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The effect of ageing and creep on the impact properties of type 316 weld metals
Materials Science and Engineering, A 132 ( 1991 ) L 1-L4
kI
Letter
The effect of ageing and creep on the impact properties of type 316 weld metals J. K. L. Lai Department of Applied Science, City Polytechnic of Hong Kong, Tat Chee Avenue, Kowloon (Hong Kong)
(Received October 18, 1990; in revised form August 20, 1990)
Abstract Hounsfield impact tests have been carried out on specimens machined from the gauge lengths and grips of type 316 weld metal creep-ruptured test pieces. The room temperature impact strength of type 316 weld metals is reduced by post-weld heat treatment at 800°C, whereas solution heat treatment at 1050°C greatly improves the impact strength relative to the aswelded condition. The room temperature impact strength is generally reduced by stress-free ageing at 600 °C. Stressed ageing causes further reduction in impact strength relative to specimens which have undergone unstressed ageing. This effect is probably caused by the enhanced rate of precipitation in the gauge lengths of the creep specimens.
1. Introduction
Stainless steel weld metals with chemical compositions similar to AISI type 316 steel are extensively used within the power-generating industry. Components with austenitic weld metals have in the past been used either in the as-welded condition or in the 1050 °C solution-treated condition. Lai and Haigh [1] studied the transformation characteristics of 6-ferrite in type 316 weld metal and suggested an intermediate heat treatment at a temperature of approximately 800 °C. It has the following advantages: (1) it can provide adequate stress relief; (2) it can improve the uniaxial creep ductility of the weld metal without 0921-5093/91/$3.50
a significant loss of rupture strength; (3) it can improve the high temperature creep crack growth resistance; (4) it is easier (and therefore cheaper) to perform than a 1050 °C solution treatment and causes less distortion in the component. To assess the long-term effects of such a treatment, creep rupture tests were performed to compare the properties of four proprietary brands of type 316 weld metal in the as-welded condition, after stress relieving at 800°C and after solution treatment at 1050 °C. A potential disadvantage of the intermediate heat treatment at approximately 800 °C is that the 6-ferrite transforms into M23C 6 carbide and intermetallic phases such as a and Z at this temperature. This results in a severe drop in the room temperature Charpy impact energy. For instance, in one case the room temperature Charpy impact energy is reduced from 90 J in the as-welded condition to 17 J when heat treated for 10 h at 800 °C [2]. A drop in impact strength of this magnitude may or may not be important depending on the component involved. However, there may be further degradation due to longterm high temperature service and this effect needs to be investigated before the intermediate heat treatment can be accepted. The objective of the present investigation is to determine the effect of ageing and creep at 600 °C, with and without stress, on the microstructure and impact properties of type 316 weld metals in various initial conditions; as-welded, stress relieved at 800 °C and solution treated at 1050 °C. 2. Experimental procedure 2.1. Materials 48 plate-to-plate butt welds, each approximately 760 m m × 150 m m x 20 mm in size were made in four batches of 12 using four types of proprietary electrodes (A, B, C and D), the chemical compositions of which are listed in Table 1. The weld metal was deposited using the manual metal arc process along a single U groove 76 cm long. A total of 13 weld runs were used to © Elsevier Sequoia/Printed in The NetherLands
L2 TABLE 1 Chemical compositions (wt.%) of weld and parent materials Element
Weld metals A
C Cr Co Cu Mn Mo Ni Nb P Si S V N B
Parent metal B
0.065 17.0 0.04 0.06 1.9 2.24 9.4 <0.1 0.017 0.42 0.006 0.076 0.045 0.004
C 0.69 17.3 0.03 0.06 1.89 2.00 9.4 <0.1 0.017 0.40 0.005 0.076 0.037 0.0003
complete the weld. No preheat was applied before welding. The bulk of the weld metal was deposited with 5 mm electrodes at an a.c. current of 160 M. Smaller electrodes were used near the root of the weld. The electrode was kept at about 80 ° to the line of travel during welding. Four joints of each batch were solution treated for 2 h at 1050°C, air cooled, and four were retained in the as-welded condition. The remaining four were heat treated for 10 h at 800 °C at a heating and cooling rate of 25 °C h-1, chosen to simulate the possible heating and cooling rates achievable on large structures. 2.2. Creep rupture tests Creep rupture specimens with a cylindrical gauge length of 39 mm and a diameter of 11 mm were machined from the weldments. The specimen orientation was parallel to the weld run, i.e. they were 100% weld metal in the gauge lengths as well as the grips. All specimens were creep tested to rupture at 600 °C and a nominal stress of 201 MPa. The rupture lives of the specimens are given in Table 2.
2.3. Hounsfield impact specimens It was decided to perform Hounsfield impact tests on the crept specimens because the latter were too small for standard size Charpy specimens to be obtained. The Hounsfield specimens were cylinders 7.94 mm in diameter and 44.45 mm in length. In order to produce these specimens from the gauge lengths and grips of the crept specimens, extension blocks were welded on by electron-beam welding.
D 0.033 18.1 0.03 0.05 0.9 2.8 11.3 <0.1 0.016 0.45 0.010 0.04 0.046 < 0.0001
0.055 18.1 0.06 0.06 1.9 1.7 9.4 <0.1 0.016 0.27 0.007 0.055 0.037 0.0006
0.045 17.37 0.21 0.14 1.45 2.52 11.85 <0.1 0.026 0.49 0.018 0.026 0.028 0.0027
TABLE 2 Creep rupture lives at 600 °C, 201 MPa Weld metal type
Pre-crept condition
Rupture life (h)
A
AW SR ST
19260 8433 13791
B
AW SR ST
7860 1729 3112
C
AW SR ST
228 275 367
D
AW SR ST
914 991 552
AW, as-welded; SR, stress relieved at 800°C for 10 h; ST, solution treated at 1050 *C for 2 h and air cooled.
Small cylinders 10 mm in diameter and 20 mm long were machined separately from the gauge length and grip of each specimen. These were carefully labelled and clamped in batches of five or six on to rectangular extension blocks of dimensions 12 mm x 20 mm x 100 mm. The cylinders were then welded to the extension blocks by electron-beam welding. Individual Hounsfield specimens were machined from the block with the pre-crept or pre-aged material at the centre of the specimen. The details of the specimen geometry are given in Table 3.
2.4. Metallography After impact testing a selected number of specimens were sectioned longitudinally for metallography. A repeat polishing and etching
L3
technique was used to examine cavitation in the pre-crept specimens. The same polishing and etching procedure was applied to the grips of the same specimens for comparison. Bulk extracts of samples from the gauge lengths and grips of the pre-crept specimens were used for X-ray analysis TABLE 3
Dimensions of Hounsfield test piece Length Diameter Notch root depth Notch angle Notch radius
44.45 7.94 5.82 45 ° 0.25
mm mm nun mm
using the quantitative technique developed by Lai and Galbraith [3]. 3. Correlation of conventional Charpy and Hounsfield results The Hounsfield impact results were correlated with standard Charpy results using the various unaged weld metals. The standard Charpy and Hounsfield specimens were machined directly from the plate-to-plate butt welds. Table 4 shows the results of the two types of impact tests. In general, specimens with high Charpy impact values also have high Hounsfield impact values and vice versa.
TABLE 4
4. Results of impact tests
Correlation of conventional Charpy and Hounsfield results (unaged samples) Weld metal
Condition
Charpy(J)
Hounsfield (J)
A
As welded
109
Fault in specimen
800 °C, 10 h 1050 °C, 2 h
78 145
19.4
As welded 800 °C, 10h 1050 °C, 2 h
91 80 151
The results of the Hounsfield impact tests are listed in Tables 4 and 5. Within the scatter obtained by the single specimen tests, the following results are obtained. (a) In the prior to creep condition, the heat treatments at 1050°C for 2 h greatly improves the room temperature impact strength relative to the as-welded condition, whereas the heat treatment at 800°C for 10 h reduces the impact strength (Table 4). (b) In general, stress-free ageing at 600°C reduces the room temperature impact strength. The extent of reduction depends on the type of weld metal, its initial condition and on the ageing time. There are, however, two exceptions. The type C weld metal in the as-welded and 800 °C
machining Not broken ( > 64) B
33.2 30.7
Not broken (>64)
C
As welded 800 °C, 10 h 1050 °C, 2 h
D
As welded 800 °C, 10 h 1050°C, 2 h
73 53 105 90 17 31.8
23.6 16.0 36.1 28.7 7.0 15.1
TABLE 5
Results of Hounsfield impact test on pre-crept samples Weld metal
Condition prior to creep test
Hounsfield impact energy (J)
Grip
Gauge length
A
As welded 800 °C, 10 h 1050 *C, 2 h
17.4 27.7 31.3
8.8 13.4 17.8
B
As welded 800 °C, 10 h 1050 °C, 2 h
21.2 25.9
Not broken ( > 64)
13.4 16.0 16.6
C
As welded 800 °C, 10 h 1050 °C, 2 h
26.7 16.6 31.4
21.7 8.0 22.0
D
As welded 800 °C, 10 h 1050 °C, 2 h
13.9 4.5
58.7 4.0 Fault in specimen machining
Creep condition, 600 °C, 201 MPa.
Not broken ( > 64)
L4 TABLE 6 Correlation of impact strength with weight of precipitates after creep testing at 600 °C, 201 MPa Weld metal
Condition
A
A
B
C
D
Hounsfield impact strength (J)
Total weight (%) of precipitates
As welded grip g.g
17.4 8.8
2.6 (0.34% a) 2.9 ( 1% a)
1050 °C, 2 h grip g.g
31.3 17.8
1 1.43
As welded grip g.g
21.2 13.4
1.63 1.95
31.4 22.0
0.39 0.42
26.7 21.7
0.98 1.15 (0.19% o)
13.9 58.7
3.47 (2% or) 4.45 (3.5% o)
1050 °C, 2h grip g.g As welded grip g.g As welded grip g.g
10 h heat-treated initial conditions showed slight increases in room temperature impact strength after ageing. In view of the short ageing times involved (200-300 h, see Table 2), the difference is probably not significant. The type A weld in the 800°C 10 h heat-treated condition showed a more substantial increase in room temperature impact strength after ageing. The reason for this behaviour is not clear. (c) Ageing with stress reduces the room temperature impact strength relative to that of unstressed ageing. Table 5 shows that the room temperature impact strength of the gauge length of the specimen is consistently below that of its grip. 5. Microstructurai studies
The microstructure in the gauge length of a crept specimen can differ from that of the grip of the same specimen in three ways: (a) creep cavitation; (b) dislocation density and structure, (c) rate of precipitation. Detailed transmission electron microscopy was not carried out to study the dislocation structure because the dislocation structure was modified by the impact tests. However, a search for creep cavities was carried out. This involved repeated etching and polishing with a 10%HCl-methanol
solution and 1/~m diamond paste. The same treatment was applied to the grip and gauge length .of the same creep specimen and they were then examined in an optical microscope (magnification x 1000). No significant creep cavitation was found in the gauge length in the region near the Hounsfield notch. This was probably because the position of the Hounsfield notch was approximately 10 mm from the creep fracture surface. A further search for creep cavities under a scanning electron microscope was also unsuccessful. Quantitative bulk extraction revealed that the total amount of precipitates is consistently greater in the gauge length compared with that of the grip (Table 6). This indicates that the precipitation process in type 316 weld metals is accelerated by creep deformation. The types of precipitates identified are M23C6, Laves phase and o phase. In general, the impact energy decreases as the amount of precipitates increase, and the lower impact energies are associated with the appearance of o phase in the material. This correlation suggests that the reduction in impact energy in the gauge length relative to the grip in the same specimen is primarily due to the enhanced rate of precipitation which is caused by creep in the gauge length. Precipitation introduces weak interfaces and this can be detrimental to the impact strength. 6. Conclusions
( 1 ) Post-weld heat treatment of type 316 weld metals at 800 °C reduces their room temperature impact strength relative to the as-welded condition. Solution treatment at 1050°C greatly improves the room temperature impact strength. (2) In general the room temperature impact strength is reduced by stress-free ageing at 600 °C. (3) Ageing with stress reduces the room temperature impact strength relative to specimens which have undergone unstressed ageing. This is likely to be due to the enhanced rate of precipitation caused by creep deformation. References 1 J. K. L. Lai and J. R. Haigh, Weld J. Res. Suppl., 58 (1979) 1-S. 2 Unpublished work. 3 J. K. L. Lai and I. G. Galbraith, J. Mater. Sci., 15 (1980) 1297.
kI
Letter
The effect of ageing and creep on the impact properties of type 316 weld metals J. K. L. Lai Department of Applied Science, City Polytechnic of Hong Kong, Tat Chee Avenue, Kowloon (Hong Kong)
(Received October 18, 1990; in revised form August 20, 1990)
Abstract Hounsfield impact tests have been carried out on specimens machined from the gauge lengths and grips of type 316 weld metal creep-ruptured test pieces. The room temperature impact strength of type 316 weld metals is reduced by post-weld heat treatment at 800°C, whereas solution heat treatment at 1050°C greatly improves the impact strength relative to the aswelded condition. The room temperature impact strength is generally reduced by stress-free ageing at 600 °C. Stressed ageing causes further reduction in impact strength relative to specimens which have undergone unstressed ageing. This effect is probably caused by the enhanced rate of precipitation in the gauge lengths of the creep specimens.
1. Introduction
Stainless steel weld metals with chemical compositions similar to AISI type 316 steel are extensively used within the power-generating industry. Components with austenitic weld metals have in the past been used either in the as-welded condition or in the 1050 °C solution-treated condition. Lai and Haigh [1] studied the transformation characteristics of 6-ferrite in type 316 weld metal and suggested an intermediate heat treatment at a temperature of approximately 800 °C. It has the following advantages: (1) it can provide adequate stress relief; (2) it can improve the uniaxial creep ductility of the weld metal without 0921-5093/91/$3.50
a significant loss of rupture strength; (3) it can improve the high temperature creep crack growth resistance; (4) it is easier (and therefore cheaper) to perform than a 1050 °C solution treatment and causes less distortion in the component. To assess the long-term effects of such a treatment, creep rupture tests were performed to compare the properties of four proprietary brands of type 316 weld metal in the as-welded condition, after stress relieving at 800°C and after solution treatment at 1050 °C. A potential disadvantage of the intermediate heat treatment at approximately 800 °C is that the 6-ferrite transforms into M23C 6 carbide and intermetallic phases such as a and Z at this temperature. This results in a severe drop in the room temperature Charpy impact energy. For instance, in one case the room temperature Charpy impact energy is reduced from 90 J in the as-welded condition to 17 J when heat treated for 10 h at 800 °C [2]. A drop in impact strength of this magnitude may or may not be important depending on the component involved. However, there may be further degradation due to longterm high temperature service and this effect needs to be investigated before the intermediate heat treatment can be accepted. The objective of the present investigation is to determine the effect of ageing and creep at 600 °C, with and without stress, on the microstructure and impact properties of type 316 weld metals in various initial conditions; as-welded, stress relieved at 800 °C and solution treated at 1050 °C. 2. Experimental procedure 2.1. Materials 48 plate-to-plate butt welds, each approximately 760 m m × 150 m m x 20 mm in size were made in four batches of 12 using four types of proprietary electrodes (A, B, C and D), the chemical compositions of which are listed in Table 1. The weld metal was deposited using the manual metal arc process along a single U groove 76 cm long. A total of 13 weld runs were used to © Elsevier Sequoia/Printed in The NetherLands
L2 TABLE 1 Chemical compositions (wt.%) of weld and parent materials Element
Weld metals A
C Cr Co Cu Mn Mo Ni Nb P Si S V N B
Parent metal B
0.065 17.0 0.04 0.06 1.9 2.24 9.4 <0.1 0.017 0.42 0.006 0.076 0.045 0.004
C 0.69 17.3 0.03 0.06 1.89 2.00 9.4 <0.1 0.017 0.40 0.005 0.076 0.037 0.0003
complete the weld. No preheat was applied before welding. The bulk of the weld metal was deposited with 5 mm electrodes at an a.c. current of 160 M. Smaller electrodes were used near the root of the weld. The electrode was kept at about 80 ° to the line of travel during welding. Four joints of each batch were solution treated for 2 h at 1050°C, air cooled, and four were retained in the as-welded condition. The remaining four were heat treated for 10 h at 800 °C at a heating and cooling rate of 25 °C h-1, chosen to simulate the possible heating and cooling rates achievable on large structures. 2.2. Creep rupture tests Creep rupture specimens with a cylindrical gauge length of 39 mm and a diameter of 11 mm were machined from the weldments. The specimen orientation was parallel to the weld run, i.e. they were 100% weld metal in the gauge lengths as well as the grips. All specimens were creep tested to rupture at 600 °C and a nominal stress of 201 MPa. The rupture lives of the specimens are given in Table 2.
2.3. Hounsfield impact specimens It was decided to perform Hounsfield impact tests on the crept specimens because the latter were too small for standard size Charpy specimens to be obtained. The Hounsfield specimens were cylinders 7.94 mm in diameter and 44.45 mm in length. In order to produce these specimens from the gauge lengths and grips of the crept specimens, extension blocks were welded on by electron-beam welding.
D 0.033 18.1 0.03 0.05 0.9 2.8 11.3 <0.1 0.016 0.45 0.010 0.04 0.046 < 0.0001
0.055 18.1 0.06 0.06 1.9 1.7 9.4 <0.1 0.016 0.27 0.007 0.055 0.037 0.0006
0.045 17.37 0.21 0.14 1.45 2.52 11.85 <0.1 0.026 0.49 0.018 0.026 0.028 0.0027
TABLE 2 Creep rupture lives at 600 °C, 201 MPa Weld metal type
Pre-crept condition
Rupture life (h)
A
AW SR ST
19260 8433 13791
B
AW SR ST
7860 1729 3112
C
AW SR ST
228 275 367
D
AW SR ST
914 991 552
AW, as-welded; SR, stress relieved at 800°C for 10 h; ST, solution treated at 1050 *C for 2 h and air cooled.
Small cylinders 10 mm in diameter and 20 mm long were machined separately from the gauge length and grip of each specimen. These were carefully labelled and clamped in batches of five or six on to rectangular extension blocks of dimensions 12 mm x 20 mm x 100 mm. The cylinders were then welded to the extension blocks by electron-beam welding. Individual Hounsfield specimens were machined from the block with the pre-crept or pre-aged material at the centre of the specimen. The details of the specimen geometry are given in Table 3.
2.4. Metallography After impact testing a selected number of specimens were sectioned longitudinally for metallography. A repeat polishing and etching
L3
technique was used to examine cavitation in the pre-crept specimens. The same polishing and etching procedure was applied to the grips of the same specimens for comparison. Bulk extracts of samples from the gauge lengths and grips of the pre-crept specimens were used for X-ray analysis TABLE 3
Dimensions of Hounsfield test piece Length Diameter Notch root depth Notch angle Notch radius
44.45 7.94 5.82 45 ° 0.25
mm mm nun mm
using the quantitative technique developed by Lai and Galbraith [3]. 3. Correlation of conventional Charpy and Hounsfield results The Hounsfield impact results were correlated with standard Charpy results using the various unaged weld metals. The standard Charpy and Hounsfield specimens were machined directly from the plate-to-plate butt welds. Table 4 shows the results of the two types of impact tests. In general, specimens with high Charpy impact values also have high Hounsfield impact values and vice versa.
TABLE 4
4. Results of impact tests
Correlation of conventional Charpy and Hounsfield results (unaged samples) Weld metal
Condition
Charpy(J)
Hounsfield (J)
A
As welded
109
Fault in specimen
800 °C, 10 h 1050 °C, 2 h
78 145
19.4
As welded 800 °C, 10h 1050 °C, 2 h
91 80 151
The results of the Hounsfield impact tests are listed in Tables 4 and 5. Within the scatter obtained by the single specimen tests, the following results are obtained. (a) In the prior to creep condition, the heat treatments at 1050°C for 2 h greatly improves the room temperature impact strength relative to the as-welded condition, whereas the heat treatment at 800°C for 10 h reduces the impact strength (Table 4). (b) In general, stress-free ageing at 600°C reduces the room temperature impact strength. The extent of reduction depends on the type of weld metal, its initial condition and on the ageing time. There are, however, two exceptions. The type C weld metal in the as-welded and 800 °C
machining Not broken ( > 64) B
33.2 30.7
Not broken (>64)
C
As welded 800 °C, 10 h 1050 °C, 2 h
D
As welded 800 °C, 10 h 1050°C, 2 h
73 53 105 90 17 31.8
23.6 16.0 36.1 28.7 7.0 15.1
TABLE 5
Results of Hounsfield impact test on pre-crept samples Weld metal
Condition prior to creep test
Hounsfield impact energy (J)
Grip
Gauge length
A
As welded 800 °C, 10 h 1050 *C, 2 h
17.4 27.7 31.3
8.8 13.4 17.8
B
As welded 800 °C, 10 h 1050 °C, 2 h
21.2 25.9
Not broken ( > 64)
13.4 16.0 16.6
C
As welded 800 °C, 10 h 1050 °C, 2 h
26.7 16.6 31.4
21.7 8.0 22.0
D
As welded 800 °C, 10 h 1050 °C, 2 h
13.9 4.5
58.7 4.0 Fault in specimen machining
Creep condition, 600 °C, 201 MPa.
Not broken ( > 64)
L4 TABLE 6 Correlation of impact strength with weight of precipitates after creep testing at 600 °C, 201 MPa Weld metal
Condition
A
A
B
C
D
Hounsfield impact strength (J)
Total weight (%) of precipitates
As welded grip g.g
17.4 8.8
2.6 (0.34% a) 2.9 ( 1% a)
1050 °C, 2 h grip g.g
31.3 17.8
1 1.43
As welded grip g.g
21.2 13.4
1.63 1.95
31.4 22.0
0.39 0.42
26.7 21.7
0.98 1.15 (0.19% o)
13.9 58.7
3.47 (2% or) 4.45 (3.5% o)
1050 °C, 2h grip g.g As welded grip g.g As welded grip g.g
10 h heat-treated initial conditions showed slight increases in room temperature impact strength after ageing. In view of the short ageing times involved (200-300 h, see Table 2), the difference is probably not significant. The type A weld in the 800°C 10 h heat-treated condition showed a more substantial increase in room temperature impact strength after ageing. The reason for this behaviour is not clear. (c) Ageing with stress reduces the room temperature impact strength relative to that of unstressed ageing. Table 5 shows that the room temperature impact strength of the gauge length of the specimen is consistently below that of its grip. 5. Microstructurai studies
The microstructure in the gauge length of a crept specimen can differ from that of the grip of the same specimen in three ways: (a) creep cavitation; (b) dislocation density and structure, (c) rate of precipitation. Detailed transmission electron microscopy was not carried out to study the dislocation structure because the dislocation structure was modified by the impact tests. However, a search for creep cavities was carried out. This involved repeated etching and polishing with a 10%HCl-methanol
solution and 1/~m diamond paste. The same treatment was applied to the grip and gauge length .of the same creep specimen and they were then examined in an optical microscope (magnification x 1000). No significant creep cavitation was found in the gauge length in the region near the Hounsfield notch. This was probably because the position of the Hounsfield notch was approximately 10 mm from the creep fracture surface. A further search for creep cavities under a scanning electron microscope was also unsuccessful. Quantitative bulk extraction revealed that the total amount of precipitates is consistently greater in the gauge length compared with that of the grip (Table 6). This indicates that the precipitation process in type 316 weld metals is accelerated by creep deformation. The types of precipitates identified are M23C6, Laves phase and o phase. In general, the impact energy decreases as the amount of precipitates increase, and the lower impact energies are associated with the appearance of o phase in the material. This correlation suggests that the reduction in impact energy in the gauge length relative to the grip in the same specimen is primarily due to the enhanced rate of precipitation which is caused by creep in the gauge length. Precipitation introduces weak interfaces and this can be detrimental to the impact strength. 6. Conclusions
( 1 ) Post-weld heat treatment of type 316 weld metals at 800 °C reduces their room temperature impact strength relative to the as-welded condition. Solution treatment at 1050°C greatly improves the room temperature impact strength. (2) In general the room temperature impact strength is reduced by stress-free ageing at 600 °C. (3) Ageing with stress reduces the room temperature impact strength relative to specimens which have undergone unstressed ageing. This is likely to be due to the enhanced rate of precipitation caused by creep deformation. References 1 J. K. L. Lai and J. R. Haigh, Weld J. Res. Suppl., 58 (1979) 1-S. 2 Unpublished work. 3 J. K. L. Lai and I. G. Galbraith, J. Mater. Sci., 15 (1980) 1297.