The four main failure modes of roller in use are wear, crack, peeling and fracture. The latter three kinds of roll failure mainly come from roll crack
The so-called crack control technology in roll use technology is to reduce the number of cracks in microstructure, reduce the sensitivity of microcrack initiation, delay the extension, connection, aggregation and crack cluster propagation of microcracks, and form crack sources by improving the metallographic structure performance, improving the roll use environment, and optimizing the roll crack detection and removal process, The technique of controlling crack source and forming destructive crack.
Title: The management of cracks on roll usage ttechnology
Keyword: metallurgical roll, usage technology, crack
- Characteristics of roll cracks
It is inevitable that the roll will crack after long-term use in the harsh environment of cyclic high pressure, high temperature and extreme large temperature difference. There are five common types of cracks in use, including surface mechanical cracks, fatigue cracks (including sub surface secondary cracks), burn cracks, defect cracks (including roll surface defect cracks, working layer defect cracks, bonding layer or transition layer cracks, core cracks, etc.), and mixed cracks.
2.1 common parts of roll cracks
The part with the largest shear stress and bending moment is often called the first dangerous section by users;
The part bearing the maximum applied load strain and the part where the residual stress is easy to attach.
Abrupt change of microstructure, continuous and complete matrix structure separated by other phases or uneven group:
The site of hardness mutation;
Cold working defect parts;
The position with the maximum absolute temperature difference of the roll body;
Parts with the most uneven wear
2.2 time period of frequent roll cracks
When the roll service environment changes significantly. For example, the cooling system in the rolling mill is faulty or unstable: the rolling process adjustment, rolling mill operation accident, guide fence, water guide plate or water cutting plate rolling mill equipment fault, and the product specification and hardness of rolled rice stock have changed greatly;
Roll adjustment period using technology. If the roll change system changes: the initial stage of roll profile or pass adjustment; Initial adjustment of reference grinding quantity;
The period when the physical quality of the roll changes: the first rolling of new materials, new sections and new suppliers; The period when the aged roll is close to scrapping;
2.3 influence range of roll crack
Different types of cracks cause different degrees of direct and indirect losses to the roll body. Among the direct losses, the first is the surface fatigue crack which affects the final surface quality of rolled products. The fatigue crack is one of the reasons for the early falling off of oxide film and the roughness of roll surface; Second, the residual mechanical cracks that increase the grinding bin of non production vehicles. In principle, it is necessary to remove those residual cracks that may cause harmful roll cracks and affect the surface quality of rolled products in the next service cycle, which will further expand, resulting in the increase of grinding bin in the next turn: Third, composite cracks cause roll surface peeling, shoulder peeling, cracking of bonding layer of composite rolls, roll breakage and other roll failure in advance. Among the indirect or related losses, first, the unplanned shutdown of the rolling mill affects the efficiency of the rolling mill and directly affects the output of the rolling mill; second, the cost of roll maintenance, inspection and management increases; Third, restrict the application of some new processes and materials by roll manufacturers and roll users: Fourth, if the incorrect crack removal method is adopted, the final cost performance of the roll will be affected.
- Naming of roll cracks
In the process of roll failure analysis, sometimes the manufacturer and user have different names for roll cracks, which often leads to misjudgment and misunderstanding. At present, roll users often name the crack by visual appearance, that is, “setting nomenclature”. Such as “large” or “small”, “long” or “short”, radial and axial, regular and irregular, closed and open, visible or invisible
Turtle crack, capillary crack, etc. “Qualitative nomenclature” is commonly used by professional technicians. For example, according to the main causes of crack formation, the crack is subdivided into burn crack and water cut-off of card steel
Injury crack, tail flick impact crack, mechanical crack, cold and hot fatigue crack, etc.
The crack name in the specification shall be named according to the two stages of roll failure analysis. Stage – the analysis process for the purpose of identifying the main failure responsibility of roll users. The positioning and setting method can be used in the description. For example, network cracks on the surface of the middle part of the roll body; In the second stage of analyzing the mechanism of crack generation, the naming method of positioning, quantitative setting and qualitative can be adopted. For example, there are local reticular cracks on the surface of the operating side of the roll body
In addition, in the failure analysis of broken roll, for the crack profile, i.e. fracture, the — kind, when determining the location of crack source, the crack can be qualitatively named according to the type of casting defect. Such as inclusion, looseness or shrinkage crack; Second, in analyzing the failure principal stress direction through the crack propagation trend, the crack setting nomenclature can be used, such as cat tongue, coast, nail, hair line, radial, shell, etc. Third, in microscopic metallographic analysis, cracks are often called intergranular, transgranular or mixed cracks. There is no absolute distinction between cracks and cracks. Generally, through cracks, cracks causing direct scrap loss of rolls or cracks that are visible on the opening day are called cracks. Crack is a kind of formal fracture, which is the most serious roll failure mode.
- Main tasks of crack management in roll application technology
The principle of roll crack control, the first is to reduce the crack initiation rate of common parts; Second, reduce the rate of crack propagation: Third, improve the roll service environment to delay crack initiation and propagation; fourth, remove harmful cracks in time and correctly.
4.1 identify, reduce and eliminate mechanical cracks
Mechanical cracks and burn cracks are generally referred to as accident cracks. The residual mechanical cracks of the original accident roll are used on the machine again, which is the biggest hidden danger of the malignant accident of the roll on the machine. Therefore, identifying, reducing and eliminating accident cracks is the focus of the roll maintenance department in the daily dynamic quality control of turnover rolls. The ways to reduce the probability of accident cracks are as follows: first, keep the rolling mill and relevant rolling equipment intact and reduce the number of accident rolls on the rolling mill; second, after the accident of abnormal rolling equipment or operation, while trying to reduce the time to deal with the steel rolling accident, we should also pay attention to avoid further damage to the accident rolls, That is, the accident treatment method of not further expanding the roll crack shall be adopted as far as possible; Third, if possible, after the mill stops in an accident, turn off the cooling water as soon as possible and make the rolled piece and the rolling mill
Roll separation, idling the roll to make the roll temperature uniform and reduce the air cooling to the appropriate temperature; Fourth, monitor whether the roll cooling system in the rolling mill is intact according to the roll temperature distribution and value of the lower roll body or the axial oxide film difference on the roll surface. Ways to deal with mechanical cracks: first, leave the rolling mill after the roll is injured, and mark the position of the last contact surface with the rolled piece before the roll surface is oxidized and rusted; Second, wait for the roll temperature to drop to normal temperature before detecting the crack. Especially for those rolls with high residual stress, the roll surface shall be covered with felt after getting off the machine to prevent the cracked roll surface from bursting due to the residual stress during the cooling process, and small fragments flying out and injuring. Inspectors: Third, check whether the crack depth has reached the scrapped roll diameter, Unnecessary processing and maintenance costs will not occur; Fourth, if it is found that the crack propagation has been at a fixed angle with the roll surface chamber, regardless of local pitting, grooving, grinding or turning, the treatment I technology to reduce the continuous inward propagation of the crack should be selected: fifth, the crack must be completely removed through nondestructive testing after treatment.
It is worth noting that, since the appearance of such cracks is similar to that of roll surface defects, it needs the experience of inspectors to distinguish and characterize accident cracks from other crack types.
4.2. Control the occurrence and propagation of fatigue cracks on the roll surface in advance
Controlling the fatigue crack growth rate is the focus of roll crack control technology. Electric crack in fatigue crack is the main cause of tissue breaking and falling off and abnormal roughness of roll surface. Serious deterioration cracking is the main reason for the increase of roll non productive grinding halo. Delaying fatigue crack initiation is a systematic engineering in roll application technology. Improving the roll service environment, optimizing the roll service technology and finding the roll material and service performance suitable for the rolling mill environment need to be carried out at the same time. The principles of controlling roll fatigue cracks are as follows: first, adjust the rolling process and optimize the initial roll shape to reduce the initiation probability of common frame and common parts; Second, dynamically monitor the cooling system in the roll service environment, or input rolling oil to control or reduce the extension collection of fatigue cracks and the number of cracks: Third, the crack resistance of roll material refers to the ability to resist crack generation and propagation, which is the roll material.
Through communication with the roll manufacturer, select high alloy rolls with high crack resistance, improve the adaptability of physical quality and roll service environment, and reduce the crack initiation sensitivity and propagation rate: Fourth, a reasonable roll ironing process, not only the use of roll residual stress distribution, but also, It can also make the roll surface form a layer of oxide film with excellent performance, that is, dense and closely combined with the body in the shortest time, which can delay the crack initiation: fifthly, a reasonable roll matching system can make the probability of fatigue cracks of the matching roll equal and reduce the amount of non production grinding caused by roll matching grinding: sixth, Reasonable matching of roll surface hardness between rolls at each stand of multi high rolling mill; Seventh, for the closed slight fatigue cracks that do not affect the use and the surface quality of rolled products, if considering the economy, they can be retained, but the principle is to remove the fatigue cracks that will expand and form harmful defects.
4.3 eliminate secondary cracks and burn cracks on the sub surface of the roll
The secondary crack, which occurs below the roll surface, on the fatigue crack and the surface defect crack of the working layer and forms a certain angle with the roll surface, is another form of fatigue crack. Pitting, small meat loss and harmful cracks are produced in the roll. The secondary cracks on the sub surface are the first cause of this kind of roll surface defects. Secondary cracks cannot be found by visual inspection because they occur in the accumulated fatigue layer or work hardening layer. At present, the advanced grinding machines are equipped with on-line nondestructive testing instruments, which can only be positioned, but not quantitative and qualitative. The control methods of secondary cracks are as follows: first, dynamically optimize the roll change cycle; second, dynamically adjust the reference grinding amount each time. After removing the damaged layer of the roll after each use, the fatigue work hardening layer must also be treated; third, improve the roll surface hardness detection and crack detection methods; fourth, select the roll metallographic structure suitable for the use environment, Delay the generation of secondary cracks. When the residual tensile stress on the roll surface is superimposed with the upper grinding tensile stress and thermal stress, it is easy to occur that the secondary cracks on the sub surface at a fixed angle with the roll surface continue to expand inward, resulting in an increase in the amount of unproductive grinding. Thus affecting the service life of the roll.
4.4 monitor roll defects and cracks during maintenance
Roll core cracks mainly include joint layer cracks, core defect cracks, core process hole defect cracks, etc., which are the first cause of major on-line accidents of rolls. There are several methods to control the core crack: first, the supplier and the buyer reach a consensus to select a “sufficiently” thin working layer or hardening layer, and the roll surface selects a “sufficiently” low hardness, which is reduced from the manufacturing process and heat treatment process
The peak value and reasonable distribution of residual tensile stress in roll core can reduce the rate of crack propagation; Second, in order to ensure that the machine is used again without malignant cracks, formulate the rationality of the dynamic core ut judgment standard according to the roll service environment, especially the most dynamic quality monitoring cycle for the quality of the bonding layer of the composite roll, and strictly formulate the U judgment standard in the aging roll stage: Third, re detect the core quality when the roll frame or frame position is changed: Fourth, For all injured accident rollers, the core shall also be rechecked after surface defect treatment before being put on the machine again; Fifth, for the original manufacturing technology
For the roll with relatively poor purity of core material, formulate measures to track the change of core quality.
4.5 increasing the tracking detection times of mixed cracks is an effective way to control the repeated failure of the original accident roll
A large proportion of roll cracks often appear in the form of composite cracks. The most common are the mixing of fatigue crack and surface defect crack, mechanical crack and fatigue crack, mechanical crack and burn crack, roll neck burn crack and fatigue crack, etc. Once this kind of composite crack appears, distinguishing the main cause from the secondary cause is the key to formulate specific control measures. After the removal of mixed cracks, special attention should be paid to the continuous tracking and monitoring of the original crack parts for the mixed cracks with mechanical crack characteristics in the next three to five use and maintenance.