| Summary: | 碩士 === 國立高雄大學 === 化學工程及材料工程學系碩士班 === 104 === The purpose of this thesis was to investigate the cold forged low carbon steel which were the main material of construction and furniture screw. The wires of SAE1022 low carbon steel were common material for small fasteners. The wires could achieve better workability after spheroidizing annealing.
There were three kinds of heat treated wires from the wire plants with stable
spheroidizing annealing quality. And the parameters of spheroidizing annealing for draw were stated as follow:
(Sample A) Heat treatment condition: The temperature of spheroidizing annealing is 715°C, and the wires were taken out after 24 hours heating and cooling.
(Sample B) Heat treatment condition: The temperature of spheroidizing annealing is 700°C, and the wires were taken out after 22 hours heating and cooling.
(Sample C) Heat treatment condition: The temperature of spheroidizing annealing is 720°C, and the wires were taken out after 24 hours heating and cooling.
The steel wires were cold forged for head forming process. Then, the mechanical properties and microstructure of unformed and formed wires were tested and analyzed if there were differences. The test items included extension test, hardness test, microstructure analysis, spheroidization rate analysis and cold forging flow line analysis.
Through the experiment result, it was obviously observed under the optical
microscope that the carbide gathered around the recess of screw head of three samples due to the cold forging stress. It was reasonable because the cold forging of these three samples had continuous plastic flow. The spheroidization rate of the unformed and formed SAE1022 spheroidizing wires with three different parameters of spheroidizing annealing were all greater than 95%. There were no big differences in the spheroidization rate. The
spheroidization rate would not have a sudden reduction after formation. The differences of mechanical properties were not apparent as well. After cold
forging, the value of tensile strength and surface hardness of spheroidized wires were increased. It stated that the work hardening would affect the tensile strength and the surface hardness. Otherwise, the values of hardness and tensile strength of unformed and formed sample B were lowest. It could be understood through microstructure observation, the ferrite structure and the pearlite structure of unformed sample B had no direction continuity. After forming process, the ferrite was finer and the it was concerned with the sample B which had lowest values of hardness and tensile strength in chapter 4.3.1.
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