Author Topic: Tribological Material Selection, Analysis Modification and Manufacturing of Mill  (Read 1604 times)

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Author : Prasad Sane, Dr. Ashesh Tiwari
International Journal of Scientific & Engineering Research Volume 2, Issue 6, June-2011
ISSN 2229-5518
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Abstract— In the industry, mill rollers are undergoing severe loading for 24 hours 7 days a week which results in formation of crack due to loading conditions, temperature and stress intensity. This leads to the demand in material selection according to its tribological needs which make the existing material to last for more time than the existing. Failures of rolls occur due to improper manufacturing and operational parameters. The samples of prematurely failed roll samples collected from steel plant were examined for their chemistry, inclusion content, microstructures, carbide characteristics, hardness and the retained austenite content. The residual stresses were also measured on the inner as well as the outer surface of the spalled roll pieces. The results obtained have been discussed in this paper. The higher content of retained austenite was primarily responsible for the spalling of indigenous rolls for which sub zero treatment has been recommended. Several suggestions have also been made for smooth operation of the mill and consequently for the extension of life of a work roll [1]. A procedure is described for the calculation of stress intensity factors for surface cracks in a section roll.  Results are given in terms of the load acting of the roll and the average pressure acting on the roll/work piece interface.  It is noted that the load acting on the roll can be determined for individual cases by experimentally measuring the deformation of the roll stands: this obviates the necessity of relying on theoretical estimates of the roll/work piece pressure.  The stresses in the uncracked roll are determined using an axis symmetric finite element model. However, for the subsequent calculations of stress intensity factors the analysis is reduced to a plane strain model. It is shown that it is necessary to carry out finite element stress analysis on cracked rolls to determine stress intensity factors and to find the point of crack initiation and crack propagation [2].

Index Terms— Finite Element Analysis (FEA), Optimization, LEFM, Life Prediction of Roll, Finite Element Model, Feasiblity, crack initiation, crack propagation.

The objective of the work is to make the model and analyze the industrial mill roll so as to make it optimized and feasible so that by putting less effort and more material saving the roll can perform its work to its best. The detailed analysis will include and develop the numerical simulation of 3 –D model analysis as well as structural as well as motion analysis. All the things of detecting the point of fracture and other are done on the software PRO-E. The model which is made can also be transferred to a CNC machine in this software by
using G- code.
In the first phase we have predicted the life of roll by changing the material specification and predicting the crack induced and critical value of crack, also predicted the amount of saving in terms of money and in terms of weight of material. In this report we have converted the physical object into its 3-D model with the help of software Pro-E Wildfire 2.0. Then we have put the forces on its surface. Then we have converted that model in its wire mesh form for its complete analysis. Then we have ana-lyzed the point of fracture with its node ID. Then the model feasibility and optimization is been checked. And with the help of it an optimized model is generated with material saving [2].

a.   Problem: Problem arise because the roll, which is 4184.5 Kg have been replaced after 24 hours, due to heavy loading condition of 20 tons in 1290 rpm and in 10-12 passes.
b.   Applied the energy balance equation to find the length of crack.
c.   In 24 hours, for varying materials.
d.   Applied life prediction method (LEFM) me-thod on different materials, and find out the best optimum material used so that life of roll should be maximum with optimum working cost.
e.   Changed the material according to loading condition and hardness, and find out the length of crack, and its permissible limit of crack.
f.   Got a suitable material which is cheap and can be used in place of cast iron without fracture for more than 24 hours.
g.   Now we have changed the design of roll for more life, according to stress intensity factor and find out a hard point from where crack will initiate.
h.   Changed the design according to node size, tetrahedrons, and isotropic material properties and found a design with no hard point, i.e. less chances of fracture from that point.
i.   Done FEA analysis on the roll, structurally and find out stress distribution factor.
j.   Now, optimized the design and checked its feasibility, then manufacturing of the roll is taken into account.

3.1 Linear elastic fracture mechanics
LEFM can be used to predict the lifetimes of rolls or to gain further understanding of any failures. The maximum permissible crack depth to avoid fracture can be calculated by LEFM approach with knowledge of the material toughness. The principal difficulty in the use of LEFM for analysis of rolls lies in the resolution of stresses: bending, torsional and thermal stresses may be present and stress concentrations due to grooves in the roll need to be considered separately for each stress component. In addition, crack geometry affects the choice of stress intensity factor, and reasonably accurate defect sizing is necessary to achieve realistic predictions [3].

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