Fracture Cause of 9Cr2Mo Steel Large Cold Roll

9Cr2Mo steel is a high-carbon alloy tool steel with excellent wear resistance, machinability, hot strength and deformation resistance, and is commonly used for cold rolling rolls both at home and abroad. Owing to its high carbon content and large quenching stress, 9Cr2Mo steel is prone to cracking during heat treatment. Therefore, sufficient preheating treatment shall be carried out for large 9Cr2Mo steel cold rolling rolls before quenching to ensure normal production.

A cold rolling roll made of 9Cr2Mo steel has a diameter of 400 mm and a height of 450 mm. Its process flow is as follows: forging → spheroidizing annealing → hydrogen diffusion treatment at 650 ℃ for 50 h → rough machining → quenching at (880±10) ℃ → tempering at 180 ℃. The roll fractured suddenly during the waiting period for tempering after quenching. A series of physical and chemical inspection methods were adopted to analyze the fracture cause of the roll, and the findings are as follows:

Analysis

There are multiple circular regions on the fracture surface of the cold rolling roll, with radial propagation morphology around them. The fracture is flat, silvery gray, and free of plastic deformation, with reflective small facets distributed on it. The interior of the circular regions is relatively smooth, with tear ridges shaped like "chicken claw marks", and the radial propagation zone exhibits the morphological characteristics of cleavage fracture. Based on the fracture characteristics, it can be determined that the circular regions are crack initiation zones, the radial regions are crack propagation zones, and the fracture is a typical hydrogen embrittlement fracture. 9Cr2Mo steel has high carbon content, high strength and high hydrogen embrittlement sensitivity. The critical hydrogen mass fraction for 9Cr2Mo steel free of hydrogen embrittlement is 0.00014%, and that free of white spots is 0.00027%. The residual hydrogen mass fraction of the roll is 0.00025%, exceeding the critical hydrogen mass fraction for hydrogen embrittlement resistance of 9Cr2Mo steel. Based on the above analysis, it is concluded that the cold rolling roll suffered hydrogen embrittlement fracture. Hydrogen mainly originates from the steel smelting process, and hydrogen absorption from the environment may also occur during subsequent processing.

Hydrogen embrittlement fracture of large forgings is usually the result of the combined action of hydrogen and stress. Hydrogen accumulates at defects under triaxial stress and gradually forms microcracks, namely the circular spots inside the cold rolling roll, leading to fracture of the roll under stress. Field investigation shows that a loud noise was emitted when the roll fractured, and multiple cracks existed besides the fracture position. Fracture of the roll under static standing indicates a very high internal stress. The roll was not tempered in time, and the stress derived from quenching stress, including thermal stress and structural stress. Thermal stress is generated by the temperature field during temperature changes, while structural stress is caused by volume changes accompanying the formation of different microstructures during phase transformation. Severe segregation and the intergrowth of pearlite and martensite in the roll resulted in considerable structural stress.

The cold rolling roll exhibits severe dendrite segregation, which is attributed to chemical composition segregation. Although the chemical composition of the roll meets the standard requirements, excessively high casting temperature or low solidification rate of the steel ingot aggravates component segregation. In the subsequent forging process, insufficient forging ratio fails to fully break up the as-cast dendrites, thus retaining the segregation. The roll contains a considerable amount of inclusions. Abundant inclusions, severe dendrite segregation and a high stress field provide channels and sites for the diffusion and aggregation of hydrogen inside the roll, hinder the escape of hydrogen to a certain extent, and thereby create sufficient conditions for hydrogen embrittlement fracture of the cold rolling roll.

Conclusion and Recommendations

The fracture of the 9Cr2Mo steel cold rolling roll is classified as hydrogen embrittlement fracture, with the crack initiation site located at the circular spots inside the roll where hydrogen accumulates. High hydrogen content and large quenching stress caused by delayed tempering are the main causes of hydrogen embrittlement fracture of the roll, while severe dendrite segregation and abundant inclusions accelerate the fracture. The recommendations are as follows:

① Improve metallurgical quality to reduce the content of inclusions and the degree of segregation in the steel;

② Increase the forging ratio to avoid dendrite segregation in the material;

③ Prolong the hydrogen diffusion time to reduce the hydrogen content of the roll;

④ Conduct tempering in a timely manner to reduce the quenching stress of the cold rolling roll.