Observations of the effects of temperature and crystallographic orientation on surface segregation in an Fe-Si-Sn-C alloy

Auger electron spectroscopy was used to study segregation of Group IVB solutes to the surfaces of individual grains of a poly crystalline Fe-3.1 pct Si-0.06 pct Sn-0.004 pct C alloy heated in UHV. As the temperature was increased C, Si, and finally Sn segregate in that order, consistent with their diffusivities in Fe, each succeeding element expelling the preceding from the surface. The interactions between C and Si and between Si and Sn appear to be strongly repulsive, while the interaction between C and Sn appears weak; this is discussed in terms of a size effect. The segregation behavior of each element depends on the crystallographic orientation of the surface; C and Sn seem to follow similar patterns, while Si behaves somewhat differently.     

The Effects of Tempering Reactions on Temper Embrittlement of Alloy Steels

The effects of tempering reactions which produce molybdenum-rich carbides on the temper embrittlement of NiCrMo, NiCrMoV, CrMo, and CrMoV steels, particularly embrittlement due to phosphorus segregation, are reviewed. Molybdenum can act as an effective scavenger for phosphorus and other embrittling impurities, but the scavenging is lost when the molybdenum is precipitated in carbides as a result of continued tempering during service at elevated temperatures. This leads to very slow embrittlement, controlled by the rates of alloy carbide formation, rather than by the diffusion of phosphorus, for example. The presence of vanadium apparently retards the embrittlement process even more by interfering with the formation of the molybdenum-rich carbides. Observations of the temper embrittlement behavior, and of the effects of service exposure, in three CrMoV steam turbine rotors are also reported and are shown to be consistent with the previous results. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Experimental Evidence for the Existence of the Ti3B4 Phase

Experimental evidence for the formation of Ti₃B₄ with an orthorhombic Ta₃B₄-type structure is presented. This phase forms by a peritectic reaction between TiB₂ and liquid at ∼2200°C and is stable to temperatures at least as low as 1690°C, the lowest temperature at which samples were annealed. No evidence was found to indicate the presence of a second high-temperature form of this phase, as has been reported in the literature.     

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 σ_th 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.     

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. Formerly a Research Fellow at the Department of Materials Science, University of Pennsylvania, Philadelphia, PA 19174.     

Temper embrittlement of Ni-Cr steel by antimony:II. Effects of addition of titanium

It is shown that the addition of 0.1 pct Ti to a low carbon Ni-Cr steel can eliminate most of the susceptibility to temper embrittlement due to both step cooling and isothermal aging. The mechanism by which Ti acts is complex and not yet clear. It suppresses carbide formation during the embrittlement heat treatment, which should retard the rate of embrittlement (cf. Part I). However, it also appears to interact with Ni and Sb by enhancing the segregation of Ni to grain boundaries and by mitigating the embrittling effect of Sb.     

EFFECT OF STEAM AND HOT WATER WITH SODA ASH AND A WETTING AGENT ON PROPERTIES OF CLAY*

The use of combined treatments was found to Ix more efficient in improving the properties of clays than individual treatments, and the best results were obtained when the clay was pugged with steam and hot water with an addition of 0.2% of soda ash. This process lowered the water of plasticity 26%, the drying shrinkage 58%, arid the absorption 46.5%. The dry and fired strengths were increased 123% and 144%, respectively.     

Surface Structure Studies Of Electron Beam Annealed Ion Implanted Silicon

A method of annealing ion implanted silicon with a scanned electron beam is described. Annealing can be achieved without visible damage to silicon or silicon oxide in resistor structures. At excessively high powers, uncontrolled melting can be seen. The structure of the implanted surface layer is shown with SED to change from amorphous to single crystal after annealing.