J. Kameda

The effects of Sb, Sn, and P on the strength of grain boundaries in a Ni-Cr Steel

The previously developed method of correlating the local intergranular fracture stress with the probable composition of the grain boundary on which fracture initiated has been applied to Ni-Cr steels doped with Sb, Sn, and P so that the relative embrittling potencies of these elements can be compared. The results are discussed in terms of a hypothesis offered to rationalize the observed embrittlement effects. Experiments on the Sb-doped steel with two different intergranular Sb distributions support the position that brittle fracture in these notched specimens is controlled by the grain boundary with the maximum Sb concentration in the highly-stressed volume of material ahead of the notch.

Hydrogen-induced cracking in 4340-type steel: Effects of composition, yield strength, and H2 pressure

Measurements of the threshold stress intensity for hydrogen-induced crack extension,Kth at room temperature were made on bolt-loaded WOL specimens of a commercial 4340 steel and of laboratory heats in which the bulk concentrations of manganese, silicon, phosphorus, and sulfur were varied. The hydrogen pressure was varied from 200 to 1600 torr (~0.03 to 0.22 MPa), and the yield strengths were varied from ~170 to 270 ksi (~1200 to 1900 MPa). Measurements ofKIc in air were also made as a function of composition and yield strength. Significant differences betweenKIc in air andKth in H2 were found only in steels containing added Mn or Si; these elements are believed to promote segregation of phosphorus and sulfur to austenite grain boundaries. TheKth values were uniquely related to the percentage of intergranular fracture and also to a parameter containing the calculated maximum hydrogen concentration and the bulk concentrations of manganese, silicon, phosphorus, and sulfur. In a high purity steel free of manganese and silicon theKth was lower thanKIc only at yield strengths greater than 200 ksi (1400 MPa). The results are consistent with an additive reduction in cohesive strength by hydrogen and metalloid impurities. It is shown that theKth depends on hydrogen fugacity, yield strength, and grain boundary purity(i.e., cohesive strength).

Solute segregation and hydrogen-induced intergranular fracture in an alloy steel

Hydrogen-induced intergranular fracture of laboratory heats of a 3.5 Ni-1.7 Cr steel doped with P, Sn, or Sb and having a yield strength of 840 MPa and a prior austenite grain size of 120 μm has been compared with that of an undoped steel at a hydrogen pressure of 0.17 MPa (1.68 atm). The intergranular concentrations of the impurities were controlled by varying the time of aging at 480 °C. Cracking of the undoped steel tested in hydrogen occurred along martensitic lath boundaries at high stresses. However, the susceptibility of the doped steels to hydrogen-induced intergranular cracking increased precipitously with impurity concentration. The susceptibility was measured in terms of the threshold stress intensity Kthfor the first detectable crack extension in precracked specimens and in terms of the threshold stress σth for microcrack formation in notched specimens. A comparison between the intergranular strength in hydrogen and in air revealed that absorption of hydrogen produced a profound intergranular weakening when the grain boundaries contained even a small amount of a segregated embrittling element. The relative embrittling potencies of P, Sn, and Sb in hydrogen gas were the same as in air. The combined effects of hydrogen and the impurities in reducing intergranular cohesion are discussed in terms of a newly proposed dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack.

Solute segregation and intergranular brittle fracture in steels

Abstract A method for determining the local tensile stress σ* required to initiate brittle fracture at a grain boundary as a function of the concentration of segregated impurity on that boundary is presented. This method has been applied to Ni-Cr steels doped with Sb, Sn, or P, and the embrittling effects have been compared. In order to determine the relationship between the measured changes in σ* and the changes in the cohesive energy γ, a fracture theory has been developed which is the analogue of the Griffith theory for a deformable crystalline solid. In this theory the processes of bond breaking and dislocation emission at the crack tip are treated as concomitant so that a relationship between the local plastic work at fracture and γ is established. This gives criteria for unstable micro crack extension in terms of the relevant plastic properties of the crystal and of the cohesive energy γ. The relationship of the foregoing to the fracture toughness K 1C is discussed. The physical basis of embrittleme...

High temperature environmental attack and mechanical degradation of coatings in gas turbine blades

Abstract This paper examines how in-service and thermal environmental attack influence the mechanical properties (22–950°C) of CoNiCrAlY coatings over Rene 80 substrates in gas turbine blades using a small punch (SP) testing technique in conjunction with scanning Auger microprobe analysis. SP tests have clearly demonstrated strong dependence of mechanical degradation of near surface coatings on the elevated temperature environmental condition. The room temperature (RT) ductility in blade coatings decreased with increasing operating time under combined fuels of kerosene and liquefied natural gas (LNG) despite softening in used coatings. All the coatings depicted lower ductility at 825°C in air than at RT but not in vacuum so that the oxidizing environment would produce deleterious effects. In-service operation under the combined fuels led to a two-fold increase in the ductile—brittle transition temperature (DBTT) over coatings observed under mainly LNG because of more extensive oxidation and grain boundary sulfidation. However, the DBTT of coating did not change during thermal ageing at 870°C in air that produced only oxidation. These findings imply that the grain boundary sulfidation would exert a stronger embrittling effect on the CoNiCiAlY coatings than the oxidation.

Effect of Solute Segregation on Fracture Toughness in a Ni-Cr Steel

A study has been made of the influence of intergranular solute segregation on fracture toughness K1c in a series of Ni-Cr steels individually doped with Sb, Sn, and P. By means of toughness measurements in steels having two different intergranular Sb distributions, of measurements of acoustic emissions and of scanning electron micrographs of a load-interrupted and post-test-fatigued specimen, the values of K1c, computed from the “pop-in” load of the loadvs clip gauge displacement curves, are found to represent the formation of many patches of contiguous intergranular microcracks ahead of the precrack. The present experiments demonstrate that in the early stage of solute segregation, K1c decreases more substantially than does the strength of grain boundaries σ* (measured in the notched bar tests), although the embrittlement effects of metalloid elements are the same order for both K1c and σ*. A proposed model for the stress-gradient-control of brittle fracture supports the finding that the measurements of K1c give a distorted view of the progress of intergranular embrittlement.

Hardening and intergranular embrittlement in neutron-irradiated ferritic alloys

Abstract The characteristics of hardening and integranular embrittlement induced during neutron irradiation (0.94 × 10 23 n m −2 at 395 °C) in ferritic alloys doped with copper, phosphorus and/or carbon have been investigated using small punch test techniques. The effect of neutron irradiation on hardening was found to be greater in alloys doped with copper and/or phosphorous than in carbon-containing alloys. It has been shown that the great magnitude of irradiation-induced hardening observed for the copper- and/or phosphorus-containing alloys is attributed to an increase in both the athermal and thermal stress components. The neutron irradiation produced a more substantial increase in the ductile-brittle transition temperature in the copper-doped alloy compared with the other alloys. Intergranular and transgranular fracture occurred in the alloys doped without and with carbon respectively. The neutron irradiation did not alter the fracture mode.

Degradation Characteristics of Intermetallic Coating on Nickel Base Superalloy Substrate in Gas Ttairbine Blade

Abstract In-service degradation of the mechanical properties (295-1223 K) and microstructure/chemistry in gas turbine blades made of CoNiCrAlY coatings and Rene 80 substrates has been studied by means of a small punch (SP) testing technique and scanning Auger microprobe (SAM). In SP tests, brittle coating cracks continuously and discretely propagated along the radial and tangential directions at 295 K and elevated temperatures, respectively. The ductility of the coating and substrate at 295 K was lowered during long time operation of the blades. The ductile-brittle transition temperature of used coatings was increased by 90 K, compared with that of unused ones while that of the substrate remained unchanged. From SAM analyses of the unused and used blades, it was found that oxidation and S segregation near the coating surface region profoundly occur in-service. The relationship between the mechanical property degradation and microstructural/chemical evolution near the coating surface is presented which ser...

Mechanical properties of aluminized CoCrAlY coatings in advanced gas turbine blades

The microstructure/composition and mechanical properties (22-950 C) in aluminized CoCrAlY coatings of advanced gas turbine blades have been examined using scanning Auger microprobe and a small punch (SP) testing method. Aluminized coatings were made of layered structure divided into four regimes: (1) Al enriched and Cr depleted region, (2) Al and Cr graded region, (3) fine grained microstructure with a mixture of Al and Cr enriched phases and (4) Ni/Co interdiffusion zone adjacent to the interface SP tests demonstrated strong dependence of the deformation and fracture behavior on the various coatings regimes. Coatings 1 and 2 showed higher microhardness and easier formation of brittle cracks in a wide temperature range, compared to coatings 3 and 4. The coating 3 had lower room temperature ductility and conversely higher elevated temperature ductility than the coating 4 due to a precipitous ductility increase above 730 C. The integrity of aluminized coatings while in-service is discussed in light of the variation in the low cycle fatigue life as well as the ductility in the layered structure.

Oxidation/carbonization/nitridation and in-service mechanical property degradation of CoCrAlY coatings in land-based gas turbine blades

This article describes variations in the microstructure/composition and mechanical properties in plasma sprayed CoCrAlY coatings and a modified René 80 substrate of gas turbine blades operated for 21,000 h under liquefied natural gas fuels. Substantial oxidation/carbonization occurred in the near surface region of concave coatings, but not in the convex coatings. Aluminum and nickel/titanium-rich nitrides formed in near interface coatings and substrates of concave side of blades, respectively. Small punch (SP) specimens were prepared from the different blade location to examine the variation of the mechanical properties in the coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave side easily initiated up to 950 °C. The convex coatings exhibited higher ductility than the concave coatings and substrate and showed a rapid increase in the ductility above 800 °C. Thus it is apparent that the oxidation/carbonization and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coatings is discussed in light of the operating temperature distribution and compared to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxidation.