The welding residual stress is caused by the uneven temperature distribution of welding parts and the thermal expansion and cold contraction of welding metal, so the welding construction will inevitably produce residual stress.
The most common method to eliminate residual stress is high temperature tempering, the weldment in heat treatment furnace heated [SCALE STEEL]to a certain temperature (below Ac1) and heat preservation time, using the material yield limit under high temperature is reduced, the internal stress of high local produce plastic flow, elastic deformation gradually reduce, plastic deformation gradually increase and reduce stress.
The influence of post-weld heat treatment on tensile strength and creep limit of metal is related to the temperature and holding time of heat treatment.The effect of post-weld heat treatment on the impact toughness of weld metal varies with different types of steel.
Purpose for PWHT:
Relax welding residual stress; 2. Stabilize the [SCALE STEEL]shape and size of the structure to reduce distortion.3. Improve the performance of base material and welding joint, including a. Improve the plasticity of weld metal.B. Reduce the hardness of heat-affected zone.C. Improve fracture toughness.D. Improve fatigue strength.E. Restore or increase the yield strength reduced during cold forming.4. Improve the ability to resist stress corrosion.5. Further release harmful gases, especially hydrogen, in the weld metal to prevent the occurrence of delayed cracks.
Methods for PWHT:
Post-Welding Heat Treatment generally adopts single high temperature tempering or normalizing and high temperature tempering treatment.Normalizing and tempering heat treatment is used for gas welding joints.This is because of the coarse grain of welding seam and heat-affected zone of gas welding, which needs to be refined, so normalizing treatment is adopted.However, single normalizing cannot eliminate the residual stress, [SCALE STEEL]so high temperature tempering is needed to eliminate the stress.The single medium tempering is only suitable for the assembly and welding of large common low carbon steel containers assembled on the site for the purpose of partially eliminating residual stress and dehydrogenation.The vast majority of occasions is the use of a single high temperature tempering.Heat treatment of heating and cooling should not be too fast, strive for internal and external wall uniformity.
The fuels used may be solid (coal), liquid (oil) and gas (gas, natural gas, liquefied petroleum gas).
There are abundant resources of coal burning heating coal, and the coal burning reflector has played a certain role in heat treatment heating [SCALE STEEL]method.The nature of the coal and the structure of the reverberatory furnace determine that the coal is not easy to be completely burned, so the coal furnace has low thermal efficiency, poor heating quality and working conditions, and soot pollutes the environment.These shortcomings make the coal heating method gradually replaced by other heating methods.
Liquid fuel heating mainly uses heavy diesel as fuel, suitable for large heating furnace heating, also used for external heating salt bath furnace heating, generally in the furnace heating outdoor wall or installed on both sides of the nozzle.When liquid fuel is used to heat an external salt bath, the nozzle is mounted on the furnace shell outside the crucible.The liquid fuel is mixed with air in a nozzle and atomized under the action of compressed air. Then the nozzle is ejected and burned in a heating chamber (or outside a crucible in [SCALE STEEL]a salt-bath furnace) to heat the workpiece (or crucible).The reasonable design and arrangement of nozzles play a key role in maintaining uniform furnace temperature and saving fuel.Atomized oil from the nozzle can also be burned in a radiator inside the furnace, heating the radiator to indirectly heat the workpiece.Fuel oil controls the heating temperature more easily than coal, and is suitable for large bulk heating and areas with sufficient oil supply.
Gaseous fuel is heated in a nozzle, where the gas is mixed with a certain proportion of air and ejected for combustion.This method can directly heat the workpiece placed in the heating chamber, or it can indirectly heat the workpiece by spraying flame into the radiation tube installed in the heating chamber.When used in salt bath furnace, the nozzle is installed on the furnace shell outside the crucible, and the flame shoots at the outside of the crucible to add hot molten salt.The gaseous fuels used for heating include gas, natural gas and liquefied petroleum gas.By adjusting the ratio of air to gas, an oxidizing [SCALE STEEL]or reducing combustion atmosphere can be obtained, thus reducing the degree of oxidizing and decarburizing when the workpiece is heated.This heating method is suitable for areas where bulk heating and gas supply is adequate.
The other way is to directly heat the surface of the workpiece with the flame of the nozzle. At this time, the nozzle and the workpiece move relative to each other. The gases used are oxygen-acetylene, oxygen-propane, oxygen-methane and so on.This heating method, flame quenching, is suitable for surface quenching of workpiece.
Electric Heating Method:
With electricity as the heat source, electric energy is converted into heat energy by various means to heat the workpiece.Electric heating, temperature easy to control, no environmental pollution, high thermal efficiency.Electric heating can be done in several ways.
The heating element uses the resistance heat generated when an alternating current of power frequency (50 ~ 60 hz) passes through the element to heat the workpiece.Electric heating elements are usually arranged around or [SCALE STEEL]on both sides of the heating room to ensure uniform temperature in the heating room.There are also elements in a radiation tube to indirectly heat the workpiece.For outer hot salt bath furnace or metal bath furnace, the heating element is arranged in the space outside the crucible and inside the shell.This heating method can also be used in a floating particle furnace for alumina particles.It is suitable for the whole workpiece heating and sufficient electric power area.
Workpiece resistance after heating the alternating current directly through the workpiece resistance, the workpiece itself resistance produces heat to increase the temperature of the workpiece.This method is suitable for the integral heating of workpiece with uniform cross section.Another way is to use the rolling copper wheel to press on the metal workpiece, through the alternating current of low voltage and[SCALE STEEL] large current, using the contact resistance between the copper wheel and the workpiece to generate heat and heat the surface of the workpiece.
Workpiece induction heating Put the workpiece in a spiral coil, the coil with a certain frequency (generally higher than the power frequency) of alternating current, so that the workpiece in the coil generated eddy current, using the resistance of the workpiece itself to generate heat and be heated.The depth of this heating can be shallower as the current frequency increases, which is called induction heating heat treatment.Induction heating is mainly used to heat the surface of the workpiece, but with a lower frequency and the workpiece diameter is small, it can also be used for overall heating.This heating method [SCALE STEEL]has high efficiency and low power consumption, and is mainly used for the heating quenching of medium and small parts.
Dielectric resistance heating leads the low voltage alternating current of industrial frequency to the electrode buried in the dielectric. The resistance heat generated when the current flows through the dielectric makes the dielectric itself reach a high temperature.When the workpiece is heated in this high temperature medium, oxidation and decarburization can be reduced or avoided.This medium is conductive, such as salt, graphite particles, etc.The furnace type of heating furnace has internal heat type salt bath furnace and graphite floating particle furnace.This heating method is mainly used for quenching medium and small parts.
The surface of workpiece is heated with a very high power density. The heating time is measured in milliseconds, and the power density can be up to 10 ~ 10 watts/cm. The heat sources used include solar energy, laser beam and electron beam.
Solar Heating- The workpiece is heated by a concentrated solar heater.
Laser Beam Heating- Utilizes the laser produced by THE CO2 continuous laser generator to produce a high-temperature beam after focusing to illuminate the workpiece, making the local surface thin layer of the workpiece instantaneously reach the quenching temperature or melting temperature.After the irradiation stopped, the surface heat rapidly passed into the substrate material and made the surface hardened [SCALE STEEL]or solidified rapidly.Laser beam heating processes include phase change hardening - hardening, surface "glazing" - rapid solidification, surface alloying and so on.This allows the reflector to change the direction of the beam, so it is best for heating the inner walls (such as cylinder liners), but the thermal efficiency is low.
Electron Beam Heating- Uses the high-speed electrons to bombard the workpiece surface, which can convert the high kinetic energy into heat energy quickly and raise the surface temperature of the workpiece to quenching temperature or melting temperature rapidly.After the irradiation stopped, the surface heat could be transferred into the cold substrate in a short time and then hardened or solidified rapidly.As with laser [SCALE STEEL]heating, electron beam heating process also has phase change hardening, surface "glazing" and surface alloying.Because the heating needs to be carried out in a vacuum chamber, the workpiece batch is limited, but the thermal efficiency is high.