Failure Analysis of the Weld Crack on Fire-Tube Glycol Reboiler

Winarto1,a, Muhammad Anis1,b 1 Metallurgy & Materials Engineering Department – University of Indonesia

Kampus Baru – UI, Depok – 16424, INDONESIA a winarto@metal.ui.ac.id, b anis@metal.ui.ac.id

Abstract. Glycol re-boiler or glycol dehydration is a unit which acts as separator for water content from TEG (Triethylene Glycol). A fire tube, which is part of glycol re-boiler, was found some cracks on the weld toe joint area during overhaul service of the glycol re-boiler after 5 year in operation. The failure occurs at the weld region which is connected the 6” flange pipe and 24” pipe. Then it was analyzed to determine the main cause of the failure of weld crack of fire tube based on the metallurgical aspects, and followed by the conclusion according to the laboratory testing results. The material testing and analyzing were started from visual test, examination of the fractography, metallography examination, hardness testing, chemical composition testing, SEM examination and finally chemical analysed by EDX. The results show that the failure of a fire tube weld crack was caused by Hydrogen-Induced Cracking (HIC) This is initiated by the presence of elongated Manganese Sulphide (MnS) inclusions at the sub-surface of 6 inch flange pipe together with diffusion of hydrogen during welding & service operation. The growth of weld -toe cracking is a time-independence with inter-granular-cracking mode, which the corrosive media and thermal cycle during services give raise the propagation of cracks.

Keywords: Failure Analysis, Fire Tube Glycol Reboiler, Weld Cracks, Hydrogen Induced Cracking.

Introduction

Glycol re-boiler or glycol dehydration is a unit which acts as separator for water content from TEG (Triethylene Glycol). Glycol itself is a liquid substance use in oil and gas industry to extract the water content from natural gas. The water is absorbed by the glycol in Absorber Unit, rich glycol then feed to the Reboiler where the water is released and the clean glycol is returned to the absorber. The glycol circulation was reheated using a direct fired re-boiler and the temperature must be maintained to avoid glycol being cracked. The failed of fire tube glycol re-boiler can be seen in Figure 1.

Figure 1. Field condition picture of failed Fire Tube Glycol Reboiler

Due to inadequate information to assess the reliability of the fire tube glycol re-boiler failure reporting, no assumption was made about the history of fire tube glycol re-boiler serviceability.

The failure analysis assessment was carried out in order to determine the main cause of the failure of fire tube glycol re-boiler based on the metallurgical aspects, and followed by the conclusion according to the laboratory testing results.

Experimental Procedures

The procedure of failure analysis based on the following stages such as: [1,2]

• Visual examination of general physical features • Chemical analysis/identification (by Sparks Spectrometer and EDS) • Mechanical properties: hardness testing • Microstructure (Metallography) analysis • Fractography examination • Discussion and conclusion

Investigation Results

a. Visual Examination of General Physical Features

The failed fire tube was visually examined and photographed in as -received condition. The following observations are made as follows:

A. The fire tube consists of several 6” pipe and 24” pipe parts joined by welding method.

B. The crack appeared at weld zone of 6” pipe as seen in Figure 2.

C. The fracture surfaces showed no significant corrosion attack. However, the fracture surfaces were having rust -color, which probably due to atmosphere contamination prior to investigation.

D. There was no obvious region of ductile necking associated with the failure.

Figure 2. Closer look of the crack appeared at 6’ flange pipe weld.

b. Chemical Analysis/Identification

Portions of the fire tube were subjected to the chemical analysis using spark emission spectrometer in order to provide its chemical specification. The result of chemical analysis revealed that the base material of fire tube is equivalent to ASTM A 106 Grade B Standard Specification for Seamless Carbon Steel Pipe for High Temperature Service [3]. The material also appeared to be easily welded steel with carbon equivalent (CE) below 0.4. [4]

c. Mechanical Properties: Hardness Testing

The sample subjected to Vickers method testing (micro hardness). The measurements were taken at several locations of the samples. The result, as can be seen in the Table 2, showed the hardness difference between 6” pipe and 24’ pipe (149 HV and 183 HV), also the hardness value between left and right side of the crack is similar (225 HV and 230 HV). While the hardness of weld metal (fusion weld) is 189 HV. The hardness near the crack area is indicated that structure near the crack has lower ductility compared to both the base metal and weld metal. The increased hardness near the crack on HAZ was due to embrittlement during long term services. [5]

Based on information from user specification, there is no mentioned the hardness value for the material. No conclusion was made as lack of mechanical properties could be as possible causes of the failure.

d. Macro -Fractography Analysis

In order to provide more information of the crack location in the weld area, a stereo-scan microscope is used to examine failed area of fire tube.

Light fractography as seen in Figure 3 showed a crack which propagated perpendicular to the surface of plate thickness of 6” flange or parallel to the weld section. The fracture was initiated at the weld toe and spread along the HAZ area with the crack length approximately half of the wall pipe thickness. Figure 4 showed the crack propagation, which were taken from failure area.

Figure 3.Light fractography of the cross section surface. The photograph showed a cross section of

flange joint consisting weld metal, HAZ and base metal for both pipes, Nital etched, 7x

Figure 4. Cross section of the crack which propagated from the surface to the center of the flange thickness, Nital etched, magnification of 50X.

e. Microstructure (Metallography) Analysis

The Microstructures of fire tube were taken at the base metal for 6’ and 24’ pipe, under magnification of 200X, which are shown in Figure 5.a and 5.b. The result from 24” pipe showed a microstructure of low-alloy carbon steel with ferrite and elongated pearlite. This structure indicated that the base material was manufactured by forming process (rolling & bending process).

(a) (b) Figure 5. Microstructure taken at the base metal of : (a) 6’ and (b) 24’`pipe consisting of banded

pearlite (black) and ferrite (white). Nital etched, magnification of 200 X

Microstructure in Figure 6.a was taken from the crack side surface revealed a microstructure which is commonly found in weldment with pearlite and acicular ferrite structure of HAZ. Figure 6.b showed that some inclusions were found adjacent to the surface of the 6‘ pipe of the base metal.

(a) (b) Figure 6. Microstructure taken at (a) adjacent to the side of the crack, Nital etched,

magnification of 200 X (b) Some inclusion (dark) was found adjacent to the surface of the 6 inch flange base metal, no- etched, magnification of 500 X

f. Micro Fractography by SEM

In order to provide more information, a Scanning Electron Microscope (SEM) was used to examine failed area of fire tube.

The examination of base material by SEM for 6 inch flange, far from the crack but at the sub-surface (65.48 micrometer from the surface), is found an elongated MnS resulted from cold working process with the length of 45.45 micrometer, (in Figure 7). The Energy Dispersive Spectrometer (EDS) analysis revealed an inclusion such Manganese Sulphide (MnS) that could be identified as source of crack initiation and the inclusions was found in the base material and in HAZ.

The examinations of fracture surface by SEM revealed an inter-granular fracture and some branched cracks (secondary cracks) as can be seen in Figure 8. The scale s (corrosion products) were observed over most of the crack path, even down to the fine tip of the crack as seen in Figure 8a. The EDS analysis was pointed on that position and it contained some corrosion products. The secondary cracks separating from the main crack was also examined using EDS analysis. The result showed that corrosion products (oxides and sulphides) were found inside the secondary cracks.

Figure 8. Scanning electron micrograph showed the length (45.45 µm) of elongated MnS inclusion

adjacent to the surface of 6 inch flange pipe.

Figure 7. SEM micrograph showed (a) the inter-granular crack & scales taken from bottom area of the main crack. (b) the MnS inclusion adjacent to the crack tip on HAZ with higher magnification

Discussion

There was no information from the user about the historical background relating to the manufacturing process of both pipes. However, the 24 inch-diameter pipe was generally made by forming process of steel-plate and then welded at the both sides of plate to form a pipe. While the 6 inch-flange (elbow) pipe seems to be made by cold-forming process of seamless pipe, the micro-structure of flange pipe was indicated that elongated ferrite with banded pearlite was due to the cold forming process. This forming process also made some inclusions such as manganese sulphide (MnS) to be elongated in shapes as can be found on the microstructure of the flange, especially near the surface as can be seen in Figure 8.

Steel flange pipe produced by cold forming process can contain a population of non-metallic inclusion (MnS) sufficient to reduce its short transverse ductility to a considerable extent as can be found on the result of increased hardness near the cracks. When such steel is welded, tearing or cracking can result if the ductility in the short transverse direction is not sufficient to accommodate the welding contraction strains. Such strains result from welding stresses, which are at a maximum in the short transverse direction in the HAZ.

The cracking or tearing process takes place in two distinct stages. Firstly, non-metallic inclusions (MnS) “decohere”, i.e. they separate from the matrix of steel in which they lie and thus form minute cracks between themselves and the steel. The second stage is for the minute cracks at the decohered inclusions to the join up, jumping from one plane in the steel to another link up with similar decohered inclusions on adjacent planes.[5]

Besides inclusions, the present of hydrogen during welding (especially using SMAW method) can increase the risk of tearing, because it reduces the ductility of the steel. In such circumstances, the increased the temperature (from preheating or heat input) gives an increased risk of a significant temperature difference between the two components being welded (24 inch pipe and 6 inch flange) and, hence, of increased stresses on cooling after welding. With continuously flange of relatively thin section (6 inch), any center -line segregates of MnS are likely to lie close to the weld HAZ. This proximity to the HAZ (making diffusion of weld hydrogen to this area eas ily), coupled with their increased composition (making them more susceptible to hydrogen cracking) as a result of segregation, makes such regions (weld toe) particularly prone to lamellar tearing, hydrogen cracking or combination of the two.

The mechanism of initial cracking formation at the fracture surface between inclusion (MnS) and hydrogen diffusion is generally due to high molecular hydrogen gas presures building up at an inclusion interface. The magnitude of pressure build up likely under weld conditions are difficult to estimate. The possible mechanism for the initial crack formation in conjunction with hydrogen-initiated decohesion at MnS inclusion can be seen in the figure below: [5]

Figure 9. The mechanism of initial cracking formation at the fracture surface between inclusion (MnS) and hydrogen diffusion.[5]

Hydrogen-induced cracking, also called stepwise cracking or blister cracking, is primarily found in lower-strength steels, typically with tensile strengths less than about 550 MPa (80 ksi). It is primarily found in line -pipe steels. [6] This type of degradation also begins with a reaction between steel and hydrogen sulfide in the presence of water. Again, hydrogen atoms enter the steel, but with HIC, as opposed to SSC, these hydrogen atoms combine to form hydrogen gas at internal defects. These internal discontinuities can be hard spots of low temperature transformation products or laminations. However, manganese sulfide inclusions are the primary sites for this to occur. These inclusions tend to become elongated during pipe manufacture and give rise to high stresses at the tip of the inclusion when hydrogen gas forms there. As cracks initiate and propagate, they begin to link up with others, and a series of stepwise cracks can propagate through the material. An applied stress is not required for this mechanism to occur.

In the subject of failure, visual examination from surface morphology indicated that the initial cracks was propagated only in the weld toe of the HAZ for the 6’ flange pipe, but not at the 24’ pipe. This is due to the 6’ flange is more prone to the combination of lamelar tearing and hydrogen cracking during welding than that of the 24’ pipe.

When the initial cracking occurred, the growth of crack or crack propagation is a time-independent process or takes along time. The subsequent of initial cracks, the environment condition and termal cycling during services tend to be more increase decohesion at the grain boundary, such as craking of glycol at the crack area and temperature process, hence, the crack is tend to be growth. This yields to be intergranular cracking phenomena (IGC). The corrosive product (ferro oxides, sulphides etc) was created at the crack tip region as the crack was growing.

Conclusions

The weld-toe cracking of fire-tube glycol re-boiler is mainly caused by the Hydrogen-Induced Cracking (HIC). This was initiated by the presence of elongated Manganese Susphide (MnS) inclusions at the sub-surface of 6 inch flange pipe together with diffusion of hydrogen during welding and also during long term services. The growth of weld-toe cracking is a time-independence and an inter -granular-cracking mode, which the corrosive media and thermal cycle during services give raise the propagation of cracks.

References

[1] ASM Metals Handbook Vol. 11, Failure Analysis and Prevention, American Society for Metals, Metals Park Ohio, 1986.

[2] ASM Metals Handbook Vol. 12, Fractography, American Society for Metals, Metals Park Ohio, 1987.

[3] ASM Metals Handbook Vol. 1, Properties and Selection: Iron and Steel, American Society for Metals, Metals Park Ohio, 1987.

[4] Norman Bailey, Weldability of Ferritic Steels, American Society for Metals (ASM International), Abington Publishing, Cambridge England, 1994.

[5] Cialone, H., and Asaro D.J, Hydrogen Assisted Fracture of Spheroidized plain carbon steel, Metal Transactions , 12A, 1373, 1981.

[6] Jeffery A. Colwell, Failures In Sour Gas Environments, ASM Metals Handbook , ASM International, 1996.