Page 346 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 33 1

Oxidation failure of radiant heater tubes

K.B. Yoona.* D.G. Jeongb
a Department of Mechanical Engineering, Chung-Ang University, 221 Huksuk, Dongjak, Seoul 156-756, Korea
Samsung Heauy Industries Co., 40-1 Woongnam, Changwon, Kyungnam 641-290, Korea
Received 30 July 1998; accepted 11 September 1998

Abstract

A radiant heater tube with a burner installed inside designed to transfer the heat generated by the burner
from the outside of the tube by radiation. Accordingly, the tube metal must endure a high temperature of
approximately 900-1 000°C. The radiant tube was manufactured by centrifugal casting with high Ni-Cr
alloy steel.
In this study, a failure analysis of a radiant heater tube was performed by careful visual inspection of the
tube cracks, metallographic observation of the near crack region and chemical analysis of tube metal and
oxide scales. It is argued that the principal cause of the cracking is progressive oxidation of the tube metal
beneath cracked thick oxide scales attached to the inside of the tube. The oxide scales are generated by
abnormally high operating temperatures which can be verified by the aged microstructure and internal void
formation. 0 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Cracks; Overheating; Oxidation; Process-plant failures.

1. Introduction

For heat treatment of moving steel plates in a hot rolling process of an iron foundry, the plates
are usually allowed to pass between two radiant heater tube rows that are located above and below
the pathway. The plates are heated by radiation from the radiant heater tubes with inside burners.
Since the burner is located inside the tube and hot flue gas pass through the tube, the radiant tubes
are continuously operated at high temperature which causes materials degradation due to thermal
aging and corrosion/erosion problems due to the combustion gas. As a result, thinning of the tube
and crack initiation occur frequently [I] and leaking of the gas through the crack may cause serious
surface finish problems to the rolled plate product as well as operating problems. Since the
operating temperature of the heater tubes is expected to be well above 900°C, they are generally
fabricated by centrifugal casting with HK or HP steels having high contents of Ni and Cr.
In this study, a failure analysis of locally fabricated radiant heater tubes was performed. A

* Corresponding author.

Reprinted from Engineering Failure Analysis 6 (2), IO 1 - 1 12 (1999)