Detection of Ni–Sn phases in temper embrittled Ni–Cr steels
A GENERIC problem in ferritic steels is the reduction in the grain boundary cohesive strength produced by the segregation of trace impurities of Groups IVB and VB of the Periodic Table. Such segregation results in a deterioration in mechanical properties produced, for example, by reversible temper embittlement1 and it also promotes enhanced susceptibility to a range of environmental enhanced cracking processes2–4. It has long been recognised that the embrittlement of steels is enhanced by the presence of certain alloy elements, that is Ni, Cr or Mn, and measurements using Auger electron spectroscopy (AES)5–7 have demonstrated that specific combinations of alloy and impurity elements cosegregate to the boundaries, for example, Ni+Sn, Ni+Sb, Cr+P. These observations can be rationalised using Guttmann's phenomenological theory8 which attributes the increased driving force for segregation to a positive interaction between alloy and impurity solute atoms. Previous investigations1,9–11 have failed to detect the presence of any three dimensional phase using conventional transmission electron microscopy and it is generally considered that the alloy element–trace impurity atoms form two dimensional phases at the boundaries. The only direct evidence for such alloy–impurity interactions and phase formation has been obtained using Mössbauer spectroscopy12,13 but it is not possible using this technique to draw any positive conclusions regarding the location of such compounds. Measurements on a pure Fe-3.5 wt% Ni alloy doped with 119Sn12 showed that Ni atoms associate preferentially with Sn atoms during tempering at 650°C and the marked change in the spectrum observed on ageing at 500°C, within the embrittling temperature range, is consistent with the formation of a Ni–Sn compound (Ni3Sn2 or Ni3Sn4). We report here some initial results from a detailed electron microscopic analysis of the microstructures of a commercial (En30A) Ni–Cr steel and a special laboratory made steel (BE 10) having similar Ni and Cr contents, but doped specifically with tin. The composition of the two steels is presented in Table 1. Both steels were oil quenched from 950°C, tempered for 1 h at 650°C and aged at 500°C for times up to 5,000 h. Carbon extraction replicas, taken from polished surfaces were examined in a Philips EM400 microscope fitted with an energy dispersive X-ray system. Selected individual precipitate particles were analysed using convergent beam microdiffraction and X-ray analysis. Attention was focused on the particles which delineated the lath packet and previous austenite boundaries, that is, high-angle boundaries.
Ni-Sn interaction in temper embrittled steel detected by Mössbauer spectroscopy
PREREQUISITES for reversible temper embrittlement of steel1 are the presence of a trace impurity2 (Sb, Sn, As or P) and a major alloying element3,4 (Ni, Cr or Mn). Auger electron spectroscopy (AES) demonstrated that in the embrittled state these elements are segregated to within the first few monolayers of the high angle boundaries5,6. The segregation is reversible and can be removed by heating at temperatures above the embrittlement range, whilst still within the α field (that is > 600 °C). It has been postulated7 that the segregation behaviour can be explained using equilibrium thermodynamics. It is proposed that the trace impurities interact more strongly with the major alloying elements than the Fe matrix and hence the chemical potential of the system is reduced by segregation to the high angle boundaries with the formation of bi-dimensional complexes (such as Ni3Sn2) Mn3P and Cr3P). No experimental evidence is available for the existence of such complexes. AES gives no information regarding the chemical bonding of the segregants but it has been shown8, using X-ray photoelectron spectroscopy (XPS), that the segregated Mn on embrittled fracture surfaces of a 2% Mn steel is in a different chemical state to that on unembrittled surfaces. We report here preliminary results from a specially prepared 3% Ni steel, doped with 119Sn, which were obtained using Mössbauer spectroscopy to monitor changes in the nearest-neighbour atoms of the Sn atoms as a function of heat treatment in the temper embrittlement temperature range.
Temper embrittlement response and toughness of a rare earth treated Ni-Cr-Mo steel
Embrittled and unembrittled transverse Charpy V-notch impact energies are compared for three commercially-produced Ni-Cr-Mo steel plates. Two of the plates were made from ingots which had been treated by the addition of mischmetal during teeming. Although the mischmetal additions were found to be ineffective in altering the susceptibility of the steel to temper embrittlement, they have a significant effect on improving the transverse toughness. It was concluded that rare earth additions, because of their beneficial effect on toughness, may provide a convenient means of coping with the problem of temper embrittlement.