Strength and Intergranular Fracture of Sintered Carbonyl Iron in Fatigue Test

Abstract

The fatigue strength of carbonyl iron compacts sintered at a temperature in the ferrite and austenite ranges and the intergranular fracture (IGF) observed in the fatigue tests were investigated.The results are as follows:(1) The fatigue strength of carbonyl iron compacts sintered for 3.6ks in the temperature range between 1073K and 1423K is higher in the ferrite and lower in the austenite field. The endurance limit ratio obtained is in the range between 0.41 and 0.47.(2) The fatigue strength of specimens sintered isothermally at 1123K and 1423K increases gradually with the increase in the sintering time, except that of specimen sintered for 172.8ks at 1123K which decreases remarkably.(3) The mode of fatigue fractured surface for specimens sintered at 1123K is a mixed trans-and intergranular fracture, while that for specimens sintered at 1423K is mainly transgranular.The fraction of IGF increases with the increase in the sintering time at 1123K and with the decrease in the carbon content. (Received July 27, 1984) Keywords: sintering conditions, carbonyl iron compact, fatigue strength, grain size, pore, intergranular fracture, transgranular fractureIt is well known that carbonyl iron compacts sintered below A3 transformation point, i.e., at the higher temperature in the ferrite field, show a different sintering behavior (1)(2) compared to other commercial iron powders.A carbonyl iron consists of fine and spherical particles with high purity, so that a fine, spherical pore structure is uniformly distributed in sintered bodies. It is the most suitable for investigating the relationship(3) between the pore structure and the mechanical properties of sintered bodies and for studying on the mechanism of sintering (2)(4).The aim of the present work is to investigate the effect of sintering time and temperature in a-and y-regionst on the fatigue strength of carbonyl iron compacts and to discuss the intergranular fracture (IGF) in these compacts sintered in a-region during fatigue test.Samples for the fatigue test were prepared from carbonyl iron powder (GAF-L, average mass%, oxygen content: 0.17mass%) mixed with 0.5mass% Zn-stearate as a powder lubricant. Solution of Zn-stearate in benzene was added to the iron powder, and then this mixture was continuously stirred by a mixer (rotating rate: 0.42r/s) for 3.6ks to evaporate the solvent completely and to recover the flowability of the powder.All compacts were dewaxed at 873K for 3.6 ks and sintered in hydrogen (dew-point: 243-248 K) under the conditions as shown in Table 1.The fatigue test specimens for this work types shown in Fig. 1(a). B and E series of specimens had the dimensions in the parenthesis. Fifteen fatigue specimens were prepared for each sintering condition. They were ground and subject to stress relief annealing at 773K for 3.6ks. The specimens were tested by means of universal fatigue testing machines