At present, long flat wire coil springs are manufactured abroad by using oil-quenched and tempered steel wire to be wound into shape and then tempered at low temperature to eliminate winding stress. In China, there is no professional factory that produces this kind of springs, so most of these special-shaped cross-section steel wires are rolled or drawn from round steel wires. It is very difficult to quench and temper after winding into springs. . After a lot of research and experimentation, we explored the resistance heating and quenching process of this long spring, and achieved preliminary results. The results of the experiment are:
1. Test equipment and results
The instruments and equipment used in the test are:
(1) One ZUDG—253 salt bath furnace transformer;
(2) A set of fixing devices for the exhausted edge of spring heating and quenching;
(3) One cooling water tank;
(4) One WGG2-302 optical pyrometer, one IR-1200 infrared measuring instrument; one stopwatch.
The quenching heating time is determined according to the resistance heating is the internal heat source heating, and the temperature is a function of time. This test uses a stopwatch, optical pyrometer and infrared thermometer to measure the continuous heating time that the specimen can reach different temperatures under certain electrical parameters. , To determine the quenching heating time. The selection of quenching medium was tested according to the three quenching mediums of water and oil polyvinyl alcohol. From the test results, these three quenching media can meet the requirements of hardness and hardenability, but the water-quenched specimens are obviously brittle, and the oil pollution is large and easy to catch fire. After a large number of experiments and comparisons, it is more appropriate to choose a polyvinyl alcohol quenching agent with a concentration of 0.5%.
2. Determination of resistance heating process plan and equipment design
According to the longer characteristics and technical requirements of this spring, we compared the following three process options, namely flame quenching, high frequency quenching and resistance heating quenching. After analysis and comparison, we believe that although the resistance heating quenching program also has certain technical difficulties, The biggest feature is professional equipment that does not require the spring to rotate during heating and does not spray coolant. Therefore, the resistance heating quenching method is determined to be tested. We deduced the function of temperature change over time on the basis of heat and electricity, and in turn deduced the change law of electric energy into thermal energy and radiant heat during resistance heating process, as the reference basis for determining the parameters of resistance heating workpiece. Since the derivation is long and does not belong to the main content of this article, it is omitted here and only a brief introduction to equipment selection.
Afterwards, our experimental results analyzed that resistance heating has a higher heating rate, which can refine the austenite grains when the steel is heated. When the heating time is properly controlled, generally there will be no overheating. For resistance heating, time is very sensitive to temperature, so the material and cross-sectional dimensions are required to be uniform. On the whole, the resistance method has a fast heating speed and proper time control will not cause overheating. The relationship between oil quenching hardness and heating time is known: Although the hardness requirement can be achieved by heating for 1.5 minutes under stable electrical parameters, but taking into account the factor that the inner side of the coil is thicker than the outer side during winding, the heating time is extended 2 minutes is more appropriate.