Field management of carburizing and quenching

This article is many years engaged in carburizing quenching front-line process personnel to explain: the process, carburizing quenching common defects, to how to burn carbon accumulation, how to fix the carbon, as well as routine maintenance of the furnace and other common problems.

In the actual production of heat treatment, often due to the neglect of details often lead to the emergence of defective products. Therefore, heat treatment work needs to uphold a serious and responsible attitude to minimize the chances of defective products.

The actual production of heat treatment process is as follows: incoming material inspection (there is no bump injury, iron chips, leakage process) – preparation (heat treatment process card, the product can be put together furnace) – loading (choose the correct way to load, mainly from the deformation aspects) –cleaning (need to brush paint paint anti-seepage, need thread anti-seepage thread anti-seepage) –pre-oxidation (mainly to make the workpiece surface activation, improve the carburization speed) — into heating furnace (the process must be selected correctly) – after cleaning – low-temperature tempering. Of course with the furnace test samples should also be available.

Carburizing quenching common process defects

Internal oxidation (IGO), carbide overload (free carbide, reticulated carbide), residual austenite overload, carburizing hardened layer in the number of bainite (NMTP) overload, coarse grains, carburizing layer after quenching micro-cracking, deviation of the hardness and carburizing depth of the heart.

Internal oxidation

Controlled atmosphere carburizing is based on the water gas reaction, CO + H20 – CO2 + H2, where CO2, H2O is a harmful gas, at high temperatures is very easy to cause the oxidation of certain alloying elements in solid solution form, in the oxidation process, oxygen adsorbed on the metal surface and then diffuse along the austenite grain boundaries to the interior, causing Oxidation of alloying elements at grain boundaries. The formation of internal oxidation of alloying elements is obtained from the austenitized solid solution, the result is a reduction of this alloying element in the austenite matrix near the oxide particles, resulting in the formation of non-ma organization at the internal oxidation after quenching, reducing the residual compressive stress on the surface of the workpiece, so to avoid the generation of internal oxidation in production.

  • 1. The workpiece should not have oil, water, rust spots before entering the furnace.
  • 2. Reasonable furnace loading to ensure that the furnace temperature recovery is fast and the furnace gas recovery should be fast to reduce the generation of internal oxidation in the heating stage.
  • 3. Strictly control the quality of carburizing auxiliary materials.
  • 4. Increase the quenching temperature and quenching cooling medium cooling rate to reduce the generation of non-horse.
  • 5. Carburizing quenching 10-30min before the passage of 5-10% NH3 can also slow down the generation of non-horse.

Carbide

20220413001507 76062 - Field management of carburizing and quenching

Carbide is mainly due to high carburizing carbon potential, poor diffusion, cooling quenching, in the sharp corner and the top part of the tooth is easy to precipitate mesh and intermittent mesh carbide. Once the precipitation of reticulated carbide rework is also difficult to eliminate, so the process must be noted.

Residual austenite

Although the residual austenite is saturated with carbon, but the hardness is very soft, when coexisting with martensite, will cause a reduction in the hardness of the seepage layer. The effect of residual austenite on the serviceability of carburized hardened gears has been the subject of academic debate in some localities.

  • 1. Residual austenite can cause a decrease in surface hardness.
  • 2. Residual austenite has an effect on the surface residual compressive stress state, the higher the residual austenite, the lower the compressive stress.
  • 3. The effect of residual austenite on fatigue strength (controversial). The alloy content of the steel, the surface carbon concentration, and the quenching temperature all directly affect the residual austenite content in the surface tissue of carburized hardened workpieces.

Amount of bainite in the carburized hardened layer

Carbide is mainly due to high carburizing carbon potential, poor diffusion, cooling quenching, in the sharp corner and the top part of the tooth is easy to precipitate mesh and intermittent mesh carbide. Once the precipitation of reticulated carbide rework is also difficult to eliminate, so the process must be noted.

Coarse grain

In actual production, there are often coarse crystals (soul crystal) phenomenon, is mainly normalized treatment of coarse crystals (soul crystal) in the carburizing quenching after being inherited (tissue inheritance). Coarse grains have a negative impact on the serviceability (increased brittleness).

Number of bainite in the carburized hardened layer

The essence of the carburizing hardened layer is caused by the lack of hardenability of the carburizing layer, all factors affecting the quenching effect will affect the amount of NMTP in the carburizing hardened layer, in the actual production of the main factors affecting:

  • 1. The size of the workpiece, the larger the size, the higher the chance of non-ma, the amount may also be large.
  • 2. The cooling medium cooling rate, hot oil quenching, hardened layer of non-horse increased, serious surface will have a certain amount of bainite appear.
  • 3. Steel hardenability. The appearance of high temperature transformation organization (bainite) in the carburized layer will directly affect the contact fatigue strength and bending fatigue strength of the carburized layer.

Microcracking of the carburized layer after quenching

Microcracking is related to the sugar content, coarse austenite grains, and H absorption on the surface of the carburizing process, which can be removed by timely tempering and partial healing of microcracking.

Heart hardness and carburizing layer depth deviation: the heart tensile strength of the workpiece after carburizing and quenching depends on the heart hardness, and the heart organization determines the heart hardness, so the heart hardness has an important impact on the bending fatigue strength of gears.

  • 1. High heart hardness reduces the surface residual compressive stress, thus reducing the bending fatigue resistance of the workpiece.
  • 2. Plastic deformation, while the surface still remains elastic, which eventually leads to the reduction of surface residual compressive stress and the increase of residual tensile stress, which greatly reduces the bending fatigue strength. Under heavy load, the core is unable to support the thin and hard surface carburized layer due to its low hardness and causes crush pineapple. For any case, the optimum core hardness must depend on the depth of the carburized layer and its surface quality. In other words, the hardness of the heart must match the depth of the carburized effective hardening layer to ensure safety in service and no premature fatigue failure.

Knowledge points

The above is a summary of the causes and hazards of gear carburizing quenching defects.
In the multi-purpose furnace gear carburizing quenching process equipment: start furnace (on the start of the furnace before the preparation work is not discussed in detail here), pre-carburizing (also called empty carburizing, flat carbon). After the completion of pre-carburization, the empty furnace carbon potential is set to 0.4-0.6, while the furnace can be loaded. Multi-use furnace production process of regular carbon burning, carbon setting is important, because they directly affect the oxygen probe measurement of the true value, and thus directly affect the quality of the product.

How to burn carbon accumulation

  • 1. Carbon burning operation method: lower the furnace temperature to 820-850℃.
  • 2. When the furnace temperature is stable, open the front door.
  • 3. Close the carbon potential control, close the methanol. Close the propane shut-off valve, close the nitrogen shut-off valve, and stop the gas supply to the back chamber.
  • 4. Close the safety nitrogen shut-off valve and stop the gas supply to the back chamber.
  • 5. Open the middle door with 50-100mm gap and keep it for about 2 hours, start to burn carbon black, pay attention to the change of furnace temperature during the process of burning carbon black. (Burning carbon black can also not open the middle door, stop the back chamber gas supply burn 4-8 hours with better results)
  • 6. burn carbon black after completion, close the middle door.
  • 7. Turn on the safety nitrogen, the flow rate control at 3-5M3 / h.
  • 8. Turn on the “gas supply on” and proceed to the next operation according to the air percolation procedure.

Note: When there is a large amount of carbon black deposited in the furnace, it will destroy the carburizing process, affect the stability of carbon potential control, affect the heat transfer of heating elements, and reduce the service life of the furnace components. If there is carbon accumulation in the furnace, it must be burned in time, it is recommended to burn carbon black once in about two weeks, if the accumulation of carbon black is faster, the cycle of burning carbon black should be appropriately shortened.

How to fix the carbon

(1) The fixed carbon piece will be carefully clamped out with tweezers, put in the cleaning bottle with acetone to fully clean, and subsequently clamped out with tweezers and fully dried with a dryer. The initial weight of the foil is then weighed out with a one-tenth of a millionth balance. A foil weight of about 1 gram is good to help reduce measurement errors. Note that if an electronic balance is used, sufficient warm-up time should be ensured to ensure the accuracy of the measurement, and a standard weight should be used for calibration if necessary.
2) Roll the steel foil into a cylinder and put it into the spiral at the front of the carbon fixing rod, pay attention not to be too loose to prevent the steel foil from falling off during the operation, load the steel foil into the furnace from the carbon fixing hole, keep it for about 30 minutes and then pull the carbon fixing rod out of the furnace, at this time the furnace gas discharged from the outlet of the sample tube will be ignited, slowly reduce the opening of the ball valve of the sample tube until the fire has been small but can still keep burning steadily to prevent the air from entering and let The steel foil is allowed to cool naturally in the sample tube for about 5 minutes before it is removed. If necessary, you can also use wet cotton yarn to cool the sample tube until it is not hot. Afterwards, remove the carbon fixing sheet and close the sampling tube. It is important to prevent oxidation of the steel foil during the whole process. A normal foil should be shiny and should be redone if a blue oxidation color appears.
Note that the steel foil needs to be kept in the furnace for a long enough time before removing it during carbon fixing to ensure penetration, and attention should be paid to check the oxygen probe purge cycle, which cannot be purged during carbon fixing, and consider temporarily turning off the purge function if necessary so as to ensure accurate measurement.
3) Generally the carburized steel foil can be carefully wiped clean with degreasing cotton and can be cleaned without cleaning. If necessary, the steel foil is cleaned and dried again with acetone and subsequently weighed out to the end weight of the steel foil.
4) The carbon potential is calculated by the formula

  • Carbon potential = (final weight initial weight) ÷ final weight × 100 + original carbon content of foil

5) Subsequently, the carbon potential can be corrected in the 2604 meter according to the calculated carbon potential.
Note: Keep the steel foil clean during the entire operation and do not allow contact with hands or other unclean objects.

Effects of poor gas tightness of the blowing gas line.

  • 1) Each time the actual flow of purge gas into the insufficient, the probe is easy to accumulate carbon black.
  • 2) Most of the time outside the purging process, furnace gas will always overflow from the oxygen probe purging channel, in the low-temperature section between the inner tube and the outer wall of the oxygen probe, it is very easy to be blocked by carbon black and oil smoke, especially in the heating section, which produces a lot of oil smoke.

Daily maintenance of multi-purpose furnace.

  • 1. Rear chamber circulating fan sealing oil cup should be added sealing oil in time, see lubrication table for periodicity.
  • 2. Rear chamber circulating fan oil self-cooling system oil level indication to maintain between 100-120, if insufficient timely add.
  • 3. Re-grease the front and rear chamber circulating fan motors and oil mixing motor every two months.
  • 4. Check the oil cooling line frequently, and clean the filter in the oil filter regularly. Before cleaning, turn off the “oil cooling” and “oil circulation” in MP270, close the hand valve of filter import and export, and turn off the “25Q1” in the control cabinet. Turn off the “25Q1” valve in the control cabinet to prevent the oil cooling pump from running automatically due to the over temperature of the front chamber.
  • 5. often check the front door seal, replace the packing when necessary. The front door leakage general treatment: repeatedly close and open the front door, check the indentation on the front door root, in the pressure is not strict under the root pad on the appropriate amount of fiber cotton, compacted repeatedly off the front door, continue to check until the four outside indentation evenly. The front door seal to quench the basic non-smoking prevail.

The boiler shall be shut down at regular intervals to keep the rear chamber warm, muffle, silicon carbide guide rail, circulating fan, etc.
1. To carry out inspection, found problems should be dealt with in a timely manner. Also should regularly check the lifting table,  push zipper. The front door and the middle door are running smoothly.
2. The control cabinet cooling air conditioner and cooling fan screen should be cleaned regularly every week.
3. The oil level of the quenching oil tank should be checked frequently. Insufficient oil should be added in time.
4. Three thermocouples in the furnace should be constantly observed (main temperature control. Over-temperature alarm couple. Below the safety temperature alarm couples) of the value, after a period of temperature stability, such as two hours, the temperature of the three thermocouples should be close to the basic, if a deviation of the value is too large, should be noted, and timely calibration.
5. Equipment operation more than six months after if the furnace stopped to room temperature, again before heating should be considered to all heating elements to clean up, remove the upper area of the existing oxide powder as well as dust, and all radiation tube turn 180 ℃ to prevent excessive bending in one direction, affecting the service life. Power outage troubleshooting.

  • 1) After the power failure, the safety nitrogen valve will open automatically. If you can be sure that the call will come soon while the nitrogen supply is sufficient, generally try to adjust the nitrogen flow rate a little less under the condition of ensuring the furnace pressure to reduce the impact on the atmosphere in the furnace.
  • 2) The workpiece is in the back room according to the charging time and the indication of each limit on the furnace body, check the current position of the material tray in the furnace, check the furnace push zipper. Lifting table. The specific position of the middle door.
  • 3) If, in the case of sufficient nitrogen supply, cool down with the furnace and wait for the power supply to be restored. If there is an exhaust gas flame out in the early stage of power failure, it should be lit by torch in time to reduce the emission of toxic gas from the furnace to the air until the combustible gas in the furnace is burned clean and extinguished automatically.
  • 4) If the workpiece is quenched in the oil pool, cool down with the furnace in the case of sufficient nitrogen supply and wait for the power supply to be restored. In general, it is forbidden to open the front door prematurely to prevent combustion caused by local superheat of quenching oil in contact with air.
  • 5) If the furnace is in the process of feeding, in the middle door position, it is recommended to manually push the workpiece to the back chamber or pull back the front chamber, close the middle door (note that when closing the middle door, it should descend slowly to reduce the possibility of negative pressure in the front chamber) and wait for the power supply to be restored. Pass high flow of nitrogen gas during operation to maintain furnace pressure.

Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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