Effect of thermo-chemical parameters on the adhesion of scale layer Technical University of Czestochowa, The Department of Industrial Furnaces and Environmental Protection EDYTA WARWAS

Introduction In the process of heating steel before plastic working the processes of chemical interaction also occur between the furnace atmosphere and the surface of the steel being heated. Scale forms as a result of oxidation. An important issue in the operation of heating furnaces is the adhesion of a scale layer to the surface of the steel being heated. The adherence of scale depends on heating technologies, such as temperature and excess combustion air ratio.

Measuring stand and testing methodology a laboratory furnace processing of results a gluing stand a stand for detaching reference specimens a stand for partial knocking off scale photographing cleaning weighting photo processing photo analysis Fig. 1. Schematic diagram of testing stands

Measuring stand and testing methodology Specimens “cooled down” to ambient temperature were subjected to adherence tests. In this case, the examined quantity was the tensile force which allows the scale to be detached from the steel core by reference specimens glued to the face walls of specimens tested. The method applied to the testing of scale adhesion to the cold charge is a quantitative method allowing the numerical determination of the force that bounds the scale with the steel core. The value of force needed for detaching the scale layer from the steel surface was measured by using a testing machine.

Measuring stand and testing methodology Fig. 2. Schematic diagram of the system for testing scale adhesion for “cooled down” specimens QrQr QrQr a glue layer cooling

Results of adhesion measurements Measurements, for cold specimens, were performed by measuring the force needed for detaching the scale layer and then by calculating the adherence expressed in MPa. It should be noted that the direct adhesion measurements were preceded by cooling the specimens down to ambient temperature, followed by joining the formed scale with reference specimens using a glue of an appropriate tensile strength. The measurements were varied out with the value of combustion air excess ratio of  =0.6  1.4.

Results of adhesion measurements Table 1 Results of scale adhesion measurements value of combustion air excess ratio α temperature [˚C] scale adhesion [MPa] 0,60,4300,7841,1262,6643,478 0,70,3610,5280,9862,4443,156 0,80,2150,3610,8331,6742,139 0,90,1870,3240,6320,9671,425 1,00,1580,2720,5210,7781,032 1,10,1340,2280,3200,5180,724 1,20,1110,1670,2780,4720,527 1,300,0270,0620,1270,158 1,400000

Effect of thermochemical parameters on the adhesion The mathematical analysis of testing results was reduced to deriving relationships describing the effect of heating parameters on the adherence of scale to the steel substrate. Several computer simulations were performed to approximate mathematical functions that could describe that effect. The general equation describing the effect of thermochemical parameters on scale adherence has been reduced to the following form:

Effect of thermochemical parameters on the adhesion The analysis of the effect of thermochemical parameters on scale adherence has been performed and suitable relationships have been derived. Due to a different behaviour of the function P=f(  ) for  1.0 and  >1.0, these relationships will take on different forms. The following forms of Equation (1) have been obtained: for  1.0 for  >1.0

Effect of thermochemical parameters on the adhesion 1000˚C 1100˚C 1200˚C 1300˚C 1330˚C 1000˚C 1100˚C 1200˚C 1300˚C 1330˚C 1000˚C 1100˚C 1200˚C 1300˚C 1330˚C measurements calculations α  1.0 α>1.0 The value of combustion air excess ratio, α Fig. 3. Relationship of adherence vs. thermochemical parameters for measurements and calculations Scale adhesion, P z [MPa]

Conclusions The performed theoretical analysis and the laboratory tests carried out allow the following conclusions to be drawn: 1.Heating parameters have an essential effect on the adhesion of scale to the steel substrate. 2.Scale adhesion increases with increasing heating temperature. 3.Scale adhesion decrease with increasing value of combustion air excess ratio.

Conclusions 4.The correlation between heating parameters (temperature and the value of combustion air excess ratio) can be described by mathematical relationships; however, a different behaviour of these relationships for  1.0 and for  >1.0 should be taken into account. 5.The performed investigation of the effect of heating parameters on the adherence of scale can form guidelines for the technology of heating steel stock before plastic working.