HOT COMPRESSION TEST OF 9 Cr1 Mo STEEL – NUMERICAL SIMULATION

This paper is focused on the evaluation of formability of heat resistant steel type 9Cr-1Mo by laboratory numerical simulation – hot compression test confirmed by laboratory hot compression test. The 9Cr-1Mo steel represents modern 9%Cr tempered martensitic steel for high temperature applications in advanced thermal power plants. Numerical simulations were computed in software Deform 3D for five proposed sample shapes. On the base of normalized Cockcroft-Latham criterion (nCL), indicating the material damage during deformation, the sample type “tapered roller with four axial notches” was found to be the most suitable for hot workability evaluation. On base of simulations, it is also evaluated the temperature range of the workability of 9Cr-1Mo. The interval of good workability according to the nCL criteria is in the temperature range from 650 to 950 °C.


Introduction
The improvement of thermal efficiency by increasing the operating temperature and pressure of boilers has recently led to the development of new grades of creep-resistant steels [1].Shi at al. [2] showed that modified 9-12% Cr steels improved properties of heat-resistant steels.The 9Cr-1Mosteel represents tempered martensitic steel with complex alloying assuring transformation, solid solution and precipitation strengthening at elevated temperatures [2].It is used for thick section components of steam generators operating in the temperature range 550 -650 C. The presence of precipitated particles formed in the microstructure of 9Cr-1Mo steels during annealing may cause formation of the voids during plastic deformation [3].This steel grade achieves better mechanical properties with the simultaneous reduction of the wall thickness.Maximum value of creep tensile strength Rmt/105h is 110 -120MPa.The common heat treatment of high Cr steels consists of normalizing and tempering.Because of high Cr content, cooling in air from austenitising temperature leads to fully martensitic microstructure [4].9Cr-1Mo steel has been widely applied to manufacture steam containers in ultra-super-critical power plants.The most research focused on 9Cr-1Mo steel investigates microstructure and its behavior during hot deformation, strengthening mechanism and long-term creep properties and corrosion resistance [7][8][9][10].9Cr-1Mo steel is suitable for welding and there are several impact studies about correlation between microstructure and conditions of welding with others materials and post-weld heat treatment [11,12].Workability and fracture limit criterions have not been determined yet.Because many kind of alloying elements exist in the steel and the microstructure evolution is complicated during hot working, it is difficult to establish the hot working regime [2].Workability can be defined as the degree of deformation that can be achieved in a particular metalworking process without creating undesirable condition and defects [13].More recently, a new approach consisting of evaluating, with finite element analysis, the void nucleation, growth and coalescence in a representative cell containing a simple cavity and embedded in a matrix was proposed.This approach has been applied to study ductile damage phenomena for several kinds of loading conditions and to characterize the propensity to cracking.The approach is currently referred to as macroscopic since the scale of the cell lies between the size of the grain (microscopic) and that of the specimen (macroscopic) [14].Ductile fracture criterions can be used to describe workability of metals.The critical value of stress at which the damage of material appears, the strain and strain rate are the basic parameters describing the limit workability of material.This property depends on the material itself which is being processed but also on the geometry of the specimen and the lubrication conditions.The simple ductile fracture criterion proposed by Cockcroft and Latham was used for theoretical analysis.This criterion states that fracture occurs when the tensile strain energy reaches a critical value [15], [16].The Cockcroft-Latham (CL) criterion is described by equation ( 1): where σ_1 [MPa]largest principal tensil stress ε [-]strain ε̅ _f [-]strain in fracture C1 [MPa]material constantvalue of the criterion at the instant of fracture initiation [17].According to this criterion, if on the material acts only compressive stress, the fracture will not occur.CL criterion does not consider the impact of hydrostatic stress.Oh et al. [18] have modified a CL criterion for uni-axial stress through normalizing of maximum principal tensile stress.This form called a normalized CL criterion (nCL) (2): where σ ̅ [MPa]effective stress C2 [MPa]material constant [17].On the basis of known material constants C1 or C2 it can be characterized workability of the material [10].Using those criteria and values we expect of fracture, but these mathematical simulations should be verified by physical and thus accurately determine their value and terms of workability of the material [17]- [19].The stress and strain distribution in upsetting deformation represents the typical stress/strain states in most bulk forming processes [15].Because of this, it was chosen a hot upsetting test.In Fig. 1 pseudo-binary phase diagram for steel grade 9Cr-1Mo is shown.Fig. 1 demonstrates that for the 9Cr-1Mo type steel with 0.1 wt.% C, the Ac1 and Ac3 critical transformation temperatures of ferrite to austenite phase transformation are 840 °C and 860 °C, respectively [20].This paper is focused on numerical simulations of hot upsetting test of 9Cr-1Mo steel and on selection of the most suitable shape of sample for this test.The suitable temperature range for good workability of studied 9Cr-1Mo steel was evaluated.

Experimental material and methods
Heat-resistant chromium steel type 9Cr-1Mo was used as experimental material.The chemical composition is given in Table 1.According to [13][14][15][21][22][23] it was designed five types of samples, as it is given at Fig. 2.
For every sample type the simulation was performed in isothermal conditions for temperature range from 700 °C to 1100 °C.The setting of each simulation included the same values of maximal strain to be 75 % with an initial strain rate  = 0,044 s -1 .Hot compression test was realized with using the last sample type -tapered with four axial notches in the temperature range T <500; 950>.In Fig. 3 cylindrical sample after 50% and 66% deformation at temperature 700°C is shown.Points P1 and P2 have been placed in the area of the highest tensile stress.Fig. 4 shows cylindrical sample with one axial notch.The highest stress is located in the middle of the notch as it acts like a stress concentrator.5 shows tapered sample.According to the simulation, it can be assumed that the first was deformed the tapered portions, wider central portion was initially deformed only slightly.After upsetting the tapered portions, deformation of the central part is visible.Deformation is centred on the perimeter at the edge of the tapered and central part.It is seen, that buckling is in this case minimized.In the fields of maximum tensile stress at 66 % strain, the nCL criterion for the simulation of tapered sample shows lower value, compared to that of the cylindrical sample with one axial notch.In Fig. 6 tapered sample with one radial notch in its central part is shown.Deformation process is similar to the process for tapered sample.Because of small buckling, the notch is pushed into the sample and does not act like a stress concentrator.2. Fig. 8 shows nCL value as a function of strain.The curve representing the tapered sample with four axial notches is above the others.It can be concluded that the most appropriate type of sample is tapered with four axial notches and the simple cylindrical sample is the least appropriate for workability evaluation through hot upsetting test.Fig. 9 depicts the individual sample types with their corresponding nCL values for strain levels of 50 % and 66 %.Fig. 10 shows that for all sample types except tapered with axial notches has nCL criterion as a function of strain constant progress.The curve representing tapered sample with axial notches is above the others.For strain ε =50% is the course of function nCL -T is approximately constant.
Only for tapered sample type with axial notches at temperature 650°C nCL value starts to decrease up to 900°C, then has the function increasing tendency again, Fig. 10.For strain ε=66% is this decrease more visible.On the basis of significant decrease in nCL value, Fig. 11 indicates suitable conditions for forming process up to temperature of 1050 °C at the strain level of 66 %.In Fig. 12 is a graphical comparison of the course of values nCL criterion dependence on temperature criteria for two types of samples which under previous simulations have been evaluated as the most valuable for hot upsetting test.For the both sample types the values of nCL criterion were evaluated for strain levels of 50 % and 66 %.For the tapered sample with axial notches the nCL value decreased at 850 °C due to the occurrence of ferrite to austenite phase transformation.At 850 °C for tapered sample with axial notches nCL value has been decreased because of phase transition.For cylindrical sample with one axial notch is the decline less pronounced.By physical simulation it is possible to evaluate the workability and compare the results with numerical simulations.According to these the good workability for 9Cr-1Mo steel is at the temperature range 650 -1050°C, depending on the required deformation.

Conclusion
The results obtained from numerical simulations of 9Cr-1Mo steel hot compression test can by summarize in the following conclusions:  According to the nCL values from simulations, the tapered roller with four axial notches was identified as the most suitable sample shape for hot workability evaluation.The cylinder with smooth surface was found to be the least appropriate. Depending on the desired deformation, the course of nCL criteria specified by the simulation indicated good workability of studied9Cr-1Mosteel in the temperature range 650 °C -1050 °C.

DOI 10 .Fig. 2
Fig. 2 The proposed shapes of samples: a) cylindrical, b) cylindrical with one axial notch, c) tapered, d) tapered with one radial notch, e) tapered with four axial notches 3 Results Values of indicator Damage have been monitored which correspond with value of nCL criterion.Color scheme has for every simulation equal interval nCL criteria <0; 0,7>.Based on these results it was possible to identify the most probable place origin of cracks and compare value of nCL criterion for individual type of sample.

Fig.
Fig.5shows tapered sample.According to the simulation, it can be assumed that the first was deformed the tapered portions, wider central portion was initially deformed only slightly.After upsetting the tapered portions, deformation of the central part is visible.Deformation is centred on the perimeter at the edge of the tapered and central part.It is seen, that buckling is in this case minimized.In the fields of maximum tensile stress at 66 % strain, the nCL criterion for the simulation of tapered sample shows lower value, compared to that of the cylindrical sample with one axial notch.In Fig.6tapered sample with one radial notch in its central part is shown.Deformation process is similar to the process for tapered sample.Because of small buckling, the notch is pushed into the sample and does not act like a stress concentrator.

Fig. 8 Fig. 9
Fig. 8 Comparison of a ε -nCL curves for different types of samples

Fig. 12
Fig. 12 Graphical comparison of the nCL criterion as a function of temperature for the most suitable samples for hot workability evaluation

Table 1
Chemical composition of steel 9Cr-1Mo in wt%

Table 2
Maximum values of nCL criterion