This article introduces the basic calculation formula for selecting electric heating equipment and determining process parameters when heat treatment of flat steel wire cylindrical spiral compression elastic yellow with a length of 50m and an outer diameter of ф60mm by resistance heating method. The test results show that this method can meet the heat treatment of springs. Requirements, saving investment, simple and easy.
The heat treatment of a flat section cylindrical spiral compression spring with a length of 50m and an outer diameter of ф60mm is a foreign aid task we accept. In foreign countries, this kind of spring is made 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 such flat section springs. Therefore, most of the special-shaped section steel wires are rolled or drawn with round steel wires. After being wound into springs, they are quenched and returned. Fire treatment. Obviously, it is very difficult to quench and temper such a long spring. After investigation and analysis and a large number of experiments, we initially explored the resistance heating and quenching process of this long spring, and achieved relatively satisfactory results.
1. 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 schemes, 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.
2. Test equipment and results
1. Test equipment
The equipment used in the test includes: (1) a ZUDG-253 salt bath furnace transformer; (2) a set of insulation fixing devices for spring heating and quenching; (3) a cooling water tank; (4) WGG2-302 optical pyrometer One IRT-1200 infrared measuring instrument; one stopwatch.
2. The main content of the test
Determine the quenching heating time and tempering time, select the quenching medium and observe the metallographic structure.
3. Test results
(1) 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, an optical pyrometer and an infrared thermometer to measure the temperature of the test piece under certain electrical parameters. Continuous heating time to determine the quenching heating time.
(2) The choice of tempering time is tested with a stopwatch and an infrared thermometer. The results show that the tempering heating time is between 25-30S.
(3) 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%.
(4) Metallographic structure. The test results show that this type of spring is properly selected in electrical parameters and can ensure stable electrical parameters. After quenching for 2 minutes, the temperature can reach 860-880°C to obtain a very fine quenched structure. Obvious martensite acicular structure characteristics can only be observed under a 1000x optical microscope. The tempering process is 380℃×28S, and the hardness can reach 39-43HRC. The metallographic structure is tempered troostite, with a small amount of tempered astringent, and there is no obvious decarburized layer, so it has good comprehensive mechanical properties.
3. Analysis of test results
1. 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, there is generally no 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, the inner side of the coil is thicker than the outer side during winding, so the heating time is extended 2min is more appropriate.
2. 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. However, when the working section changes significantly, it may cause the local temperature to be too high, or even burn out. Therefore, when using resistance heating equipment to heat the workpiece, especially when handling such special-shaped section springs, prepare an air compressor. The machine performs partial cooling to reduce the temperature difference; at the same time, the rust on the surface of the workpiece must be removed before heating to prevent poor electrical heating from contacting the workpiece and generating electric sparks to burn the workpiece.
3. This test uses polyvinyl alcohol fire agent, which can meet the requirements of spring quenching hardness, reduce deformation and out-of-line phenomenon. The cooling rate of this kind of medium can be between water and oil, which can greatly reduce the austenite to Martensite The thermal stress and phase change stress of the volume transformation reduce the deformation and cracking of the workpiece.