- Email: [email protected]
30 brass tubes with tensile and combined tensile and hoop stresses
Corrosion
Science,
1967,
Vol.
7, pp. 537 to 540. Pergamon
SHORT STRESS-CORROSION AND COMBINED
Press Ltd.
Printed
in Great
Britam
CO MMUNICATION
OF 70/30 BRASS TUBES WITH TENSILE TENSILE AND HOOP STRESSES* K. c.
zb4MSON
Royal Naval College, Greenwich, England Abstract-Commercial 70/30 brass tubes were tested in a Mattson type solution. For a given tensile stress it was found that the addition of a hoop stress increased the amount of stress-corrosion damage in cold drawn tubes, reduced the amount of damage in tubes air-cooled from an annealing temperature well above the recrystallization temperature, but could either slightly increase or decrease the damage in tubes annealed at a similar temperature when furnace cooled. R&sum&On a essayt des tubes en laiton commercial 70/30 dans une solution de type Mattson. On a constate que I’addition a un effort de traction donnt d’une contrainte en flexion augmente les degats par corrosion sous tension pour les tubes etires I froid, les diminue pour les tubes recuits au-dessus de la temperature de recristallisation, pouis refroidis a Pair, mais qu’elle peut soit legerement les augmenter, soit les diminuer pour les tubes recuits et refroidis au four. Zusammenfassnng-Technische 70/30-Messingrohre wurden in Mattson’scher Losung untersucht. Dabei ergab sich, daR bei konstanter Zugspannung eine iiberlagerte Umfangsspannung dieAnf;illigkeit von kaltgezogenen Rohren gegen Spannungskorrosion steigerte, dagegen die Anfalligkeit von gegliihten und luftabgekiihlten Rohren erniedrigte. Wenn die Abkiihlung nach dem Gliihen im Ofen durchgefiihrt wurde, wurde durch die iiberlagerte Umfangsspannung die Anfllligkeit der Rohren teils gesteigert, teils vermindert. k@epaT ~~pOMbII‘IJIelIlIbIe TPY6bI M3 JIaTyHU 70/30 MCIIbITbIBaJIWCb B paCTnOpe &~aTCOHa. Ehno IIaRnerlo, 'IT0 IIpIl AaHHOhl paCTRIY%BaIo~eM yCHnHli HaJIOWeHIie TaHIWi~AaJIbHoro HaIIpFMKeHllfl JW~JIII'lIlBaJIO KOJIM9eCTBO llOBpWXL(eHHii O T IiOp~O~ll~l II0J-I HallpRHieHLl?hl B XOjIO~IIOTRHJ'TbIX Tpy6aX, yMeHbLUaJI0 rroppoarrio ~py6, oxnamzaenmx Ha Boazyxe 0T macIIiTenbH0 IIpeBbIIIIaIoII(efl Tehineparypy nepeHpr~cTannlr3aqnri~~~, OnHaIiO TeMnepaTypbI, JIPlIIIb CJIa60 )‘CllJIllBaJIO WIM yMeHbLIIaJI0 KOppO8IUO Tpy6, OTTOif(eHHbIX IIpkI TeX WI? Tehlnepa TypaX, t10 0xnamAaehmx B nem.
INTRODUCTION
of brass has been extensively investigated as is clearly shown by a recent bibliographyland review. 2 However, little attention appears to have been given to the effect of stresssystem on cracking. In fundamental work wire specimens with tensile loading are usually favoured and although notches are often used to initiate failure it is impossible to separate the effects of complex stressingand stress concentration. With service failures it is difficult to know the stress conditions, particularly if internal stressesare present. The experiments described in this paper were carried out to see if the stresssystem present affected stress-corrosion of 70/30 brass. THE
STRESS-CORROSION
*Manuscript
received 2 September 1966. 537
K. C.
538
ADAMSON
EXPERIMENTAL
Thin brass tubular specimenswere used (I.D. 0.52 in., wall thickness 0.02 in.). Tensile stresseswere applied by direct loading and, when required, a hoop stresswas produced by using an internal hydraulic pressure. The stressesquoted must be regarded as nominal since they were calculated from simple elastic formulae and once the cracks start to penetrate into the metal the stresslevel will rise. A nitric-phosphoric acid pickle3 was used after machining to remove the work hardened layer and give a smooth reproducible finish. In the initial experiments the tensile load was applied by a butcher’s steelyard and the specimen was partly immersed in a Mattson* solution of pH 7.3. Perforation of the specimenwas used asthe criterion of failure. Using fully recrystallized 70/30 brass, five specimenswere tested with a tensile stressof 14,000 p.s.i. and the average perforation time was found to be 12 h 52 mm, whilst with a tensile and hoop stressboth equal to 14,000 p.s.i. the average time for perforation of a further five specimens was 11 h 14 min. There was considerable scatter: this may in part have been due to the apparatus as it was difficult to ensuretrue axiality of loading and there was a suspicion that the non-return valve to the hydraulic pump sometimesleaked. In the subsequentteststhe specimenwas loaded in a Hounsfield tensometer and a hand-operated isolating valve was fitted on the hydraulic side. A bandage was wrapped round the centre of the gauge length and kept wet by controlled dripping of the Mattson solution. The test was run for 4 h; the load, pressure and drip rate were adjusted back to the initial values every half hour, if necessary. The specimen was then washed, dried and broken in the tensometer. The loss in U.T.S., compared with that of an uncorroded specimen,was usedas a measureof the amount of stresscorrosion damage. SERIEs
II
EXPERIMENTS
Annealed at 600°C for 1 h: furnace cooled Nominal tensile stress psi
Nominal hoop stress psi
0%loss in U.T.S.
10,ooo 10,004 15,000 15,000 17.500 17,500 20,000 20,ooo
10,ooo 15,ooo 17,500
1.8 2.1 8.5 3.9 12.0 12.5
20,ooo
24.8 28.1
SERlEs
111 EXPERIMENTS
Annealed at 650°C for 1 h: furnace cooled Nominal tensile stress psi
Nominal hoop stress psi
y. loss in U.T.S.
15,450 15,450
15,450
45.7 42.7
Short communication SERIES
Iv
539
EXPERIMENTS
Annealed at 650°C for 1 h: air cooled Nominal tensile stress psi
Nominal hoop stress psi
y6 loss in U.T.S.
17,500 17,500 17,500 17,500
8750 17,500 21,880
40.2 35.8 23.5 20.7
In Series III and Series IV Experiments the ratio of the tensile test stress to the U.T.S. is the same (0.3852).
Many of the specimenswere crack detected using a dye penetrant technique in conjunction with a binocular low power microscope. With simple tensile stressingthe cracks were all at right angles to the applied stress. A superimposed hoop stress introduced a new set of cracks perpendicular to the hoop stress so that a cellular crack pattern was developed. Specimensrepresentative of tensile and hoop stressing were sectioned and examined: in all cases the crack path appeared to be intergranular. SERIES
v
EXPERIMENTS
As received condiion Cold drawn and probably stress relieved Nominal tensile stress psi
Nominal hoop stress psi
y. loss in U.T.S.
20,000 20,000 22,000 22,000
20,000 22,000
4.8 7.0 3.3 8.4*
*A fine pin hole developed after 31 h and the pressure began to fall over the last half-hour. DISCUSSION
From the results presented above it would appear that with commercial 70/30 type brass the addition of a hoop stressto an applied tensile stressmay increase, decrease or have little effect on the stress-corrosion damage depending on the metallurgical history of the brass. With material air cooled from a temperature above the recrystallization temperature the addition of a hoop stressleads to a decreasein the amount of stress-corrosion damage, with material furnace cooled from a similar temperature the general trend is either a small increaseor decreasein the amount of stress-corrosion damage, whilst in the caseof cold drawn tubes the amount of stress-corrosion damage increases, although in this last case the situation may well be complicated by the presenceof unknown internal stresses.In all casesthe addition of a hoop stressresults in a new system of fine cracks orientated at right anglesto the hoop stress. For a given tensile stress, as the hoop stressincreases the shear component will decreaseand as a result the amount of dislocation movement will be reduced. This could well retard the initiation of a crack since moving dislocations are necessaryto
540
K. C.
ADAMSON
start and maintain the anodic reactions in the grain boundary zone,5,6 and the tensile component required to crack the cuprous oxide formed in the grain boundaries depends on the pile-up of dislocations at the grain boundary.’ As the initiation of the crack is a relatively slow process,*this retardation of the crack initiation stagecould well account for the reduction in stresscorrosion damage experienced with specimensair cooled from the annealing temperature. It is very hard to seehow reduced dislocation movement could accelerate the initiation stage in cracking and it is therefore thought that in the other two conditions examined the slow initiation stage must be counteracted by a more rapid crack propagation stage. In cold worked material the dislocation pattern must be very complex and the reduced dislocation movement would make it difficult to blunt the tip of the crack which would then travel more rapidly though the inherently more brittle material. The difference between the furnace and air cooled annealed specimensis most surprising. The degree of ordering in the furnace cooled material is presumably higher and, using an idea put forward by Forty,” this could reduce the rate of dislocation movement and thus allow the crack to spread rapidly along the grain boundary. Any theoretical discussion of these results is speculative as it is impossible to separate crack initiation from crack propagation and also no attempt was made to find out the dislocation patterns present in the brassin the various conditions studied. The results do, however, once again show the importance of metallurgical aspects in controlling stress-corrosion behaviour. Ackno*,/e~ge,nenls-It gives the author great pleasure to acknowledge the encouragement of Dr. R. S. Thornhill to start these experiments and to thank Mr. R. Kent and Mr. R. Woodgate for their help in the laboratory. REFERENCES 1. S. RASK, Recent Advances in S!ress-Corrosion. AKE BRE~LE (Ed.), p. 8 I. Royal Swedish Academy of Engineering Sciences (1961). 2. A. R. BAILEY, Aletall. Rev. 6, 21 (1961). 3. H. ROSENTHAL and J. MAZIA, A.S.T.M.IA.I.M.E., Synposiwn on Stress Corrosion of Metals, p. 128 (1944). 4. E. MAI-IXON, Electrochim. Act. 3, 279 (1961). 5. H. W. PICKERING and P. R. SWANN, Corrosion 19, 373t (1963). 6. D. TROMANS and J. NUTTING, Fracfftre of Solids. GILMAN and DRUCKER (Eds.), p. 637. Interscience, New York (1963). 7. W. D. ROBERTSONand A. S. TETELMAN, Strengthening Mechanism in Solids. The American Society for Metals (1960). 8. T. P. HOAR and C. J. L. BOOKER, Corros. Sci. 5, 821 (1965). 9. A. J. FORIY, Recenf Advances in Stress Corrosion. AKE BRE~LE (Ed.), p. 22. Royal Swedish Academy of Engineering Sciences (1961).
Science,
1967,
Vol.
7, pp. 537 to 540. Pergamon
SHORT STRESS-CORROSION AND COMBINED
Press Ltd.
Printed
in Great
Britam
CO MMUNICATION
OF 70/30 BRASS TUBES WITH TENSILE TENSILE AND HOOP STRESSES* K. c.
zb4MSON
Royal Naval College, Greenwich, England Abstract-Commercial 70/30 brass tubes were tested in a Mattson type solution. For a given tensile stress it was found that the addition of a hoop stress increased the amount of stress-corrosion damage in cold drawn tubes, reduced the amount of damage in tubes air-cooled from an annealing temperature well above the recrystallization temperature, but could either slightly increase or decrease the damage in tubes annealed at a similar temperature when furnace cooled. R&sum&On a essayt des tubes en laiton commercial 70/30 dans une solution de type Mattson. On a constate que I’addition a un effort de traction donnt d’une contrainte en flexion augmente les degats par corrosion sous tension pour les tubes etires I froid, les diminue pour les tubes recuits au-dessus de la temperature de recristallisation, pouis refroidis a Pair, mais qu’elle peut soit legerement les augmenter, soit les diminuer pour les tubes recuits et refroidis au four. Zusammenfassnng-Technische 70/30-Messingrohre wurden in Mattson’scher Losung untersucht. Dabei ergab sich, daR bei konstanter Zugspannung eine iiberlagerte Umfangsspannung dieAnf;illigkeit von kaltgezogenen Rohren gegen Spannungskorrosion steigerte, dagegen die Anfalligkeit von gegliihten und luftabgekiihlten Rohren erniedrigte. Wenn die Abkiihlung nach dem Gliihen im Ofen durchgefiihrt wurde, wurde durch die iiberlagerte Umfangsspannung die Anfllligkeit der Rohren teils gesteigert, teils vermindert. k@epaT ~~pOMbII‘IJIelIlIbIe TPY6bI M3 JIaTyHU 70/30 MCIIbITbIBaJIWCb B paCTnOpe &~aTCOHa. Ehno IIaRnerlo, 'IT0 IIpIl AaHHOhl paCTRIY%BaIo~eM yCHnHli HaJIOWeHIie TaHIWi~AaJIbHoro HaIIpFMKeHllfl JW~JIII'lIlBaJIO KOJIM9eCTBO llOBpWXL(eHHii O T IiOp~O~ll~l II0J-I HallpRHieHLl?hl B XOjIO~IIOTRHJ'TbIX Tpy6aX, yMeHbLUaJI0 rroppoarrio ~py6, oxnamzaenmx Ha Boazyxe 0T macIIiTenbH0 IIpeBbIIIIaIoII(efl Tehineparypy nepeHpr~cTannlr3aqnri~~~, OnHaIiO TeMnepaTypbI, JIPlIIIb CJIa60 )‘CllJIllBaJIO WIM yMeHbLIIaJI0 KOppO8IUO Tpy6, OTTOif(eHHbIX IIpkI TeX WI? Tehlnepa TypaX, t10 0xnamAaehmx B nem.
INTRODUCTION
of brass has been extensively investigated as is clearly shown by a recent bibliographyland review. 2 However, little attention appears to have been given to the effect of stresssystem on cracking. In fundamental work wire specimens with tensile loading are usually favoured and although notches are often used to initiate failure it is impossible to separate the effects of complex stressingand stress concentration. With service failures it is difficult to know the stress conditions, particularly if internal stressesare present. The experiments described in this paper were carried out to see if the stresssystem present affected stress-corrosion of 70/30 brass. THE
STRESS-CORROSION
*Manuscript
received 2 September 1966. 537
K. C.
538
ADAMSON
EXPERIMENTAL
Thin brass tubular specimenswere used (I.D. 0.52 in., wall thickness 0.02 in.). Tensile stresseswere applied by direct loading and, when required, a hoop stresswas produced by using an internal hydraulic pressure. The stressesquoted must be regarded as nominal since they were calculated from simple elastic formulae and once the cracks start to penetrate into the metal the stresslevel will rise. A nitric-phosphoric acid pickle3 was used after machining to remove the work hardened layer and give a smooth reproducible finish. In the initial experiments the tensile load was applied by a butcher’s steelyard and the specimen was partly immersed in a Mattson* solution of pH 7.3. Perforation of the specimenwas used asthe criterion of failure. Using fully recrystallized 70/30 brass, five specimenswere tested with a tensile stressof 14,000 p.s.i. and the average perforation time was found to be 12 h 52 mm, whilst with a tensile and hoop stressboth equal to 14,000 p.s.i. the average time for perforation of a further five specimens was 11 h 14 min. There was considerable scatter: this may in part have been due to the apparatus as it was difficult to ensuretrue axiality of loading and there was a suspicion that the non-return valve to the hydraulic pump sometimesleaked. In the subsequentteststhe specimenwas loaded in a Hounsfield tensometer and a hand-operated isolating valve was fitted on the hydraulic side. A bandage was wrapped round the centre of the gauge length and kept wet by controlled dripping of the Mattson solution. The test was run for 4 h; the load, pressure and drip rate were adjusted back to the initial values every half hour, if necessary. The specimen was then washed, dried and broken in the tensometer. The loss in U.T.S., compared with that of an uncorroded specimen,was usedas a measureof the amount of stresscorrosion damage. SERIEs
II
EXPERIMENTS
Annealed at 600°C for 1 h: furnace cooled Nominal tensile stress psi
Nominal hoop stress psi
0%loss in U.T.S.
10,ooo 10,004 15,000 15,000 17.500 17,500 20,000 20,ooo
10,ooo 15,ooo 17,500
1.8 2.1 8.5 3.9 12.0 12.5
20,ooo
24.8 28.1
SERlEs
111 EXPERIMENTS
Annealed at 650°C for 1 h: furnace cooled Nominal tensile stress psi
Nominal hoop stress psi
y. loss in U.T.S.
15,450 15,450
15,450
45.7 42.7
Short communication SERIES
Iv
539
EXPERIMENTS
Annealed at 650°C for 1 h: air cooled Nominal tensile stress psi
Nominal hoop stress psi
y6 loss in U.T.S.
17,500 17,500 17,500 17,500
8750 17,500 21,880
40.2 35.8 23.5 20.7
In Series III and Series IV Experiments the ratio of the tensile test stress to the U.T.S. is the same (0.3852).
Many of the specimenswere crack detected using a dye penetrant technique in conjunction with a binocular low power microscope. With simple tensile stressingthe cracks were all at right angles to the applied stress. A superimposed hoop stress introduced a new set of cracks perpendicular to the hoop stress so that a cellular crack pattern was developed. Specimensrepresentative of tensile and hoop stressing were sectioned and examined: in all cases the crack path appeared to be intergranular. SERIES
v
EXPERIMENTS
As received condiion Cold drawn and probably stress relieved Nominal tensile stress psi
Nominal hoop stress psi
y. loss in U.T.S.
20,000 20,000 22,000 22,000
20,000 22,000
4.8 7.0 3.3 8.4*
*A fine pin hole developed after 31 h and the pressure began to fall over the last half-hour. DISCUSSION
From the results presented above it would appear that with commercial 70/30 type brass the addition of a hoop stressto an applied tensile stressmay increase, decrease or have little effect on the stress-corrosion damage depending on the metallurgical history of the brass. With material air cooled from a temperature above the recrystallization temperature the addition of a hoop stressleads to a decreasein the amount of stress-corrosion damage, with material furnace cooled from a similar temperature the general trend is either a small increaseor decreasein the amount of stress-corrosion damage, whilst in the caseof cold drawn tubes the amount of stress-corrosion damage increases, although in this last case the situation may well be complicated by the presenceof unknown internal stresses.In all casesthe addition of a hoop stressresults in a new system of fine cracks orientated at right anglesto the hoop stress. For a given tensile stress, as the hoop stressincreases the shear component will decreaseand as a result the amount of dislocation movement will be reduced. This could well retard the initiation of a crack since moving dislocations are necessaryto
540
K. C.
ADAMSON
start and maintain the anodic reactions in the grain boundary zone,5,6 and the tensile component required to crack the cuprous oxide formed in the grain boundaries depends on the pile-up of dislocations at the grain boundary.’ As the initiation of the crack is a relatively slow process,*this retardation of the crack initiation stagecould well account for the reduction in stresscorrosion damage experienced with specimensair cooled from the annealing temperature. It is very hard to seehow reduced dislocation movement could accelerate the initiation stage in cracking and it is therefore thought that in the other two conditions examined the slow initiation stage must be counteracted by a more rapid crack propagation stage. In cold worked material the dislocation pattern must be very complex and the reduced dislocation movement would make it difficult to blunt the tip of the crack which would then travel more rapidly though the inherently more brittle material. The difference between the furnace and air cooled annealed specimensis most surprising. The degree of ordering in the furnace cooled material is presumably higher and, using an idea put forward by Forty,” this could reduce the rate of dislocation movement and thus allow the crack to spread rapidly along the grain boundary. Any theoretical discussion of these results is speculative as it is impossible to separate crack initiation from crack propagation and also no attempt was made to find out the dislocation patterns present in the brassin the various conditions studied. The results do, however, once again show the importance of metallurgical aspects in controlling stress-corrosion behaviour. Ackno*,/e~ge,nenls-It gives the author great pleasure to acknowledge the encouragement of Dr. R. S. Thornhill to start these experiments and to thank Mr. R. Kent and Mr. R. Woodgate for their help in the laboratory. REFERENCES 1. S. RASK, Recent Advances in S!ress-Corrosion. AKE BRE~LE (Ed.), p. 8 I. Royal Swedish Academy of Engineering Sciences (1961). 2. A. R. BAILEY, Aletall. Rev. 6, 21 (1961). 3. H. ROSENTHAL and J. MAZIA, A.S.T.M.IA.I.M.E., Synposiwn on Stress Corrosion of Metals, p. 128 (1944). 4. E. MAI-IXON, Electrochim. Act. 3, 279 (1961). 5. H. W. PICKERING and P. R. SWANN, Corrosion 19, 373t (1963). 6. D. TROMANS and J. NUTTING, Fracfftre of Solids. GILMAN and DRUCKER (Eds.), p. 637. Interscience, New York (1963). 7. W. D. ROBERTSONand A. S. TETELMAN, Strengthening Mechanism in Solids. The American Society for Metals (1960). 8. T. P. HOAR and C. J. L. BOOKER, Corros. Sci. 5, 821 (1965). 9. A. J. FORIY, Recenf Advances in Stress Corrosion. AKE BRE~LE (Ed.), p. 22. Royal Swedish Academy of Engineering Sciences (1961).