Several air heat exchangers failed in service in a pulp and paper operation. The tubes were made from AISI 316 stainless steel with an extruded aluminum fin mechanically bonded to the outside. Originally, the failures were blamed on poor tube to header welds. The units were sent back to the manufacturer for repair. Some of the units failed the hydrostatic test after they were repaired. Microscopic examination revealed the presence of branched transgranular cracks characteristic of stress-corrosion cracking. Only some of the tubes failed and these did so by stress-corrosion cracking. The most probable primary cause of the stress-corrosion cracking was local high residual stresses indicated by the areas of high hardness in the tubes. Low halogens in the water and airborne corrodents found normally in a pulp and paper mill were all that were required in the presence of high residual stresses in the tubes to initiate stress-corrosion cracking. Use of a low-carbon grade of stainless steel such as 316L was recommended to facilitate formation of the tube without producing excessive residual stresses. It was recommended also that failed units be segregated until it can be determined if the failure was related to operating pressure or some other unique cause.

A steam-condensate line (type 316 stainless steel tubing) began leaking after five to six years in service. The line carried steam condensate at 120 deg C (250 deg F) with a two hour heat-up/cool-down cycle. No chemical treatment had been given to either the condensate or the boiler water. To check for chlorides, the inside of the tubing was rinsed with distilled water, and the rinse water was collected in a clean beaker. A few drops of silver nitrate solution were added to the rinse water, which clouded slightly because of the formation of insoluble silver chloride. This and additional investigation (visual inspection, and 250x micrograph etched with aqua regia) supported the conclusion that the tubing failed by chloride SCC. Chlorides in the steam condensate also caused corrosion of the inner surface of the tubing. Stress was produced when the tubing was bent during installation. Recommendations included providing water treatment to remove chlorides from the system. Continuous flow should be maintained throughout the entire tubing system to prevent concentration of chlorides. No chloride-containing water should be permitted to remain in the system during shutdown periods, and bending of tubing during installation should be avoided to reduce residual stress.

A 680,000 kg (750 ton) per day ammonia unit was shut down following a fire near the outlet of the waste heat exchanger. The fire had resulted from leakage of ammonia from the type 316 stainless steel outlet piping. The outlet piping immediately downstream from the waste heat exchanger consisted of a flange made from a casting, and a reducing cone, a short length of pipe, and a 90 deg elbow, all made of 13 mm thick plate. A liner wrapped with insulation was welded to the smaller end of the reducing cone. All of the piping up to the flange was wrapped with insulation. Investigation (visual inspection, 10x unetched images, liquid-penetrant inspection, and chemical analysis of the insulation) supported the conclusion that the failure occurred in the area of the flange-to-cone weld by SCC as the result of aqueous chlorides leached from the insulation around the liner by condensate. Recommendations included eliminating the chlorides from the system, maintaining the temperature of the outlet stream above the dewpoint at all times, or that replacing the type 316 stainless steel with an alloy such as Incoloy 800 that is more resistant to chloride attack.

Field metallography and replication were performed on a type 316 stainless steel column in diglycol amine vacuum service to determine the cause of visible OD pitting on the column in several areas above the insulation support rings. The examination revealed transgranular stress-corrosion cracking beneath the pitted areas on the OD. The likely cause of the cracking was chloride stress corrosion, with chlorides deriving from the marine atmosphere and concentrating under the insulation around the support rings. A complete insulation evaluation, including repair or replacement, was recommended to prevent chloride buildup. Painting of the steel surface with an epoxy-phenolic or epoxy-coal tar was also suggested.

A narrow bone plate made of type 316 stainless steel and used to stabilize an open midshaft femur fracture failed. A crack at a plate hole next to the fracture site had been revealed by a radiograph taken 13 weeks after the operation. The plate was revealed to be slightly bent in the horizontal plane, and the fracture gap was considerably open. The screws and plates supplied by different manufacturers were revealed to be different with respect to microcleanliness (primary inclusion content) of the materials and only one of them was found to be according to specifications. The local crack formation was influenced by the presence of larger inclusions. The screw failed was revealed to have failed through a fatigue mechanism by the presence of striations in the scanning electron micrograph. The crack in the plate was revealed to have originated at the upper, outer corner of the plate by the beach marks which indicated the action of asymmetric bending and rotational forces.

A type 316 stainless steel pipe reducer section failed in service of bleached pulp stock transfer within 2 years in a pulp and paper mill. The reducer section fractured in the heat-affected zone of the flange-to-pipe weld on the flange side. The pipe reducer section consisted of 250 and 200 mm (10 and 8 in.) diam flanges welded to a tapered pipe section. The tapered pipe section was 3.3 mm (0.13 in.) thick type 316 stainless steel sheet, and the flanges were 5 mm (0.2 in.) thick CF8M (type 316) stainless steel castings. Visual and metallographic analysis indicated that the fracture was caused by intergranular corrosion/stress-corrosion cracks that initiated from the external surface of the pipe reducer section. Contributory factors were the sensitized condition of the flange and the concentration of corrosive elements from the bleach stock plant environment on the external surface. In the absence of the sensitized condition of the flange, the service of the pipe reducer section was acceptable. A type 316L stainless steel reducer section was recommended to replace the 316 component because of its superior resistance to sensitization.