On the Formation of Internal Cracks in Heating Process of High Carbon Steel Ingots.

Study of thermal stress cracks in steel ingots-IV

Abstract

Through a series of experiments and theoretidal calculations we disovered that 1·4 t circularingots (400mm_??_×1, 500mm) and 1.7t square ingots (400mm_??_×1, 500mm) of highcarbon-chromium steel suffer cracking due to to thermal stress when heated to the rollingtemperature in a reheating furnace.
The results obtained are as follows:
(1) To find out the period and the surface-center temperature difference at which theinternal cracking may take place, we heated the ingots at varios heating speeds in a Batchtype furnace. Cracks developed only in the ingots in which transformation had been completed, and whose surface-center temperature difference at the completion of transformationwas over 230°C-240°in calculated value. And this critical temperature difference wasidentical with both of the circular and the spuare ingots.
(2) From the inspection of many of the ingots of the above two types which had undergone cracking after heating in a continuous reheating furnace, we could find again thatcracking took place only in the ingots whose core had completed transformation with surfacecentertemperature difference of more than about 240°.
Thus we can say that thecontinuous furnace of this type must have the heating capacity smaller than 30t/h to preventcracking.
(3) With several typical examples of heating curves of ingots in the continuous furnace, wemade calculations of the thermal stress which occurs inside the ingots and found that elasticplasticstress and plastic strain increase violently at the core of the ingot when the corecompletes the transformation. But in all the calculations of rapid heating the maximumthermal stress value remains in this case constant at 0.1kg/mm² with the exception of theincrease of plastic strain.
On the other hand, if the increase of plastic flow delays in proportion to the increase ofplastic strain and strain rate because the plastic flow is retarded by the triaxial tension stressesof the core part, the increase of the stress in the flow state becomes greater than thefracture stress of 13kg/mm² and consequently it causes thermal stress crackings.
Thus we can explain the occurrence of thermal stress crackings in rapid heating.
(4) From the results of the above experiments and calculations, we could devise the wayof heating effectively and without cracking the ingots which are very liable to crack likethose of high-carbon-chromium steel.By the use of this method we are now able to obtaina heating capacity of more than 40t/h without any crackings.