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CASE STUDY 1

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

The cable supporting an aluminum cage which brought welders up to the working location on a pressure vessel boiler failed in the first few minutes of the working day, while on assent and before the two welders inside started welding. The cable was attached to a crane, and in the past, observers indicated that many times the cable would rub on the tank. At times the cage would actually hang up on the tank on assent and would release with a jerk. On this day, a gang of 18 welders were on duty in the construction of this vessel, but they were all busy on their jobs and none saw if the cage had "hung up" this day.

Investigation

The cable used to support the cage was 0.25 inch diameter (6 bundles of 31 strands) steel cable with a rubber hose insulation surrounding the cable OD. In addition, the cables were insulated from the boiler steel by insulating sleeves; however the insulation had been breached in several spots.

The welding cables being used by both welders in the cage were also in disrepair. Several spots on the aluminum cage had copper contamination arc spots and the cables had aluminum at several spots, indicating that some arcing between welding cables and cage had occurred sometimes in the past. However, it was reported that the welding power supplies for these two welders are not even turned on until the welder reach the job location position.

Examination of the steel support cable in locations near the fracture indicated that the rubber insulating hose had been breached as indicated in the picture below. Laboratory analysis of the hose indicated that loads in excess of 2050-3150 pounds were necessary to perforate the rubber hose. Welders and equipment total were less than 1000 pounds.

In examining the 186 individual strands, the vast majority appears to have separated due to melting. Some additional strands had a very hard martensitic structure right at the fracture location and failed in a brittle manner. Softer structures were found back away from the fracture. The remaining 12-14 strands failed due to tensile overload.

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CASE STUDY 2

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

A cylindrical monoethanolamine absorber (MEA) vessel built in 1969 catastrophically ruptured in 1984 releasing explosive gasses and killing 17 people and doing over $100 million in property damage. The cylinder was 8 1/2 feet in diameter and 55 feet tall, designed and constructed in accordance with ASME Code Section VIII. The vessel was made from 1 inch thick A516-Gr 70 steel (nominal hardness RC 22) as illustrated in the diagram. Welding was done by full penetration submerged arc welding before standing upright into service. No post weld heat treatment was required according to code. In service, the vessel contained mostly propane and hydrogen sulfide at 100 F and 200 psi and was used to remove the hydrogen sulfide from the propane by internally distributing the MEA.

Investigation

Hydrogen blisters began to form soon after the vessel was placed in service but they were not sufficient to threaten the integrity of the vessel. In 1974, as a measure of convenience, the second course was cut out and replaced with a preformed ring and field welded with SMAW, again without pre or post heat in accordance with ASME Section IX. Corrosion continued to plague the vessel and in 1976 a monel liner was installed from the top down to the bottom two courses (not including the bottom two courses).

In 1984 when the failure occurred, the operator noticed a 6 in crack along the weld between the first and second course. Before he could completely get the gas supply shut off, the crack had propagated and the flammable gas ignited, sending the tank 3500 feet.

Examination of the failure indicated that 4 major pre-rupture cracks occurred in the HAZ of the lower weld as noted on the diagram. The cracks originated in the HAZ of the repair weld (hardness 45 RC) and encompassed a circumferential length of about 9 feet. The remainder of the crack was a fast running brittle crack.

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CASE STUDY 3

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

A fracture occurred in a railroad locomotive axle after it had been in service for only three months. The fracture occurred near the journal as illustrated below. The axle was made from cast ASTM A 21-78 material which had been cast and heat treated to a hardness of about 180 VHN and machined to its final shape.

Investigation

The fracture surface on the longer piece was so badly abused that no useful information could be obtained from that surface. Drillings were taken from that part, however, and the chemistry performed is listed in the table. The fracture surface on the shorter end is presented below and shows some striations. The fracture surface was then sectioned and examined metallographically. Away from the fracture surface, the structure was ferrite-pearlite structure typical of normalized material. Around the outer edge of the fracture surface to a depth of about 9 mm was a hardened region with acicular structure having a hardness of 330 VHN. The hydrogen content in the bulk of the axle was measured at 0.84-0.86 ppm. The hydrogen in the acicular structure region was 2.6-3.5 ppm. X-ray techniques were used to measure surface stresses in the axle. On the surface in the ferrite pearlite region, stress was low; but in the circumferential region around the outer part of the fracture where the acicular structure existed, the residual stress was very high.

Sketch showing location of fracture

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CASE STUDY 4

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

Tubing made from AISI 1035 with 0.125 wall thickness showed failures near the welds, running parallel to the welds and going down into the tube cross section at an angle. The cracks appeared when the tubing was being fabricated into fairly high stressed parts. The part was put into service caring warm water at 100 psi, at which time the cracking caused leakage.

The original tubing was made on a tube mill which formed sheet into the tube shape and then low frequency upset welded the tubing. A second set of tubes from a different chemistry performed acceptably. The chemistries of both good and bad tubes are presented below.

Investigation

The fracture surfaces of the cracks were examined with scanning electron microscopy but no identifiable features were found on the fracture surface. Failed tubing was cut across the pipe, mounted near the weld and polished. Photomicrographs at 100X in regions mildly and excessively cracked are presented.

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CASE STUDY 5

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

A GTAW weld was made on 0.062 inch thick type 321 stainless steel sheet forming a tube which was subsequently formed into a bellows for use in very sensitive medical pressure gauges. The problem is that one heat of stainless steel (whose chemistry is presented below with the balance being iron) showed weld porosity and was not able to be welded as well as all other heats previously obtained. The steel company was consulted and indicated that the only different thing noted about this heat was that it was ingot cast with a powder hot top. They noted an excessive "boil" when the hot top was added.

Investigation

A radiograph of the weld is presented. It shows two rows of porosity on either side of the weld at the fusion line. The majority of these pores were just below the top weld surface and not observed with visual inspection. A scanning electron micrograph of a pore surface is presented in the figure. Energy dispersive analysis showed that the pore wall contained Fe, Cr, Ti, Ni, Al, and Si in that order of concentration.

C Mn S Si Cu Cr Ni V Mo Al Co Zr Nb Ti N O

0.05 1.6 .001 .57 .22 17.0 11.0 .09 .28 .04 .14 <.01 <.01 .27 .0102 .0023

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Radiograph of As-Received Weld and SEM of pore in weld.

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CASE STUDY 6

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

Weld failures have been noted in the GTAW stator cores of electric motors. These cores were made with laminations of electrical steel produced by a conventional and a calcium-silicon melt practices; with bulk chemistry variations and anti-stick and c-5a coatings typically used on electrical steels. The base metal, the coatings and the weld fractures were all examined, and results are presented in the attached tables.

Slit coils of electrical steel are supplied to the motor manufacturer in the dry condition. The strip is then given a coating of "vanishing oil" which is applied to decrease punch wear and the laminations are punched. The oil is a 50/50 mixture of Gillube metal forming compound and mineral spirits. This oil has a high vapor pressure and was selected because no oil cleaning is needed before further processing. The laminations are then hand stacked, put in a welding jig and GTAW down the outside of the laminations at six locations. Low silicon containing materials are given one weld at each location, while the high silicon materials are given two passes. The following conditions are used: fixture clamping pressure = 1800 pounds, weld direction = vertically down, weld speed = 18 ipm, shield gas = argon at 18 cfh, weld current = unknown. The welded cores are then given an "oxidize anneal" at 1550F for 1-1/2 hours, followed by a vigorous bumping on a vibrating table to release any laminations which may be stuck together. The assembly is then tested for core loss properties.

Weld cracks are noted most prevalently after the bump cycle. Assembles which contain more than three cracks are rejected.

Investigation

Metallographic analysis of the base materials indicated that all materials had similar equiaxed grains.

SEM analysis of the coatings was performed and the anti-stick coating on sample 3 appears to have a very soothe thin coating. Attempts were made to measure coating thickness but were unsuccessful except to note that the sample 3 coating was much thinner than that of the others. The other coating appeared to be somewhat porous as well (even the sample 1 anti-stick coating). Analysis was performed on the coatings. The anti-stick coatings contained H3PO4 and the other coatings contained Monoaluminum Phosphate.

Metallographic cross sections of the welds were performed and are also presented. Pores were noted in samples where cracking occurred. Fratographic analysis from samples 1, 2, and 4 indicated the cracks propagated from the pores in a brittle

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manner. Sample 3 material was mechanically broken and found to have mixed ductile-brittle fracture.

The top surface of the weld sample 1 was also examined and found to contain Fe, Al, and Si.

Sample 1

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Sample 2

Sample 3

Sample 4

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CASE STUDY 7

a) Define the Problem b) Determine what data is missing and how to get data c) Plan the course of action

PROBLEM

Overview

Parts of a 21Cr-6Ni-9Mn stainless steel that had been forged at about 815 C were gas tungsten arc welded. During post weld inspection, cracks were found in the heat-affected zone of the welds. These cracks were small and somewhat perpendicular to the weld.

Investigation

Sections of the weld were taken parallel to the weld and perpendicular to the cracks. In SEM examination the cracks appeared as intergranular and extended to a depth of approximately 0.05 inches (see figure). Residual stresses in the weld away from the cracks were measured at 50,000 psi in tension.