252 PHILlPS TECHNICAL REVIEW VOLUME 24

HYDROGEN IN IRON AND STEEL

-IT. FRACTURING

by J. D. FAST *) and D. J. van OOIJEN *). 539.42:546.11:669.14

The first part of this article made it clear that if hydrogen is present in iron and steel it maycause various harmful effects. Hydrogen taken up during the enamelling of steel may causecracking of the enamel layer, and hydrogen that penetrates during pickling may give rise tosurface blistering. Part 11 below offers some insight into the origin of the often highly dangerousfracturing of iron and steel under the influence of hydrogen.

On the origin of fracturesDuring plastic deformation of a metal there arise

not only lattice imperfections in' the form of dis-locations and point defects but also crack nucleiwhich, under unfavourable conditions, may developinto real cracks. There are various theories on theorigin of these nuclei. The hypothesis common tothem all is that each crack nucleus is produced bythe piling up and coalescence of a number of dislo-cations under the influence of external shear stresses.In the picture which Zener and Stroh 1) give ofthis process, the dislocations moving along a slipplane are piled-up against some obstacle or other,e.g. a grain boundary or inclusion. The dislocationsat the head of such a piled-up group experience con-siderable pressure from the dislocations comingalong behind them. As a result they can he forcedso close together as to merge to form a single dis-location having a large Burgers vector (see fig. I).After exceeding a certain value of the Burgers vec-tor a wedge-shaped void is formed, which can act asa crack nucleus.

For iron Cottrell 2) suggested a somewhat differ-ent mechanism, in, which dislocations move towardseach other along two intersecting slip planes andcoalesce along the junction of the two planes. Thisgives rise to a wedge-shaped crack nucleus in acleavage plane (see fig. 2). This nucleus will be largerthe more dislocations are involved in the coales-cence 3).Various the'ories also exist as to the factors that

play an essential p~rt in the catastrophic growth of

*) Philips Research Laboratories, Eindhoven.1) C. Zener, Fracturing of metals, Amer. Soc. Metals, Cleve-

lando(Ohio) 1948, pp. 3-31; A. N. Stroh, Adv. Phys. 6,418,1957.

2) A. H. Cottrell, Trans. AIME 212, 192, 1958; see also hisarticle in the congress book "Fracture", Proc. internat.Conf. on the atomic mechanisms of fracture, Swampscott(Mass.) 1959.

3) For a refinement of this theory, see A.W. Sleeswijk, Twin-ning and the origin of cleavage nuclei in a iron, Acta metal-lurgica 10, 803-812, 1962 (No. 9).

000000~000000000000000000o 0 0 O-P 0 0 0 0 0 0OOOO-l~o 0 0 0-00 0 0 0 0 000000000000OOO~OOOOOOO000

Fig. 1. Illustrating the mechanism by which a crack nucleusforms, as proposed by Zener and Stroh 1). The dislocations movein the direction of the arrow along a slip plane (left) and thefirst are stopped by an obstacle, such as a grain boundary orinclusion. Under pressure from the following dislocations, threeof them coalesce to form a wedge-shaped void - a cracknucleus (right).

a crack nucleus. The formation of a crack nucleusis always accompanied by the building-up of largestresses in its vicinity. The larger these are, thegreater is of course the chance that the nucleuswill develop into a macrocrack. Among the influen-tial factors is the behaviour of the neighbouringdislocation sources. If these sources are easily acti-vated, the plastic deformation will continue and thestresses will diminish before macrocracks have beenable to form. If, however, the sources are stronglypinned by foreign atoms, there is a chance that thesources will not become active early enough, withthe result that cracks appear. Itis known that nitro-

(101) (101)

~)')')'

">- (:..:.001)

'y'y

'y

~(IOij

Fig. 2. According to Cottrell ê) crack nuclei form in iron on {lOO}planes (the cleavage planes) by the coalescence of dislocationsmoving towards each other along two intersecting {llO} planes(the slip planes).

1962/63, No. 8 HYDROGEN IN IRON AND STEEL, 11 253

gen and carbon may cause embrittlement in thisway, and it has been assumed that the same appliesto the embrittling effect of hydrogen in steel.

Apart from this hypothesis there are two othersworth mentioning. In Part I of this article 4) it wasstated that high-pressure molecular hydrogen mayform in microcavities of iron or steel which is super-saturated with hydrogen. It has been assumed thatH2 might form similarly in any crack nucleus. Theresultant internal pressure increases the chance of thenucleus growing catastrophically into a macrocrack.A third hypothesis concerning the embrittling

behaviour of hydrogen envisages an adsorptioneffect. The adsorption of hydrogen slightly reducesthe surface tension of iron, as a result of which theformation of new surfaces - in this case the growthof crack nuclei - will cost less energy in steel con-taining hydrogen than in hydrogen-free steel.

The question arises to what extent the three above-mentioned effects of hydrogen might contribute tothe embrittlement of steel. In an attempt to answerthis question we measured the electrical resistance ofsoft-annealed and also of plastically deformed ironwire before and after charging with hydrogen, on theunderlying assumption that the electrical resistanceof hydrogen-charged iron wire depends on the formin which the hydrogen is present in the metal, i.e.whether it is dissolved interstitially, whether it isbound to dislocations, and so on. To prevent in-terference from inclusions and other impurities,our measurements were made on very pure iron 5).The results of this investigation, which will be

discussed under the next heading, not only leadto important conclusions with regard to the mecha-nism of the embrittling behaviour of hydrogen insteel, but also throws new light on the work of otherresearch workers. At the end of this article we shalldiscuss the consequences of our findings, and con-sider among other things how the likelihood of frac-ture is influenced by the rate of deformation and bythe temperature. In conclusion we shall examine thatdangerous form of steel failure known as delayedfailure.

Effects of the electrolytic charging of iron withhydrogen

It is known that the electrical resistivity of ironthat contains both dislocations and carbon atomsis higher in the state in which these imperfections

4) J.D. Fast and D. J. van Ooijen, Hydrogen in iron and steel,I. Solution and precipitation, Philips tech. Rev. 24, 221.227,1962/63 (No. 7).

5). Made by the method described by J. D. Fast, A. I. Luteijnand E. Overbosch, Philips tech. Rev. 15, 114-121, 1953/54.

occur separately than in the state in which they arebound to one another 6).

Originally our measurements of the electricalresistance of pure iron wires electrolytically chargedwith hydrogen (in some cases preceded by plasticdeformation), seemed to indicate that there is alsostrong interaction of hydrogen and dislocations.Further investigation, however, revealed the COlIl.-

plicating factor that charging with hydrogen leadsto macroscopically measurable, permanent changesin the dimensions of the wires 7). Examination undera light microscope showed that these dimensionalchanges are caused by microcavities and micro-cracks along the grain boundaries (see figs 3 and 4).

Fig. 3. Soft-annealed iron wire after electrolytic charging withhydrogen. Cracks have formed at grain boundaries owing tothe evolution of molecular hydrogen of high pressure. Thecracks and grain boundaries were made visible by polishing andetching a longitudinal cross-section. Magnification 100 x.

Fig. 4. Cold-worked iron wire after electrolytic charging withhydrogen. The cracks generated, made visible in the same wayas in fig. 3, lie here mainly parallel to the long axis of the wire.Magnification 50 X .

6) A. B. Bhatia, Proc. Phys. Soc. B 62, 229, 194,9; A. H.CottreH and A. T. Churchman, J. Iron Steel Inst. 162, 271,1949.

7) For further details of these experiments, see: D. J. vanOoijen and J. D. Fast, Electrical resistance of hydrogen-charged wires, Acta metallurgica 11, 211-216,1963 (No. 3).

254 PI-IILIPS TECHNICAL REVIEW VOLUME 24

The only possible explanation for their presence isthat H2 molecules were formed at these boundaries,giving rise to local gas pressures which exceeded thelocal cohesive strength of the metal. These H2 mole-cules are most likely to form at high-angle grainboundaries, i.e. at the boundaries between crystalsthat differ considerably in orientation. Fig. Sashows a model of such a boundary; in fig. Sb canbe seen a model of a boundary between crystalsdiffering only slightly in orientation.

Description. and discussion. of some of our own experi-ments

Fig. 6 shows for two iron wires (the same as de-picted in figs 3 and 4) the relative resistance incre-ments (fJRjRb measured at 77 oK as a function ofthe charging time (at room temperature). One wirewas a soft-annealed wire of 0.5 mm diameter, the

a

b

76168

Fig. 5. a) Model of a high-angle grain boundary, obtained usingsoap bubbles. The boundary is one between two "crystals"that differ considerably in orientation. For iron containinghydrogen it is thought probable that molecular hydrogen formsin the cavities of atomic dimensions existing at such boundaries.b) By the soap bubble method it is also a simple matter tobring together two "crystals" with a small difference in orien ta-tion. The difference is then bridged by a series of dislocations.(After W. M. Lomer and J. F. Nye, Proc. Roy. Soc. (London)A 212,576. 1952.)

0,06 v~7\/

1

/l)~Î-l "I

x

0,02

00 20 30 40 50-+t

60 70mm10

Fig. 6. Relative increase ofresistance (LJRjR)77 at 77 "K as afunction of charging time t for soft-annealed and cold-workediron wire (circles and crosses respectively). The much smallerresistance increase of the cold-worked wire is shown to he duenot so much to the interaction of hydrogen and dislocations,as we originally thought, but to the micro cracks generated,visible in figs. 3 and 4.The diameter of the first wire was 0.5 mm; the second wire

was drawn from 0.5 to 0.3 mm diameter. The wires werecharged at room temperature and at constant current density.The quantity of hydrogen ultimately taken up was about 10-3at. HJat. Fe.

other a wire drawn at room temperature from 0.5mm to 0.3 mm diameter. The dislocation density inthe latter ("cold-worked") wire was considerablygreater than in the soft-annealed specimen (theeffect of cold-working being to introduce dislocations,whereas soft-annealing removes thcm). The ultimatepercentage of absorbed hydrogen was found byvacuum extraction to be about 0.1 at. Oio for bothwires.

From fig. 6 it can be scen that the relative resist-ance increase of the soft-annealed wire is manytimes greater than that of the cold-worked wire.Until we discovered the formation of microcracksalong the grain boundaries, it seemed obvious tointerpret these observations in the same way asthe above-mentioned results found for the presenceof carbon instead of hydrogen. Our original conclu-sion was therefore that the resistivity is increasedby interstitially dissolved hydrogen, and that thisincrease is lower the more dissolved hydrogen isbound to dislocations.

However, after we had discovered that chargingwith hydrogen causes changes in dimensions, it wasplain that this conclusion was no longer valid.Since not only any dissolved hydrogen but also thedimensional changes will affect the resistance ofthe wires, we must first eliminate from our results theinfluence of the changes in dimensions.

The resistance R of a wire is given by the formula:

(1)

r---:----------------------~---~---.- ---

1962/63, No. 8 HYDROGEN IN IRON AND STEEL, II 255

where e is the resistivity, I is the length and A thecross-sectional area of the wire. The repeatedlyoccuring quotient I/A will be denoted henceforth byG (from "geometry"). After charging, the increasedresistance can be written as:

R + LlR = (e + LIe) (G + LIG).

The relative resistance increment is then given by:

LlR/R = Lle/e + LlG/G.

The two contributions Lle/e and LlG/G, from whichthe relative resistance increment is built up, can beseparately determined by performing the resistancemeasurements at two temperatures both before andafter charging. The values of LlG/G and (Lle/ebfound in this way are presented in Table I for atemperature of 77 °1(,

The proof of the above is as follows. Before charging, thedifference of the resistances at the temperatures Tl and T2is given by the equation:

.... (4)

where G is regarded as independent of temperature. Thisis permissible because the change of G, due to thermalexpansion, in the transition from T2 (approx. 77 OK) to Tl(approx. 300 OK), is very small compared with the change ofG resulting from the hydrogen-charging. After charging withhydrogen, we have:

R'(TI) - R'(T2) = {(Q + Lla)TI - (Q + LlQ)T2} (G + LJG). (5)

According to Matthiessen's rule, the increase in the resistivityof a metal, caused by foreign atoms and other lattice imper-fections (dislocàtions, vacancies, etc.) is independent of tem-perature. Assuming that this rule also applies to the caseunderconsideration (in the article cited under 7) we demonstrate thatthis is in fact the case), we can write (5) as:

R'(T1) -R'(T2) = {a(TI) -a(T2)}(G + LlG). .. (6)Dividing (6) by (4) yields:

R'(TI) - R'(T2) _ 1 AGR(TI) - R(T2) - +G . . . . . (7)

We can thus calculate LJG/G from the results of the measure-ments. Since LJR/R has been measured, fla/a can then be foundwith the aid of equation (3).

In reality, flG/G and fla/a are determined by a somewhatdifferent procedure, since after charging it is difficult to obtainexactly the same temperatures Tl and T2 as before charging;for this procedure see 7).

Compared with fig. 6, Table I gives a much lesspronounced indication of the existence of an inter-action between hydrogen and dislocations. Thistable shows that the difference between the increasesof resistance of the two wires is not primarily dueto a difference in the resistivity changes but to thedifference in the dimensional changes (LIG/G). Thisdifference can be explained from the fact that themicro cracks III the cold-worked wire do not have

(2)

the same orientation as in the soft-annealed wire.As a result of drawing, the grain boundaries of thecold-worked wire are roughly parallel to the longaxis of the wire, and so too are the majority ofmicrocracks produced by charging with hydrogen.This can be seen in fig. 4, which presents a longi-tudinal cross-section of the cold-worked wire aftercharging. In the soft-annealed wire the cracks arerandomly oriented with respect to the axis of thewire (fig. 3), and therefore have a much greater in-fluence on the resistance.

After the effect of the dimensional changes hasbeen eliminated, there remains a relatively smalldifference in (Lle/e)77 between the two wires whichmight be thought to be attributable to the inter-action of hydrogen with dislocations. The followingexperiment, however, makes even this seem ratherimprobable.

We kept the soft-annealed wire from the previousexperiment for 24 hours at room temperature.During this time the wire lost at least 95% of it~hydrogen without showing any change in (Lle/e)77'Annealing for two hours at 350°C was necessary todecrease the value of (LIe/e)?? appreciably, viz-from1.1% (Table I) to 0.6%. However, the same decreaseis found if an iron wire completely free of hydrogenis given the same heat-treatment after it has beenplastically deformed to such a degree as to show thesame value of 1.1% for (LIe/e)??The higher resistivity measured after charging

with hydrogen therefore seems to be "due not somuch to dissolved hydrogen but rather to the plasticdeformation of the lattice in the neighbourhood ofthe microcracks. One can appreciate that this effectwould be more pronounced in the soft-annealedwire, which contained far fewer dislocations beforecharging than the cold-worked" wire. This offers asatisfactory explanation for the differences in(LIe/e)?? between the two wires, although one can-not entirely exclude the possibility that dissolvedhydrogen also plays some part in this connection,though an insignificant one.

(3)

Table I.' Relative increase of the resistance of pureïron dueto hydrogen-charging (oné hour in 0.1 n H2SO'1 doped with50 mg/litre As203, to promote hydrogen absorption, at a currentdensity of 0.12 Afcm2) for a soft-annealed and a cold-workedwire. (LJR/R)n is the total change at 77oK; LlG/G is the contri-bution from changes in dimensions; (Lla/e)77 is the contributionfrom changes in resistivity.

(LJR/R)n%

LlG/G%

Soft-annealed 5.40 4.27 1.08

Cold-worked 0.86 0.37 0.49

256 PHILlPS TECHNICAL REVIEW VOLUME 24

Let us now ;turn to the increase of resistance due tochanges in dimensions. This increase is a kind of "shadoweffect" since it depends on the fact that no current is carriedby small regions of the conductor just in front of and justbehind a cavity (seen in the direction of the current). Therelative increase of resistance of a wire caused by sphericalcavities having a relative volume LJV/V follows' from cal-culations by Landauer 8):

L1R L1V]f"= 1.5V' (8)

where R' is the resistance of the same wire in the absenceof the cavities. For the soft-annealed wire L1V/V was foundto be 2.4% (inerease of diameter 1.2%, inerease in lengthnegligible). Randomly-orientated lenticular cavities, havinga relative volume of 2.40%, will cause LJR/R' to be greaterthan given by equation (8): (LJR/R/lthcor> 3.6% with re-spect to a wire with the same (enlarged) diameter but with-out cavities. The experimental value of the relative increaseof resistance of the soft-annealed wire caused by cavities(LJG/G = 4.27%; Table I) relates to the original diameterof the wire. Its value with respect to the enlarged diameteris obtained by correcting for the observed increase in cross-sectional area: (LJR/R/)cxp = (4.27 + 2.4)% = 6.7%. Theagreement between the theoretical and experimental valuesis not unsatisfactory.

Experiments of other research workers

From the above experiments and from otherscarried out by us 7) it may be concluded that thechange in the resistance of an iron wire, measuredafter charging with hydrogen, is not or to no sig-nificant extent, due to the presence of dissolvedhydrogen. The cause of the change of resistance israther the permanent damage which precipitatedmolecular hydrogen inflicts on the metal in the formof microcracks and plastic deformation. In com-plete agreement with this conclusion are the resultsof an investigation into the cause of the broadeningof Xsray diffraction lines after electrolyticallycharging iron with hydrogen 9). It has been foundthat this line-broadening agrees entirely with thatproduced by a few per cent cold working. The "re-covery" (elimination of the line-broadening) duringheating at 425 oe or 475 oe follows the same patternin both cases and proceeds with the same activationenergy. The latter is very much greater than thatof the diffusion of hydrogen in iron, and greater toothan that of the surface reactions involved in theescape of hydrogen from iron. This demonstratesthat the recovery in question does not depend onthe expulsion of hydrogen.It is interesting to note that the occurrence of

permanent damage to iron after charging with

8) R. Landauer, J. appl. Phys. 23, 779, 1952.0) A. S. Tetelma~, C. N. J. Wagner and W. D. Robertson,

Acta metallurgica 9, 205, 1961 (No. 3).

hydrogen was inferred from magnetic measurementsas long ago as thirty years by Reber 10). He electro-lytically charged flat rings of magnetically soft ironwith hydrogen and observed that this caused aconsiderable increase in magnetic hardness (themaximum permeability and remanent magnetiza-tion dropped, and the coercivity rose). Expulsionof the hydrogen left the change in the magneticproperties virtually unaffected.

The cracks found by us along the grain boundariesof iron as a result of electrolytic charging withhydrogen also provide an explanation for certainremarkable phenomena found by other researchworkers. As an example we mention an extensiveinvestigation undertaken by Simone Besnard 11).She charged iron electrolytically with hydrogenfrom a bath to which Na2S had been added to pro-mote the take-up ofhydrogen. After charging, it waspossible to demonstrate the presence of sulphuralong the grain boundaries to a considerable depthinside the metal. The phenomenon was carefullystudied by partly replacing the sulphur in Na2S bya radioisotope (sulphur 35). No satisfactory expla-nation was gi~en, however, for the penetrationof sulphur. Our own experiments immediately sug-gest the explanation that in Besnard's experimentsliquid from the electrolytic bath entered the metalvia cracks along the grain boundaries.

It was long ago demonstrated by Bardenheueret al. 12) that hydrogen may cause cracks along grainboundaries in commercial steel. They offered an ex-planation, for example, of the damage produced insteel upon dip-soldering in molten brass, after thesteel had been pickled to obtain a clean surface.When the steel is dipped in molten brass, the hydro-gen taken up during the pickling process is expelledso rapidly that large cracks form along the grainboundaries, thus enabling the brass to penetratedeep into the steel (fig. 7).What conclusions can we now draw from the

foregoing regarding the embrittling behaviour ofhydrogen in iron and steel? May we assume, as wedid earlier, that this embrittlement is a consequenceof interaction between hydrogen and dislocations?From our own experiments, which gave no indica-tions of any strong interaction, this assumption doesnot seem to be warranted. It seems far more reason-able to suppose that the embrittlement is the conse-

10) R. K. Reber, Physics 5,297,1934.11) S. Besnard, Ann. de Chimie 6, 245, 1961; S. Besnard and

J. TaIbot, Colloquesur la diffusion à l'état solide, Saclay,1958, North Holland Publ. Co., Amsterdam 1959, p. 147.

12) P. Bardenheuer and H. Ploum, Mitt. K. W. I. Eisenforsch.16,129 and 137, 1934; P. Bardenheuer, Metall 6,351,1952.

1962/63, No. 8 HYDROGEN IN IRON AND STEEL, 11 257

quence of molecular hydrogen forming at high pres-sure in crack nuclei. This possibility is very plausiblenow that we know that hydrogen is evolved notonly in relatively large internal cavities, but also inthe very small cavities existing at grain boundaries.As regards the third possibility referred to, namelythat the embrittlement is due to adsorption of hy-drogen, we can only say here that this adsorptioncan be shown on various grounds to have only aninsignificant influence 13).

The conclusion that the formation of molecularhydrogen in crack nuclei is by far the most impor-tant of the three effects mentioned, clarifies muchthat was previously difficult to understand.

standabIe: if hydrogen is to exercise its harmfulaction by forming high pressure H2 in the cracknuclei (produced during deformation), the rate ofdeformation must be low enough and the tempera-ture high enough to give the H atoms an opportunityto reach the crack nuclei by diffusion.

For some part these hydrogen atoms are perhapsonly formed during plastic deformation, from hydro-gen molecules that had previously precipitated inpores or other defects. This view is suggested fromexperiments of Hofmann et al. 14) on the ductilityof unalloyed steel (0.22% C) in air and in hydrogen.Tensile test bars of this metal that exhibited highductility in a normal tensile test in air were found

Fig. 7. Hydrogen-charged steel wire after dipping in molten brass (magnification 500 x).The brass (yellow) penetrates deep into the steel along micro-fissures (Cl), and fills internalcavities (b). (After Bardenheuer and Ploum 12).)

a

The influence of hydrogen on the ductility and hrittlefracturing of steel

If a steel contains no hydrogen, the chance ofbrittle fracturing is greater the higher the rate ofdeformation and the lower the tem.perature. Theexplanation is found in the fact that the dislocationsin steel are as a rule pinned by foreign atoms (e.g.nitrogen atoms), and that the breaking away ofthese dislocations is a thermally activated process(see the discussion on page 253).Where the steel contains hydrogen, however, it

may show brittleness which (within limits) is morepronounced the lower the rate of deformation andthe higher the temperature. This is now under-

13) See e.g. B. A. Bilby and J. Hewitt, Acta metallurgica 10,587, 1962 (No. 6).

b

to have very low ductility in an atmosphere of purehydrogen at high pressure. The experiments dem-onstrate that the metal absorbed hydrogen duringplastic defonnation in that gas. This makes it reason-able to assume that also the high-pressure hydrogencontained in the pores may dissolve during plasticdeformation and later precipitate in crack nuclei.In the following we shall consider experiments of

various research workers that illustrate the pointsjust discussed. There can be no question, however,of giving anything like a complete survey of thenumerous investigations into the brittle fracturingof steeL The literature on this subject has assumedenormous proportions in the last twenty years,

14) W. Hofmann and W. Rauls, Arch. Eisenhüttenw. 32, 169,1961; W. Hofmann, W. Rauls and J.Vogt, Acta metallurgica10,688, 1962 (No. 7).

258 PHILlPS TECHNICAL REVIEW VOLUME 24

largely owing to the occurrence of fractures in manywelded-steel ships during and after the secondworld war. These fractures were frequently of avery serious kind, so serious in fact that severalships broke in two. A case in point, which attractedconsiderable attention at the time, was the tankerSchenectady which, in 1943, while lying at anchoroff Portland quay (Oregon), suddenly snapped intwo with a bang that could be heard several kilo-metres away. It is not known whether hydrogenplayed any part in this special case, but it is cer-tainly not inconceivable.

Brittleness appearing under test

The following experiments are of special interestconcerning the influence of the deformation rate andtemperature on the ductility of steel containinghydrogen 15). Test bars of a particular type of com-mercial steel (spheroidized SAE 1020 steel) wereelectrolytically charged with hydrogen for one hourin 4% H2S04• After charging, the hydrogen con-tent was about 10 cm" per 100 grams of metal,which was not, however, distributed uniformly overthe whole cross-section of the bar. Fig. 8 shows the

1,2

o- r---- j---1 .1--9_ -0/)(

)(1/.1/

.,/)/x- l..--'

0

in AaA

Î0,8

0,4

Fig. 8. Influence of deformation rate è on the ductility of un-charged (circles) and of hydrogen-charged steel (crosses) atroom temperature. As the deformation rate increases, theductility of the uncharged steel decreases, but that of thecharged steel rises rapidly. The latter can be explained fromthe fact that at higher deformation rates the hydrogen has lesstime to diffuse to the crack nuclei.As a measure of the ductility the true tensile strain In ActA

is plotted as ordinate, Ao being the initial and A the final cross-section of the test bar at the position of fracture. (After Brownand Baldwin 15).)

ductility of charged and uncharged steel at roomtemperature as a function of the deformation rate.The measure of ductility adopted was the truetensile strain, given by the natural logarithm ofAo/A, where Ao is the initial and A the :final cross-section of the bar at the position of fracture. At lowdeformation rates the absorbed hydrogen has amarked embrittling effect. If the deformation rateis high enough, however, the charged steel exhibitsthe same ductility as the uncharged steel.

Fig.9 gives a plot of the ductilities of charged anduncharged steel versus température for four differ-ent deformation rates. Figures IDa and b show howthe ductility is affected both by temperature and thedeformation rate (in a for uncharged and in b forcharged steel). As can be seen, there is a region infig. lOb, i.e. surface c, in which the ductility of thecharged steel - unlike that of the uncharged steel- increases with increasing deformation rate anddecreases as the temperature rises. In the light ofthe foregoing considerations, this behaviour is un-derstandable. To the left of curve i the temperatures,and hence the hydrogen diffusion rates, are so lowcompared with the deformation rates that no appre-ciable H2 pressures can form in the crack nucleiduring the deformation. To the right of curve i, insurface c, the influence of hydrogen becomes morenoticeable the higher is the temperature and thelower is the deformation rate.

According to figures 9 and 10, at more elevatedtemperatures a point is reached where the embritt-ling action of hydrogen gradually decreases againwith increasing temperature. In surface d (fig. lOb)the ductility increases both with increasing deform-ation rate and with rising temperature. This isprecisely as expected. In the first place the equili-brium H2 pressure pertaining to a given hydrogencontent drops rapidly as the temperature rises. Inthe second place the metal loses its hydrogenrapidly at temperatures above 100°C. A conse-quence of these facts is that at a given deformationrate noticeable embrittlement occurs only in alimited temperature range.In fig. 9 the limiting temperatures of the brittle

zone correspond to the points C and C' (the latterpoint, at c, has been obtained by extrapolation).Fig. 11 gives these limiting temperatures as a func-tion of the deformation rate for two hydrogen con-tents. The presence of the hydrogen is noticeableonly in the area between the two branches of thecurve.

15) J. T. Brown and W. M. Baldwin, Trans. AIME 200, 298,1954;Taiji Toh and W. M. Baldwin, Stress corrosion crack- Delayed failureing and embrittlement (editor W. D. Robertson), Wiley,New York 1956, p. 176. .' Particularly notorious are the fractures that may

1962/63, No. 8 HYDROGEN IN IRON AND S~EEL, II 259

a

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Fig. 9. True tensile strain In Ao/A versus temperature at de-formation rates of 5%/min (a), 104%/min (b), 5X105%/min (c)and 1.9X106%/min (d). G and G' give the temperatures belowwhich and above which there is no difference in ductility be-tween uncharged (circles) and charged steel (crosses). Athigher deformation rates the "brittle zone", due to the actionof hydrogen, becomes smaller. (Mter Brown and Baldwin 15).)

Fig. 10. True tensile strain In Ao/A versus temperature Tanddeformation rate è for uncharged steel (a) and hydrogen-charged steel (b). Left of the curve i the charged steel behaveslike uncharged steel, which is understandable since in thisregion the diffusion rates of hydrogen are low compared withthe deformation rates and therefore high H2 pressures have notime to form in the crack nuclei during deformation. Right ofthe curve i, in surface c, high pressures are able to form, so thatbrittleness increases with rising temperature. In surface d thebrittleness decreases again with rising temperature. This is dueto the fact that the equilibrium pressure pertaining to a givenhydrogen content decreases rapidly as the temperature in-creases. Moreover, at these elevated temperatures the metalrapidly loses its hydrogen. (After Taiji Toh and Baldwin 15).)

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Fig. 11. The limiting temperatures below or above which un-charged and hydrogen-charged steel exhibit the same truetensile strain, as a function of deformation rate è : For the solidline the charging time was 1 hour, for the broken line û minutes.The hydrogen makes its influence felt onlyin the areas enclosedby the curves. (After Taiji Toh and Baldwin 15).)

VOLUME 24260 PHILIPS TECHNICAL REVIEW

700x TO-8S2.

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occur in certain high-tensile steels (e.g. AISI 434·0and HIl) as used in aircraft construction. To pro-tect these metals against corrosion they were origi-nally given an electrolytic coating of cadmium.Hydrogen taken up by the metal during this platingprocess gave rise in many cases to fractures whenthe metal was subj ected to external or internalstresses. Characteristic of s_uchfractures is firstlythat they may occur under the influence of a staticstress far below the yield point, and secondly thatthe fracturing is often preceded by a long delayperiod. These fractures can be avoided by ensuringthat the metal can absorb no hydrogen, e.g. byapplying the cadmiuin coating by vacuum evapo-ration.

It seems obvious to assume that the stresses willgive rise to slight dislocation movements, leadingto the formation of crack nuclei. If hydrogen ispresent, high-pressure molecular hydrogen can formin these nuclei, which may thus develop into actualcracks.Of particular interest in this connection are the

following experiments in which measurements weremade of the electrical resistance of notched steelbars (AISI 4340) that were electrolytically chargedwith hydrogen and submitted to a constant load 16).Fig. 12 presents the results of the measurements(increase of resistance as a function of time) for

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-+tFig. 12. Resistance increase AR of notched steel bars (AIS!4340) after electrolytic charging with hydrogen, versns time tat constant load (127kg/mm2) and a temperature of -18°C.After an incubation period of 16minutes the resistance sudden-. ly increases, presumably owing to the generation of a smallcrack, after which the resistance stays constant for a while.The phenomenon repeats itself several times before full frac-ture of the bar (at the arrow). The broken curves representparts of the curves from figs 13and 14. (The three figures 12,13 and 14 are due to Steigerwald, SchalIer and Troiano 16).)

16) E. A. Steigerwald, F. W. Schaller and A.R. Troiano, Trans.AIME 215, 1048, 1959.

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Fig. 13. The same experiment as in fig. 12, at a higher tem-perature of 27°C. The incubation timc is here much shorter(about 3 minutes) after which the crack grows continuously.

a temperature of -18°C and a tensile stress of127 kgfmm2• When the stress is applied the resist-ance increases by a certain amount and then re-mains constant for some time. After this incubationperiod there occurs a sudden rise, followed by ashort period of constant resistance. This phenom-enon is repeated several times. It may he assumedthat each sudden rise in resistance is caused by acrack nucleus growing into a real crack of minutedimensions.At a higher temperature (27°C) the incubation

time is seen to be shorter, and to be followed by aperiod of continuous crack growth (fig. 13). In thiscase the discontinuities have apparently become toosmall to be measured. On the other hand at a lower

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Fig. 14.The same experiment as in fig. 12, at a lower tempera-ture of -4.6 °C. The incubation period is now much longer,about 3.5hours, followed by a sudden total fracture of the bar.

1962/63, No. 8 HYDROGEN IN IRON AND STEEL, rr 261

ABSTRACTS OF RECENT SCIENTIFIC PUBLICATIONS BY THE STAFF OFN.V. PHILIPS' GLOEILAMPENFABRIEKEN

Reprints of these papers not marked with an asterisk * can be obtained free of chargeupon application to the Philips Research Laboratories, Eindhoven, Netherlands, wherea limited number of reprints are available for distribution.

temperature (-46 0c), the incubation period isfound to he longer, and to be followed by a suddencomplete fracture of the bar (fig. 14) - the firstcrack that forms propagates throughout the wholebar with great velocity.

In conclusion it should be noted that there aremany other cases of fracturing which are due to thepresence ofhydrogen in steel, typical examples being"flakes" or "shatter cracks" and "fish eyes" 17). Oneof the reasons why these phenomena are not dis-cussed in this article is that our understanding ofthem is as yet too limited.

17) An extensive review will be found in: E. IIoudremont,IIandbuch der Sonderstahlkunde, Springer, Berlin 1956,p. 1375-1390.For an investigation in this field, carried outin this laboratory, see P. C. van der Willigen, Schweissenund Schneiden 9,517,1957.

3036: W. van GooI and A. P. Cleiren: Bemerkungenzur Natur der Leuchtzentren in aktivator-freien ZnS-Luminophoren (Z. Naturf. 16a,948-950, 1961, No. 9). (Remarks on thenature of the fluorescence centres in activa-tor-free ZnS; in German.)

ZnS can easily he made fluorescent by calciningit together with a halide. The nature of the latticeimperfection which is responsible for the blue fluo-rescence which is produced on irradiation withultraviolet light is not yet known with any certainty.The authors mention five possible imperfections,and discuss why the experimental data are insuf-ficient to allowan unambiguous choice betweenthem. See also 2988.

3037: C. Wansdronk: Miniature condenser micro-phones (Proc. 3rd int. congress on acoustics,-Stuttgart 1959, Vol. Il, pp. 638-640, Elsevier,Amsterdam 1961).

The author discusses the properties of two smallcondenser microphones (8 mm in diameter and55 mm long) which he designed; such microphonesare needed for acoustic work and for use in film andtelevision studios. One of the microphones has apreferred direction for the sensitivity, while theother is equally sensitive in all directions.

Summary. In order to get some insight into the causes of theembrittling effect of hydrogen in steel, the authors study thechange in electrical resistance of pure iron wires as a result ofelectrolytic charging with hydrogen. The resistance of soft-annealed wire is shown to increase much more than that ofcold-worked wire. This difference is not, or only to a smallextent, caused by the interaction of dissolved hydrogen anddislocations but must primarily be ascribed to the differentorientation of cracks which form along the grain boundariesduring charging. The formation of cracks is shown to be accom-panied by plastic deformation of the metal and by changes indimensions. Based on this invcstlgntion and on the existingliterature the authors conclude that the deleterious influenceof hydrogen on the ductility of iron and steel is mainly due tothe formation of molecular hydrogen of high pressure in micro-voids, especially in crack nuclei formed by coalescence of dis-locations. This gives an explanation for the well-known factsthat the chance of brittle fracturing of steel containing hydro-gen is greater (within certain 'limits) the lower the rate ofdeformation 'and the higher the temperature, the hydrogenatoms needing sufficient time to diffuse to the crack nucleiformed during plastic deformation. Apart from brittle fracturesoccurring during testing, attention is also paid to the delayedbrittle fractures that can have such serious practical conse-quences.

3038: D. L. A. Tjaden: Some considerations on therecording process on magnetic tape withapplication of HF bias (as 3037, pp. 758-760).

Measurements of the sensitivity of magneticrecording tape as a function of the amplitude ofthe signal, and of the "thickness losses" as a func-tion ofthe recording depth. The results are in reason-able agreement with the theory.

3039: J. Hornstra: Rectificatie van krommen(Chem. Weekblad 57, 541-544, 1961, No. 4.2).(Plotting results as straight-line graphs; inDutch.)

When plotting experimentally determined rela-tionships between physical quantities, it is oftenpreferable to plot not the experimentally deter-mined quantities but suitable functions of these,so chosen as to make the graph a straight line.Parameters occurring in the relationship can thenhe determined with greater ease and accuracy. Thisis illustrated by reference to a number of examples;a discussion of the calculation of the error and ofthelimits of applicability of the method follows. Theauthor hopes that this publication will fill a gapin the literature which became apparent to himwhen examining chemical analysts in advanced

262 PHILlPS TECHNICAL REVIEW VOLUME 24

mathematics: most of the candidates had no know-ledge of this very useful method.

3040: W. J. Oosterkamp and Th. G. Schut: Mag-netische Speicherung von Röntgenbildern(Elektromedizin 6, 147-152, 1961, No. 3).(Magnetic recording of X-ray images; inGerman.)

A brief discussion of the use of television tech-niques in medical X-ray diagnostics. Particularmention is made of the magnetic image memorizer,on which a number of X-ray images can be storedsimultaneously. See also 3016 and Philips tech. Rev.22, 1-10, 1960/61.

3041: P. A. H. Hart and C. Weber: A transmis-sion-line coupler for a fast wave transversevelocity electron beam amplifier (Nachr.-techno Fachber. 22, 358-361, 1961).

Theoretical treatment of a low-noise parametrieamplifying tube, in which the signal transmission iseffected with the aid of an electron beam. Theéuccia transmission-line coupler and the travelling-wave coupler are compared.

3042: M. T. Vlaardingerbroek: Comparison of noisein microwave triodes and in electron beams(Nachr.-techn. Fachber. 22, 399-4,02, 1961).

A brief survey of the subject matter which theauthor dealt with in detail in his thesis (R 393).

3043: G. G. J. Bos: Enkele aspecten van voorraad-ketens (Statistica neerl. 15, 489-500, 1961,No. 4,). (Some aspects of warehouse chains; inDutch.)

When goods are not supplied directly from thefactory to the customer, but via a number of dis-persed warehouses, it is necessary to ensure that eachwarehouse has sufficient stocks to meet customers'demands. In this paper the author considers thequestion of when stocks should be ordered for thedispersed warehouses, and in what quantities. A

distinction is made between two systems of organi-zation. In the fust there is a chain of warehousesbetween the factory and the customer; in the secondthe products are directed to the dispersed ware-houses by a central store,smanagement.

3044: J. F. Schouten: Der Reaktionsablauf beim, ,

Menschen (published in Aufnahme und Ver-arbeitung von Nachrichten durch Organis-men, proceedings of NTG-Fachtagung,Karlsruhe 1961, pp. 49-55, Hirzel, Stuttgart'1961). (Measurement of human reactiontimes; in German.)

In the Institute for Perception Research at Eind-hoven an installation has been developed (called"DONDERS") which makes it possible to carry outin a simple way the numerous observations requiredfor measuring human reaction times. The "DON-DERS" is used in combination with a "histometer",an instrument developed at the same time whichsorts the observations and presents them in anordered form. The installation is an illustrativeexample of the potentialities of electronic 'engineer-ing in the field of psychophysics.

3045: E. Roeder and G. D. Rieck: Walz- undRekristallisationstexturen von dünnem Wolf-ramblech mit mid ohne Zusatz (Z. Metall-kunde 52, 572-576, 1961, No. 9). (Rollingand recrystallization textures of thin dopedand undoped tungsten foil; in German.)

Investigation of the structure of rolled tungstenfoil, in which the properties of doped and undopedmaterial are compared. The observations were madewith an X-ray diffractometer and a metallurgicalmicroscope. Both the same microstructure and thesame texture (a rolling texture centred around the[110] direction) are found in the doped and undopedmaterial. The two materials show differences inregard to recrystallization temperature and theshape and dimensions of the grains after secondaryrecrystallization.