Another failure in a turbogenerator, similar to the accidents in Toronto described in Metal Progress in July 1956, was due to the presence of fatigue cracks at ventilating holes. These acted as stress-raisers during temporary and minor overspeeding, inducing an almost instantaneous brittle failure which wrecked the machine, fortunately without human casualty.

The safety valve on a steam turbogenerator was set to open when the steam pressure reaches 2400 kPa (348 psi). The pressure had not exceeded 1790 kPa (260 psi) when the safety-valve spring shattered into 12 pieces. The steam temperature in the line varied from about 330 to 400 deg C (625 to 750 deg F). Because the spring was enclosed and mounted above the valve, its temperature was probably slightly lower. The 195 mm (7 in.) OD x 305 mm (12 in.) long spring was made from a 35 mm (1 in.) diam rod of H21 hot-work tool steel. It had been in service for about four years and had been subjected to mildly fluctuating stresses. Analysis (visual inspection, 0.3x photographs, 0.7x light fractographs, and metallographic examination) supported the conclusions that the spring failed by corrosion fatigue that resulted from application of a fluctuating load in the presence of a moisture-laden atmosphere. Recommendations included replacing all safety valves in the system with new open-top valves that had shot-peened and galvanized steel springs. Alternatively, the valve springs could be made from a corrosion-resistant metal-for example, a 300 series austenitic stainless steel or a nickel-base alloy, such as Hastelloy B or C.

Admiralty brass (Alloy C44300) cooling tubes which were part of a heat exchanger in a turbogenerator that provided electricity to a manufacturing plant failed. A mixture of non-recirculating city and “spring pit” water flowed through bundles of tubes to cool the oil in which they are immersed. However, a problem developed when several of the brass tubes cracked transversely, allowing cooling water to mix with the oil. The presence of a tensile stress, intergranular cracks, and a corrosion product suggested the tube failures resulted from stress-corrosion cracking. The main corrosion product was cupric hydroxychloride. In addition to switching to a more corrosion-resistant alloy, extreme care should be taken in the manufacturing of the replacement tube bundles to avoid imparting any residual tensile stresses in the tubing. Analyses of city and spring-pit water were recommended also, to determine which contained the least-harmful corrosive chemicals.

The silver layer on a thrust bearing face experienced electrostatic discharge attack (the bombardment of an in-line series of individual sparks onto the soft bearing face), which destroyed the integrity of the bearing surface. The electrical attack appeared as scratches to the naked eye. Macrophotography showed that the attack was more severe at one edge of each pad, resulting in deeper grooving and a buildup of deposits, mostly silver sulfides. Microstructural analysis of a cross section indicated that the interface between the silver overlay and the substrate (beryllium copper) was sound and free of voids and foreign material. Corrosion products contained a large quantity of sulfur. The probable cause of the attack was the presence of electrical current within the system, with sulfides a possible contributing factor. Elimination of residual magnetism and grounding of the rotating system at appropriate locations were recommended.

Hydrogen embrittlement is the brittleness affecting copper and copper alloys containing oxygen which develops during heat treatment at temperatures of about 400 deg C (752 deg F) and above in an atmosphere containing hydrogen. The phenomenon of hydrogen embrittlement of copper and its alloys is illustrated by examples from practice and reference is made to data from recent publications on the subject. Embrittlement due to this cause can only be identified by microscopic examination because other modes of failure in copper; e.g., from heat cracking, mechanical overload, the formation of low melting point eutectics or corrosion; show a similar appearance when investigated on a macroscopic scale.

Four cadmium-plated ASTM A193 grade B studs from a steam line connector associated with a power turbine failed unexpectedly in a nil-ductility manner. Fracture surfaces were covered with a light-colored, lustrous deposit. Optical microscope, SEM, and EDS analyses were conducted on sections from one of the studs and revealed that the coating on the fracture surface was cadmium. The fracture had multiple origins, and secondary cracks also contained cadmium. The fracture topography was intergranular. The failures were attributed to liquid metal embrittlement caused by the presence of a cadmium plating and operating temperatures at approximately the melting point of cadmium. It was recommended that components exposed to the cadmium be replaced.

A 44.5 mm (1.75 in.) diam type 321 stainless steel seamless tube in a power-generating turbine failed after 19,000 h in service. The tube was used to carry a mixture of approximately 25% steam and 75% hot air. Three fractured pieces and part of the tube containing the mating fracture surface were examined. Both fractographic and metallographic features revealed that the failure was by thermal fatigue caused by the presence of biaxial thermal stresses on the inner surface of the tube. It was recommended that the steam and air be thoroughly mixed prior to entering the tube to decrease the temperature fluctuations of the inner surface.

Copper shortening has been found to occur in the rotor windings of turbo alternators and takes the form of a progressive reduction in the length of the coils leading to distortion of the end windings. The trouble results from the high loading which develops between successive layers of the strip conductor due to centrifugal force. This leads to a high frictional binding force between turns and prevents axial expansion under normal heating in service. Rotor trouble which proved to be due to copper shortening was found in a set rated at 27.5 MW. It was manufactured in 1934 at which time silver-bearing copper was not available. The use of hard-drawn silver-bearing copper for a rewind, in conjunction with special attention to blocking up the end windings, is confidently expected to effect a complete cure.

A combination of adverse factors was present in the disruption of a turbo-alternator gearbox. The major cause was the imposition of a gross overload far in excess of that for which the gearbox was designed. The contributory factors were a rim material (EN9 steel) that was inherently notch-sensitive and liable to rupture in a brittle manner. Discontinuities were present in the rims formed by the drain holes drilled in their abutting faces, and possibly enhanced by the stress-raising effect of microcracks in the smeared metal at their surfaces It is probable that the load reached a value in excess of the yield point within the delay time of the material so when the fracture was initiated, it was preceded by several microcracks giving rise to the propagation of a brittle fracture.

A cracked, martensitic stainless steel, low-pressure turbine blade from a 623 MW turbine generator was found to exhibit fatigue cracks during a routine turbine inspection. The blade was cracked at the first notch of the fir tree and the cracks initiated at pits induced by chloride attack. Examination of the blade microstructure at the fracture origins revealed oxide-filled pits and transgranular cracks. The oxide filled cracks appeared to have originated at small surface pits and probably propagated in a fatigue or corrosion-fatigue fracture mode. It was recommended that the sources of the chlorides be eliminated and that the remaining blades be inspected at regular maintenance intervals for evidence of cracking.