Determination of Hydrogen in Steel Comparison of Tin-Fusion and Hot-Extraction Techniques CHESTER C. CARSON Materials and Processes laboratory, Large Steam Turbine-Generator Department, General Electric Co., Schenectady, N . Y.
b Tests on specimens obtained from a molten bath and from large rotor forgings of low-alloy steels show that the same hydrogen content i s obtained b y the tin-fusion technique a t 1 150" C. and b y the hot-extraction technique at either 975" or 650" C. The precision of the hot-extraction technique was found to be much better. For samples weighing about 3 grams the time for the extraction of all the measurable 15 minutes) hydrogen i s similar for the tin-fusion technique and for the hot-extraction technique a t 975" C., while for hot-extraction at 650" C. i t i s about 30 minutes. The hotextraction technique is preferable not only because of its better precision but also from the point of view of simplicity of apparatus and convenience of operation.
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APPARATUS
I. 1lie npparntus used for the hotcrtraction technique is shown in Figure 1. Except for two quartz parts the sybtcm is constructrd of borosilicate glass. 1Iost of tlic tuhing is 1 '2 inch in diameter. Tlie copper oxid(, oxidant, K , wis prcpared according to Cook aiid Speight (4) and is niaintaincd at' 450" C. A4cc,tonr s t less tlim -85" C. is placed around trap G to remove the n-ator vapor t1i:it may be introduced with the collccted gases or formed subsequently upon osidation of the ext,ractctl hydrogen. Thc trap also protects the Pirani gage from mercury and other vapors. Acetone is cooled by careful addition of small amounts of liquid nitrogen until a temperature of less than -85" C. is reached. A low
tciiipcrature is niaiiitaincd sat'isfactorily n-ithout the need for purging that accompanies the use of dry ice and aceton(>. Small amounts of liquid nitrogen are added every 30 to 60 minutw to assure a 1011-enough temperature. T ~ I Poxidant can be isolated from the rest of the system by closing stopcocks 3. 4. and 8: so that it can be kept under a v:icuum n-hcn some adjaccnt portion has to be opened to the atmosphere. This is an aid \rhen shutdown time has to be kept a t a miniiiiuiii! because tlie oxidant requires a longer outgassing time than any other part of the system. All stopcocks are grcased with hpiczon IK stopcock grrase. The esccllent vacuum obtained with thc prccisioii stopcocks prorided no incentivtl to csperiinent with mercury cutoffs. Ground-glass joints are sealed ivith hpiezon TY was. The apparatus for the tin-fusion tcchiiique is the same, except that the qultrtz tube and hinged resistance furnace are replaced by tlie assenihly shown in Figure 2. The furnace section qh0n.n in Figure 2 differs from that of Carnq- et al. ( 2 ) chiefly in that their is no n-ater-cooled jacket and tlie dcsign is simpler. The length of the outer quartz tube is eaqily sufficient to confine the deposition of tin vapor to the quartz section
Ivithout resorting to 11-atcr rooling. 'rlic t'in bath was heated by a highfrrqumcy heatcr vr-ith a 1 5 - k ~rating, , which is niorc powrful than nrcessary but happcned to he available. AIthough the temperature of the tin bat'li \T:E niaintaind at 1150' C., a high ttmpcmture for quartz under a vacuum, t h r tcnipcmture of tlic, outside quartz tuhc in the vicinity of the tin bath niust have been safely below this tempcmturc liccausr thcw 11-as no sign of w a k ( ~ n i n gof the t ub(1. The furnacacl dosign in Figure 2 also pc~mitted tests to hi, conducted to dvt,crininr whethc>r a resistance heater c*ould be substituted for the relatively expensive induction heater. These trsts indicated that an induction heater is to be prcferrrd. rhieflj- because thr w e of a resistance heater resulted in much higher and more variable blanks (0.2 to 0.6 p.p.ni. 11s. 0.1 p.p.m. for an induction heater).
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PROCEDURES
Hot-Extraction Technique. Kith hot11 fuinaces brought u p to tcinperat u i c :ill -topcocks open, and the thicc-n a! Stopcock 2 ogcn ton a i d tlicl cx\tiai tion fuinacc, the system i n Figuiv 1 is outga-cd until the reading on t h c l\IcIJc~otlgag(' is TT e11 brlon 0.01 micion of nicicuiv (sticbing vacuum). This dcgree of outgassing normally eliminates the need for blank 4 and 9 aic ( ~ o i r c ~ t i o n c Ptopc'ocks . tl1cn (loscd. Unlcss othcrn is(' noted, t h p specimens T\ orc solid steel cylinders about '3 inch i n diameter and about 1 inch long us(d
:ind neighing about 3 grams. For :malwis, the ipecimen i q taken out of liquid nitrogen storage, brought to room tc~mperaturc~ in notatone, dried in a warm air stream, filed Iightlv, n eighed, nnqhed nitli :Iceton(,, dried again, and introduced into th(> SI stein via thc mtrcurj lift, A . I n the system the spccinicn is inovecl bv a inagnct if it is ferromagnetic or b j magnetic pusher if it is not I n about 2 minutes thc gas introduced n ith thc m n p l c is sufficiently iciiiovcti for thca aiialvsiq. Thus. the w n p l c is e \ p o ~ c dto room tcnipcraturc f o i lcss than 10 minutw prior to anal\ sis, :mi the 10'5 of hvtlrogcii in thi- time n :IS found to be small (
