PROBLEMS OF MACHINING STEEL P91 SVOČ FST 2010

Transcription

1 PROBLEMS OF MACHINING STEEL P91 SVOČ FST 2010 Ing. Jaroslava Fulemová, West Bohemia University, Univerzitni 8, Pilsen, Czech Republic Ing. Zdeněk Janda, West Bohemia university, Univerzitni 8, Pilsen, Czech Republic ABSTRACT This article is solved in terms of project, which is called: Increasing of cutting productivity at slabbing of steel P91. The project is solved in conjunction with firm Škoda Power. Material P91 (ferritic/martensit steel) is used, for example, for production of a metal-clad body of turbines, whose application is given at temperatures C, especially for energy industry. Among basic benefits of using this materials belong to: increased corrosive resistivity in environment of hydrogen, vapour and fouling, high heat conductivity and low thermal expansivity. Above-mentioned properties differ from good machinability these steels. The goal of this project will be finding fit combination of cutting material and cutting conditions, for improving of current state. KEYWORDS P91, ferritic/martensitic steel, machining, tool life INTRODUCTION To increase the thermal efficiency and to reduce the environmental pollution from power generating plants (fossil, thermal and nuclear systems), it is necessary to use higher steam temperature and pressure (600ºC/30MPa). At present it is used temperature and pressure 568 ºC/ 16,9MPa. In using of higher temperature and pressure is demand on materials with improved properties at high temperatures. It is desirable to use high Cr (9 12% Cr) ferritic steels because of their better resistance to stress-corrosion cracking, higher thermal conductivity and lower thermal expansion coefficient as compared with austenitic stainless steels. Among steels with high content of chromium belong to modified 9Cr 1Mo. This steel is marked as Grade 91 (T91 for tube and P91 for plate) and it is with minor additions of niobium, vanadium and nitrogen. The high creep strength of P91 enables higher design stresses than that is permissible with 2.25Cr 1Mo (P22) or X20 CrMoV 12 1 (12Cr 1Mo steel) for the entire temperature range and higher stresses up to 625ºC. Next properties of the steel P91 are: good oxidation resistance and resistance to hot hydrogen attack, adequate fracture toughness and affordable price as well [1]. Interest in tempered ferritic/martensitic steels for nuclear applications (generation IV, fusion reactors) caused considerable improvement in last years. Competition in the electricity industry has also meant more frequent operation of power plant in cyclic mode and the need to reduce damage to components due to ensuing thermal fatigue. Here high strength steels can offer an additional benefit in that the reduced section thickness increases pipework flexibility and reduces the level of through wall temperature gradient in thick section components. Use of such steel commonly reduces wall thickness by 54% and weight by 65% compared to conventional steels, e.g. 2.25Cr 1Mo steel, this in turn increases the thermal fatigue life by a factor of But all these superior properties of grade 91 steel depend on the creation of a precise microstructure by initial heat treatment and stabilizing the same throughout its service life. Due to improper heat treatment or during the operations such as the hot bending, forging, and welding will seriously degrade its high temperature properties lead to failure of the material [2] and [3]. MACHINING OF STEEL P91 IN COMPANY ŠKODA POWER Currently there is machined material, which is called P91, in Škoda Power. This material is used for metal-clad bodies of turbines. This metal-clad body is compound from two parts. These parts are each other connected with bolting. Our task is optimalization process of dividing plane machining. This surface is plane. A top part of body is in the Fig.1. The top part and the bottom part are different in design of a dividing plane. There are reliefs in the top part of the body. These reliefs are created in form of supporting and sealing surfaces. The surfaces are in Fig.1 and they are highlighted with red colour. The dividing plane at the bottom part of the body has full profile; this means more material to cutting. Dimensions one's part are 5485 x 2820 x 1720 mm, weight is 33t and it is cast. The metal-clad body is machined with a milling cutter. The milling cutter has diameter 315mm and has 30 inserts and 4 positions for finishing inserts. The tool is in the Fig.2. The tool is fixed in machine by the help of method "fixing on the face"; this means it provides high stiffness of fixing. A machining allowance is 5mm. Half-finished product is rough, semi-finished and finished. A depth

2 of cut for semi-finishing is 0.4mm and for finishing cut is 0.02mm. The workpiece is sent to a verification of surface and subsurface cracks after finishing cut. If there are cracks on the workpiece, they are welded and the workpiece is again machined. Roughness of final surface after finishing cut is Ra = 0.8μm (this value is written on the production drawing). This roughness must be kept. It is for reasons, that if it is achieved well final roughness of machined surface, so top part and bottom part of the body may glide each other and will not be established vapour barrier of wiring. Fig.1: The top part of the body Fig.2: A cutter head Final quality (roughness) of the machined surface is achieved by present way of machining approximately Ra = 0,2μm. That is why machined surface must be roughened. Time costingness of machining of this part is as follows: fix of body on the work-table of the portal milling machine = approximately 12 hours roughing = approximately 6 hours semi-finishing and finishing cut = approximately 6 hours Some basic deficiencies of machining P91 steel was realized during observation of real machining of this workpiece: Edges of workpiece were not cut off - it was brightly seen that before semi-finishing and finishing cut there were flashes after previous machining. These flashes can negatively influences tool life of inserts and final integrity of machined surface. The tool was not specified - at present, 4 cutter heads are available for machining of dividing plane. Differences among tools are in: diameter, number of inserts and cutting geometry. It is necessary to brightly define which tool should be used. NC programs are not compact - operator can finish this program on machine tool during machining. In result happens to situation, when operator votes the tool and cutting conditions for machining. Operator does not check state of insert - exchange of inserts is done after very expressive plastic deformation of cutting edge. It is not keep roughness of final machined surface how was already noted above. In drawing is specified value Ra = 0,8μm but resulting roughness is Ra = 0,2μm.

3 APPROACH TO THE EXPERIMENTAL SOLUTION Cubes of material P91 in size 300x400x10 were delivered to Department of Machining Technology. Experiment was done on them under laboratory conditions. Aim of these experiments is to find suitable combinations of cutting conditions. These conditions will be used for optimisation of current way of machining in company Škoda Power. Other attendant phenomenon of cutting process will be also monitored. It is necessary to cooperate with Department of Material Science and Technology because of special machined material for power industry. Influence of cutting conditions on resulting quality of machined surface will be monitored in cooperation with this department. Measured characteristics by KTO Measured characteristics by KMM cutting forces chemical constitution cutting temperature metallographical analysis vibration at cutting microhardness tool life grain size surface roughness surface topography surface hardness surface and subsurface defects Experiment is divided into 4 parts: Part 1: includes selection of acceptable kinds of cutting material for semi-finishing cut. 7 types of sintered carbides are available. Only 2 types will be chosen for further investigation. Part 2: acceptable cutting conditions for selected carbides will be selected (in semi-finishing cut). Part 3: includes selection of acceptable kinds of cutting material for finishing cut. Some types of sintered carbides are available. Part 4: acceptable cutting conditions for selected carbides will be selecting (in finishing cut). EXPERIMENTAL MACHINING - PART 1 - COMPARING IN LIGHT OF TOOL LIFE Experimental machining was done on three-axis vertical milling center. Workpiece (steel P91) X10CrMoVNb91 was machined with a cutter head with tangential inserts made from sintered carbide. Diameter of cutter is 80 mm. Only one insert was used at first. Reason of this solution is to achieve extreme cutting conditions and load on one cutting edge. Afterwards 4 insert was used because of fluency of cut (minimal number of cutting edges in cut is equal 2). Tool life, surface roughness, surface hardness and topography of surface were monitored during experiment. In Fig.3 is workspace of the machine tool. Cutting and technical conditions are mentioned in Table 1. Diagram of milling is visible in Fig.4. Cutting conditions were similar with conditions used in company Škoda Power. Milling type: Axial depth of cut a p [mm]: Cutting speed v c [m/min]: Climb milling, outer and inner cooling 0,8 v c = 180 Feed rate f z [mm/zub]: f z = 0,15 Machine tool: Tool: MCV 750 A Producer Kovosvit a.s. Sezimovo Ústí Cutter head Ingersoll, ø80mm, possibility to fix 8 inserts Spindle speed: [ot/min] Radial width of cut a e [mm]: Criterial value of tool wear: Workpiece: n = a e = 50 -VB b =0,3mm (uniform tool wear) -VB n =0,5mm (notch wear) Measurement system: X10CrMoVNb91 ( HB) marked P91 Tab.1: Cutting and technical conditions Scleroscope WHU 330 (direct display scales HRB, HRC, HV, HB, HS, HL) Digital microscopic camera DIGITUS (fluent zoom 10x 200x) Device for measuring roughness DIAVITE DH-5 (measured values Ra, Rz, Max, R3z, Rt, Rq) Microscope MULTICHECK PC 500 (zoom 10x, 30x, 75x a 150x)

4 Fig.3: Workspace of the machine tool Fig.4: Scheme of milling First pre-experiment The cutter head was stepped with only a 1 insert namely from reason testing extreme cutting conditions. These cutting conditions worked on cutting edge. In terms of this first part was watched only tool life of cutting edge. Results of tested sintered carbides are following. Sintered carbides IN6510, IN6515 and IN2040 achieved the lowest values of tool life namely also at repeated tests. Sintered carbides IN2015 and IN2030 achieved similar results and their position in light of usability for future testing is middlemost. Among the best sintered carbides belongs to IN2005 and IN2004.There are values of achieved tool life in Tab.2. These values are divided in two columns, where first column means value of the lowest tool life during cutting and in the second column are mentioned values of on the average reached tool wear. These values are outspread to criterial values of tool wear on the flank of the cutting edge. The criterial value of tool wear was given on 0.5mm (VB n = notch wear on the flank of the cutting edge). Tool life Sintered Carbide The lowest achieved tool life [min] Average value of tool life [min] IN6510-1,5 IN6515-3,7 IN IN ,5 IN2030 7,5 9,2 IN2005 7,8 15 IN ,6 20 Tab.2: Values of achieved tool life for each kind of sintered carbide There is tool life depending on time of machining for representative sample of sintered carbide type of IN2005 in the Fig.5. Green curve describes value of the lowest achieved tool life; this means curve passes over value of 0.5mm at time around 7.8min. In the same way was analysed average value of tool life with it, that red curve was not included to the result. Insert 1VBD-2Hr. reached the highest value of tool life of all measuring. Unfortunately it was not achieve reproducibility of hereof result, so the value of the red curve was not included to the value of average tool life. Second pre-experiment The cutter head was stepped with 4 inserts, cutting conditions stayed identical like hereinbefore case (except value of f min, that it was 4 times higher, so equal with 432mm/mim). The reason of repetition of the test with more inserts is achieved fluency of tool cut. This last pre-experiment should confirm results, which were achieved by previous testing and excluded 4 or 5 types of sintered carbides, which will not be tested in the future. There is a graph in the Fig.6. There is visible dependence of tool wear insert during cutting for individual type of sintered carbides. Dependence is created for insert No.4, because this insert was worn mostly during cutting process. There is time on the x- axis. All 4 inserts were cutting during this time (this means, that the values, which you can read from the graph, you must divide 4, so that received time, that is comparable with records featured at first pre-experiment).resulting values are outspread to insert No.4 (insert, which was placed in seating on four position, this means, to insert which finished tests on the ground of achieving of criterial values of tool wear).

5 4000 Sintered Carbide IN2005 VBb/VBn [μm] VBD-1Hr. 2.VBD-1Hr. 3VBD-1Hr. 1.VBD-2Hr. 2.VBD-2Hr. 3VBD.-2Hr Time [min] Fig.5: Tool wear depending on time of machining VBb/VBn [μm] Tool wear depending on time of machining (insert No.4) IN2040 IN2005 IN2004 IN6515 IN6510 IN2030 IN Time [min] Fig. 6: Tool wear depending on time of machining CONCLUSION The present state of machining steel P91 in company Škoda Power is more or less non-productive. It is caused by unprecedented material on the Czech market, that nobody knows how to machine it. Both machine operator and technology office do not know, how they should write a production method. As exclusive supplier of tools in to Škoda Power was elected company Ingersoll. This company supplies us all tools equipment that are tested in laboratory conditions. If we look at achieved results, we can see that cutting material for semi-finishing will be sintered carbide IN2004 and IN2005. These sintered carbides will be further tested to found fit combination of cutting conditions for maximal usage hereof cutting material.

6 REFERENCES [1] ARIVAZHAGAN B., SUNDARESAN S., KAMARAJ M. A study on influence of shielding gas composition on toughness of flux-cored arc weld of modified 9Cr-1Mo (P91) steel (2009) Journal of Materials Processing Technology, 209 (12-13), pp [2] SHIBLI A., STARR F. Some aspects of plant and research experience in the use of new high strength martensitic steel P91 (2007) International Journal of Pressure Vessels and Piping, 84 (1-2), pp [3] KUMAR H., MOHAPATRA J.N., ROY R.K., JOSEYPHUS R. MITRA A: Evaluation of tempering behaviour in modified 9Cr-1Mo steel by magnetic non-destructive techniques (2010). Journal of Materials Processing Technology, 210, pp