5 FAQs About Hardness: I Bet You Don’t Know

1. What if it is quenched but not hardened?

At the quenching site, it is often heard that the parts are not hardened after quenching.

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 that it is quenched, but actually, there is no quenching inside.

Related reading: 10 Types of Quenching Methods in Heat Treatment Process

There are three reasons for quenching but not hardening:

(1) The quenching temperature (austenitizing temperature) is insufficient or the holding time is insufficient, and the austenitizing is not complete;

(2) Insufficient cooling speed;

(3) Mix.

To this end, the following three measures can be adopted:

(1) It is necessary to determine whether the austenitizing temperature of the hardened layer on the part surface is sufficient.

The temperature indicated on the thermometer or record 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 harden the steel, it must be cooled at a speed above the critical cooling rate.

Among them, the critical area (about 550 ℃) must be cooled at a speed of more than 160C/S, and the temperature of the quenching liquid and the mixing method should also be checked.

In addition, the cooling effects of the parts placed in the tank or cage for batch quenching (group quenching) and single part quenching are different, so the charging method must be paid attention to.

(3) Mixing problem.

Mixing is the most dangerous in heat treatment. If mixing occurs, the expected quenching effect cannot be achieved.

To find out whether the mixture is mixed, the simplest method is to use the spark test.

The spark test method can check the mixture according to the spark flow line and the carbon drying.

Therefore, heat treatment personnel should master this method. Before heat treatment, spark inspection must be carried out to check the material first.

While taking the above measures, pay attention to the surface decarburization or oxidation of the treated parts.

If there is carburizing atmosphere in the furnace, it is better to add nitrogen.

The oxide skin can be removed by shot peening.

In addition, when there is more residual austenite, the quenching hardness will be insufficient.

In this case, it is better to conduct cold treatment to improve the hardness.

2. What causes the soft spot?

The hardness of parts after heat treatment (quenching and tempering) is usually inspected by sampling.

In the measured hardness, if the values are uneven, it indicates that there are soft spots.

The basic reason for soft spots is uneven quenching rather than uneven tempering temperature.

It is not so much the uneven quenching temperature as the uneven cooling.

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 local decarburization or local carburization occurs, even if the cooling is uniform, it will also produce quenched soft spots, 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 and the heat conduction of steel are considered together, the difference of tempering temperature in steel is not so severe.

Therefore, soft spots are mainly caused by uneven cooling. Since the problem of uneven hardness will not occur during tempering, if there are soft spots, the negative consequences of uneven quenching should be considered first.

It goes without saying that the hardness of parts after quenching is higher than the specified hardness, which indicates that it is caused by insufficient tempering;

On the contrary, if the hardness is low, it means that it is caused by over tempering.

In a word, uneven quenching hardness of parts is caused by uneven quenching, so the problem of uniform quenching should be paid attention to first.

It should be noted that the accuracy of the Rockwell hardness tester should be: HRC ± 1;

The accuracy of Shore hardness shall be Hs ± 3.

3. How is insufficient annealing softening caused?

There are examples of non-softening during annealing, which is a bad phenomenon of annealing.

Poor annealing is mostly due to improper cooling speed.

Generally speaking, annealing is furnace cooling in principle.

However, for small parts or steel parts with good hardenability, furnace cooling will also cause insufficient softening due to unexpectedly fast cooling speed.

The cooling rate from austenitizing temperature to about 550 ℃ can play the most important role in softening, and it is important to cool the steel slowly within this temperature range.

Under special circumstances, it is better to adopt isothermal annealing, that is, put the austenitizing temperature into a constant temperature furnace at 650~680 ℃ for isothermal treatment.

If isothermal annealing is used, no matter how good the hardenability is, the steel can be fully annealed and softened.

4. What is the reason for surface softening of surface hardened 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 there is residual austenite in the surface layer.

There is residual stress (σr) on the surface layer of surface quenched parts, so the surface layer should have been hard, but it is soft because of the existence of residual austenite.

The cold treatment of the surface will harden and increase the hardness, which is evidence of the existence of residual austenite.

Therefore, when the surface-hardened parts need wear resistance, they generally have to be cold-treated.

Naturally, it must be tempered at the specified temperature (180~200 ℃) after cold treatment.

If the surface is decarburized, the surface hardness will certainly decrease.

The surface hardened parts are mostly finished parts, so the cause of surface softening is not so much decarburization as residual austenite.

Is the surface softening caused by decarburization or residual austenite? As long as it is cold-treated, it will be clear.

If the hardness increases after cold treatment, it is the softening caused by residual austenite;

If the hardness remains unchanged, it can be concluded that the softening is caused by decarburization.

5. How to check whether it is tempered at high temperature?

High-temperature tempering (500~700 ℃) is generally used for secondary tempering hardening of high-speed steel (SKH) and mold shell steel (SKOIl).

However, it is difficult to determine whether the part is quenched without tempering or hardened by secondary tempering only by checking the hardness of the part.

In this case, as long as the part is tempered at 400 ℃: that is, if the hardening (HRC) does not change after tempering at 400 ℃, it is proved that the part has been tempered twice (high temperature tempering);

On the contrary, if the hardness decreases, it proves that it has not been tempered at high temperature. As shown in the figure below.

5 FAQs About Hardness: I Bet You Don't Know 1
Tempering hardness curve

The steel that can be hardened by secondary tempering, tempered at 400 ℃, has the lowest hardness, which is lower than that of quenching, and also lower than that of high-temperature tempering.

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 50 ℃ higher than the operating temperature.

Without such high-temperature tempering, not only the hardness will be reduced, but also the size of the tool will change, causing quality accidents.

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