5 questions about hardness
1. What about quenching but not hardening?
It is often said that the parts are quenched but not hardened at the quenching site. The purpose of quenching is to improve the hardness of parts. If it is not hard, it is not called quenching. From the appearance, it seems to be quenched, but the actual interior is not quenched.
There are three reasons for quenching but not hardening:
- (1) Insufficient quenching temperature (austenitizing temperature) or holding time, and incomplete austenitization;
- (2) Insufficient cooling speed;
- (3) Mix.
To this end, the following three measures can be taken:
- (1) Consider whether the austenitizing temperature of the surface hardening layer of the part is sufficient,
- It is necessary to determine this. The temperature indicated on the thermometer or recording paper is the temperature at the top of the thermocouple, not the temperature of the part. It must be noted that what we want is the temperature of the part itself.
- (2) Consider whether rapid cooling is sufficient. In order to quench and harden the steel, it must be cooled at a speed above the critical cooling speed. Among them, in the critical area (about 550 ℃), it must be cooled at a speed of more than 160 C/S. The temperature of quenching liquid and mixing method shall also be checked. In addition, the parts are quenched in batches (Group quenching) in the tank or cage, which has different cooling effects from the quenching of single parts. Therefore, attention must be paid to the charging mode.
- (3) What is the problem of mixing? Mixing is the most dangerous in heat treatment. If mixing occurs, the expected quenching effect cannot be achieved. The easiest way to find out whether the mixture is mixed or not is to use spark test. The spark test method can check the mixture according to the spark flow line and the dryness of carbon. Therefore, heat treatment personnel should master this method. Spark inspection must be done before heat treatment to check the material first.
While taking the above measures, pay attention to the decarburization or oxidation on the surface of the treated parts and the adhesion of the scale. If there is carburizing atmosphere in the furnace, it is best to add nitrogen.
The oxide scale can be removed by shot peening. In addition, when there is much residual austenite, it will also lead to insufficient quenching hardness. In this case, it is best to carry out cold treatment. Improve the hardness by cold treatment
2. What are the causes of soft spots?
The hardness of parts after heat treatment (quenching and tempering) usually adopts sampling inspection. In the measured hardness, if the values are uneven, it indicates that there are soft spots. The basic reason of soft spot is uneven quenching rather than uneven tempering temperature. It is better to say that most of the quenching temperature is not uniform than uneven. If it is considered that the steel has good thermal conductivity, it is more appropriate to consider that the uneven cooling rate leads to soft spots. Of course, if partial decarburization or partial carburization occurs, even if the cooling is uniform, quenching soft spots will be produced, resulting in uneven hardness. As long as the material is not uneven, it can be considered that the uneven hardness is caused by uneven cooling.
Although the tempering temperature is uneven and soft spots appear, as long as the tempering temperature is combined with the heat conduction of the steel, the uneven tempering temperature in the steel is not so severe. Therefore, the soft spots are mainly caused by uneven cooling. Since tempering will not cause the problem of uneven hardness, if there are soft spots, we should first consider the consequences left by uneven quenching It goes without saying that if the hardness of the parts after quenching is higher than the specified hardness, it indicates that it is caused by insufficient tempering; On the contrary, if the hardness is low, it indicates that it is caused by excessive tempering. In short, the uneven quenching hardness of parts is caused by uneven quenching. Therefore, we should first pay attention to the problem of uniform quenching.
It should be noted that the accuracy of Rockwell hardness tester should be: HRC soil 1; The accuracy of shore hardness should be: HS ± 3.
3. How is insufficient annealing softening caused?
There are instances of non softening in annealing, which is a phenomenon of poor annealing. Most of the reasons for poor annealing are improper cooling speed Generally speaking, in principle, annealing is cold in furnace. However, for small parts or steel parts with good hardenability, furnace cooling will also cause softening due to the unexpectedly fast cooling speed. In softening, the cooling rate from austenitizing temperature to about 550 ℃ plays the most important role. It is important to make the steel parts cool down slowly in this temperature range.
Under special circumstances, it is best to adopt isothermal annealing, that is, put the austenitizing temperature into a constant temperature furnace at 650 ~ 680c for isothermal treatment. If isothermal annealing is used, the steel can be fully annealed and softened no matter how good the hardenability is.
4. What are the reasons for surface softening of surface quenched parts?
Generally speaking, the surface hardness of surface carburizing quenched and high-frequency quenched parts is softer than that of the sub surface near the surface, which is mainly due to the residual austenite in the surface layer. There is residual stress on the surface of surface hardened parts（ σ r) Therefore, the surface should have been hard, but the reason why it is soft is due to the existence of retained austenite.
Cold treatment of this surface will harden and increase the hardness, which is the evidence of the existence of retained austenite.
Therefore, when surface hardened parts need wear resistance, cold treatment is generally adopted. Naturally, the cold place must be tempered at the specified temperature (180 ~ 200 ℃).
If the surface decarburizes, the surface hardness will certainly decrease. Most of the surface hardened parts are finished parts. Therefore, the reason for surface softening is not so much decarburization as residual austenite. Is surface softening caused by decarburization or retained austenite? As long as it’s cold treated.
If the hardness increases after cold treatment, it is the softening caused by retained austenite; if the hardness remains unchanged, it can be determined that it is the softening caused by decarburization.
5. How to check whether it has been tempered at high temperature?
Secondary tempering and hardening of high speed steel (SKH) and skoil- Generally, high temperature tempering (500 ~ 700C) is adopted However, only checking the hardness of parts is difficult to determine whether it is the parts without tempering after quenching or the parts hardened by secondary tempering? In this case, as long as it is tempered at 400 ℃: that is, after tempering at 400 ℃, if the hardening (HRC) does not change, it is proved that the parts have been tempered twice (high temperature tempering); On the contrary, if the hardness decreases, it proves that it has not been tempered at high temperature.
For the steel that can be tempered and hardened twice, tempering at 400 ℃ has the lowest hardness, which is lower than quenching hardness and high-temperature tempering hardness. If it has been tempered and hardened twice, the hardness will not change due to tempering at 400 ℃.
Tools used in high-temperature environment must be tempered at a temperature 50 degrees higher than the service temperature. If this high-temperature tempering is not carried out, not only the hardness will be reduced, but also the size of tools will change, resulting in quality accidents.
Source: China Stainless Steel Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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