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Mechanism of Embrittlement and De-Embrittlement for 2. 25Cr-1Mo Steel
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ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2011, 18(3) : 47-51
Mechanism of Embrittlement and De-Embrittlement for 2.25Cr-lMo Steel ZHANG Xi-liang ,
ZHOU Chang-yu
(School of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, China) Abstract : The constant embrittlement curve for constant segregation concentration on grain boundary of impurity element P and relationship between equilibrium grain boundary segregation concentration and operation time for 2. 25Cr1Mo steel were derived based on the theory of equilibrium grain boundary segregation. The mechanism of step-cooling test and mechanism of de-embrittlement for 2. 25Cr-1Mo steel were explained. The segregation rate will increase but equilibrium grain boundary segregation concentration of impurity element P will decrease as temperature increases in the range of temper embrittlement temperature. There is one critical temperature of embrittlement corresponding to each embrittlement degree. When the further heat treating temperature is higher than critical temperature, the heat treating will become a deembrittlement process; otherwise, it will be an embrittlement process. The critical temperature of embrittlement will shift to the direction of low temperature as further embrittlement. A s a result, some stages of step-cooling test would change into a deembrittlement process. The grain boundary desegregation function of impurity element P was deduced based on the theory of element diffusion, and the theoretical calculation and experimental results show that the further embrittlement or deembrittlement mechanism can be interpreted qualitatively and quantitatively by combining the theory of equilibrium grain boundary segregation with constant embrittlement curve. Key words: stepcooling test; critical embrittlement temperature; embrittlement mechanism; grain boundary desegregation
Safety assessment and life prediction for pressure vessels which serviced in the environment of high temperature become very important in engineering and Embrittlement treating was one of the indispensable and important means for the study of material deterioration and life prediction. However, further embrittlement treating process becomes more difficult for the embrittled material. A B DoucetC3]wanted to make a further embrittlement for embrittled 2. 25Cr-1Mo steel by using of step-cooling test, but he did not succeed. Domestic scholar made a temper treating using the matrix from hydrogenation reactor after 5 years, and the result is that the matrix became more de-embrittlementC4’. Though people have made some progress in phenomenon of temper embrittlement , influencing factors and assessment method and so on, theoretical breakthrough has not been achieved, so the problems could not be thoroughly solvedC5p61.Taking 2. 25Cr1Mo steel of hydrogenation reactor’s matrix as an
example, the theories of equilibrium grain boundary segregation and atom diffusion are applied to analyze the mechanism of embrittlement and de-embrittlement of material, which provides certain theoretical support to the further work.
1 The Constant Embrittlement Curve Based on the Theory of Equilibrium Grain Boundary Segregation According to the theory of equilibrium grain boundary segregation, temper embrittlement results from segregating of impurity element to the grain boundary in the range of the temperature of temper embrittlement , which causes intergranular embrittlement. Temper embrittlement for the 2. 25Cr-1Mo steel is due to the segregation of impurity element P, As, Sb, Sn to the grain boundary, and segregation of the impurity element P is the major factor‘7p81. Mclean studied on the phenomenon of equilibrium grain boundary in the 1950s. Concentration C, (T)of
Foundation Item: Item Sponsored by Graduate Student Scientific Innovation Project of Jiangsu Province of China (CXO9B-131Z) Biography: ZHANG XiGliang(1983-), Male, Doctor; E-mail: changyu-zhouC3163. corn; Received Date: January 8 , 2010
48
Journal of Iron and Steel Research, International
equilibrium grain boundary segregation of impurity element idg1: AC, exp( AG/k T ) (1) cgbm ( T )= l+AC,exp( AG/RT) where, C, is average concentration of impurity element P in the transgranular (in atomic percent), % ; A is a constant which is related t o the vibrational entropy of transgranular; K is Boltzmann constant, J/K; and T is absolute temperature, K. Dynamics formula of grain segregation was derived by M ~ l e a n [ ~: ~ " ~
17. 5 (in atomic percent, % I , respectively.
2 Mechanism of Embrittlement or De-Embrittlement for Material and Calculation for Critical Embrittlement Temperature Taking the 2. 25Cr-1Mo steel of hydrogenation reactor matrix as an example, the P concentrations on grain boundary in the different holding time are calculated in this paper. T h e calculation parameters are shown in Table lcll-lzl. Table 1
where, C,b(t) is concentration of grain boundary of impurity element P when holding time is equal to t ; CbgO is the initial P concentration on grain boundary; c g h m is concentration of equilibrium grain boundary of impurity element P (in atomic percent), % ; D, is diffusion coefficient of impurity element P, mz/s; a = Cgboo/Cg;d is intergraular width, m ; and erfc is error function complement. P concentration on grain boundary in the ,different holding time and each temperature in the range of temper temperature is first calculated according to Eqn. ( 2 ) , and then the holding time to be abscissa and concentration of segregation on grain boundary of impurity element P is to be longitudinal coordinate, and the constant embrittlement curve will be obtained when linking the different temperature spots that have equal segregation concentration. According to the prior period work["', the constant embrittlement curve for 2. 25Cr-1Mo steel is shown in Fig. 1. Each curve of C shape has equal P concentration on grain boundary, in other words, each curve has same embrittlement degree. The concentration of segregation on grain boundary of impurity element P for curve A-G is 4 . 9 , 6.2, 8.1, 10.0, 13.5, 15.0, and
1
10
100
1OW 10 o(X) 100 (xw) 1 0 0 0 000
Timeih
Fig. 1 The time-temperature diagram for the constant segregation concentration of P in 2.25CrlMo
Vol. 18
Parameters for calculation
1.38X l o r z 3
0.021 6
0.775
45
10-9
Note: 1) In atomic percent.
From Eqn. (1) and Table 1, one obtains the relationship between saturation concentration ( in atomic percent) (99.999% saturation concentration as target value) of equilibrium grain boundary segregation and holding temperature as shown in Fig. 2.
01
600
Fig. 2
650
700
I , 750 800 850 Temperaturex
900 950
Relationship between saturated concentration of equilibrium grain boundary segregation and holding temperature
Fig. 2 shows that there is a concentration of equilibrium grain boundary segregation corresponding to each holding temperature, and it will increase by reducing holding temperatures. When P concentration on grain boundary exceeds the concentration of equilibrium grain boundary segregation at a certain temperature for embrittlement material, impurity element P will desegregate to transgranular from grain boundary if further heat treating was made at this temperature or above this temperature. Fig. 1 shows that there are two temperatures in the constant embrittlement curve for designated time, which are the temperatures corresponding to upper level curve and
Mechanism of Embrittlement and De-Embrittlement for 2. 25Cr-1Mo Steel -
Issue 3
that to lower level curve, respectively. For the former, the concentration of grain boundary segregation drives to saturation, and the material can not be embrittled anymore. Only further heat treating below this temperature can make the constant embrittlement curve shift to right-hand, which will be an embrittlement process. Further heat treating over this temperature can make the constant embrittlement curve shift to left-hand, which will be a de-embrittlement process. In this paper the critical embrittlement temperature is defined as the temperature corresponding to upper level curve of each constant embrittlement curve. T h e critical embrittlement temperature can be obtained from Fig. 2.
3 Mechanism of Embrittlement and DeEmbrittlement of Step-Cooling Test According to Eqn. (1) and Eqn. ( 2 ) , the relationship between holding time of saturation P concentration of equilibrium grain boundary segregation and tempering temperature can be obtained, as shown in Fig. 3.
600
Fig. 3
650
700
750 800 850 TemperatureK
900 950
Relationship between the holding time of saturated concentration of equilibrium grain boundary segregation and tempering temperature
Fig. 2 and Fig. 3 show that when temperature is rising, saturation P concentration on grain boundary will reduce and segregation rate will increase; otherwise, P concentration on grain boundary will increase and segregation rate will decrease. T h e common step-cooling test is shown in Fig. 4, which includes five thermostatic stages and five continuous cooling stages. T h e result above reflects designed idea of step-cooling test that holding time of temper embrittlement is shorter as the temperature is higher, and holding time of temper embrittlement is longer as the temperature is lower. As Fig. 1 shows, there
49
Timeh
Fig. 4
Curve of stepcooling test
is a critical embrittlement temperature corresponding to each embrittlement degree for a material, so there is also a critical embrittlement degree to each temperature in the range of temper embrittlement temperature. Further heat treating in this temperature will be a de-embrittlement process if embrittlement degree of material exceeds the critical embrittlement degree corresponding to the one temperature. Such as temperature stage 593 "C of step-cooling test, whose critical embrittlement concentration (in atomic percent) is 8. 2 % according to Fig. 2. This stage will become de-embrittlement process if P concentration on grain boundary exceeds 8.2 %. Any stage of step-cooling test may become a de-embrittlement process as embrittlement degree of material is higher than the corresponding critical embrittlement degree at its relative temperature stage. A conclusion will be reached that step-cooling test can not forever accelerate material embrittlement.
4 Grain Boundary Desegregation Model of Impurity Element P T h e grain boundary desegregation function of impurity element P was derived based on the theories of element diffusion and equilibrium grain boundary. T h e relation between concentration of grain boundary desegregation of impurity element P and holding time is:
(3) where CgMis P concentration on grain boundary before thermal retardation for embrittlement material (in atomic percent, %>. Ref. [4] and this paper made further heat tlieating using the matrix from hydrogenation reactor after 5 and 12 years, respectively. T h e concentrations (in atomic percent) of grain boundary segregation of impurity element P are respectively 14. 22 % and
16. 65% for the matrix without any heat treating. T h e critical embrittlement temperatures for the matrix after 5 and 12 years are respectively 513 and 498 "Caccording to Fig. 2. Based on this paper's theory, further heat treating at 513 "Cfor the matrix after 5 years is a de-embrittlement process, and further Table 2
heat treating at 498 'C is a de-embrittlement process for the matrix after 12 years. Ref. [4] and this paper's experimental results ( t h e matrix from Hydrogenation reactor after 1 2 year) and theoretical calculation according to the theory of this paper are shown in Table 2.
The calculated and experimental results
Test material
Heat treating scheme
VTr54. 2/ ~cC4l
AVTr54. 2/ .C
Segregation of P element/ %
Difference of senrenation/ ?4
T h e matrix from hydrogenation reactor after 5 aC41
Matrix of received 593 'C X 1 h 538 'C X15 h
-51. 6 -52.6 -59.6
-
-1 -8
14.220 11.614 8. 515
-2.606 -5. 705
T h e matrix from hydrogenation reactor after 12 a
Matrix of received 524 C X 28 h 524 C X60 h
-41. 4 - 52 -56. 7
-10.6 -15.3
16.650 10.357 8. 676
-6.293 -7.974
T h e grain boundary segregation in Table 2 is result of theoretical calculation by this paper's theory, others are experimental results. As is known, the critical embrittlement temperature for the matrix after 5 and 1 2 years are respectively 513 and 498 'C. Further heat treating of 593 "Cfor 1 h and 538 O C for 15 h for the matrix after 5 years are all de-embrittlement processes and further heat treating of 524 "C for 28 h and 524 'Cfor 60 h for the matrix after 12 years are all deembrittlement processes. The grain boundary desegregation function is used to calculate concentration of grain boundary after further heat treating. T h e concentration (in atomic percent) of grain boundary of impurity element P after processes of 593 'C for 1 h and 538 "Cfor 15 h are respectively 11. 614% and 8. 515%, for the same reason the concentration after processes of 524 O C for 28 h and 524 'C for 60 h are respectively 10. 357% and 8. 676%. T h e theoretical calculation and experimental results make out that the further embrittlement or de-embrittlement mechanism can be explained qualitatively and quantitatively with combination of theory of grain boundary desegregation in this paper and constant embrittlement curve.
5
Vol. 18
Journal of Iron and Steel Research, International
50
Conclusions
1) There is a concentration of equilibrium grain boundary segregation corresponding to each temperature in the range of temper embrittlement temperature, and the P concentration on grain boundary will decrease and segregation rate will increase with the increase of temperature. 2 ) There is a critical embrittlement temperature corresponding to every embrittlement degree, further
-
/
/
heat treating above or equal to the critical embrittlement temperature is a de-embrittlement process ; otherwise, it will be an embrittlement process. 3 ) T h e critical embrittlement temperature will decrease as the embrittlement increases; the critical embrittlement temperature can be calculated by Fig. 2 and Eqn. (1). 4) Step-cooling test can not always be a process of acceleration embrittlement for material, and some heat treating stages or all stages of step-cooling test will become de-embrittlement process as embrittlement reaches certain degree. 5) T h e further embrittlement and de-embrittlement mechanism of material can be explained qualitatively and quantitatively by combining the theory of grain boundary desegregation with constant embrittlement curve. References: Christien F, Le Gall R, Saindrenan G. Phosphorus Grain Boundary Segregation in Steel 17-4PH [J]. Scripta Materillia, 2003, 48: 11. The Japan Steel Works. Technical Service for Safe Operation of High-pressure Vessel [R]. Tokyo: The Japan Steel Works, 1995. Doucet Anne B. Effects of Service Exposure of 2 1/4 Cr-1Mo Pressure Vessel Steel in A Heavy Oil Hydrocracking [Cl // Firness-in-Service and Decisions for Petroleum and Chemicial Equipment. PVP-Vol. 315. New York: ASME, 1995: 407. TAN Jin-zhu, HUANG Wen-long. Application of Step-Cooling Test Method for Temper Embrittlement of 2. 25Cr-1Mo Steel [J]. Journal of Nanjing University of Chemical Technology, 1998, ZO(Supp1ement): 17 (in Chinese). Masaaki Katsumata, Hisashi Takada, Hiromichi Hrano, et al, Temper Embrittlement in Pressure Vessel Steel [J]. 1981, 19 (3): 120. Scott T E. Application of 2 1/4Cr-Mo Steel for Thick-Wall Pressure Vessels [MI. Baltimore: American Society for Tes-
Issue 3
[7]
[8]
[9]
Mechanism of Embrittlement and De-Embrittlement for 2. 2 5 C r l M o Steel
ting and Materials, 1982. Viswanathan R , Jaffee R I. 2 1/4Cr-lMo Steels for Coal Conversion Pressure Vessels [J]. ASME Journal of Engineering Material and Technology, 1982, 104: 220. McMahon C J , Genter D H , Ucisk A H. An Investigation of Grain Size and Hardness in Temper-Embrittled 2 1 / 4 C r l M o Steel [J]. ASME Journal of Engineering Material and Technology, 1984, 106: 66. Mclean D. Grain Boundaries in Metals [D]. London: Oxford University Press, 1957.
[lo]
[ 111
[12]
-
51
XU Ting-dong, CHENG Bu-yuan. Kinetics of Non-Equilibriurn Grain-boundary Segregation [ J]. Progress in Materials Science, 2004, 49: 109. ZHANG Xi-liang, ZHOU Chang-yu. Study on Further Embrittlement Mechanism Based on the Theory of Equilibrium Grain Boundary Segregation [J]. Transactions of Materials and Heat Treatment, 2009, 30(4): 194. Ogura T. A Method for Evaluation of the Mount of GrainBoundary Segregation During Quenching [ J]. Trans Japan Inst Met, 1981, 22(2): 109.
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2011, 18(3) : 47-51
Mechanism of Embrittlement and De-Embrittlement for 2.25Cr-lMo Steel ZHANG Xi-liang ,
ZHOU Chang-yu
(School of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, China) Abstract : The constant embrittlement curve for constant segregation concentration on grain boundary of impurity element P and relationship between equilibrium grain boundary segregation concentration and operation time for 2. 25Cr1Mo steel were derived based on the theory of equilibrium grain boundary segregation. The mechanism of step-cooling test and mechanism of de-embrittlement for 2. 25Cr-1Mo steel were explained. The segregation rate will increase but equilibrium grain boundary segregation concentration of impurity element P will decrease as temperature increases in the range of temper embrittlement temperature. There is one critical temperature of embrittlement corresponding to each embrittlement degree. When the further heat treating temperature is higher than critical temperature, the heat treating will become a deembrittlement process; otherwise, it will be an embrittlement process. The critical temperature of embrittlement will shift to the direction of low temperature as further embrittlement. A s a result, some stages of step-cooling test would change into a deembrittlement process. The grain boundary desegregation function of impurity element P was deduced based on the theory of element diffusion, and the theoretical calculation and experimental results show that the further embrittlement or deembrittlement mechanism can be interpreted qualitatively and quantitatively by combining the theory of equilibrium grain boundary segregation with constant embrittlement curve. Key words: stepcooling test; critical embrittlement temperature; embrittlement mechanism; grain boundary desegregation
Safety assessment and life prediction for pressure vessels which serviced in the environment of high temperature become very important in engineering and Embrittlement treating was one of the indispensable and important means for the study of material deterioration and life prediction. However, further embrittlement treating process becomes more difficult for the embrittled material. A B DoucetC3]wanted to make a further embrittlement for embrittled 2. 25Cr-1Mo steel by using of step-cooling test, but he did not succeed. Domestic scholar made a temper treating using the matrix from hydrogenation reactor after 5 years, and the result is that the matrix became more de-embrittlementC4’. Though people have made some progress in phenomenon of temper embrittlement , influencing factors and assessment method and so on, theoretical breakthrough has not been achieved, so the problems could not be thoroughly solvedC5p61.Taking 2. 25Cr1Mo steel of hydrogenation reactor’s matrix as an
example, the theories of equilibrium grain boundary segregation and atom diffusion are applied to analyze the mechanism of embrittlement and de-embrittlement of material, which provides certain theoretical support to the further work.
1 The Constant Embrittlement Curve Based on the Theory of Equilibrium Grain Boundary Segregation According to the theory of equilibrium grain boundary segregation, temper embrittlement results from segregating of impurity element to the grain boundary in the range of the temperature of temper embrittlement , which causes intergranular embrittlement. Temper embrittlement for the 2. 25Cr-1Mo steel is due to the segregation of impurity element P, As, Sb, Sn to the grain boundary, and segregation of the impurity element P is the major factor‘7p81. Mclean studied on the phenomenon of equilibrium grain boundary in the 1950s. Concentration C, (T)of
Foundation Item: Item Sponsored by Graduate Student Scientific Innovation Project of Jiangsu Province of China (CXO9B-131Z) Biography: ZHANG XiGliang(1983-), Male, Doctor; E-mail: changyu-zhouC3163. corn; Received Date: January 8 , 2010
48
Journal of Iron and Steel Research, International
equilibrium grain boundary segregation of impurity element idg1: AC, exp( AG/k T ) (1) cgbm ( T )= l+AC,exp( AG/RT) where, C, is average concentration of impurity element P in the transgranular (in atomic percent), % ; A is a constant which is related t o the vibrational entropy of transgranular; K is Boltzmann constant, J/K; and T is absolute temperature, K. Dynamics formula of grain segregation was derived by M ~ l e a n [ ~: ~ " ~
17. 5 (in atomic percent, % I , respectively.
2 Mechanism of Embrittlement or De-Embrittlement for Material and Calculation for Critical Embrittlement Temperature Taking the 2. 25Cr-1Mo steel of hydrogenation reactor matrix as an example, the P concentrations on grain boundary in the different holding time are calculated in this paper. T h e calculation parameters are shown in Table lcll-lzl. Table 1
where, C,b(t) is concentration of grain boundary of impurity element P when holding time is equal to t ; CbgO is the initial P concentration on grain boundary; c g h m is concentration of equilibrium grain boundary of impurity element P (in atomic percent), % ; D, is diffusion coefficient of impurity element P, mz/s; a = Cgboo/Cg;d is intergraular width, m ; and erfc is error function complement. P concentration on grain boundary in the ,different holding time and each temperature in the range of temper temperature is first calculated according to Eqn. ( 2 ) , and then the holding time to be abscissa and concentration of segregation on grain boundary of impurity element P is to be longitudinal coordinate, and the constant embrittlement curve will be obtained when linking the different temperature spots that have equal segregation concentration. According to the prior period work["', the constant embrittlement curve for 2. 25Cr-1Mo steel is shown in Fig. 1. Each curve of C shape has equal P concentration on grain boundary, in other words, each curve has same embrittlement degree. The concentration of segregation on grain boundary of impurity element P for curve A-G is 4 . 9 , 6.2, 8.1, 10.0, 13.5, 15.0, and
1
10
100
1OW 10 o(X) 100 (xw) 1 0 0 0 000
Timeih
Fig. 1 The time-temperature diagram for the constant segregation concentration of P in 2.25CrlMo
Vol. 18
Parameters for calculation
1.38X l o r z 3
0.021 6
0.775
45
10-9
Note: 1) In atomic percent.
From Eqn. (1) and Table 1, one obtains the relationship between saturation concentration ( in atomic percent) (99.999% saturation concentration as target value) of equilibrium grain boundary segregation and holding temperature as shown in Fig. 2.
01
600
Fig. 2
650
700
I , 750 800 850 Temperaturex
900 950
Relationship between saturated concentration of equilibrium grain boundary segregation and holding temperature
Fig. 2 shows that there is a concentration of equilibrium grain boundary segregation corresponding to each holding temperature, and it will increase by reducing holding temperatures. When P concentration on grain boundary exceeds the concentration of equilibrium grain boundary segregation at a certain temperature for embrittlement material, impurity element P will desegregate to transgranular from grain boundary if further heat treating was made at this temperature or above this temperature. Fig. 1 shows that there are two temperatures in the constant embrittlement curve for designated time, which are the temperatures corresponding to upper level curve and
Mechanism of Embrittlement and De-Embrittlement for 2. 25Cr-1Mo Steel -
Issue 3
that to lower level curve, respectively. For the former, the concentration of grain boundary segregation drives to saturation, and the material can not be embrittled anymore. Only further heat treating below this temperature can make the constant embrittlement curve shift to right-hand, which will be an embrittlement process. Further heat treating over this temperature can make the constant embrittlement curve shift to left-hand, which will be a de-embrittlement process. In this paper the critical embrittlement temperature is defined as the temperature corresponding to upper level curve of each constant embrittlement curve. T h e critical embrittlement temperature can be obtained from Fig. 2.
3 Mechanism of Embrittlement and DeEmbrittlement of Step-Cooling Test According to Eqn. (1) and Eqn. ( 2 ) , the relationship between holding time of saturation P concentration of equilibrium grain boundary segregation and tempering temperature can be obtained, as shown in Fig. 3.
600
Fig. 3
650
700
750 800 850 TemperatureK
900 950
Relationship between the holding time of saturated concentration of equilibrium grain boundary segregation and tempering temperature
Fig. 2 and Fig. 3 show that when temperature is rising, saturation P concentration on grain boundary will reduce and segregation rate will increase; otherwise, P concentration on grain boundary will increase and segregation rate will decrease. T h e common step-cooling test is shown in Fig. 4, which includes five thermostatic stages and five continuous cooling stages. T h e result above reflects designed idea of step-cooling test that holding time of temper embrittlement is shorter as the temperature is higher, and holding time of temper embrittlement is longer as the temperature is lower. As Fig. 1 shows, there
49
Timeh
Fig. 4
Curve of stepcooling test
is a critical embrittlement temperature corresponding to each embrittlement degree for a material, so there is also a critical embrittlement degree to each temperature in the range of temper embrittlement temperature. Further heat treating in this temperature will be a de-embrittlement process if embrittlement degree of material exceeds the critical embrittlement degree corresponding to the one temperature. Such as temperature stage 593 "C of step-cooling test, whose critical embrittlement concentration (in atomic percent) is 8. 2 % according to Fig. 2. This stage will become de-embrittlement process if P concentration on grain boundary exceeds 8.2 %. Any stage of step-cooling test may become a de-embrittlement process as embrittlement degree of material is higher than the corresponding critical embrittlement degree at its relative temperature stage. A conclusion will be reached that step-cooling test can not forever accelerate material embrittlement.
4 Grain Boundary Desegregation Model of Impurity Element P T h e grain boundary desegregation function of impurity element P was derived based on the theories of element diffusion and equilibrium grain boundary. T h e relation between concentration of grain boundary desegregation of impurity element P and holding time is:
(3) where CgMis P concentration on grain boundary before thermal retardation for embrittlement material (in atomic percent, %>. Ref. [4] and this paper made further heat tlieating using the matrix from hydrogenation reactor after 5 and 12 years, respectively. T h e concentrations (in atomic percent) of grain boundary segregation of impurity element P are respectively 14. 22 % and
16. 65% for the matrix without any heat treating. T h e critical embrittlement temperatures for the matrix after 5 and 12 years are respectively 513 and 498 "Caccording to Fig. 2. Based on this paper's theory, further heat treating at 513 "Cfor the matrix after 5 years is a de-embrittlement process, and further Table 2
heat treating at 498 'C is a de-embrittlement process for the matrix after 12 years. Ref. [4] and this paper's experimental results ( t h e matrix from Hydrogenation reactor after 1 2 year) and theoretical calculation according to the theory of this paper are shown in Table 2.
The calculated and experimental results
Test material
Heat treating scheme
VTr54. 2/ ~cC4l
AVTr54. 2/ .C
Segregation of P element/ %
Difference of senrenation/ ?4
T h e matrix from hydrogenation reactor after 5 aC41
Matrix of received 593 'C X 1 h 538 'C X15 h
-51. 6 -52.6 -59.6
-
-1 -8
14.220 11.614 8. 515
-2.606 -5. 705
T h e matrix from hydrogenation reactor after 12 a
Matrix of received 524 C X 28 h 524 C X60 h
-41. 4 - 52 -56. 7
-10.6 -15.3
16.650 10.357 8. 676
-6.293 -7.974
T h e grain boundary segregation in Table 2 is result of theoretical calculation by this paper's theory, others are experimental results. As is known, the critical embrittlement temperature for the matrix after 5 and 1 2 years are respectively 513 and 498 'C. Further heat treating of 593 "Cfor 1 h and 538 O C for 15 h for the matrix after 5 years are all de-embrittlement processes and further heat treating of 524 "C for 28 h and 524 'Cfor 60 h for the matrix after 12 years are all deembrittlement processes. The grain boundary desegregation function is used to calculate concentration of grain boundary after further heat treating. T h e concentration (in atomic percent) of grain boundary of impurity element P after processes of 593 'C for 1 h and 538 "Cfor 15 h are respectively 11. 614% and 8. 515%, for the same reason the concentration after processes of 524 O C for 28 h and 524 'C for 60 h are respectively 10. 357% and 8. 676%. T h e theoretical calculation and experimental results make out that the further embrittlement or de-embrittlement mechanism can be explained qualitatively and quantitatively with combination of theory of grain boundary desegregation in this paper and constant embrittlement curve.
5
Vol. 18
Journal of Iron and Steel Research, International
50
Conclusions
1) There is a concentration of equilibrium grain boundary segregation corresponding to each temperature in the range of temper embrittlement temperature, and the P concentration on grain boundary will decrease and segregation rate will increase with the increase of temperature. 2 ) There is a critical embrittlement temperature corresponding to every embrittlement degree, further
-
/
/
heat treating above or equal to the critical embrittlement temperature is a de-embrittlement process ; otherwise, it will be an embrittlement process. 3 ) T h e critical embrittlement temperature will decrease as the embrittlement increases; the critical embrittlement temperature can be calculated by Fig. 2 and Eqn. (1). 4) Step-cooling test can not always be a process of acceleration embrittlement for material, and some heat treating stages or all stages of step-cooling test will become de-embrittlement process as embrittlement reaches certain degree. 5) T h e further embrittlement and de-embrittlement mechanism of material can be explained qualitatively and quantitatively by combining the theory of grain boundary desegregation with constant embrittlement curve. References: Christien F, Le Gall R, Saindrenan G. Phosphorus Grain Boundary Segregation in Steel 17-4PH [J]. Scripta Materillia, 2003, 48: 11. The Japan Steel Works. Technical Service for Safe Operation of High-pressure Vessel [R]. Tokyo: The Japan Steel Works, 1995. Doucet Anne B. Effects of Service Exposure of 2 1/4 Cr-1Mo Pressure Vessel Steel in A Heavy Oil Hydrocracking [Cl // Firness-in-Service and Decisions for Petroleum and Chemicial Equipment. PVP-Vol. 315. New York: ASME, 1995: 407. TAN Jin-zhu, HUANG Wen-long. Application of Step-Cooling Test Method for Temper Embrittlement of 2. 25Cr-1Mo Steel [J]. Journal of Nanjing University of Chemical Technology, 1998, ZO(Supp1ement): 17 (in Chinese). Masaaki Katsumata, Hisashi Takada, Hiromichi Hrano, et al, Temper Embrittlement in Pressure Vessel Steel [J]. 1981, 19 (3): 120. Scott T E. Application of 2 1/4Cr-Mo Steel for Thick-Wall Pressure Vessels [MI. Baltimore: American Society for Tes-
Issue 3
[7]
[8]
[9]
Mechanism of Embrittlement and De-Embrittlement for 2. 2 5 C r l M o Steel
ting and Materials, 1982. Viswanathan R , Jaffee R I. 2 1/4Cr-lMo Steels for Coal Conversion Pressure Vessels [J]. ASME Journal of Engineering Material and Technology, 1982, 104: 220. McMahon C J , Genter D H , Ucisk A H. An Investigation of Grain Size and Hardness in Temper-Embrittled 2 1 / 4 C r l M o Steel [J]. ASME Journal of Engineering Material and Technology, 1984, 106: 66. Mclean D. Grain Boundaries in Metals [D]. London: Oxford University Press, 1957.
[lo]
[ 111
[12]
-
51
XU Ting-dong, CHENG Bu-yuan. Kinetics of Non-Equilibriurn Grain-boundary Segregation [ J]. Progress in Materials Science, 2004, 49: 109. ZHANG Xi-liang, ZHOU Chang-yu. Study on Further Embrittlement Mechanism Based on the Theory of Equilibrium Grain Boundary Segregation [J]. Transactions of Materials and Heat Treatment, 2009, 30(4): 194. Ogura T. A Method for Evaluation of the Mount of GrainBoundary Segregation During Quenching [ J]. Trans Japan Inst Met, 1981, 22(2): 109.