# Patent

## Title
Site conditions thick-wall P92 pipe local heat treatment method

## Abstract

The present invention published a large walled field conditions P92 long pipeline local heat treatment processes. Firstly, according to the specific conditions for the on-site grade heat-resistant steel (P92 pipe), combining of the finite element method and test, the Heated Band (HB) width and Gradient Control Band (GCB) was calculated. The pipe was then locally heated using the flexible ceramic resistive heater. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h. The pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h). Lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device. Finally, the local heat treatment process can be obtained. The present invention can effectively decrease the temperature difference between the outer and inner wall, the residual stress in the welded joints. Thus the service properties can be guaranteed.

## Background
TECHNICAL FIELD
The present invention relates to a high-grade heat-resistant steels local heat treatment process, particularly, it relates to a large walled field conditions P92 long pipeline local heat treatment processes.
BACKGROUND ART
Currently, the electricity industry is facing unprecedented challenges. To accelerate the pace of development of electric power, it is faced with the power to accelerate the pace of structural adjustment. The most realistic and feasible way is to accelerate the construction of ultra-supercritical units. Supercritical pressure parameter is the effective use of thermal energy in a new technology. Through its steam pressure and temperature increased to some extent, to improve the thermal efficiency of the unit. To ensure that the unit at a higher temperature and pressure stable operation, it is necessary to use a higher temperature strength of steel, or the wall thickness of the member must be increased exponentially to meet service requirements. Currently the temperature of P92 steel used in the boiler main steam was raised to 610° C., reheat temperatures up to 625° C. P92 steel has been widely used in the USC units set box and main steam piping and other components. P92 steel was used widely in China without of foreign experience.
On economic and technological grounds, P92 pipes are usually fabricated from separate pieces by welding girth joints. However, after welding process the joints are marked by residual welding stress and nonuniform microstructures and properties, which will impair the operational reliability of components. Hence, PWHT should be instantly carried out. For large-sized vessels and apparatuses, placing the entire component in a furnace or oven is impractical. In such case, the weld metal, heat-affected zone (HAZ), and a limited volume of base metal adjacent to the weld might be subjected to heating using the local heat treatment (LHT) technology. Compared with the overall heat treatment, it saves fuel and material resources substantially. The primary benefit of local PWHT is relaxation of residual stresses. Other significant benefits are reducing susceptibility to brittle fracture and improving notch toughness of welded joints. Heated band consists of the soak band plus adjacent base metal over which the heat source is applied to. Because of the inherent radial temperature gradient, the band should be large enough to achieve the required temperature and limit the induced stresses within the soak band. ASME Boiler and Pressure Vessel Code, Rules for Construction of Pressure Vessels: Alternative Rules, Section VIII, Division 2 As a minimum, the soak band shall contain the weld, heat affected zone, and a portion of base metal adjacent to the weld being heat treated. The minimum width of this volume is the widest width of weld plus the nominal thickness defined in paragraph 6.4.2.7 or 50 mm (2 in.), whichever is less, on each side or end of the weld. The equation of Heated Band (HB) width and Gradient Control Band (GCB) width has not provided. In summary, various standards or codes have different methods to define the HB width and GCB width. Hence it may bring confusion in engineering practice.
Results of the literature search indicate that no study has been conducted to take the field conditions during local PWHT of P92 welded joints into consideration. Under field conditions, effect of local PWHT may be influenced by air convection situation, material types, insulation condition, et al. and determination of the local PWHT criterion for ASME P92 pipes becomes a more complicated problem.
Currently many welding joints cracks of P92 pipe was observed in many service after LHT. It should be inappropriate for local heat treatment process. Therefore, it is significant to choose the right LHT technology for the protection of local heat treatment P92 safe operation of pipelines.
SUMMARY OF THE INVENTION
The object of the present invention is to overcome the deficiencies of the prior art. According to the specific conditions for the on-site grade heat-resistant steel (P92 pipe), combining of the finite element method and test, a local heat treatment process was obtained. Using this process, temperature difference between the inside and outside wall temperature process is within 25° C. or less. And residual stress relieving after heat treatment was good.
Technical object of the invention is realized by the following technical solutions
Site conditions walled pipe P92 local heat treatment method was implemented, according to the following steps
Step 1, Calculating the Heated Band (HB) width and Gradient Control Band (GCB)

HB=15.6×√{square root over (Rt)}−3.556×R−372

Where R is pipe inside radius, t is pipe wall thickness

GCB=1.4×HB+226

Step 2, In accordance with the P92 steel pipe welding process for welding.
Step 3, after welding, when the P92 pipe was cool down to 120-150° C. using the calculation results in Step 1 the respective heating device, insulation device were installed outside P92 pipes and the thermocouple was arranged. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h.
Step 4, the pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h)
Step 5, lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device.
In the technical solution of the present invention, the flexible ceramic resistive heater (e.g. WDK-6360 type flexible ceramic resistance heater) was used to heat the steel pipe, and a thermocouple temperature, cotton insulation and a resistance heater were used to control the heating rate and cooling speed control, in order to meet the needs of the heat treatment method.
After P92 pipe on site welding, the X-ray method was employed to measure the residual stress at different distances from the center of weld. After the completion of local heat treatment, re-use the same approach to test the residual stress; contrast between the two can be analyzed to obtain the effect of local heat treatment on residual stress.
During monitoring of the temperature, three temperature zones are set in the invention: 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall. The monitoring thermocouples mentioned above were used to measure the temperature during the heat treatment in the axial direction gradient.
The invention relates to the field conditions presents a thick-walled pipe P92 local heat treatment method to achieve better results. To this end, the method using experimental and numerical simulation method, a topical heat treatment process for thick-walled pipes P92 under scene conditions has been obtained, ie how to determine the HB and GCB using the pipe diameter and wall thickness and get a good effect of heat treatment (better temperature distribution and elimination of residual stress effects). Compared with the prior art, the present invention can achieve an effective reduction PWHT P92 pipe's outer wall during constant temperature (less than 25° C.) and heat treatment of the residual stress of the welded joint, to ensure the use of materials properties; Additionally, the method belong to different thickness and diameter specifications of the general specification, expanding the scope of application of local heat treatment process.



BRIEF DESCRIPTION OF THE INVENTION
FIG. 1 The stress measuring points specimen diagram. The test point interval in the weld zone is 3 mm (the 3 points near heat affected zone), in the heat affected zone is 1.5 mm, in the base material is 3 mm spacing for the first 3 points (close to the heat-affected zone three points), the outermost is 50 mm from a fusion line.
FIG. 2 Heating band and the insulation layer arrangement
FIG. 3 Thermocouple installation location and distribution diagram. 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall.



SPECIFIC EMBODIMENTS
Below with reference to specific embodiments described in more detail the technical solution of the present invention, in embodiments of the present application carried out under the premise, gives a detailed embodiments and procedures, the protection scope of the present invention is not limited to the following embodiments case.
EXAMPLE 1
First of all, the P92 pipes with inner diameter of 538 mm and thickness of 94.5 mm were welded according to the welding procedure. Before welding, the monitoring thermcouples was install at the inner wall of the pipe.
The HB and GCB were calculated (HB=1230 mm, GCB=1950 mm). Then the Heater and insulation cotton Specifications were customized. WDK-6360-type flexible ceramic resistance heater was chosen.
The X-ray method was employed to measure the residual stress at different distances from the center of weld. The test point interval in the weld zone is 3 mm (the 3 points near heat affected zone), in the heat affected zone is 1.5 mm, in the base material is 3 mm spacing for the first 3 points (close to the heat-affected zone three points), the outermost is 50 mm from a fusion line (See FIG. 1).
After welding, the residual stress measurement was conducted. According to the diagram shown in FIG. 2, the heating and heat-retaining devices were installed to confirm the GCB and HB. During monitoring of the temperature, three temperature zones are set in the invention: 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall. The monitoring thermocouples mentioned above were used to measure the temperature during the heat treatment in the axial direction gradient (See FIG. 3).
Then the local heat treatment process was conducted on the welded pipe. After welding, when the P92 pipe was cool down to 120-150° C., the respective heating device, insulation device were installed outside P92 pipes and the thermocouple was arranged. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h.
The pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h).
Lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device.
After local heat treatment, the residual stress measurement was conducted using X-ray method (iXRD Portable residual stress measuring instrument).
Measuring temperature difference at different distances from the weld during the constant temperature between the inner wall (12 o'clock position), the results are shown as follows













Error between average temperature zone



(50 mm from weld edge) and inner weld (° C.)










Time
Outer wall #9
Inner wall#17
Temperature Difference













765° C.*1 h
748
733
15


765° C.*2 h
752
735
17


765° C.*3 h
754
740
14


765° C.*4 h
755
742
13


765° C.*5 h
755
743
12


765° C.*6 h
756
745
11


765° C.*7 h
755
744
11


765° C.*8 h
755
745
10









The residual stress before and after local heat treatment was measured and the results were shown as follows.














Before LHT
After LHT















Y

Y




X
(Circum-
X
(Circum-


Location
No.
(Axial)
ferential)
(Axial)
ferential)















Base metal
20
112
76
74
13



19
198
191
79
10



18
158
200
75
20



17
108
188
14
11


HAZ
16
168
200
36
47



15
139
105
−36
−14



14
171
119
20
−5


Weld metal
13
145
99
51
18



12
118
188
107
23



11
139
80
113
76



10
125
103
118
98



9
117
146
44
84



8
114
96
43
39


HAZ
7
125
74
61
57



6
129
146
72
41



5
134
187
80
18


Base Metal
4
253
179
37
35



3
239
164
65
55



2
200
124
87
68



1
144
77
62
5









EXAMPLE 2
First of all, the P92 pipes with inner diameter of 538 mm and thickness of 91 mm were welded according to the welding procedure. Before welding, the monitoring thermcouples was install at the inner wall of the pipe.
The HB and GCB were calculated (HB=1167 mm, GCB=1859 mm). Then the Heater and insulation cotton Specifications were customized. WDK-6360-type flexible ceramic resistance heater was chosen.
The X-ray method was employed to measure the residual stress at different distances from the center of weld. The test point interval in the weld zone is 3 mm (the 3 points near heat affected zone), in the heat affected zone is 1.5 mm, in the base material is 3 mm spacing for the first 3 points (close to the heat-affected zone three points), the outermost is 50 mm from a fusion line (See FIG. 1).
After welding, the residual stress measurement was conducted. According to the diagram shown in FIG. 2, the heating and heat-retaining devices were installed to confirm the GCB and HB. During monitoring of the temperature, three temperature zones are set in the invention: 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall. The monitoring thermocouples mentioned above were used to measure the temperature during the heat treatment in the axial direction gradient (See FIG. 3).
Then the local heat treatment process was conducted on the welded pipe. After welding, when the P92 pipe was cool down to 120-150° C., the respective heating device, insulation device were installed outside P92 pipes and the thermocouple was arranged. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h.
The pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h).
Lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device.
After local heat treatment, the residual stress measurement was conducted using X-ray method (iXRD Portable residual stress measuring instrument).
Measuring temperature difference at different distances from the weld during the constant temperature between the inner wall (12 o'clock position), the results are shown as follows













Error between average temperature zone



(50 mm from weld edge) and inner weld (° C.)










Time
Outer wall #9
Inner wall#17
Temperature Difference













765° C.*1 h
748
735
13


765° C.*2 h
752
740
12


765° C.*3 h
754
741
13


765° C.*4 h
755
745
10


765° C.*5 h
755
748
7


765° C.*6 h
756
743
13


765° C.*7 h
755
746
9


765° C.*8 h
755
747
8









The residual stress before and after local heat treatment was measured and the results were shown as follows.














Before LHT
After LHT















Y

Y




X
(Circum-
X
(Circum-


Location
No.
(Axial)
ferential)
(Axial)
ferential)















Base metal
20
124
83
70
51



19
158
185
78
79



18
148
186
73
142



17
109
193
57
111


HAZ
16
151
208
52
147



15
179
114
82
82



14
173
123
71
95


Weld metal
13
155
142
78
118



12
127
152
100
120



11
135
112
103
76



10
129
99
108
85



9
124
141
75
80



8
124
107
59
78


HAZ
7
127
86
78
51



6
120
149
75
102



5
139
179
81
140


Base Metal
4
199
174
51
135



3
189
173
79
59



2
207
139
80
76



1
157
97
90
40









EXAMPLE 3
First of all, the P92 pipes with inner diameter of 550 mm and thickness of 80 mm were welded according to the welding procedure. Before welding, the monitoring thermcouples was install at the inner wall of the pipe.
The HB and GCB were calculated (HB=944 mm, GCB=1548 mm). Then the Heater and insulation cotton Specifications were customized. WDK-6360-type flexible ceramic resistance heater was chosen.
The X-ray method was employed to measure the residual stress at different distances from the center of weld. The test point interval in the weld zone is 3 mm (the 3 points near heat affected zone), in the heat affected zone is 1.5 mm, in the base material is 3 mm spacing for the first 3 points (close to the heat-affected zone three points), the outermost is 50 mm from a fusion line (See FIG. 1).
After welding, the residual stress measurement was conducted. According to the diagram shown in FIG. 2, the heating and heat-retaining devices were installed to confirm the GCB and HB. During monitoring of the temperature, three temperature zones are set in the invention: 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall. The monitoring thermocouples mentioned above were used to measure the temperature during the heat treatment in the axial direction gradient (See FIG. 3).
Then the local heat treatment process was conducted on the welded pipe. After welding, when the P92 pipe was cool down to 120-150° C., the respective heating device, insulation device were installed outside P92 pipes and the thermocouple was arranged. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h.
The pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h).
Lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device.
After local heat treatment, the residual stress measurement was conducted using X-ray method (iXRD Portable residual stress measuring instrument).
Measuring temperature difference at different distances from the weld during the constant temperature between the inner wall (12 o'clock position), the results are shown as follows













Error between average temperature zone



(50 mm from weld edge) and inner weld (° C.)










Time
Outer wall #9
Inner wall #17
Temperature difference













765° C.*1 h
747
737
10


765° C.*2 h
751
741
10


765° C.*3 h
755
743
12


765° C.*4 h
754
745
9


765° C.*5 h
754
747
7


765° C.*6 h
754
744
10


765° C.*7 h
755
748
7


765° C.*8 h
755
745
10









The residual stress before and after local heat treatment was measured and the results were shown as follows.














Before LHT
After LHT















Y

Y




X
(Circum-
X
(Circum-


Location
No.
(Axial)
ferential)
(Axial)
ferential)















Base metal
20
134
99
78
58



19
160
188
89
91



18
149
178
78
120



17
114
181
81
109


HAZ
16
147
204
64
149



15
165
123
93
91



14
178
129
99
115


Weld metal
13
159
131
85
110



12
134
140
107
104



11
139
109
91
79



10
139
103
113
71



9
207
138
114
83



8
139
108
70
79


HAZ
7
145
79
72
41



6
137
145
78
110



5
141
171
89
132


Base Metal
4
179
140
57
137



3
193
157
74
78



2
205
131
88
81



1
167
106
95
53









EXAMPLE 4
First of all, the P92 pipes with inner diameter of 433 mm and thickness of 72 mm were welded according to the welding procedure. Before welding, the monitoring thermcouples was install at the inner wall of the pipe.
The HB and GCB were calculated (HB=843 mm, GCB=1406 mm). Then the Heater and insulation cotton Specifications were customized. WDK-6360-type flexible ceramic resistance heater was chosen.
The X-ray method was employed to measure the residual stress at different distances from the center of weld. The test point interval in the weld zone is 3 mm (the 3 points near heat affected zone), in the heat affected zone is 1.5 mm, in the base material is 3 mm spacing for the first 3 points (close to the heat-affected zone three points), the outermost is 50 mm from a fusion line (See FIG. 1).
After welding, the residual stress measurement was conducted. According to the diagram shown in FIG. 2, the heating and heat-retaining devices were installed to confirm the GCB and HB. During monitoring of the temperature, three temperature zones are set in the invention: 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall. The monitoring thermocouples mentioned above were used to measure the temperature during the heat treatment in the axial direction gradient (See FIG. 3).
Then the local heat treatment process was conducted on the welded pipe. After welding, when the P92 pipe was cool down to 120-150° C., the respective heating device, insulation device were installed outside P92 pipes and the thermocouple was arranged. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h.
The pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h).
Lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device.
After local heat treatment, the residual stress measurement was conducted using X-ray method (iXRD Portable residual stress measuring instrument).
Measuring temperature difference at different distances from the weld during the constant temperature between the inner wall (12 o'clock position), the results are shown as follows













Error between average temperature zone



(50 mm from weld edge) and inner weld (° C.)










Time
Outer wall #9
Inner wall #17
Temperature difference













765° C.*1 h
745
738
7


765° C.*2 h
750
740
10


765° C.*3 h
755
741
14


765° C.*4 h
753
741
12


765° C.*5 h
755
745
10


765° C.*6 h
754
747
7


765° C.*7 h
753
748
5


765° C.*8 h
755
747
8









The residual stress before and after local heat treatment was measured and the results were shown as follows.














Before LHT
After LHT















Y

Y




X
(Circum-
X
(Circum-


Location
No.
(Axial)
ferential)
(Axial)
ferential)















Base metal
20
132
91
84
58



19
164
171
92
91



18
147
171
81
120



17
118
172
78
114


HAZ
16
149
199
69
102



15
164
134
98
97



14
170
131
91
101


Weld metal
13
160
138
88
95



12
139
142
101
93



11
141
110
97
89



10
147
117
115
84



9
199
134
119
89



8
140
119
79
78


HAZ
7
149
88
77
59



6
143
114
83
78



5
169
165
92
102


Base Metal
4
185
107
61
80



3
179
151
78
90



2
189
134
80
89



1
165
110
97
76









EXAMPLE 5
First of all, the P92 pipes with inner diameter of 355 mm and thickness of 42 mm were welded according to the welding procedure. Before welding, the monitoring thermcouples was install at the inner wall of the pipe.
The HB and GCB were calculated (HB=270 mm, GCB=605 mm). Then the Heater and insulation cotton Specifications were customized. WDK-6360-type flexible ceramic resistance heater was chosen.
The X-ray method was employed to measure the residual stress at different distances from the center of weld. The test point interval in the weld zone is 3 mm (the 3 points near heat affected zone), in the heat affected zone is 1.5 mm, in the base material is 3 mm spacing for the first 3 points (close to the heat-affected zone three points), the outermost is 50 mm from a fusion line (See FIG. 1).
After welding, the residual stress measurement was conducted. According to the diagram shown in FIG. 2, the heating and heat-retaining devices were installed to confirm the GCB and HB. During monitoring of the temperature, three temperature zones are set in the invention: 1#, 2#, 3# are the thermocouples for temperature controlling locating at 1 o'clock, 11 o'clock, 6 o'clock of the center of the inner weld. Others are thermocouples for monitoring. Among them, 5# (12 o'clock) locates at the center of the outer weld. The distance between 9#, 11#, 13#, 15# thermocouples (12 o'clock) and weld edge was 50 mm, 1.5t, 2t, 3t respectively (t was the pipe thickness). The distance between 10#, 12#, 14#, 16# thermocouples (6 o'clock) and weld edge was t, 1.5t, 2t, 3t respectively. 6#, 17#, 19# thermocouples locate at 12 o'clock of inner wall corresponding with 5#, 9# 13# thermocouples at the outer wall. 4#, 18#, 20# thermocouples locate at 6 o'clock of inner wall corresponding with 3#, 10# 14# thermocouples at the outer wall. The monitoring thermocouples mentioned above were used to measure the temperature during the heat treatment in the axial direction gradient (See FIG. 3).
Then the local heat treatment process was conducted on the welded pipe. After welding, when the P92 pipe was cool down to 120-150° C., the respective heating device, insulation device were installed outside P92 pipes and the thermocouple was arranged. When the temperature reaches 80-100° C., the pipe was heated to constant temperature at 80-100° C. for 2 h.
The pipe was heated to 300° C. at the rate of 150° C./h. When the temperature reaches 300° C., rising the temperature to 765° C. at the rate of 80° C./h. Holding time is calculated by the wall thickness (5 min/mm, no less than 4 h).
Lowering the temperature to 300° C. at the rate of 100° C./h and then natural cooling to room temperature, finally removing the heating device.
After local heat treatment, the residual stress measurement was conducted using X-ray method (iXRD Portable residual stress measuring instrument).
Measuring temperature difference at different distances from the weld during the constant temperature between the inner wall (12 o'clock position), the results are shown as follows













Error between average temperature zone



(50 mm from weld edge) and inner weld (° C.)










Time
Outer wall #9
Inner wall #17
Temperature difference













765° C.*1 h
744
736
8


765° C.*2 h
747
739
8


765° C.*3 h
751
740
11


765° C.*4 h
753
740
13


765° C.*5 h
754
742
12


765° C.*6 h
755
746
9


765° C.*7 h
755
747
8


765° C.*8 h
755
749
6









The residual stress before and after local heat treatment was measured and the results were shown as follows.














Before LHT
After LHT















Y

Y




X
(Circum-
X
(Circum-


Location
No.
(Axial)
ferential)
(Axial)
ferential)















Base metal
20
139
90
80
56



19
165
173
97
96



18
149
175
89
114



17
129
177
72
112


HAZ
16
153
184
68
97



15
165
137
99
95



14
173
138
97
102


Weld metal
13
164
139
89
94



12
142
143
100
90



11
143
112
89
83



10
149
119
97
80



9
187
140
111
81



8
142
134
84
74


HAZ
7
140
108
79
62



6
139
101
87
74



5
171
169
91
98


Base Metal
4
187
108
67
84



3
182
153
79
97



2
179
130
87
90



1
169
117
91
77









The test results of the above embodiment shows that the local heat treatment method of the present invention can be widely applied in various diameters and wall thickness of steel pipe P92 conditions at the site of the local heat treatment after welding, and are able to effectively reduce the axial and circumferential directions residual stress, and can achieve effectively reduce the ASME Code P92 pipeline during PWHT temperature inside and outside wall temperature (less than 25° C.).

## Claims

1. A method for local heat treatment of a walled steel pipe following welding, comprising steps:
(1) obtaining a heated band width, HB as a parameter for local heat treatment by carrying out the following calculation:


HB=15.6×√{square root over (Rt)}−3.556×R−372


where R in mm is the inner radius of the walled steel pipe and t in mm is the wall thickness of the walled steel pipe;
(2) obtaining a gradient control band, GCB as another parameter for local heat treatment by carrying out the following calculation:


GCB=1.4×HB+226;


(3) performing a welding process on the walled steel pipe;
(4) after welding, when the temperature of the walled steel pipe is cooled down to 120-150° C., setting up a heating device and an insulation device according to the HB and GCB values calculated in step 1 and, when the temperature of the walled steel pipe is further cooled down to 80-100° C., the heating device is turned on to maintain the temperature of the wall steel pipe at 80-100° C. for 2 hours;
(5) heating the walled steel pipe, at a first heating rate of 150° C. per hour, to reach a temperature of 300° C. then further heating the walled steel pipe, at a second heating rate of 80° C. per hour to reach a temperature of 765° C. and maintaining the temperature at 765° C. for 4 hours or a duration based on the following calculation, whichever is longer,


D=5×t, 


where D is duration in minute and t is the wall thickness in mm;
(6) lowering the temperature to 300° C. at a cooling rate of 100° C. per hour and then turning off the heating device and letting the walled steel pipe naturally cool down to room temperature before removing the heating device and insulating device and completing the local heat treatment.

2. The method for local heat treatment of a walled steel pipe according to claim 1, wherein the heating device is a flexible ceramic resistive heater, which, together with a thermocouple and cotton insulation are used to control the first and second heating rates and the cooling rate.

3. The method for local heat treatment of a walled steel pipe according to claim 1, wherein a plurality of temperature zones are set on the walled steel pipe in and around the area subject to welding, each with a thermocouple for measuring the temperature during local heat treatment.