FLOW STRESS BEHAVIOR AND MATHEMATICS MODELING OF SEMI-HOT FORMING FOR CF53 STEEL

Transcription

1 FLOW STRESS BEHAVIOR AND MATHEMATICS MODELING OF SEMI-HOT FORMING FOR CF53 STEEL FANXIN KONG, ZHENHONG LI School of Material Engineering, Nanjing Institute of Technology, Nanjing, 67, China ABSTRACT Using the isothermal comression exeriment, the article study the flow stress change law in the semi-hot temerature range for Cf53, discuss the change mechanisms of flow stress, and develo a flow stress mathematical model alied to Cf53 steel semi-hot forming based on cree theory and statistical analysis which rovides the basis for reasonable controlling of numerical simulation and thermal arameters for Cf53 steel semi-hot forming. The exeriment arameter variation of deformation temerature (from to 95 ) and strain rate (from.s- to s-) have been realized. Pressure, temerature and dislacement during deformation are directly recorded by a comuter and automatically transformed into the true strain and true stress, and draw the true stress - true strain curves. Keywords: Cf53 Steel, Semi-Hot Forming, Flow Stress, Cree Theory. INTRODUCTION The forming of structure steel is mainly divided as cold forming, warm forming and hot forming. Warming forming is in a temerature range of 65 ~85 and combines advantages of cold and hot forming: high recision, low oxidation and decarburization tendency, long life of the mold, good formability and without softening treatment ste[]. These advantages have made warm forming achieve automatic roduction and therefore being widely used[]. However there are some kinds of steel still can not get good formability in the warm forming temerature, the formed art are normally defective caused by oor liquidity. For this reason, slightly higher temerature can be used to form these steel arts[3, 4]. Under this condition, lower heating temerature can imrove the recision of formed art and also higher heating temerature can conserve energy because light tonnage ress is caable to form steel arts. This semi-hot forming technique, which the temerature is between warm forming and hot forming, has its articular advantages for recision forming manufacturing. By controlling billet heating, lubrication condition, accuracy of the mold and deformation arameters during the rocess, semi-hot forming can roduce recision metal arts which with good surface quality and small dimensional tolerance. Most dimensions even all of the arts do not need subsequent machine working, which saves in row material and energy consumtion. Semi-hot forming has some roerties differ from warm forming and thermo forming because of different heating temerature. For a better understanding of the deformation mechanism of this advanced forming technique and exerting its advantages, the roerties of semi-hot forming need to be researched[5]. CF53 is a kind of common steel used as truck transmission shaft art, so the use of semi-hot recision forming instead of existing hot forming can considerably imrove roduction efficiency and reduce roduction cost. But there is little information about the research of semi-hot deformation mechanical roerties, and lack of scientific basis to develo the rocess technology. Using the isothermal comression exeriment, this aer studied the change law and mechanism of flow stress of Cf53 at temerature of, 85, 9, 95 and, a reliable frame of reference is made to establish the semi-hot forming rocess of this material[4].. EXPERIMENTAL METHOD Test material is Cf53 steel, its chemical comosition is given in table : Select cylinder comression samle, aly lubricant at the ends of samle to ensure uniformity of deformation. Samle size is Φ8mm mm before deformation. 97

2 Table : Chemical Comosition of Cf53 (wt% ) C Si Mn P S Hot comression exeriment was carried out on the Gleeble-35 thermo-simulated tester[6]. The tester automatically gives alied rocess strain (reduction), strain rate and temerature by rogram. Heating technique: the samles directly heated to, 85, 9 and 95, and then began to deform after ket for 5 min (as Figure ). Parameter variation of deformation temerature (from to 95 ) and strain rate (from.s- to s-) have been realized. Pressure, temerature and dislacement during deformation are directly recorded by a comuter and automatically transformed into the true strain and true stress, and draw the true stress - true strain curves. Figure : Heating Technique Of Hot Comression 3. EXPERIMENT RESULTS AND ANALYSIS 3. True Stress - True Strain Curves Figure shows true stress - true strain curves at different temerature and different strain rate. () Under the same deformation condition, work hardening caused by the increase of strain, flow stress raidly increased while strain was increasing. Then dynamic recrystallization softening was carried out after deforming over a certain oint, the flow stress reached the maximum oint when softening rate balanced with hardening rate; softening rate became greater than hardening rate with the rogress of dynamic recrystallization and the flow stress decreased gradually; after erfect dynamic recrystallization true-stress continually but not significantly increased, as shown steady state flow characteristics; true stress increased with deformation at the same temerature. () Under the same strain rate condition, the higher the deformation temerature, the lower the flow stress during steady deforming stage; when the deformation temerature remained unchanged, the lower the strain rate, the lower the flow stress during steady deforming stage. (3) Figure (a) shows the most magnificently softening henomena of flow stress curve aears when the strain rate was.s -. This is because softening was fully erformed when the deformation rate was low and it offset the work hardening caused by deformation increasing. (4) Peak strain increased as T decreased and increased, indicating that dynamic recrystallization blocked when T decreased, the eak stress moved towards the direction of increased, also indicating the imact trend which T had on the critical deformation occurred while dynamic recrystallizing: the smaller the T, the greater the eak strain, the more difficult for recrystallization. 3. Cree Equation And Prediction Method Of Semi-Hot Deformation Flow Stress Zener and Sellars[7] thought flow stress is related to deformation temerature, deformation seed and strain in a rogress of high temerature deformation and ut the general exression of material flow stress model: σ = f ( T, &, ) () Where σ is the flow stress (MPa), & is the equivalent strain rate (s-), is the equivalent strain, T is the absolute deformation temerature. The relation between T and & can exressed by arameter Z Z = & ex( / ) () Where Z is Zener-Hollomon arameter, its hysical meaning is the deformation rate factor of temerature comensation; is the hot deformation activation energy, it reflects the ease of material hot deformation and is an imortant mechanical roerty during hot deformation; R is the gas constant, its value is 8.34J mol- K-. There are three exression form of Z. n & ex( / ) σ (3) Z = C Z = & ex( / ) C ex( n σ ) (4) [ ασ ] Z = & ex( / ) C sinh( ) n (5) 98

3 t= (a) & =.s (d) & = s t= Figure : True Stress - True Strain Curves At Different Temerature And Different Strain Rate (b) & =.5s t= t= (c) & = 5s Take the logarithm to formula (3), (4) and (5): ln & = ln C + n lnσ (6) ln & = ln C + nσ (7) ln & = ln C + nln[ sinh( ασ )] (8) Where C, C, C, n and n are the constant values related to deformation, n is the stress exonent, α is the material constant and its value changes little, α is taken as. according to the references [8]. Research of different hot-forming data indicates the relation[7] between flow stress σ and Z can be described as exonential relationshi under low stress, ower exonential relationshi under high stress, and hyerbolic function relationshi under the whole rocess of stress change, which resectively corresond to formula (6), (7) and (8). Because the theoretical basis of formula (5) is cree deformation, formula (5) commonly used to described steady flow stress σ and eak flow stress σ. By analyzing the semi-hot flow stress of steel Cr and steel Cf53, we find that they are consistent with the hyerbolic function model of formula (8). This study used formula (8), cree equation in the form of hyerbolic function, to describe the flow stress of continuous deformation rocess of Cf53, the flow stress used in this formula is eak stress. s 99

4 When we kee & constant, according to formula (8), by requiring artial derivative of /T, we get: ln sinh( ασ ) = Rn = Rnβ (9) (/ T ) & When we kee T constant, according to formula (8), value n can be obtained by requiring artial derivative of ln & : ln & n = ln sinh( ασ ) T () Equation () shows the linear relationshi between ln sinh( ασ ) and ln &, using least squares regression, the average sloe of relation curve is n. By taking ln sinh( ασ ) and /T as coordinates to establish relation curve and then require average sloe β of the curve. Substitute the resultant value β, n and gas constant R in the formula (9), and then the deformation activation energy of Cf53 under different stress condition can be calculated. According to formula (8), we can exress flow stress by using Zener-Hollomon arameter as follows: Z n Z n σ = ln + + () α C C 3. 3 Semi-Hot Deformation Steady-State Stress Model Of Cf53 Figure 3 is the relational grah of Cf53 steel strain rate and ln sinh( ασ ), Figure 4 is the relational grah of /T o f Cf53 steel and ln sinh( ασ ). By averaging the straight sloe of Figure 3 we get n, by averaging the straight sloe of Figure 4 we get β, and then deformation activation energy of Cf53 steel can be worked out. Given a deforming condition as T=73K and & =5s-, constant value C can be worked out. Calculated values of each arameter shown as table. Substitute deformation activation energy in the formula (), the sub-exression of arameter Z in the semi-hot temerature range of Cf53 steel can be obtained: Z = & ex( ) () Substitute constant value C and index n in formula (), the flow stress model for Cf53 steel in semi-hot temerature range can be obtained: Z 4.73 Z 4.73 σ = ln (3) Table 3 calculated value ln Z corresonded to different exerimental condition by the formula ():. ln ln(sinh(ασ )) T=73K T=3K T=73K T=3K T=73K ln(sin h (ασ )) lnln sin h Figure 3: Peak Stress Versus Strain Rate For The Cf s -.5s - 5s - s ) / T ( K - Figure 4: Peak Stress Versus Temerature For The Cf53 Table The Parameters In The Flow Stress Model Of The Cf53 Parameter arameter value n 4.73 β K MPa 9.78 KJ mol - C.98 s - 3

5 & (s - ) T(K) Table 3 Values Of Lnz At Different Conditions For The Cf CONCLUSIONS [5]Zhiliang Zhang, Warm Forming Technology, (Shanghai Science and Technology Publishing House, 986). [6] Xuchuan Huang, Thermal simulation technology of tester GLEEBLE-35, Meishan Technology, Vol., No., 6, [7]Sellars CM, Hot deformation rocessing. Materials Science and Technology, Vol., No.8, 99, [8] A. Laasraoui, J.J.Jonas, Prediction of temerature distribution, flow stress and microstructure during the multiass hot rolling of steel late and stri, ISIJ International, Vol.8, No.3, 99, According to the Cf53 comression exeriment carried out in a semi-hot temerature range of ~, this aer has analyzed its forming roerty and flow stress, conclusions as follows: Peak strain increased as T decreased and increased. When the strain rate was.s-, because softening was fully erformed as the deformation rate was low and it offset the work hardening caused by deformation increasing. Flow stress formula in the semi-hot temerature range for Cf53 has been worked out based on cree theory by doing laboratory exeriment. It can be used to calculate and redict flow stress in roduction, to guide the design of rocess and mold. REFERENCES: [] Zhenhong Li, Research on the forming rocess simulation and die wear of semi-hot temerature range, Ph.D. Thesis, Det. Plasticity Technology, Shanghai Jiao Tong University, Shanghai, China, 9. [] T.Wu, G.Ren. Research on warm forging/cold orbital forming technology for the straight bevel gear in differential case,china Mechanical Engineering, Vol.6, No.6, 5,.6-9. [3] Hui Yang and Zhenhong Li, Investigation on Zener-Hollomon in the warm-hot deformation behavior of CrMnTi, Journal of Zhejiang University,Vol.7, No.8, 6, [4]Yang Hui, Li Zhenhong, Investigation on Zener- Hollomon in the warm-hot deformation of Cf53, Journal of Shanghai Jiaotong University, Vol., No.3, 7,