- Email: [email protected]
Microstructural transformations in stress relieved type 316 stainless steel weld metal
59
METALLOGRAPH Y 13:5%70 (1980)
Microstructural Transformations in Stress Relieved Type 316 Stainless Steel Weld Metal
G. F. S L A T T E R Y ANt) P. O ' R I O R D A N
Risley Nuclear Power Development Laboratory, Risley, Warrington WA3 6AT, England
The heat treatment of Type 316L weld metal containing small amounts of ferrite results in the transformation of the ferrite to X or o- phases after aging for only 3 hr at 850°C. The x-phase transformation is associated with a transitional M2sC6 carbide, whereas the o-phase transforms directly from the ferrite.
Introduction Type 316 austenitic stainless steel is widely used in the power generating industries because of its corrosion resistance and its high-temperature strength. The composition of the steel is adjusted when it is used as weld metal to give a small percentage of ferrite, which counteracts hot cracking on cooling [1-3]. During high temperature aging, carbides and intermetallic phases, such as tr and )¢, precipitate in type 316 steel, with significant effects on mechanical properties [4-6]. Small amounts of ferrite are known to accelerate the formation of o- phase [7-8], and therefore weld metal is more likely to contain the detrimental intermetallic phases than the parent 316 material. It has also been reported that carbide precipitation precedes and is associated with the formation of ophase [5,8,9]. Large components are invariably stress relieved to reduce the residual stresses generated in welding and a typical treatment is 850°C for 6 hr. In the present work, an investigation of the effect of stress-relieving treatments on the microstructure of duplex austenite-ferrite type 316 weld metal has shown that intermetallic phases are produced in the relatively short aging times of stress-relieving heat treatments. © Elsevier North Holland, Inc., 1980
0026-0800/80/010059 + 12501.75
60
G. F. Slatterv and P. O'Riordan
Experimental Procedure Table 1 gives the chemical composition (weight %) of the Type 316L weld metal investigated. The relatively low nickel content in this lowcarbon grade of 316 results in the presence of about 6% ferrite (8) in a semicontinuous "vermicular" distribution [10] (Fig. 1). It should be remembered that in the as-deposited weld metal, the ferrite is a metastable phase and wrought material of the same composition would contain less ferrite [3]. The welds are produced by a manual metal-arc process and samples of the weld were heat treated (stress relieved) at 850°C for 3, 6, and 24 hr and then air cooled. Specimens 10 mm × 10 mm × 5 mm were characterized microstructurally by optical metallography. They were prepared for examination by conventional grinding and diamond polishing and final relief polishing was carried out using y alumina for examination unetched by the Nomarski interference contrast technique on a Reichert Mef2 microscope with interference contrast attachment [11]. Using this technique, ferrite (6), o-, X, and carbide (M2aC6) could all be observed in the unetched condition (Fig. 2). To differentiate between the phases, a selective electrolytic etching procedure has been established using aqueous 10% oxalic acid at 5 V and a stainless steel cathode (Fig. 2). This procedure for the identification of phases is supported by electron microscopy, electron diffraction, and x-ray analysis [11]. It can be seen from Fig. 2 that the phases are attacked at different rates, being outlined and eventually dissolved in the sequence: carbide, tr, X, and ferrite. The carbide interface and not the actual carbides are attacked resulting in the carbides falling out as shown in Fig. 2.
TABLE I Steel Composition Element Carbon Chromium Nickel Molybdenum Manganese Silicon Sulphur Phosphorus
Wt% 0.043 17.1 10.8 2.34 1.22 0.33 0.00 I 0.027
Steel Weld Metal
(a)
61
/
k~ ~
7t (b) FIc. l(a). Vermicular distribution of ferrite in two phase (~ and 30 region of Type 316 steel weld. (b). Vermicular distribution of ferrite with M2aC6 carbide precipitate at the ferrite-austenite interfaces.
.
¢hi
.
.
.
.
.
.
FIG. 2.
.
2
]-§seconds
i
7
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5 lOseconds I0 20secor~ds:20 30seconds
Summary of the etching characteristics of the precipitate phases in type 316 steel•
.
0 ]:2 ~econd ~:2 lsecond
~out
around carbides allowing smaller i particles to drop
chi phase more resistant to
completely after | second
sigma dissolves
ferrite o u t l m ~ via attack ~t oustenite interface
~°
IxJ
Steel Weld Metal
63
Experimental Results AS WELDED Weld structures are notoriously heterogeneous and two quite distinct types of structure were observed, (Fig. 1). In some areas, the structure consisted simply of the vermicular distribution of about 6% ferrite in austenite with no other grain-boundary structure apparent, i.e., there were no austenite or ferrite grain boundaries apart from the austeniteferrite interfaces [Fig. l(a)]. Other regions showed a similar distribution of ferrite, but the ferrite-austenite interface was heavily decorated with carbides [Fig. l(b)]. No explanation can be given for this apparent microsegregation of carbides unless it was caused by local carbon segregation or local heating effects to precipitate the carbides. Metallographic and magnetic measurements indicated an identical amount of ferrite in both types of structure. STRESS RELIEVED The selective etching procedure revealed that the welds that were aged for 3, 6, and 24 hr all contained evidence of M~3C6 carbide, o-, and X phases together with a small amount of untransformed ferrite (Figs. 36). There appeared to be little effect of time at 850°C, all the tranformations seemingly occurring after 3 hr. A major observation, however, was that the areas that initially contained interfacial carbides had transformed to X phase with some untransformed ferrite, but with no evidence for o-. Additionally, the amount of X phase formed was less than that of the original ferrite, which suggested that some of the ferrite had transformed to austenite. Where carbide was not present originally, the ferrite had apparently transformed to cr phase with some untransformed ferrite. The total oplus ferrite produced by heat treatment was again less than that of the original ferrite, indicating that there had been some transformation of ferrite to austenite in the tr precipitation process. It was also of interest to observe that the ferrite in the weld metal adjacent to the parent plate appeared to totally resist any transformation to intermetallic phases even after aging for 24 hr at 850°C (Fig. 7).
Discussion It is well known that the sequence of carbide and intermetallic phase precipitation in Type 316 steel is extremely complex. The presence of
64
(a)
(b)
(c) FIG. 3. M~aC6carbide precipitated within the ferrite. Weld stress relieved at 850°C for: (a). 3 hr. (b). 6 hr. (c). 24 hr.
Steel Weld Metal
65
Q
/% (a)
j~
/
(b)
~%
:~
~ i ¸•
X J
.....
~
~ili!!~i ~ ~~i....
X
X
(c) FIG. 4. X p h a s e formed in association with M23C6 in the ferrite. Weld stress relieved at 85ff'C for: (a) 3 hr. (b). 6 hr. (c). 24 hr. Etched 1-3 sec in 10% oxalic acid.
G. F. Slattery and P. O'Riordan
66
%
N
N
(a)
U
Q
(b)
• 51s,. (c)
FIG. 5. The o- phase formed; in ferrite. Weld stress relieved at 850°C for (a). 3 hr. (b). 6 hr. (c). 24 hr. Left hand side photographs etched for a fraction of a second in 10% oxalic acid. Right hand side photographs etched for 1 sec in 10% oxalic acid.
ferrite, segregation effects, and cold working can all vary the rates of precipitation and the types of precipitate occurring. In the present work, the 8 ferrite transforms to either X or tr phase by apparently different precipitation processes on aging at 850°C. Very little work has been done to examine the mechanisms of inter-
Steel Weld Metal
67
L,
.t&
D
(a)
(b)
::
(c)
~
4
..... ; a t ~
FIG. 6. Presence of untransformed ferrite revealed by etching in 10% oxalic acid for 10 sec. Weld stress relieved at 850 ° for: (a). 3 hr. (b). 6 hr. (c). 24 hr.
68
G. F. Slattery and P. O'Riordan
%
.
PARENT METAL'~~
WELD ................
Fro. 7. Weld stress relieved at 850°C for 24 hr. Region of untransformed ferrite in the weld zone adjacent to the parent metal. Etched for 10 sec in 10% oxalic acid.
metallic phase formation in duplex steels apart from that reported by Beckitt [8] who used transmission electron microscopy to study cr -phase formation in a 25% Cr, 8% Ni steel. Beckitt showed that o- is preceded by M23C6 precipitation at the austenite-ferrite interface which then is associated with a cellular reaction as the ferrite is replaced by an M23Cdaustenite lamellar structure. Fe-Cr o- then precipitates from the remaining ferrite which is presumably enriched with the ferrite-stabilizing element chromium. Beckitt showed no evidence for X phase, but his steel contained no molybdenum which is an essential constituent of X phase [12]. Apparently x-phase precipitation from ferrite has not been investigated in any detail, although the phase is frequently observed in the Mo-containing Type 316 steel. In the present work, optical microscopy has shown the following quite separate transformation reactions in type 316 welds: 1. 6 ferrite --> o-, 2. 6 ferrite + M23C6--~ X + M23C6. In both cases, the ferrite was never fully transformed but the total amount of transformed and untransformed ferrite after aging was always less than the initial amount of ferrite, inferring that some of the ferrite is replaced
Steel Weld Metal
69
by austenite. No microscopical evidence for austenite precipitation within the ferrite was obtained so that the transformation to austenite must have occurred by migration of the austenite-ferrite interface. To explain the different transformation reactions to o- and to X phase, it is n e c e s s a r y to consider why some areas of the weld showed interfacial carbides to p r o m o t e the X reaction. Probably the best explanation is that successive deposits of weld metal produce a local tempering of the underlying material, sufficient to cause precipitation of the M23C6 phase in the heat-affected areas of the weld metal. This will deplete the ferrite of Cr and to a lesser extent Mo (the composition of M23C6 has been shown to be (Cr,6FesMoz)C6 in T y p e 316 steel [5]) which will m e a n that the ferrite composition is different f r o m the ferrite in carbide-free areas and this m a y be the reason why o- forms in some areas and X is o b s e r v e d in others. Current w o r k on T y p e 316L welds containing different amounts of ferrite is showing that the transformation of ferrite is closely related to the composition of the ferrite [13]. Lai and Haigh have recently reported [14] that a stress relief treatment at 600-650°C results in the transformation of ferrite primarily to M~3C6 carbide whereas at 750800°C, X, M23C6, and s o m e o- phase are formed. The present work agrees well with these findings.
Conclusions An examination of stress-relieved T y p e 316L weld metal has been carried out using a selective electrolytic etching technique to identify the precipitating phases. It has been shown that: 1. The 6 ferrite partially transforms to X or o- phases after aging for only 3 hr at 850°C. 2. X phase is p r o d u c e d in the ferrite when the original " a s - w e l d e d " structure contained M23C6 at the austenite-ferrite interfaces. 3. I f there is no M23C6 associated with the austenite-ferrite boundaries the ferrite transforms to o- phase. The authors would like to express their appreciation f o r valuable discussion with Dr. S. R. K e o w n .
References 1. F. C. Hull, Effect of delta-ferrite on the hot cracking of stainless steel. Welding J. 46:339-S (1967). 2. H. Astrom, B. Loberg, B. Bengtsson, and K. E, Easterling. Hot cracking and microsegregation in 18-10 stainless steel welds. Metal Sci. 10(7):225 (1976).
70
G. F . S l a t t e r v a n d P . O ' R i o r d a n
3. T. Boniszewski. Austenitic stainless steels: welding must take account of service conditions. Met. Mater. Dec/Jan, 41 (1978/79). 4. L. K. Poole, Sigma--an unwanted constituent in stainless weld metal. Met. Progr. 65:108 (1954). 5. B. Weiss and R. Stickler. Phase instabilities during high temperature exposure of 316 austenitic stainless steel. Met. Trans. 3:851 (1972). 6. J. K. Lai and A. Wickens. Microstructural changes and variations in creep ductility of three casts of Type 316 stainless steel. Acta Met. 27:217 (1979). 7. H. J. Shirley. Microstructural Characteristics of Acid Corrosion in 18% Cr, 8-14 % Ni, 3% Mo steels. J. Iron Steel Inst. 174:242 (1953). 8. F. R. Beckitt. The formation of sigma-phase from delta-ferrite in a stainless steel. J. Iron Steel Inst. 207:632 (1969). 9. W. E. White and I. LeMay. Metallographic observations on the formation and occurrence of ferrite, sigma phase, and carbides in austenitic stainless steels. Part II: Studies of AISI Type 316 stainless steel. Metallography 3:51 (1970). 10. T. Takalo, N. Suutala, and T. Moisio. Influence of ferrite content on its morphology in some austenitic weld metals. Met. Trans. 7A:1591 (1976). 11. G. F. Slattery, P. O'Riordan, M. E. Lambert, and S. M. Green, (unpublished work). 12. K. W. Andrews, A new intermetallic phase in alloy steels, Nature No. 4180 (Dec):1015 (1949). 13. G. F. Slattery, S. R. Keown, P. O~Riordan and M. E. Lambert, (unpublished work). 14. J. K. Lai and J. R. Haigh. Delta-ferrite transformations in a Type 316 weld metal. Welding Res. (suppl. to Welding J. 58(1):Is, (Jan 1979). Received July 1979.
METALLOGRAPH Y 13:5%70 (1980)
Microstructural Transformations in Stress Relieved Type 316 Stainless Steel Weld Metal
G. F. S L A T T E R Y ANt) P. O ' R I O R D A N
Risley Nuclear Power Development Laboratory, Risley, Warrington WA3 6AT, England
The heat treatment of Type 316L weld metal containing small amounts of ferrite results in the transformation of the ferrite to X or o- phases after aging for only 3 hr at 850°C. The x-phase transformation is associated with a transitional M2sC6 carbide, whereas the o-phase transforms directly from the ferrite.
Introduction Type 316 austenitic stainless steel is widely used in the power generating industries because of its corrosion resistance and its high-temperature strength. The composition of the steel is adjusted when it is used as weld metal to give a small percentage of ferrite, which counteracts hot cracking on cooling [1-3]. During high temperature aging, carbides and intermetallic phases, such as tr and )¢, precipitate in type 316 steel, with significant effects on mechanical properties [4-6]. Small amounts of ferrite are known to accelerate the formation of o- phase [7-8], and therefore weld metal is more likely to contain the detrimental intermetallic phases than the parent 316 material. It has also been reported that carbide precipitation precedes and is associated with the formation of ophase [5,8,9]. Large components are invariably stress relieved to reduce the residual stresses generated in welding and a typical treatment is 850°C for 6 hr. In the present work, an investigation of the effect of stress-relieving treatments on the microstructure of duplex austenite-ferrite type 316 weld metal has shown that intermetallic phases are produced in the relatively short aging times of stress-relieving heat treatments. © Elsevier North Holland, Inc., 1980
0026-0800/80/010059 + 12501.75
60
G. F. Slatterv and P. O'Riordan
Experimental Procedure Table 1 gives the chemical composition (weight %) of the Type 316L weld metal investigated. The relatively low nickel content in this lowcarbon grade of 316 results in the presence of about 6% ferrite (8) in a semicontinuous "vermicular" distribution [10] (Fig. 1). It should be remembered that in the as-deposited weld metal, the ferrite is a metastable phase and wrought material of the same composition would contain less ferrite [3]. The welds are produced by a manual metal-arc process and samples of the weld were heat treated (stress relieved) at 850°C for 3, 6, and 24 hr and then air cooled. Specimens 10 mm × 10 mm × 5 mm were characterized microstructurally by optical metallography. They were prepared for examination by conventional grinding and diamond polishing and final relief polishing was carried out using y alumina for examination unetched by the Nomarski interference contrast technique on a Reichert Mef2 microscope with interference contrast attachment [11]. Using this technique, ferrite (6), o-, X, and carbide (M2aC6) could all be observed in the unetched condition (Fig. 2). To differentiate between the phases, a selective electrolytic etching procedure has been established using aqueous 10% oxalic acid at 5 V and a stainless steel cathode (Fig. 2). This procedure for the identification of phases is supported by electron microscopy, electron diffraction, and x-ray analysis [11]. It can be seen from Fig. 2 that the phases are attacked at different rates, being outlined and eventually dissolved in the sequence: carbide, tr, X, and ferrite. The carbide interface and not the actual carbides are attacked resulting in the carbides falling out as shown in Fig. 2.
TABLE I Steel Composition Element Carbon Chromium Nickel Molybdenum Manganese Silicon Sulphur Phosphorus
Wt% 0.043 17.1 10.8 2.34 1.22 0.33 0.00 I 0.027
Steel Weld Metal
(a)
61
/
k~ ~
7t (b) FIc. l(a). Vermicular distribution of ferrite in two phase (~ and 30 region of Type 316 steel weld. (b). Vermicular distribution of ferrite with M2aC6 carbide precipitate at the ferrite-austenite interfaces.
.
¢hi
.
.
.
.
.
.
FIG. 2.
.
2
]-§seconds
i
7
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5 lOseconds I0 20secor~ds:20 30seconds
Summary of the etching characteristics of the precipitate phases in type 316 steel•
.
0 ]:2 ~econd ~:2 lsecond
~out
around carbides allowing smaller i particles to drop
chi phase more resistant to
completely after | second
sigma dissolves
ferrite o u t l m ~ via attack ~t oustenite interface
~°
IxJ
Steel Weld Metal
63
Experimental Results AS WELDED Weld structures are notoriously heterogeneous and two quite distinct types of structure were observed, (Fig. 1). In some areas, the structure consisted simply of the vermicular distribution of about 6% ferrite in austenite with no other grain-boundary structure apparent, i.e., there were no austenite or ferrite grain boundaries apart from the austeniteferrite interfaces [Fig. l(a)]. Other regions showed a similar distribution of ferrite, but the ferrite-austenite interface was heavily decorated with carbides [Fig. l(b)]. No explanation can be given for this apparent microsegregation of carbides unless it was caused by local carbon segregation or local heating effects to precipitate the carbides. Metallographic and magnetic measurements indicated an identical amount of ferrite in both types of structure. STRESS RELIEVED The selective etching procedure revealed that the welds that were aged for 3, 6, and 24 hr all contained evidence of M~3C6 carbide, o-, and X phases together with a small amount of untransformed ferrite (Figs. 36). There appeared to be little effect of time at 850°C, all the tranformations seemingly occurring after 3 hr. A major observation, however, was that the areas that initially contained interfacial carbides had transformed to X phase with some untransformed ferrite, but with no evidence for o-. Additionally, the amount of X phase formed was less than that of the original ferrite, which suggested that some of the ferrite had transformed to austenite. Where carbide was not present originally, the ferrite had apparently transformed to cr phase with some untransformed ferrite. The total oplus ferrite produced by heat treatment was again less than that of the original ferrite, indicating that there had been some transformation of ferrite to austenite in the tr precipitation process. It was also of interest to observe that the ferrite in the weld metal adjacent to the parent plate appeared to totally resist any transformation to intermetallic phases even after aging for 24 hr at 850°C (Fig. 7).
Discussion It is well known that the sequence of carbide and intermetallic phase precipitation in Type 316 steel is extremely complex. The presence of
64
(a)
(b)
(c) FIG. 3. M~aC6carbide precipitated within the ferrite. Weld stress relieved at 850°C for: (a). 3 hr. (b). 6 hr. (c). 24 hr.
Steel Weld Metal
65
Q
/% (a)
j~
/
(b)
~%
:~
~ i ¸•
X J
.....
~
~ili!!~i ~ ~~i....
X
X
(c) FIG. 4. X p h a s e formed in association with M23C6 in the ferrite. Weld stress relieved at 85ff'C for: (a) 3 hr. (b). 6 hr. (c). 24 hr. Etched 1-3 sec in 10% oxalic acid.
G. F. Slattery and P. O'Riordan
66
%
N
N
(a)
U
Q
(b)
• 51s,. (c)
FIG. 5. The o- phase formed; in ferrite. Weld stress relieved at 850°C for (a). 3 hr. (b). 6 hr. (c). 24 hr. Left hand side photographs etched for a fraction of a second in 10% oxalic acid. Right hand side photographs etched for 1 sec in 10% oxalic acid.
ferrite, segregation effects, and cold working can all vary the rates of precipitation and the types of precipitate occurring. In the present work, the 8 ferrite transforms to either X or tr phase by apparently different precipitation processes on aging at 850°C. Very little work has been done to examine the mechanisms of inter-
Steel Weld Metal
67
L,
.t&
D
(a)
(b)
::
(c)
~
4
..... ; a t ~
FIG. 6. Presence of untransformed ferrite revealed by etching in 10% oxalic acid for 10 sec. Weld stress relieved at 850 ° for: (a). 3 hr. (b). 6 hr. (c). 24 hr.
68
G. F. Slattery and P. O'Riordan
%
.
PARENT METAL'~~
WELD ................
Fro. 7. Weld stress relieved at 850°C for 24 hr. Region of untransformed ferrite in the weld zone adjacent to the parent metal. Etched for 10 sec in 10% oxalic acid.
metallic phase formation in duplex steels apart from that reported by Beckitt [8] who used transmission electron microscopy to study cr -phase formation in a 25% Cr, 8% Ni steel. Beckitt showed that o- is preceded by M23C6 precipitation at the austenite-ferrite interface which then is associated with a cellular reaction as the ferrite is replaced by an M23Cdaustenite lamellar structure. Fe-Cr o- then precipitates from the remaining ferrite which is presumably enriched with the ferrite-stabilizing element chromium. Beckitt showed no evidence for X phase, but his steel contained no molybdenum which is an essential constituent of X phase [12]. Apparently x-phase precipitation from ferrite has not been investigated in any detail, although the phase is frequently observed in the Mo-containing Type 316 steel. In the present work, optical microscopy has shown the following quite separate transformation reactions in type 316 welds: 1. 6 ferrite --> o-, 2. 6 ferrite + M23C6--~ X + M23C6. In both cases, the ferrite was never fully transformed but the total amount of transformed and untransformed ferrite after aging was always less than the initial amount of ferrite, inferring that some of the ferrite is replaced
Steel Weld Metal
69
by austenite. No microscopical evidence for austenite precipitation within the ferrite was obtained so that the transformation to austenite must have occurred by migration of the austenite-ferrite interface. To explain the different transformation reactions to o- and to X phase, it is n e c e s s a r y to consider why some areas of the weld showed interfacial carbides to p r o m o t e the X reaction. Probably the best explanation is that successive deposits of weld metal produce a local tempering of the underlying material, sufficient to cause precipitation of the M23C6 phase in the heat-affected areas of the weld metal. This will deplete the ferrite of Cr and to a lesser extent Mo (the composition of M23C6 has been shown to be (Cr,6FesMoz)C6 in T y p e 316 steel [5]) which will m e a n that the ferrite composition is different f r o m the ferrite in carbide-free areas and this m a y be the reason why o- forms in some areas and X is o b s e r v e d in others. Current w o r k on T y p e 316L welds containing different amounts of ferrite is showing that the transformation of ferrite is closely related to the composition of the ferrite [13]. Lai and Haigh have recently reported [14] that a stress relief treatment at 600-650°C results in the transformation of ferrite primarily to M~3C6 carbide whereas at 750800°C, X, M23C6, and s o m e o- phase are formed. The present work agrees well with these findings.
Conclusions An examination of stress-relieved T y p e 316L weld metal has been carried out using a selective electrolytic etching technique to identify the precipitating phases. It has been shown that: 1. The 6 ferrite partially transforms to X or o- phases after aging for only 3 hr at 850°C. 2. X phase is p r o d u c e d in the ferrite when the original " a s - w e l d e d " structure contained M23C6 at the austenite-ferrite interfaces. 3. I f there is no M23C6 associated with the austenite-ferrite boundaries the ferrite transforms to o- phase. The authors would like to express their appreciation f o r valuable discussion with Dr. S. R. K e o w n .
References 1. F. C. Hull, Effect of delta-ferrite on the hot cracking of stainless steel. Welding J. 46:339-S (1967). 2. H. Astrom, B. Loberg, B. Bengtsson, and K. E, Easterling. Hot cracking and microsegregation in 18-10 stainless steel welds. Metal Sci. 10(7):225 (1976).
70
G. F . S l a t t e r v a n d P . O ' R i o r d a n
3. T. Boniszewski. Austenitic stainless steels: welding must take account of service conditions. Met. Mater. Dec/Jan, 41 (1978/79). 4. L. K. Poole, Sigma--an unwanted constituent in stainless weld metal. Met. Progr. 65:108 (1954). 5. B. Weiss and R. Stickler. Phase instabilities during high temperature exposure of 316 austenitic stainless steel. Met. Trans. 3:851 (1972). 6. J. K. Lai and A. Wickens. Microstructural changes and variations in creep ductility of three casts of Type 316 stainless steel. Acta Met. 27:217 (1979). 7. H. J. Shirley. Microstructural Characteristics of Acid Corrosion in 18% Cr, 8-14 % Ni, 3% Mo steels. J. Iron Steel Inst. 174:242 (1953). 8. F. R. Beckitt. The formation of sigma-phase from delta-ferrite in a stainless steel. J. Iron Steel Inst. 207:632 (1969). 9. W. E. White and I. LeMay. Metallographic observations on the formation and occurrence of ferrite, sigma phase, and carbides in austenitic stainless steels. Part II: Studies of AISI Type 316 stainless steel. Metallography 3:51 (1970). 10. T. Takalo, N. Suutala, and T. Moisio. Influence of ferrite content on its morphology in some austenitic weld metals. Met. Trans. 7A:1591 (1976). 11. G. F. Slattery, P. O'Riordan, M. E. Lambert, and S. M. Green, (unpublished work). 12. K. W. Andrews, A new intermetallic phase in alloy steels, Nature No. 4180 (Dec):1015 (1949). 13. G. F. Slattery, S. R. Keown, P. O~Riordan and M. E. Lambert, (unpublished work). 14. J. K. Lai and J. R. Haigh. Delta-ferrite transformations in a Type 316 weld metal. Welding Res. (suppl. to Welding J. 58(1):Is, (Jan 1979). Received July 1979.