Development of Low Cost Carbon Steel Tube for Sugar Mill Evaporators in South Africa

Due to the recent droughts, the retube of Sezela evaporators was delayed for a few years, resulting in a need for a low cost carbon steel tube to retube five evaporators. A recent failure highlighted the importance of the heat treatment and expansion characteristics of these tubes. This paper discusses the failure of a carbon steel tube, the development of the heat treatment of locally manufactured tubing, the expansion procedure for the tubes and other related problems with large Scale vessel retubing. Introduction Sezela intended cascading carbon steel tubes from the 6,8 m Kestner vessels to lower order evaporators when the Kestner was to be retubed with stainless steel tubes for the 1996-97 season. However, the Kestner retube was cancelled in November 1995 and an alternative supply of carbon steel tubes had to be found. Pongola mill had used an inexpensive heat treated carbon steel tube (SAE 10110) without problems but, due to the long delivery times (on average three months) the best that could be sourced was an SAE 10110 carbon steel tube, without heat treatment. This was considered satisfactory at the time. (At the time of writing, these heat-treated tubes had been in service for four years at Pongola mill without a failure.) SAE 10/10 tubing and expansion problems Problems were experienced when expanding the SAE 10110 tubes into the tube plates, and tubes were reportedly harder than other carbon steel tubes. After repeated re-expansion of certain tubes during the season by mill staff and contractors, it was decided to replace the tubes. Metallurgical examination of SAE IO/IO tubes Tube samples of SAE 10110 were sent to the co-author (Bartholemew) at Natal University's Mechanical Engineering Department for analysis to determine the cause of failure. A tensile test indicated that the tube was still in cold fmished condition. Microscopic examinations of the electric resistance welded joint and the parent material were done. The microstructure of the parent metal of all the specimens was found to consist of slightly deformed grains of ferrite, with small areas of pearlite at the grain boundaries. The ASTM grain size number was estimated to be nine or 10. The structure of the weld was similar although finer and containing some widmanstatten ferrite. The microstructures were typical for this type of tube in the cold finished condition. Vickers hardness tests, using a 10 kg load, were performed on the microspecimens, which yielded the following results: Parent metal : 150-152 HV Weld metal : 202-209 HV. Discussion The results of the tests and examinations performed indicated that the tubes were not in a condition suitable for expanding. The tubes were in the cold finished condition and therefore exhibited yield point values that were too high and too close to the ultimate tensile strength of the material, i.e. tensile to yield ratios of 1,l to 1,O. A more acceptable ratio would be, say, 1,7 to l ,O (refer to Figure 1). When the yield strength and the tensile strength are close to each other, the control required for tube expansion into the tube plate becomes unacceptably tight. This is because the tubes need to be expanded sufficiently to put the steel into the plastic range, but without encroaching on the ultimate tensile strength or, in other words, fracturing the tube end. The second point to be discussed here is the effect of the electric resistance weld. As can be seen from the results of the hardness tests, the weldment was 50 to 60 HV points higher than the parent metal. A much higher stress would be required to permanently expand this area on the periphery of the tube.