JOURNAL O F RESEARCH of the National Bureau of Standards-C. Engineering and Instrumentation Vol. 66C, No. 4, October- De cember 1962 Studies of the Stress-Corrosion Cracking of Low-Carbon Steels Hugh 1. Logan (Ju ne 6, 1962) The mechanism of st ress-co rr osion crack ing in low- car bon steels was in vestigate d usi ng not ched specimen s, st ressed in te nsion to pr odu ce a sl ight amount of plast ic deformat ion, and exposed in a boiling 20 percent aq ueous solution of NH,N0 3. Ini ti at i on a nd pr opagaLion of crae ks were st udi ed by removing specimens for m eta ll ograp hi c exa min at io n after s tated periods of expos ur e an d by simul taneo usly reco rdin g extensions and elect rochemi ca l solu tion potentia ls. Cr ac ks were initiated af ter abo ut 20 minutes' expos ur e and complete failur es occurred in 200 minutes or more. Both crack initiat ion and propagation were post ul ated to res ult fr om an electroc hemi cal pr ocess in which t he ano dic areas were submicroscopic. Tensile fracture s were initiated in the corroded specimens several times before compl ete fa ilur e occ urr ed but were prevent ed from going to compl et ion because of the energy involved in formin g the s ur faces of t he cracks a nd t he st rain h ar dening and t he st ra in ag ing of t he steel at the t ips of the cracks. 1. Introduction Fail ur es of riv eted steel pr essurc vessels hav e frequ e ntl y b een attributed to stress-corrosion c ra ck- ing, called in these in stan ces "cau stic embritt lemen t." The construction of pressur e vessels by welding has pr event ed the leaks th at were a f actor in causing the cracking of riveted s tru ct ures but has by no means eliminated st ress-colTo sion cracking of low-carbon steel vessels. Th e m ech ani sm of the initiation and propagat ion of st rcss-corrosion cracking in low-carbon steels is not wcll und erstood . Sevcral theories hav e b een advanccd to explain the m echani sm ; non e are universally accepted. Th e da ta reportcd in this paper are used for the reexamin at ion and refinement of som e of thc curr ent theories of stress- co rrosion crackin g. A bri ef review of co nditions under which st r ess- corrosion cracking has b een reported in low-carbon steels was given in an carlicr papcr [ 1) .1 Conod c nt s have included hydroxides, nitrates , cyanidcs, com- pl ex organics, and even humid weather co nditiol1 s. 2 In our earlier l aboratory st udy of the stress- co rrosion cracking of low-carbon steels [1) extensive exposure pcriods were requu·cd to produce failures in conventional ( bu t subsiz e) tensile specun ens in a boiling aqueous solu tion of N"H4N03. Failures of specimens in lcss t han 300 hI' were rare and some sp ecimens were remov ed from the corrodent after 1,8 00 hr expos ur e with no evidence of the develop- ment of str ess-corro sion cracks. Failur es, when they occurr ed, were in regions of str ess con cen trat ions resulting from the combin ed action of stress and the general attack of the corrodent. A typical failed sp ecimen is shown in figure 1. Anodic polarization of specimens produced prefer- ent ial grain boundary attack at a small numb er of boundaries, whether or not the specimens were 1 Figures in brackets indicate the literature references at the end of thi s paper. 2 Failure of steel hooks, odesigned to h ol d in place the concrcte block facing on the B ureau of Engraving and Printing Annex, \Vashington, D.C ., was attributed to stress·corrosion cracking [2]. subjected to externally applied st resses. In the stressed specimens the penetra tion of the corrodent along the grain boundari es as a r es ult of the appli ed curr e nt (average curr e nt dens it y, 0.06 ma /cm 2 ) eventuall y caused such stress co n ce ntra tions t ha t fai l ur es occurred in regions havin g fairly uniform cross-sectional areas. Again, however, the exposure periods to failure were mor e than 200 hr. Th ese data suggested that s pecim ens d es igned to co n tain dcfinite and reprodu cible regions of str ess co n ce ntration, e.g., not ches, would mo st probabl y fail after relatively short exposure periods. Accord- ingly, a notched specimen was designed t ha t sub- sequently fa il ed aILer a few hours' exposure und er stress in the cOl"rod ent. In earlier st udi es time lapse pho togr aphi c records of tbe exte nsion of a co nventional specimen, that failed in 315 hI" , sh owed that mo st of t he strain indica- tive of the development of stress-co rro sion cracks OCCUlTed during the l ast 3 hI' of the life of the speci- men. " Wi th a specimcn th at would fail in a few homs it was feasible to determine the incubation pcri od to the initiat ion of cracking and to dctermine the exten- sion-time and electrochemical solll tion pot en tial-time relationships during the life of the specimen. In the earl y work [1] with finc-grained ma terials the expos ur e periods to failu re were much longer for a "ship plate stecl" 3 h aving a high tra nsition temper- ature from brit tle to du ct il e fracture than fo[, a similar steel havin g a lower trans i 1,ion temperature. Although the temperaturc of the corroden t was well above the higher transition temp erat ur e, it seemed d es u'able to comparc tbese materials as notched specimens wh ere the inheren t brittl eness of tbe steels, if pr esent, wou ld be expected to aff ect the res ult s. Two steels, one sili co n killed, the other 3 Pl ates had bee n rcmovcd from welded ships. A crack had run completely across one of these plates (high transition temperature from brittle to duct ile fractur e) while the crack had penetrated in to and stopped in the second plate (low transition te mpcrat ure from bri ttlc to ductilc fracture). For a report of the NBS wor k on "ship pl a te/ ' see uCorrelation of metallurgical properties and se rvice performance of steel plates fro m fractured shi ps" by JVLorgan L. \V illiam s, Welding J. Rcscarch Supp!. , p. 445, (Oct. 1958) . 347
Studies of the Stress-Corrosion Cracking of Low-Carbon Steels
May 17, 2023
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