Although ferritic steels can exhibit cleavage fractures at reduced temperatures, experience suggests that certain types may be used with safety at temperatures down to about −150 F provided that appropriate precautions are taken in design and fabrication. While common mild steels have been in use in ammonia refrigeration plants for upwards of a century, the employment of welded joints in modern construction, as compared with the earlier use of fully machined components with screwed and bolted joints, requires a qualification such as this. Early refrigeration engineers were familiar with cast iron, which is always brittle to a degree, and either disregarded or did not know of the existence of transition temperatures; their attitude was justified by the results achieved. It is perhaps curious that the present-day concern with transition temperatures owes more to failures outside the refrigeration industry. Nevertheless, transition temperatures are usefully, if indirectly, related to lowest safe temperatures for service. In the determination of transition temperature it is first necessary to consider quality control tests, such as the Charpy test with either the V or the U notch, whose primary purpose is to ensure uniformity of day-to-day steel production. The degree of success that has been additionally achieved with such tests in correlating a measured value of energy absorption, ductility, or appearance of fracture with the temperatures of brittle fracture casualties—notably ships—is tempered when the range of steels involved is diversified. Thus the 15 ft-lb energy absorption that arose as a criterion for transition temperature for war-built ships has had to be raised by international agreement with the inclusion of varied steels of postwar ship construction, mainly because the latter are equally brittle as far as the structure is concerned at higher energy absorptions, albeit at lower temperatures.