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Heat treatment technology to improve the quality of die and machinery
1. Mold manufacturing accuracy
Uneven and incomplete structural transformation and excessive residual stress caused by heat treatment cause deformation in the processing, assembly and use of the mold after heat treatment, thus reducing the precision of the mold or even scrap.
2. The strength of the mold
The strength (hardness) of the treated mold fails to meet the design requirements due to improper heat treatment process, improper heat treatment operation, or incomplete heat treatment equipment.
3. Working life of molds
The unreasonable structure and excessive grain size caused by heat treatment lead to the decrease of major properties such as toughness, cold and hot fatigue performance and anti-wear performance of the mold, and affect the working life of the mold.
4. Manufacturing cost of molds
As the intermediate or final process of mold manufacturing process, cracking, deformation and performance of the die caused by heat treatment are out of tolerance. In most cases, the die will be scrapped. Even if the die can still be used through repair, it will increase working hours, extend delivery time and increase the manufacturing cost of the die.
It is the heat treatment technology and mold quality has a very close correlation, so that the two technologies in the process of modernization, promote each other, improve together. In recent years, the rapid development of die heat treatment technology in the world is the vacuum heat treatment technology, die surface strengthening technology and die material pre-hardening technology.
Vacuum heat treatment technology of molds
Vacuum heat treatment technology is a new heat treatment technology developed in recent years. It has the characteristics that are urgently needed in mold manufacturing, such as preventing oxidation and non-decarburization, air degidation or degidation, eliminating hydrogen embrittlement, thus improving the plasticity, toughness and fatigue strength of materials (parts).
Degrees. The factors such as slow vacuum heating and small temperature difference between inside and outside of the parts determine the small deformation of the parts caused by vacuum heat treatment process.
According to different cooling medium, air quenching can be divided into: vacuum oil cold quenching, vacuum air cold quenching, air water cold quenching and vacuum saltpeter isothermal quenching. In the vacuum heat treatment of molds, the main applications are vacuum oil cold quenching, vacuum air cold quenching and vacuum tempering. In order to maintain the excellent vacuum heating characteristics of the workpiece (such as the mold), the choice and formulation of coolant and cooling process is very important. The mold quenching process mainly adopts oil cooling and air cooling.
For the die face that is no longer machined after heat treatment, vacuum tempering should be used as far as possible after quenching, especially for the workpiece (die) with vacuum quenching, which can improve the mechanical properties related to the surface quality, such as fatigue performance, surface brightness, corrosion resistance and so on.
The successful development and application of computer simulation technology of heat treatment process (including organization simulation and performance prediction technology) makes the intelligent heat treatment of die possible. The characteristics of small batch (or even single piece), multiple varieties, high requirement for heat treatment performance and no waste are allowed in the mold production make the intelligent treatment of the mold a must.
Intelligent heat treatment of molds includes:
1. Define the structure, material and heat treatment performance requirements of the mold;
2. Mold heating: computer simulation of process temperature field and stress field distribution;
3. Computer simulation of temperature field, phase transition process and stress field distribution during mold cooling process;
4. Simulation of heating and cooling process;
5. Formulation of quenching process;
6. Automatic control technology of heat treatment equipment.
Foreign industrial developed countries, such as the United States, Japan and so on, have carried out the research and development of vacuum high-pressure gas quenching technology in this aspect, mainly targeting mold.
Surface treatment technology of molds
In the work of plastic mold, in addition to the requirement that the matrix has a reasonable fit of sufficient strength and toughness, the surface performance is crucial to the working performance and life of the mold. These surface properties refer to: wear resistance, corrosion resistance, friction coefficient, fatigue performance, etc. The improvement of these properties is very limited and uneconomical only depending on the improvement and improvement of matrix materials, while the surface treatment technology can often get twice the result with half the effort, which is also the reason for the rapid development of surface treatment technology.
The surface treatment technology of mold is a systematic engineering that changes the morphology, chemical composition, structure and stress state of mold surface through surface coating, surface modification or composite treatment technology, so as to obtain the required surface performance. From the way of surface treatment, it can be divided into chemical method, physical method, physical chemical method and mechanical method. Although new treatment technologies are emerging to improve the surface performance of molds, the main applications of nitriding, carburizing and hardening film deposition are more common in mold manufacturing.
Nitriding technology includes gas nitriding, ion nitriding and liquid nitriding. In each kind of nitriding method, there are several kinds of nitriding technology, which can adapt to the requirements of different kinds of steel and different workpieces. Nitriding technology can form a good performance of the surface, and nitriding I art and the quenching process of mold steel has a good coordination, at the same time, low nitriding temperature, after nitriding does not need to be hot cooling, the mold deformation is minimal, therefore, the surface hardening of the mold is the use of nitriding technology earlier, is the most widely used.
The purpose of carburizing the mold is mainly to improve the overall strength and toughness of the mold, that is, the working surface of the mold has high strength and wear resistance. The technical idea is to replace the higher material with the lower material by carburizing and quenching, so as to reduce the manufacturing cost.
3. Hardening film deposition.
At present, CVD and PVD are more mature technologies for hardening film deposition. In order to increase the bonding strength between the film layer and the workpiece surface, various enhanced CVD and PVD technologies have been developed.
Hardening film deposition technology was first used in tools (cutting tools, cutting tools, tools, etc.), with excellent results, a variety of tools have coated hardening film as a standard process.
Since the 1980s, the mould has adopted the technology of coating hardening film. Under the current technical conditions, the hardening film deposition technology (mainly equipment) has a high cost, and is still only used in some precision, long-life molds. If the heat treatment center is established, the cost of coating hardening film will be greatly reduced. If more moulds adopt this technology, it can improve the overall level of mould manufacturing in China.
The prehardening technology of mold materials
In the manufacturing process of mold, heat treatment is a process used for a long time by most molds. Since the 1970s, the idea of pre-hardening has been put forward internationally, but due to the constraints of the rigidity of the machining machine tool and cutting tool, the hardness of pre-hardening cannot reach the hardness of the mold, so the research and development of pre-hardening technology is not invested much.
With the improvement of the performance of the machine tools and cutting tools, mold material hardening technology development speed, to the 80 s, the international industrial developed countries on the plastic mould materials using the proportion of the pre hardened module has reached to 30% (currently at more than 60%), our country began in the late 1990’s the pre hardened module (mainly use imported products).
SSAB Swedish steel group, research and development of pre hardened steel TOOL I have OX extension of steel, heat treatment has the world’s leading technology, out is hard to 45-48 HRC, 29-33 HRC, 100% before delivery to various performance test, the Swedish royal institute of technology is “the perfect combination of hardness and toughness”, put the steel to the maximum performance, reduce the heat treatment time and risk for the customer.
The prehardening technology of die material is mainly developed and implemented in die material manufacturers. By adjusting the chemical composition of the steel and equipping it with appropriate heat treatment equipment, pre-hardening modules of stable quality can be produced in large quantities. Our country in the mold material pre-hardening technology, starting late, small scale, can not meet the requirements of domestic mold manufacturing.
Adopting the material of pre-hardening mould can simplify the mould manufacturing process, shorten the mould manufacturing period and improve the mould manufacturing precision. It can be expected that as processing technology improves, pre-hardening mold materials will be used for more mold types.
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