Page 399 - Failure Analysis Case Studies II

Failure Analysis Case Studies I1
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 383

HYDROGEN CRACKING OF FERRITIC STAINLESS
STEEL THERMAL STORAGE TANKS

SHINJI KONOSU* and TSWOSHI NAKANIWA
Department of Mechanical Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 3 16, Japan
(Receiced 28 January 1998)

Abstract-A ferritic stainless steel (SUS436L), which was subjected to various kinds of reduction ratio was
precharged with hydrogen at 40°C in 15% HCI solution by employing galvanic reaction with zinc. Tensile
tests were performed in air at room temperature on both uncharged and charge specimens. Finite element
method (FEM) analyses were carried out to obtain strain at panel comers under various different internal
radii in a thermal storage tank when it was subjected to internal pressure. As a result, it was found that the
value of internal comer radius/thickness of the panel (R/t) should be more than about 2 in order to prevent
hydrogen embrittlement cracking. Q 1998 Published by Elsevier Science Ltd. All rights reserved.
Keywords: Cleavage fracture, embrittlement, heat-exchanger failures, hydrogen-assisted cracking. tanks (fail-
ures).

I. INTRODUCTION

Ferritic stainless steel is frequently used in the manufacture of waterheating appliances due to its
excellent formability and its extremely high resistance to such shortcomings as pitting and stress
corrosion cracking associated with austenitic stainless steel. However, because ferritic stainless steel
possesses high hydrogen embrittlement susceptibility, investigations are being conductcd [I] on the
effect of hydrogen on the mechanical properties of the material. Meanwhile, numerous accidents
thought to be due to hydrogen embrittlement are occurring at the corner portions of cold-bent
ferritic stainless steel (SUS436L) panels used in thermal storage tanks. It is believed that fracture
elongation in hydrogen-charged ferritic stainless steel is largely due to the influence of cold working
and, further, that the strain occurring on the inside of bent portions during water proof tests is
attributable to the influence of the inner radius of the bent portion.
Hence, using ferritic stainless steel in the current series of investigations, the influence of cold
working on fracture elongation in hydrogen-charged specimens was determined experimentally and
the limits of hydrogen embrittlement cracking on the inside of cold-bent portions were studied and
clarified by analyses employing the finite element method.

2. FAILURE OF THERMAL STORAGE TANK

Figure 1 shows a portion of a thermal storage tank assembly measuring 4 m in height, 3 m in
width and 10 m in length. It consists of 4 banks of tanks stacked vertically, with three rows arranged
in the longitudinal direction. The tank panel is made from ferritic stainless steel (SUS436L), with
the corner portions being bent by cold forming, as shown in Fig. 2. Hot-dip zinc-coated steel tubes
are laid inside the tank. Coolant is passed through these tubes to freeze the water in the tank during
the night and the heat of melting is utilized during the day by means of air conditioners.
After water proof tests were conducted at the respective pressures concerned (Case 1: hydraulic
pressure 3.52 x IO-’ MPa, Case 2: 6.36 x IO-’ MPa), cracks were found in the corner portions (inner
radius R = I .62 mm). The appearance of the cracked portion in Case 2 is shown in Fig. 3. It can be
seen that the crack has propagated from the inside of the panel toward the outside.

*Author to whom correspondence should bc addressed.
Reprinted from Engineering Failure Analysis 5 (4), 323-33 1 (1998)