Kenneth Hamberg

The influence of grain size and fracture surface geometry on the near-threshold fatigue crack growth in ferritic steels

Fatigue crack growth in the threshold region is very much affected by the microstructure. A large portion of this microstructural influence is due to changes in the crack closure stress intensity Kcl. The present paper is concerned with the influence of the grain size on the near-threshold fatigue crack growth in ferritic steels, with special emphasis on the influence of the microstructurally induced fracture surface roughness on the crack closure stress intensity. The experiments were performed on ferritic steels with grain sizes varying between 8 and 82 μm. An increase in the nominal and effective fatigue crack growth threshold values with increasing grain size was found. Quantitative fractographic measurements showed that the fracture surface roughness rises with the grain size independently of the stress intensity level. The results also indicate that Kcl is proportional to H13, where H is the fracture surface roughness as expressed by the mean standard deviation of height. By rationalizing the experimental data, ΔKth has been quantified in terms of grain size effects and the fracture surface roughness.

Crack initiation in ductile cast irons

Two different types of ductile irons have been investigated in this study, one ordinary ferritic/pearlitic grade 500 material (SS 140727), and one ductile iron solution hardened with silicon (SS 140725). Initiation sites for fatigue cracks and early fatigue crack propagation have been studied for the two irons. The irons have been examined both with conventional methods and with in situ experiments (DISMAP) on machined and polished specimens. The main result of the investigation was that the initiation sight was at highly strained points in the microstructure. In this case the micro porosities dominated over the bigger and often degenerated graphite nodules as preferred initiation locations for fatigue cracks. During propagation, the softer ferrite phase was under high strain and guiding the crack from nodule to nodule. The reason for that is that ferrite area in front of the crack tip plastifies. Control of the ferrite properties is therefore of utmost importance.

Variation of Tensile Properties of High Silicon Ductile Iron

The casting processes are characterized by complex relationships between predictors and responses. It is the fundamental understanding of these complex relationships that often involves hundreds of factors, which improves quality without losing productivity and raising cost. In this work, cast solid solution strengthened ferritic spheroidal graphite irons GJS-500-14 and GJS-600-10 (EN 1563:2012) have been evaluated. These materials offer stronger components with good machinability owing to their even hardness properties. In this case the predictors are chemical composition, gating layout, foundry set-up, testing procedure and equipment etc. and the responses are the tensile properties (Rp0.2, Rm, A5). Here 200 tensile specimens compiled from industrial foundry melts from over 30 years of research have created a state-of-the-art platform for statistical engineering in order to perform Exploratory Data Analysis (EDA) and data visualization. This statistical platform has provided new insight on how foundries should treat complex relationships between predictors and responses in order to identify sources of variation and interaction effects.