Volume 49, Issue 8

High-strength steels exhibit different fracture surface characteristics under cyclic load when failure is initiated by non-metallic inclusions depending on the lifespan. These can be attributed to different crack propagation mechanisms. These characteristics are correlated with the short and long crack propagation. As far as the short crack is concerned, the mechanism of the discontinous decohesion of carbides in the immediate vicinity of the inclusions and the merging of the resulting microcracks to form a macrocrack is discussed. This postulated mechanism is investigated by means of a combination of scanning electron microscope (SEM) analyses of the fracture surface and target preparation by means of focused ion beam (FIB) by a detailed resolution of the local microstructure around the crack-initiating inclusion and the crack path.

Characterization and evaluation of diffusion bonding process temperatures of AISI 1010 plain carbon steel (PCS) with AISI 430 ferritic and AISI 420 martensitic stainless steels were experimentally studied. The diffusion bonding of AISI 1010 with AISI 430/420 steel couples were held for 40 min, at 800°C–900°C–1000°C– 1100°C, under protective atmosphere, with 10 MPa load blow those that would cause macroscopic deformation to the contacting surfaces. Microstructure morphologic evaluations were carried out with the help of SEM. Shear tests and weld efficiency were calculated at interface for mechanical properties characterization. Test results and morphologic evaluations showed that under given diffusion bonding conditions, welding/bonding integrity were slightly increased at elevated temperatures and highest values were traced at 1000°–1100°C process temperatures.

After starting the austempering nucleation of bainitic ferrite was observed within a very short time. Samples that austempered for 2 min.showed martensite in the microstructure. By increasing the austempering time from 30 min to 90 min the retained austenite decomposes and X-ray diffraction observations revealed the greatest volume fraction of retained austenite up to 17.3 and 23.8 percent when austempered for 30 min. Micro-hardness of the bainitic ferrite increased up to 370 and 500 HV and micro-hardness of the retained austenite increased up to 300 and 400 HV at the austempering temperatures of 300°C and 400°C respectively. In comparison to austempered ductile iron, the austempered compacted graphite cast iron shows higher rate of bainitic reaction. Bainite formation driving force and consequently the rate of austempering process are higher in compacted graphite cast irons.

Damage by corrosion was discovered on a dropped handlebar made of AA7075 during an inspection at a workshop. The possible causes are presented and the corrosion types encountered are interpreted. Stress corrosion or corrosion fatigue under the effect of chloride can be concluded from the damage pattern and reconstructed conditions of use. The heterogeneous microstructure containing a large number of intermetallic precipitates accelerates the start of corrosion because microgalvanic elements are formed. Although aluminium alloys, when in an urban environment, do not exhibit a pronounced tendency to corrosion the conditions of corrosion can locally change in use to such an extent that corrosion damage will occur. In the present case, a remedy can primarily consist in inspections at well-adapted intervals and the exchange of the handlebar tape in good time.