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Preventing Decarburisation in Hot Rolling
S. P. Shenoy
Decarburisation or depletion of surface carbon content takes place when steel is heated to temperatures above 650OC. It progresses as a function of time, temperature and furnace atmosphere. Typical reactions involved are :
O2 + C D CO2
O2 + Fe3C 3 Fe + CO2
CO2 + C 2 CO
CO2 + Fe3C 2 CO + 3 Fe
H2O + Fe3C CO + H2 + 3 Fe
The equilibrium relationship depends on the ratio of carbon dioxide to carbon monoxide. It is neutral to a given carbon content at a given temperature.
NEED TO PREVENT DECARBURISATION
Users of special grades like rail steel, high speed steel, tol steels, alloy steels, spring steel, ball bearing steel and cold heading quality (CHQ) steel demand very low level of decarburisation.
Decarburisation has the following harmful effects :
1. Non-uniform metallurgical transformation during heat treatment.
2. Lowered surface hardness and strength.
3. Reduced fatigue strength of the heat-treated product.
4. Fatal accidents in the case of rail steel.
5. Premature tool failure leading to production losses.
PREVENTING DECARBURISATION IN HOT ROLLING
1. Good rolling practice, free form interruptions, normally helps to minimize decarburisation. This is too theoretical and ideal. In practice, due to unexpected breakdowns, some billets remain in the reheating furnace for a longer time. The rolled products corresponding to these billets show higher level of decarburisation. It is very difficult to segregate them.
2. Use of controlled atmospheres like cracked ammonia, exothermic / endothermic gas, nitrogen, liquid hydrocarbon, etc. effectively controls decarburisation. But they cannot be used to the need of additional capital investment, highly skilled operators and safety regulations. Also, frequent entry and exit of rolling stock into and out of the re-heating furnace makes controlled atmosphere ineffective since ingress of air cannot be prevented.
3. Use of a barrier coating which can prevent the basic reactions of decarburisation is the most economical method. Decarburisation and scaling are simultaneous phenomenon; controlling the former leads to a free bonus of controlling the latter without any extra cost.
Steel Plant Specialities, a Mumbai based firm, has been conducting research on the subject of preventing scale loss and decarburisation for the past fifteen years. It has introduced a range of coatings. Anti-decarb coating enables to guarantee very low level of decarburisation consistently. Here in an example. :
Grade of Steel : A1S1 4140
Type of Furnace : Pusher type
Fuel used : Furnace oil
Product forged : Connecting rod
Depth of decarburisation when forging blanks are not coated : 0.5/0.6mm,Occasionally 1.0mm
Depth of decarburisation when forging blanks are coated with anti-decarb paste, ADP : 0.15 / .020mm (partial)
Every rolling mill is unique by itself. Hence it is a challenging task to develop a coating to fulfill its exact techno-commercial requirements.
The coatings developed by Steel Plant Specialities have accelerated many metallurgical developments in the defence, public and private sector establishments. They have made significant contributions to cost reduction and quality improvement in hot rolling, hot forging and heat treatment.