In production, sometimes there is insufficient hardness after quenching, which is a common defect in the process of heat treatment and quenching.
”There are two manifestations of “insufficient hardness”, one is that the hardness of the whole workpiece is low, the other is that the local hardness is insufficient or soft spots appear.
When the phenomenon of insufficient hardness occurs, we should use hardness test or metallographic analysis to analyze which kind of “insufficient hardness”, and then find out the reasons from raw materials, heating process, cooling medium, cooling method and tempering temperature, so as to find out the solutions.
1. Improper selection of raw materials or wrong material distribution
If the parts should be made of medium carbon steel or high carbon steel instead of low carbon steel, and the parts should be made of alloy tool steel instead of ordinary high carbon steel, the hardness will be insufficient or soft spots will appear.
Example 1: the gear should be made of #45 steel, its quenching hardness should be about 60HRC, and the wrong choice of #25 steel, the result is about 380hbs hardness;
Example 2: the mould made of 9mn2v should be used instead of T8 steel. Because the sparks of 9mn2v and T8 steel are difficult to distinguish, the quenching process of 9mn2v is mistakenly followed and oil cooling is adopted. As a result, the hardness is only about 50HRC.
The above two cases belong to the overall insufficient hardness, which can be determined by hardness test or metallographic test.
- Appropriate materials should be selected in design;
- Strengthen the material management, chemical analysis before the material into the warehouse, and then classification marking, can effectively avoid the wrong material;
- The heat treatment operator shall conduct spark analysis before operation to roughly identify whether the part material meets the drawing requirements;
- When the cross section of the workpiece is large or the thickness of the workpiece is wide, if the tool steel is used, because of its poor hardenability, the internal hardness of the large cross section will be low. At this time, the alloy steel with good hardenability should be used.
2. The uneven microstructure of raw materials results in insufficient local hardness or soft spots
The microstructure appears one of the following conditions: carbide segregation or aggregation, such as ferrite aggregation, graphite, severe widmanstatten structure, etc., will appear hardness deficiency or soft spots.
- The microstructure is homogenized by repeated forging or preheat treatment (such as normalizing or homogenizing annealing) before quenching.
1. The quenching temperature is low and the holding time is short
For example, when the heating temperature of hypoeutectoid steel is between AC3 and AC1 (for example, the quenching heating temperature of # 25 steel is lower than 860e), because the ferrite does not completely dissolve into austenite, the uniform martensite can not be obtained after quenching, but ferrite and martensite are obtained, which affects the hardness of the workpiece.
The undissolved ferrite can be seen from metallographic analysis (as shown in Fig. 3).
For high carbon steel, especially for high alloy steel, if heating or holding time is not enough, pearlite can not transform to austenite, but martensite can not be obtained.
In the actual production, the above situation is often due to the deviation of instrument indication (indicating high temperature) or uneven furnace temperature, so that the actual temperature of the workpiece is low;
The wrong estimation of the thickness of the workpiece results in the short holding time.
- Control the heating speed, avoid heating too fast, resulting in uneven furnace temperature, at the same time will cause premature holding time, so that the holding time is insufficient;
- Frequently check whether the temperature indicating instrument is in good condition and accurate, so as to avoid the phenomenon that the actual temperature is insufficient when the indicating instrument reaches the temperature;
- The quenching heating speed and temperature shall be determined in strict accordance with the material manual to prevent the quenching temperature from being too low or too high;
- Correct estimation of material thickness, especially for special-shaped parts.
2. Quenching heating temperature is too high, holding time is too long
For tool steel (such as T8 steel), when the quenching temperature is 780e, austenite and carbide (Fe3C) are obtained.
At this time, the amount of carbon dissolved in austenite is slightly higher than 0.77%. After cooling, austenite transforms into martensite.
If the heating temperature is too high or the holding time is too long, a large amount of carbon in carbide (Fe3C) will dissolve into austenite
At the same time, it greatly increases its stability, makes austenite transform to martensite (AYM), and the temperature begins to drop.
Therefore, a large amount of retained austenite (AC) is retained in the workpiece after quenching, and the microstructure obtained is m + AC
Because retained austenite has austenitic properties, that is, low hardness, the hardness decreases after quenching.
The influence of heating temperature and tempering temperature on the content of retained austenite.
- Strictly control the quenching temperature and holding time to prevent excessive carbon from dissolving into austenite (a);
- By reducing the cooling rate of quenching, or by using step quenching, the undercooled austenite can transform into martensite;
- After cold treatment, retained austenite transforms into martensite;
- When high temperature tempering is used to reduce the retained austenite, the hardness will increase.
3. During quenching and heating, the surface of workpiece decarburizes
After quenching, the surface of #45 steel is ferrite and low carbon martensite by metallographic analysis.
After the decarburization layer is removed, the hardness meets the requirements.
This situation often occurs in the box furnace without protection or poor protection, or heating in the salt bath with poor deoxidation, resulting in the reaction of oxygen and carbon atoms in the workpiece to form CO, which reduces the carbon content on the surface of the workpiece to make its surface hardness insufficient.
- Non oxidation heating furnace with protective atmosphere is used, such as the protective atmosphere of alcohol and methanol cracking;
- Vacuum heating quenching is adopted;
- For general box type furnace, it can be packed and sealed with scrap iron or charcoal;
- The surface of the workpiece is coated with anti-oxidation coating;
- Charcoal is put in the furnace, and the workpiece is heated after being coated with boric acid and alcohol solution.
Cooling process problems
1. Improper selection of quenching medium
When oil cooling is used for the workpiece quenched by water or salt bath, the hardness of the workpiece is low due to the insufficient cooling capacity and slow cooling rate, the transformation of austenite to pearlite (AYP) and the absence of martensite (m), especially in the core of the workpiece.
For example, the hardness of T10 hand hammer quenched in oil is only about 45HRC. Through metallographic analysis, it can be seen that it is troostite rather than martensite.
- The proper cooling medium must be selected according to the material, shape and size of the workpiece.
2. Influence of quenching medium temperature
During water quenching, a large number of parts are continuously quenched. If there is no circulating cooling system, the water temperature will rise and the cooling capacity will drop, resulting in the phenomenon of hardening failure.
When the oil is cold, due to the low temperature and poor fluidity of the oil at the beginning of quenching, the cooling capacity is not strong, resulting in hardening.
- During water quenching, the circulating cooling system should be adopted and the water temperature should be kept at about 20E;
- When the oil is cold, especially at the beginning, it should be heated properly to make the temperature above 80e, which is the reason of “cold water and hot oil” in quenching.
3. The quenching medium is too old
When there are more impurities in the alkali (salt) bath or too little water, it is easy to produce quenching soft spots.
- The quenching medium should be changed in time and the water content in alkali (salt) bath should be controlled.
4. Improper cooling time control
When carbon steel is used to make switch parts with complex or large cross section, water quenching and oil cooling are used to prevent deformation and cracking.
Because of the high temperature of the part itself, especially the slow cooling rate of the core, the uniform and complete martensite can not be obtained.
- Properly control the water cooling time. If the workpiece is clamped with pliers, when the hand can’t feel the vibration, it will immediately turn into the oil;
- For the larger cavity, the waste should be removed first to reduce the thickness of the workpiece, and then quenched.
- During step quenching, bainite transformation occurs when the residence time in the salt bath is too long, resulting in insufficient hardness.
In short, the phenomenon of insufficient quenching often occurs, the operator should find out the reasons to solve the problem according to different situations and specific analysis.