Insufficient hardness when quenching steel
Insufficient hardness when quenching steel
What are the solutions to insufficient hardness during steel quenching?
What are the solutions to insufficient hardness during steel quenching? Today we will discuss together: In production, sometimes the hardness after quenching is insufficient, which is a common defect in the heat treatment quenching process. "Insufficient hardness" has two manifestations, one is that the entire workpiece has a low hardness value, and the other is that the local hardness is not enough or soft spots appear. When the phenomenon of insufficient hardness occurs, the hardness test or metallographic analysis should be used to analyze which kind of "hardness is insufficient", and then find the cause from the raw materials, heating process, cooling medium, cooling method and tempering temperature, etc. Work out a solution.
1 Raw materials
1.1 Improper selection of raw materials or wrong materials
Parts made of medium carbon steel or high carbon steel should be misused and low carbon steel should be used. Parts made of alloy tool steel should be misused and common high carbon steel misused, which will cause insufficient hardness or soft spots. Example 1: A gear made of 45 steel should be used, and its quenching hardness should be about 60HRC, and the wrong choice of 25 steel, the result is a hardness of about 380HBS.
Example 2: The mold 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 was wrongly followed by the quenching process of 9Mn2V and oil cooling was used. The hardness was only about 50HRC.
The above two cases are insufficient overall hardness, you can use hardness test or metallographic test (25 steel quenched to obtain low carbon martensite, as shown in Figure 1, 45 steel quenched to obtain medium carbon martensite, as shown in Figure 2) To judge.
Solution:
1) Choose appropriate materials when designing;
2) Strengthen the management of materials. Chemical analysis is carried out before the materials are put into the warehouse, and then they are classified for marking, which can effectively avoid sending wrong materials;
3) The heat treatment operator should conduct a spark analysis before the operation to roughly identify whether the material of the part meets the requirements of the drawing;
4) When the workpiece cross-section is large or the workpiece cross-section is thick and thin, if tool steel is used, due to its poor hardenability, it will cause a low internal hardness at the large cross-section. In this case, alloy steel with good hardenability should be used. .
1.2 The uneven microstructure of the raw material causes local hardness insufficient or soft spots
One of the following conditions occurs in the microstructure: carbide segregation or aggregation phenomena, such as ferrite aggregation, graphite, and severe Weissite structure, etc., will have insufficient hardness or soft spots.
Solution: Repeat forging or preliminary heat treatment (such as normalizing or homogenizing annealing) before quenching to homogenize the structure.
2 Heating process
2.1 The quenching heating temperature is low, the holding time is insufficient
Such as hypoeutectoid steel, when the heating temperature is between Ac3 and Ac1 (for example, 25# steel quenching heating temperature is lower than 860 ℃), because the ferrite is not completely dissolved in the austenite, after quenching can not get a uniform horse Austenite, which is ferrite and martensite, affects the hardness of the workpiece. Metallographic analysis shows undissolved ferrite (as shown in Figure 3).
For high carbon steel, especially high alloy steel, if the heating or holding time is insufficient, the pearlite cannot be transformed into austenite, and martensite cannot be obtained. In actual production, the above situation is often due to the deviation of the instrument indication (the indication temperature is too high) or the uneven furnace temperature, which makes the actual temperature of the workpiece low; the estimation of the thickness of the workpiece is wrong, causing the holding time to be too short.
Solution:
1) Control the heating speed to avoid the heating speed is too fast, resulting in uneven furnace temperature, at the same time will cause premature insulation timing, making the insulation time insufficient;
2) Always check whether the temperature indicating instrument is intact and accurate, to avoid the phenomenon that the indicating instrument displays the reached temperature and the actual temperature is insufficient;
3) Determine the quenching heating speed and heating temperature strictly according to the material manual to prevent the quenching temperature from being too low or too high;
4) Correctly estimate the material thickness, especially the shaped parts.
2.2 The quenching heating temperature is too high, the holding time is too long
For tool steel (such as T8 steel), when the quenching heating temperature is 780 ℃, austenite and carbide (Fe3C) are obtained. At this time, the amount of austenite dissolved carbon is slightly higher than 0.77%. After cooling, austenite Transformation to martensite. If the heating temperature is too high or the holding time is too long, it will cause a large amount of carbon in the carbide (Fe3C) to dissolve into the austenite, resulting in a high amount of austenite dissolved carbon, while greatly increasing its stability, making the austenite Martensite (A→M) transforms and the temperature begins to decrease, so a large amount of residual austenite remains in the workpiece after quenching, and the resulting structure is M+A'. Because the residual austenite has austenite properties, that is, hardness Low, so the hardness decreases after quenching. The effect of heating temperature and tempering temperature on the residual austenite content (see Figure 4).
Solution:
1) Strictly control the quenching heating temperature and holding time to prevent excessive carbon from dissolving into austenite, and controlling the heating temperature is more important;
2) Reduce the quenching cooling rate, or use graded quenching to make the supercooled austenite fully transform to martensite;
3) Use cold treatment to transform residual austenite to martensite;
4) Use high temperature tempering to reduce residual austenite, but the hardness will increase.
2.3 When quenching and heating, the surface of the workpiece is decarburized
After quenching of 45# steel, through the metallographic analysis, the surface is ferrite and low carbon martensite, and after removing the surface decarburization layer, the hardness meets the requirements. This situation often occurs in the box furnace without protection. Or poor protection, or heating in a salt bath with poor deoxidation, causing oxygen to react with the 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.
Solution:
1) Use a non-oxidizing heating furnace with a protective atmosphere, such as a protective atmosphere for cracking with alcohol and methanol;
2) Use vacuum heating quenching;
3) For general box furnaces, pig iron or charcoal can be used for box sealing; the surface of the workpiece is coated with anti-oxidation coating; charcoal is placed in the furnace; the workpiece is coated with boric acid and alcohol solution and then heated.
3 Cooling process issues
3.1 Improper selection of quenching medium
Oil-cooled workpieces that are quenched with water or salt bath, because of insufficient cooling capacity, the cooling rate is too slow, during the cooling process, austenite changes to pearlite type structure (A→P), but Martensite cannot be obtained. Body (M) (especially at the core of the workpiece), which causes the hardness of the workpiece to be low. For example, the hand hammer made by T 10 is quenched in oil, and the hardness is only about 45HRC. Through the metallographic analysis, it can be seen that the structure is a trinite. Not martensite.
Solution: The proper cooling medium must be selected according to the workpiece material, shape and size.
3.2 Effect of quenching medium temperature
During water quenching, a large number of parts are continuously quenched. If there is no circulating cooling system, the temperature of the water will increase and the cooling capacity will decrease (see Table 1). When the oil is cooled, the temperature of the oil is low and the fluidity is poor at the beginning of quenching, so the cooling capacity is not strong, resulting in hardening.
Solution: A circulating cooling system should be used during water quenching, and the water temperature should be kept at about 20℃; when oil cooling, especially at the beginning, it should be properly heated to make it reach a temperature above 80℃, which is often said during quenching The principle of "cold water and hot oil".
3.3 The medium is too old during quenching
When the alkali (salt) bath containing more impurities or too little water, it is easy to produce quenching soft spots.
Solution: The quenching medium should be replaced in time and the moisture in the alkali (salt) bath should be controlled.
3.4 Improper cooling time control
When using carbon steel to manufacture parts with complicated switches or large cross-sections, water quenching-oil cooling is used to prevent deformation and cracking. If the part stays in water for too short or after being taken out of water, it stays in the air for too long and then transferred into oil. Due to the high temperature of the part, especially the core cooling rate is slow, uniform and complete martensite cannot be obtained.
Solution: Properly control the water cooling time. If you grip the workpiece with pliers, when the hand does not feel the vibration, immediately turn it into the oil; for the mold with a large cavity, the scrap should be removed first, reduce the thickness of the workpiece, and then quench . During the staged 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, and the operator should analyze it according to different situations and find out the reasons that can be overcome.
