AbstractThe effect of hot isostatic pressing (HIP) and electrochemical machining (ECM) on the microstructure and mechanical properties of cast steel, particularly the fatigue strength, has been investigated.
It is shown that microporosity in cast steel reduces the elongation, reduction in area, and the fatigue strength. However, hot isostatic pressing at an argon pressure of 103MN/m2 and at temperatures varying from 930 °C. to 1210°C. was effective in closing internal microporosity and improving the mechanical properties, particularly the fatigue strength, by up to 70% in a cast low alloy steel. The contribution made by homogenisation of microconstituents to the improvement in the fatigue strength was determined and is shown to be only marginal. The HIP of edge specimens having columnar crystals resulted in an improvement in the fatigue strength. This is attributed to the removal of the anisotropic columnar crystals by isostatic hot working.
The electrochemical machining of wrought and cast steels in a 10% sodium nitrate solution has been carried out. Both the surface finish and the fatigue strength are reduced after ECM and are strongly dependent upon the current density used. It is shown that in spite of their greater heterogeneity and inferior surface finish the reduction in the fatigue strength of cast steels is less than that of wrought steels. The stress relief annealing of mechanically polished specimens resulted in a reduction in fatigue strength of the same order as that obtained by ECM at a high current density. Clearly, ECM produces a surface free from microcracks and compressive stresses. ECM at a low current density similar to that of "stray machining" causes selective attack of the microconstituents with an increased reduction in fatigue strength. However, light shot peening of the surface increased the fatigue strength to a level higher than that of the base metal.
|Date of Award||Feb 1984|