High quality low alloy steel s17Cr2Ni2Mo is the first choice steel for hard tooth surface gear and gear shaft. In a factory in Tangshan, there is a serious quality problem that the gear shaft and gear of s17cr2ni2mo hard tooth surface gear reducer continuously crack after the finished product (or the basic finished product). Entrusted by the factory, we conducted a detailed field investigation and analysis, summed up the basic characteristics and phenomena of cracks, and found the main causes of cracks in the approval (including the axle wheel): first, the defects of the blank are serious, Secondly, the residual stress (including temperature stress, structure stress and other deformation stress) produced in the subsequent processes (such as forging, cooling after forging, heat treatment, processing, etc.) leads to the aggravation and expansion of internal defects, and finally leads to the workpiece cracking. According to these reasons, the corresponding forging and cooling processes are established to solve the problem of cracking.
Basic composition and forging performance of s17Cr2Ni2Mo material
It can be seen from the chemical composition of s17cr2ni2mo alloy steel that its carbon content and impurity sulfur and phosphorus content are lower or similar to 20 × carbon steel. In fact, s17cr2ni2mo alloy steel is a kind of forging steel with excellent properties because of its high content of Cr and Ni elements, which are beneficial to refine grains and improve the strength and toughness of steel.
Main causes of cracks
The state of crack distribution observed in the field. For the axial parts, the cracks are distributed in a zigzag shape along the axial direction, and the cracks originate from the stress concentration of some steps or through cracks at the central hole of the shaft end. For gear disk parts, the cracks are distributed along the outer circumference of the wheel in a plane perpendicular to the axis (wheel) centerline. The cracks along the surface of the inner circle can also be observed at the inner hole of the gear.
According to the field investigation, the batch of heads with quality problems were found and checked. From the cutting end face and the outside, the defects of the material were serious. It can be seen that the shallow surface was mixed with pits and shallow micro cracks, and the structure of the head end and the incision was rough. Based on the above crack morphology and the time and mode of crack generation, the possible causes of the final crack generation are analyzed, which are summarized as follows.
Original defect of raw material white spot
White spot is a fine crack on the section of steel, which mainly occurs in low alloy structural steel and Cr, Ni steel sensitive to white spot. It is a serious defect of high-quality steel, which is easy to produce in smelting or ingot first hot working process. The raw material used in the plant is the long bar after hot processing in the rolling mill. The defect is easy to occur in the dense zone of the middle structure of the rolled or forged material, but generally does not appear in the loose zone of the surface or the center of the forging. The mechanical properties of the steel are greatly reduced by white spots, especially the plasticity. White spot, a potential crack, often leads to sudden cracks in parts when they are quenched or placed after quenching, as well as during use. The location, shape and time of the cracks are consistent with the characteristics of the defects.
As for the cause of white spots, the consistent theory is that hydrogen enters the molten steel in an atomic state during smelting, and the solubility of hydrogen in the steel decreases with the decrease of temperature, so hydrogen will be separated out when the steel is cooled by high temperature. For example, after forging, the hydrogen atoms can diffuse to the surface. If the cooling rate is too fast, the hydrogen atoms will not diffuse and precipitate to the surface of the steel, the hydrogen molecules will form at the grain boundary of the steel and generate huge pressure, which will eventually lead to the formation of white spots in the internal structure of the steel. In addition, the microstructure stress, thermal stress, segregation and non-metallic inclusions in the cooling process of the steel all contribute to the white spot.
Another important factor leading to cracking – stress (temperature stress, tissue stress, residual stress)
The stress can be produced not only in forging process, but also in heat treatment process such as quenching. In the process of forging, due to the uneven degree and speed of material deformation, different parts restrict each other, resulting in residual stress.
Cooling after forging is an important part to ensure the quality of forgings. If the cooling is not proper, it will lead to the aggravation of various stresses and eventually the generation of cracks. The stress produced in the cooling process mainly includes: (1) temperature stress: due to the inconsistent cooling speed inside and outside the workpiece, there is a temperature difference between the inside and outside, so the thermal expansion and contraction inside and outside the metal are not synchronous, which eventually leads to the generation of internal temperature stress. (2) Structure stress: This is due to the change of structure when the steel is cooled, and different structures have different volumes (due to the different density of the structure). For example, the volume will increase by 1% when austenite transforms to pearlite. Therefore, the volume will change with the change of the structure. Because of the temperature difference between the inside and outside of the workpiece, the change of the surface structure is not synchronous with the center. In this way, the internal and external volume changes are not synchronous, and then the tissue stress is formed. In addition, in the process of carburizing and quenching after forging, there will be residual stress.
In fact, cooling after forging and subsequent heat treatment are the main causes of residual stress. If the comprehensive results of these stresses exceed the strength limit of the steel itself, it will cause cracks, so that the workpiece cracks. Therefore, in order to prevent the workpiece from cracking due to stress, strict measures should be taken in the process of cooling after forging and quenching and tempering to control the uniformity of temperature change.
Other possible causes of cracks – Blank
From the crack shape, we can also notice a common feature: the crack is zigzag distribution, and defects such as coarse structure and inclusions are indistinctly visible at the fracture, which can be concluded that the incoming materials have serious defects in the smelting process, and the subsequent process can not be improved. In addition to the defect of “white spot” mentioned above, there are also serious inclusions, segregation, subcutaneous bubbles, etc. in the material itself, and because the defect is oxidized during heating, it can not be forged (welded) during forging, which eventually becomes a potential crack source. Under the combined action of various stresses, these crack sources lead to cracking during aging.
After forging, the parts are naturally air-cooled in the workshop. The natural cooling speed is quite different in winter and summer. In addition, the “wind through” effect in winter makes the cooling speed greatly accelerated in winter, thus increasing the thermal stress, which is also a factor that can not be ignored. From the past experience, winter (November to January) is also the season when alloy steel forgings have more problems. Similar problems have occurred on the 120m cold belt machine previously produced by the factory.