the area of the fractured surface revealed a flat and shiny surface typical of brittle fracture. 2.2. Scanning electron microscopy The scanning electron microscopy was performed using a JEOL SEM model 25KV. Fig. 2 shows the fractographic examination performed by SEM. Examination of the fractured surface showed grain boundary embrittlement represented as a typical brittle fracture [3]. 2.3. Metallography of the centerline cracked area Several axial metallographic sections cut from zones close to the centerline crack were prepared to examine the microstructural characteristics and crack propagation shape.
Fig. 1. Visual examination of crack propagation and forced fracture surface.
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Fig. 2. Fractured surface of the cracked centerline of the column examined by SEM.
Two different metallographic sections can be seen in Fig. 3. Fig. 3(a) shows a macro photograph taken without etching the metallographic sample, while Fig. 3(b) consists of a micrographic montage performed after etching the sample. In both cases it can be observed that there are main cracks through the middle of the column thickness, which is called the centerline crack. 2.4. Microstructure and microhardness The microstructure and the microhardness, with a load of 100 g, were studied in axial metallographic sections from zones close to the cracks and from zones far from the cracks ( 300 mm away). Table 1 gives chemical compositions of SM490A, average column sample and fracture surface. In the section from zones close to the centerline cracks, the microstructure is the typical one of martensite in accordance with the thermal treatments of quenching suffered by the material SM490A, as shown in Fig. 4. The average value of the microhardness in these zones is 380 HV, typical of the martensite phase of material. In contrast, the microstructure and hardness results obtained from zones far from the centerline cracks were different. In this section the microstructure is composed of a normal structure (ferrite/pearlite) typical of the mast column steel in the as-received condition, as shown in Fig. 5. The microhardness shows lower Table 1 Chemical compositions of SM490A, average column sample and fracture surface
SM90a Column sample Fracture surface
C
P
S
Mn
0.2 max 0.16 0.40
0.035 max 0.003 0.021
0.035 max 0.007 0.005
1.6 max 1.4 1.85
320
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Fig. 3. Optical microstructures showing the crack propagation through the centerline of the column.
values in this zone far from the centerline. The average value of the microhardness was 180 HV, typical of the ferrite and pearlite phases of the material.
3. Discussion—martensite and crack formation The destructive examination of the mast column has shown that the crack failure occurred at the center of the bar, progressing along the bar centerline. Metallographically, important variations of microstructure have also been observed: the microstructure is composed of martensite close to the centerline cracks. On the contrary, the normal steel structure of ferrite and pearlite was observed in zones far from the centerline cracks. Moreover, the microhardness of the material close to the cracks is very high, reaching values up to 380 HV. All these observations, related to the changes in the microstructure and in the microhardness, in the column close to the centerline cracks, point to both having been subjected to high temperature heating and
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Fig. 4. Optical microstructure of martensite phase in a centerline of the column, which reveals the fast cooling condition.
Fig. 5. Optical microstructure showing the general steel structure of ferrite (white) and pearlite (black) from zones far from the centerline crack (about 300 mm away).
fast cooling processes. It is well known that one of the clear indications that the steel has been fast cooled is the microstructure of martensite, as in the structure close to the centerline cracks [1,2]. The characteristics detected in the column, macroscopically and microscopically, suggest that the cracking originated in quench cracks due to localized formation of martensite. This process results from stresses produced as a consequence of the volume increase accompanying the austenite to martensite transformation [1,2]. When a steel is cooled, the hard and brittle martensite can be formed along the segregation of
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Fig. 6. Optical microstructure after heat treatment of 930 C/1 h and air cooling, which shows two phases of martensite close to the centerline and pearlite far from the center.
solute atoms in the steel. Fig. 6 shows an optical microstructure sampled in a sound zone of the column after heat treatment of 930 /1 h and air cooling condition, which is similar to the thermal manufacturing process of the column. The microstructure of the sound column is composed of two distinct ones, martensite close to the corner of the column and ferrite/pearlite far from the centerline of the column. It reveals that the column has the martensite even in non-cracked areas. The separation of the microstructure, martensite and ferrite/pearlite, is caused by the segregation of solute atoms like C or Mn, which are in the chemical composition in the steel. In Table 1, a difference of
Fig. 7. Continuous cooling transformation (CCT) curves of carbon steel heat treated at 900 C/30 min: (a) 0.45 wt.% and (b) 0.16 wt.% carbon steel [4].
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chemical compositions, particularly in carbon and manganese, exists between the zones close to the centerline and the zones far from the centerline of the column. The segregation of the solute atoms like C and Mn to the centerline causes the nose to shift to the right in the CCT (continuous cooling transformation) curve, as shown in Fig. 7. Particularly the carbon content has a great influence on the nose shift. The carbon content of 0.45 wt.% in the steel makes the nose time shift to the right side, which means easier formation of the martensite in the cooling process than with the carbon content of 0.16 wt.% [4]. Moreover, this resultant martensite is very hard and brittle, making the material very susceptible to cracking [1,2]. Taking into account that the manufacturing temperature conditions cannot normally produce fast enough cooling for martensite formation, the martensite should be caused by the centerline segregation of solute atoms in steel making.
4. Conclusions The present failure is caused by the presence of martensite near the center of the column. This martensite is considered an abnormal structure in steel making, and is induced due to the segregation of solute atoms like C and Mn. The brittle and hard martensite in the centerline of the column causes a thermal crack in this region.
References [1] [2] [3] [4]
Lancha AM, Serrano M, Briceno DG. Engineering Failure Analysis 1999;6:337–53. Failure analysis and prevention, ASM metals handbook, vol. 11. 9th ed. Choe B-H. Failure analysis. Jin-young Book Company; 2001. Metallography and heat treatment diagram of steels. Won-Chang Book Company; 1991.
Failure analysis of a mast column Jong-Hoon Leea,*, Wee-Do Youa, Jin-Woo Kimb, Jae-Ho Choib, Byung-Hak Choeb a
Korea Institute of Machinery and Materials, Kyungnam, Changwon 641-010, South Korea b Department of Metallurgical Engineering, Kangnung National University, Kangnung kangwon-do 210-702, South Korea Received 12 August 2002; accepted 14 October 2002
Abstract This paper presents the failure analysis of a low carbon steel mast column which failed in service. Optical microscopy was performed to evaluate the basic microstructure and crack propagation of the as-received material. The fracture surface was examined in a scanning electron microscope. Detailed microscopic studies have indicated that the failure was due to the presence of martensite in the center of the column. The cracking may be caused by the hardened martensite, which is induced by centerline segregation of carbon or Mn during cooling after hot rolling. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Centerline segregation; Martensite; Brittle fracture; Thermal stress; Hardness testing
1. Introduction The basic function of the mast column is to support a roof plate above it. The mast columns are originally produced from a hot-rolled bar of SM490A steel as a kind of low carbon Mn steel. The columns had a total length of 20 m, with width and thickness of 1.2 m and 0.1 m, respectively. Centerline segregation during continuous casting and hot rolled processing in steel making may have a great influence on the failure mechanism of the column. Moreover, the segregation of solute atoms like C, Mn and so on in the centerline of a hot rolled bar can cause martensite. A crack can be easily initiated and propagated in the martensite structure because its structure is too hard to be deformable under the thermal stress of the column during cooling down after hot rolling. This paper describes the detailed metallurgical investigations carried out on the fractured or cracked mast column sample, which included visual examination, optical microscopy, scanning electron microscopy and chemical composition analysis.
* Corresponding author.
1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00078-X
318
J.-H. Lee et al. / Engineering Failure Analysis 10 (2003) 317–323
2. Experimental procedure 2.1. Visual examination Visual examination of the mast column revealed a straight line crack in the middle of bar, which is called a ‘‘centerline crack’’, and a flat fracture surface was also observed as shown in Fig. 1. In the fracture surface of Fig. 1, the dark area shows the pre-existing and the bright area is a fractured surface. All the area of the fractured surface revealed a flat and shiny surface typical of brittle fracture. 2.2. Scanning electron microscopy The scanning electron microscopy was performed using a JEOL SEM model 25KV. Fig. 2 shows the fractographic examination performed by SEM. Examination of the fractured surface showed grain boundary embrittlement represented as a typical brittle fracture [3]. 2.3. Metallography of the centerline cracked area Several axial metallographic sections cut from zones close to the centerline crack were prepared to examine the microstructural characteristics and crack propagation shape.
Fig. 1. Visual examination of crack propagation and forced fracture surface.
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Fig. 2. Fractured surface of the cracked centerline of the column examined by SEM.
Two different metallographic sections can be seen in Fig. 3. Fig. 3(a) shows a macro photograph taken without etching the metallographic sample, while Fig. 3(b) consists of a micrographic montage performed after etching the sample. In both cases it can be observed that there are main cracks through the middle of the column thickness, which is called the centerline crack. 2.4. Microstructure and microhardness The microstructure and the microhardness, with a load of 100 g, were studied in axial metallographic sections from zones close to the cracks and from zones far from the cracks ( 300 mm away). Table 1 gives chemical compositions of SM490A, average column sample and fracture surface. In the section from zones close to the centerline cracks, the microstructure is the typical one of martensite in accordance with the thermal treatments of quenching suffered by the material SM490A, as shown in Fig. 4. The average value of the microhardness in these zones is 380 HV, typical of the martensite phase of material. In contrast, the microstructure and hardness results obtained from zones far from the centerline cracks were different. In this section the microstructure is composed of a normal structure (ferrite/pearlite) typical of the mast column steel in the as-received condition, as shown in Fig. 5. The microhardness shows lower Table 1 Chemical compositions of SM490A, average column sample and fracture surface
SM90a Column sample Fracture surface
C
P
S
Mn
0.2 max 0.16 0.40
0.035 max 0.003 0.021
0.035 max 0.007 0.005
1.6 max 1.4 1.85
320
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Fig. 3. Optical microstructures showing the crack propagation through the centerline of the column.
values in this zone far from the centerline. The average value of the microhardness was 180 HV, typical of the ferrite and pearlite phases of the material.
3. Discussion—martensite and crack formation The destructive examination of the mast column has shown that the crack failure occurred at the center of the bar, progressing along the bar centerline. Metallographically, important variations of microstructure have also been observed: the microstructure is composed of martensite close to the centerline cracks. On the contrary, the normal steel structure of ferrite and pearlite was observed in zones far from the centerline cracks. Moreover, the microhardness of the material close to the cracks is very high, reaching values up to 380 HV. All these observations, related to the changes in the microstructure and in the microhardness, in the column close to the centerline cracks, point to both having been subjected to high temperature heating and
J.-H. Lee et al. / Engineering Failure Analysis 10 (2003) 317–323
321
Fig. 4. Optical microstructure of martensite phase in a centerline of the column, which reveals the fast cooling condition.
Fig. 5. Optical microstructure showing the general steel structure of ferrite (white) and pearlite (black) from zones far from the centerline crack (about 300 mm away).
fast cooling processes. It is well known that one of the clear indications that the steel has been fast cooled is the microstructure of martensite, as in the structure close to the centerline cracks [1,2]. The characteristics detected in the column, macroscopically and microscopically, suggest that the cracking originated in quench cracks due to localized formation of martensite. This process results from stresses produced as a consequence of the volume increase accompanying the austenite to martensite transformation [1,2]. When a steel is cooled, the hard and brittle martensite can be formed along the segregation of
322
J.-H. Lee et al. / Engineering Failure Analysis 10 (2003) 317–323
Fig. 6. Optical microstructure after heat treatment of 930 C/1 h and air cooling, which shows two phases of martensite close to the centerline and pearlite far from the center.
solute atoms in the steel. Fig. 6 shows an optical microstructure sampled in a sound zone of the column after heat treatment of 930 /1 h and air cooling condition, which is similar to the thermal manufacturing process of the column. The microstructure of the sound column is composed of two distinct ones, martensite close to the corner of the column and ferrite/pearlite far from the centerline of the column. It reveals that the column has the martensite even in non-cracked areas. The separation of the microstructure, martensite and ferrite/pearlite, is caused by the segregation of solute atoms like C or Mn, which are in the chemical composition in the steel. In Table 1, a difference of
Fig. 7. Continuous cooling transformation (CCT) curves of carbon steel heat treated at 900 C/30 min: (a) 0.45 wt.% and (b) 0.16 wt.% carbon steel [4].
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chemical compositions, particularly in carbon and manganese, exists between the zones close to the centerline and the zones far from the centerline of the column. The segregation of the solute atoms like C and Mn to the centerline causes the nose to shift to the right in the CCT (continuous cooling transformation) curve, as shown in Fig. 7. Particularly the carbon content has a great influence on the nose shift. The carbon content of 0.45 wt.% in the steel makes the nose time shift to the right side, which means easier formation of the martensite in the cooling process than with the carbon content of 0.16 wt.% [4]. Moreover, this resultant martensite is very hard and brittle, making the material very susceptible to cracking [1,2]. Taking into account that the manufacturing temperature conditions cannot normally produce fast enough cooling for martensite formation, the martensite should be caused by the centerline segregation of solute atoms in steel making.
4. Conclusions The present failure is caused by the presence of martensite near the center of the column. This martensite is considered an abnormal structure in steel making, and is induced due to the segregation of solute atoms like C and Mn. The brittle and hard martensite in the centerline of the column causes a thermal crack in this region.
References [1] [2] [3] [4]
Lancha AM, Serrano M, Briceno DG. Engineering Failure Analysis 1999;6:337–53. Failure analysis and prevention, ASM metals handbook, vol. 11. 9th ed. Choe B-H. Failure analysis. Jin-young Book Company; 2001. Metallography and heat treatment diagram of steels. Won-Chang Book Company; 1991.