What is hot straightening

What is hot straightening?

Hot straightening is an operation to correct the deformation by applying external force or changing the internal stress state of steel parts in various ways with the help of the characteristics of good plasticity of steel parts at a certain temperature. Due to the heating state with a certain temperature, it is easy to correct, so the thermal straightening method is widely used.

Why does the workpiece need to be straightened?

  • 1) After heat treatment, the workpiece is bent or warped due to thermal stress and structural stress, which is beyond the scope and must be straightened. The deformation of the straightened workpiece shall meet the process requirements.
  • 2) Straightening methods include: cold pressing method, hot pressing method, hot spot method, pressure tempering method, tempering waste heat method and counterattack method. It can be selected according to different conditions and requirements such as workpiece shape, material and hardness.
  • 3) The workpiece with large size and large deformation resistance shall be pressurized by hydraulic press; Small workpieces can be pressurized by manual screw press; Small workpieces with low hardness and good toughness can be straightened by hand hammer. Batch workpieces shall be equipped with special straightening fixture to improve work efficiency.

Preparation for workpiece alignment

  • 1) The measuring part and center hole of the workpiece to be straightened must be clean;
  • 2) Prepare the tooling and instruments for alignment according to the process regulations, such as dial gauge, feeler gauge, V-shaped iron, knife edge ruler and other tools;
  • 3) Master the technical requirements of drawings / process documents, detect the deformation of workpieces, make marks, and select appropriate straightening methods as required;
  • 4) Make preparations and safety work for the oxygen acetylene gas device for hot spot straightening according to the operating procedures.

Operation of workpiece straightening

Cold press straightening

For workpieces with simple shape and hardness lower than 35hrc, if the depth of hardened layer of carburizing or surface quenching is less than 1 / 5 of the diameter or thickness of workpieces, they can be straightened by pressurized cold straightening;

  • 1) When pressing or hammering with a press for straightening, pay attention to placing both ends of the workpiece flat / firmly padded Master the degree of pressurization. For long workpieces with large deformation, pressurize them point by point in sections to avoid forming double peaks or distortions that are difficult to calibrate;
  • 2) The pin groove, edge thread and other parts of precision workpiece shall be padded with copper sheet or aluminum sheet when pressurized.

Thermal straightening

Workpieces requiring high hardness, such as alloy tool steel, CrWMn lead screw and 9mn2v steel guide rail; Measuring bar \ high speed steel broach, milling cutter, reamer, friction plate and alloy steel machine tool spindle can be thermally straightened;

  • 1) The workpiece is quenched and cooled to the MS point (about 200 ℃ or the temperature range specified in the process document), and there is still a lot of good plasticity of residual austenite, or superplasticity (transformation induced plasticity) during the transformation of supercooled austenite to martensite for pressure straightening. The action shall be rapid during operation. After the calibration meets the requirements, the workpiece shall be hung for air cooling.
  • 2) The above workpieces can also be pressed in a special pressure fixture with large pressure for cooling and phase transformation. The cooling time is calculated according to the effective thickness of the workpiece 1-1.5min/mm.
  • 3) Pay attention to control the final calibration temperature, generally not less than 60 ℃, otherwise it is easy to break the workpiece. If the hot pressing straightening is too late or the deformation is still unqualified after tempering, the pressure tempering method shall be used to remedy it.

Hot spot straightening

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For workpieces whose hardness is required to be higher than 35hrc after quenching and tempering, when local soft areas are allowed or the deformed convex surface is a non working surface, the hot spot method can be used for straightening.

  • 1) Pad the deformed convex surface of the workpiece upward stably, quickly heat it to more than 700 ℃ at the high point of the convex surface with oxygen acetylene flame (neutral flame), then cover it with wet cotton yarn for cooling, and straighten the workpiece with the volume shrinkage after local tempering;
  • 2) The hot spot shall be implemented at the unimportant part of the workpiece. The heating shall be rapid and the range shall be small and shallow. If it is not possible to heat a few more points, but it is not suitable to heat repeatedly at the same position;
  • 3) The hot spot temperature shall not cause phase transformation of the workpiece, otherwise it is easy to crack during cooling;
  • 4) During hot spot straightening, the workpiece can be pressurized at the same time, but the convenience of operation should be considered.

Pressure tempering straightening

The principle of hot press alignment is basically consistent with that of cold press alignment. The plastic deformation ability of steel parts is better than that of cold pressing only when hot pressing is straightened. Therefore, it is easy to operate and not easy to break. Hot press straightening, that is, the steel parts can be heated as a whole, It is also possible to locally heat the parts with the greatest stress (such as oxygen acetylene flame heating or high-frequency induction heating), and the heating temperature depends on the hardness required by the steel parts. Generally speaking, for the steel parts with quenching tempering, the temperature of hot pressing straightening shall not be higher than the tempering temperature. Only in the unimportant parts or parts where the hardness is allowed to be reduced, the temperature can be slightly increased for correction.
In addition, the hot press straightening can also be pressurized in the convex part and heated in the concave part.
The heating temperature should not exceed the tempering temperature. The purpose of heating is to improve the plasticity on one hand and to facilitate the relaxation of compressive stress in the concave part on the other hand.
During hot pressing straightening operation, attention should be paid to the relationship between heating temperature and applied pressure and duration. Practice has proved that when the steel is in the austenitic (red hot) state above AC1 point, relatively large pressure can be used to remove the external force for a short time. In the pearlite state below AC1 point (warm), it is appropriate to use relatively small pressure for a long time. This is safe (not easy to break) and efficient. Secondly, when using local heating, the heat of the flame should not involve a larger area, otherwise it will affect the hardness of the non heating zone.
For the hot pressing straightening with low temperature, in order to prevent the steel parts with large deformation from breaking, it can be corrected to a certain extent, sent to the furnace at the same temperature as the tempering temperature of the parts, heated to remove the stress and restore the sliding and distorted lattice, and then continue the pressure correction (for example, this method is used for the straightening of the quality regulating parts).
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Hot spot straightening method is widely used in steel parts with hardness requirements above HRC35-40. The workpiece with carbon content of 0.35 ~ 1.3% carbon steel is subject to hot spot straightening after quenching and low-temperature tempering. The carburized parts of low carbon steel and its low alloy steel with hardness above HRC35 ~ 40 can also be corrected by hot spot, and the effect is poor when the carburized layer is too thin. Generally, if the infiltration layer is less than 0.8-0.9mm, it is not easy to correct. The hot spot correction effect of steel parts with high hardness is good. The hot spot correction effect of alloy tool steel is better than that of carbon steel. Pay attention to prevent cracks in the hot spot correction of high alloy steel.

Friction blade, saw blade, milling blade, scissors blade and other thin-walled or flat workpieces with simple shape are easy to be clamped. They are straightened by using the plasticity and stress relaxation in the tempering process, and the effect of high tempering temperature is good.

  • 1) Clamp the deformed workpiece in the fixture, do not press it too tightly at first, heat it in the furnace for a short time, that is, press the fixture quickly while it is hot out of the furnace, and then continue tempering; The workpiece with too large deformation should be heated repeatedly and gradually under the condition of lower than the tempering temperature, and continue tempering after straightening;
  • 2) The tempering temperature shall not exceed the specified value to avoid unqualified workpiece hardness. After pressure tempering, except that the workpiece to avoid tempering brittleness should be cooled quickly, the fixture should be loosened after slow cooling.

When hot spot straightening is adopted, the following points shall be paid attention to: the hot spot straightening of quenched steel parts shall be carried out after tempering, otherwise it is easy to cause cracks due to large internal stress. Practice has proved that hot spot straightening of quenched steel (without tempering) is also difficult (not easy to correct). Alloy tool steel is easy to crack due to improper cooling during hot spot correction. Therefore, the deformed steel parts are often preheated at 180 ℃ in advance. Then perform hot spot operation. Head practice also proved that the hot spot operation with point temperature of steel parts is better. Therefore, for steel parts with great difficulty, the bending degree shall be checked immediately after tempering and discharging, and the well shall be marked for hot spot correction. The heating temperature of hot spot is generally above 550 ℃, and different effects can be obtained. However, 700 ~ 850 ℃ is usually used. If the heating temperature is lower than the phase transformation point (AC1), the hardness of the hot spot area after cooling will be low and can not play the role of re quenching. If the heating temperature is very high, it may cause overheating due to excessive heating, resulting in cracks after cooling. The heating time of hot spots should not be too long, otherwise the range of hot spots will expand and affect the quality of steel parts. The commonly used acceleration time is about 3 ~ 5 seconds, and the heating area at the hot spot shall be determined according to the cross-section size of the steel part, generally in φ 10~ φ In the range of 20 mm, the larger the section of the steel part, the larger the heating area of the hot spot area. The number of hot spot heating points shall be determined according to the bending degree of steel parts. Generally, it starts from the highest point of the bulge and continues symmetrically to the left and right sides. After several hot spots, check the deformation. If it is still unqualified, continue to make more points. Care should be taken to avoid repeated hot spots at any place. Especially alloy tool steel, even secondary repeated hot spots may produce cracks. In addition, non working parts should be selected as far as possible.

Tempering waste heat straightening

The workpiece tempered at a temperature above 300 ℃ shall be straightened by thermoplastic pressure when tempering out. During straightening, the workpiece can be discharged one by one and straightened under the press. The pressurization method is the same as the cold pressing method. The straightened workpiece continues to be air cooled.

Counterattack straightening

Counterattack correction is carried out with the help of the principle that the pressure is easy to relax at higher temperature. The operation process and knocking position are the same as the cold back knocking straightening method.
The hot back knock straightening method is only applicable to high alloy steel with red hardness after quenching and tempering, such as high-speed steel and high chromium steel parts. As we all know, these steels also have high enough hardness in the red hot state after high temperature heating, quenching and tempering. Therefore, hot counterattack correction is different from hot pressing correction principle, that is, it is not corrected by good plasticity in hot state, but by knocking to relax its stress and straighten the deformation. High speed steel parts are finally tempered in the second and third air cooling processes. If it is hot knocked after the first tempering, it is easy to knock out scars because more residual austenite is not transformed. It should be noted that the corresponding “hot normal knock straightening method” is not commonly used in production practice. Because ordinary carbon steel or low alloy steel has low hardness and good plasticity in the heating state, there are two disadvantages of applying external force by hammering: first, the size of hammering force is not easy to grasp, which is very easy to cause reverse bending or insufficient stress; Another point is that the hammering trace is large, which damages the surface of steel parts. Therefore, “hot normal knock straightening” is not as good as hot press straightening.

For thin and slender workpieces with high hardness, carefully knock the deformed concave surface with a small cemented carbide hammer to make the workpiece produce extended extrusion force and correct the deformation.
For the hammer used in the counterattack method, the cemented carbide hammer surface shall be polished smooth and flat, and the calibrated work piece shall be padded directly against the striking part without gap, otherwise the work piece will be broken. Counterattack can be carried out at many places on the deformed concave surface of the workpiece. Until the workpiece is straightened.

Quality requirements for workpiece alignment

  • 1) All straightened workpieces shall be 100% inspected for deformation and rechecked for hardness. Check whether the included angle, small hole, keyway, thread and other parts of the parts are damaged. If necessary, the flaw detection method shall be used to check whether the workpiece has cracks, and special attention shall be paid to the micro cracks at the pressurized and hammered parts.
  • 2) All straightened workpieces shall be subject to stress relief tempering. The tempering temperature shall not cause the decrease of hardness, and the tempering time shall exceed 2 hours. Except that workpieces with tempering brittleness shall be cooled quickly, air cooling is generally used.

Safety precautions in the operation of workpiece straightening

  • 1) Operators shall wear labor protection articles. When heating and straightening, they shall wear asbestos gloves to prevent scalding;
  • 2) Straightening with sizing block and V-shaped iron is strictly forbidden to be made of cast iron or materials with high hardness and brittleness, so as to avoid cracking and flying out during work and hurting people;
  • 3) In order to prevent the workpiece from breaking and flying out to hurt people, people are not allowed to stand at both longitudinal ends of the workpiece, and baffle shall be used for protection when necessary;
  • 4) When pressurizing with a press, the workpiece shall be padded stably, and the pressurization shall be stable without impact;
  • 5) The operator shall be familiar with the safety knowledge of using oxygen acetylene gas device.

Source:  China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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