Ei-ichi Furubayashi

Crack propagation of high strength steels in a gaseous hydrogen atmosphere

The crack propagation behavior in delayed fracture was investigated at room temperature under pressure up to 784 kPa of hydrogen gas for high strength steels with the tensile strength of 1500 or 2000 MPa. For specimens with the tensile strength of 1500 MPa, the crack propagation rateda/dt increased with C content from 0.21 to 0.42 wt pct, and then it decreased with increasing C content up to 0.53 wt pct.da/dt increased rapidly with Mn content from 0.009 to 0.84 wt pct, and it increased gradually with Mn content from 0.84 to 2.13 wt pct. The permeation flow of hydrogen from the crack tip surface could be qualitatively estimated from the dependence ofda /dt upon the hydrogen pressure based on the simple assumption. In addition to the grain boundary embrittlement caused by tempering and/or hydrogen, the permeation flow of hydrogen was used to explain qualitatively the dependence ofda/dt upon C or Mn content.

Effect of Specimen Size on the Variant Selection in Martensitic Transformation

In thin sheet the anisotropy of the internal stress due to constraint by the surrounding parent phase which will be abbreviated as “constraint stress,” seems to occur during the martensitic transformation and the variant selection is expected also in the transformation without external stress. To verify this, the textures were measured by X-ray pole figure method on the γ and α phase which were developed in thin sheet of Fe-30 Ni alloy prepared by severe cold rolling and annealing followed by subzero quenching to liquid Nitrogen. Experimental data were analyzed with conventional variant selection models. The results obtained are as follows: (a) Variant selection phenomenon was clearly observed and the results could be explained with conventional variant selection models. (b) It is expected that different results occur between the conventional variant selection models when the martensitic transformation occurs accompanying the strain component of |e1|≦|e2|, where e1 is the strain along the Bain compression axis and e2 is along the perpendicular axis.

Transformation Texture Analysis of BCC and BCT Ferrous Martensite

The theoretical prediction of transformation textures, that developed in thin sheet without external stress, was carried out based on the Bain-Strain and Twinning-Shear model, respectively. To examine the validity of the prediction, the experiments were conducted with X-ray pole figure method on Fe-30Ni and Fe-30Ni-0.8C alloy sheets. The results obtained are as follows:

Effect of Stress on the Variant Selection in Martensitic Transformation

The anisotropic constraint stress will be induced in the specimens of thin sheets, when they are quenched below Ms temperature without external stress. The characteristics of variant selection phenomenon were studied under these conditions. The texture of y and of ferrous alloys were analyzed with X-ray method. The results obtained could be explained satisfactorily with the Bain Strain model as follows(1) Variant selection phenomenon depends on the orientation of the parent phase and on the Bain strain associated with martensitic transformation. (2) Such variants that have the largest Bain strain components to the sheet normal are selected predominantly, because the occurrence of variants are prevented least of all under the anisotropic constraint stress.

Tensile Properties of an Age-hardenable Fe-16Ni-4Si Alloy with Duplex Microstructures

An Fe-16%Ni-4%Si alloy having a large hardenability due to a maraging mechanism was used to study the effect of microstructures present prior to aging upon the properties of tensile strength. In an attempt to obtain microduplex structures showing a good combination of strength and toughness, the present alloy was cold rolled by 80% at a martensitic state (i.e., marforming) after conventional solution treatment in the γ field, and then annealed at the temperatures in the α+γ two phase range followed by air cooling, and finally aged.The marforming was found to increase both the tensile strength (σu) and the reduction of area (ψ) due to the formation of proper duplex structures. Large uniform elongation (eu) was observed after annealing at the temperatures around 570°C, and this was attributable to a transformation plasticity of precipitated austenite which was stabilized thermally. by partitioning of Ni. The strength, however, was rather low and σu=120kg/mm2. When annealed at a temperature range between 600°C and 650°C (still in the two-phase range), the reverted austenite was transformed on cooling into martensite which was strengthened by agingproportionally to the martensite fraction; the resultant duplex (martensite+ferrite) structure had a strength of σu=158kg/mm2, and a plasticity ofψ=27% and eu=1%, which were better than the combination, σu=132kg/mm2, ψ=25% and eu=1% in the same alloy that was age-hardened conventionally without marforming and annealing.The marforming had a dominant effect of reducing transgranular brittle fracture as well as intergranular fracture associated with preexisting coarse austenite grains.