Common Defects and Preventive Measures of Heat Treated Workpieces

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During the heat treatment of the workpiece, due to improper process measures or other various factors, various defects will occur, and the existence of these defects directly affects the performance of the workpiece.

  During the heat treatment of the workpiece, due to improper process measures or other various factors, various defects will occur, and the existence of these defects directly affects the performance of the workpiece.
  1. Cracks: Heat treatment cracks mainly include quenching cracks, tempering cracks and grinding cracks. When heating some large-sized steel parts and alloy blanks, if the cooling speed is too fast during the cooling process, it will be very fast. Cracks are prone to occur, and in severe cases, there will be cracks in the section. Once the workpiece cracks, it is irreparable and can only be reprocessed. Therefore, in the heat treatment process, uniform heating and correct heating should be achieved as far as possible, appropriate cooling medium and cooling method should be selected, and the correct method of immersing the workpiece in the quenching medium and the direction of the workpiece should be selected.
  2. Overheating and overburning: Overheating refers to the phenomenon that the crystal grains are significantly coarsened due to excessive heating temperature or excessive holding time. The result is that coarse needle martensite is obtained after quenching, which causes a decrease in mechanical properties, especially a decrease in impact toughness, and an increase in brittleness, resulting in a decrease in fatigue strength. For workpieces that are not seriously overheated, carbon structural steel and alloy structural steel should generally be reheated and re-quenched after normalizing or annealing. For high carbon steel, alloy steel and tool steel, it should be annealed, normalized several times, and then re-quenched according to the correct quenching process.
  3. Overburning: that is, the temperature of the metal workpiece is too high during the heating process or the time of heat preservation and heating is too long. Oxygen and other oxidizing gases in the furnace penetrate into the gaps between the metal grains and combine with iron, sulfur, carbon Wait for oxidation to form a eutectic of fusible oxides, which destroys the connection between grains and reduces the plasticity of the material. In severe cases, it will crack at one blow, and the workpiece cannot be rescued after too little.
  4. Oxidation and decarburization: that is to say, the carbon on the surface of the metal workpiece is oxidized at high temperature, thereby reducing the carbon content on the surface of the workpiece, and the depth of the decarburization layer is related to the composition of the steel, the composition of the furnace gas, the temperature and the temperature. The holding time at this temperature is related. If an oxidizing atmosphere is used to heat high-carbon steel and steel with a high silicon content, it will be easy to decarburize. The effect of decarburization is to reduce the strength and fatigue of the workpiece. Measures to prevent oxidation and reduce decarburization include: workpiece surface coating, sealing and heating with stainless steel foil packaging, heating with salt bath furnace, heating with protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), flame combustion furnace (to make the furnace gas reductive) and other measures.
  5. Carburization: Generally, lost foam castings are prone to carburization defects, that is, carburization often occurs on the surface or part of the surface of workpieces heated by oil furnaces. This is due to the poor mixing of oil and air. Due to incomplete combustion, once the carburization phenomenon occurs in the workpiece, the machining performance of the forging will be deteriorated.
  Prevention of carbon increase can be considered 1. In the design of casting mold and pouring process, it is necessary to accelerate the gasification of mold material, reduce and stagger the contact and reaction time of liquid phase and solid phase in its decomposition products, and reduce or avoid carbon absorption of steel parts. 2. The control of the negative pressure must match the speed of the entire pouring process. If the negative pressure is too high, sand sticking and other defects will easily occur. If the negative pressure is too small, the pyrolysis products will not be easily discharged, resulting in carbon increase. 3. Add some anti-carburization catalysts such as alkali metal salts and limestone powder to the coating. After pouring, the coating can decompose enough carbon dioxide gas to absorb carbon; Inert gas prevents decomposed carbon from reducing or penetrating into steel castings.
  6. Copper brittleness: Copper brittleness generally occurs when forgings are heated. One type is the residual copper or copper oxide shavings in the furnace, and the steel penetrates into the austenite grain boundary at high temperature. The other type is high-copper steel during high-temperature forging. Copper is enriched at the grain boundary, forming copper brittleness, which weakens the connection between grains and cracks on the surface of the forging.
  7. Hydrogen embrittlement phenomenon: The phenomenon of reduced plasticity and toughness of high-strength steel during heat treatment in a hydrogen-rich atmosphere is called hydrogen embrittlement. The higher the carbon content of the steel, the more prone to hydrogen embrittlement under the same temperature and pressure conditions. serious. The workpiece with hydrogen embrittlement can also be eliminated by hydrogen removal treatment (such as tempering, aging, etc.). Heating in vacuum, low hydrogen atmosphere or inert atmosphere can avoid hydrogen embrittlement, or add chromium, titanium, vanadium, etc. to the steel. Elements that can prevent the generation of hydrogen embrittlement.