Solution to insufficient quenching hardness
What is insufficient quenching hardness?
In production, sometimes there is a lack of hardness after quenching, which is a common defect in the quenching process of heat treatment. There are two types of “hardness deficiency”, one manifestation is the whole workpiece hardness value is low, the other manifestation is local hardness is not enough or appear soft spot. When the phenomenon of insufficient hardness, to use hardness test or metallographic analysis and other methods to analyze which is “insufficient hardness”, and then from raw materials, heating process, cooling media, cooling methods and tempering temperature to find the cause, so as to find a solution.
1. Raw material
1.1. Improper selection of raw materials or send the wrong material
Should be made of carbon steel or high carbon steel parts and the wrong use of low carbon steel, should be made of alloy tool steel parts and the wrong use of ordinary high carbon steel can cause a lack of hardness or soft spots.
Example 1: should use 45 # steel manufacturing gear, the hardening hardness should be about 55 HRC, and the wrong choice for 25 # steel, the result is only about 380 HBS hardness.
Example 2: should be made of 9Mn2V mold, and wrongly used into T8 steel, due to 9Mn2V and T8 steel sparks are difficult to distinguish, quenching mistakenly according to 9Mn2V quenching process quenching, using oil cooling, the result is only about 50HRC hardness.
The above two cases belong to the overall hardness is not enough, you can use hardness test or metallographic test to determine.
Solutions: the design should choose the right material; strengthen material management, materials into the warehouse before the chemical analysis, and then classified for marking, can effectively avoid sending the wrong material; heat treatment operators should be in operation before the spark analysis, to roughly identify whether the parts material in line with the requirements of the drawings; when the workpiece cross-section is large or workpiece cross-section thickness disparity, if the use of tool steel, due to its poor hardenability, will cause large Section at the internal hardness of the phenomenon of low, then should be replaced with good hardenability of alloy steel.
1.2 Uneven microstructure of the raw material causes insufficient local hardness or soft spots
One of the following microstructure: carbide segregation or aggregation phenomena, such as ferrite aggregation, graphite, severe Weinstein’s organization, etc. will appear hardness or soft spots.
Solution: Repeated forging or preparatory heat treatment (such as normalizing or homogenizing annealing) before quenching to homogenize the organization.
2. Heating process aspects
2.1 Quenching heating temperature is low, insufficient holding time
Such as sub-eutectoid steel, when the heating temperature between Ac3 and Ac1 (such as 25 # steel quenching heating temperature below 860 ℃), because the ferrite is not completely dissolved in austenite, quenching can not get uniform and consistent martensite, ferrite and martensite, affecting the hardness of the workpiece, visible from the metallographic analysis of undissolved ferrite. For high-carbon steel, especially high-alloy steel, if the heating or holding time is not enough to cause pearlite can not be transformed to austenite, and not martensite. In actual production, the above situation is often due to deviations in the instrument indication (indication of high temperature) or uneven furnace temperature, so that the actual temperature of the workpiece is low; the thickness of the workpiece is incorrectly estimated, causing the insulation time is too short.
Solution: control the heating speed to avoid heating too fast, resulting in uneven furnace temperature, while it will cause premature insulation time, so that the insulation time is not enough; often check whether the temperature indicator meter is intact and accurate, to avoid the indication of the meter to reach the temperature, and the actual temperature is not enough; strictly according to the materials manual to determine the quenching heating speed, heating temperature, to prevent quenching temperature is low or high; the correct estimate material thickness, especially for shaped parts.
2.2 Quenching heating temperature is too high, holding time is too long
For tool steel (such as T8 steel), when its quenching heating temperature at 780 ℃ to get austenite and carbide (Fe3C), at this time the austenite dissolved carbon amount is slightly higher than 0.77%, after cooling austenite transformation to martensite. If the heating temperature is too high or holding time is too long, it will cause the carbide (Fe3C) in a large amount of carbon dissolved in austenite, resulting in a high amount of austenite dissolved carbon, while greatly increasing its stability, so that the austenite to martensite (AyM) transformation, the temperature began to fall, and thus after quenching the workpiece retains a large amount of residual austenite (Ac), the organization obtained for M + Ac, due to the residual austenite has austenite properties, i.e., low hardness, thus causing a decrease in hardness after quenching. The effect of heating temperature and tempering temperature on the residual austenite content is shown in the figure below.
Heating temperature and tempering temperature on the effect of residual austenite content
Solutions: strict control of quenching heating temperature and holding time to prevent excessive carbon dissolution into the austenite (A), control the heating temperature is more important; reduce the quenching and cooling rate, or the use of graded quenching, so that the full transformation of subcooled austenite to martensite; the use of cold treatment, so that the residual austenite to martensite transformation; the use of high-temperature tempering, reduce residual austenite, hardness will increase instead.
2.3 Quenching and heating, the workpiece surface decarburization
45 # steel quenching, through metallographic analysis, its surface ferrite and low carbon martensite, and grinding the surface decarburization layer, the hardness meets the requirements, this situation often occurs in the box furnace without protection or poor protection, or in the poor deoxidation of the salt bath heating, resulting in oxygen and carbon atoms in the workpiece reaction to generate CO, so that the surface carbon content of the workpiece decreased, resulting in its surface hardness is not enough.
Solutions: the use of a protective atmosphere without oxidation heating furnace, such as the use of alcohol, methanol cracking protective atmosphere and other methods; the use of vacuum heating quenching; for general box-type furnace can be applied pig iron chips or charcoal boxing seal; the surface of the workpiece coated with anti-oxidation coating; furnace put charcoal; workpiece coated with boric acid, alcohol solution before heating.
3. Cooling process issues
3.1 Improper selection of quenching medium
The water quenching or salt bath workpiece and the use of oil cooling, it is not enough cooling capacity, cooling speed is too slow, in the cooling process austenite occurs to the pearlite type of tissue transformation (AyP), and do not get martensite (M) (especially in the core of the workpiece), resulting in low hardness of the workpiece, such as T10 production of hand hammer quenching in oil, the hardness is only about 45HRC, through metallographic analysis, visible to get taustenite organization and not martensite.
Solution: The workpiece material, shape and size must be selected according to the appropriate cooling medium.
3.2 The effect of quenching medium temperature
In the water quenching, a large number of parts quenched continuously, if there is no circulating cooling system, resulting in a rise in water temperature, cooling capacity decreases, there will be quenched hard phenomenon. In the oil cooling, the beginning of quenching due to the low temperature of oil, poor liquidity, and thus the cooling capacity is not strong, resulting in quenching hard.
Solution: water quenching should be used to circulate the cooling system to maintain the water temperature at about 20 ℃ is appropriate; oil cooling, especially at the beginning of the appropriate heating, so that its temperature reaches 80 ℃ or more is appropriate, which is often said when quenching “cold water hot oil” reason.
3.3 Quenching medium is too old
When containing more impurities in the alkali (salt) bath or moisture is too little, easy to produce quenching soft spot.
Solution: the quenching medium should be replaced in a timely manner and control the alkali (salt) bath moisture.
3.4 Improper control of cooling time
When using carbon steel manufacturing switch complex or large cross-section parts, in order to prevent its deformation and cracking, using water quenching oil cooling. Parts in the water for too short a time or removed from the water in the air for too long and then transferred to the oil, due to the parts of their own high temperature, especially the core cooling rate is slow, and can not get a uniform and complete martensite.
Solution: proper control of the water cooling time, if the workpiece is held in a vice, when the hand does not feel the vibration, immediately transferred into the oil; for cavity larger mold, the scrap should be excluded first, reduce the thickness of the workpiece, and then quenched. Graded quenching, in the salt bath for too long a stay in the bainite transformation caused by insufficient hardness.
In short, the quenching phenomenon often occurs, the operator should be analyzed according to different circumstances, to find the causes to overcome.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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