What to do if the steel is easily deformed by heat treatment
What to do if the steel is easily deformed by heat treatment
Based on the mechanism of heat treatment deformation and its influencing factors, this article discusses the prevention and control of heat treatment deformation and the later method of machining remediation
One. Factors leading to heat treatment deformation
1. Carbon content and its effect on heat treatment variation
As the yield strength of high carbon steel increases, the amount of deformation is smaller than that of medium carbon steel. For carbon steel, in most cases, the deformation of T7A steel is the smallest. When the mass fraction of carbon is greater than 0.7%, it tends to shrink; but when the mass fraction of carbon is less than 0.7%, both the inner diameter and the outer diameter tend to expand.
The yield strength of carbon steel itself is relatively low, so the carbon steel parts with inner holes (or cavities) are deformed more and the inner holes (or cavities) tend to expand. Alloy steel has higher strength, lower Ms point and more residual austenite, so the quenching deformation is smaller, and it mainly shows thermal stress type deformation, and the inner hole (or cavity) of the steel parts tends to shrink. Therefore, when quenched under the same conditions as medium-carbon steel, the workpieces of high-carbon steel and high-alloy steel tend to shrink mainly in the inner hole.
2. The effect of alloying elements on heat treatment deformation
The influence of alloying elements on the heat treatment deformation of the workpiece is mainly reflected in the influence on the Ms point and hardenability of the steel. Most alloying elements, such as manganese, chromium, silicon, nickel, molybdenum, boron, etc., reduce the Ms point of the steel and increase the amount of residual austenite, which reduces the specific volume change and microstructure stress of the steel during quenching. Reduce the quenching deformation of the workpiece. Alloying elements significantly improve the hardenability of the steel, thereby increasing the volumetric deformation and structural stress of the steel, resulting in an increased tendency of the workpiece to undergo heat treatment deformation. In addition, because alloying elements improve the hardenability of steel and reduce the critical quenching cooling rate, in actual production, a mild quenching medium can be used for quenching, thereby reducing thermal stress and reducing heat treatment deformation of the workpiece. Silicon has little effect on the Ms point and only reduces the deformation of the sample; tungsten and vanadium have little effect on the hardenability and Ms point, and have little effect on the heat treatment deformation of the workpiece. Therefore, the so-called micro-deformed steel in the industry contains a large amount of alloying elements such as silicon, tungsten and vanadium.
3. The effect of original structure and stress state on heat treatment deformation
The original structure of the workpiece before quenching, for example, the morphology, size, quantity and distribution of carbides, segregation of alloying elements, and the direction of fibers formed by forging and rolling all have certain effects on the heat treatment deformation of the workpiece. Spherical pearlite is larger in volume and higher in strength than sheet pearlite, so the quenched deformation of the workpiece that has been pre-spheroidized is relatively small. For some high-carbon alloy tool steels, for example, the spheroidization grades of 9Mn2V, CrWMn and GCr15 steels have a great influence on the deformation correction after heat treatment and the deformation after quenching, and the 2.5-5 grade spheroidization structure is usually suitable. Tempering treatment not only reduces the absolute value of the deformation of the workpiece, but also makes the quenching deformation of the workpiece more regular, which is conducive to the control of the deformation.
The strip carbide distribution has a great influence on the heat treatment deformation of the workpiece. After quenching, the workpiece expands parallel to the direction of the carbide strip, and shrinks in the direction perpendicular to the carbide strip. The coarser the carbide particles are, the greater the expansion in the strip direction. For the Cr12 steel and high-speed steel and other leestenite steels, the influence of carbide morphology and distribution on quenching deformation is particularly significant.
In short, the more uniform the original structure of the workpiece, the smaller the heat treatment deformation, the more regular the deformation, and the easier it is to control.
4. The stress state of the workpiece itself before quenching has an important influence on the deformation. Especially for the workpieces with complex shape and large cutting feed, if the residual stress is not eliminated, it will have a great influence on the quenching deformation.
5. The influence of workpiece geometry on heat treatment deformation
Workpieces with complex geometric shapes and asymmetrical cross-sectional shapes, such as shafts with keyways, keyway broaches, tower-shaped workpieces, etc., are quenched and cooled with one side dissipating fast and the other side dissipating slowly, which is an uneven cooling. If the deformation caused by uneven cooling above Ms predominates, the side with the fastest cooling is concave. The transformation of austenite makes the retained austenite more stable. Reducing the transformation of martensite in air cooling can significantly reduce the deformation of the workpiece.
6. Effect of process parameters on heat treatment deformation
Whether it is a conventional heat treatment or a special heat treatment, heat treatment deformation may occur. When analyzing the influence of heat treatment process parameters on heat treatment deformation, the most important thing is to analyze the influence of heating process and cooling process. The main parameters of the heating process are heating uniformity, heating temperature and heating speed. The main parameters of the cooling process are cooling uniformity and cooling rate. The effect of uneven cooling on the quenching deformation is the same as the uneven cooling caused by the asymmetry of the cross-sectional shape of the workpiece. This section mainly discusses the influence of other process parameters.
Two. Prevention and control methods of heat treatment deformation
1) Reverse compression bending method: According to the heat treatment deformation rule of shaft parts, a stress can be pre-added before quenching, that is, reverse compression bending in the bending direction of the part to compensate for the bending deformation generated after quenching, which can be reduced Straightening workload. It is suitable for parts with obvious uneven section and severe deformation.
2) Static quenching method: the temperature of the quenching coolant must be uniform, and it is in a static state immediately after being homogenized before quenching. Clamp the part with pliers and quench into the coolant. This method can be much smaller than the quenching deformation of lead wire suspension.
3) The parts are designed to be even and symmetrical: the cross-sectional shape of the parts should be designed to be as uniform and symmetrical as possible, and if necessary, slots can be opened for the process. For example, there are two symmetrical grooves on the boring bar, in fact, only one is used, and the other is designed to reduce heat treatment deformation.
4) Use special quenching jig for quenching: if the cross section of the part is symmetrical, it can be put into a special jig after the furnace is released, and then the cooling liquid is quenched in the vertical direction. Since the deformation of the parts is limited by the fixture, it can generally be controlled within the reserved margin.
5) Heating by the embedded salt bath furnace: the plug-in salt bath furnace is heated, the parts are heated quickly on one side, and it is easy to produce bending deformation, and the embedded salt bath furnace has a more uniform temperature and energy saving, and a flow particle furnace can also be used .
6) Straight hanging horizontally: For the storage of long parts before and after quenching, care should be taken not to bend the parts due to their own weight, it is best to use a straight hanging frame. For long-distance transportation, multiple plastic airbags can be used, which not only can automatically balance the parts, but also has a shock absorption effect.
7) Stress relief before quenching: used for important parts that are easily deformed, such as precision long screw rods. Annealing or normalizing before quenching to refine grains and make the structure uniform, reduce internal stress. Strictly control the heating temperature during quenching and heating.
Third, the machining correction method of heat treatment deformation
Taking the carburizing and quenching gear shaft as an example, the machining allowance before carburizing and after quenching is very large according to the standard. It is not economical and inconvenient to directly grind teeth, and the hardness is large. Is there any method for direct processing?
The non-metallic binder cubic boron nitride tool BN-S20 grade can be used to process a hardened workpiece with a large margin in one pass. The BN-S20 grade superhard cutter can cut the hardened layer with a large margin to replace the large excess The amount of rough grinding processing can save the traditional processing method of secondary quenching after annealing, saving processing costs and corresponding process costs, and greatly improving production efficiency.
