Investigation of fracture of carbon steel springs used for furniture application
The article deals with the investigation of fracture reasons of zigzag springs made from patented and tempered hypereutectoid steel wires. Microscopy analysis was performed for identification of quality of springs’ material. It was specified that index i of non-metallic inclusions was the lowest according standard EN 10270-1:2011, i.e. i = 0.5 and surface defects such as decarburization and roughness examining twelve occasionally collected samples did not exceed the permissible limits. A number of wire rods with cleaned surface from phosphate coating were tempered: 1) in industrial furnace together with spring pallets, 2) in laboratory furnace with welded thermocouples for precise temperature determination. Comparison of colours of oxides films revealed that during tempering of spring pallets in industrial furnace they did not reach the required temperature of 300 °C. The average of temperature inside the chamber of industrial furnace reached just 200-240 °C. The tensile test of steel wires tempered at different temperatures heating for various durations allowed selection of the optimal regime of heat treatment of hypereutectoid steel springs.