Hydrogen-assisted stress-corrosion cracking failure occurred in four AISI 4137 chromium molybdenum steel bolts having a hardness of 42 HRC. The normal service temperature (400 deg C, or 750 deg F) was too high for hydrogen embrittlement but, the bolts were subjected also to extended shutdown periods at ambient temperatures. The corrosive environment contained trace hydrogen chloride and acetic acid vapors as well as calcium chloride if leaks occurred. The exact service life was unknown. The bolt surfaces showed extensive corrosion deposits. Cracks had initiated at both the thread roots and the fillet under the bolt head. Multiple, branched cracking was present in a longitudinal section through the failed end of one bolt, typical of hydrogen-assisted SCC in hardened steels. Chlorides were detected within the cracks and on the fracture surface. The failed bolts were replaced with 17-4 PH stainless steel bolts (Condition H 1150M) having a hardness of 22 HRC.