In this work, the effects of phosphorus(P) and sulphur(S) on the environment-assisted cracking in high strength steels have been studied. The present work consists of the effects of P segregated at grain boundaries on the stress corrosion crack initiation and propagation, the effects of tempering temperature on the hydrogen-assisted crack propagation response and crack paths, and the role of sulphidic inclusions in hydrogen-assisted cracking(HAC). In addition, the characteristics of hydrogen trapping at sulphidic inclusions-matrix interfaces in mild steel have been studied to understand the underlying concepts of the resistance to the HAC of high strength steel.
Chapter III is concerned with the effects of phosphorus segregated at grain boundaries on intergranular corrosion(IGC) by an electrochemical method. The stress corrosion crack initiation and propagation of rotor steels has been studied as a function of applied stress in boiling 40wt.% NaOH solution at an applied passive potential of -530 $mV_SHE$ using the electrical potential method. Phosphorus segregation leads to IGC susceptibility. Intergranular attack by anodic dissolution is responsible for the crack initiation, irrespective of the applied stress and P-doping. The decrease in stress corrosion crack propagation time due to P segregated at grain boundaries is enhanced with increasing applied stress and is presumably related to crack branching. Fringe patterns on the intergranular facets of the fracture surface are found to be associated with martensite lath boundaries and are inferred to be caused by straininduced dissolution.
In chapter IV, effects of tempering temperature(250℃, 350℃ and 450℃) on the stress-corrosion (SC) crack propagation response and crack paths have been investigated with P-undoped and P-doped 3.5NiCrV and 3.5NiCrMoV steels in 3.5 wt.% NaCl solution at 30℃. The P-undoped specimens showed decreased SC crack propagation rates as the tempering temperature increased. However, the P-doped specimens showed the highest SC crack propagation rate on tempering at 350℃. It is concluded that the P segregation at prior austenite grain boundaries is primarily responsible for the two conflicting relationships between SC crack propagation rate and tempering temperature. As the applied stress intensity and the tempering temperature increased, the fracute mode changed from intergranular or quasi-cleavage mode to the microvoid coalescence mode. This is discussed in terms of our understanding of the micromechanisms for HAC of the respective fracture modes.
In chapter V, the role of sulphidic inclusions in HAC of low-alloy steels with different sulphur contents has been studied. The steels were previously cathodically charge in sulphuric acid solution. Evaluating the susceptibility to HAC in terms of the reduction in area values, the high sulphur steel is less susceptible than the low sulphur steel. The fracture surface of the low sulphur steel appears rather brittle, with local quasi-cleavage fractures around large intergranular fracture facets. In contrast, the fracture surface of the high sulphur steel is characterized predominantly by the microvoid coalescence mode, with a great number of manganese sulphide inclusions. The difference between the two sulphur steels is based upon the concept that the interfaces between sulphidic inclusions and matrix act trapping sites for hydrogen and thus hydrogen is uniformly distributed over the sulphidic inclusions.
Chapter VI is concerned with those factors which determine the hydrogen trapping at the interfaces between spheroidized and elongated sulphidic inclusions, and matrix in mild steel by using gas-phase charging and electrochemical detection techniques. Three kinds of specimens A, B and C were prepared from the calcium-treated mild steel by water quench from 950℃, and from the ordinary mild steel by water quench from 950℃ and 1150℃, respectively. Specimen A was characterized by the interface between the spheroidized sulphidic inclusions and matrix, but the specimens B and C were characterized by the elongated sulphide-matrix interface. The values of time-lagdecreased with increasing hydrogen input pressure for the specimens A, B and C. The results indicated that the defects produced at the interfaces act as saturable trap sites for hydrogen. The hydrogen trap density and binding energy were obtained from the plot of $[(t_T/t_L)-1]$ vs. $P_H2^{-1/2}$. The trap densities for the specimens A, B and C were found to be about 5.0 × $10^{-8}$, 2.1 × $10^{-7}$ and 5.0 × $10^{-7}$ mol ㎤, respectively. The trap-binding energy was determined to be -(56.4 ± 1.1) kJ $mol^{-1}$ for the specimens A, B and C as well. The experimental results indicated that the nature of the interfaces is determined by the number of defects produced in the interfaces per unit volume, regardless of the inclusion shape. The defects distributed around the interfaces included namely microvoids and water-quench-created dislocations which act as deep trap sites for hydrogen.
