The Influence of Oxide and Sulphide Inclusions in Microalloyed Structural Steels on the Mechanism of Hydrogen Induced Cracking
Vpliv oksidnih in sulfidnih vključkov v mikrolegiranih konstrukcijskih jeklih na mehanizem z vodikom induciranega pokanja
Vehovar L1., IMT Ljubljana S. Ažman, ACRONI Jesenice
The effect of different microstructures and oxide or sulphide inclusions on the corrosion and hydrogen uptaken of tvvo fine-grained microalloyed steel grades has been investigated. In the practice, fine-grained structural steels appear to be more or less susceptible to hydrogen induced cracking, probably because of various strong interactions betvveen the absorbed hydrogen and the various alloying elements or inclusions. aeting as traps. Such effect vvas examined by cathodic polarization, and by SEM examination of the fracture surface.
Key vvords: microalloyed steels, oxide, sulphide traps, hydrogen embrittlement
Preiskovan je bil vpliv mikrostrukture oz. oksidnih in sulfidnih vključkov na korozijo ter navzemanje dveh mikrolegiranih jekel za vodik. V praksi se je pokazalo, da so finozrnata konstrukcijska jekla več ali manj občutljiva na z vodikom inducirano pokanje, verjetno zaradi močne interakcije med absorbirnim vodikom in različnimi legirnirni elementi ali vključki, ki delujejo kot pasti. Takšen vpliv je bil preiskovan s katodno polarizacijo in z SEM analizo prelomnih površin.
Ključne besede: mikrolegirana jekla, oksidne, sulfidne pasti, vodikova krhkost
1. Experimental, results and discussion
/./ Cathodic polarization cxperimcnts
The aim of the work was to investigate the resistanee of tvvo microalloyed steel grades vvith ferrite + bainite (F + B). and ferrite + pearlite (F + P) microstructure to hvdrogen embrittlement (HE) due to the aetion of trapped internal atomic hydrogen. This hydrogen vvas produced bv cathodic charging at room temperature in deaerated IN H.SO,. containing 10 mg/l of As,O, as poison.
The chemical composition and microstructure of the investigated steel grades are listed in Table 1.
The seleetion betvveen different steel grades and the studv of meehanisms vvere performed electrochemically by cathodic polarization in the above-mentioned solution al a current densitv of 3.7 mA/cnr. Smooth tension specimens vvith a diameter of lOmni vvere stressed at a constant load equal to 60 U of their
prol. dr. Leopold VF.HOVAR
Institut /a ko\ mske materiale in tehnologije
Lepi pot I I. 610011 I iublana
vield strength. After cathodic charging for different periods of tirne, ali the specimens vvere taken out of the corrosion celi and finallv tested bv means of a tensile test. vvith the intention of de-termining the reduetion of area and of fractographic analvsis.
Table 1: Chemical composition and type of microstructure
Tvpeof microstructure				Composilion (\\l. %)		
	C	Mn	Si	P S Cr Mo	Ni V	Nb Cu
Fine-grained ferrile+bainite	0,08	0.36	0.34	0,011 0.004 0.56 0.27	0.17 -	0,03 0.35
Fine-grained ferrite+perlite	0.17	1.48	0.41	0,012 0.016 -	- 0.1 IS	- 0.21
Tests of the microalloved lovv-carbon steel grade vvith F + B microstructure. alloved vvith small amounts of chromium, molybdenum. eopper and niobium, shovved that the fine-grained ferrite-bainite microstructure has a high resistanee lo HE. despite the moderate quantity of oxide particles. Il is obvious that, from the HE point of vievv, these oxides do not plav an important part.
it can bc considered that almost ali these traps were occupicd hv hvdrogen after 2h of cathodic charging. since the reduetion of area (from 79 to 711 'it) at this time is the same as after 3 hours of charging.
The fine-grained microalloved steel with F + I' microstructure shovved a high degree of susceptibilitv to HE. dne to the ac-t ion of various oxide and sulphidc particles in the ferrite-pcarlite microstructure. In addition. vanadiunt carbides (this kind of steel is ulloved vvith manganesc and vanadium) probably represent ver\ attractive traps. because of the strong interaction betvveen the traps and the absorbed h v drogen. Reduetion of area \vas dras-ticallv reduced. from 76 ' i to values between I is to 23 H . after 3 hours of charging. Moreovcr. the results shovv that the steadv-state stage had not vet been achieved. and a further lowering of reduetion ol area might stili be expected.
/.2 Fmctographic c.\aminution
The findings ot fractographic analvsis of the fraeture sur-taces of specimens which vvere. after several hours of cathodic charging. tested to failurc in the tensile testing machine agree vvell vvith the results of reduetion of area. In the čase of the (F + B) microalloved steel, vvith its high resistance to hvdrogen embrittlement. the fraeture surface bas a predominantlv duetile appearance. Hovvcver, in a fevv places characteristic spots (Fig. 11 can be observed. vvhieh. in the čase of the less resistant (F + P) steel. are considerablv more pronottnced and more nu-merous (Fig. 21.
Figure I: "i hc fraeture surlace of the (F+Bt steel, vvith the spots occurring after 3" of cathodic charging Slika 1: Prelomna površina (F+B) jekla s pegami, ki se pojavljajo po 3 urah katodne polarizacije
In ncarlv ali cases, in the middle of the spot an oxide inclu-sion occurs (Fig. 3), vvhieh obviouslv operates as an eftective trap for hvdrogen. In the vicinilv of sueh inclusions the fraeture surface has a transgranuktr appearance (Fig. 4 and 5). In the čase of the (F + B) steel these places onlv occur rarelv. vvhieh has an important effect on the measured reduetion of area. vvhieh re-mains very high. even after 3 hours of charging.
Figure 2: The fraeture surface of the l F+P i steel. vv ith larger and more numerous spots after 3" of cathodic charging Slika 2: Prelomna površina (F+Pi jekla z večjimi in številnejšimi pegami po 3 urah katodne polarizacije
Figure 3: (F+P) steel. after charging. vv ith a characteristic spot and inclusion ir, the middle Slika 3: (F+P) jeklo po polarizaciji s karakteristično pego in v ključkom v njeni sredini
The mechanism of forming the pots. vvhieh have sueh fateful consequences for the toughness of steel and its resistance to hvdrogen embrittlement. could bc the follovving. The spots mav occur in those cases vvlien there is a high enough local concentra-tion ol absorbed hvdrogen in the vicinitv ot' (e.g.) an oxide inclusion, vvhieh acts as a trap. Sueh a condition can be quickly established in the čase of cathodic polarization. vvhere the fugac-
atomic hvdrogen into molecular hvdrogen according to Zappfe's pressure theory ). Due to the sufficiently effective trap. with a high interaetion encrgy w ith hvdrogen. a critical concentration o!' hvdrogen builds up at the interiace between the trap and the lat-tice. As a result. at a certain distance vvithin the vicinitv of the spheroidal inclusion. a sub-microscopic initial crack (Fig. 6b) oc-curs in the lattice. growing in a direetion at right-angles to the di-reetion of loading of the material. Due to the presence of a con-stant stress in the material, a tri-axial state-of-stress occurs at the end of the initial crack. resulting in numerous dislocations.
it\ of hv drogen is great. orclse possibly. over longer intervals. in the čase of corrosion processes. In the čase of cathodic polariza-tion. the spots always occur in a plane which is at right-angles to the axis of loading of the specimen (Fig. 6a): for this reason. it is necessarv that for their formation there is a stress in the material (the /ig-zag cracks observed in stepwise cracking occur vvithout the presence of these stresses bv means of a re-combination of
Figure 4: Magnification of Fig. 3. The transgranular mode of fraetures can be seen in the v icinity of the inclusion Slika 4: SI. 3 pri večji povečavi. V bližini vključka se pojavlja transkristalna oblika preloma
Fracture area
Figure 6: A schematic" presentation of the mechanism vvhich makes possible the concentric grovvth of cracks around a central trap I inclusion)
Slika 6: Shematičen prikaz mehanizma, ki omogoča koncentrično rast razpok okoli centralne pasti (vključka)
© - Trap
H - Hydrogen
1 - Dislocation
gg Crack ~ initiation region
Figure 5: The transgranular nature of the fracture surface of a spot in the (F + B) steel. vvith a characteristic oxide inclusion in the middle of the spot
Slika 5: Transkristalna narava prelomne površine pege v (F+B) jeklu s karakterističnim oksidnim vključkom v njeni sredini
Figure 7: (F+P) steel after cathodic polarization vvith a fragmented sulphide inclusion and a cleavage fracture surface in its vicinitv Slika 7: (F+P) jeklo po katodni polarizaciji s fragmentiranim sulfidnim vključkom in cepilno naravo prelomne površine v njegovi bližini
Figure 9: Spots in the shape of an ellipse in (F+P) steel Slika 9: Pege v obliki elipse \ (F+P) jeklu
2. Conclusions
The investigations clcarlv show the strong influence of different oxide or sulphide inclusions on hvdrogen uptaken in mi-croallovcd structural steels with ferrite-bainite or ferrite-pearlite microstructure. Tests performed electroehemicallv by cathodic polarization have shovvn that ferrite-bainite microstructure has a high resistance to hvdrogen embrittlement, despite of moderate quantity of oxide particles. On the contrarv ferrite-pearlite microstructure showed a high degree of susceptibilitv. This lovver-ing of susceptibilitv could be attributed to various oxide. sulphide and vanadium carbide particles, liable to occur quasi cleavage fraetures.
In the vicinitv of oxide or sulphide inclusions aeting as traps for hvdrogen. characteristic brittle spots took plače, vvhich in the čase of the less resistant ferrite-pearlite steel. are considerablv more pronounced and more numerious. The final fracture of the prestressed steel occurs along the plane vvhere the greatest deco-hezion is present due to the presence of the greatest number of spots.
References
1 E. Riecke: \Vcrkst. Korros. 39. 1988. 525 II. K. Birnbaum: Proceedings of the symposium: Environment-Sensitive Fracture of Engineering Materials. Chicago. 1977. 326 K. Kiuchi. R. B. McLellon: Perspectives in Hvdrogen in Metals. Perganton Press. 1986. 46 4 B. Craig: Metals Handbook 9th Edition. Vol. 13. Corrosion. ASM International. 1(>87. 167
vv hicli. as is vv cll-knovv n. can act as sufficiently effective traps for hvdrogen. Thus. at the head of the initial crack a nevv eritieal quantity of atomic hvdrogen builds up. vv hich can act according to the pressure theorv. or. even more probably, according to the Troiano-Oriani decohesional mechanism. According to the latter. atomic hvdrogen reduces the binding force betvveen atoms of iron. causing the further grovvth of the crack, vvhose progress is made possible through the constant diffusion of hv drogen at the crack lip. vvhere onlv dislocations are present. It is the latter vvhich, after the establishment of the initial crack, are the main driving force in the further development of the crack. Thus the cvcles of hvdrogen aecumulation and trans-erv slallic splitting keep repeating themselves. vvhich makes possible a roughlv concentric grovvth of the crack around the inclusion. giving the cleavage nature of fracture surface. During the tensile testing of
Figure S: The microstructure of (F+P) steel. vv ith characteristic strips
Slika 8: Mikrostruktura (F+P) jekla s karakterističnimi trakovi
such damaged material, the final fracture occurs along the plane vvhere the greatest decohesion is present due to the presence of the greatesl number of spots (Fig. 6a).
In the čase of (F + P) steel. apart front concentric grovvth of spots due to the presence of oxide inclusions. grovvth in the shape of an ellipse vv as also observed (Fig. 2). The same mechanism can be applied for this mode of spot. The deviation from a more or less perfectlv circular shape can be aseribed to the presence of sulphide inclusions (MnS), vvhich are spread out along the di-reetion of rolling (sulphide inclusions represent very effective ir-reversible traps - Fig. 7. vvith an energy ol' interaetion betvveen them and hydrogen of approximatelv 0,98 eV14'. The deviation could also, even more likelv, be aseribed to the striplike microstructure (Fig. S), vvhich is typical for steels alloyed vvith manganese (F + P steel contain 1,48 '<; of Mn). The ellipsoidal shape of the spots could also occur along strips vvith an increased quantity of segregated manganese. vv hich tends to catalv se the formation of bainitic oreven martensitic nests. It is probable that such nests in the ferrite-pearlite microstructure may represent traps. around vvhich. taking into account the same hypothesis. othervvise round spots are formed, but vvith their coalescence just one oblong elliptical shape occurs (Fig. 9).