The heat treatment method of high frequency induction heating USES electric energy, and CO2 emission is less than common heat treatment such as carburizing and quenching, so it is a clean heat treatment method.High-frequency induction heating can be used for rapid short-time heating, local heating and surface heating of the processed workpiece, so it is used for surface hardening treatment and plays a great role in the miniaturization of mechanical parts.High-frequency heat treatment is the online treatment of heating and cooling of workpiece one by one, so stable heat treatment quality can be obtained, and the workpiece after heat treatment has small deformation, which is a heat treatment method with stable and small deformation characteristics.SRIQ (super-fast short time heating quenching), using high power high precision high frequency heat treatment device, with only 0.5s can make the workpiece surface austenitic, and quench.SRIQ?It not only has the above characteristics of stability and small deformation but also can add high residual compressive stress to the surface of the workpiece and make the surface of the workpiece finely crystallized to improve the fatigue strength of the workpiece.
However, high frequency heat treatment and other single heat treatment methods to improve the performance of parts is limited. Therefore, in order to further improve the performance of parts, the composite heat treatment and surface modification treatment technology were studied.
2. Composite heat treatment technology formed by SRIQ combined with surface heat treatment
2.1 “PALNIP” processing
PALNIP (salt bath soft nitriding SRIQ) is a composite heat treatment technique of salt bath soft nitriding and surface anti-oxidation treatment combined with SRIQ. Taking the tempered steel SCM440 treated by PALNIP as an example, it was observed from the section tissues that the effect of surface anti-oxidation treatment and SRIQ treatment made the workpiece hardened at high frequency, but the nitride layer remained the same state as that of salt-bath soft nitriding treatment.SCM440 is nitrified before SRIQ to reduce the austenite transition point of FE-C-N system. Therefore, SRIQ treatment can be conducted at a temperature lower than the original SRIQ heating temperature, which has a great effect on the residual nitride layer on the surface of the workpiece.
Roll denudation tests were performed on the above PALNIP treated materials. In order to make a comparison, the sample (N material) of only salt-bath soft nitride treatment for SCM440 steel and the sample (VCQ material) of vacuum carburizing and quenching treatment for SCM420 steel were prepared. The nitrogen diffusion hardened layer thickness of N material is about 0.4mm, and the surface hardness is about 600HV.PALNIP material treated with antioxidant SRIQ increased its surface hardness to 800HV due to the quenching and hardening effect of diffuse nitrogen.PALNIP material formed a uniform compound layer and hardened layer on the surface due to soft nitriding in a salt bath, and the hardened layer thickness was increased due to SRIQ treatment. In addition, the PALNIP surface has a higher residual compressive stress than other heat-treated materials.
Test results of roll denudation of various materials (test conditions: rotation number 1500r/min; The sliding rate is 40%; The oil temperature 80 ℃; Oil for Nissan ATF D – Ⅲ; The material of large roll is SCM420 carburized and quenched surface grinding. The big roller is raised 300mm. Under the pressure of each surface, the denudation life of N material is less than that of other heat treated materials. The reason for the short denudation life of N materials is that although N materials have compound layers, the hardening layer is very thin.PALNIP material has a thick hardening layer, so the strength and denudation life of the material is greatly improved, which is equal to or better than VCQ material. In addition, the cross-section of the PALNIP material specimen, which was rolled 107 times and the maximum surface pressure was 2950MPa, was tested to confirm the presence of compound layers on the entire surface of the specimen. Therefore, the reasons for the increased strength and life of PALNIP material are as follows: 1) The compound layer has the effect of reducing the friction coefficient during the roll pressure test;2) When the temperature rises in the roller pressure test, the -Fe2n and -Fe3n iron nitrides are extracted from the hardened FE-C-N Martensite layer, which can inhibit the softening of the material by tempering.
The PALNIP treatment technology increases the thickness of the hardened layer while providing a homogeneous compound layer on the surface of the treated material, thus providing good resistance to denudation. Because of SRIQ treatment, a large thickness hardened layer is obtained, so that the treated material has excellent fatigue resistance.The PALNIP process is already being used in some automotive components and is moving towards further application.
2.2 Super processing SRIQ
SRIQ processing is a composite heat treatment technology that USES friction processing method to process the processed material with super strong processing and then SRIQ processing. Friction machining is a surface modification method that presses the machining tool on the surface of the processed material, performs friction, and forms nanocrystalline ultrastructure on the surface of the processed material. The super-strong processing SRIQ method is a new composite processing process in which the ultrastructure of nanometer grains is maintained after SRIQ treatment and a deep hardening layer is formed under the ultrastructure due to SRIQ treatment. The rotational fatigue life of the treated materials can be improved by using this process.S45C steel was processed with CNC lathe for super strong processing (lathe rotation number 1600r/min, compression load 1500N) and then SRIQ processing was carried out. Although austenitic quenching with SRIQ treatment, the ultrafine nanocrystalline structure formed by super strong processing is still inherited. Although the treated steel is S45C, its hardness reaches 900HV due to the existence of an ultra-fine nanocrystalline structure. The SRIQ treated material has an effective hardened layer thickness of up to 0.9mm, which cannot be obtained only by ultra-high strength processing. It can be seen from the results of the roller denudation test that the rotational fatigue life of S45C processed by SRIQ composite treatment with ultra-high strength processing is significantly higher than that of S45C treated by SRIQ alone.
2.3 SRIQ DLC processing
SRIQ DLC treatment is a composite heat treatment technology of DLC (diamond-like coating film) treatment after SRIQ treatment. The gear can be treated by DLC using the UBMS (non-equilibrium magnetron sputtering) which can be treated at low temperature. The tempered steel S45C was processed into the dynamic circulating gear fatigue sample (flat gear with modulus 3 and pitch circle diameter of 99mm), and the sample was SRIQ treated to make the effective hardening layer depth at the tooth bottom of the gear sample 0.6mm. The gear samples were then UBMS treated to form a DLC coating film with a thickness of 3 m. The formation conditions of THE DLC coating film were controlled so that the structure of the DLC coating film was a composite layer of me-DLC pure DLC with a composition gradient of Wadded in 2 m thick and a composite layer of ME-DLC pure DLC with a thickness of 10%W in 1 m thick. The contrast gear used in the fatigue test is the sample of S45C tempered at the same low temperature as DLC coating temperature (150℃) after being treated by SRIQ. The DLC coating temperature of 150℃ is equivalent to the tempering temperature with furnace after high-frequency quenching, so the high hardness, high residual compressive stress, and micro-fine structure obtained by SRIQ treatment are still preserved after DLC treatment. Therefore, the treated material has both high fatigue strength after SRIQ treatment and excellent friction characteristics of SRIQ coating film. The fatigue test results of dynamic circulating gear show that DLC treatment improves the fatigue resistance of gear. In addition, the roller denudation test also proves that DLC treatment can improve the material’s resistance to denudation. The results of section observation and FEM analysis show that DLC treatment has the above effect because of the low friction coefficient of coating, which reduces the stress load on the material surface treated by DLC, and inhibits the occurrence of surface microscopic denudation.
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