Hiroo Ohtani

Temper embrittlement of Ni-Cr steel by antimony I. Embrittlement at low carbon concentration

A detailed study of temper embrittlement by isothermal aging between 450 and 590°C and by step cooling has been made in anFe, 3.5 pctNi, 1.7 pctCr, 0.008 pctC alloy doped with 700 ppm Sb. Matrix hardness was eliminated as a variable by comparing embrittlement in the fully recrystallized starting condition with that in the quenched and tempered condition. Embrittlement in the recrystallized condition was analogous to that in the quenched and tempered condition, except that the kinetics were slower in the former. AES measurements showed that the transition temperature was directly proportional to the intergranular Sb concentration, which varied with the aging time and temperature. The maximum rate of embrittlement occurred at or above 520°C. The embrittlement appeared to be stable, and it appeared that a steady state was approached at aging temperatures ≥560°C in times ≤8500 h. It was concluded that Ni and Sb enter the grain boundary separately and exist there in solution, rather than as a stoichiometric compound. However, this build-up of Ni and Sb is due to a cooperative process, since a definite relationship between the two concentrations was found; the results are consistent with the recent thermodynamic analysis of Guttmann.

Morphology and properties of low-carbon bainite

Morphology of low-carbon bainite in commercial-grade high-tensile-strength steels in both isothermal transformation and continuous cooling transformation is lathlike ferrite elongated in the 〈11l〉b direction. Based on carbide distribution, three types of bainites are classified: Type I, is carbide-free, Type II has fine carbide platelets lying between laths, and Type III has carbides parallel to a specific ferrite plane. At the initial stage of transformation, upper bainitic ferrite forms a subunit elongated in the [−101]f which is nearly parallel to the [lll]b direction with the cross section a parallelogram shape. Coalescence of the subunit yields the lathlike bainite with the [−101]f growth direction and the habit plane between (232)f and (lll)f. Cementite particles precipitate on the sidewise growth tips of the Type II bainitic ferrite subunit. This results in the cementite platelet aligning parallel to a specific ferrite plane in the laths after coalescence. These morphologies of bainites are the same in various kinds of low-carbon high-strength steels. The lowest brittle-ductile transition temperature and the highest strength were obtained either by Type III bainite or bainite/martensite duplex structure because of the crack path limited by fine unit microstructure. It should also be noted that the tempered duplex structure has higher strength than the tempered martensite in the tempering temperature range between 200 °C and 500 °C. In the case of controlled rolling, the accelerated cooling afterward produces a complex structure comprised of ferrite, cementite, and martensite as well as BI-type bainite. Type I bainite in this structure is refined by controlled rolling and plays a very important role in improving the strength and toughness of low-carbon steels.

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.

Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowBs and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.

Steel Plates for Pressure Vessels in Sour Environment Applications

It is well known that wet hydrogen sulphide (H2S) can cause embrittlement of steels, hydrogen induced cracking (HIC) and sulphide stress corrosion cracking (SSCC). Several fractures of pipelines handling sour crude oil or gas led to vigorous researches on these problems. As similar failures have also been experienced in petroleum refinery equipment, degradation of steel by hydrogen sulphide is now recognized as a serious environmental problem. The paper considers the mechanism and factors involved in HIC. This type of cracking occurs mainly in the parent steels. The susceptibility of steels to cracking is influenced strongly by inhomogeneities such as the shape and distribution of non-metallic inclusions, and segregation of alloying elements. These have a significant effect on HIC because they modify the microstructures in the segregated regions. With reference to environmental factors, these mainly concern the influence of H2S partial pressures, pH of the solutions and other phenomena relevant to the absorption of hydrogen by the steel. SSCC poses problems in weld zones. It can occur especially in heat affected zones (HAZ) with high hardnesses. Such cracking can be prevented by the control of hardness by a suitable selection of the chemical composition of the steel and the welding conditions. Nevertheless, countermeasures similar to those described for the prevention of HIC are necessary to prevent SSCC in HAZ even with relatively low hardness. Research on factors influencing HIC and SSCC has resulted in the development of steels which are highly resistant to wet H2S cracking. These steels have been supplied in plate form for pressure vessels. Experience has confirmed the good performance of welded constructions in aggressive service environments.

The development of normalized and tempered A1-B treated 1 1/4Cr-1/2Mo steel plate

Strength and toughness of normalized and tempered 1 1/4Cr-1/2Mo steel plate decrease to a certain extent after stress relief annealing during fabrication. The addition of Al and small amounts of B increases the hardenability of 1 1/4Cr-1/2Mo steel for normalizing treatments and improves the strength and toughness so that enlargement of stress relief annealing condition up to 720°C × 50 h (temper parameter 21.5 × 103) is possible for plate thicknesses of 200 mm. The increase of hardenability during normalizing by a very slight amount of total boron is due to the redistribution of boron atoms during slow cooling. Such behavior of boron atoms is explained by the Gibbs absorption equation.

Effect of direct-quench processing on delayed fracture of high-strength steel

In this work the effect of direct-quench pocessing on the delayed fracture of high-strength steel has been investigated from the standpoint of impurity reduction