NEED FOR QUALITY CHECKS ON RAW MATERIALS BEYOND TEST CERTIFICATE - S.RAMANUJAM - K.S.WAGH

Transcription

1 INTRODUCTION:- NEED FOR QUALITY CHECKS ON RAW MATERIALS BEYOND TEST CERTIFICATE - S.RAMANUJAM - K.S.WAGH Industrial structures are normally made either of Reinforced Cement Concrete or of Structural Steel Sections depending upon suitability, location and functional requirements. Quality checks on Reinforced Cement Concrete is possible at various stages of construction, whereas quality of the Structural Steel Sections procured, have always been verified with the test certificates that accompany them. Major producers of steel sec-tions being M/s. SAIL, TISCO, etc. test certificates furnished by them are taken for granted and additional cross checks are not frequently done. The authors present a case study highlight-ing the importance of such quality checks of steel materials procured even from renowned producers. Ammonia based Heavy Water Plants consist of Main Plant structure, compressor house, cracker units, catalyst preparation unit, pipe racks, etc. which are structures fabricated out of rolled steel sections. The total quantity of these structures in a typical two stream plant similar to the ones at Thai and Hazira, is about 7,000 Tonne. The main plant structure which consumes a major portion of about 5,000 T of steel sections, is 135 m X 16 m in plan, divided into 15 bays, the height being 60 m. in view of the 100 T - EOT crane situated at 53 m level and large seismic load from the Exchange tower, the structure is quite heavy. During fabrication and erection of the main plant structure at Hazira Plant following incidents highlighted the importance of checks in the quality of steel materials :- SEQUENCE OF INCIDENTS 1) During the progress of work on it was reported that a beam of ISMB 450 during fabrication burst into 3 pieces as shown in Figs.l & 2, without any apparent reason. Only a small notch at one end was cut by gas torch prior to that. 2) Subsequently a box beam consisting of 2 beams ISMB 400 placed side by side with webs parallel, with 2 flange plates developed a crack at one end. (Fig.3) 3) In a beam of ISMB-600 a longitudinal crack of about 2 M length at the centre of a flange was noticed. 4) A column of ISMB-400 of im height erected on the structure at 18.8M level developed a crack after welding of the support to connect a bracing.

2 INVESTIGATION WORK 6,5 By the time the first incident was observed about 2,000 T of steel sections had already been fabricated and erected and an equal quantity was under various stages of fabrication and so it was a very disturbing incident. Various possibilities were thought of regarding the reasons for the development of cracks. 1):Development of internal stresses due to fault rollingn.;arld- cooling. : "!: -;,-, :-:r ft.,. -.. :" ; ''JfJ 2) H ifaulty handling of materials by fabricators. - n;s!l( 3) >Eaulty chemical composition. c To firid out the exact reasons, the samples of Cracked w' ISMB-*450 and ISMB-400 were subjected to chemical analysis;!,^ The^eracked structure of ISMB-450 was examined by Physical' Metallurgy Division of BARC. The crack surface of ISMB 450 which' Was brittle in nature was subjected to micrographic analysis. The Chemical analysis revealed that the carbon contents is <asmb-45-0 and 400 were, 0.49% and 0.52% respectively as against a maximum permissible percentage of 0.25% (including 0.02% tolerance) as per IS-226. In addition, presence of, ' manganese, an alloying metal to the extent of 1.25% and :v> 1.35% respectively was detected even though this is ri6t! "'' specif ied. in IS-226. For additional information more ean-,'- pies'were taken from members which had not developed cracks and tested. The Chemical analysis results in such cases conformed to IS-226. This indicated that abnormalities werv;' note'present, in all structural steel sections. iiml >" ' " ''' " ' '.'' i K Preliminary discussions with the main producers, froiir'/ whom the particular sections were procured, revealed tnat'in'"' their same Rolling mill rails and heavy beams are rolled. ' While the blooms sections conform to IS:226 that for rails are of high carbon steel. The high carbon steels are brittle and are not of weldable quality for the normal procedure'" of welding of structures. It was opined that the blooms meant for rails must have got inadvertently mixed up with J the ones meant for structural steel sections. ' ' - ; i j :i-.- ' -.- ri:. :trt OEder to ensure that steel od required specifica- ; tions have gone into the work, drilling samples were cbllected from the members already erected and tested for : chemical.composition. The results are tabulated below:- : " : M?. U.

3 S.No. Carbon content No. of samples Remarks 1. Upto 0.1% 2. Between 0.10* & 0.25% 3. Eatween 0.25% & 0.30% 4. Batween 0.30% & 0.40% Within IS 226 Within ASTM. limits of -dolimits Higher than permissible -do- of 5. Above 0.40% 6 -do- -do- Total 1112 Out of the 1112 samples, 1087 were found to be within limits of IS-226 and hence acceptable. 16 samples against SI. No.3 eventhough indicated carbon content slightly more than that specified in IS-226, were accepted based on specifications of ASTM SA 36 for structural steel which permitted carbon upto 0.3%. 3 samples against SI. No. 4 were further tested for tensile, bending and weldability characteristics and were found unsatisfactory and hence rejected. The 6 samples against SI No.5 which had shown carbon content of. more than 0.4% were rejected out-right. Thus in all 9 samples out of 1112 (0.81%) had to be rejected and as such refabrication and erection had to be done. Regarding incident No.3 wherein a crack was observed at the centre of the flange of ISMB-600, as a first step chemical analysis was conducted results of which was found normal. The sample was sent to BARC for micrographic examination. It was concluded that there was some localised segregation and inclusions near the crack and the hardness near the crack was very high as compared to other places. The suppliers admitted that the particular piece which should have been rejected at factory level itself was delivered by mistake. In order to ensure that such defects did not exist in the columns whose flanges were already covered with flange plates, a procedure was finalised in consultation with AFD BARC to detect the existence, if any, of similar cracks through ultra sonic testing. The procedure was adopted to test the entire structure and trouble spots were identified. Defects were observed at 4 places out of about 4000 spots tested. As regards incid' it No. 4 it was concluded after chemical analysis that cracking was due to high carbon content.

4 REMEDIAL MEASURES :- 1) To over come problem of high carbon content the only option was to remove the members having carbon content more than permissible and return the same to manufacturer for replacement. As an additional safeguard after the completion of structure, EOT Crane with 100T load was made to traverse and retain the load over each column for some specified amount of time and the structure was observed for satisfactory performance. 2) The matter of existence of cracks in ISMB-600 at 4 spots was referred to M/s.PDIL the consultants and reinforcing the structure at such places was done as per their recommendations. CONCLUSION:- Industrial structures involve use of large quantity of structural steel sections, and hence there exists a need for more stringent quality control procedure even for the sections procured through renowned producers. Following measures merit consideration :- '.D Planning of procurement well in advance. (2) Review of production procedures at the factory. (3) Further testing of structural steel sections for chemical composition and mechanical properties on their receipt at site. (4) To avoid mix up of ateel manufactured from different lots manufacturers must be insisted to have identification of batch, heat nos. embossed on every piece.

5 ISMB450 OVERALL VIEW OF CRACKS FIG. 1

6 ISMB kso & CLOSER VIEW 2 FIG.2

7 ISMB 400 CLOSER VIEW 1 FIG. 3

8 ISMB 450 CHEMICAL COMPOSITION C-0.49% S-0.027% P-0.037% Mn-1.25 % CLOSER VIEW 3 FIG.4