Failures of rolls occur due to improper manufacturing and operational parameters. The samples of prematurely failed roll samples collected from steel plant were examined for their chemistry, inclusion content, microstructures, carbide characteristics, hardness and the retained auste¬nite content. The residual stresses were also measured on the inner as well as the outer surface of the spalled roll pieces. The results obtained have been discussed in this paper. The higher content of retained austenite was primarily responsible for the spalling of indigenous rolls for which sub zero treatment has been recommended. Several suggestions have also been made for smooth operation of the mill and consequently for the ex¬tension of life of a work roll

Chaudhary, P N

Mishra, K K

Ray, Ajoy K

English

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PROCEEDINGS : COFA-1997©NML JAMSHEDPUR; pp. 37-46Failure analysis of rolls of coldrolling mill in steel plantA. K. RAY, K. K. MISHRA & P. N. CHAUDHARYNational Metallurgical Laboratory, Jamshedpur - 831 007.ABSTRACTFailures of rolls occur due to improper manufacturing and operationalparameters . The samples of prematurely failed roll samples collectedfrom steel plant were examined for their chemistry, inclusion content,microstructures, carbide characteristics , hardness and the retained auste-nite content. The residual stresses were also measured on the inner as wellas the outer surface of the spalled roll pieces . The results obtained havebeen discussed in this paper. The higher content of retained austenite wasprimarily responsible for the spalling of indigenous rolls for which subzero treatment has been recommended . Several suggestions have alsobeen made for smooth operation of the mill and consequently for the ex-tension of life of a work roll.INTRODUCTIONThe work rolls used in the cold rolling mills are expected to perform undervery arduous operating conditions. With the advent of faster mills and the in-crease in load, the quality of roll needs continuous improvement. Cold mill rollsshould withstand mechanical and thermal shock, must have excellent wear resis-tance and very little plastic deformation.The premature failure of a cold mill roll increases not only the cost of a rollbut also down time of mill affecting the productivity of mill. The reasons of thepremature failure can be attributed to the combined effects of mechanical as wellas metallurgical factors. While the mechanical factors have concentrated on themisalignment, uneven roll surface and bearing seizure, metallurgists have con-sidered the failure as arising out of localised overloading, temperature gradient,the presence of non-metallic inclusions and phase transformation [11.The objective of the present work was to analyze the reasons of failure frommanufacturing as well as operational point of view and to provide suitablerecommendations for extending the life of a cold rolling tandem mill work roll37A. K. RAY, K. K. MISHRA and P N. CHA UDHAR Yof an Indian Steel Plant..BACKGROUNDIt was observed that the three contributing factors responsible for poor life ofa work roll were following :a) Spallingb) Crackingc) Metal pick up & wrappingFailures of type `a' and `b' are partly manufacturing and partly operationalwhereas `C' is mostly operational. Amongst these failures, spalling in consideredto be the major cause of roll failure in cold rolling mill. A work roll in service issubjected to set of stresses. First of all these are the internal stresses resultingfrom the heat treatment. On top of these internal stresses are those caused byrolling operation. Spalling is mainly classified into two types [31.a) Spatting due to fatigue fracture of which origin starts from roll barrel sur-face or just below the surfaceb) Spalling due to fatigue fracture of which origin starts inside the roll body.Failure of type (a) is related to problems of operational origin whereas type(b) is mostly of manufacturing origin. Factors [41 responsible for the spatting aregiven below.Operational factors :1. Overloading2. Insufficient stock removal3. Thermal stresses due to insufficient roll cooling4. Severe bruising of rolls5. Mill chatter, improper crowning and mechanical alignment problems6. Sharp temperature gradient on the surface of the rolls (e.g.) near stripedge.7. Pickling quality of hot rolled coil8. Wrong grinding Operation9. Hydrogen pickup from soluble oil used as a roll coolantManufacturer's factors :1. Non metallic Inclusion2. Casting and pouring defects such as coarse grained structure or clusteredcarbides.38A. K. RAY, K. K. MISHRA and P. N. CHAUDHARY3. Heat treatment faults such as poor carbide morphology, coarse martensite,retained austenite and residual stress.Cracks develop in regions between top hardened surface and subsurface, afew microns below during final grinding stage [5.61. Due to thermal gradient crackpropagates and failure occurs. Crack is also initiated due to compressive stressesexperienced during contacts between roll and strip while rolling. They grow un-der fatigue condition and failure in not usually premature [']. It is also believedthat the inclusions are centres from where the cracks start building up and propa-gate resulting in spall [3].Failure also occurs due to metal pick up and wrap-up of coil with roll. Due tobreakage of strip, the material gets wrapped up and the temperature increasesmuch above the specified temperature leading to failure.METHODOLOGYThe detail specification of roll investigated in the present work has beengiven in Table 1. These rolls are induction hardened forged steel quality and areused in stand 2 to 5 of five stand 4 Hi tandem mill. The data on the lives obtainedfrom the roll manufactured by various suppliers,indigenous as well as importedalong with the cause of premature failures were collected. These data are shownin table 2Samples of prematurely failed rolls from indigenous as well as importedmake were collected for examination at NML. The methodology adoptedfocusses on metallographic features. The etching reagent (4% picral + CH OH)was used to see the retained austenite. Inclusion analysis was done to compareindigenous rolls with imported rolls. Residual stress was measured using ax-ray stress analyser AST 2001. Besides this, mill operation was carefully ob-served in order to suggest the ways and means to extend the life of a work roll.RESULTSFig. 1 shows the damaged portion of an imported as well as indigenous workroll. Typical fatigue lines as well as origin of the crack close to the surface arerevealed. These were examined for the following.1. MicrostructureThe microstructure (Fig. 2a) of the indigenous roll shows the presence ofmartensite needles, which was perhaps not converted fully to tempered marten-site during heat treatment whereas the microstructure (Fig. 2b) of imported rollsreveal tempered martensite.39A. K. RAY, K. K. MISHRA and P N. CHAUDHARYTable I : Detailed specification of investigated rollRoll SizeScrap SizeSurface HardnessDepth of Hardened layerNo. of standsTypes of standsAverage no of dressingson a roll before scrappedBearingsDriveCooling waterLubricant with cooling water :Maxm. SpeedPickling PracticeRoll Material585 x 1420 mm B. L.555 x 1420 mm B.L.95-97 Sh D (Amr)15-20 mm5All 4 H45-46Morogoil Bearings on Back up rollsTaper Bearings on the work rolls.All rolls are driven by 7500 HP DC MotorsQuantity & Pressure 18001/mm, 6-12 kg/cm2with a pH of 7-8Palm Oil System A - 8 to 12% System B - 15 to20%1830 m /minSulphuric acid with inhibitors at a line of80-120 m/min.Forged Steel (Induction hardened) A typicalcomposition is as follows : C=0.85, Mn=0.35,Si =0.50, Cr = 2-3.5, M = 0.30Table 2 : Performance data fro the year 1993INDIGENOUS ROLL IMPORTED ROLLDate of Life Reason of Date of Life Reson ofScrapping obtained scrapping scrapping obtained scrapping15.2.93 20,116 Small spall 2.2.93 51.088 Depression21.2.93 4,536 spalled atmiddle9.2.93 56,167 -do-4.3.93 3,822 -do- 16.2.93 59,415 Metal pick up28:6.93 9,419 -do- 9.3.93 39,457 Dep.6.7.93 25,484 Small spall 29.3.93 52,226 Pickup &spall12.7.93 5,593 Spalled atmiddle5.4.93 40,359 Low hardness23.7.93 2,684 Spalled at 22.4.93 13,031 Pickup &middle depression19.9.93 3,162 Spalled at 31.7.93 32,371 Low hardness21.9.93 9,990 -do- 23.8.93 9,118 Small spall22.9.93 2,367 Spalled at 31.8.93 8,438 Spalled atdrive end one end9.10.93 6,374 Spalled atat one end23.9.93 24,218 -do-24.10.93 10,557 Small spall 10.11.93 42,465 Pickup4.11.93 22,816 -do- 10.12.93 23,770 Dep.5.11.93 14,601 Spalled atone end21.12.93 49,737 -do-40A. K. RAY, K. K. MISHRA and P. N. CHAUDHARYThermally spatted indigenous roll Fig. 1 Thermally spalled imported rollx 1000 Fig. 2 x1000Presence of macteneritic needles and Presence of tempered martensite gad lowerhigher percentage of retained austenite % retained austenite in imported rollin indigenous rollH. Retained AusteniteRetained austenite was attempted on X-Ray diffractometer and the patternsobtained (Fig. 3) were carefully examined. The values obtained on the outer sur-face of different samples of indigenous rolls varied from 6-15% by volumewhereas for the imported rolls it was 10.94%.% retained austenite was also measured on the inner surface by XRD tech-nique using a computer software, at IGCAR Kalpakkam in RIGAKUdiffractometer. It varied between 13-15% for indigenous rolls whereas it wasabout 10.5% for imported rolls.Retained austenite being a metastable phase, appears to be the main reason41A. K. RAY, K. K. MISHRA and P N. CHAUDHARYS•.tewry..ly'dfx^•041M1+.^J'tn7'1-&-E)Fig. 3 : XRD pattern of an Indigenous roll (outer surface)for spalling of indigenous rolls. During rolling retained austenite may be con-verted to martensite resulting in expansion of volume which create external pres-sure and cause spalling.III. CarbidesCarbides were not found to be evenly distributed and globular in shape inindeigenous rolls compared to imported rolls (Fig. 4). This may be due to inad-equate soaking during the heat treatment operation, where all the carbides maynot dissolve in austenite and the precipitation during quenching may not result inproper distribution and shape and size.IV. Non-Metallic InclusionsThe steel from imported roll was found to be cleaner than that of indigenousroll. The inclusion content in the imported roll varied in the range of 0.02 to0.15% by volume. Whereas, in the case of indigenous rolls it was 0.16 to 0.24%.It is believed that the inclusions are the centres from where the cracks start build-ing up, Fig. 5 shows the presence of inclusion on the spalled surface of the rolls& multiple surface cracks initiating & emanating from an inclusion on the sur-face of imported roll (Fig. 5).42A. K. RAY, K. K. MISHRA and P. N. CHA UDHARY(a)(a)(b)Fig. 4 x 800Carbides evenly disttributed in importedroll carbides are glolular in shape(h)x800Indigeneous rollSlag inclusion indigenouson surface of rollV. Residual StressFig. 5 Multiple cracks from aninclusion in imported rollResidual stresses were measured on the spalled piece along two directions,(a) radial & (b) transverse direction. Fig. 6 shows that residual stresses weretensile on the spalled (inner) surface for the indigenous roll. Whereas for theimported roll, compressive residual stresses were observed on both the surfaces(inner & outer) which are not considered determinantal. However, in-situ mea-surement of residual stresses during rolling was most important because stressesmight have got relieved after spalling.OBSERVATIONS & DISCUSSION1. The major cause of failure of indigenous roll was due to spalling. The contentof retained austenite is more in indigenous rolls compared to imported rolls.43A. K. RAY, K. K. MISHRA and P N. CHAUDHARYOn spalled surface all units in Ksi-40.3e.0On top (upper surface)Fig. 6a : Residual stress measurement on failedsample (due to spalling) for indigenous roll(All units in Ks_)On spalled surface all units in Ksi On top (upper surface)Fig. 6b : Residual stress measurement on failedsample (due to spalling) for an imported roll(All units in Ks.)During rolling retained austenite might be converted to martensite resultingin volume expansion. Thus creating internal pressure to cause spalling. Rec-ommendation was made to go for sub-zero annealing. If it is installed almanufacturer's premises, there is further liberty for trying better composi-tions and higher alloying with chromium and other elements resulting in bet-ter performance.2. Spalling is also originated from subsurface fatigue crack. The excessivestress concentration takes place on the roll surface at the portion where sharrstrip edge contacts the roll. This stress concentration may cause generation offatigue crack and result in spalling of roll. Therefore, it is advisable to mea-44A. K. RAY, K. K. MISHRA and P. N. CHA UDHARYsure the residual stress at different stages of operation.3. Because of the severe hardening process during manufacturing, high residualstress is retained in the roll which may affect the performance of a roll. Theresidual stress of freshly manufactured roll should have been measured inorder to suggest the modalities for extending the life of CRM roll.4. Another cause of roll failure is due to mill abuse, number of mill wrecks oraccidents occuring during operation. These accidents are caused by stripbreakages and the stock material gets wrapped up around the roll.There may also be local metal pick up by the rolls which gets welded with thesheet where the temperature increases much above the specified temperature.Roll fails due to thermal stress. This can be avoided by maintaining properpressure of the cooling water and its quantity.Therefore, smooth mill operation is necessary for the extension of the life ofCRM rolls.5. The performance of the roll also depends on quality of rolling stock and itssurface condition. There may be variation in hardness, gauge thickness,surface condition. If strip is under pickled, oxide layer will be present whichbecomes the source of roll failure.6. The surface of the roll sometimes get bruised and develops cracks and elimi-nation of these cracks result in scrapping of the roll because diameter goesdown.7. Route of the roll was studied carefully from the roll card. The expected ton-nage on stand no. 2 is 500 tons per campaign and 2000 tons per campaign onstand No. 5. If the proper inventory is maintained and the inspection of rollsurface is made frequently, it would be possible to increase the life of roll toan optimum tonnage of 40,000-50,000 tons.CONCLUSIONS1. It was found that the retained austenite content is more in indigenous rollscompared to imported rolls. This was responsible for the failure of rolls dueto spalling. Austenite being the metastable phase in the steel deserved to beconverted to stable phases by subzero annealing. It was recommended to in-stal this facility at manufacturer's.2. The quality of input (hot rolled strip) is not uniform with regards to its metal-lurgical characteristics. This needs attention for improving the performanceof a roll.3. Recommendations have also been made for smooth operation of mill. Theserecommendations have already increased the awareness amongst the rolling45A. K. RAY, K. K. MISHRA and P N. CHAUDHARYcrew during the tenure of the project and there is substantial reduction infailure of rolls and down time in the mill.ACKNOWLEDGEMENTThe authors are greatful to Prof. P. Ramachandra Rao, Director, NML, for hispermission to publish this paper. The authors are also thankful to Mr. RaviKumar and Dr. G. Das for their assistance in residual stress measurements and toDr. (Mrs.) Aruna Bahadur for her help in XRD analysis.REFERENCES[1] A. N. Sinha and C. S. S. Sivaramakrishnan : Ironmaking and steelmaking , 1994 vol.21 No. 2.[2] A. K. Ray et. al : Internal Report of NKL/CIEP/SSP/95, 1995.[3] B. Mohanty and K. Bishoji "Premature failure of cold mill work rolls with specialreference to indigenous rolls". Paper presented during Forged Rolls User meelRanchi - 6-7th November, 1993.14] K. Chopra : Quality assurance in Forged Steel Rolls Paper presented during ForgedRolls User meet Ranchi, 6-7th November 1993.[5] J. M. Chilton and M. J. Robert, : Iron Steel Engg., January 1981, 58, 77.[6] M. Nakagawa, Ahoshi, A. A. Sano and Y. Nausher : Iron Steel Eng., March 1981, 58.44.[7] H. Swahn elal : Scand. J. Metall. 1975; 4, 263.46

Failure analysis of rolls of cold rolling mill in steel plant

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Failure analysis of rolls of cold rolling mill in steel plant

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