Heat treatment misunderstanding, cause and prevention of deformation

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Misunderstanding of heat treatment
1. The workpiece that is quenched is not cooled to room temperature and cannot be tempered？
Some people think that after quenching, they can’t enter the tempering process when they are not cooled to room temperature. In fact, many steel grades, especially low- and medium-carbon steels, have a martensite transformation point that is higher than room temperature. When it is cooled to room temperature, it is easy to crack. After quenching, it can be transferred to the tempering process as soon as possible.

2. The workpiece that has been quenched must be warmed and tempered?
This kind of practice is not advisable. It is necessary to determine the temperature before the tempering after quenching according to the martensite transformation point of the steel grade! In order to prevent quenching and cracking, we cannot add speculation, and we adopt a method of warming and tempering in a general way!

3. The product size processing has been completed, and the heat treatment is required to ensure no deformation？
In order to save product processing costs, some people finish all the dimensions before heat treatment, and then heat-treating and quenching. The heat treatment is required to ensure that there is no deformation during the heat treatment, or that only the amount of deformation is allowed within the tolerance value of the last cold working! The process of heat treatment is essentially a stage of tissue deformation, and the accumulation of deformation on the microscopic level. Who can guarantee that it will not be represented as a dimensional deformation at the macroscopic level?

4. Can an annealed workpiece form an equiaxed grain?
In the low carbon steel annealing process, many people believe that equiaxed grains can be obtained. In fact, equiaxed grain size is easily obtained in boiling steel. It is difficult to achieve equiaxed grain structure in Al-aluminum killed steel. Especially after the cold-extruded deformed parts are annealed, the grains are obviously deformed and extruded. It is difficult to achieve equiaxed grains even at an annealing temperature of 950 ℃or higher.

5, the lower the hardness, the better the extrusion deformation, the easier?
The direct thinking of people is that the lower the hardness, the easier it is to squash and deform. In the extrusion process of steel, the pearlite spheroidized state has the highest deformation ability, but this tissue state is generally higher than the hardness of the flaky pearlite, so the original structure of the extrusion is required to be a pearlite spheroidized structure. It is required to use the lowest hardness sheet-like pearlite structure.

Heat treatment deformation reasons and prevention methods
1, the cause of heat treatment deformation
Steel may be deformed or even cracked during the heating and cooling process of heat treatment because of the existence of quenching stress. Quenching stress is divided into thermal stress and tissue stress. Due to thermal stress and tissue stress, the residual stress is generated in the parts after heat treatment, which may cause deformation. When the stress is greater than the yield strength of the material, deformation occurs. Therefore, the quenching deformation is also related to the yield strength of the steel. The greater the plastic deformation resistance of the material, the smaller the degree of deformation.

1.1, thermal stress
During heating and cooling, thermal stress is generated due to the inconsistency of thermal expansion and contraction due to the temperature difference in the parts table. When the parts are cooled by high temperature, the surface dissipates quickly and the temperature is lower than the core. Therefore, the surface has a larger volume contraction tendency than the core, but the surface is subjected to tensile stress by the core, and the core is subjected to compressive stress. The temperature difference in the table increases the stress.

1.2, tissue stress
The tissue stress is due to the different specific volume of austenite and its transformation products, and the surface transition time of the part and the heart or parts of the part are different. Since the austenite specific volume is the smallest, an increase in volume is inevitable when quenching and cooling. When quenching, the surface begins to transform martensite first, the volume increases, and the austenite volume remains unchanged. Since the core obstructs the surface volume, the surface generates compressive stress and the core generates tensile stress.

2. methods for reducing and controlling heat treatment deformation

2.1. Reasonable material selection and hardness improvement
For parts with complex shapes and large differences in cross-sectional dimensions and requiring less deformation, materials with better hardenability should be selected for quenching with a milder quenching cooling medium. For thin-plate precision parts, bi-directional rolled sheets should be used to make the fiber direction of the parts symmetrical. For the hardness requirements of the parts, under the premise of meeting the requirements of use, try to choose the lower limit hardness.

2.2. the correct design of parts
The shape of the part should be as simple, uniform and structurally symmetrical as possible so as to avoid the uneven tendency of deformation and cracking due to uneven cooling. Try to avoid sudden changes in section size, reduce grooves and thin edges, and do not have sharp edges and corners. Avoid deeper holes. Long parts avoid cross sections with a cross section.

2.3. Arrange the production route reasonably and coordinate the relationship between hot and cold processing and heat treatment.
For parts with complex shapes and high precision requirements, pre-treatment between roughing and finishing should be carried out, such as stress relief and annealing. Prepare the blank for heat treatment to make the tissue more uniform.

2.4. Improve heat treatment process and operation
(2.4.1) Control of heat treatment temperature
In the case of meeting the heat treatment process requirements, the quenching heating temperature is minimized and the holding time is shortened, so that the high temperature strength loss of the parts is less. The plastic resistance is enhanced, and the comprehensive resistance of the parts to stress deformation and anti-quenching deformation is enhanced, thereby reducing deformation.

(2.4.2) Select a milder quenching cooling medium
2.4.2.1 Cooling is the key process of quenching. It is related to the quality of quenching, and it is also the most prone to problems in quenching process. The effect of cooling process on deformation after metal quenching is also a very important reason. Hot oil quenching is less deformed than quenching of cold oil, and is generally controlled at about 90 °C. Under the premise of ensuring hardness, oily medium is used as much as possible. The cooling rate of oily medium is slower, while the cooling rate of aqueous medium is relatively faster, and the change of water temperature has a great influence on the cooling characteristics of aqueous medium.

2.4.2.2 The use of graded quenching can significantly reduce the thermal stress and microstructure stress of the cattle during quenching, which is an effective way to reduce some complex parts. This quenching method is effective because the temperature of each part of the part has become uniform before the martensite transformation and the martensite transformation is completed under slow cooling conditions, which not only reduces the quenching thermal stress but also significantly reduces the structural stress, thus effectively Reduce or pre-prevent parts from quenching deformation.

2.4.2.3 Austempering can also significantly reduce the deformation of the part. The difference between austempering and fractional quenching is that the former obtains the lower bainite structure. Because the lower bainite structure has higher strength and hardness, and the toughness is good, the specific heat capacity is martensite. The specific heat capacity is small, and the temperature inside and outside the part is consistent when the structure is changed, so the quenching structure stress is also small. Austempering and staged quenching can only be applied to smaller parts.

(2.4.3) Correctly master the heat treatment operation method
In order to reduce and control the heat treatment deformation, it is necessary to correctly grasp the heat treatment operation method. When the section is evenly cooled, if the elongated shaft (screw, shaft) is vertically quenched, moving up and down, there is also vertical gradual quenching and standing still. When the section is unevenly cooled, the horizontal rapid quenching or oblique quenching, such as uneven thickness parts, should be thickened first. For sheets with lateral entry, for holes with and for concave parts, blind holes and concave surfaces should be quenched upwards to facilitate bubble discharge. In short, to make the parts have a uniform cooling rate, some parts that need protection are protected before quenching.

3, the salvage method of the deformed parts
Although the deformation reduction measures are taken, the deformation is still inevitable, so it is necessary to carry out remediation. The commonly used methods are cold straightening and thermal straightening. If the hardness is less than 40HRC after quenching and tempering, it can be directly corrected by cold straightening. If the metal is not cooled to the martensite transformation temperature point during the quenching and cooling process, the plasticity of the part is good, and the pressurization alignment effect is good.

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