Common defects and preventive measures in heat treatment of seamless pipes

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

1. Crack
Heat treatment cracks mainly include quenching cracks, tempering cracks and grinding cracks. When heating some large-sized steel parts and alloy billets, if the cooling rate is too fast during the cooling process, cracks will easily occur. Seriously, there will be cracks in the section, and once the workpiece is cracked, it cannot be repaired, and it can only be re-processed. Therefore, during the heat treatment process, try to achieve uniform heating and correct heating, select the appropriate cooling medium and cooling method, and select the correct method for immersing the workpiece in the quenching medium and the direction of the workpiece.

2. Overheating
Overheating refers to the phenomenon that the grains are significantly coarsened due to excessive heating temperature or long holding time. As a result, thick needle martensite is obtained after quenching, resulting in a decrease in mechanical properties, especially a decrease in impact toughness, an increase in brittleness, and a decrease in fatigue strength. For workpieces that are not overheated seriously, carbon structural steel and alloy structural steel should generally be normalized or annealed once and then reheated and quenched again. For high carbon steel and alloy steel and tool steel, it should be annealed, normalized for several times, and then re-quenched according to the correct quenching process.

3. Overburning
Overburning refers to when the heating temperature of steel far exceeds the normal heating temperature, resulting in melting and oxidation of grain boundaries. The overfired structure of steel has extremely coarse grains, and there is an oxide network on the grain boundary, and the mechanical properties deteriorate sharply. This defect is difficult to remedy and should be avoided as much as possible. Therefore, the heating temperature should be strictly controlled, the holding time at high temperature should be shortened as much as possible, and the cooling speed should be properly controlled to avoid adopting a moderate cooling speed.

4. Oxidation and decarbonization
That is to say, the carbon in the lower layer of the metal workpiece is oxidized at high temperature, so that the carbon content on the surface of the workpiece is reduced, and the depth of the decarburized layer is related to the composition of the steel, the composition of the furnace gas, the temperature and the holding time at this temperature. . If an oxidizing atmosphere is used to heat high carbon steel and steel with a high silicon content, it will be easy to decarburize, and the effect of decarburization is to reduce the strength and fatigue of the workpiece. Measures to prevent oxidation and reduce decarburization include: coating the workpiece surface, sealing and heating with stainless steel foil, heating with a salt bath furnace, heating with a protective atmosphere (such as purified inert gas, control of carbon potential in the furnace), flame burning furnace (to make the furnace gas reducing) and other measures.

5. Carbon increase
Generally, lost foam castings are prone to carburization defects, that is, the workpiece heated by the oil furnace often has carburization on the surface or part of the surface, which is caused by the poor mixing of oil and air and incomplete combustion. Yes, once the workpiece appears carbonization, it will deteriorate the machinability of the forging.

To prevent carbon increase, consider:
1. The mold and pouring process design should be able to accelerate the gasification of the mold material, reduce and stagger the contact and reaction time of the liquid and solid phases in the decomposition products, and reduce or avoid carbon absorption of the steel.
2. The control of the degree of negative pressure must be matched with the speed of the whole pouring process. If the negative pressure is too large, sand sticking and other defects will easily occur.

3. Add some anti-carburization catalysts such as alkali metal salts and limestone powder to the coating. After pouring, the coating can decompose a sufficient amount of carbon dioxide gas to absorb carbon; It can prevent the decomposed carbon from reducing or infiltrating the steel casting.

6. Copper brittle
Copper brittleness usually occurs when forgings are heated. One is the residual copper or copper oxide chips in the furnace, and the steel penetrates into the austenite grain boundary at high temperature. Boundary, forming copper brittleness, copper brittleness weakens the connection between grains, and appears as a crack on the surface of the forging.

7. Hydrogen embrittlement
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 severe the tendency of hydrogen embrittlement is under the same temperature and pressure conditions. Hydrogen embrittlement can also be eliminated by hydrogen embrittlement treatment (such as tempering, aging, etc.) for workpieces with hydrogen embrittlement. Hydrogen embrittlement can be avoided by heating in vacuum, low hydrogen atmosphere or inert atmosphere, or adding chromium, titanium, vanadium, etc. to steel element that can prevent hydrogen embrittlement.