Page 380 - Failure Analysis Case Studies II

Faiilrre Analysis Case Studies 11
D.R.H. Jones (Editor)
0 200 1 Elsevier Science Ltd. All rights reserved 365

PIk S1350-6307(98)000094

FAILURE ANALYSIS OF CARRIER CHAIN PINS

G. A. SLABBERT,* J. J. McEWAN and R. PATON
Physical Metallurgy Division, Mintek, Private Bag X3015, Randburg 2194, Republic of South Africa
(Received 3 February 1998)

Abstract-A sugar plant is designed to operate with the minimum disruption during the sugar cane harvesting
season, and equipment must be maintained in a high standard of repair. When failure of equipment does take
place, it is important to identify the cause to minimize the likelihood of any future problems. This paper details
the analysis of a failure of conveyor chain pins that had operated for only six weeks. The pins had been heat
treated so that they had become susceptible to hydrogen embrittlement, and had cracked. The source of
hydrogen was attributed to corrosion of other steel components in the system. In order to minimize the
likelihood of future failures, it was recommended that the source of the corroded components be identified. In
addition, it was recommended that the authors should liaise with the plant personnel in order to recommend
a suitable heat treatment schedule for future pins. 0 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Sugar plant, hydrogen ernbrittlement, stress corrosion cracking, corrosion.

1. INTRODUCTION
A sugar mill operates continuously during the sugar cane cropping season which, in South Africa,
runs for more than six months. It is important that the plant down-time is maintained at an absolute
minimum during this period. Unforeseen failure of critical components does occur, however, and
repair and maintenance is done rapidly in order to get the plant back into full production. In many
instances, it is imperative that the cause of failure be ascertained to avoid future problems and
minimize costly breakdowns. This paper details the failure of carrier chain pins from a conveyor
used to transport moist sugar cane pulp.

1.1. Background
The sugar mill was commissioned in 1994 and the original chain conveyor operated for three
years before it had to be replaced due to excessive wear. The new carrier chain had only been
operating for about six weeks when the chain pins started failing.
Sugar cane pulp is conveyed at a temperature of about 80°C. This temperature is maintained by
means of steam, which has a temperature of approximately 90°C. The material specification for the
chain was either AISI 431 or BS 970 grade 431329 ('En 57') stainless steel. The hardness was
specified as WRC 43 to 45 (Rockwell 'C') although the material specification did not give any details
on the heat treatment required to obtain this range. AISI 431 and BS 970 grade 431829 are related
specifications, but there are differences, as shown in Tables 1 and 2.

2. INVESTIGATION
2.1. Visual examination
Twelve pins were examined, three of which had fractured. All the pins examined had been in
operation in the conveyor for about six weeks. The three failed pins had fractured at an angle of
approximately 45" (Fig. 1). Although pin C failed close to the bottom of the pin, plant personnel
reported that the majority of fractures were in the middle portion of the pin.

*Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (2), 121-128 (1998)