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The influence of tempering temperature, minor alloying elements, and cathodic polarization on the low frequency fatigue resistance of 18% Ni maraging steel
Corrosion Science, 1976, Vol. 16, pp, 545 to 549. Pergamon Press. Printed in Great Britain
THE INFLUENCE OF TEMPERING TEMPERATURE, MINOR ALLOYING ELEMENTS, AND CATHODIC POLARIZATION ON THE LOW FREQUENCY FATIGUE RESISTANCE OF 18% Ni MARAGING STEEL* J. HUTCHINGS and G. SANDERSON Fulmer Research Institute Limited, Stoke Poges, Bucks. Abstract--The low frequency fatigue resistance of 18~o Ni maraging steel (Gll0) under environmental conditions conducive to hydrogen embrittlement was altered by changes in the S, P, Mn and Cu minor element contents. The detrimental effect of increasing the S, P or Mn contents independently could be reversed when they were in combination, owing to beneficial interactions between Mn and S and Cu and P. Tempering the Gll0 maraging steel at higher temperatures (500-650°C) reduced the hardness and increased the amount of retained austenite, with a consequent improvement in the hydrogen embrittlement resistance. INTRODUCTION PREVIOUS work by the present authors and others 1-6 has shown that at potentials < -- 0.8V (S.C.E.), slow cycle (0.083Hz) corrosion fatigue and s.c.c, of 18~oNi maraging steel occurs by a hydrogen embrittlement mechanism. Carter, 6,7 investigating the s.c.c, characteristics of high strength maraging steel, found that reverted austenite reduced the U.T.S. but increased the K1scc value. At ageing temperatures < 480°C the steel was very susceptible to s.c.c, which was thought to occur by a hydrogen embrittlement mechanism. The low ageing temperature would result in less reverted austenite, which may provide an explanation for this behaviour. Stavros and Paxton a also found that low temperature ageing (426°C) resulted in shorter s.c.c, failure times than higher temperature ageing (482°C); however, in contrast they found Kiscc remained constant. The chemical and electrochemical properties of the phases making up an alloy structure will also have an important influence on its corrosion behaviour. Wranglen 9 has described how sulphur in iron can increase its corrosion rate by 1 0 0 x ; the Fe-FeS couple promotes pitting and adsorpton of sulphide ions, the latter catalysing both anodic dissolution and hydrogen adsorption. Additions of manganese were found to be beneficial because MnS reduces sulphide ion adsorption and is a less efficient cathode than FeS. Copper reduces sulphide ion adsorption still further by forming insoluble copper sulphide at the iron/electrolyte interface. The aim of the present work is to determine the influence of tempering temperature on the low frequency fatigue resistance in 3 ~o NaCl solution and correlate this with austenite content and hardness. In addition the influence of the minor elements Mn, Cu, S and P on the resistance has also been studied. *Manuscript received 7 June 1974; in revised form 6 January 1975. 545
546
J. HUTCH1NG$ and G. SANDERSON EXPERIMENTAL
METHOD
Tempering experiments The material was rolled plate of black-finished 18 ~ Ni maraging steel of the following composition : C 0.02
Si 0.16
Mn 0.02
S 0.008
Ni 17.66
P 0.006
Co 7.61
Mo 4.41
Ti 0.65
AI 0.14
Sample,s of the plate were solution treated for 1 h at 820°C and tempered for 3 h at 500, 565, 600 and 650°C to give the hardness values and austenite contents shown in Table 1. TABLE 1. Tempering Temp.
Hardness Hv/30
Vol. Yo Retained Austenitc
580 520 445 379
2 l0 25 50
500°C 565°C 600°C 650°C
Longitudinal specimens were cut from the heat treated plates, notched and given a 0.127Ftm finish by fine grinding. They were then corrosion-fatigue tested at 0.083 H, and :!: 772 M N / M 2 under reverse bend trapezoidal loading. The solution was aerated 3 % NaCl at ambient temperature,and the specimens were potentiostatically polarized to - 1.00V and - 1.30V (SCE) in order to induce failure by hydrogen embrittlement.
Alloying experiments The base material finish and composition was the same as that used in the tempering experiments. Sixteen different vacuum remelted casts of 18 ~o Ni maraging steel, representing a 24 factorial experimental design, were prepared by making various additions of S, P, Mn and Cu to stock material of the following composition: S 0.004
P 0.004
Mn 0.03
Cu 0.11
After hot rolling, the alloys were solution treated at 820°C for 1 ~ air cooled and aged for 3 h at 500°C. The compositions and hardnesses of the alloys are given in Table 2. TABLE 2. Alloy G G G G G G G G G G G G G G G G
(1)* (ab) (ac) (be) (ad) (bd) (cd) (abed) (a) (b) (c) (abe) (d) (abd) (acd) Cocd)
COMPOSITIONAND HARDNESSOF MARAGINGSTEELALLOYS S~
P ~o
0.004 0.008 0.008 0.004 0.009 0.004 0.005 0.008 0.011 0.004 0.004 0.012 0.003 0.012 0.015 0.004
0.004 0.008 0.004 0.009 0.004 0.007 0.005 0.008 0.006 0.016 0.006 0.015 0.006 0.014 0.005 0.015
*Original base material. tAverage of 5 fatigue specimens from each alloy.
M n ~o
Cu ~o
Hv/30t
0.03 0.02 0.07 0.09 0.02 0.02 0.11 0.11 0.02 0.02 0.08 0.10 0.02 0.02 0.07 0.09
0.11 0.12 0.10 0.09 0.50 0.50 0.51 0.52 0.12 0.11 0.11 0.11 0.63 0.63 0.68 0.68
537 536 541 541 524 528 528 527 540 539 532 542 526 524 530 529
18 ~o Ni maraging steel
547
In order to ensure failure by hydrogen embrittlement, the alloys were tested under reverse bend sinusoidal loading at 4- 772 M-N/M 2 and 0.083 H, under the following environmental conditions: (i) Aerated 3 ~. NaCI, pH3, freely corroding ambient temp. (ii) Aerated 3 ~o NaCI, pHs, potentiostatically cathodically polarized to -- 1.0V w.r.t. SCE at ambient temperature. (iii) Aerated 3 ~. NaCI, pH3 + 50 p.p.m, sulphur added as Na2S cathodically polarized to -- 1.0V (SCE) at ambient temperature. RESULTS
Tempering experiments T h e results are r e p r e s e n t e d g r a p h i c a l l y in Fig. 1, w h i c h shows loglo N/NA p l o t t e d a g a i n s t vol. ~o a u s t e n i t e a n d H v / 3 0 respectively where N is the a q u e o u s c o r r o s i o n fatigue life a n d N A is the air fatigue life.
Alloying experiments T h e results are s h o w n in T a b l e 3. TABLE 3.
Alloy
G (1)
CORROSION FATIGUE LIVES OF MARAGING STEEL ALLOYS
Corrosion Fatigue Life (Cycles) Freely Cathodically Polarized Corroding -- 1.0V (SCE) Nil Sulphur Nil Sulphur 50 ppm Sulphur 10,402 10,782
5,508
4,324
2,373 3,906 3,388 1,902 2,852 1,527 6,531 2,390 6,800 6,579 6,457
1,628 4,284 2,502 1,782 2,153 1,207 3,144 1,870 3,087 2,850 1,660
6,487
3,883
6,529 7,669 6,704 4,487 5,992
4,779
11,162
G G G G G G G G G
(ab) (ac) (bc) (ad) (bd) (cd) (abed) (a) (b)
G (d)
8,823 10,404 9,156 11,072 10,109 10,345 15,093 4,040 8,612 9,259 5,772 6,696 9,764 9,065 6,911
G (abd) G (aed) G (bcd)
9,000 7,951 10,257
G (c) G (abe)
3,232 4,338 1,980
These were analysed by computer, using multiple regression analysis, and the following function was the best obtained: Life (L)
= 11,051 + 5.833 × 106 [Mn] IS] + 0.325 × 10a [Cu] [PI + 0.248 × 106 G[P] --0.326 x 106 [ S ] - 0.126 × 106 [P] -- 0.027 × 106 [Mn] --0.038 × 10e GIMn] [S] - - 0.228 × 106 F.G. [P] --0.006 × 106 G Where [x] = wt ~. x G = 0, specimen freely corroding G = 1, specimen cathodically polarized F = 0, sulphur-free solution F = 1, sulphur contaminated solution. The probability that this equation has arisen by chance is < 0.001 ~ .
548
J. HtrrCmNGS and G. SANDERSON 50
600
P
40
55O
3O
5O0
>. "r tn 2O
450
/ \,"i 400
o/
0 0.01
I 0.I0
I LO0
350 I0.00
N/N A
N/NA vsvol. ~ a u s t e n i t e and hardness for 18yo maraging steel after various heat treatments. [] 1.0V Cathodic polarization; • 0.5V Cathodic polarization; - - - Volume ~ 7 vs N/Na; -Hardness vs N/NA.
FiG. 1.
DISCUSSION
The effect of tempering temperature The improvements in the low frequency fatigue resistance, as a result of raising the tempering temperature, were associated with both a decrease in hardness and an increase in the amount of retained austenite. The latter phase is tougher (more resistant to brittle cleavage) than martensite and has a higher solubility for hydrogen. The austenite may thus act as a sink for the absorbed hydrogen thus preventing it embrittling the martensitic phase. The decrease in hardn~-ss will allow some of the local stresses built up by the adsorbed hydrogen to be alleviated by plastic deformation before cleavage fracture can occur. Tempering at 650°C, to produce a hardness < 350 Hv/30 and > 50 vol. ~ austenite, rendered the maraging steel immune to low frequency fatigue failure as a result of hydrogen embrittlement.
The influence of minor alloying elements The equation relating corrosion fatigure life with composition and environmental conditions, showed that increasing the sulphur alone and increasing the manganese alone decreased the life under all three environmental conditions. The detrimental effect of sulphur and phosphorus may be a result of the dissolution of the corresponding sulphides and phosphides, thus releasing these ions into solution where they become adsorbed on to the steel surface. These can strongly catalyse the hydrogen absorption process by a mechanism thought to involve the intermediate formation of iron hydride,z° The beneficial effect of phosphorus additions when the steel was cathodically polarized in unpolluted solution remains to be explained. The detrimental effect of manganese additions on impact and fracture toughness properties has been reported in the literature; n in the case of fracture toughness it was a combination of 0.14 9/o manganese and 0.15 ~o silicon which caused a sharp drop
18 ~ Ni maraging steel
549
in K i t ; the alloys of the present study contained 0.16% silicon. The beneficial effect of the manganese-sulphur interaction is thought to be a result of the replacement of the iron sulphide, formed because there was insufficient manganese to combine with the increased sulphur, by the thermodynamically more stable manganese sulphide. The latter is known to be a less efficient promoter of hydrogen absorption than iron sulphide) Increasing the copper alone increased the life under all three environmental conditions and, in addition, between 0.40 and 0.68 % copper there was a beneficial copper-phosphorus interaction. This may be due to the formation of acid insoluble copper phosphide, thus preventing the adsorption of phosphide ions onto the steel surface. CONCLUSIONS The low frequency fatigue resistance of 18 % maraging steel, both freely corroding and cathodically polarized in 3 % NaCI solution with and without Na2S additions, was affected by its S, P, Cu and Mn composition as follows: (i) Increasing the S and Mn independently in the ranges 0.003--0.015% and 0.02-0.11% respectively decreased the resistance under all conditions. (ii) Increasing the P alone in the range 0.004-0.015~o decreased the resistance when the steel was freely corroding or cathodically polarized in sulphur polluted solution. In un-polluted solution it had a beneficial effect under cathodic polarization. (iii) Increasing the Cu alone in the range 0.09-0.68 ~o increased the resistance under all conditions, and between 0.40 and 0.68 % there was a beneficial interaction with the phosphorus additions. (iv) When the Mn is in the range 0.06-0.11%, increasing the S content was no longer deleterious, owing to a beneficial interaction between the two elements. (v) Tempering the G l l 0 maraging steel at higher temperatures (500-650°C) reduced the hardness and increased the retained austenite with a consequent improvement in the low frequency corrosion fatigue resistance under conditions conducive to hydrogen embrittlement. Acknowledgement
This work was carried out with the support of Procurement Executive, Ministry
of Defence.
REFERENCES 1. J. HtrrcmNGSand G. SANDERSON.To be published. 2. B. C. SYa.ETT,Corrosion 27, 7, 270 (1971). 3. N. KENYON,W. W. KInK and D. VAN ROOYEN,Corrosion 27, 390 (1971). 4. R. N. PARKINSand E. G. HANEY,Trans. metall. Soc. A.LM.E. 242, 1943 (1968). 5. C. LANG,J. HAVRANEKand O. BLAHOZ,Kovore Mat. 9, 57 (1971). 6. C. S. CARTER, Met. Trans. 1, 1551 (1970). 7. C. S. CARTER,Met. Trans. 2, 1621 (1971). 8. A. J. STAVROSand H. W. PAXTON,Met. Trans. 1, 3049 0970). 9. G. WRANGLEN,Corros. Sci. 9, 585 0969). 10. J. F. NEWMANand L. L. SHREIR,Corros. Sci. 9, 631 (1969). 11. R. L. CAIRNSand C. J. NOVAK,Met. Trans. 2, 1837 (1971).
THE INFLUENCE OF TEMPERING TEMPERATURE, MINOR ALLOYING ELEMENTS, AND CATHODIC POLARIZATION ON THE LOW FREQUENCY FATIGUE RESISTANCE OF 18% Ni MARAGING STEEL* J. HUTCHINGS and G. SANDERSON Fulmer Research Institute Limited, Stoke Poges, Bucks. Abstract--The low frequency fatigue resistance of 18~o Ni maraging steel (Gll0) under environmental conditions conducive to hydrogen embrittlement was altered by changes in the S, P, Mn and Cu minor element contents. The detrimental effect of increasing the S, P or Mn contents independently could be reversed when they were in combination, owing to beneficial interactions between Mn and S and Cu and P. Tempering the Gll0 maraging steel at higher temperatures (500-650°C) reduced the hardness and increased the amount of retained austenite, with a consequent improvement in the hydrogen embrittlement resistance. INTRODUCTION PREVIOUS work by the present authors and others 1-6 has shown that at potentials < -- 0.8V (S.C.E.), slow cycle (0.083Hz) corrosion fatigue and s.c.c, of 18~oNi maraging steel occurs by a hydrogen embrittlement mechanism. Carter, 6,7 investigating the s.c.c, characteristics of high strength maraging steel, found that reverted austenite reduced the U.T.S. but increased the K1scc value. At ageing temperatures < 480°C the steel was very susceptible to s.c.c, which was thought to occur by a hydrogen embrittlement mechanism. The low ageing temperature would result in less reverted austenite, which may provide an explanation for this behaviour. Stavros and Paxton a also found that low temperature ageing (426°C) resulted in shorter s.c.c, failure times than higher temperature ageing (482°C); however, in contrast they found Kiscc remained constant. The chemical and electrochemical properties of the phases making up an alloy structure will also have an important influence on its corrosion behaviour. Wranglen 9 has described how sulphur in iron can increase its corrosion rate by 1 0 0 x ; the Fe-FeS couple promotes pitting and adsorpton of sulphide ions, the latter catalysing both anodic dissolution and hydrogen adsorption. Additions of manganese were found to be beneficial because MnS reduces sulphide ion adsorption and is a less efficient cathode than FeS. Copper reduces sulphide ion adsorption still further by forming insoluble copper sulphide at the iron/electrolyte interface. The aim of the present work is to determine the influence of tempering temperature on the low frequency fatigue resistance in 3 ~o NaCl solution and correlate this with austenite content and hardness. In addition the influence of the minor elements Mn, Cu, S and P on the resistance has also been studied. *Manuscript received 7 June 1974; in revised form 6 January 1975. 545
546
J. HUTCH1NG$ and G. SANDERSON EXPERIMENTAL
METHOD
Tempering experiments The material was rolled plate of black-finished 18 ~ Ni maraging steel of the following composition : C 0.02
Si 0.16
Mn 0.02
S 0.008
Ni 17.66
P 0.006
Co 7.61
Mo 4.41
Ti 0.65
AI 0.14
Sample,s of the plate were solution treated for 1 h at 820°C and tempered for 3 h at 500, 565, 600 and 650°C to give the hardness values and austenite contents shown in Table 1. TABLE 1. Tempering Temp.
Hardness Hv/30
Vol. Yo Retained Austenitc
580 520 445 379
2 l0 25 50
500°C 565°C 600°C 650°C
Longitudinal specimens were cut from the heat treated plates, notched and given a 0.127Ftm finish by fine grinding. They were then corrosion-fatigue tested at 0.083 H, and :!: 772 M N / M 2 under reverse bend trapezoidal loading. The solution was aerated 3 % NaCl at ambient temperature,and the specimens were potentiostatically polarized to - 1.00V and - 1.30V (SCE) in order to induce failure by hydrogen embrittlement.
Alloying experiments The base material finish and composition was the same as that used in the tempering experiments. Sixteen different vacuum remelted casts of 18 ~o Ni maraging steel, representing a 24 factorial experimental design, were prepared by making various additions of S, P, Mn and Cu to stock material of the following composition: S 0.004
P 0.004
Mn 0.03
Cu 0.11
After hot rolling, the alloys were solution treated at 820°C for 1 ~ air cooled and aged for 3 h at 500°C. The compositions and hardnesses of the alloys are given in Table 2. TABLE 2. Alloy G G G G G G G G G G G G G G G G
(1)* (ab) (ac) (be) (ad) (bd) (cd) (abed) (a) (b) (c) (abe) (d) (abd) (acd) Cocd)
COMPOSITIONAND HARDNESSOF MARAGINGSTEELALLOYS S~
P ~o
0.004 0.008 0.008 0.004 0.009 0.004 0.005 0.008 0.011 0.004 0.004 0.012 0.003 0.012 0.015 0.004
0.004 0.008 0.004 0.009 0.004 0.007 0.005 0.008 0.006 0.016 0.006 0.015 0.006 0.014 0.005 0.015
*Original base material. tAverage of 5 fatigue specimens from each alloy.
M n ~o
Cu ~o
Hv/30t
0.03 0.02 0.07 0.09 0.02 0.02 0.11 0.11 0.02 0.02 0.08 0.10 0.02 0.02 0.07 0.09
0.11 0.12 0.10 0.09 0.50 0.50 0.51 0.52 0.12 0.11 0.11 0.11 0.63 0.63 0.68 0.68
537 536 541 541 524 528 528 527 540 539 532 542 526 524 530 529
18 ~o Ni maraging steel
547
In order to ensure failure by hydrogen embrittlement, the alloys were tested under reverse bend sinusoidal loading at 4- 772 M-N/M 2 and 0.083 H, under the following environmental conditions: (i) Aerated 3 ~. NaCI, pH3, freely corroding ambient temp. (ii) Aerated 3 ~o NaCI, pHs, potentiostatically cathodically polarized to -- 1.0V w.r.t. SCE at ambient temperature. (iii) Aerated 3 ~. NaCI, pH3 + 50 p.p.m, sulphur added as Na2S cathodically polarized to -- 1.0V (SCE) at ambient temperature. RESULTS
Tempering experiments T h e results are r e p r e s e n t e d g r a p h i c a l l y in Fig. 1, w h i c h shows loglo N/NA p l o t t e d a g a i n s t vol. ~o a u s t e n i t e a n d H v / 3 0 respectively where N is the a q u e o u s c o r r o s i o n fatigue life a n d N A is the air fatigue life.
Alloying experiments T h e results are s h o w n in T a b l e 3. TABLE 3.
Alloy
G (1)
CORROSION FATIGUE LIVES OF MARAGING STEEL ALLOYS
Corrosion Fatigue Life (Cycles) Freely Cathodically Polarized Corroding -- 1.0V (SCE) Nil Sulphur Nil Sulphur 50 ppm Sulphur 10,402 10,782
5,508
4,324
2,373 3,906 3,388 1,902 2,852 1,527 6,531 2,390 6,800 6,579 6,457
1,628 4,284 2,502 1,782 2,153 1,207 3,144 1,870 3,087 2,850 1,660
6,487
3,883
6,529 7,669 6,704 4,487 5,992
4,779
11,162
G G G G G G G G G
(ab) (ac) (bc) (ad) (bd) (cd) (abed) (a) (b)
G (d)
8,823 10,404 9,156 11,072 10,109 10,345 15,093 4,040 8,612 9,259 5,772 6,696 9,764 9,065 6,911
G (abd) G (aed) G (bcd)
9,000 7,951 10,257
G (c) G (abe)
3,232 4,338 1,980
These were analysed by computer, using multiple regression analysis, and the following function was the best obtained: Life (L)
= 11,051 + 5.833 × 106 [Mn] IS] + 0.325 × 10a [Cu] [PI + 0.248 × 106 G[P] --0.326 x 106 [ S ] - 0.126 × 106 [P] -- 0.027 × 106 [Mn] --0.038 × 10e GIMn] [S] - - 0.228 × 106 F.G. [P] --0.006 × 106 G Where [x] = wt ~. x G = 0, specimen freely corroding G = 1, specimen cathodically polarized F = 0, sulphur-free solution F = 1, sulphur contaminated solution. The probability that this equation has arisen by chance is < 0.001 ~ .
548
J. HtrrCmNGS and G. SANDERSON 50
600
P
40
55O
3O
5O0
>. "r tn 2O
450
/ \,"i 400
o/
0 0.01
I 0.I0
I LO0
350 I0.00
N/N A
N/NA vsvol. ~ a u s t e n i t e and hardness for 18yo maraging steel after various heat treatments. [] 1.0V Cathodic polarization; • 0.5V Cathodic polarization; - - - Volume ~ 7 vs N/Na; -Hardness vs N/NA.
FiG. 1.
DISCUSSION
The effect of tempering temperature The improvements in the low frequency fatigue resistance, as a result of raising the tempering temperature, were associated with both a decrease in hardness and an increase in the amount of retained austenite. The latter phase is tougher (more resistant to brittle cleavage) than martensite and has a higher solubility for hydrogen. The austenite may thus act as a sink for the absorbed hydrogen thus preventing it embrittling the martensitic phase. The decrease in hardn~-ss will allow some of the local stresses built up by the adsorbed hydrogen to be alleviated by plastic deformation before cleavage fracture can occur. Tempering at 650°C, to produce a hardness < 350 Hv/30 and > 50 vol. ~ austenite, rendered the maraging steel immune to low frequency fatigue failure as a result of hydrogen embrittlement.
The influence of minor alloying elements The equation relating corrosion fatigure life with composition and environmental conditions, showed that increasing the sulphur alone and increasing the manganese alone decreased the life under all three environmental conditions. The detrimental effect of sulphur and phosphorus may be a result of the dissolution of the corresponding sulphides and phosphides, thus releasing these ions into solution where they become adsorbed on to the steel surface. These can strongly catalyse the hydrogen absorption process by a mechanism thought to involve the intermediate formation of iron hydride,z° The beneficial effect of phosphorus additions when the steel was cathodically polarized in unpolluted solution remains to be explained. The detrimental effect of manganese additions on impact and fracture toughness properties has been reported in the literature; n in the case of fracture toughness it was a combination of 0.14 9/o manganese and 0.15 ~o silicon which caused a sharp drop
18 ~ Ni maraging steel
549
in K i t ; the alloys of the present study contained 0.16% silicon. The beneficial effect of the manganese-sulphur interaction is thought to be a result of the replacement of the iron sulphide, formed because there was insufficient manganese to combine with the increased sulphur, by the thermodynamically more stable manganese sulphide. The latter is known to be a less efficient promoter of hydrogen absorption than iron sulphide) Increasing the copper alone increased the life under all three environmental conditions and, in addition, between 0.40 and 0.68 % copper there was a beneficial copper-phosphorus interaction. This may be due to the formation of acid insoluble copper phosphide, thus preventing the adsorption of phosphide ions onto the steel surface. CONCLUSIONS The low frequency fatigue resistance of 18 % maraging steel, both freely corroding and cathodically polarized in 3 % NaCI solution with and without Na2S additions, was affected by its S, P, Cu and Mn composition as follows: (i) Increasing the S and Mn independently in the ranges 0.003--0.015% and 0.02-0.11% respectively decreased the resistance under all conditions. (ii) Increasing the P alone in the range 0.004-0.015~o decreased the resistance when the steel was freely corroding or cathodically polarized in sulphur polluted solution. In un-polluted solution it had a beneficial effect under cathodic polarization. (iii) Increasing the Cu alone in the range 0.09-0.68 ~o increased the resistance under all conditions, and between 0.40 and 0.68 % there was a beneficial interaction with the phosphorus additions. (iv) When the Mn is in the range 0.06-0.11%, increasing the S content was no longer deleterious, owing to a beneficial interaction between the two elements. (v) Tempering the G l l 0 maraging steel at higher temperatures (500-650°C) reduced the hardness and increased the retained austenite with a consequent improvement in the low frequency corrosion fatigue resistance under conditions conducive to hydrogen embrittlement. Acknowledgement
This work was carried out with the support of Procurement Executive, Ministry
of Defence.
REFERENCES 1. J. HtrrcmNGSand G. SANDERSON.To be published. 2. B. C. SYa.ETT,Corrosion 27, 7, 270 (1971). 3. N. KENYON,W. W. KInK and D. VAN ROOYEN,Corrosion 27, 390 (1971). 4. R. N. PARKINSand E. G. HANEY,Trans. metall. Soc. A.LM.E. 242, 1943 (1968). 5. C. LANG,J. HAVRANEKand O. BLAHOZ,Kovore Mat. 9, 57 (1971). 6. C. S. CARTER, Met. Trans. 1, 1551 (1970). 7. C. S. CARTER,Met. Trans. 2, 1621 (1971). 8. A. J. STAVROSand H. W. PAXTON,Met. Trans. 1, 3049 0970). 9. G. WRANGLEN,Corros. Sci. 9, 585 0969). 10. J. F. NEWMANand L. L. SHREIR,Corros. Sci. 9, 631 (1969). 11. R. L. CAIRNSand C. J. NOVAK,Met. Trans. 2, 1837 (1971).