Case Study of Weld Cracking in 2 ¼ Cr-1Mo-1/4V Heavy ......included preheating and PWHT...

Case Study of Weld Cracking in 2 ¼ Cr-1Mo-1/4V

Heavy Wall Ammonia Converter

The use of advanced 2 1/4Cr-1Mo-1/4V (22 V) Vanadium enhanced alloys are becoming common

practice for high temperature heavy wall equipment in the petrochemical industry due to the high

allowable stress (and reduced thickness) allowed by ASME Code. This paper covers a description of

the various grades of Cr-Mo alloys used in ammonia converter service and their

advantage/disadvantage over other alloys.

Also included is a case study of a through wall weld crack that occurred in a heavy wall ammonia

converter owned and operated by TACC, a Chinese company.

Included are the inspections that were performed in the field, repairs required to put the vessel back

in service, and the results of a root cause evaluation. Fabrication issues which contributed to the weld

failure will be discussed along with recommendations to prevent future repeat of the issues.

John Czerwinski, John Li, Darrell Svec

KBR Inc, Houston, Texas, USA

Ling Jiang

TACC: Yunnan Tian-An Chemical Company, China

Introduction

BR was awarded a contract by TACC to provide basic engineering design for a 1660 metric ton/day ammonia

synloop. The plant is based on coal feed. Part of KBR’s scope was to provide technical assistance to TACC in the Detailed Engineering and Procurement phases of the project. TACC contracted a European fabricator to manufacture a heavy wall (105 mm) horizontal ammonia converter fabricated from advanced 2 1/4Cr-1Mo-1/4V (22V) alloy material. The vessel could have been fabricated from conventional 1 1/4Cr or 2 1/4Cr materials, but the 22 V material was selected because of its high stress allowed by the code. The 22 V material allowed for a vessel of much thinner wall thickness with a substantial savings in vessel weight (refer to

Table 1 which shows the differences in Code allowable stresses, wall thicknesses, and weights for the conventional and advanced Cr-Mo materials). A lighter weight was required for this vessel because of long overseas and land travel restrictions to its final inland destination. The converter is owned and operated by TACC in China and was commissioned in 2009. The converter operated at 200°C (392°F) and 16.68 MPa (2419 psi) until a leak was detected in May 2011. The cracking occurred in a circumferential weld identified as (W20) a closure weld that was completed in the shop during the final fabrication steps. The crack was confined to a submerged arc narrow gap weld, and was transverse to the welding direction as shown in Figure 1. The vessel had seen 28 start-

K

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ups and shut-downs in the last 1 1/2 years of operation due to upstream Gasifier problems.

Field Examinations

The insulation and weather proofing were removed from the vessel so that all welds could be non-destructively examined. All welds were power tool cleaned to provide a suitable surface for examination. Time of Flight Diffraction (TOFD) ultrasonic volumetric examination was carried out on all longitudinal and circumferential shell welds. Wet Fluorescent Magnetic Particle Testing (WFMT) was also performed on all longitudinal and circumferential welds, and all nozzle connection welds. Ultrasonic (UT) shear wave examination was performed on the cracked weld (W20) as well as most circumferential and longitudinal weld junctions. No additional cracking or recordable indications were detected with any of the non-destructive examination methods used on the vessel welds. Leeb hardness testing was carried out on the weld metal deposit and base material in the cracked area at 500 mm (20 inches) intervals over an area approximately 2500 mm (100 inches) in length. The hardness values were converted to a Brinell hardness scale. The highest hardness (380 HB) was measured in the weld metal adjacent to the crack. There were two (2) other areas where the hardness exceeded API RP 934-A requirements of 235 HB maximum for advanced steels. Repair Procedure

KBR’s recommendation to repair the cracked weld was to excavate the cracked weld area to the ID surface, and perform a full penetration weld repair. To perform this method of repair, the weld would have to be back welded from the ID surface. This would require removal of the converter head and pulling the internal catalyst

basket which would have added four to six days to the unscheduled outage. In order to save time, the owner decided to use a core drill to remove the crack area, and weld a false nozzle overtop of the cored hole. In an effort to prevent the crack from propagating during the coring operation the tips of the crack were drilled out with a 10 mm (0.39 inches) drill bit. The core bit used to remove the plug sample was 38 mm (1.50 inches) in diameter. After the core sample was removed, Liquid Penetrant (LP) and Ultrasonic (UT) examinations were used to assure that the entire crack had been removed before the repair procedure was started. A dehydrogenation heat treatment (DHT) procedure was carried out per API RP 934-A requirements before any welding was done on the vessel. Ceramic resistance heating pads were used to perform the DHT. The entire circumference of the weld (W20) was heated during the DHT. After the DHT was completed, the false nozzle was tacked into place and the nozzle was welded into place with an ASME Section IX qualified procedure qualification record (PQR) and welder. The design details of the nozzle placement are shown in Figure 2. The PQR included preheating and PWHT requirements that followed API RP 934-A recommendations. After the welding was completed the attachment weld was examined with UT and MT, and the ID of the nozzle with PT before and after post weld heat treatment (PWHT) (see Figure 3). Metallurgical Examinations

The Materials Property Council (MPC) and the University of Tennessee Materials Joining Group (UT MJG) were contracted by KBR to perform a root cause analysis on the cracked weld plug sample. The sample was sectioned

152 2011AMMONIA TECHNICAL MANUAL


into two pieces by the owner and one piece was sent to KBR, so a section of the full thickness was missing from the sample. Also, a portion of the OD surface of the plug was missing because of counter bore drilling used to stabilize the core drill. The two core sections totaled 67 mm (2 5/8 inches) in thickness compared to the full weld thickness of 105 mm (4 1/8 inches), as shown in Figure 4. The cut faces closest to the center of the weld were polished and etched with nital to reveal the position of the crack in relation to the weld to aid in sectioning the samples (refer to Figure 5). The samples show the crack was transverse to the welding and was arrested in the base metal. Sectioning was performed on the sample to achieve the following: Return half of the plug sample to the owner

as requested. Prepare metallographic samples for

evaluation in the longitudinal and transverse direction in relation to the welding direction.

Scanning Electron Microscope (SEM) fractographic analysis of the crack surfaces.

To obtain a section of the weld deposit for chemical analysis.

Electric Discharge Machining (EDM) was used to section the plug for different testing techniques (Figure 6). The plug was sectioned in the through thickness direction so that the owner could have an equal half of the plug sample to perform their own evaluation. Metallographic sections were mounted, polished, and etched to disclose the micro and macro structure of the weld (refer to Figure 7). One of the sections was used to expose the fracture surfaces using a cryo-cracking technique (Figure 8). Metallographic examination was performed on longitudinal and transverse specimens removed from the weld. SEM examination was done on the fracture surfaces to identify the fracture morphology to 10,000X. Chemical composition of the weld deposit was completed to determine

any elemental anomalies that may have contributed to the failure. Hardness results were acquired with Rockwell and Vickers hardness test methods. The mechanical properties (Tensile, Yield, Impact) could not be determined due to the small size of the sample. The information obtained from the above examination techniques was used to determine that cause of failure. Results

Metallographic examination was performed at magnifications up to 1,000X and detected a primary crack front with some secondary cracking. The cracking was intergranular and transgranular. No unusual conditions were seen in the weld which could have contributed to the failure. The microstructure in all samples is typical of 22 V weldments with no substantial heat treatment. This condition is noted by the contrast between fine and coarse grained microstructures in the tempered regions which indicate lack of any significant heat treatment. Photomicrographs of the weld metal and the cracking are shown in Figure 9. SEM analysis of the fracture surface detected a surface morphology of both cleavage and tear dimples (Figure 10). The fracture surface is consistent with an overload fracture in a 22 V weld deposit, and is not typical of reheat cracking. The weld deposit chemical composition was determined with optical emissions spectroscopy (OES) and inductively coupled plasma mass spectrometry (ICP-MS). The carbon and sulfur content were analyzed with LECO equipment. The ICP-MS analyzer is capable of analyzing metal components down to the part per trillion ranges. The weld deposit chemical composition conforms to 22 V material. There are no anomalies in the trace and tramp elements that would indicate any susceptibility to reheat cracking (Table 2).



Hardness tests were taken on the weld deposit, heat affected zone (HAZ), and base materials using both Rockwell and Vickers equipment. The weld deposit was 38-39 HRC or 400-410 HV. The HAZ had a hardness of 423 HV (43 HRC). In comparison with extensive research at UT, the hardness in the weld deposit and HAZ is similar to values of a weld in the as-welded condition. The base material shows a measured hardness of 7-9 HRC or 175-185 HV which is typical of other 22 V base metal hardnesses. The hardnesses can be translated into approximate tensile strengths of 1213.5 MPa (176,000 psi) for the weld deposit and 599.8 MPa (87,000 psi) for the base material. Discussion and Conclusions It should be noted that the analysis of the plug sample was performed on less than 50% of the as received plug sample. The plug sample was sectioned so that the owner could retain half of the sample. The fracture surface of the plug sample was examined with a stereo microscope to try and identify the fracture origin as shown in Figure 8. The chevron patterns on the samples examined indicate the fracture initiation site most likely is located in the sample portion that was returned to TACC. At the start of the investigation it was difficult to determine if the weld had been exposed to adequate PWHT temperatures to provide the expected thermal softening or if a repair had been made in the crack area and was not appropriately PWHT. Examination of the macro etched specimens revealed narrow gap weld geometry in the crack area which indicated a repair was not made in this area. From the investigation performed by MPC and UT MJG, the following conclusions can be made as to the root cause of the weld crack that resulted in the leak.

Cracking in the weld appears to be caused by the combined effects of high residual stresses that remained in the weld because of improper or lack of adequate PWHT. This coupled with the stresses of operation and the possibility of hydrogen being present in the weld, led to the fracture in the weld. The high hardness in the weld deposit is an indication of improper PWHT. Based on extensive studies of 22 V weld deposits, hardnesses of 16HRC (222 HV) can be expected after eight hours of heat treatment at 705°C (1301°F). It is believed the residual stresses from welding were close to the ultimate strength and combining the hoop stresses of operation results in residual plus operational stresses above the yield/ultimate tensile stress. At the normal temperatures of operation, 200°C (392°F) the vessel would not have experienced any stress relief. The crack morphology contained transgranular cleavage and tear dimples which are not consistent with reheat cracking which would have resulted in intergranular cracking. To determine if hydrogen contributed to the cracking in the weld, the trapped and diffusible hydrogen was measured using the American Welding Society (AWS) method. At 45°C (113°F) for 72 hours, diffusible hydrogen was measured at 0.1ppm. The trapped content was determined using a hot extraction method at 700°C (1292°F) for 72 hours. This method revealed a hydrogen level of 3.5 ppm. It should be understood that due to the small size of the sample and the instability of hydrogen, some of the hydrogen may have diffused out of the sample. It should also be recognized that the service conditions of the vessel of 200°C (400°F) in ammonia service, hydrogen can diffuse into the hardened microstructure of the weld deposit and could have contributed to the cracking mechanism. However, the crack morphology did not show any signs of intergranular fracture which is consistent with cracking in hardened, hydrogen-charged material. This evaluation determined that the



lack of effective PWHT resulted in a hardened, hydrogen-susceptible microstructure susceptible to fracture which would not have occurred if the weld was properly heat treated. Examination of the vessel data book determined that the localized PWHT of the closing seam (W20) was done with the use of three (3) thermocouples. This would have resulted in a thermocouple spacing of approximately 10 feet (3048 mm). At this spacing it is conceivable that one or more heating pads could have malfunctioned resulting in less than adequate PWHT temperatures in an isolated area. Welding Research Council (WRC) bulletin 452 provides recommended monitoring thermocouple locations for PWHT practices.

In January 2012, a thorough examination of the repair area was conducted with MT and UT. No defects were detected during this examination. TACC will continue to monitor the repair area during their yearly turnaround to ensure the operational safety of the vessel. Acknowledgement We would like to give special thanks and recognition to Dr. Martin Prager of MPC and Dr. Carl D Lundin and Max Trent of the University of Tennessee Materials Joining Group for the metallurgical evaluation of the weld metal plug sample.

Table 1 - COMPARISON OF CONVENTIONAL AND ADVANCED Cr-Mo STEEL

Construction Material 1.25Cr - 0.5Mo 2.25Cr - 1Mo 2.25Cr - 1Mo - 0.25V Approx Weight of Converter, kg ~575,000 ~560,000 ~400,000

Allowable Stress @ Design Temperature (MPa) 165 170 244

Normal Wall Thickness (Shell, mm) 176 170 118

Overall Weight Savings, kg n/a 15,000 175,000



Table 2 - CHEMICAL RESULTS OF 22 V PLUG SAMPLE

Chemical Analysis of TACC 22 V Plug Element TACC 22V Plug Detection Method Carbon 0.11 wt% LECO

Iron 95.02 w% Difference Manganese 0.73 wt% OES

Silicon 0.16 wt% OES Chromium 2.45 wt% OES Vanadium 0.25 wt% ICP-MS

Molybdenum 1.04 wt% OES Antimony 9 ppm ICP-MS

Lead 0.5 ppm ICP-MS Bismuth <0.2 ppm ICP-MS Calcium <5 ppm OES Niobium 110 ppm OES

Phosphorus 60 ppm OES Tin 42 ppm ICP-MS

Titanium <0.2 ppm ICP-MS Aluminum 63 ppm ICP-MS

Copper 670 ppm ICP-MS Boron 16 ppm ICP-MS

Arsenic 18 ppm ICP-MS Sulfur 40 ppm LECO Nickel 0.13 wt% ICP-MS Cobalt 64 ppm ICP-MS



FIGURE 1 - TRANSVERSE CRACK IN CIRCUMFERENTIAL WELD (W20)



Figure 2 - DETAILED DESIGN OF FALSE NOZZLE

Figure 3 - PT EXAMINATION OF NOZZLE AFTER WELDING



Figure 4 - A sketch of the plug material provided to UT MJG. Note that approximately 48% of

the through thickness was provided for examination.



Figure 5 - Macrograph of plane near the center of the plug sample. The cracking terminates

in the base metal and is transverse to the weld direction, 4X Nital Etch



Figure 6 - Samples sectioned in the through thickness direction, one 1/2 for return to Owner

and the other 1/2 for analysis by UT MJG.



Figure 7a - Macrograph of cracking in the center of the plug. 2X Nital Etch.



Figure 7b - Macrograph taken transverse to welding direction. Note the profile of the narrow

gap weld. 2X Nital Etch.



Figure 8 - Shows the crack surfaces disclosed with a Cry-cracking technique. Chevron

patters on the fracture surface indicate the crack initiation most likely occurred in the sample returned to the Owner.



Figure 9 - Macrograph and Micrograph of crack in 22 V weld deposit. Both the primary and

secondary cracks have propagated intergranularly and transgranularly. Nital Etch.



Figure 10 - Fractographs of the exposed crack surface show a cleavage and dimple tearing

fracture mode. The dimple tearing facture mode indicates hydrogen was not a major contributor to the fracture, and even with the high hardness measured in the fracture area, the weld still contained some ductility.



Case Study of Weld Cracking in 2 ¼ Cr-1Mo-1/4V Heavy ......included preheating and PWHT...

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