C.J. McMahon

Initiation of cleavage in polycrystalline iron

Abstract The initiation of cleavage microcracks in two coarse-grained vacuum-melted ferrites containing 0.035 and 0.007 per cent carbon was studied by means of tensile tests carried out between room temperature and −195°c in conjunction with special metallographic procedures. The latter involved surface replication of prepolished tensile specimens at various stages along the stress-strain curve as well as progressive sectioning of such specimens to reveal the internal details. In this way, the roles of carbides, mechanical twinning and slip in the initiation of cleavage were determined. Cleavage microcracks develop in ferrite throughout the strain-hardening portion of the stress-strain curve at subzero temperatures, and are mainly initiated by the cracking of carbides. Twinning does not play an important part in crack initiation in these materials at temperatures down to − 195°c. Although carbides may crack copiously during plastic deformation, they lead to microcracking of the ferrite only when the applied stress is high enough to permit the carbide cracks to act as griffith cracks. A model for microcrack initiation by such carbide cracking is proposed.

A microscopic theory of brittle fracture in deformable solids: A relation between ideal work to fracture and plastic work

A criterion is developed for brittle fracture of a crystalline solid which is capable of being plastically deformed. The theory starts with the experimental fact that during extension of a brittle crack, energy is consumed not only by bond stretching and breaking, but also by dislocation emission from the crack tip. The latter is the “plastic work”. γp, which in the present theory depends on the ideal work of fracture. An empirical relationship between stress and dislocation velocity is employed to calculate the work connected with dislocation emission; then an approximation of the dynamics of bond stretching is made. Combination of the two allows the development of a Griffith-type thermodynamic fracture criterion which is applicable to any deformable solid. This theory contains all the basic features of ductile vs brittle behavior of solids, regardless of whether the brittle mode is transgranular cleavage or intergranular fracture. As a consequence, a relationship between γp and γ is obtained which enables one to estimate γ from measurements of local fracture stress. The use of this is illustrated in the form of estimates of reductions in γ due to segregation of several impurities, calculated from measured local integranular fracture stresses in experimental steels.

Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann.

Intergranular Fracture in 4340-Type Steels: Effects of Impurities and Hydrogen.

A study has been made of the conditions which lead to intergranular brittle fracture in 4340-type steels at an ultra high yield strength level (200 ksi, 380 MPa) in both an ambi-ent environment and gaseous hydrogen. By means of Charpy impact tests on commercial and high purity steels, and by Auger electron spectroscopy of fracture surfaces, it is con-cluded that one-step temper embrittlement (OSTE or “500°F embrittlement”), and low K intergranular cracking in gaseous hydrogen are primarily the result of segregation of P to prior austenite grain boundaries. Segregation of N may also contribute to OSTE. Most, if not all, segregation apparently occurs during austenitization, rather than during tem-pering. Elimination of impurity effects by use of a high purity NiCrMoC steel results in an increase inKth for hydrogen-induced cracking by about a factor of five (to the range 130 to 140 MNm-3/2). These observations are discussed in terms of our understanding of the mechanisms of OSTE and hydrogen-assisted cracking.

The cooperative relation between temper embrittlement and hydrogen embrittlement in a high strength steel

A sample plate of HY 130 steel (5 pet Ni-0.5 pet Cr-0.5 pet Mo-0.1 pet V-0.1 pet C) was found to be quite susceptible to temper embrittlement. Step-cooling produced a shift in transition temperature of 583 K (310°C). In the step-cooled condition the plane strain stress intensity threshold for crack growth in 0.1 N H2SO2 was about 22 MNm-3/2 (20 ksi √in. ) and the fracture mode was inter granular, whereas in the unembrittled condition the threshold for a 1.27 cm (1/2 in.) plate (not fully plane strain) was around 104.5 MNm-3/2 (95 ksi Vin. ) and the fracture mode was mixed cleavage and microvoid coalescence. The interaction between the impurity-induced and the hydrogen embrittlement is discussed in terms of Oriani’s theory of hydrogen embrittlement.

Hydrogen-induced intergranular fracture of steels

Abstract The phenomenon of hydrogen-induced intergranular brittle fracture of steels is a result of the combination of hydrogen dissolved in the crystal lattice and embrittling impurities that had previously segregated at grain boundaries. This type of brittle cracking is a stress-controlled phenomenon and can be demonstrated in steels that have been embrittled by the elements that are responsible for temper embrittlement and tempered-martensite embrittlement. The presence of mobile hydrogen essentially facilitates the formation of an intergranular microcrack that can initiate brittle fracture. In the absence of sufficient segregated impurity, hydrogen produces enhanced cracking at relatively high stresses by concentrated plastic flow, a displacement-controlled phenomenon. Thus, the two types of hydrogen-induced cracking are fundamentally different.

The effects of segregated impurities on intergranular fracture energy

Abstract The segregation of metalloid impurities to grain boundaries of steel can result in reductions of fracture toughness of the order of 95%. It is shown that this can be caused by reductions in the work of ideally brittle fracture (i.e. the cohesive energy) of the order of 10%. This is based on the exponential dependence of the plastic work on the local stress field ahead of a brittle crack, which in turn can be related to the force-displacement curve for intergranular separation.

Oxygen-induced intergranular cracking of a Ni-base alloy at elevated temperatures: An example of dynamic embrittlement

Abstract The brittle intergranular cracking of Alloy 718 at temperatures between 550 and 650°C and at oxygen pressures from atmospheric to 10−5 torr has been studied by the use of single-edge-notched specimens loaded in pure bending at fixed displacement. The cracking rate was calculated from the load relaxation and a compliance–calibration curve. Rates in the range 10−9 to 10−5 m/s were observed, depending on temperature and oxygen pressure. The phenomenon appears similar to the oxygen effects observed by others in cyclic loading and also to the dynamic embrittlement observed in other materials and environments. The mechanism recently modeled for dynamic embrittlement can be applied here directly, and the special characteristics of polycrystalline (as opposed to bicrystalline) behavior, as well as the effects of varying the oxygen pressure, can be explained.

Intergranular embrittlement of iron-carbon alloys by impurities

It is shown that plain Fe-C alloys can be embrittled by certain heat treatments if they are doped with Sb, Sn, As or P. Embrittlement occurs when these elements are rejected from precipitating carbides during cooling, and it results from a piling-up of the elements ahead of the carbide and the concomitant lowering of cohesion along the carbide-ferrite interface. It is a transient (non-equilibrium) condition and disappears upon continued holding at elevated temperatures. Embrittlement by equilibrium segregation of these impurities in unalloyed ferrite apparently does not occur. A model for this transient embrittlement is proposed and tested. This model is relevant to the phenomenon of “500°F” embrittlement in alloy steels, and it is also consistent with most aspects of temper embrittlement.

Embrittlement of a 5 Pct Nickel high strength steel by impurities and their effects on hydrogen-induced cracking

Temper embrittlement induced by isothermal aging at 480°C (753 K) in a commercially produced heat of HY 130 steel is shown by Auger electron spectroscopy to be due mainly to intergranular segregation of Si, with additional contributions by P, N, and Sn. Studies of rates of crack growth in gaseous hydrogen at controlled temperature and pressure in specimens aged various amounts showed that, as the impurity concentration increased, the stress intensity for crack growth Kth dropped precipitously, the cracking mode changed from cleavage to intergranular, and the crack growth rate atK > Kth increased. Hydrogen-induced crack growth occurred in a step-wise, rather than continuous, fashion. The impurity-plus-hydrogen effect is discussed in terms of a model adapted from an earlier theory of Oriani.