Despite the successful development of 1000 MPa grade ferritic HSLA steel strengthened with a high number density of ultra-fine Cu-rich precipitates through minimal Ni additions, the steel requires a peak aging at 550 °C [8] that falls within the 300–600 °C temper-embrittlement regime. The 300–600 °C temper-embrittlement still remains an unsolved issue [1,10] in HSLA steels, limiting their widespread in various engineering applications, especially in turbine rotors applications. Temper-embrittlement is a problem not only for the HSLA steels but for all ferritic and martensitic steels. The sinking of the Titanic and the burst of a nuclear power plant in Hinkley point in 1969 [11] are the two well-known metallurgical catastrophes due to temper-embrittlement that are worth contemplating. The temper-embrittlement is also known as reversible or double-step embrittlement. It is characterized by the loss of impact toughness, right shift of ductile-to-brittle transition temperature (DBTT), and brittle intergranular impact fracture along prior austenite grain boundaries. The fracture path corresponds well to the segregation of tramp elements such as phosphorus (P), antimony (Sb), tin (Sn), sulphur (S), and arsenic (As) [12] at the prior austenite grain boundaries after tempering between 300 and 600 °C or at even higher temperatures followed by slow cooling [13]. The embrittlement is even worse in the presence of Cu, Cr, Ni, and Mn [[14], [15], [16]].

1. Introduction – history of failure in transition region and case studies
One of the first low temperature fracture problems which involved the academic community, was the failure of Liberty ships. Out of 4964 ships of this type, made by welding during and before the Second World War, 233 literally broke in half, whereas 1289 had suffered severe damage. These fractures were brittle with very little or no plastic strain. Initially, it was believed that the cause of failure was the increased brittleness of welded joints compared to riveted structures, or that it was caused by quenching of material near the necks [1]. However, it was determined that, in addition to welding effects and residual stresses, the real cause was the toughness transition temperature of sheets from which the ships were made. While the sheets of ships that were destroyed had shown transition temperature between 15 and 70 °C, for sheets of non-damaged ships, this temperature ranged from 15 to 55 °C [2], [3], [4]. Due to this, the chemical composition of the used steel sheets have been changed. For example, the carbon content has been reduced and the manganese content has been increased, which led to the decreasing of transition temperatures as well as transition region lowering between −25 °C and 25 °C. This research then influenced studies about the sinking of the Titanic and other ships from that era.

During exploitation, pressure equipment are subjected to various working and environmental conditions, including work pressure and temperature, corrosion, etc., which could lead to brittle fracture. One of the first major disasters related to pressure vessels took place in 1919. Just 4 years after it was built in the Boston, a giant storage tank exploded, releasing a flood of molasses. After appropriate investigation, factors that lead to this catastrofic accident are: poor fabrication practices, low material toughness, and design practices contributed to the failures [5], [6]. In this period, the term transition temperature was still not known, since studies of this problem started with Liberty ships failures. Another catastrofic event was happened in South Africa 1973. A pressurized ammonia bullet-type tank that was operated near ambient conditions had required weld repairs near a dished head seam. The tank had not been stress relieved at the time of manufacture, nor after the weld repairs. Later material testing also indicated poor toughness properties with the transition temperature above ambient conditions [5]. During sevice, an ammonia leak developed in the dished head, which apparently chilled the area so that a 4-foot portion of the dished head failed “in an explosive manner” [5], [7]. A series of studies followed, for the purpose of determining and undertanding why certain disasters happened, so that they could be avoided in the future. Studies by Filipović [8], [9] can be used as examples, in which the analysis of failure of two pressure vessels revealed that it was caused by environmental conditions. The fracture mechanisms of both vessel was quite different, brittle in the first one, and completely ductile in the second, although the base metal (which was ferritic) of both vessel that they were made off was of the same thickness and sufficient specified ductility. For first case it is supposed that during the proof pressure test, the underground storage tank had been exposed to the temperature lower than the nill ductility transition temperature, and the initiation of the britlle crack in plane strain situation can be attributed to this testing condition.