1. How to quench but not hard?
It is often heard in the quenching site that parts are quenched without hardening. The purpose of quenching is to improve the hardness of parts, not hard is not called quenching. It appears to be quenched from the outside, but the inside is not quenched. There are three reasons for quenching rather than hardening:
(1) the quenching temperature (austenitizing temperature) is not enough or the holding time is insufficient, and the austenitizing is not complete;
(2) insufficient cooling rate;
(3) mixture.
The following three measures can be adopted for this purpose:
(1) Considering whether the austenitizing temperature of the hardening layer on the surface of the parts is sufficient,
It is necessary to determine this point. The temperature indicated on the thermometer or recording paper is the temperature of the tip of the thermocouple, not the temperature of the part. It must be noted that we want the temperature of the parts themselves.
(2) Consider whether rapid cooling is sufficient. In order to quench and harden steel, it must be cooled at a rate above the critical cooling rate. Among them, in the critical area city (about 550℃) must be cooled with a speed of about 160C/S or more. The temperature of the quench and the mixing method should also be checked. In addition, parts placed in the tank or cage for batch quenching (group quenching), and single parts quenching two cooling effect is different, so we must pay attention to the way of furnace.
(3) What is the problem of mixing? Mixing is the most dangerous in heat treatment. If mixing occurs, the expected quenching effect can not be achieved. The easiest way to find out whether the mixture is mixed is to use the spark test. The spark test method can check the mixture according to the spark streamline and the dry bloom of carbon. So heat treatment personnel to master this method, before heat treatment must do the spark test, to the material first check.
Take the above measures at the same time, but also pay attention to the treatment of the surface decarbonization or oxidation, the attachment of the skin. If the furnace is carburizing atmosphere, it is best to add nitrogen.
The method of shot peening can be used to remove the oxide skin. In addition, when the residual austenite is more, it will also lead to the lack of quenching hardness, which is the best case of cold treatment. The hardness is improved by cold treatment.
2. What causes soft spots?
The hardness of the parts after heat treatment (quenching, tempering) is usually checked by sampling. In the measured hardness, if the value is uneven, it indicates that there are soft spots. The basic cause of soft spot is uneven quenching rather than uneven tempering temperature. It is not so much that the quenching temperature is uneven, but that the cooling is mostly uneven. If the steel is considered to have good thermal conductivity, then uneven cooling rate resulting in soft spots, this consideration is more appropriate. Of course, if the occurrence of local decarbonization, or local carburizing, even if the cooling is uniform, will also produce quenching soft point, resulting in uneven hardness. As long as it is not uneven material, it can be considered that uneven hardness is caused by uneven cooling.
Although the tempering temperature has uneven and soft points, but as long as the tempering temperature and steel heat conduction combined to consider, steel tempering temperature uneven is not so severe. So the soft spots are mainly caused by uneven cooling. Because tempering will not occur the problem of uneven hardness, so if there is a soft spot, the first consideration should be the evil result of uneven quenching. It is self-evident that the hardness of parts quenched from the whole is higher than the prescribed hardness, which is caused by insufficient tempering; On the contrary, if the hardness is low, it is caused by excessive tempering. In a word, the uneven hardness of parts quenching is caused by the uneven quenching. So the first thing to pay attention to is the uniform quenching.
As a side note, the accuracy of Rockwell hardness tester should be: HRC soil 1; The accuracy of Shaw hardness should be Hs 3.
3. How is inadequate annealing and softening caused?
There are instances of non - softening in annealing, which is the phenomenon of poor annealing. Most of the reasons for poor annealing are improper cooling rate. . Generally speaking, in principle, annealing is done by furnace cooling. But for small parts, or good hardenability of steel parts, with furnace cooling will be unexpectedly fast and cause softening small enough. The cooling rate from austenization temperature to about 550 ° C is most effective in softening, and it is important that the steel is cooled very slowly in this temperature range. In special cases, it is best to use isothermal annealing, that is, from the austenitizing temperature into the 650~680C constant temperature furnace for isothermal processing, if the use of isothermal annealing, no matter how good hardenability, steel can be fully annealed softening.
4. What is the reason for the surface softening of the surface quenched parts?
Generally speaking, the surface hardness of the surface carburized and high-frequency quenched parts is softer than that of the subsurface near the surface, mainly because of the residual austenite in the surface layer. There is residual stress (σr) on the surface of the surface quenched part, so the surface should be hard, but it is soft because of the presence of residual austenite.
Cold treatment of this surface hardens and increases hardness, which is evidence of residual austenite.
Therefore, surface quenched parts need wear resistance, generally have to use cold treatment. Naturally, the cold place must be tempered at the specified temperature (180~200℃).
If you decarbonize the surface, of course the surface hardness decreases. Surface hardened parts are mostly finished parts, so the cause of surface softening is not so much decarbonization as residual austenite. Is the cause of surface softening surface decarbonization or residual austenite? As long as the cold treatment will understand.
If the hardness increases after cold treatment, it is the softening caused by residual austenite: if the hardness is unchanged, it can be concluded that the softening caused by decarbonization.
5. How to check whether it has been tempered at high temperature?
Secondary tempering hardening of high speed steel (SKH) and die shell steel (SKOIl). - Generally adopt high temperature tempering (500~700C). However, it is difficult to determine whether it is the parts without tempering after quenching or the parts hardened by secondary tempering only by checking the hardness of the parts. In this case, as long as the tempering at 400℃ : that is, through 400℃ tempering, if the hardening (HRC) does not change, it is proved that the parts have undergone two tempering (high temperature tempering); On the contrary, if the hardness is reduced, it is proved that it has not been tempered by high temperature.
