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Fastener materials and heat treatment technology hotspots

Apr 02, 2018

1. Can I have a direct water quenching of alloy steel?


How to choose quenching medium? Water quenching actually uses the water solution, uses the PAG quenching liquid "to the water substitution oil" and realizes the intense quenching control technology, has obtained the fastener enterprise Heat treatment colleague approval, uses the polymer quenching Water solution (hereinafter referred to as PAG) replaces the mineral oil as the base quick quenching oil and so on. It depends on the diameter of the bolt and the material grade. General 40Cr, 35CrMo, SCM435 steel diameter above 30mm, 42CrMo, 40CrMnMo, B7 Steel in 48mm above, should have no problem. But the proportion of concentration must be paid attention to.


Pag concentration of 12±1% or above, the use of temperature generally controlled between 30~45℃, according to the steel and process needs to choose different temperature quenching solution. The molecular structure of water is H2O, where O is negatively charged and H is positive. When many water molecules gather together, o and other molecules of H are attracted to each other, forming molecular bonds---hydrogen bonds. In the process of water quenching, the hydrogen bond decreases greatly with the increase of water temperature. Water evaporation accelerated, so the general water use temperature can not exceed 40 ℃, the highest not more than 45 ℃, otherwise easy to appear soft points. Pag is also related to hydrogen bonding, PAG is polyoxyethylene polymer, there is polarity, low temperature and water form hydrogen bond, dissolved in water. When the temperature rises, the hydrogen bond breaks and the Pag precipitates from the water.


Therefore, the use of Pag, the best water temperature not more than 45 ℃, otherwise it may appear steam membrane time, soft points and so on.


2, why Pag "start to use, with a long time can not control the concentration and cause cracking or hardening" and so on?


This mainly is the medium pollution metamorphism, the cooling characteristic curve changes rapidly, especially in the low-temperature area cooling rate rise, causes the workpiece under the same heating condition deformation, the crack is serious, and after quenching hardness obviously enhances or is insufficient. Pag changes are sometimes strange, not a linear process. No matter which brand media, there will be a sudden occurrence of a batch of waste, of course, there may be other reasons for the scrap, but the possibility of quenching media problems is very high. Therefore, to strengthen the day-to-day monitoring of the PAG is the most important, strict management of the quality of the hardening of the fastener is of paramount importance. First of all to determine the regular detection, such as the weekly cooling characteristics of the value, feel pag abnormalities should be intensified testing. Pag is a water-based organic medium, both with bacteria. Pag manufacturers of raw materials are the same, mainly resin, the difference lies in the preservative, sterilization agent. Pag sterilization cycle with the production of varieties, production frequency and day-to-day management, high-grade high-precision bolt cycle is short.


The more good things, the more strict management. There are two ways to determine whether there are bacteria breeding: one is to measure the ph value, this method is simple and easy to do. Bacterial breeding generally causes the ph to drop. If the ph is higher, the basic can not worry about the bacterial problem, conversely, if the ph value is lower, bacteria are more likely to breed. This also has an erroneous zone, some factory once discovered the ph value reduces, tries to adjust the ph value, the result discovers is not high, this time is likely the bacterium breeding problem, does not sterilize also adjusts the bad ph value. It is noteworthy that if the ph value above 9, PAG will not have the problem of bacterial breeding.


The color of the Pag is not directly related to the bacteria, can be used to test the bacteria, the surface is generally one side of bacteria, the other side of the fungus, compared to know.


3, Pag quenching ability, how to avoid quenching cracking? Selection of Pag, cooling capacity, generally using the lower limit of steel quenching heating temperature, the selection of fast quenching oil, cooling capacity of the medium, can improve quenching temperature.


such as swrch35k steel M12 bolts with Pag quenching 800~820℃, with rapid quenching oil can be used 850~870℃. Previously, a large number of production research is mainly in the quenching medium without stirring state, and the actual quenching process in almost all of the solution is in the mixing state, due to the inconsistent chemical composition of the steel, the composition of the mill inconsistent control, in mass production, we must carry out sample test production, and then adjust the production process,


To reduce workpiece deformation, to avoid quenching cracks, improve the hardness uniformity. In order to reduce the possibility of quenching deformation and quenching cracking, the best way is to control the concentration by cooling characteristics. The purpose of using Pag is to add water as an additive to water to regulate its cooling properties. Therefore, the cooling characteristics of quenching medium should also be controlled in heat treatment production.


Practice has proved that one is to adjust the concentration by experience from the quenching effect of the fastener, and the other is to control the concentration by the cooling characteristic instrument. The practice of adjusting the concentration by experience is to record the hardening hardness of the bolts and make a chart, which can be used to observe the hardness change trend of similar bolts. When it is found that the hardening hardness of the bolts is continuously high to a certain degree, it is possible to reduce the actual cooling rate by adopting the method of increasing concentration in time to prevent the quenching cracking. When the hardness of the bolts is found to be low or uneven, and when there is a large deformation, it can be considered to reduce the quenching medium concentration to increase the cooling rate.


Experience method needs experience and strict management, need to be responsible, but very effective, very reliable, suitable for engineering and technical personnel to do.


Use the cooling characteristic instrument to control the concentration, purchase a portable cooling medium performance tester, the concrete method is to measure the cooling characteristic of quenching medium regularly, mainly by its 300 ℃ cold speed to determine the effective concentration of solution, then use the refractometer to control the concentration by the proportion.


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