17CrNiMo6 round steel for sale 17CrNiMo6 special steel in stock

It is a German-standard steel grade, and the European standard is. The corresponding B is not B material, but B material. The specific standard number is: B∕-large gears, ring gear forgings
and the difference:
The toughness is much better
The carbon content is different, the alloying elements are different, and the mechanical properties are different. Thermal processability is also different.
The performance of the former is superior, and the latter type of gearbox gears and differential gears are commonly used.
Chemical composition Carbon (): .,
Silicon: .,
Manganese: .,
Sulfur: .3,
Phosphorus: .3,
, Chromium: .,
Nickel: .,
Molybdenum: .3,
Analysis of steel gear carburizing slow cooling cracks and prevention measures Summary: In view of the crack problem of steel gear slow cooling, the analysis The causes of cracks and preventive measures are proposed.
Foreword
In 2008, when a factory was producing reducers for primary and intermediate rolling mills for Masteel's bar rolling mill, the gears made of steel developed cracks after carburizing and slow cooling. In order to find out the cracks We analyzed and discussed the reasons for this occurrence with the guidance and help of experts from the Chinese Academy of Sciences.
Causes of slow cooling cracks
The main cause of cracks is the uneven phase transformation of the infiltration layer during the cooling process. There are large pieces of cementite and continuous network carbides in the carburized layer. The metallographic structure of the carburized layer is three layers. The outermost layer is lower bainite and network carbides. The middle layer is quenched martensite and lower bainite. The third layer of solid and network carbides is lower bainite plus ferrite. The hardness check from the surface to the inside is shown in the table below.
Inspect the carburized layer base material
Outer layer, middle layer transition layer
Hardness.... ..
Phase change is affected by the following factors:
. Temperature Influence of carbon
Since the solubility of carbon in ferrite is small (up to about.), in the austenite state, the higher the carburizing temperature, the greater the diffusion coefficient of carbon in it, which means carburizing The greater the speed. However, the temperature should not be too high, otherwise the service life of the carburizing equipment will be significantly reduced or damaged, and if the temperature is too high for too long, it will cause defects such as coarse carburized layer structure and excessive carbide levels. Usually, ℃ and 3℃ carburizing are used in actual production.
. Effect of carbon concentration
Slow cooling cracks are related to the carbon potential during carburization.
In the early stage of carburizing, due to the poor carbon on the surface of the workpiece, the ability to accept active carbon atoms is very strong, and the carburizing speed is fast. At this time, the carbon potential in the furnace is low, and a large amount of penetrating agent needs to be passed into the furnace to Maintaining the carbon potential in the furnace is also related to the amount of furnace charging. If the carburizing agent cannot be replenished in time at this time, it may cause defects such as the carburizing time being too long and the carbon concentration distribution curve being concave. However, it cannot be too strong, otherwise it may occur. A large amount of network carbide cannot be eliminated.
When the carbon content on the surface of the workpiece continues to increase and the carbon potential continues to build, the addition of penetrating agent should be gradually reduced, and carburizing enters the diffusion stage. If a large dose of penetrating agent is still maintained at this time, a carburizing agent will form. The network carbide on the surface reduces the strength of the infiltrated layer and increases its brittleness, especially the decrease in tensile strength, which is very detrimental to preventing slow cooling cracks.
.Influence of carburizing time
When the carburizing temperature and carbon potential are determined, the carburizing time mainly depends on the depth of the effective hardened layer. The longer the carburizing time, the deeper the hardened layer, and vice versa. For workpieces with steel hardened layers in -, if the diffusion period is not well controlled and the time is too short, it may cause the carbon concentration distribution curve of the carburized layer to be too steep, and slow cooling cracks will be formed during the subsequent slow cooling process.
. The influence of slow cooling speed
Slow cooling is generally carried out in a cooling well, and its cooling speed should be slower than air cooling in order to obtain a more balanced structure as much as possible. If for some reason, the slow cooling speed is equal to the air cooling speed, slow cooling cracks will appear. The analysis results also show that when the carbon content on the surface of the carburized layer reaches above the eutectoid composition, the hardenability of the carburized layer is not exactly the same. At a specific slow cooling rate, uneven phase transformation occurs, and the Martens of the middle layer The larger specific volume puts the surface under tensile stress. Due to the deterioration of the surface layer, it cannot withstand the large tensile force and cracks.
Measures to prevent slow cooling cracks
From the above analysis, it can be seen that the conditions for the occurrence of slow cooling cracks are: first, the presence of a large number of massive and network carbides in the infiltration layer, which deteriorates its performance; second, the conditions in the infiltration layer are: Uneven phase change occurs. Preventive measures are: first, avoid the generation of a large amount of network carbide in the infiltration layer. For this kind of steel containing strong carbide-forming elements, the carbon potential cannot be too high during carburization, especially during the diffusion period. The carbon potential must be reduced to about . In addition, it is necessary to avoid the generation of martensite in the middle layer. When the slow cooling effect is relatively good, the general structure is relatively balanced and there is no uneven phase change. However, because the cooling well is relatively humid and has a large amount of water, the cooling rate increases and cracks occur. If the ambient temperature is relatively low in winter and the amount of workpieces loaded into the furnace is small, even though it is in a cooling well, the cooling rate is still very fast and slow cooling cracks are prone to occur.