A recuperator for blast heating of a cupola furnace became unserviceable because of the brittle fracture of several finned tubes made of heat resistant cast steel containing 1.4C, 2.3Si and 28Cr. The service temperature was reported as 850 deg C. This led to the suspicion that the fracturing had something to do with the precipitation of sigma phase. Metallographic examination showed that the multiaxial stresses caused by sigma phase formation and the related embrittlement was the cause for the fracture of the recuperator. A steel of lower chromium content with no or little tendency for sigma phase formation would have had adequate corrosion resistance at the relatively low service temperature.

Three radially-cracked disks that circulated the protective gases in a bell-type annealing furnace were examined. During service they had been heated in cycles of 48 h to 720 deg C for 3 h each time, then were kept at temperature for 15 h followed by cooling to 40 deg C in 30 h, while rotating at 1750 rpm. Two disks were cracked at the inner face of the sheet metal rim while the rim of the third was completely cracked through. An analysis of the sheet metal rim of one of the disks showed the following composition: 0.06C, 1.98Si, 25.8Cr, and 35.8Ni. A steel of such high chromium content was susceptible to s-phase formation when annealed under 800 deg C. The material selected was therefore unsuitable for the stress to be anticipated. In view of the required oxidation resistance, a chromium-silicon or chromium-aluminum steel with 6 or 13% Cr would have been adequate. If the high temperature strength of these steels proved inadequate, an alloy lower in chromium would have been preferable.

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.