Erosion-corrosion of laser and thermally deposited coatings exposed in fluidised bed combustion plants

Coated steel tubes were exposed in two power plants (CFB and PFBC) fired with wood chips and coal with a small addition of olive seeds, respectively. Laser coating and two thermal spray techniques were used. The samples in the PFBC plant were immersed in the fluidising bed at material temperatures 450 \ub0C and 400 \ub0C. The exposure lasted for a total time of 8089 h. In the CFB plant a probe at the cyclone entrance had a material temperature of 630 \ub0C for the first 2100 h and then 480 \ub0C for 920 h. The material wastage was determined from tube cross-sections. The corrosion products and deposits were analysed by SEM/EDX, Auger spectroscopy, XPS and XRD. Cobalt based coatings show the best performance in both plants, while nickel based coatings are very sensitive to erosion but resistant to a corrosive atmosphere. The totally degraded chromium carbide containing coating in the CFB plant shows that the degree of corrosion is much larger in this environment. The same coating was excellent in the fluidised bed due to its high erosion resistance. The material wastage is independent of the deposition method. The coating technicques had no significant effect on the spallation behaviour

Microstructural Development in the Heat-Affected Zone of Laser-Cladded Steel

The surface of a medium carbon steel sheet was laser cladded with a Co-Cr alloy in two layers by a multi-pass technique. This study is concerned with the heat-affected zone (HAZ) created. Within this zone, austenitisation and martensite formation take place repeatedly twice or three times followed by later tempering. This change in microstructure is associated with a hardening and a softening. The microstructure consists of tempered martensite close to the heat source, an intermediate zone of tempered martensite/bainite and a partially transformed zone. However, the hardness is constant throughout the HAZ and decreases in the partially transformed zone to the substrate hardness. The analysis of the microstructure and the hardness are used to re-establish the temperature history encountered by the material in the HAZ. It could be shown that the HAZ has a translational symmetry that can be approximated by a 2-D sheet structure. \ua9 2005 Carl Hanser Verlag, M\ufcnchen

Erosion-corrosion of laser and thermally deposited coatings exposed in fluidised bed combustion plants

Coated steel tubes were exposed in two power plants (CFB and PFBC) fired with wood chips and coal with a small addition of olive seeds, respectively. Laser coating and two thermal spray techniques were used. The samples in the PFBC plant were immersed in the fluidising bed at material temperatures 450 \ub0C and 400 \ub0C. The exposure lasted for a total time of 8089 h. In the CFB plant a probe at the cyclone entrance had a material temperature of 630 \ub0C for the first 2100 h and then 480 \ub0C for 920 h. The material wastage was determined from tube cross-sections. The corrosion products and deposits were analysed by SEM/EDX, Auger spectroscopy, XPS and XRD. Cobalt based coatings show the best performance in both plants, while nickel based coatings are very sensitive to erosion but resistant to a corrosive atmosphere. The totally degraded chromium carbide containing coating in the CFB plant shows that the degree of corrosion is much larger in this environment. The same coating was excellent in the fluidised bed due to its high erosion resistance. The material wastage is independent of the deposition method. The coating technicques had no significant effect on the spallation behaviour

Comparison of the microstructures in continuous-cooled and quench-tempered pre-hardened mould steels

The increased demand for plastic mould steels in pre-hardened condition has drawn the attention to this specific type of steel. As a result, more investigations are performed to understand microstructure and properties. In this work, the microstructures of two pre-hardened plastic mould steels, one quench-tempered (Uddeholm Impax HH) and the other continuously cooled (Uddeholm Nimax), are studied in delivery condition by means of different microscopy techniques and are linked to their production procedure. The results show that the quench-tempered material contains large amounts of M3C carbides formed within the martensite plates as well as at the lath- and prior austenite grain boundaries. A few coarser Cr-rich M7C3 carbides have also been found. In comparison, the microstructure of the continuously cooled material consists of mainly bainite with much lower density and finer cementite particles. The hardness is with similar to 40 HRC more or less constant over the cross section of both materials

An Experimental Study to Understand the Effect of Cooling Media in Metal Cutting

Dry and Semi-Dry machining has gained interest in the last years because of the new environmental legislations and the possibility to decrease the machining cost. However, the role of cutting media such as emulsion, compressed air and Minimum Quantity Lubrication (MQL) on the chip formation has not been understood yet. The mechanical and thermo dynamical conditions in the cutting zone affects the properties of work piece material under cutting and these conditions change with the application of cutting media. In this work medium alloyed forged steel was machined with assistance of emulsion, MQL and only compressed air and the results were compared to dry machining in terms of chip color, micro structure, white-etching band and contact area between chip and the insert rake face. It was observed that white-etching band, which is believed to be ferrite, forms both on the chip upper free side and chip back side for all conditions. In emulsion cutting, this band has finest grains and with dry cutting it has largest grains proving a slower cooling with dry cutting. Not a big difference was observed on white-etching band when MQL, compressed air and dry machining were compared. However, it was observed that thermo-mechanical effect of air in MQL and compressed air supply has affected the contact area when the media was applied on rake face

An Experimental Study to Understand the Effect of Cooling Media in Metal Cutting

Dry and Semi-Dry machining has gained interest in the last years because of the new environmental legislations and the possibility to decrease the machining cost. However, the role of cutting media such as emulsion, compressed air and Minimum Quantity Lubrication (MQL) on the chip formation has not been understood yet. The mechanical and thermo dynamical conditions in the cutting zone affects the properties of work piece material under cutting and these conditions change with the application of cutting media. In this work medium alloyed forged steel was machined with assistance of emulsion, MQL and only compressed air and the results were compared to dry machining in terms of chip color, micro structure, white-etching band and contact area between chip and the insert rake face. It was observed that white-etching band, which is believed to be ferrite, forms both on the chip upper free side and chip back side for all conditions. In emulsion cutting, this band has finest grains and with dry cutting it has largest grains proving a slower cooling with dry cutting. Not a big difference was observed on white-etching band when MQL, compressed air and dry machining were compared. However, it was observed that thermo-mechanical effect of air in MQL and compressed air supply has affected the contact area when the media was applied on rake face

An Experimental Study to Understand the Effect of Cooling Media at Metal Cutting

Dry and Semi-Dry machining has gained interest in the last years because of the new environmental legislations and the possibility to decrease the machining cost. However, the role of cutting media such as emulsion, compressed air and Minimum Quantity Lubrication (MQL) on the chip formation has not been understood yet. The mechanical and thermodynamical conditions in the cutting zone affects the properties of work piece material under cutting and these conditions change with the application of cutting media. In this work a medium alloyed forged steel was machined with assistance of emulsion, MQL and only compressed air and the results are compared to dry machining in terms of chip colour, micro structure, white-etching band and contact area between chip and the insert rake face. It was observed that white-etching band, which is believed to be ferrite, forms both on the chip upper free side and chip back side for all conditions. In emulsion cutting, this band has finest grains and with dry cutting it has largest grains proving a slower cooling with dry cutting. Not a big difference was observed on white-etching band when MQL, compressed air and dry machining are compared. However, it was observed that cooling ability of MQL and compressed air has affected the contact area when the media was applied on rake face

Development and manufacture of brazed sintered part for wear resistant application

To replace a conventionally machined and brazed component the manufacture of a sintered cog-wheel Cu-brazed to a steel cup has been studied. The sintered steel is fabricated from water-atomized 0.85% Mo steel powder admixed with 3% Cu-powder and 0.8% graphite to a green density of 7.0 g/cm3 followed by sintering at 1120\ub0C for 30 min in nitrogen-bearing atmosphere with carbon control at -65\ub0C dew point. After sintering, brazing and partial Cu infiltration in endogas preceded the final oil hardening, which through hardened the material to 680 \ub1 30 HV10. Component field testing showed that the sintered parts only experienced mild abrasive wear, thereby surpassing the required life time performance. Studies of different field tested parts showed that failure or too short life time was associated with brazing problems and not to the performance of the sintered part itself further demonstrating the potential of the studied sintered part assembly