This article describes some of the welding discontinuities and flaws characterized by nondestructive examinations. It focuses on nondestructive inspection methods used in the welding industry. The sources of weld discontinuities and defects as they relate to service failures or rejection in new construction inspection are also discussed. The article discusses the types of base metal cracks and metallurgical weld cracking. The article discusses the processes involved in the analysis of in-service weld failures. It briefly reviews the general types of process-related discontinuities of arc welds. Mechanical and environmental failure origins related to other types of welding processes are also described. The article explains the cause and effects of process-related discontinuities including weld porosity, inclusions, incomplete fusion, and incomplete penetration. Different fitness-for-service assessment methodologies for calculating allowable or critical flaw sizes are also discussed.

Because of the tough engineering environment of the railroad industry, fatigue is a primary mode of failure. The increased competitiveness in the industry has led to increased loads, reducing the safety factor with respect to fatigue life. Therefore, the existence of corrosion pitting and manufacturing defects has become more important. This article presents case histories that are intended as an overview of the unique types of failures encountered in the freight railroad industry. The discussion covers failures of axle journals, bearings, wheels, couplers, rails and rail welds, and track equipment.

The T-section cross member of the lifting sling failed in service while lifting a 966 kg (2130 lb) load. The L-section sling body and the cross member were made of aluminum alloy 5083 or 5086 and were joined by welding using aluminum alloy 4043 filler metal. The fracture was found by visual examination to have occurred at the weld joining the sling body and the cross member. Inadequate joint penetration and porosity was revealed by macrographic examination of the weld. Lower silicon content and a higher magnesium and manganese content than the normal for alloy 4043 filler metal were found during chemical analysis. It was revealed by examination of the ends of the failed cross member that a rotational force that had been applied on the cross member caused it to fracture near the sling body. It was concluded that brittle fracture at the weld was caused by overloading which was attributed to the misalignment of the sling during loading. Aluminum alloy 5183 or 5356 filler metal was recommended to be used to avoid brittle welds.

A resistance-welded chain link made from 16 mm diam 4615 steel failed while lowering a 9070 kg load of billets into a rail car after being in service for 13 months. Beach marks, typical of fatigue were found to have originated at the inside of the link which broke at the weld. Cracks in the weld zone (up to 1.2 mm deep) were revealed during metallographic examination of a section through the fracture surface. The cracks were filled with scale which indicated that they had formed during resistance welding of the link. The defect was thus attributed to the weld defects which initiated the fatigue failure by acting as stress raisers. The welding method was changed by the manufacturer and all chains were replaced with defect free chains.

Following three similar failures of load chains on manually operated geared pulley-blocks of 1-ton capacity, a portion of one of the chains was obtained for examination. The chain was made of mild steel and the links had been electrically butt-welded at one side. In the case of the sample obtained, the failure in service had resulted from fracture of one of the links in the plane of the weld. Six of the other links in the vicinity showed cracks in the welds in various stages of development. Microscope examination showed a crack in an early stage of development and also from an apparently sound link, the prepared surfaces lying in the planes of the links. This examination revealed that the welds were initially defective. Discontinuities were present in both cases adjacent to the insides of the links, of a type indicative of either inadequate fusion or incomplete expulsion of oxide, etc., at the time of the upset, i.e. the pressing together of the ends of the links to complete the welding. It was evident from the examination that the service failures were due to the use of chain that was initially defective.

A crane hook of 200T rated capacity failed suddenly at an indicated load of 143T, while the crane was undergoing a load test. Fracture took place through the intrados of the hook at the region of maximum stress. The jib and other portions suffered subsequent damage following the sudden release of the load. Fracture was wholly of the brittle cleavage type except for a small crescent shaped lip at the top right-hand side. In this zone, fracture occurred at an angle of 45 deg to the general plane of fracture, indicative of failure in shear. Failure of the hook had taken place where a deposit of weld metal had been made, probably to eliminate a surface defect but apparently, without complete removal of the defect down to sound metal prior to welding. On many occasions it is preferable to blend out surface defects by local dressing. The effect of the resulting loss of strength is insignificant compared with the increased chance of failure associated with a weld repair.

Failure occurred by fatigue cracking of links from chains which were used to replace the ropes on grabs of the multirope type. In the first example, the links were made from high tensile steel rod. The fracture in the side of the link was duplex in appearance one half of the surface being discolored, indicative of a preexisting crack of the fatigue type, whilst the remaining portion was brightly crystalline, resulting from brittle fracture at the time of the mishap. In the second example, the fracture took place at a similar location adjacent to one of the butt welds situated at the mid-length of the sides. Brinell hardness values confirmed that the link was made from the higher tensile grade of material. The cracks were due to fatigue, there being no indications that the weld was initially defective.

Weld-decay and stress-corrosion cracking developed in several similar all-welded vessels fabricated from austenitic stainless steel. During a periodic examination cracks were revealed at the external surface of one of the vessels. External patch welds had been applied at these and several other corresponding locations. Cracks visible on the external surface developed from the inside in a region close to the toe of the internal fillet weld to the deflector plate, and another deep crack associated with a weld cavity is visible slightly to the right of the main fissure. Microscopic examination revealed that precipitation of carbides at the grain boundaries had taken place in the vicinity of the cracks, but that the paths of the cracks were not wholly intergranular. Conditions present in the vicinity of the internal fillet weld must have been such as to favor both inter- and transgranular cracking. It is probable that the heating associated with the repair welds made from time to time also contributed to the trouble. The transgranular cracks, however, were indicative of stress-corrosion cracking.

A failed crosshead of an industrial compressor was examined using optical and SEM. The crosshead was an ASTM A148 grade 105-85 steel casting. On the basis of the observations reported and available background information, it was concluded that the failure began with the initiation of cracks at slag inclusions and sharp fillets in weld-repair areas in the casting. The weld-repair procedures were unsatisfactory. The cracks propagated in a fatigue mode. he casting quality was judged unacceptable because of the presence of excessive shrinkage porosity. It was recommended that crosshead castings be properly inspected before machining. Revision of foundry practice to reduce or eliminate porosity was also recommended.

The onset of leakage adjacent to two butt welds in a 2 in. bore pipe was traced to the development of fine cracks. The pipe carried 40% sodium hydroxide solution. The actual temperature was not known, but the pipeline was steam traced at a pressure of 30 psi, equivalent to a temperature of 130 deg C (266 deg F). Magnetic crack detection revealed circumferential crack-like indications situated a short distance from the butt weld. Cracking originated on the bore surfaces of the tube and was of an intergranular nature reminiscent of caustic cracking in steam boilers. The strength of the solution of caustic soda and possibly the temperature also were in the range known to produce stress-corrosion cracking of mild steels in the presence of stresses of sufficient magnitude. In this instance the location of the cracking suggested that residual stresses from welding, which approach yield point magnitude, were responsible. As all other welds were suspect, the remedy was to remove the joints and to reweld followed by local stress relief.

A sample tube was removed from a reformer furnace for life assessment after 69,000 h of service. Sections were cut from the tube, which was a spindle cast A297 Grade HK 40 (25 Cr, 20 Ni, 0.4 C) austenitic steel of 122.5 mm OD and 10.5 mm nominal wall thickness. They were examined metallographically on transverse sections and on longitudinal sections through the butt welds joining the separate cast segments of the tube. Creep damage was mainly concentrated within the inner one third of the wall thickness. The use of damage assessment parameters in evaluating the reformer tube remaining life showed the welds to be inadequate, and to have a strength and creep resistance below those of the base metal.

A front-wheel drive hatchback automobile was involved in a severe front end impact. Failure analysis of the automobile revealed only a single sound spot weld in each of two 66 cm (26 in.) sections of both upper and lower floor sill flanges. Consequently, upon impact, the floor pan separated from the rocker panel, buckled and rotated upward and forward. This introduced slack in the seat belts since their retractors, being anchored to the floor pan, also rotated forward. Although not contributory to the accident itself, the faulty welds were responsible in part for the severity of the injuries sustained by the driver. The faulty welds in the unit body were apparently a consequence of improper settings of parameters on a multihead electrical resistance spot welding machine. Lack of appreciation of the hazard associated with failure of this weldment may have contributed to the low frequency of their physical inspection during production. A similar case involving faulty welds in a fuel delivery truck is also discussed.

The fan used to cool a diesel engine fractured catastrophically after approximately 100 h of operation. The fan failed at a spider, which was resistance spot welded to a shim placed between two circular spiders of 3 mm thickness. The detailed analysis of the fracture indicated that the premature failure of the fan was due to inadequate bonding between the sheets at the weld nugget. The fracture was initiated from the nugget-plate interface. The inadequate penetration and lack of fusion between the steel sheets during resistance spot welding led to poor weld strength and the fracture during operation. The propensity to crack initiation and failure was accentuated by improper cleaning of the surfaces prior to welding and to inadequate nugget-to-sheet edge distance.

The piston rod of a steering damper on a single decker bus fractured after 100,000 miles of service in the fully-extended left full-lock position. The steering damper, which is similar in shape and operation to a telescopic shock absorber, was secured by ball joints locked with slotted nuts. The steel piston rod fractured at the axle end leaving approximately 5 mm of rod welded to a securing ferrule. The failure was caused by a fatigue mechanism. Small surface cracks formed during welding in the heat-affected zone close to an unradiused shoulder in the piston. Under alternating stresses in normal service these cracks propagated through the piston rod made less tough by the extended weld heat-affected zone.

A suspension bushing separated from the disk on the rear wheel suspension of a racing vehicle while under operation on an express highway, causing the wheel to detach from the car. Visual inspection showed fresh turning grooves at four built-up fillet welds on the torus of the outside of the disk. The welding heat and rapid cooling caused the base material of the disk – already with a martensitic structure – to harden throughout the torus, both of which caused cracking in the inner and outer fillets at the transition from bushing to disk. Visual inspection of the other rear wheel showed similar stress cracks in the hardened base material of the transition region as well as the same four welds. Rough finishing and the sharp-edged formation of the cross section transition may have also contributed to the failure. The results of the investigation suggested that machine shops neither execute nor permit repair-welds on highly stressed machine parts and especially vehicle components.

A type 321 stainless steel bellows expansion joint on a 17-cm (6 in.) OD inlet line (347 stainless) in a gas-turbine test facility cracked during operation. The line carried high-purity nitrogen gas at 1034 kPa (150 psi) with a flow rate of 5.4 to 8.2 kg/s (12 to 18 lb/s). Cracking occurred in welded joints and in unwelded portions of the bellows. The bellows were made by forming the convolution halves from stainless steel sheet, then welding the convolutions together. Evidence from visual examination, liquid penetrant inspection chemical analysis, hardness tests, and metallographic examination of sections etched with Vilella's reagent supports the conclusions that failure of the bellows occurred by intergranular fatigue cracking. Secondary degrading effects on the piping existed as well. Recommendations included the acceptability of Type 321 stainless steel (provided open-cycle testing does not result in surface oxidation and crevices) Although type 347 stainless steel would be better, and Inconel 600 would be an even better choice. Welds would also need modified processing for reheating and annealing. Prevention of oil leakage into the system would minimize carburization of the piping and bellows.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

The top tube of a horizontal superheater bank in the reheat furnace of a steam generator ruptured after seven years in service. The rupture was found to have occurred in the ferritic steel tubing (2.25Cr-1Mo steel (ASME SA-213, grade T-22)) near the joint where it was welded to austenitic stainless steel tubing (type 321 stainless steel (ASME SA-213, grade TP321H)). The surface temperature of the tube was found to be higher than operating temperature in use earlier. The ferritic steel portion of the tube was found to be longitudinally split and heavily corroded in the region of the rupture. A red and white deposit was found on the sides and bottom of the tube in the rupture area. The deposit was produced by attack of the steel by the alkali acid sulfate and had thinned the tube wall. It was concluded that rupture of the tube had occurred due to thinning of the wall by coal-ash corrosion. The thinned tubes were reinforced by pad welding. Type 304 stainless steel shields were welded to the stainless steel portions of the top reheater tubes and were held in place about the chromium-molybdenum steel portions of the tubes by steel bands.

The case and stiffener of an inner-combustion-chamber case assembly failed by completely fracturing circumferentially around the edge of a groove arc weld joining the case and stiffener to the flange. The assembly consisted of a cylindrical stiffener inserted into a cylindrical case that were both welded to a flange. The case, stiffener, flange, and weld deposit were all of nickel-base alloy 718. It was observed that a manual arc weld repair had been made along almost the entire circumference of the original weld. Investigation (visual inspection, 0.5x macrographs, and 10x etched with 2% chromic acid plus HCl views) supported the conclusions that failure was by fatigue from multiple origins caused by welding defects. Ultimate failure was by tensile overload of the sections partly separated by the fatigue cracks. Recommendations included correct fit-up of the case, stiffener, and flange and more skillful welding techniques to avoid undercutting and unfused interfaces.

An outer fan-duct assembly of titanium alloy Ti-5Al-2.5Sn (AMS 4910) for a gas-turbine fan section cracked 75 mm (3 in.) circumferentially through a repair weld in an arc weld in the front flange-duct segment. Examination of the crack with a binocular microscope revealed no evidence of fatigue. A blue etch-anodize inspection showed the presence of an alpha case along the edges of the repair weld. The alpha case, a brittle oxide-enriched layer, forms when welds are inadequately shielded from the atmosphere during deposition. The brittleness of this layer caused transgranular cracks to form and propagate in tension under the thermal stresses created by the repair-weld heat input. The crack resulted from contamination and embrittlement of a repair weld that had received inadequate gas shielding. Thermal stresses cracked the oxide-rich layer that formed. The gas-shielding accessories of the welding torch were overhauled to ensure that leak-in or entrainment of air was eliminated. Also, the purity of the shielding-gas supplies was rechecked to make certain that these had not become contaminated.