# Studies on Influence of Chromium Layer on Inner Surface of Steel Tube on Heat Transfer
## 1 Introduction
The tube was divided into 30 zones--Figure 1. The heat transfer coefficient \(h_{i}(t)\) and the gas temperature \(T_{g}(t)\) were calculated in 30 cross-sections, P\({}_{1}\) to P\({}_{30}\), in the middle of zones S1 to S30 (Table 1). The functions \(h_{i}(t)\) in cross-sections P\({}_{i}\) are valid in the zones Si (e.g., function \(h_{1}(t)\) in section P\({}_{1}\) is valid in zone S1). The zone in the range from 0 to 216 mm, which requires a different heat transfer model, was omitted in the calculations. It was assumed that there is a galvanic protective chrome coating on the inner surface of the tube. The calculations were performed taking into account thermal conductivity, specific heat and density as a function of temperature (from room temperature--RT to 1000 \({}^{\circ}\)C) of 30 HN2 MFA steel and galvanic chromium Cr [15; 34; 35; 36]. Various chromium layer thicknesses were considered in the calculations, i.e., 10, 20, 30 and 200 \(\upmu\)m. Particular attention was paid to the numerical calculations of the heat transfer in the wall of the tube with protective coatings of different thicknesses for a single and a sequence of heat impulses. An additional idea was to perform numerical simulations of the temperature distribution along the entire length of the tube after sequences of seven heat impulses. When operating the considered tube, destruction of the inner surface of the tube due to overheating is often observed. This damage is manifested by a network of cracks and degradation of the protective chrome coating [4; 14; 15; 16; 17]. Thermal loads in the tube of 30 HN2 MFA steel lead to changes in its structure, which then cause a change in the mechanical properties of the tube. These structural changes are related to the ferrite-austenite phase transition in the tube steel at temperatures above 750 \({}^{\circ}\)C [14; 15]. As is known, such a transition during the heating of steel is associated with a sharp change in volume. In the above case, the shrinkage is caused by the phase transition from the BCC crystallographic lattice to the FCC [37]. As the  
Figure 1: Heat transfer zones S1 to S30 input to the calculations. heat impulses are repeated, the cracks appearing in the tube grow bigger. After a series of heat impulses, the tube is cooled, and the austenite is converted back to ferrite. In addition, high pressure shifts the transformation of austenite to ferrite towards a lower temperature. In summary, the combined action of heat and pressure causes deeper cracks to form and spread in the steel of the tube and, consequently, leads to the destruction of the chrome coating. Early authors' tests of the temperature-dependent thermophysical characteristics of 30 HN2 MFA steel showed that the ferrite-austenite phase transition occurs at a temperature of approximately 750 \({}^{\circ}\)C [15]. In this paper, it was checked whether the temperature of the substrate of the tube with a chrome coating of various thicknesses would exceed 750 \({}^{\circ}\)C after a series of heat impulses. According to the manufacturer's suggestion, the number of heat impulses was limited to seven. The literature query showed that shrinkage of the substrate material of a steel tube covered with a layer of chromium subjected to a series of heat impulses had not been analyzed so far. Numerical simulations of heat transfer in a tube composed of 30 HN2 MFA steel obtained a temperature distribution on the inner surface of the tube. An exemplary temperature distribution on the inner surface of the tube during a single heat impulse is shown in Figure 2. This paper shows that the lowest peak base temperature of the inner surface of the chrome-plated tube is actually the temperature of the inner wall of the tube to a depth of 0.5 mm.