The solution to insufficient quenching hardness of steel pipes

Sometimes, insufficient hardness occurs after quenching in production, which is a common defect in the heat treatment quenching process. There are two manifestations of "insufficient hardness", one is that the hardness value of the entire workpiece is low, and the other is that the local hardness is insufficient or soft spots appear. When insufficient hardness occurs, it is necessary to use hardness tests or metallographic analysis methods to analyze which type of "insufficient hardness" it is, and then find the reasons from the aspects of raw materials, heating process, cooling medium, cooling method, and tempering temperature, to find solutions.

1. Raw materials

1.1 Improper selection of raw materials or wrong materials: parts that should be made of medium carbon steel or high carbon steel are mistakenly made of low carbon steel, and parts that should be made of alloy tool steel are mistakenly made of ordinary high carbon steel, which will cause insufficient hardness or soft spots.

Example 1: The gear should be made of 45# steel, and its quenching hardness should be about 55HRC, but it is mistakenly selected as 25# steel, resulting in a hardness of only about 380HBS.

Example 2: The mold should be made of 9Mn2V, but T8 steel is used by mistake. Since the sparks of 9Mn2V and T8 steel are difficult to distinguish, the quenching process of 9Mn2V is mistakenly used during quenching, and oil cooling is used. As a result, the hardness is only about 50HRC.

The above two situations belong to insufficient overall hardness, which can be determined by a hardness test or metallographic test.

Solution: Choose appropriate materials during design; strengthen material management, conduct chemical analysis before materials enter the warehouse, and then classify and mark them, which can effectively avoid sending wrong materials; heat treatment operators should conduct spark analysis before operation to roughly identify whether the part material meets the requirements of the drawing; when the cross-section of the workpiece is large or the thickness of the workpiece cross-section is very different, if tool steel is used, due to its poor hardenability, it will cause the internal hardness of the large cross-section to be low. At this time, alloy steel with good hardenability should be used instead.

1.2 The uneven microstructure of the raw materials causes insufficient hardness or soft spots in the local area: If the microstructure has one of the following conditions: carbide segregation or aggregation, such as ferrite aggregation, graphite, severe widmanstatten structure, etc., insufficient hardness or soft spots will occur.

Solution: Repeated forging or preparatory heat treatment (such as normalizing or homogenizing annealing) before quenching to make the structure uniform.

2. Heating process

2.1 Low quenching heating temperature and insufficient holding time: For example, for hypereutectoid steel, when the heating temperature is between Ac3 and Ac1 (for example, the quenching heating temperature of 25# steel is lower than 860℃), because the ferrite is not completely dissolved in the austenite, uniform martensite cannot be obtained after quenching, and ferrite and martensite are obtained, which affects the hardness of the workpiece. From the metallographic analysis, it can be seen that the ferrite is not dissolved. 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 caused by deviation in instrument indication (indicated temperature is too high) or uneven furnace temperature, which makes the actual temperature of the workpiece too low; the thickness of the workpiece is estimated incorrectly, resulting in too short a holding time.

Solution: Control the heating speed to avoid being too fast heating speed, which will cause uneven furnace temperature, and will also cause premature holding time, resulting in insufficient holding time; frequently check whether the temperature indicator is intact and accurate to avoid the phenomenon that the indicator shows the reached temperature but the actual temperature is insufficient; strictly determine the quenching heating speed and heating temperature according to the material manual to prevent the quenching temperature from being too low or too high; correctly estimate the material thickness, especially for special-shaped parts.

2.2 The quenching heating temperature is too high and 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 carbon content of austenite is slightly higher than 0.77%, and austenite is transformed into martensite after cooling. If the heating temperature is too high or the holding time is too long, a large amount of carbon in the carbide (Fe3C) will dissolve into austenite, resulting in high carbon content in austenite, and greatly increasing its stability, causing austenite to transform into martensite (AyM). The temperature begins to drop, so a large amount of residual austenite (Ac) is retained in the workpiece after quenching, and the resulting structure is M+Ac. Since the residual austenite has austenite properties, that is, low hardness, the hardness decreases after quenching.

Effect of heating temperature and tempering temperature on the content of residual austenite

Solution: Strictly control the quenching heating temperature and holding time to prevent excessive carbon from dissolving into austenite (A). It is more important to control the heating temperature; reduce the quenching cooling rate, or use graded quenching to fully transform the supercooled austenite into martensite; use cold treatment to transform the residual austenite into martensite; use high-temperature tempering to reduce the residual austenite, and the hardness will increase instead.

2.3 During quenching and heating, the workpiece surface is decarburized: After quenching 45# steel, through metallographic analysis, its surface is ferrite and low-carbon martensite, and after grinding off 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 poorly deoxidized salt bath, causing oxygen to react with carbon atoms in the workpiece to generate CO, which reduces the carbon content on the workpiece surface and causes its surface hardness to be insufficient.

Solution: Use a non-oxidizing heating furnace with a protective atmosphere, such as a protective atmosphere of alcohol or methanol cracking; use vacuum heating quenching; for general box furnaces, pig iron filings or charcoal can be used for box sealing; the anti-oxidation coating is applied to the surface of the workpiece; charcoal is placed in the furnace; the workpiece is coated with boric acid or alcohol solution before heating.

3. Cooling process problems

3.1 Improper selection of quenching medium: If the workpiece that should be quenched in water or salt bath is cooled in oil, the cooling capacity is insufficient and the cooling speed is too slow. During the cooling process, austenite transforms into a pearlite-type structure (AyP), and martensite (M) cannot be obtained (especially in the core of the workpiece), resulting in low hardness of the workpiece. For example, the hardness of the hand hammer made of T10 is only about 45HRC when quenched in oil. Through metallographic analysis, it can be seen that the obtained structure is troostite instead of martensite.

Solution: The appropriate cooling medium must be selected according to the material, shape, and size of the workpiece.

3.2 Influence of quenching medium temperature: When quenching in water, a large number of parts are quenched continuously. If there is no circulating cooling system, the water temperature will rise, the cooling capacity will decrease, and the quenching will not be hardened. When cooling in oil, the oil temperature is low and the fluidity is poor at the beginning of quenching, so the cooling capacity is not strong, resulting in quenching failure.

Solution: When water quenching, a circulating cooling system should be used to keep the water temperature at around 20°C; when oil cooling, especially at the beginning, it should be properly heated to a temperature of more than 80°C. This is the reason for "cold water and hot oil" when quenching.

3.3 The medium is too old during quenching: when the alkali (salt) bath contains more impurities or too little water, quenching soft spots are easily produced.

Solution: The quenching medium should be replaced in time and the water content in the alkali (salt) bath should be controlled.

3.4 Improper cooling time control: When carbon steel is used to make switches with complex or large cross-section parts, water quenching and oil cooling are used to prevent deformation and cracking. The parts stay in the water for too short a time or stay in the air for too long after being taken out of the water and then transferred to the oil. Due to the high temperature of the parts themselves, especially the slow cooling speed of the core, uniform and complete martensite cannot be obtained.

Solution: Properly control the water cooling time. If you clamp the workpiece with pliers, immediately transfer it to the oil when you can no longer feel the vibration. For molds with larger cavities, the waste should be removed first to reduce the thickness of the workpiece before quenching. During graded quenching, the bainite transformation occurs when the workpiece stays in the salt bath for too long, resulting in insufficient hardness.

In short, insufficient quenching often occurs, and the operator should analyze the specific situation according to different situations, find out the reasons, and overcome them.