Abstract
Spalling of back-up rolls is a matter of great concern for both roll users and manufacturers because it makes the unit cost of a roll higher and gives unfavorable effect upon the surface of the rolled products.
In order to make this problem clear, measurements of wear, temperature and mill load are required and further it seems to be desirable to test rolls in practical mills. But as these measurements and tests take much cost and long time, the short-cut to the solution of this problem will be to design and manufacture a test mill and to observe the development of the damages by reproducing the spalling on the model roll.
There are many factors which affect the generation of spalling of back-up rolls from the standpoints of both application and materials and such factors seem to act on each other and to make the phenomena of spalling complex. Of these many factors, heat treatment of roll materials, especially their hardness, is considered to be the most important for the roll manufacturing.
With the intention of clarifying the effect of the hardness of back-up roll materials on the damages due to rolling contact, a new 3-high rolling contact testing machine was designed and constructed.
This testing machine consists of two hardend rolls (Hs 85∼90, 140mm_??_) and a specimen (70mm_??_) with 15mm effective width which is placed between these two hardened rolls and is rolled under various loads.“Damage index” was defined to evaluate the defects on the surface of a specimen by roll on roll under heavy load.
The specimens are heated to 850°C and quenched in oil. Then they are tempered to various hardnesses.The rolling on roll test was carried out under 120kg/mm2 of Hertz maximum stress.
The main results obtained are as follows:(1) As the hardness of materials become higher, resistan ceto rolling contact damages increases.(2) The surface layers of low hardness specimens are strengthend by rolling contact, so it seems to make the life of these specimens longer.
