Product Introduction:

17CrNiMo6 is an alloy structural steel, identified as steel grade DIN 17210-(86) in Germany, with the European standard being 18CrNiMo7-6. The corresponding domestic steel grade is 17Cr2Ni2Mo.

Broadly used in transmission gears, featuring high bending strength and contact fatigue resistance, 17CrNiMo6 also boasts high hardness and wear resistance, with a core that exhibits high hardness and toughness. It offers a superior overall mechanical performance.

Chemical Composition:

Carbon (C) 0.14-0.19

Silicon (Si) 0.17-0.35

Manganese (Mn) 0.4-0.6

Sulfur (S) <= 0.030

Phosphorus (P) <= 0.035

Chromium (Cr) 1.5-1.8

Nickel (Ni) 1.4-1.7

Molybdenum (Mo) 0.25-0.35

3. Analysis and Preventive Measures

1. Introduction

In 1997, during the production of primary and intermediate mill reducers for Ma Steel's bar rolling mill, gears made of 17CrNiMo6 steel developed cracks after quenching and tempering. To identify the cause of the cracks, we conducted an analysis and discussion with the guidance and assistance of experts from the Chinese Academy of Sciences.

2 Causes of Delayed Cooling Cracks

The cause of cracking is primarily due to uneven phase transformation during the cooling process of the diffused layer. The diffused layer contains large carbide particles and continuous reticulated carbides, with the microstructure consisting of three layers: the outermost layer is lower bainite and reticulated carbides; the middle layer is quenched martensite, lower bainite, and reticulated carbides; and the third layer is lower bainite mixed with ferrite. The hardness check from the surface to the interior is as shown in the following table.

Inspection Locations: Carbonized Layer, Base Material

Outer Layer, Intermediate Layer, Transition Layer

Hardness (HL) 420.43, 345.83, 458.51, 501.47, 479.49, 249.47, 914.31, 837.30, 730.73

Thermochromic change is influenced by the following factors:

2.1 Impact of Temperature

Due to the lower solubility of carbon in ferrite (up to approximately 0.025% in ultra-high purity iron), and as the carbon diffusion coefficient increases with higher渗碳 temperatures in the austenitic state, the渗碳 rate also increases. However, excessive temperatures are not advisable as they significantly reduce the lifespan or damage the渗碳 equipment. Moreover, prolonged high temperatures can lead to coarse microstructure in the渗碳 layer and exceed the permissible level of carbides. Typically, carbonizing at 900℃ and 930℃ is practiced in production.

2.2 The Impact of Carbon Concentration

The delayed cooling cracks are related to the carbon potential during carburization.

During the early stages of carburization, due to the surface of the workpiece being highly carbon-poor, it has a strong ability to absorb active carbon atoms, leading to a rapid carburization rate. At this point, the carbon potential in the furnace is low, and a substantial amount of infiltrant needs to be introduced into the furnace to maintain the carbon potential. This is also related to the amount of material loaded into the furnace. If the infiltrant is not replenished promptly, it may result in excessive carburization time and defects such as concave carbon concentration distribution curves. However, it should not be overly strong, as this may cause a large number of reticulate carbides that are difficult to eliminate.

As the carbon content on the workpiece surface continuously increases and the carbon potential is established, the addition of the infiltrant should be gradually reduced. The carbon infiltration enters the diffusion stage. If a large amount of infiltrant is still maintained at this point, surface reticular carbides will form, causing a decrease in the strength of the infiltration layer and an increase in brittleness, especially a decrease in tensile strength, which is quite unfavorable for preventing slow cooling cracks.

2.3 Influence of Carbonitriding Time

Once the carburizing temperature and carbon potential are established, the carburizing time primarily depends on the depth of the effective hardening layer. The longer the carburizing time, the deeper the hardening layer; conversely, it will be shallower. For workpieces made of 17NiCrMo6 steel with a hardening layer thickness of 10-15μm, if the diffusion period is not controlled properly and is too short, it may cause the carbon concentration distribution curve of the carburizing layer to be excessively steep. During the subsequent slow cooling process, it could result in slow cooling cracks.

2.4 Effects of Slow Cooling Rate

The slow cooling process is typically conducted in a cooling well, with a slower cooling rate than air cooling to achieve a more balanced structure. If, for some reason, the slow cooling rate is equivalent to the air cooling rate, slow cooling cracks may occur. Analysis results also indicate that when the carbon content on the surface of the carburizing layer exceeds the eutectoid composition, the hardenability of the carburized layer is not identical. Under specific slow cooling rates, uneven phase transformations occur, resulting in a larger volume of martensite in the intermediate layer, which causes tensile stress on the surface. Due to the deterioration of the surface, it cannot withstand significant tensile forces and thus cracks.

Prevent Cooling Crack Measures

Through the above analysis, it is known that the conditions for the formation of delayed cooling cracks are: first, the presence of a large amount of blocky and reticular carbides in the diffusion layer, which deteriorates its properties; second, the occurrence of uneven phase transformations in the diffusion layer. Preventive measures include: first, avoiding the formation of a large amount of reticular carbides in the diffusion layer. For steels like 17CrNiMo6, which contain Cr and Mo, strong carbide-forming elements, the carbon potential during carburization should not be too high, especially during the diffusion stage. It is essential to reduce the carbon potential to about 0.9%C and maintain it for a certain period to prevent the formation of carbides. Additionally, efforts should be made to avoid the formation of martensite in the intermediate layer. When the delayed cooling effect is good, the microstructure is generally balanced, without uneven phase transformations. However, due to the high humidity and water content in the cooling well, the cooling rate is increased, leading to cracks. If the ambient temperature is low during winter and the quantity of workpieces loaded into the furnace is small, even though they are in the cooling well, the cooling rate is still rapid, and delayed cooling cracks are more likely to occur.